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Analysis of the Feasibility of Replacing Asbestos in Automobile and Truck Brakes
prepared by THE ASME EXPERT PANEL ON ALTERNATIVES TO ASBESTOS IN BRAKES
prepared for
'
THE ENVIRONMENTAL PROTECTION AGENCY
THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS
United Engineering Center
345 East 47th Street
New York. N.Y. 10017
This report was prepared by ASME's Expert Panel on Alternatives to Asbestos in Brakes. It represents the considered judgement of this Panel rather than an official position of ASME. ASME's Bylaws declare, "The Society shall not be responsible for . statements or opinions advanced ... in its, publications." (7.1.3) The report was prepared under contract to an agency of the United States Government. None of its employees, contractors, subcontractors, or their employees make any warranty, expressed or implied, or assumes any legal liability or responsibility for any third party's use of the results or such use of any information, apparatus, product, or process disclosed in this report, or represents that its use by such third party would not infringe on privately owned rights. Publication of the data in this doucment does not signify that the contents necessarily reflect the joint or separate views and policies of each sponsoring agency. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.
Copyright 1988 by THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS
All Right! Reserved Printed In U.S.A.
This is a report of an expert panel assembled by ASHE to address the technical issues associated with the removal of asbestos from vehicle friction products. This panel convened on December s and 4, 1986, to discuss issues of vehicle safety, friction material availability and per formance, and future trends in the development of asbestos-free friction products.
Panel Members
Panel Chairman
Dr. L. S. Fletcher Associate Director Texas Engrg. Experiment Station The Texas ASM University System
Members.
.
Mr. Arnold E. Anderson \ Ford Motor Company
Mr. John Fobian Director, Automotive Engineering American Automobile Association
Prof. Serge Gratch General Motors Institute
Hr. Jerry McCullough
.
Head, Development Section
Lyndon B. Johnson Space Center
Hr. Robert Nelson Manager, Technical Service ABEX Corporation
Or. Michael J. Rabins Assoc. Dean-Graduate Engrg. Programs Wayne State University
Hr. Richard Radlinski Vehicle Stability and Control Branch Nat'l Hwy Traffic Safety Admin.
Dr. Ernest Rabinowicz Prof, of Mechanical Engineering Mass. Institute of Technology
Report Prepared by:
Hr. Scott Barber Mr. Jeff Hadden
Mr. Joseph Hoess Hr. Keith Dufrane
Battelle Columbus Division 505 King Avenue Columbus, Ohio 43201
'
Mr. Jerry Francis
iii
TABLE OF CONTENTS
Pafle
EXECUTIVE SUMMARY................... ... ................................................................. .
1
1. INTRODUCTION........................................................ . . ............................ 7
2. REVIEW OF VEHICLE BRAKING SYSTEMS ......................................................... 11
2.1 Section Summary........................................................................... . 11
2.2 Braking Performance Requirements................................................. 12
2.3 Influence of Brake Friction Material Characteristics on Brake System Response........................ ....................................... 14
2.4 Influence of Brake Design on BrakeEffectiveness ..................... 15
2.5 Influence of Brake Lining-Brake Drum Pressure Distribution on Brake Effectiveness .... ........................... 20
2.6 Issues of Friction-Induced Vibration and BrakeNoise . . ..; 22
2.6.1 Stick Slip............................................................
24
2.6.2 Negative Slope of the Friction Velocity Curve .... 24
2.7 Comparison of 4-Wheel Disc Brake Systems With Front Wheel Disc-Rear Wheel Drum System .................................. 24
3. EVALUATION OF FRICTION MATERIAL PERFORMANCE ...................................... 26 3.1 Section Summary...................................... ,........................................26
3.2 General Evaluation Criteria ..................................................... 27 3.2.1 Fade Resistance ................................................. ..... 28
3.2.2 Fade Recovery..........................
30
3.2.3 Delayed Fade............... ... . . ..........................................30
3.2.4 Brake Effectiveness Versus . Speed Characteristics ......................................................... 30
3.2.5 Friction Stability ............................................................. 32
v
TABLE OF CONTENTS (Continued!
3.2.6 Wet Friction.............................................
P22
32
3.2.7 Moisture Sensitivity.................................................
34
i
3.2.8 Lining Wear Rate.......................................... ......................34
3.2.9 Friction Material Qualification ...................................... 36 1
3.3 Laboratory and Vehicle Friction Material Evaluation .... 36
i
.
3.3.1 Friction Assessment Screening Test (FAST) ................... 38
3.3.2 Friction Materials Test Machine (FMTH) ................... / 38
3.3.3 Girling Scale Dynamometer .. j. ..... .................39
3.3.4 Full Brake Inertia Dynamometer ...................................... 40
3.4 Correlation of Laboratory Test Results With Vehicle Test Results ................ .......................................... 42
3.5 Federal Braking Requirements and Other Brake Tests ................ 43
3.5.1
Federal Motor Vehicle Safety Standard 105 .................... 43
i 3.5.1.1 Stopping Distance Requirements....................... 44
i' 3.5.1.2 Parking Brake Requirements...............................44
3.5.2
Federal Motor Vehicle Safety Standard 121
46
i
3.5.2.1 Vehicle Braking Experiments...........................46
i 3.5.2.2 Parking Brake Test .............................................. 47
3.5.2.3 Dynamometer Testing for FMVSS 121 ..... 47
3.5.3 Federal Motor Vehicle Safety Standard 135 ..................... 47 '1
3.5.3.1 Front Brake Biasing .......................................... 48
3.5.3.2 Control Forces for Br^ke Application .... 48
3.5.3.3 Parking Brake Performance............... ...
48
vi
TABLE OF CONTEXTS (Continued)
Page
3.5.4 SAE Recommended Practices for Evaluating Brake Systems and Friction Materials ........................... 48
4. PERFORMANCE ATTRIBUTES OF FRICTION MATERIALS NOW IN USE .............. 51
4J1 Section Summary . ........................................................................... 51
4.2 Introduction to Friction Material Formulations ...... 52
4.3 Asbestos-Based Friction Materials .............................................. 53
4.4 Non-Asbestos Friction Materials ...... .............................. 55
t
4.4.1 Semimetallic Friction Materials...........
56
4.4.2 Non-Asbestos Organic Friction Materials ................... 57
4.4.3 Sintered Metallic Friction Materia-ls...............................58
4.4.4 Carbon-Carbon Friction Materials ........................... . . 59
4.5 Aftermarket Vehicle Considerations . . . .................................. 59 4.5.1 Drum Brakes................................................................................59
4.5.2 Disc Brakes......................................... . ...........................60
4.5.3 Factors Affecting Substitution of Friction Materials................................................
60
4.6 Direct Comparison of OEM arid Aftermarket . Brake Linings............................................. .....................................65
4.6.1 Dynamometer Test Data ......................................................... 65
4.6.2 Vehicle Test Data.....................................................................68
4.7 Issues of Consumer Acceptability of
Non-Asbestos Materials
............................................................ 70
5. REVIEW OF VEHICLES AFFECTED BY PROPOSED BAN ........................................ 72
5.1 Section Summary ....................................................................................72
5.2 Brake System Trends in PassengerCars, Trucks, and Equipment ......................................................... ... . 73
vii
TABLE OF CONTENTS (Continued)
ESS
5.2.1 Passenger Cars .................................................................... 73
5.2.2 On-Highway Trucks
................... .......................... 75
5.2.2.1 Light Trucks . . .'..........................
77
5.2.2.2 Medium Trucks . ...................................................... 78 5.2.2.3 Heavy Trucks . . ....................................................79 5.2.3 Off-Highway Trucks and Equipment ........................... ... . 79
5.3 Current European and Japanese Expedience in Brake Design and Friction Material Selection (Automobiles and Light Trucks)......................................................... ..
80
6. SUMMARY OF INDUSTRY RESPONSES TO PROPOSED BAN . ...............................84
6.1 Section Summary ................................................. ........ 84
6.2 Concerns of Respondents to Issues of Brake Performance ... 84
6.3 Concern of Respondents to Issues of Friction Material Availability ................... .............................................. 87
6.4 Concerns of Respondents to Feasibility of the Proposed Phase-Down Schedule ...................................... .................. 88
7. REFERENCES................................................. !................................................ 90
APPENDIX A ,
VEHICLE AND DYNAMOMETER TEST PROCEDURES FOR QUALIFYING FRICTION. MATERIALS AND BRAKE SYSTEMS UNDER FMVSS 105 AND FMVSS 121 .................... 93
APPENDIX B ,
CONCERNS OF RESPONDENTS TO ISSUES OF BRAKE PERFORMANCE ....................... 101
APPENDIX C ;
CONCERNS OF RESPONDENTS TO ISSUES OF FRICTIOll MATERIAL AVAILABILITY ................................................................................... .109
APPENDIX 0
CONCERNS OF RESPONDENTS REGARDING FEASIBILITY* OF PROPOSED PHASE-DOWN SCHEDULE ................................................................
viii .
115
LIST OF TABLES
EaaS
Table 1. Motor Vehicles Affected by Proposed Ban ................................... 10
Table 2. Influence of Some Brake Lining Property Changes on Brake System Response With Design Modifications to Compensate............... 1........................................ 16
Table 3. Machines For Friction Material Evaluation (Reference 3).......................................... ... .................. 37
Table 4. Test and Performance Criteria Specified Under
FMVSS 105...................................
1...................................... 45
Table 5. Selected SAE Vehicle Brake Test Codes...................................... 50
Table 6. Producers of`Brake Linings for Light and Medium Vehicles....................... . .......................................... 62
Table 7. Producers of Brake Linings for Heavy Vehicles ....................... 64
Table 8. New Vehicle Sales and Vehicle Registration For 1984 (Ref. 151.................................. ,.......................................74
Table 9. List of 1985 American Automobiles Equipped With 4-Wheel Disc Brakes (Ref. 16) . ,...................................... .76
Table 10. List of 1984 European Automobiles Equipped With . 4-Wheel Disc Brakes (Ref. 14)....................................................... 81
Table 11. List of 1984 Japanese Automobiles Equipped With 4-Wheel Disc Brakes (Ref. 10)....................................................... 83
Table 12. List of Respondents to Federal Register Notice Regarding Proposed EPA Action . . . j...................................... 85
Table 13. Summary of Respondent's Comments to Proposed EPA Plan.............................................
86
LIST OF FIGURES
Figure 1. Schematic of Generalized Braking System ............................... 13
Figure 2. Forces Acting on Brake Shoes of Two Common Designs .... 18
Figure 3. Relationship Between Brake Effectiveness and Brake Design for Various Lining Friction Levels ........................... 19
ix
LIST OF FIGURES (Continued!
Figure 4.
Page
I Effect of Lining Pressure Distribution on Brake Effectiveness for a Leading Shoe prum Brake ....................... 21
Figure S.
' Figure 6.
Fade Characteristics of Good and j>oor Brake
Friction Materials....................... '.......................................... .
"i Fade Recovery Characteristics of feood and Poor Brake Friction Materials ....................... . ...................................
29 29
Figure 7. Delayed Fade Characteristics of Friction Materials .... 31
Figure 8. Speed Versus Braking Performance Characteristics for Good and Poor Friction Materials ...................................... 31
Figure 9. Friction Stability Characteristic! of Both Good and Poor Friction Materials . . i.......................................... 33
Figure 10. Wet Braking Performance for Good'and Poor Performance Braking Materials .4 ........................................... 33
Figure 11. Effect of Moisture Sensitivity oil Braking Performance.................................. ^..................................................35
Figure 12. Wear Performance for Braking Materials ............................... 35
Figure 13. Schematic of Inertia Dynamometer1.......................................... 41
Figure 14. Comparison of Dynamometer Test Results for Four Different Friction Materials, 30p F After Burnish,
12 x 3 Duo Servo Drum Brake ......................................................67
Figure 15. Vehicle Test Results Comparing tije Performance of Aftermarket Friction Materials .'...................
69
1 1 x
EXECUTIVE SUMMARY
Due to reported health problems associated with the use of
asbestos, the Office of Toxic Substances has proposed a regulation to ban
the use of asbestos over a 10-year phase-down period. Since asbestos is
currently a critical constituent in some vehicle friction products, the
Environmental Protection Agency is interested in determining whether the
proposed ban could have adverse effects on vehicle! braking safety.
In order to assess the potential effects; of the proposed ban on
vehicle brake system operation and vehicle safety, a panel of individuals
knowledgeable in the various aspects of vehicle brjakes and friction mate
rials was assembled by the American Society of Mechanical Engineers (ASHE).
The panel addressed technological issues associate)! with the removal of
asbestos from friction materials, specifically: \
(1) Identification of substitute brakes]and systems,
i
(2) Compatibility of the ban with motor vehicle safety standards,
' (3) Problems associated with replacing asbestos friction mate
rials in aftermarket vehicles, and j
(4} Pace of research and commercialization and effects on phase-out.
This report addresses the technical issues related to the proposed
ban considered by the panel to be the most important.
'.
I*
. Conclusions
Issue 1: Identification of Substitute Brakes and Systems i
Vehicle controllability during braking ife. affected by both driver and brake system performance. Generally, vehicle Ifront-to-rear braking ratios are adjusted on a per-model basis to provide optimal braking. Too much braking on an axle results in premature wheel1 lock-up, and this can lead to loss of steering control (front lock-up) oH spin-out (rear lock up). While the effectiveness of disc brakes is directly proportional to pad-disc friction levels, the effectiveness of druln brakes can be greatly affected by changes in lining-drum friction. Friction materials that exhibit reduced effectiveness also can result in brakes having insufficient
torque needed to stop the vehicle. Different drum brake designs exhibit dramatic differences in effectiveness.
Because disc brakes maintain effectiveness at high braking speeds, 4-wheel disc brakes have been used in the past on high performance Euro pean vehicles designed to operate at high speeds on unregulated European highways. This Is one reason behind the European use of 4-wheel disc systems. In addition, non-asbestos drum brake materials for automobile drum brakes have been difficult to qualify in the past. For these rea sons, several European automakers use 4-wheel' disc brakes in conjunction with semimetallic pad materials as a means of eliminating asbestos.
The majority of the automobiles sold in the United States now are fitted with front disc brakes and rear drum brakes, and this trend will most likely continue for some time. 8ecause of differences between quali fication standards, cost, driver preference, ind driving conditions in the United States and Europe, American and Japanese automakers are utilizing rear drum brakes. Suitable non-asbestos materials are not available for all of these applications, and industry-wide substitution of non-asbestos materials in all existing brake designs would! require considerable develop ment. It is unrealistic to assume that all automakers will redesign all passenger car and truck braking systems around disc brakes in order to utilize semimetallic materials.
Issue 2: Compatibility of the Ban With Motor Vehicle Safety Standards
The qualification of original equipment braking systems is regu lated under Federal Motor Vehicle Safety Standards (FMVSS) 105 and 121. For many original equipment manufacturers, these requirements represent minimal standards of performance, and supplemental qualifications are usually satisfied due to customer demand. Satisfactory compliance with FMVSS 105 and 121 requires vehicle and inertial dynamometer test facilities to evaluate brake system performance on a perrmodel basis. To test compli ance with these requirements, vehicle manufacturers submit braking systems and friction products to numerous levels of qualification tests. No simple bench-top tests are available to evaluate the, performance characteristics of friction materials or to demonstrate their, compliance with Motor Vehicle Safety Standards. Results obtained using scaled-down laboratory apparatus,
2
such as friction material testing machines (FHTH) or friction assessment screening test (FAST) machines, have been shown to>correlate poorly with vehicle test results. The performance of aftermarket friction materials is not regulated by law.
In comparison with current federal safety standards which specify required stopping distances and deceleration, the proposed FHVSS 135 for passenger cars and light trucks will require fronts braking bias in the event of a wheel lock-up during braking. This situation requires that the front wheels lock-up before the rear wheels.
The adoption of FHVSS 135 and the failurt to qualify rear brake drum linings exhibiting consistent levels of friction over a wide range of performance conditions may affect the design philosophy of American and Japanese automotive engineers. Forced to ensure front bias in the system, automakers may move to using less effective disc brakes on the rear and increasing the front-to-rear braking ratio to ensure consistent performance, if proven rear lining materials are not found.
Issue 3: Problems Associated With Replacing Asbestos
Friction Materials in Aftermarket Vehicles
'
. The substitution of unqualified non-asbestos friction materials in the aftermarket poses' the largest potential safety issue. Because vehicle controllability during braking is stronglyiaffected by friction material performance, unqualified substitution of friction materials in either vehicle disc pads or drum linings may have an adverse effect on vehicle brake balance and controllability. A large number of older vehicles in the United States have brake systems designed for asbestos friction products. The use of unproven materials in place of proven asbestos mate rials in existing systems could result In a loss of vehicle controllability during braking.
Currently, there are no required performance tests for aftermarket brake friction materials. In addition, most of the aftermarket, non asbestos material suppliers lack the facilities to: evaluate their materials under dynamometer and vehicle braking test conditions.
Non-asbestos friction materials that have been developed to date have provided some new and unexpected failure modes and mechanisms due to their unique combinations of new raw materials and manufacturing processes.
3
Mandating aftermarket non-asbestos friction materials for vehicles that were originally equipped with asbestos-based linings could lead to a poten tially serious customer safety risk, unless stringent friction material qualification specification tests are Included.
Issue 4: Pace of Research and Commeircialization and Effects on Phase-Out
Non-asbestos friction material technology is advancing rapidly. Several new asbestos-free car, light truck, apd heavy vehicle brake systems have been released for OEM applications in the past 3 years. More new vehicle brake systems will be released with npn-asbestos friction materials for the 1988 model year.
Adequate non-asbestos friction material formulations presently are not available for all vehicle systems. Hpwever, at the present rate of technical progress, most new passenger cars could be equipped with totally non-asbestos frictional systems by 1991, and most light trucks and heavy trucks with S-cam brakes, by 1992. ; However, a few low-volume new vehicle applications may not have acceptable non-asbestos friction
materials at that time. Heavy truck wedge brake blocks, medium truck . drum brake linings and many off-road vehicle brake linings may not be developed by 1992.
New classes of non-asbestos friction materials have unique compo sitions which may have unexpected failure mechanisms. In many cases, unique failure modes are revealed only through extensive vehicle testing.
Numerous automobile, truck, and friction material manufacturers responded with written comments to the Federal Register announcement of . the proposed EPA action. While most of the automobile and truck manufac turers were optimistic about the future availability of asbestos-free materials, they were unanimous in their opposition to a proposed phasedown schedule in which the amount of allowable asbestos would be phased out over a 10-year period. Instead, they favored a 5-year lead time prior to any ban. In addition, they were unanimously opposed to banning asbestos' containing materials for older aftermarket vehicles. Although suitable substitutes have been found for automobile front disc brakes, material qualification for rear automobile drum brakes and medium truck drum brakes is still in progress.
4
Friction product manufacturers gave mixed response to the issues.
One manufacturer indicated that for many applications, suitable substitutes
are available, although for many vehicle applications no qualified materials
exist. Another manufacturer indicated that suitable substitutes were
produced by their company, although no FHVSS 105 or FHVSS 121 qualification
capabilities existed at their facility.
j" i,
. Recommended Future Mork '
Industry trends for the next 5 years appear to be directed toward the elimination of asbestos in all new vehicles. For these applications, the use of qualified non-asbestos friction materials may not present a vehicle safety problem over the life of the friction product. However, the use of unqualified non-asbestos materials for the aftermarket still
i
remains a safety issue. If the eventual elimination of all asbestos in friction products is to be accomplished, additional future studies are required. Several possible research tasks are briefly outlined in the following sections.
Task 1: Determine Populations of Aftermarket Vehicle Classes and Brake System Designs in the United Staties
The purpose of this task would be to determine and project, over the next 10 years, the general classes of brake designs found on older vehicles (passenger cars and light trucks, medium trucks, and heavy trucks) that will require aftermarket friction materials. For example, it would be useful to determine how many passenger cars in the 3000-3500 1b weight range are fitted with duo-servo drum brake and how many utilize leading-trailing drum brakes. In addition, the population of vehicles having 4-wheel drum-brakes as opposed to rear wheel drum-front wheel disc brakes could be determined. Thus, vehicle populations would be assessed with respect to weight range, brake design, and front-to-rear braking balance.
This information would help the EPA to determine where the quali fication of non-asbestos friction materials for the aftermarket would have the greatest impact on asbestos elimination. For example, if the results indicate that 80 percent of the passenger vehicles using asbestos
5
materials are fitted with duo-servo rear driira brakes and front disc brakes, and are within the 3300-3800 lbs weight range, then the requalification of non-asbestos materials for these vehicles would have the greatest Impact on asbestos elimination.
Task 2. Conduct Dynamometer and Vehicle
Qua!Ification tests on Non-Asbestos Materials
using Representative Vehicles
"
The purpose of this task would be to determine whether current non-asbestos materials could be safely used in aftermarket vehicles designed for use with asbestos friction products. This task would draw upon the results of Task 1 by limiting the vehicle,studies to these vehicles known to represent the majority of vehicles now in service. In this way, the study could be conducted In a more cost-effective manner.
This experimental study would use dynamometer and vehicle tests to determine friction product effectiveness under vehicle service condi tions. The ability of brake systems and friction materials to satisfy Federal motor vehicle safety standards would also be assessed.
This second task would determine (1) whether safe effective substitute materials are available for some [I aftermarket applications and (2) what range of effectiveness currently exists among suppliers of after market non-asbestos materials.
i
6
1. INTRODUCTION
The Environmental Protection Agency, under the authority granted
by the Toxic Substance Control Act (TSCA), has proposed a ban on the pro
duction and importation of asbestos and asbestos-containing products.
According to documents published in the Federal Register (Vol. 51, No.
19, Wednesday, January 29, 1986) the EPA has proposed several alternative
approaches to eventually eliminate or drastically reduce the amount of
asbestos used in the United States. The proposed actions apply to products
containing asbestos, as well as raw asbestos mined or imported for incor
poration into products. .
.
The proposed ban on asbestos would have a direct effect on fric
tion materials used in automotive, truck, transit bus and train brake
systems. At the present time, many friction products contain asbestos.
Asbestos provides several performance attributes for the brake lining
both during its intended use as well as during its manufacture and assem
bly. Removal of asbestos from these materials raises the following
questions:
(1) Will asbestos-free friction products allow motor vehicles to meet current and proposed braking, standards? Non-asbestos friction materials have added new complexl ties to brake system design and development because of their new, different, and sometimes unexpected functional proper ties. Furthermore, brake system development work becomes more difficult with the present shortage of fully evalu ated, documented non-asbestos brake lining formulations. However, fully non-asbestos formulations for passenger cars, light trucks, and heavy trucks have been produced and more have been released for production. There is con siderable controversy whether acceptable asbestos-free substitute friction materials have been found for some vehicle brakes, in particular some o,f the larger passenger car/medium truck drum brakes.
7
(2) Will the use of asbestos-free materials require redesign of braking components, such as actuation cylinders, distribution valves, and reservoirs and accumulators, to permit safe use of non-asbestos brake linings? This question arises due to the proposed requirement for applying asbestos-free friction materials to existing vehicles that have braking systems designed for use with asbestos-containing friction materials. When non-asbestos friction materials are installed In existing vehicle systems with no alteration of the rest of the braking system, stopping distances may increase, required brake actuation forces may increase, resistance to fade may decrease, and undesirable brake stability proI blems may arisen
' It would appear to be prohibitively expensive to replace !
brake system components to accommodate friction materials with different frictional characteristics because of the extensive redesign and retesting required. To assess the impact of the proposed ban, the feasibility will have to
be addressed for the direct replacement of asbestos brake linings on existing vehicles with non-asbestos friction materials.
(3) What schedule for asbestos phase-out can be implemented without compromising vehicle braking performance and safety? This question arises due to the possibility that in some applications acceptable substitute materials may not be available because of performance, manufacturability, or profitability Issues. For some specialized vehicles, the market for friction products may be. so small as to restrict the profitability of developing and manufacturing suitable asbestos-free friction materials and required associated hardware.
Objectives of this study were to: (1) Define the technological issues associated with the EPA's
8
plan to disallow the use of asbestos in automobile and truck braking systems. Such issues would Include:
Identification of substitute brakes and systems, Influence of the ban on motor vehicle safety standards, Problems associated with replacing asbestos brakes on
existing vehicles with alternative materials, and Pace of research and commercialization as they affect
the proposed time for phase-out of asbestos. (2) Collect relevant technical information required to resolve
those issues. (3) Present the findings and conclusions of the study regarding
those issues where the state of existing knowledge is suffi cient to make a sound engineering judgement. (4) Define the limits of current knowledge and identify priority ` research topics where available information is not sufficient to draw firm conclusions.
This report addresses these and other issues created by the
proposed EPA action against asbestos. Section 2 of the report describes
vehicle braking systems now In use In both on-highway and off-highway
vehicles. Table 1 lists the general vehicle classifications using these
braking systems and vehicle examples affected by the ban. Section 3 of
the report describes how friction materials are evaluated and qualified
for vehicle use under Federal Motor Vehicle Standards and Society of Auto
motive Engineers (SAE) practices. Section 4 describes the performance of
the various friction products. Section 5 covers vehicle applications
affected by the proposed ban. Finally, Section 6 outlines the responses
of the various industries to the EPA's proposed ban.
.
9
*
TABLE 1. MOTOR VEHICLES AFFECTED BY PROPOSEO BAN
Vehicle Category
On highway (low weight)
On highway (high weight)
Example
5
! Representative Brake Systems
Automobiles and
light trucks
'
4 wheel drum Rear wheel drum and front
wheel disc
4 wheel disc
Heavy trucks Tractor-trailer
combinations Buses .
Concrete mixers Tank trucks
; .
i
Hydraulic drum S-cam air drum Wedge air drum
Air disc
Off highway
Logging trucks Mining trucks Agricultural
equipment
.; S-cam air drum Air disc Wet (oil) disc
1 r-- 1 TTi a c-
--------- '"ra
2. REVIEW OF VEHICLE BRAKIH6 SYSTEMS
2.1 Section Summary
A review of vehicle braking systems is presented in this section and includes discussions of the performance requirements, factors influ encing performance, and general design characteristics of the main braking systems. The main purpose of this section is to examine how vehicle braking performance is affected by the friction characteristics of both pad and lining materials. This is of particular importance, in the aftermarket in which asbestos materials may be replaced with non-jasbestos materials having different frictional properties. The main points of the section are:
Vehicle brakes must provide consistent, and dependable per formance over a wide range of environmental and operating conditions, causing complex design and, development challenges and necessitating performance tradeoffs.
' Brake lining friction and wear characteristics, along with thermal and mechanical properties, all, affect the brake torque output, especially with high-servo-factor drum brakes. The performance of these brake systems, which are common on many American-made automobiles, is especially sensitive to the frictional properties of the brake lining material.
Vehicle controllability and stability during braking is depen dent upon the stability and consistency of the brake system. For example, an alteration In the front-to-rear braking balance due to a change in either the front or rear friction material may decrease vehicle controllability. The use of friction materials having different wear, stiffness, friction, or thermal expansion characteristics may cause a degradation in braking performance.
Structural and vibrational behavior of, brake assemblies can be significantly affected by the frictional, elastic, and structural properties of the friction materials. Brake squeal occurs when the brake and associated structural components are excited to vibrate due to friction. Some friction materials naturally damp or dissipate the vibration, while other mate
11
rials accentuate vibration. The tendency for squeal is deter mined mainly through vehicle testing because prediction by analytical means is difficult.
2.2 Braking Performance Requirements
The primary requirement of a vehicle braking system is to provide
the capability to decelerate the vehicle in a controlled manner at an
acceptably high rate over the full range of in-service operating and envi
ronmental conditions. The brake system must provide consistent and depend
able frictional behavior for all reasonable conditions. These conditions
Include:
' Environmental Conditions.
.
. - temperature
..
- humidity
- road conditions (debris, wet, oily, etc.)
- barometric pressure
- contamination level
:
Operating conditions
.
- vehicle speed
.
- vehicle trajectory
.
- tire condition
- brake system component condition
- brake component temperatures
- brake prior usage history
- road condition and grade
- traffic.pattern
A conceptual schematic of a braking system Is provided in Figure 1. The figure illustrates the feedback control nature of a braking operation, in which the driver is a critical controlling element. The . driver must process information about this operating environment, based upon which he must respond by depressing the foot pedal with a degree of effort that he believes will decelerate the vehicle as he desires. The actual braking performance of the vehicle depends upon several factors, the more important of which are:
12
Environmental and Operating
Conditioni
FIGURE 1. SCHEMATIC OF GENERALIZED BRAKING SYSTEM
. Accuracy of the driver's perception of the operating . environment,
Dynamic response characteristics of the driver, Predictability and reliability of the total brake system, Vehicle/road characteristics, and Response characteristics of the; braking system. In order to stop a vehicle safely, tjhe brake system should provide repeatable, uniform deceleration for the same brake pedal input and provide braking force in proportion to pedal forceJ Further, vehicle path direction
should be readily controllable by the driver during braking. * The limitation to decelerate a vehicle depends ultimately on
road/tire traction, which varies with wheel! slip. Maximum braking requires the brake torque to be just enough so that the resulting wheel-slip gener ates peak traction force available at the tire-road interface. Excessive brake torque causes a progressive increase in wheel slip and decrease in adhesion, resulting in wheel lockup and skidding. Skidding may produce an unsafe condition because directional control of the vehicle is reduced substantially and stopping distances may be increased. Rear-wheel lock up during braking results in severe vehicle uncontrollability. For this reason, current European braking standards and proposed American braking standards require that in the event of wheel lock-up, the front wheels must lock before the rear wheels. To ensure this condition throughout the life of the friction material, both front and rear brake system per formance must not be altered by friction material wear or by other changes In the material friction characteristics. Changes in the friction material properties, either due to replacing the brake shoes with different friction materials, or due to wear, prior operating history, temperature, and load effects, will change the response characteristics of the braking system.
2.3 Influence of Brake Friction Material Characteristics on Brake System Response
There are several scenarios in which an existing hydraulic or pneumatic system would require modification or redesign to accommodate changes in the properties of the brake friction material. These are sunwarized in Table 2. As indicated in the table, substitution of non-asbestos
14
friction materials for original asbestos linings requires characteriza tion of the substitute material properties, followed by an evaluation or verification of the performance of the existing brake system with the non-asbestos materials. Due to brake system design differences, this must be done on a model-by-model basis.
For new vehicles, non-asbestos friction materials may be incorpor ated into a new brake system design. Depending on the characteristics of the non-asbestos friction materials, compared with those of the asbestos materials, the new brake system design could involve minor modifications . of existing designs or completely new brake components.
In order to achieve adequate braking performance, brakes are designed primarily on the basis of wear, stability, brake or shoe factor, (ratio of brake torque to applied force) and pedal travel.(1) These parameters are interrelated. For example, lining material thermal expansion may require added running clearances to avoid parasitic drag. This then results in the need for greater brake pedal travel, which may lead to a master cylinder resizing. If pedal forces then become too great, a booster may need to be added or increased in size. Some of these relations are dis cussed in the following sections.
2.4. Influence of Brake Design on Brake Effectiveness
As mentioned previously, it is important that a vehicle have the ability to be decelerated rapidly and controllably during emergency braking. Thus the response of the vehicle to the driver's brake "commands" must be predictable, repeatable, and fast enough to stop the vehicle in a short distance, but slow enough that the driver can respond effectively with subsequent corrective braking and steering commands, e.g., to avoid skidding or "fishtailing".
The relationships between braking (frictional) torque, applied force, and the coefficients of friction have a strong influence on vehicle stability. The parameter that most strongly affects the stability of brakes is the shoe or brake factor (also called effectiveness). The brake effec tiveness is defined as the ratio of the brake friction torque to the applied force and is used commonly to describe the performance of brakes. -
15
TABLE 2. INFLUENCE OF SOME BRAKE LINING PROPERTY
CHANGES ON BRAKE SYSTEM RESPONSE WITH DESIGN MODIFICATIONS TO COMPENSATE
F ric tio n
Properties
Change of Brake Lining Properties
Potential Response of Brake System
Lower Friction Coefficients
- Front brakes - Rear brakes - Both front/rear
i
Greater Pedal Forces Required, Leading to:
. - Early rear skid - Early front skid - Low brake capacity
Higher Friction Coefficients - Front brakes - Rear brakes - Both front/rear
Lower Brake Pedal Force Required. Leading to:
- Early front skid - Early rear skid - "Touchy" pedal
Poor Fade Characteristics -'Green fade - Thermal fade - Water fade - Flash fade
Inconsistent Friction Level - Green linings - Usage history - Wear depth - Within batch - Batch-to-batch
High Brake Pedal Force Required For: - Initial hot brake - Any hot brake, - Wet brakes - High speed stops
Varied Brake Pedal Forces Needed With:
' - New brake linings - Temperature change - Mileage driven - Brake Imbalance - Car-car variations
Environmental Sensitivity - Water/water vapor, - Road dust - Oily contaminants - Oxide/rust effects
Brake Pedal Force
Sensitive to: - Humidity - Dust pickup - Road rain splash
- Moist storage
Lower Compression Modulus
Higher Brake Pedal Travel
Higher Compression Modulus
Noise and Uneven Effectiveness
Higher Thermal Expansion/Growth
Brake Dragging and Hot-Spotting
Lower Tensile Strength
Lining Fracture, Cast Iron Scoring
Lower Toughness
Handling Breakage
16
Needed Design Modifications
Larger Booster or Small Master Cyl. - Big front w.c. - Big rear w.c-. - System redesign
Smaller Booster/ Big Master Cyl. - Small front w.c. - Small rear w.c. - System redesign
Recalibrate/Add Booster - Scorch linings - Modify linings - Shield brakes - Modify linings
Change Linings/ Redesign Brake - Modify linings - Modify linings - Modify linings - Improve process - QC improvement
Change Linings/ Redesign Brake . - Modify linings - Shield brakes - Shield brakes - Materials change
Redesign/Stiffen Actuation System
Reduce Shoe/Drum Stiffness
Add Clearance or Thin Linings
Use More Rivets, Bond/Mold Linings
Alter Processes
Mechanical
Properties
Figure 2a shows the forces acting In the shoes of a simple lead ing-trailing drum brake. As shown, the friction force on the leading shoe causes it to be further loaded against the drum, increasing its effec tiveness, while the friction force on the trailing shoe causes it to oppose the application force, decreasing its effectiveness;
In the duo-servo design. Figure 2b, the friction force of the primary shoe is used to apply the secondary shoe, thereby markedly amplifying the application force. The increase in brake loading due to friction is referred to as self-actuation, and this phenomenon can be used to Increase the brake effectiveness. This design has been and still is used extensively on American cars using asbestos-containing brake linings.
Although the phenomenon of self-actuation can increase brake effectiveness, the effectiveness is strongly influenced by the frictional properties of the lining material. Figure 3 shows the relationship between brake effectiveness and 1ining friction coefficients for different drum brake designs and for an automobile disc brake. Duo-servo drum brakes are widely used in American automobiles, while leading-leading systems are found on many transit buses and heavy trucks. Leading-trailing systems are also found on most heavy trucks and on many automobiles. Assuming a nominal friction coefficient of 0.4, a 12-percent change in friction coeffi cient can alter the brake effectiveness by approximately 44 percent for a duo-servo drum brake, 33 percent for the leading-trailIng drum brake, and only 12 percent for a disc brake. Figure 3 also shows that for automobile rear drum brake systems, friction materials qualified for use with duo- . servo systems may not perform satisfactorily when used with leading-trailing systems. Conversely, materials qualified for use in leading-trailing systems may precipitate rear wheel lock-up when used with a more effective duo-servo system.
Aftermarket friction materials from different manufacturers can exhibit a wide variance in friction characteristics. This can lead to variable and unpredictable brake performance, even with materials containing asbestos. Under current law, aftermarket friction materials do not have to meet Federal Motor Vehicle Safety Standards.
As more major OEM friction material suppliers phase out asbestos materials, the burden of supplying materials for the aftermarket will fall on these secondary aftermarket suppliers. Under the proposed ban on
17
Friction
Force on Trailing Shoe
Wheel Cylinder
Hydraulic Force on Leading Shoe
Leading Shoe
Friction Force on Leading Shoe
Shoe Pivots
a. leading-trailing
Application force
through linkage
Drum rotation b. duo-servo FIGURE 2. FORCES ACTING ON BRAKE SHOES OF TWO COMMON DESIGNS
18
0.2 0.3 0.4 Lining Coefficient of Friction FIGURE 3. RELATIONSHIP BETWEEN BRAKE EFFECTIVENESS AND 8RAKE DESIGN FOR VARIOUS LINING FRICTION LEVELS
19
asbestos-based linings, the brake system engineer would have two choices. First, the drum brake could be redesigned and redeveloped for acceptable performance with non-asbestos linings. 'Second,.the designer could wait for a lining formulation to be developed that can be used directly in existing drum brake designs. This is a critical decision. Redesign of a brake assembly is costly, time consuming, and probably unrealistic. How ever, waiting for the development of a suitable universal non-asbestos replacement friction material--that has.yet to be made--involves the risk of not having acceptable aftermarket components if asbestos-based brake linings are banned.
2.5 Influence of Brake Lining-Brake Drum Pressure Distribution on Brake Effectiveness
For drum brakes of the same design, the use of different materials exhibiting similar friction coefficients is not sufficient to ensure con sistent and even braking throughout the life of the material. The use of a material that exhibits different wear or thermal properties can be suffi cient to change the effectiveness of the brake system. Figure 4 shows the variation in effectiveness for the same brake design but for different brake lining pressure distributions. Note that large changes of brake . effectiveness are possible for the same value of friction coefficient due to the effect of lining pressure distribution.
Several factors can lead to uneven and inconsistent brake liningbrake drum pressure distribution, but the most significant are wear and thermal distortion. Wear tends to shift the center of pressure location. Thus, substitute friction materials with different wear rates from the original asbestos materials may also influence stability by changing the center of pressure. A reduction in brake effectiveness, or an increase in brake effectiveness leading to wheel lock-up, could be possible conse quences of improper friction material selection.
The profiles of the brake linings also are affected by thermal distortion. Brake lining expansion through the friction material thickness is often much greater for non-asbestos brake linings, and these friction materials also tend to be much stiffer. Therefore, the contact geometry and pressure distributions will be different, which can affect the friction
20
Effectiveness
FIGURE 4. EFFECT OF LINING PRESSURE DISTRIBUTION ON BRAKE EFFECTIVENESS FOR A LEADING SHOE DRUM BRAKE
21
forces generated at the contact area, especially on the larger drum brake
assemblies. For this reason, friction materials with higher thermal expan
sion rates may cause inconsistent braking performance, especially with
varying brake temperatures.
:
An added complication results from the greater thermal expansion
rate and compression stiffness of many non^asbestos friction materials.
This Is called hot-spotting, a thermoelastlc Instability that causes the
lining pressure distributions to become localized, with resultant high
thermal stresses and erratic friction values. The basic principles are
simple, but the total system effects can be extremely complex.
For example, one particular brake lining location may have a greater
clamping load than average, providing greater friction and heat generation.
This causes that location to expand, further increasing its excess load.
If the lining wear rate is low, this cycle'can continue for some time,
leading to the formation of a hot spot. Hot spotting tends to occur during
low temperature brake usage, with light brake applications, and during
braking from highway speeds. Since this brake usage is not severe, it can
inadvertently be overlooked in the early stages of the brake development
process.
,
2.6 Issues of Friction-Induced Vibration and Brake Noise
.
Vehicle braking systems generally include several structural components, e.g., calipers, pedal linkages, etc. Forces generated during braking can cause high stresses in these brake components and in vehicTeassociated components, e.g., axles and suspension elements. These stresses result from the nominal braking torque as well as from thermal and other dynamic loads. Sufficiently large dynamic and thermal stresses superimposed on the nominal braking stresses might promote fractures or fatigue failures In some vehicle structures, which then could result in an unsafe operating conditions. The brake friction material properties strongly influence the dynamic behavior of the vehicle structure during braking because these properties determine the magnitude and frequency of the braking torques generated at the friction interface. For example, phenomena such as brake squeal, chatter, and groan produce structural vibrations of vehicle com
22
ponents which may be either reduced or increased by substituting different brake friction materials. The influence of a particular braking material on structural vibrations depends strongly upon the specific brake system design. Therefore, replacement materials should be evaluated carefully for critical vehicle applications.
Another structural consideration occurs in attaching the friction material to the brake shoe. This can be done by riveting, bonding, or integrally molding the lining to the brake shoe. Non-asbestos linings generally are stiffer, more brittle, and more highly anisotropic, i.e., their properties are sensitive to orientation, which can provide attachment challenges.
Friction-induced vibration and noise can occur under some condi tions with vehicle braking systems. The phenomenon has received consider able attention, and attempts are normally made to quantify the parameters that influence its initiation. In the final analysis, however, this phe nomenon is usually evaluated experimentally on a model-by-model basis.
Although the principle of sliding friction is extremely compli cated and not totally understood at the present time, It is known that friction is not a steady-state process. Vibration occurs because of this variation in friction force in the related components. It is also known that friction-induced vibration occurs via several different yet distinct mechanisms. It is possible to have more than one vibration-exciting mechanism active at a given time. This makes brake vibration corrections complex. Some non-asbestos friction materials tend to be more prone to generate friction excited oscillations, for as yet uncertain reasons. It is felt that their generally greater energy storage modulus (stiffness) and lower energy loss modulus (dampening] are probable sources of greater tendencies for vibration and noise. Asbestos is much like many strands of rope in a brake lining formulation, although much finer in size. This is believed to provide the greater inherent dampening of asbestos friction materials. Fiberglass, aramid fiber (Kevlar), steel wool, wolastonite, and other non-asbestos reinforcing fibers typically are found as single solid elements, contributing minimally to the lining damping factor.
To clarify some of the mechanisms of friction-Induced vibration, the following sections discuss these mechanisms in more detail.
23
2.6.1 Stick-Slip
At low brake application speeds, it is possible for the brake lining and drum or disc to stick together for a short time, twisting up the vehicle suspension assembly. When the restoring torque builds up to the static breakaway value,the lining again slips against the drum/disc. This stick and slip cycle repeats rapidly, generating brake chatter, groan, or squeal vibration. This mechanism is attributed to a difference in the static and kinetic frictional torques and the spring-like restoring forces acting on brake components. The stick-slip mechanism produces a self-excited vibra tion with displacement-versus-time characteristics similar to a saw-shape form. This is the usual mechanism for low-speed brake chatter, although it can also exist in the form of a higher frequency squeal.
2.6.2 Negative Slope of the Friction Velocity Curve
This vibration inducing mechanism arises from a negative slope of the friction-versus-sliding velocity curve characteristic of certain mate rials in specific speed ranges. This causes instability in the system and excites vibrating components into nearly sinusoidal waveforms. An explanation is based on observations that the brake drum or rotor surface roughness asperities require a finite amount of time to produce equilibrium deformation of the brake lining material. The time that is available to deform the lining is decreased with increasing speed, restricting the time to compress the asperities in the friction material. Thermal effects, such as interfacial softening and changes of the chemical composition of surface layers, also have been offered as an explanation for the cause of the negative slope of friction versus velocity.
2.7 Comparison of 4-Wheel Disc Brake Systems TTfth Front Wheel Disc-Rear Wheel Drum System
The almost standard use of front wheel disc brakes for automobiles has been due to some performance characteristics provided by this design. Because these brakes and the friction materials used with them (notably semi-metallics) can operate at higher temperatures than drum brakes, the
24
vehicle's braking balance can be shifted toward the front, reducing the likelihood of rear wheel lock-up during severe braking conditions. In addition, since automobile front brakes are exposed to water, disc brakes provide for more effective wet braking under many road conditions.
Many European automobiles imported to the United States have been fitted with 4-wheel disc brakes. There appears to be three reasons for implementing these brake systems. First, the effectiveness of disc brakes is less affected by friction material performance than the effectiveness of drum brakes. Since European safety standards require front brake biasing to prevent rear wheel lock-up, this situation is more readily ensured by using similar brakes on all four wheels and adjusting the front-rear pressure proportioning valve accordingly. Front brake biasing can be accomplished with vehicles outfitted with rear drum brakes, but the drum brakes require friction materials that exhibit very stable friction performance in orderto ensure consistent performance (Figure 3). Second, in the absence of suitable non-asbestos linings, some European manufacturers elected to use proven semi-metallic materials and disc brakes as a means of eliminating asbestos. Third, in the United States, there is a perceived performance advantage provided by the term "4-wheel disc brakes", and so such systems are usually provided on higher priced imports. More inexpen sive automobiles produced for wide-spread consumption in Europe (Fiat, VW, Renault, etc.) are still equipped with rear wheel drum brakes.
Rear wheel disc brakes have disadvantages. Currently used mate rials, semi-metallies, exhibit poor friction performance at low temperatures and optimum performance at high temperatures. The effectiveness of the parking brake can be adversely affected, since these brakes are usually applied when the brakes are hot and then required to hold as the brakes cool down. Also, the pad-disc normal load for the parking brakes can decrease as the pads cool down and contract from their hot, expanded geome try. The rear disc braking surfaces also are more prone to contamination by mud and road debris thrown from the front wheels.
25
3. EVALUATION OF FRICTION MATERIAL PERFORMANCE
3.1 Section Summary
The performance of friction materials, as applied to automotive brake systems, is reviewed in this section along with the pertinent friction material testing devices and relevant testing procedures and standards. Main points presented are:
Friction materials are expected to perform satisfactorily over a wide range of operating and environmental conditions as an important element of the total brake system.
Many critical performance considerations, such as fade and fade recovery, are sensitive both to the friction material and to the brake system in which it is tested. Therefore, full-system testing is required for proper evaluation.
Friction stability can vary with both thermal and mechanical history. Therefore, short-term evaluations and those with narrow ranges of brake temperatures can produce Incomplete data leading to erroneous conclusions on brake compatibility
and performance. Laboratory specimen test machines, such as FAST and FMTM, may be appropriate for quality control tests and material screening, but they are not able to determine the accepta bility of substitute friction materials or to compare differ ent friction materials. Full brake Inertial dynamometers, properly instrumented and utilized, can be used to screen friction materials and determine some component performance behavior. Typically, these large and expensive devices do not adequately simulate the airflow over the brake and the environmental conditions of vehicle service. Good brakes are those with few minor faults. Brake devel opment testing therefore is time consuming and costly, since a wide range of conditions must be evaluated in the process of brake development.
26
Federal Motor Vehicle Safety Standards (FMVSS 105, 121, and the proposed 135) are severe-usage brake tests which are only one of many standards that new (OEM) vehicle brake systems must meet. No standards need be met by aftermarket friction materials.
3.2 General Evaluation Criteria
Friction material performance for vehicle brake systems may be
qualified by meeting the requirements of the Federal Safety Standards,
Society of Automotive Engineers (SAE) recommended practices, along with
brake component and vehicle manufacturers' standards. These experiments
are directed to maximize safety, dependability, and customer satisfaction
over a variety of brake operating conditions. For new vehicles, compliance
with federal and state motor vehicle safety standards is only the starting
point for acceptance of a brake system.
This section of the report discusses the criteria and techniques
used to determine friction material suitability and performance. Full
evaluation of a friction material requires installation into a complete
brake system. It is only in the full brake system that many of the impor
tant brake lining attributes such as fade, fade recovery, environmental
effects, and varying service factors may be properly determined.
Vehicle brakes are required to operate under a wide range of
conditions, from steep downhill grades with a heavily loaded vehicle to
minimal loads on interstate highways. Vehicle brakes must be completely
reliable and must be minimally affected by temperature, water, or contami
nation. Brake actuation forces should be properly distributed, and brake
friction must be consistent throughout the life of the friction material.
Pedal actuation forces should not exceed the capabilities of a wide range
of drivers.
.
Vehicle braking performance can be closely related to loss of
directional control in vehicles. Under skidding conditions, the tendency
for wheel lockup in some brake systems can make controlling the vehicle more
difficult. Commercial vehicles with lower brake effectiveness have been
shown to be less likely to encounter loss of control in accidents, presum
ably due to lower usage speeds and reduced tendency for wheel lockup (2).
27
Variation in the front-to-rear brake balance, due to changes in frictional performance, may adversely affect safety through reduced directional control and stopping efficiency. The subject of vehicle brake systems is discussed in more detail in Section 2.
Section 3.2 of the report describes some of the more important friction material performance characteristics that are evaluated during original equipment manufacturer (OEM) brake system qualification experi ments. These include: Fade Resistance, Fade Recovery, Delayed Fade, Effectiveness Versus Speed, Friction Stability, Wet Friction, Moisture Sensitivity, and Wear Rate. Each of these important performance charac teristics will be highlighted in the following sections.
3.2.1 Fade Resistance
Brake fade refers to a loss of brake effectiveness, generally as the result of excessive brake temperatures. Such excessive temperatures may occur under hard brake usage conditions or under less stringent condi tions, should component cooling be restricted. Brake fade may occur under particularly hazardous driving conditions, such as descending steep and winding mountain roads. Five types of brake fade have been described:
Thermal--due to high brake system bulk temperatures, De1ayed--due to resin migration during brake cooling, Blister--due to effects of near-surface lining blisters, FIash--due to high speed, high torque demand braking, and Water--due to partial lubrication from water contamination. Figure 5 shows a performance comparison between a good friction material and a poor material. The poorer friction material exhibits a more rapid drop-off of brake effectiveness, compared with the better mate rial. Since poor brake fade behavior could also be exhibited by high quality materials if used In inappropriate brake applications, brake lining fade behavior is meaningful primarily in the context of a particular brake lining, brake, and vehicle application. It should be noted that although these curves are continuous, the driver applies the brakes at discrete points along the curve. If the last application happens to be on the "knee1* of a performance curve, then the next brake application will yield different and unexpected brake system response.
28
Effectiveness
Effectiveness ------- --
FIGURE 5. FADE CHARACTERISTICS OF GOOD AND POOR BRAKE FRICTION MATERIALS
FIGURE 6. FADE RECOVERY CHARACTERISTICS OF GOOD AND POOR BRAKE FRICTION MATERIALS
29
3.2.2 Fade Recovery
Fade recovery refers to the ability of the friction material to quickly regain normal effectiveness after fade. This recovery is shown in Figure 6. As the brake cools with time after experiencing fade, the friction level should return rapidly to approximately the pre-fade level. Poor friction materials may exhibit slow recovery, compared with good friction materials, and may produce either a decrease or increase of brake effectiveness as a lasting consequence of the fade.
3.2.3 Delayed Fade
Delayed fade is a phenomenon which may occur with some friction materials. This phenomenon is illustrated in Figure 7. During fade recov-'. ery, brake effectiveness may drop unexpectedly, causing a temporary but potentially hazardous increase of required brake pedal force. This "delayed fade" is insidious in that it is often totally unexpected. It occurs well after a period of hard brake usage and usually with no warning signs.
3.2.4 Brake Effectiveness Versus Speed Characteristics
To ensure even braking over a wide range, of stopping speeds, the brakes on each axle should exhibit similar effectiveness characteristics at all vehicle speeds. In general, brake effectiveness decreases with increasing speed. Consequently, a brake application from. 60 mph usually requires greater brake pedal effort than from 20 mph. Good brake linings provide less speed spread, or difference in effectiveness at different braking speeds, and good brake systems employ friction materials that provide a proper balance of front-to-rear brake effectiveness at any speed. Figure 8 shows the general relationship between brake effectiveness and deceleration for brakes with poor and good "speed spread" performance. As illustrated, the effectiveness at 75 mph is markedly different from that at 25 mph for this brake, using a relatively poor friction material. Disc brakes are inherently less speed sensitive than high-servo-factor drum brakes, so they typically show less speed spread.
30
FIGURE 7. DELAYED FADE CHARACTERISTICS OF FRICTION MATERIALS
FIGURE 8. SPEED VERSUS BRAKING PERFORMANCE CHARACTERISTICS FOR GOOD AND POOR FRICTION MATERIALS
31
3.2.5 Friction Stability
To ensure consistent vehicle braking performance, the brake effec tiveness characteristics should be stable throughout the life of the brake linings. Figure 9 shows the difference between a poor material and a good material in this regard. The brake effectiveness of poor friction materials can deteriorate with accumulated use history and wear, as Indicated.
On vehicles equipped with a balanced set of friction materials on all four brakes, a gradual reduction in brake effectiveness may be marked only by a slight increase in pedal pressure to decelerate the vehicle, but brake stability will be essentially unaffected. However, on vehicles employing two unmatched friction materials on the front and rear axles, a shift in effectiveness of one braking axle relative to the other will alter braking balance and could adversely affect controllability of the vehicle during hard braking.
3.2.6 Wet Friction
The performance of vehicle brakes when wet is a significant safety concern. Disc brakes usually are less affected by water than are drum brakes largely because of the lower Inherent servo factor in disc brakes. However, both disc and drum brakes can show large effectiveness losses when wet.
As expected, poor friction materials can provide a greater loss of effectiveness and take a considerably longer time to recover friction capability, when wetted, than experienced by good materials. This is shown in Figure 10. Permeability, heterogeneity, and compression stiffness are some of the brake lining properties which determine wet friction response. Complete understanding of this behavior is not known, so full brake dynamometer and vehicle tests are used to establish the wet friction behaviors of a brake system.
32
Effectiveness
0 25 50 75 100 Lining Worn, percent -----------------
FIGURE 9. FRICTION STABILITY CHARACTERISTICS OF BOTH GOOD AND POOR FRICTION MATERIALS
FIGURE 10. WET BRAKING PERFORMANCE FOR GOOD AND POOR PERFORMANCE BRAKING MATERIALS
33 . >.
3.2.7 Moisture Sensitivity
Friction materials are porous, fiber-reinforced composites that are capable of absorbing atmospheric moisture when a vehicle is parked, such as overnight. For some friction materials, this moisture has been shown to lower brake effectiveness temporarily, leading to a phenomenon called "morning sickness". Other morning sickness effects result from rusting of the cast iron disc/drum surface when a vehicle is parked for some time, causing abnormally high initial brake effectiveness. Under some conditions, it is possible for the friction material to become rustbonded to the cast iron. Substantial driving torque may be required to break the Interface free, and the rusted surface of the brake drum or disc may generate a temporarily uneven brake torque.
' Figure 11 depicts the effect of moisture sensitivity on brake effectiveness. The ideal brake assembly exhibits little or no moisture sensitivity. When present, it usually persists for a few brake applications and disappears when brake heat drives the moisture from the brake lining or wear removes the surface rust.
3.2.8 Lining Wear Rate
Wear rates of friction materials depend upon temperature, prior
use, speed, and load. In general, wear is directly proportional to applied
normal load and speed. At moderate brake drum and disc temperatures,
friction material wear rates are not affected greatly by temperature.
However, at high brake temperatures, wear of the friction material may .
increase exponentially due to thermally induced degradation of the organic
resin binder material(3).
Figure 12 shows representative wear performance for three differ
ent types of friction materials. Low quality materials may utilize an
inferior binder resin (with poor heat resistance), providing a wear curve
like that labelled "A". This may give acceptable wear rates at low brake
temperatures, but rapid wear rates at higher brake temperatures. OEM
type materials behave like curve "B", and heavy duty brake linings wear like
curve "C". Note this heavy-duty lining does not improve wear life, except
at the higher brake temperatures.
34
Effectiveness
Lining Wear Rate --------
FIGURE 11. EFFECT OF MOISTURE SENSITIVITY ON BRAKING PERFORMANCE
FIGURE 12. WEAR PERFORMANCE FOR BRAKING MATERIALS
35
Since the full brake assembly represents a tribological system,
surface conditions at each contacting interface (brake drum/lining and
drum/disc) will influence brake lining wear behavior. For example, the
use of abrasives in the friction material may promote brake drum and/or
brake disc wear, but the use of such materials may be required to achieve
a needed brake effectiveness or to remove lining transfer layers or rust
from the cast iron surface.
.
3.2.9 Friction Material Qualification
Qualification of friction materials for vehicle service usually involves both dynamometer and vehicle testing. Dynamometer experiments permit controlled, fully instrumented brake lining testing in a safe and cost-effective manner. Vehicle tests are conducted to verify the results obtained through full brake dynamometer experiments and to include the many conditions that are not suitably evaluated on a brake dynamometer. The next section of the report discusses test procedures to qualify friction materials.
3.3 Laboratory and Vehicle Friction Material Evaluation
Laboratory specimen testing machines are commonly used to charac terize and audit the quality of friction materials, using specimens of full brake linings. The advantages of using laboratory systems for evalu ation include: (1) automated testing, (2) careful control of operating conditions, and (3) faster measurement of brake lining characteristics. In addition, laboratory evaluations using a specimen test are less costly than full-scale brake dynamometer or vehicle experiments.
. Numerous laboratory machine designs are used to determine friction properties^). Table 3 lists four of the most common test machines and test procedures generally used with each of these machines. The next section of the report briefly describes these four test machines.
36
TABLE 3. MACHINES FOR FRICTION MATERIAL EVALUATION (REFERENCE 3)
Apparatus
(1) Friction assessment screening test machine (FAST)
(2) Friction materials test (FMTM)
(3) Girling scale dynamometer (4) Full brake inertia dynamometer
General Test Procedure
90 minute FAST QC test
SAE 0661a quality control machine test procedure Simulated vehicle road test Simulated vehicle road test
37
3.3.1 Friction Assessment Screening Test (FAST)
The Friction Assessment Screening Test.(FAST) machine was devel oped by Ford Motor Company specifically for rapid "fingerprinting" of friction material specimens. It is used for in-plant quality control of clutch facings and brake linings and some specialized friction material screening and diagnostic tests. A 0.50-inch-square specimen generally is used, but lining samples up to 1.00-inch-square can be tested. The small, flat sample assures rapid specimen seating to the flat, cast iron test disc (1.5-inch thick/7.1-inch diameter). Quality control (QC) tests run for 90 minutes, with the friction drag held to a constant value. With a constant rubbing speed (880 rpm) and constant friction drag, the horsepower dissipated is also constant, providing repeatable temperature-time histories (70 to 560 F for QC). QC tests are often run on every batch of friction material as the linings are finished at their manufacturing sites.
However, the small FAST specimen precludes confidently making correlations of laboratory performance with full brake behavior. Thus this machine is used primarily for routine brake lining QC testing. Specialized characterization test procedures are mostly proprietary, and require exper tise for proper evaluation.
While this machine reportedly has been used to perform friction material screening tests, it never has been recommended as a substitute for full-scale brake evaluations.
3.3.2 Friction Materials Test Machine (FHTH)
This apparatus, developed by T. P. Chase of General Motors, uses
an arc-shaped 1-inch-square specimen of brake lining material, which Is
forced against the internal surface of a rotating, 11-inch diameter, cast
' iron brake drum. Auxiliary heaters and air blowers are used to provide con
trolled brake drum heating and cooling rates.
The FHTH is also used for quality control testing. SAE has
developed a recommended practice (SAE J661a) that is classified as a quality
control test procedure on the FHTH. This test requires more test time
and expense than the FAST QC procedure, since it includes simulated burnish,
wear, effectiveness, fade, and recovery procedures. It is used more for
38
periodic QC surveillance testing, than for routine production batch testing.
General Motors and others also have developed specialized testing procedures
for evaluating brake lining materials on the FHTM.
Based on the SAE J661a procedure, a brake lining friction rating
specification has been required by some states for many years. However,
this brake lining rating system has been shown to be clearly inadequate for
meaningful comparative testing with different types of classes of friction
materials. The SAE recommended practice J866 (revised in March 1985),
includes this caution against such uses:
Note: It is emphasized that this recommended practice does not
establish friction requirements for brake linings, nor does it
designate significant characteristics of brake linings which
must be considered in overall brake performance. Due to other .
factors that include brake system design and operating environ
ment, the friction coefficients obtained from this recommended
practice cannot be reliably used to predict brake system
performance."
Technical papers have pointed out that laboratory specimen tests,
such as those which use the FMTM, do not provide acceptable correlation
with actual vehicle or brake dynamometer service when non-asbestos or
asbestos linings are evaluated. Different classes of non-asbestos brake
linings, such as non-asbestos organic (NAO) and semimetallic (semimet),
provide conflicting trends as well as different absolute values of friction
on the Chase MachineC5).
3.3.3 Girling Scale Dynamometer
The Girling Scale Dynamometer is an apparatus which employs
scaled-down brake components. A small brake disc is mounted on the end '
of a rotating shaft which carries inertia discs sized to ensure that the
scaled disc pad will absorb the same amount of energy per unit area as a
full-sized brake disc pad. The friction sample is pneumatically loaded
against the rotating disc and a torque control system is used to produce
a repetitive, constant deceleration drag.
.
Some brake characteristics scale by geometry and others are
governed by absolute physical size. Consequently, experience is required
39
when using a scale device to be assured that the scaling process Itself has
not altered performance characteristics of the brake assembly. Therefore
this device, as well as the other specimen test devices, can produce good
test results only when utilized by a person with expertise in such
specialized tests.
>
3.3.4 Full Brake Inertia Oynamometer
A full brake dynamometer simulates vehicle braking by mounting
a complete brake assembly to a large rotating, inertlally loaded shaft.
The shaft's inertial loading is usually adjusted to simulate the actual
road Inertia. Figure 13 shows a schematic of an inertial dynamometer.
Brake inertial dynamometers are designated by the number of ends, stations,
or brake assemblies that can be tested at one time. Most are single ended
for testing a single brake.
Double-ended dynamometers, while capable of simultaneous testing
of two brakes, often are used as single-ended brake dynamometers so one
test assembly can be installfed as the other is being tested. Only a few
four-ended brake dynamometers have been built. It is difficult on multiple
station dynamometers to control the air flow for balanced brake cooling.
Most brake dynamometers place the brake assemblies in closed ducts, both
to expedite cooling and .to control smoke and odor. Since faster cooling
rates hasten testing, most dynamometer tests have much greater air flow
and resultant cooling rates than are found in on-road testing. Conse
quently, brake dynamometers are used mostly for controlled wear tests,
basic effectiveness tests, initial fade/recovery tests, and parking brake
tests.
.
.
Full brake dynamometers are available In a range of sizes with
inertial capacities ranging from "minicar" to "maxitruck". There are no
standards, so almost every unit is different. Dynamometer-to-dynamometer
differences in test results can be significant, even for carefully matched
linings and brakes tested to the same inertia loading. Consequently, com
parative brake lining test data is preferably obtained from a single brake
dynamometer.
Much of the data difference between various dynamometers results
from using brake lining thermocouples to control the brake test, as required
40
BD
Tnertia dynamometer. A: Hydraulic cylinder; B: Torque arm; C: Brake assembly; D: Flywheels; E: Motor.
FIGURE 13. SCHEMATIC OF INERTIA DYNAMOMETER
in the FMVSS 121 dynamometer procedure and as specified in most other procedures. Significant brake drum temperature variations occur with semimet and semimetallic linings, compared with conventional asbestos-based materials when control is by the lining temperature. Even different formu lations of the same general type exhibit test drum temperature differ ences, especially if they vary in metallic content. Current brake testing practice now favors measurement of the cast iron temperature for test control. In this way differences of lining thermal conductivity do not significantly alter the temperature of the major heat sink, the drum or disc mass. Drum temperature measurement requires infrared pyrometry or thermo couple slip-ring assemblies. Host good brake dynamometers now permit one . or both of these instrument refinements.
Burnishing conditions also affect the test results with most drum brake linings and blocks being sensitive to their initial usage history. When drum temperature control of the burnishing operation is provided, ' the test results are clearly better.
It now appears possible, with proper instrumentation and good control on the burnishing procedure, to perform meaningful friction material screening tests on full brake dynamometers.
Except for replicating airflow over the brake and environmental conditions, such as water and dust contamination, full brake dynamometers now can closely duplicate most vehicle in-service braking conditions and are invaluable for brake diagnostic testing. They can be excellent for Initial brake lining screening tests. However, they are not sufficient to fully evaluate the acceptability of substitute brake linings.
3.4 Correlation of Laboratory Test ftesults With Vehicle Test Results
Numerous studies have been done to determine the correlation between laboratory friction material test results and actual vehicle test results(^56,7,8). in general, the only good analogy of a vehicle brake is the brake itself. Consequently, there are no specimen or scale test devices that can consistently yield test data that correlates with full vehicle performance data. This does not mean that such laboratory tests are useless--just that they should not be used to predict field perform
42
ance behavior. They can and have been used to screen friction material
for undesirable performance flaws.
Even tests on full brake dynamometers are difficult to correlate
with on-road vehicle brake performance unless careful instrumentation and
test controls are used. It should be possible to correlate full brake dyna
mometer test results with on-road data without need for questionable cor
rection factors. This has been attempted by many, published by few, but
not yet verified by anyone.
.
3.S Federal Braking Requirements and Other Brake Tests
As indicated in previous sections, the qualification of vehicle brake systems and friction materials can involve numerous experiments to determine effectiveness, stability, fade resistance, moisture sensitivity, wet friction, and other performance parameters. Some of the brake per formance criteria are determined by Federal Motor Vehicle Standards (105 83 and proposed 135 for hydraulic brakes, 121 for air brakes), while other performance standards are determined principally by the standards of the vehicle and friction product manufacturers.
This section of the report discusses the Federal Motor Vehicle Safety Standards now in effect and proposed for hydraulic and air brakes. In addition, some of the numerous SAE brake test procedures used to evaluate brake system and friction product performance will be discussed.
3.5.1 Federal Motor Vehicle Safety Standard 105
This federal standard mandates hydraulic and parking brake per formance under specific vehicle operating conditions. Under the provisions of this standard vehicles under 10,000 lbs gross vehicle weight are tested to one set of procedures, while vehicles over 10,000 lbs gross vehicle weight are tested to a different set of procedures. General specifications are designed around stopping distances for braking with minimaltire skid ding. Brake use tends to modify friction material performance. For this reason specifications are listed for new (pre-burnished) and conditioned (burnished) friction materials.
43
FNVSS 105 outlines braking requirements for vehicles braking under level, straight, dry, clean pavement conditions. Requirements for vehicle wet braking as well as parking brake performance on grades are also outlined. Table 4 lists the general categories of tests and the evaluation criteria used to determine brake performance. Specific braking requirements are outlined in the following section.
3.5.1.1 Stopping Distance Requirements. FMVSS 105 outlines a series of tests for brake effectiveness and dictates required stopping distances without wheel lock-up for passenger cars, vehicles (non-passenger cars) with GVWR of less than 8,000 lbs, vehicles weighing between 8,000 and 10,000 lbs, and vehicles with a GVWR of greater than 10,000 lbs. The procedures describe required performance for new brake linings and for burnished linings that have accumulated a specified history of brake per formance. Performance requirements for partially disabled brake systems also are outlined.
Appendix A lists the procedures for evaluating braking system stopping distances under "normal" braking duty. The performance under brake fade conditions is determined by measuring the brake pedal force required to stop the vehicle under severe braking conditions.
3.5.1.2 Parking Brake Requirements. The vehicle parking brake must be capable of holding the vehicle stationary for 5 minutes on a grade (30 percent for cars, 20 percent for light trucks and vehicles over 10,000 lbs). This can be accomplished in part by using the transmission to brake the vehicle, provided the parking mechanism in the transmission must be engaged before the ignition key can be removed.
With respect to replacement friction materials, the ability of alternative materials to provide static-friction levels sufficient to satisfy these criteria can only be assessed by actual vehicle tests.
44
TABLE 4. TESTS AND PERFORMANCE CRITERIA SPECIFIED UNDER FHVSS 105
Test
Performance Criteria
General Test Conditions
(1) Effectiveness Stopping Distance
(2) Fade tests
Pedal force and deceleration
(3) Wet brakes
. Pedal force and deceleration
(4) Parking brake Pedal force/lever force
Before and after burnish GVWR and empty With and without failures Before and after spike stops Before and after fade
Pedal force needed to hold required deceleration in repeated stops to heat brakes
Pedal force during cool down (recovery)
(Did brakes return to normal?)
Pedal force needed to hold deceleration after brakes are wet
(Did brakes return to normal?)
Hold on grade with specified application force
45
3.5l2 Federal Motor Vehicle Safety Standard 121
Federal Motor Vehicle Safety Standard 121 lists the performance requirements for air braking systems. These systems are commonly used in heavy trucks, tractor-trailer combinations, off-road vehicles, and transit buses. This federal standard specifies requirements for stopping distance, brake effectiveness, fade and recovery, brake actuation time, brake release time, and parking brake operation. For these vehicles., brake loading can vary considerably, depending upon the service conditions and cargo load carried by the truck or trailer.
This.section of the report will be concerned with stopping dis tance requirements and vehicle/dynamometer experiments needed to determine these requirements. Currently under FMVSS 121 only intercity and transit buses are required to meet stopping distance requirements. All other vehicles are exempt and do not require vehicle tests to satisfy FMVSS 121.
3.5.2.1 Vehicle Braking Experiments. Vehicle braking tests are conducted on level surfaces exhibiting different friction properties (as indicated by pavement skid numbers). For example, an 80 skid number refers to a generally dry, concrete surface, while a 30 skid number refers to a wet, polished concrete surface. Vehicle loads are adjusted to replicate heavy (loaded) or light (unloaded) conditions. Brakes are evaluated by braking the vehicle from 60 mph and 30 mph on a dry surface with a skid number of 81 and by braking the vehicle on a wet surface with a skid number of 30. Both dry and wet pavement braking are conducted under empty and fully loaded conditions.
Braking road test procedures for FMVSS 121 are listed in Appendix A. Note that in contrast with FMVSS 105 no decelerations are designated. Road tests with new brakes are preceded by a brake burnishing procedure consisting of 500 brake applications. During burnishing, brake, lining temperatures are restricted to 500 F 50 F (Note that brake drum temperature is not controlled--see 3.2.4).
, In addition to the regular system brakes, the vehicles must have emergency braking systems capable of stopping the vehicle in the event of partial brake system failure. This requirement is similar to the FMVSS
46
105 requirements describing brake operation in the event of partial loss of hydraulic fluid.
3.5.2.2 Parking Brake Test. The parking brakes for trucks, buses, and tractor-trailer combinations must be capable of holding the vehicle on a 20 percent grade, on a concrete surface, under both empty or fully loaded conditions in both directions. Initial brake application can be achieved using air or hydraulic activators. Once actuated the application braking loads must be maintained solely by mechanical means.
3.5.2.3 Dynamometer Testing for FHVSS 121. FMVSS 121 describes procedures for inertia dynamometer evaluation of friction materials. These procedures require the installation of a complete air brake assembly on the inertia dynamometer. Since stopping distance requirements cannot be measured using an inertia dynamometer, brake performance is determined by measuring brake torques and deceleration rates as a function of air pressure in the actuator. Dynamometer inertia is determined by using the Inertial equivalent to the vehicle load on each axle.
The dynamometer test sequence for determining brake retardation, brake power, and brake recovery is listed in Appendix A. Trailers are only required to pass brake retardation requirements. Service line pressure and calculated brake retardation factor are used to determine acceptability of the materials.
3.5.3 Proposed Hotor Vehicle Vehicle Safety Standard 135
This standard would replace FMVSS 105 for hydraulic brakes for passenger cars only. It contains a shortened test procedure designed to be more harmonized with European regulations.. Requirements posed by the new standard which may affect friction material qualification are described in the following sections. This proposed standard can be found in Federal Register, Hay 10, 1985, Vol. 50, No. 91, pp. 19744 through 19760. Later revisions are in FR, Jan 14, 1987, Vol. 52, No. 9, pp 1474 through 1474. '
47
3.5.3.1 Front Brake Biasing. Under FMVSS 135 standard, brake balancing would have to be adjusted to ensure that in the event of a brakinc situation resulting in wheel lock-up, the front wheels will lock first, for both a lightly-loaded vehicle and a fully-loaded vehicle. Currently, FHVSS 105 does not specify a wheel lock-up sequence. The braking situation involves careful selection of friction materials exhibiting stable, repeat able friction performance coupled with a properly adjusted proportioning valve. A vehicle designed with front-bias braking could become unsafe over time if the brake balance significantly changed.
3.5.3.2 Control Forces for Brake Application. Under the proposed standards, the allowable pedal forces required to brake the vehicle under specified stopping conditions will be reduced. Lower control forces may necessitate redesign of brake components on some cars to provide greater mechanical advantage, particularly to meet the requirements for performance with a failed power assist unit.
3.5.3.3 Parking Brake Performance. Under the proposed standard,
the test gradient would be reduced from 30 percent to 20 percent, and a
dynamic test has been added. The allowable control force for applying the
parking brake would be reduced. This particular requirement may not place
additional restrictions on friction material performance. A grade reduction
from 30 percent to 20 percent represents a 30 percent reduction in load
applied via gravity In the direction of slope. In contrast, the proposed
reduction in allowable control force for the parking brake is between 10
and 20 percent.
'
3.5.4 SAE Recommended Practices for Evaluating Brake Systems and friction Materials
The Society of Automotive Engineers (SAE) has developed about 26 recommended practices for checking the performance of brake lining systems. These standards cover automobile, truck, and trailer brake system tests using both vehicles and dynamometers.
Prior to the adoption of FMVSS 105 and 121, the SAE procedures were intended to give some suggested standard guidelines to brake system
48
evaluations. The enactment of FMVSS 105 as a requirement for brake system
certification shifted the emphasis of these SAE procedures to the role of
supplementary tests that could be used to further qualify vehicle brakes
and braking systems. .
-
Table 5 lists some of the SAE brake test code procedures. Various
SAE documents outline test procedures, while others outline performance
requirements for various vehicle classes.
49
TABLE 5. SELECTEO SAE VEHICLE BRAKE TEST COOES
SAE Code
Description
J661a
Laboratory procedure for evaluating friction
J667 J843d
Laboratory procedure for evaluating friction material performance using an Inertial dynamometer
Vehicle test procedure-for evaluating brake system performance
J201
In-use vehicle test procedure for brake system of cars, light trucks, and passenger vehicles up to
10,000 GVWR
J212 JUMBO
J880 HAR8S
Laboratory test procedure for dual* end Inertia dynamometer
Brake system rating test code
Comments
Generally shown to have poor correlation with vehicle performance
first SAE recommended practice for Inertia dynamometer testing
Intended to replicate conditions of vehicle tests required by
FNVSS 10S
An Inexpensive quick test designed to examine, in a cursory fashion vehicle parking brakes and service brakes; set up for state Inspection stations
Laboratory (Inertia dynamometer) version of SAE J843b
Rates power absorption capablllty of heavy duty vehicle brakes
-
Vehicle Affected
Passenger cars and light duty trucks Passenger cars and light duty trucks
Passenger cars and light duty trucks Commercial vehicles
Approval Date 5/1953 - revised 9/1971 4/195J - revised 6/1961 1/1963 - revised 3/1973 4/1976
6/1980 3/1985
4. PERFORMANCE ATTRIBUTES OF FRICTION MATERIALS NOW IN USE
4.1 Section Summary
This section presents some performance characteristics of known asbestos and asbestos-free friction materials and discusses the feasibility of replacing the asbestos-based brake linings that are currently used.
Main points brought out in this section include:
Friction materials are proprietary formulations, made from specific combinations of binder resins, reinforcing agents, fillers, and friction modifiers to provide acceptable performance in some brake applications.
Chrysotile asbestos-based friction products have been highly . developed and refined over the past 80 years. Their field
performance attributes are well known. Non-asbestos friction materials presently are under intensive
development, but most have been conceived within the past decade. Limited field service data is available. . Four classes of non-asbestos friction products exist: non asbestos organic (NA0), resin-bonded metallic (semimetal 11c), sintered metallic, and carbon-carbon. Only the semimetallic and NAO materials have shown premise for common automotive brake lining applications. Semimetallic linings are not readily applicable to drum brakes, except for heavy truck brake blocks. Non-asbestos organic linings offer the best potential for most automotive friction material applications. However, development of NAO materials Is hampered by a lack of under standing of the new raw materials, specifically their spe cial processing requirements and their full-service perform ance behavior. Since lining formulation is a proprietary process, there is very little Information interchange on non-asbestos lining technology. . No performance requirements exist for aftermarket friction products. Consequently, non-asbestos brake linings are
51
commercially available, but they lack field service data to assure satisfactory performance for the full range of automotive applications.
4.2 Introduction to Friction Material Formulations
Little has been published about the specific formulations of friction materials, since they are considered to be proprietary compositions by the friction material manufacturers. This section of the report reviews the more common forms of both asbestos and non-asbestos lining materials and presents data regarding performance under vehicle or simulated vehicle braking conditions. .
Friction materials for automobiles contain four general types of ingredients: reinforcing agents--usually fibers, friction modifiers, fillers, and binders. Most automotive friction materials use thermosetting resins in their binder systems. These resins, often of the two-step (Novolac) phenolic type and generally modified for both processing and functional purposes, provide the matrix to bond the various constituents together. Internal pressures generated during processing and fade testing can reach 1000 psi, so binder resins require good tensile strength at elevated temp eratures. Binder resins provide more than just structural attributes to the brake lining. Thermal stability, friction level, fade, fade recovery, dimensional stability, wear life, and other performance characteristics of the brake lining are determined, at least in part, by the choice and amount of binder resin.
Reinforcing agents provide the structural elements to support the friction material in service. Brake linings experience a range of loadings that require strength, stiffness, and toughness. The reinforcing agents contribute to the stiffness and strength of the friction material composite. Usually tHese agents are fibrous and most often provide other attributes to the brake lining, such as wear resistance and improved dimen sional stability.
Since chrysotile asbestos has been used as a reinforcing agent in friction materials for about 80 years, its performance attributes are fairly well known. Chrysotile asbestos was chosen because of its unique combination of physical, thermal, mechanical, tribological, processing.
52
and economic properties. It is a unique mineral, processed to provide the desired fiber length distribution and fiber "openness" (degree of fiber fluffing or opening of the fiber bundles) needed for specific appli cations. Usually two or more grades of chrysotile asbestos are blended to produce the desired properties for the brake lining.
Non-asbestos friction materials use a blend from several hundred potential fiber and other structural agents that have been tried in brake linings. No one fiber replaces asbestos, so a "fiber cocktail" is developed to provide the needed processing, structural, functional, and permeability attributes for the brake lining at an acceptable cost.
Friction modifiers and fillers, as the names suggest, are added to provide the needed performance modifications and cost control to the binder/fiber system. Hundreds of organic and inorganic constituents may be used, in varying amounts, distributions, and particle sizes to achieve the intended results. Since friction material formulation is more of an art than science, it is common for these materials to be sequentially added to the formulation to "correct" different performance shortcomings In the lining development process.
The asbestos-free friction materials have four developmental classes: non-asbestos organic (NAO), semimetallic (semimet), sintered metallic, and carbon-carbon. Semimetallic and NAO linings appear to be the best suited for most automotive brake applications, but all four types will be discussed in Section 4.4.
4.3 Asbestos-Based Friction Materials
Asbestos has been used in brake linings and other friction pro ducts since the turn of the century, when metals, leather, and wood no longer were adequate. The predominant type of asbestos used in brake linings Is chrysotile, a hydrated magnesium silicate. The ultimate chryso tile fibril is about one millionth of an inch in diameter, so a fiber bundle about the size of a human hair may contain a million fibrils. The larger fiber bundles, called "crudes" constitute the visible asbestos in a friction material. Smaller fibers, especially when fully wetted by the binder resin, are virtually impossible to see in the brake lining. Many lengths and fiber diameters are used in making the asbestos-based friction
53
materials, usually by selection of an appropriate commercial grade or grades and by proper blending. Asbestos provides many desirable attributes to the linings, both in manufacture and in service usage.
During the mixing and molding process, asbestos helps to hold the materials together. This is referred to as "green strength" in the preforming and molding operations. During molding, and also during usage, the asbestos provides permeability to the lining, letting internally gener ated volatiles escape before they can produce high Internal pressures and . possibly blister or crack the brake linings.
Chrysotlle asbestos, being hydrated, loses this water as the
;
temperature rises. At around 650 degrees Celsius (about 1200 degrees F) this dehydroxyl atIon causes about a 14-percent weight loss to the asbestos. At slightly higher temperatures, the chrysotile asbestos converts to an amorphous or glassy phase. One common product of this conversion is called forsterite. In this converted phase, the chrysotile asbestos normally has been transformed from a fiber to a very fine powder. At very high temperatures, forsterite can deposit in smeared layers over the disc or drum braking surfaces, but this is quite uncommon and undesired.
Each vehicle application requires friction materials exhibiting different frictional and mechanical properties. Passenger car and light truck drum brake linings, called segments, usually contain from 30 to 70 percent by weight of asbestos. If held together with a liquid resin, these are referred to as wet mixes. Such materials are commonly rollmolded to their basic cross-sectional shape, but also may be formed by extrusion or other molding processes. When a powdered binder resin Is used, the mix is referred to as a dry mix. These generally require an initial molding or briquetting operation to produce the required shape. Both are cured by heat in a process that is both time and temperature dependent.
A wide range of friction, wear, and other properties can be built into such materials, particularly with over 70 years of formulation development time. Asbestos provides good static friction properties, important to drum brakes with integral parking brakes, and Is easily formed, by many processes, to make durable and dimensionally stable segments.
Passenger car and light truck disc brake linings are almost exclusively made with powdered binder resins in dry mix processes that
54
involve being individually molded to nearly final dimensions. Asbestos content usually is a bit lower and longer fiber lengths are used in disc brake linings because of their greater temperatures and pressures in service usage. The permeability, or "breathing" characteristic, of chrysotile asbestos Is important to disc brakes to prevent rapid friction loss with Increasing temperatures and during hard, high speed stops.
Heavy truck drum brake linings are called blocks and are gener ally molded In slabs. Longer fiber length asbestos and added crude fiber content is needed in these blocks, for reasons similar to those for disc brakes. Truck blocks are thick, about 0.75 inches (19 mm) so they require sufficient permeability to release volatile materials from manufacturing as well as from hard brake usage. Inadequate "breathing" causes blistered or delaminated brake linings. Truck brake blocks have been continuously refined over the years, with special formulations for most of the unique usage conditions.
4.4 Hon-Asbestos Friction Materials
Less than twenty years have been devoted to the development of non-asbestos brake linings, with the most intensive effort over the past ten years. Various alternative fibers have been studied since cotton was replaced by asbestos 80 years ago. However, none provided the requisite performance to challenge asbestos until the semimetallic linings were developed for hard service disc brake usage In the 1970's.
Concerns about asbestos fiber toxicity. Increasingly stringent air quality standards in asbestos manufacturing plants, and rising insurance costs all stimulated the development of substitute materials. Substantial progress appears to have been made in the past few years, as many new vehicle models have been released with fully asbestos-free brake systems. Several aftermarket friction products have been advertised as asbestosfree, but little is known of their actual service performance characteristics.
Dynamometer and vehicle performance test data for non-asbestos friction materials were not made available to us during the preparation of this report, so a complete evaluation of these materials was not possible. A variety of non-asbestos materials is available for aftermarket applica tions. No replacement friction materials, except qualified OEH linings.
55
are known to have undergone qualification tests under FMVSS 105. In addi
tion, most aftermarket suppliers lack the facilities required to conduct
FMVSS 105 and FMVSS 121 test procedures.
.
The following discussion, though not complete, provides at least,
an introduction to the different types of non-asbestos friction materials
that are presently available or under development. Some of their basic
functional characteristics are also given, when sufficient information
was available.
Despite substantial engineering efforts, non-asbestos replacement
friction materials are not available, at a proven quality and performance
level that is equivalent'to that of the original brake linings, for vehicles
which originally were released with asbestos-based brake linings.
Non-asbestos friction materials technology is advancing rapidly,
with many new car, light truck, heavy truck, and off-road brake systems
being released for new vehicle production. Materials are under development
for all four general classes.
4.4.1 Semimetallic Friction Materials
Semimetal lie and re.sin bonded metallic (RBM) are all names for this popular class of friction material. Seraimetallics utilize steel wool, some form of iron powder, graphite, binder resin, and various other con stituents in their formulations.
Semimetal.lics have been used as disc brake linings on passenger cars and light trucks for about a decade. Presently they are the most common friction material used with original equipment manufacturer (OEM) disc brakes in the United States. Although originally produced with a resin-asbestos backing layer, most semimetallic linings now use no backing layer, or one of a non-asbestos organic (NAO) composition. Semimetallic linings also have had limited usage in heavy truck drum and disc brakes.
These friction materials are usually hot-pressed to finished dimensions. Consequently, they are not readily made into drum brake seg ments, due to the needed large lining curvature. Since semimetallies also tend to be low in strength and stiffness, they are better suited to the thicker disc brake lining and heavy truck block configurations.
Semimetallic linings have several unique performance character istics. For one, they have a high initial wear rate, at least until a
56
ferrous transfer layer is built up onto the drum or disc cast iron surface.
This formation is rapid at high temperatures, but can be quite slow for
brakes that operate at low temperatures, low speed, and light pressures.
Lining wear life is greatest for moderate temperature service. Poor lining
life can result from very low usage temperatures, so brakes are often
designed to run hotter with seraimetallic linings. Most friction materials
have a lower friction level at higher rubbing speeds, but semimets do
not. These materials have nearly constant friction levels from about 30
mph to beyond 100 mph, providing potential front-to-rear brake balancing
difficulties for some systems. While the friction Is nearly constant at
high speeds, the lining wear rate is not. Seraimetallic linings have high
wear rates, per unit work done, at higher rubbing speeds. Thus they seldom
are used for racing car applications.
'
Water affects many friction materials greatly, but has very little
Influence on semimetallics--unless accompanied by oil. Water and oil can
act to reduce the friction of semimet materials in contact with cast iron
discs. Since road splashing generally contains some.oil, this can cause
a loss of friction. Disc brakes are less water sensitive than are most drum
brakes and often run warm enough to dry quickly, so water effects usually
are not critical or long lasting. Cool and humid ambient air conditions
affect semimetallic linings significantly, causing a temporarily low brake
effectiveness called "morning sickness".
4.4.2 Mon-Asbestos Organic Friction Materials
.
Non-asbestos organic (NAO) materials utilize a combination of fibers and other ingredients to fulfill the functions that chrysotile fibers had performed in resin-asbestos linings. Aramid (DuPont's Kevlar), fiberglass, mineral wool, wollastonite, steel wool, and processed mineral fiber are some of the common reinforcement fibers used along with the binder resin and various fillers and friction modifiers in NAO brake linings.
NAO brake lining formulations presently are used in some OEM drum brake applications for passenger cars and light trucks and increas-
57
ingly are used in brake blocks on heavy truck drum brakes. NAO disc brake
linings have been released for many OEM disc brake applications. Devel
opment efforts of NAO drum brake segments for the remaining OEH appli
cations remains active.
~
This class of friction materials offers the greatest hope as an
effective asbestos replacement, but also provides the greatest problems
to develop. Literally hundreds of fibers and reinforcing agents have
been used, generally in combination with several others. Finding the
best combination for lining processability, friction level, friction sta
bility, wear life, fade resistance, recovery, contamination sensitivity,
and mechanical properties is a formidable task, even using statistically
designed experiments. Since there is essentially no technical communication
or cooperation among the lining suppliers, each is working virtually
independently at this task. It appears likely that new NAO materials
will be developed that are superior to the best of the asbestos-based
linings. Presently, NAO materials tend to be hard, brittle, low in perme
ability, highly anisotropic, and prone to hot spot, blister, and crack in
service.
4.4.3 Sintered Metallic Friction Materials
Usually sintered, these heavy-duty materials typically are of
iron or copper base, but they may generally contain inorganic filler and
friction modifiers as minor constituents.
Sintered ferrous drum brake linings were released for a few OEM
passenger car applications two decades ago. They tend to be environmentally
sensitive, both to temperature and moisture, which limits commercial appli
cations. However, they are used for special service aftermarket automo
biles, some severe service commercial vehicles, and aircraft disc brake
applications.
Sintered copper-based friction materials have been used in heavy
duty brakes and clutches for around three decades. Often with another
metal, forming a bronze, and a refractory, such as mullite, these materials
can perform well in hard service usage. However they too are environmentally
58
sensitive and can cause severe galvanic corrosion in wet environments,
when used against the typical grey cast iron countersurface materials.
Aircraft disc brakes and heavy duty truck/tractor clutches are present
uses for the sintered bronze friction materials.
No known new application of these relatively old and well devel
oped friction materials has resulted from the search for asbestos substi
tutes, largely due to their cost and sensitivity to light-duty environmental
conditions^).
.
4.4.4 Carbon-Carbon Friction Materials
These materials are space-age composites of carbon (graphite) fiber, held in a matrix of amorphous carbon using a costly and time-con suming manufacturing process.
Military aircraft, race cars, and some commercial aircraft now sometimes use the carbon-carbon friction materials for both the stationary and rotating elements of disc brakes. High cost and environmental sensi tivity have limited additional applications^)- Recent developments of lower cost carbon fibers may increase their use in vehicle applications.
4.5 Aftermarket Vehicle Considerations
4.5.1 Drum Brakes
8rake design also affects the suitability of using some friction materials. The duo-servo drum brake used In older U.S. vehicles usually employs two different types of linings, with different friction properties, on a single brake to maximize overall brake performance. Aftermarket
friction materials must be capable of replicating original design friction levels and friction stability to achieve acceptable vehicle braking performance. . The leading-trailing drum brake used on the rear axle of the new, smaller front wheel drive vehicles does not require friction materials with such stringent friction stability properties. In both applications.
59
the static friction properties of the material are important due to the use of rear drum brakes as parking brakes.
4.5.2 Oise Brakes
.
Disc brakes generally operate at higher temperatures than do
drum brakes, and the friction material used for this application must
exhibit stable friction behavior over a wide temperature range. U.S.
passenger cars and light truck disc brakes which used asbestos-phenolic
friction materials were designed to operate at lower temperatures than do
present vehicles that use semimetallic linings. These asbestos-based
linings generally had higher friction, especially at the lower brake temper
atures, than do the semimetallic linings.
.
4.5.3 Factors Affecting Substitution of Friction Materials
The use of asbestos-free materials as direct substitutes in vehicles designed for asbestos-based linings may be restricted for the following reasons:
(1) Braking balance between front and rear brakes may be adversely affected. With few exceptions, semimetallic and NAO linings presently are the only non-asbestos friction materials available for use on disc brakes. Use of semimetallic linings would decrease the front brake effectiveness, espe cially at the lower temperatures. Since many of the front brakes were designed to operate at lower temperatures than are optimal for semimetalllcs, the semimetallic linings would not provide proper friction and wear behavior for many users. The available NAO rear brake linings differ in theirfriction properties, especially In the low temperature region, and also may be humidity sensitive. Front-rear brake balance at low temperature could be unsatisfactory.
60
unless matched front and rear linings both were installed during brake servicing. Regrettably, many users have only one set of linings replaced at a time. This offers a sub stantial opportunity for unbalanced braking between the front and rear brakes.
No known balanced non-asbestos brake lining sets are available for the aftermarket applications that originally were asbestos-based. The front-rear balance with replacement asbestos-free linings probably will be sensitive to the brake temperature, vehicle speed, and brake line pressure.
(2) Parking brake capacity may be reduced. Many of the non-asbestos organic (NAO) drum brake
linings provide low effectiveness at low temperatures. In addition, they also generally have high thermal expansion coefficients and are stiffer in compression. This can lead to a loss of parking brake capacity both from the lower effectiveness values and from larger losses of input cable forces, due to lining contraction.
(3) No meaningful brake lining effectiveness ratings exist. NAO friction materials are currently produced domes
tically in both brake pad and brake drum configurations. Tables 6 and 7 list current U.S. producers of brake pads and linings for passenger cars and trucks(lO). Only a third of the manufacturers of NAO brake linings for light and medium vehicles are outfitted with the facilities needed to evaluate friction products under true vehicle test conditions or simulated vehicle tests (full brake inertia dynamometers). Similarly, only a third of the manufacturers of heavy vehicle drum brake blocks have appropriate test facilities for evaluating their NAO friction materials. All of these suppliers rate their linings using a rating methodology that has been shown to be deficient (see Section 3.2.2).
61
TABLE 6. PRODUCERS OF BRAKE LININGS FOR LIGHT ANO NEOIUN VEHICLES
Company
Abe* Corp.
Bendi* Corp.
Friction Dlv. Products, Inc.
General Motors (Oelco Moraine Division) General Motors (Inland Olv.)
H. Krasne Nfg. Co. Inc. Lear Slegler, Incorporated .
Nuturn
Auto Friction Corp.
Brake Systems, Inc.
Location
Market
Winchester, VA Troy, NY
. OEM and aftermarket
OEM and aftermarket
Trenton, NJ
OEM and aftermarket
Dayton, OH
OEM and aftermarket
Dayton, OH
OEM and aftermarket
Los Angeles, CA Aftermarket for Imported cars
Danville, KY
Primarily
aftermarket:
some OEM for medium trucks
Smlthvllle, TN OEM and aftermarket
Lawrence, HA
Aftermarket,
Imported and domestic vehicles
Stratford, CT . OEM' and aftermarket
Products
Disk Brake Pads
Semi-
.
Asbestos Metallic
NAO
Drum Brake
Llnlnas Asbestos . NAD
1983 Sales (million
dollars)
Employees
Comments
XX
X X 12 200
X
X
X
X. X
75
1,200
Aftermarket pro
ducts contain
-
asbestos
X
X
' 10-12*
N/A
1 million disc pads
400-500 thousand
drum pieces pro
duced per month
XX
400 4,800
OEM for GM and Saab
.
XX
'X
X
H/A H/A Also produces semi metal 11c drum brake lining
N/A N/A
XX
50* 400*
XX XX
XX XX
12 200 8 100
X X XX
N/A N/A
f
TABLE 6. PRODUCERS OF BRAKE LININGS FOR LIGHT AND NEDIUN VEHICLES (Continued)
Company
Location
Market
Products
Disk Brake Pads
semi- .
Asbestos Metallic
NA0
Drum Brake
Llnlnos
Asbestos
NAQ
1983
Sales (million dollars)
Employees
Comments
U.S. Automotive Hfg. Co.
Tappahannock, VA
Aftermarket
X
.
Carlisle
Ridgeway, PA
Aftermarket
Chrysler
Wayne, HI
OEM and
XX
aftermarket
Virginia Friction Products
'
Walkerton, VA
Aftermarket X X
X
X X X
N/A N/A
Figures provided by company representatives
Sources: ICF 198S Survey of Primary and Secondary Processors of Asbestos Products. Telephone conversations with company representatives.
Sales and employees Ward's Directory of 51,000 Largest U.S. Corporations and Ward's Directory of 49,000 Private U.S. Companies. 1984. Petaluma, CA; Baldwin H. Ward Publications.
TABLE 7. PRODUCERS OF BRAKE LININGS FOR HEAVY VEHICLES
Company
Abex Corp.
Bend lx Corp. Heavy Vehi cles Systems Carlisle Corporation H.K Porter Co. Nuturn
Ordnance Parts and Engineering
Brake Sys tems, Inc. Scan Pac
WheelIng Brake Block Nfg. Co. Raytnark
Standco Ind. Palmer. Prod. Friction Prod.
Location Winchester, VA Cleveland, TN
Market
OEM and aftermarket . Aftermarket
Rldgway, PA Huntington, IN
OEM and aftermarket
Newcastle, IN Fort Worth, TX
Stratford, CT Heguon, WI Wheeling, WV
OEM and aftermarket
OEM and aftermarket, primarily military vehicle
OEM and aftermarket
OEM and aftermarket
OEM and aftermarket
Crawfordsvllle, IN
Houston, TX Louisville, KY Medina, OH
OEM and aftermarket
N/A
N/A
OEM and aftermarket
Figures provided by company representative
Oise Brake Pads Seal-Hetal'ITc
'
X
X
Asbestos X. X
Orum Brake Blocks NAO Full-Metallic
Metallic (Asbestos-
Free)
1983 Sales
(million dollars)
Employees
X
12 200
XXX
N/A N/A
XX X
XX X
X. .
25 400 IB 300 12 200 2* N/A
X
X
/ N/A
X N/A
X XX
N/A X
N/A N/A N/A N/A
5 " 100
This test, with 1 inch square specimens, can give particu larly misleading friction data for some NAO and semimet brake linings.
The brake lining/drum or disc/pad system is a tribological system in which alterations in either the pad surface or the rotor surface affect system frictional behavior. Under elevated temperatures, cast iron sur faces can form iron oxides or develop transfer films which exhibit lower coefficients of friction than do plain cast iron surfaces. The abrasive nature of asbestos-phenolic friction materials acts to continually remove these surface films, thus restoring system friction and brake effectiveness. Semi-metallies, on the other hand, depend on a transfer film that is known to exhibit low friction under cold and moist conditions.
4.6 Direct Comparison of OEH and Aftermarket Brake Linings
Complete brake dynamometer and vehicle performance data are not generally available to use for comparisons of different friction materials, making it difficult to support or refute claims made about the availability of acceptable, asbestos-free replacement brake linings. When available, interpretation and evaluation of the test data can be difficult.
The next two sections present both dynamometer data and vehicle test data that highlight observed performance limitations posed by the use of non-asbestos friction materials.
4.6.1 Dynamometer Test Data
.
One example set of data follows which supports the claim that no acceptable asbestos-free friction materials have yet been developed for certain drum brakes. In this case a full brake dynamometer was used to test four different sets of drum brake linings on a 12-inch duo-servo drum brake with a 3-inch lining width. Identical procedures and matching test components were used, so the brake linings were the only known variables(11).
65
The linings tested have been coded, a condition for the availa bility of this test data. For simplicity of presentation, only the initial lining burnish test data is shown.
Figure 14a shows this data for lining A, a NAO drum brake lining set. The test data is presented as brake line pressure versus equivalent vehicle speed. The brake line pressure relates to the amount of brake pedal effort exerted. All brake stops, after stop 3, were performed from the same initial brake drum temperature of 300 degrees F. All of the stops were from the same initial speed (40 mph) and controlled to provide an equivalent of 8 feet per second per second deceleration--a "normal braking" application. Note the wide range of pressures required to make the same stop. Even after 100 burnish applications, the required hydraulic pressures varied by a factor of two for adjacent stops. Also, the required brake pressures for this normal type of brake stop were almost to the limit for the power brake booster.
Brake lining A would be unacceptable for any 0EH brake usage on this brake assembly. However, this lining formulation was released for production application on a different, small.er-si zed drum brake. With a smaller brake and attendant thinner linings, lining A met all of the service requirements for an 0EH brake lining. This demonstrates how the accept ability of a friction material is not just dependent upon the properties of the brake lining, but also depends on the specific choice of brake, . vehicle, and use conditions.
Figure 14b shows performance curves for a NAO material B. Lining B was a candidate NAO material, but was not released for production. It used a different non-asbestos fiber system from that used in A. This lining provided, on average, higher brake effectiveness, but with a greater range of brake application pressures for the same test. Although not illustrated here, other test data for this lining show it to have an under sirable "morning sickness" with very high initial effectiveness for the first few cold stops. A lining like this on the rear drum brakes, used with a semimetal lie front disc brake lining, would provide a vehicle brake system with a very strong tendency for rear wheel lockup (skidding) when cold.
66
<T>
a. NAO Lining A
Equivalent Vehicle Speed, mph
b. NAO Lining B
c, Asbestos Production Lining C
Note: Numbers on curves represent stop numbers in test.
d. Asbestos OEM Lining D
FIGURE 14. COMPARISON OF DYNAMOMETER TEST RESULTS FOR FOUR DIFFERENT FRICTION MATERIALS, 300 F AFTER BURNISH, 12 x 3 DUO SERVO DRUM BRAKE (REF 11)
Figure 14c shows performance curves for a material C. Lining C, a "premium quality" aftermarket asbestos-based lining set, has been used for many years. Note the narrow band of brake line pressures for this burnish sequence. In particular, the stops from 40 through 100 were virtually identical. This consistency of brake effectiveness, although not guaranteed by the use of asbestos-based friction materials, is not yet available from any known NAO drum brake lining on this size of drum brake. Lining C had slightly higher (about 25 percent) lining wear rates than the two non-asbestos materials shown.
Figure 14d shows performance curves for asbestos material D. Lining 0 was the OEM released material for this brake system. It had the highest average effectiveness of the four linings and also the narrowest band of brake line pressures. In this case, the second 100 burnish-stop data was included to illustrate the consistency of performance that can be obtained from this brake. About 45 brake applications were required to obtain steady-state frictional behavior. After these, the test data were very repeatable, and the variation of effectiveness with speed was quite acceptable for a duo-servo drum brake.
4.6.2 Vehicle Test Data
For a vehicle test study conducted.by the National Highway and Traffic Safety Administration (NHTSA), the-performance of aftermarket brake linings was compared to the performance of OEM llningsC12). These experiments were conducted using a compact size passenger car equipped with front disc brakes with semi-metallic pads and duo-servo drum brakes on the rear. Twenty-three different aftermarket rear drum linings were obtained by purchasing them from automotive service centers and automobile parts outlet stores. These materials were identified with a letter code of A through W.
Experiments were conducted under conditions similar to those specified in FMVSS 105 (see Appendix A). To generate data representative of OEM lining performance, 10 sets of OEM-qualified linings were also obtained and tested to generate a comparison baseline.
68
S
lining Coda
a. 30 mph pre-burnish rear brake effectiveness results
c. 30 mph post burnish rear brake effectiveness results
d. 60 mph post burnish rear brake effectiveness results
FIGURE 15. VEHICLE TEST RESULTS COMPARING THE PERFORMANCE OF AFTERMARKET FRICTION MATERIALS (REF 12}
69
Figure 15 shows the measured brake effectiveness for the various aftermarket linings under four different test conditions. At least two separate sets and as many as four separate sets of the same linings were tested; variations in performance are illustrated by the range in effective ness exhibited among different sets of identical materials. Figures 15c and 15d indicate the wide range of effectiveness variability among the aftermarket materials. Lining material "R" exhibited the highest vari ability in effectiveness. This material was the only non-asbestos material tested of the 23 aftermarket materials tested. This data is consistent with the dynamometer test results presented in Figure 14. The remaining 22 test results also illustrate how wide variations in effectiveness can exist even for asbestos materials.
4.7 Issues of Consumer Acceptability of Non-Asbestos Materials
Consumer acceptability of non-asbestos friction materials, as with asbestos friction materials, will be affected by material wear per formance and by the tendency to induce brake noise. Excessive brake wear could lead to sooty tire sidewalls, while brake noise continues to be an annoyance to many consumers.
Some brake materials used for front disc brakes have created a "dirty wheel syndrome" from wear debris(^). Also known as "dusting", this situation arises when fine, sooty wear debris is deposited on the outside wheel and the surfaces. This situation makes routine tire main tenance more undesirable since the black film Is tenacious and usually makes the car owner's hands dirty. To prevent this situation, some car owners switch to aftermarket disc pads that are advertised as being "dusting resistant". Auto owners apparently assume that these materials are equally effective as braking materials as other materials more prone to dusting.
Aluminum "shields" are also available to shroud the disc and prevent debris from being deposited on the outer wheel surfaces. These shields are mounted between the disc and wheel of the brake and wheel assembly. Two adverse situations may arise from the use of such shrouds. First, the discs may operate at higher temperatures due to the decreased
70
convection and radiation from the disc surface. Second, the wheel lug
nuts may eventually loosen due to plastic deformation'of the aluminum shroud
in the vicinity of the lugs.
The "dirty wheel syndrome" has been reported more often by owners
of 6erman luxury automobiles, such as BMW and, Mercedes, for many years.
. Owners of such vehicles tend to be more fastidious about the car's appear
ance than owners of other less expensive vehicles which utilize other
materials. It is now becoming more of a problem on American and Japanese
cars as more non-asbestos materials are utilized.
'
. Nevertheless, the various techniques used by owners to eliminate
the problem more clearly illustrate the public's viewpoint that vehicle
brakes are largely unaffected by consumer neglect or tampering. Unless
prohibited by legislation, OEN-qualified non-asbestos materials that exhibit
excessive brake squeal or "dusting" could be replaced by materials provided
by an unregulated aftermarket supplier, that need not meet any certification.
71
5. REVIEW OF VEHICLES AFFECTED BY PROPOSED BAN
. 5.1 Section Summary
The purpose of this section was to examine the various vehicle
classes using friction materials and identify trends in braking systems
in the various vehicle classses. The following observations were made:
. The overwhelming majority of American passenger cars are
- now designed with front disc brakes and rear drum brakes.
There is not a significant trend toward wider use of 4-wheel
disc brakes: American manufacturers are directing efforts
toward developing suitable asbestos-free lining materials
for rear automotive drum brakes.
Because disc brakes maintain effectiveness at high braking
. speeds, 4-wheel disc brakes have been used in Europe on
some high performance vehicles before asbestos became a
concern. Now some European manufacturers use 4-wheel disc
brakes and semi-metallic brake pads as a means of eliminating
asbestos from passenger cars. Differences in consumer
acceptance between Europe and the United States, specifically
brake wear and brake noise, may restrict the use of 4-wheel
disc systems in the United States.
Trends in light truck braking systems follow those in
passenger cars.
. Most medium trucks (American, European, and Japanese) use
drum brakes on all four wheels. Acceptable qualified sub
stitutes for current asbestos lining materials have not
been found in many cases. These drum brake systems use
segments, a thin lining material similar in geometry to auto
mobile brake linings. However, the relatively severe braking
requirements of this application prevent a simple retrofit
of automobile materials to the medium truck market. A
small percentage of medium trucks are now being produced
with disc brakes and semi-metallic pads.
.
Acceptable substitutes for asbestos materials have been
found in some cases in the heavy truck segment. These
72
brake systems use thick molded brake blocks which are bolted
or riveted, to shoes. Some non-asbestos materials have
been found to exhibit sufficient mechanical strength and
frictional properties for this application.
'
5.2 Brake System Trends in Passenger Cars, Trucks, and Equipment
For this study, motor vehicles have been divided into three classes: passenger cars, on-highway trucks, and off-highway vehicles. Motorcycles, transit buses and trains, arid aircraft'have not been included.
The overwhelming majority of vehicles in the United States are, as expected, passenger cars (Table 8). The majority of the 1984 American passenger cars were fitted with a rear-drum and front disc brake system; 33 percent of 1984 imported automobiles were fitted with 4-wheel disc brake systems(14). Medium and heavy trucks continue to use traditional drum brakes. Light trucks use front disc brakes with rear drum brakes.
5.2.1 Passenger Cars
'
Passenger cars are the largest single market for friction products
in the U.S. As indicated previously, there were about 130 million regis
tered passenger cars in 1984 in the United States.
Prior to 1965, almost all American passenger cars were equipped
with 4-wheel drum brakes with asbestos linings. In 1965, some vehicles
(Lincoln Continental, and Chevrolet Corvette) were equipped with front
disc brakes. Widespread use of front disc brakes across most product
.
lines of American manufacturers began in the early 1970s.
There are a substantial number of vehicles that have drum brake
systems designed for use with asbestos friction materials. In 1984, there
were still 11.5 million cars in operation that were manufactured prior to
1970, and the majority of these presumably were fitted with 4-wheel drum
brakes. Prior to 1982, the majority of automobiles with front-wheel disc
brakes had been designed for use with asbestos brake pads. Since the
mean life of passenger cars is now about 7.6 years(^), automobiles fitted
with brakes designed for use with asbestos friction products will continue
to be in operation for several more years. Vehicles manufactured in 1986
73
TABLE 8. NEW VEHICLE SALES AND VEHICLE REGISTRATION FOR 1984 (REF. 15)
Passenqer Cars
Domestic Imported
On Hiqhway Trucks
Light Medium Heavy
Off-Highway Vehicles
Farm equipment Construction equipment
U.S. Calendar Year Sales (Millions)
10.4 8.0 2.4
4.1 3.8 0.06 0.20
0.17* 0.05**
1986 estimate ** 1985 estimate
74
with asbestos friction materials can be expected to require aftermarket brake components for at least another 11 years.
Current American automobiles are almost exclusively designed with front wheel disc brakes. As shown in Table 9, American manufacturers do not appear to be embracing 4-wheel disc brakes as a means of changing the braking characteristics of automobiles. Currently, American manufac turers appear to be retaining rear wheel drum brakes. These brakes are attractive for the following reasons:
(1) Easier implementation of parking brake mechanism, (2) Less susceptability to debris, (3) Less susceptibility to corrosion, (4) Less weight, and (5) Less cost. At the present time, there appears to be no distinct performance advantage under American driving speeds to rear wheel disc brakes other than reduced time required for maintenance. The use of rear disc brakes also simplifies the setting of front-to-rear braking balance. The current trend among American automakers Is to implement asbestos-free brake lining and brake pad materials in new major model changeovers. Brake systems on existing models are never routinely redesigned. New models, such as the Ford Taurus, are equipped with semlmetallic front disc brake pads and non-asbestos organic rear drum linings. Rather than redesign brake systems with 4-wheel disc brakes using semimetallic pads, American automobile manufacturers have elected to retain rear drum brakes and direct development efforts toward new, non-asbestos rear drum linings. Qualification of non-asbestos materials through vehicle testing may take 2 years or longer, assuming a suitable material exists for a specific vehicle application. The redesign of a vehicle's rear brake system from drum brakes to disc brakes could take 5 to 7 years, with little guarantee.of significantly improved performance.
5.2.2 On-Highway Trucks
On-highway trucks are generally classified with respect to gross vehicle weight class, i.e., by the vehicle curb weight plus cargo. Trucks can be considered single chassis vehicles or can be used in tractor-trailer
75
r
TABLE 9. LIST OF 1985 AMERICAN AUTOMOBILES EQUIPPED WITH 4-WHEEL DISC BRAKES (REFERENCE 16)
General Motors Ford Chrysler American Motors Corporation Volkswagen of America Honda
Number of Units Sold With 4-Wheel
Disc Brakes
164,301 46,608 0 0 82,797 0
Percentage of output
3.6 2.7 0 0 100 .0
76
combinations. Eight weight classes currently exist, although for the purposes of this study, the following classifications are used:
Classification Light
Medium
Heavy
GW Class
Group 1 Group 2 Group 3
Group 4 6roup 5 Group 6
Group 7 Group 8
GVW Range fibs)
6,000 and less 6,001 and 10,000 10,001 - 14,000
14,001 - 16,000 16,001 - 19,500 19,501 - 26,000
26,001 - 33,000 33,001 - over
New truck registrations for 1984 indicate that light trucks make up approximately 94 percent of the total truck population, which reflects the popularity of small pick-up trucks and mini vans. In contrast; medium trucks account for about 1 percent of the population, while heavy trucks make up the remaining 5 percent(^).
The following sections outline current brake systems and trends in vehicle brake materials for the three classes o.f on-highway trucks.
5.2.2.1 Light Trucks. The "light truck" designation refers to both pick-up trucks and compact vans used in light hauling and passenger transport. About 3.9 million light trucks were sold in 1984 in the U.S. As of 1984, almost 31 million light trucks were registered in the United States.
Trends in light truck braking systems follow closely those of passenger cars. In 1984, more than 94 percent of light trucks manufactured in the United States were equipped with front wheel disc brakes. Ford and GM 1987 light trucks are using variable proportioning valves for rear brakes or anti-lock systems to Improve braking under a wide range of truck loads. Calibration of a variable proportioning valve is affected by changes in rear brake lining material. In this respect, trucks brakes are following European passenger car trends in that design steps are being taken to ensure front wheel brake biasing.
77
Many light truck brakes manufactured in the United states are now using asbestos-free semimetallic materials in the front disc brakes and non-asbestos organic linings for the rear drum brakes. In 1986, the Ford Aerostar and Ranger models were fitted with all non-asbestos friction materials. The automotive Industry Is in the process of converting existing light truck models over to asbestos-free friction products.
Although new truck brake systems appear to be heading toward asbestos-free friction materials, a large population of older vehicles requiring aftermarket friction products still exists. Between 1970 and 1982, the median age of light trucks was 7.1 yearsU5).
5.2.2.2 Medium Trucks. The "medium truck" designation generally refers to single chassis trucks with a GVW of between 14,001 lbs and 26,000 lbs. These trucks usually are used as enclosed delivery trucks or open, flat-bed trucks. Manufacturers usually provide the engine, drive-train, cab, and chassis, which are custom modified to suit the customers' needs. Brake systems on this truck classification can be either hydraulic or air operated, depending upon customer selection and intended service. The majority of the systems are hydraulic due to lower initial costs associ ated with these systems. In 1982, there were 1.4 million medium trucks in service in the United States(115).
Current manufacturers and marketers of medium trucks in the United States are Ford, General Motors, Chrysler, Mack, and Mercedes-Benz, and others. These manufacturers typically purchase axle and brake assem blies from "foundation" axle and brake manufacturers and assemble them, along with the engine and chassis, into the finished truck. These.brake systems are almost exclusively drum brakes on both the front and rear brakes. These brakes use strip linings similar to automotive drum brakes. However, due to the more severe braking conditions found in these trucks, qualified asbestos-free materials have not been found for some medium trucks.
Disc brakes for medium trucks are now under development, although the number now in service is very small(IT). Some of the advantages cited for truck disc brakes are light weight and increased ease of maintenance. However, problems with excessive rotor and pad temperatures continue to limit their use in potentially severe braking conditions. European medium
78
trucks produced by IVECO (Italy) and Saab-Scania (Sweden) are exclusively fitted with hydraulically actuated drum brakes(17*^,19).
S.2,2.3. Heavy Trucks. The "heavy truck" designation refers to either single-chassis trucks or segmented tractor-trailers having a 6VWR of greater than 26,000 lbs. In general, the single chassis trucks have ' hydraulically-actuated brake systems, while the brakes of tractor-trailer combinations are generally air actuated.
Large truck brake systems are almost exclusively drum type, either of the leading-trailing type (Figure 3a) or the leading-leading type. For these severe braking requirements, thick block segments rather than thin strips of friction materials are bolted or riveted to shoes. Some success has been achieved in qualifying non-asbestos block materials for heavy truck applications. This success has been due in part to the ability to mold the non-asbestos materials Into rigid blocks possessing sufficient mechanical strength.
The use of disc brakes in the heavy truck market has received some attention, but the overwhelming majority of the systems is still centered around the use of drum brakes. Because drum brakes still offer some performance advantages over disc brakes, a complete conversion to disc brakes in this vehicle market is not expected.
Qualification of non-asbestos materials for truck and trailer service is progressing through the combined efforts of foundation brake manufacturers, truck manufacturers, and friction product manufacturers. Most of the aftermarket friction product manufacturers do not possess the facilities to evaluate the performance of their products under vehicle service conditions.
5.2.3 Off-Highway Trucks and Equipment
Farm equipment and construction equipment have braking needs different from those of on-highway trucks. Braking speeds will be lower and required brake torques will be higher.
Examples of off-highway equipment include road scrapers, road graders, excavators, haulers, cranes, mobile drilling rigs, and other large
79
pieces of equipment. Farm equipment can range from small tractors to
large harvesting combines.
.
Qualification of friction products for these applications Is
usually the responsibility of the foundation brake manufacturer and the
equipment manufacturer since no Federal braking requirements exist. Hate-
rials found to perform satisfactorily through years of customer and manu
facturers' experience are rarely changed since the usage conditions and
equipment change only slightly.
The development of qualified asbestos replacements for this
market segment may be difficult due to the extended time required for
development, coupled with the relatively small market represented by the
farm and construction equipment Industry.
5.3 Current European and Japanese Experience In Brake Design and Friction Material Selection (Automobiles and Light Trucks)
The bulk of the European import automobile and truck market is comprised of vehicles manufactured in 6ermany, Great Britain, France, ' Sweden, and Italy. As indicated previously, most of the recent imported European automobiles are fitted with front wheel disc brakes which use semimetallie asbestos-free friction pads. There is a strong trend among European automakers to use 4-wheel disc brakes, along with semimetallie materials. Table 10 lists the European automakers and the percentage of their product lines that are equipped with 4-wheel disc brakes. Although the percentage of new European 4-wheel disc brake imports is high (80 percent), the total number of units imported is small compared with United States automobile production.
European drivers have different preferences with respect to brake performance than do American drivers. In general, Europeans are more tolerant of brake squeal and brake wear than are American drivers, and materials judged acceptable from a standpoint of stopping performance alone may not be acceptable to the American public. In addition, the European safety standards are different from those in the United States but are difficult to compare directly. Generally, the ECE/EEC regulations allow lower levels of braking efficiency than do the United States regu lations, but they do require calculations to show that the "design intent" brake balance is front bias, i.e., front wheels lock first. The emphasis
80
TABLE 10. LIST OF 1984 EUROPEAN-MANUFACTURED AUTOMOBILES EQUIPPED WITH 4-WHEEL DISC BRAKES (REF. 14)
Number of Units Sold In the U.S.
Percentage With 4-Wheel
Disc Brakes
Germany
Mercedes-Benz Volkswagen BMW Audi Porsche
58,017 64,405 64,525 54,590 14,708
100 100
58.6 15 100
Great Britain Jaguar
18,044
100
France
Renault Peugeot
10,277 14,792
0 100
ItaTjt Alfa Romeo
3,399
100
Sweden
Volvo Saab
TOTAL
96,422 25,146
424,325
100 100
83
81
of front biasing makes 4-wheel disc brakes attractive because the effective
ness of disc brakes are less affected by slight changes In friction material
performance. Automakers can help ensure that the brakes are front biased
In spite of unforseen friction product behavior by using proportioning
valves to force the front discs to handle most of the vehicle braking.
Virtually all of the Japanese automobiles Imported to the United
States are fitted with front disc brakes, although some models are also
equipped with 4-wheel disc brakes. Table 11 lists the percentages of
Japanese vehicles equipped with 4-wheel disc brakes. In 19B4, only 20
percent of the 1 million units sold In the United States were so equipped.
The eventual adoption of FMVSS 135 and the failure to qualify
rear automotive brake drum linings exhibiting consistent levels of friction
over a wide range of performance conditions may affect the design philosophy
of American and Japanese automobile engineers. Forced to ensure front
bias in the system, automakers may eventually move to using less-effective
disc brakes on the rear axle and increasing the front-to-rear braking
ratio to ensure consistent system performance, if proven rear drum lining
materials are not found.
.
82
TABLE 11. LIST OF 1984 JAPANESE AUTOMOBILES . SOLO IN THE UNITEO STATES EQUIPPEO WITH 4-WHEEL DISC BRAKES (REF 10)
Number of - Units Sold
JAPAN Honda Nissan Toyota Subaru Mitsubishi Isuzu Mazda TOTAL
374,819 372,633 306,900 148,880
39,104 17,233 128,197 1,387,766
Percentage With 4-Wheel Disc Brakes
17.9 32.7 24.9
0 >0 >0
o >20
83
6.0 SUMMARY OF INDUSTRY RESPONSES TO PROPOSED BAM
6.1 Section Summary
Various concerns representing automobile manufacturers, truck and equipment manufacturers, friction product producers, and environmental groups responded with written comments to the Environmental Protection Agency's proposal outlined in the Federal Register. Table 12 lists the respondents.
In general, respondents commented on numerous facets of the proposal, but the comments most pertinent to this study were those dealing with issues of brake performance, application availability, and feasibility of the proposed phase-down schedule. The comments of respondents have been cate gorized in the following sections with respect to the issues addressed.
Table 13 summarizes in a very general sense the responses of the various industries and groups to the main features of the proposed EPA ban.
6.2 Concerns of Respondents to Issues of Brake Performance
The comments of the respondents to this issue are summarized in Appendix B. The automobile and truck manufacturers were consistent in their opposition to the proposed ban on asbestos in aftermarket friction products.These manufacturers contend that vehicle safety may be compromised if unproven friction products are used in place of vehicle-tested products containing asbestos. One truck manufacturer (Navistar) claims that brake and friction product qualification can take up to 48 months, so the time and cost of re-qualifying materials for other aftermarket products would be prohibitive.
Some manufacturers question the long-term performance of some asbestos-free materials and subsequently feel that further development will be necessary before a phase-out can be feasible. Ford Motor Company indi cated that friction products must be qualified under long term conditions more stringent than federal motor vehicle safety standards in order to determine the sensitivity of materials to "hot-spotting" and "morning sickness". Rockwell International inferred that current non-asbestos
84
TABLE 12. LIST OF RESPONDENTS TO FEDERAL REGISTER NOTICE REGARDING PROPOSED EPA ACTION
Automobile Manufacturers
Japanese Automakers
Toyota Technical Center, U.S.A., Inc. Subaru of America, Inc. American Honda Motor Company, Inc. Mitsubishi Motors Corporation Nissan Research and Development, Inc.
American Automakers
Chrysler Corporation General Motors Corporation
Ford Motor Company American Motors Corporation
European Automakers
Volkswagen of America Mercedes-Benz of North America, Inc. Austin Rover Group Limited
Trade Associations
Automobile Importers of America, Inc. National Automobile Dealers Association Motor Vehicle Manufacturers Association
Environmental Lobbyists
.
National Resources Defense Council, Inc.
Truck and Equipment Manufacturers Foundation Brake and Axle Manufacturers Rockwell International
Truck Frame and Chassis Builders
Navistar International Corporation Freight!iner Corporation
Farm Equipment
John Deere and Company
Friction Product Manufacturers
OEM Compounders
Allied/Bendix Corporation Abex Corporation
.
Aftermarket Suppliers
Wagner Scan-Pac Original Quality, Inc.
Trade Association
Friction Materials Standards Inst.
Fiber Suppliers
DuPont
85
TABLE 13. SUMMARY OF RESPONDENTS' COMMENTS TO PROPOSED EPA PLAN
Brake Performance
Substitute Availability
Feasibility of Proposed Phase Down
Automobile
aftermarket for older
Manufacturers . vehicles should be
exempt
materials for front disc brakes available
materials for rear drum
brakes not qualified sufficiently
phase-down plan opposed
5-year lead time prior to ban favored .
Truck and
Equipment Manufacturer
aftermarket for older
vehicles should be exempt
materials for heavy on-road phase-down plan opposed
and off-road vehicles not available or qualified.as
yet
5-year lead time prior to ban favored
Friction Product
Manufacturers
performance of some
many vehicle applications
materials still needs
have no qualified materials
qualification (Allied/
(Allied/Bendix)
Bendlx)
performance of substi substitutes available for
tutes as good dr better all applications (Scan-Pac)
than asbestos (Scan-Pac)
many substitutes will be available within 5-years (Allied/Bendix)
substitutes now available (Scan-Pac)
Environmental Lobbyists
--
substitutes available for all applications
Immediate ban on current and aftermarket asbestos friction materials
materials designated for off-highway heavy-duty applications give unaccep table performance and durability.
Friction product manufacturers gave mixed reports concerning product performance. Allied/Bendix indicated that development work was still progressing, and substitute formulations need to be evaluated using vehicle tests and dynamometer tests. Scan-Pac, on the other hand, claimed to have products available for all heavy-duty vehicle applications, although no in-house inertia dynamometer or vehicle test facilities are available at Scan-Pac`s facility for qualification of materials.
6.3 Concern of Respondents to Issues of Friction Material Availability
The comments of the respondents to this issue are summarized in Appendix C. U. S. and Japanese automobile manufacturers indicate that non asbestos front disc brake pads are now being used extensively.on new car models. Chrysler indicated that all but one vehicle manufactured by Chrysler is fitted with semi-metallic front brake pads, while General Motors acknowledged that for front disc brakes, asbestos-free replacement materials are now available for new model cars. Asbestos-free rear drum brake materials are still not available for automotive applications. "i"
Volkswagen Indicated that in model year 1987 all products will be fitted with non-asbestos linings,-while Daimler-Benz indicated that new 300 series models will be fitted with asbestos-free friction materials. Daim ler-Benz did indicate that brake systems of other vehicles In their product line are undergoing lengthy and extensive redesign and requalification to be fitted with non-asbestos materials.
Since most heavy duty truck and equipment brakes use drum brakes, the concerns of truck and equipment manufacturers were different from those of the automobile manufacturers. Since truck life Is considerably longer than automobile life, aftermarket availability for older vehicles remains a strong concern. Frelghtliner and Rockwell indicated that friction product suppliers have been unable to provide qualified non-asbestos linings for heavy-duty on-highway and off-highway applications. John Deere indi-
87
cated that replacement materials have not been qualified for use in tractor transmission'clutches and axle brakes.
Friction product manufacturers gave mixed reports concerning the availability of asbestos-free materials for the various applications. Allied/ Bendix indicated that they had been successful in developing asbes tos-free products for some applications, although many vehicle applica tions currently remain without qualified asbestos-free materials. ScanPac and DuPont both indicated that substitute materials are available for all applications, although Scan-Pac has concentrated on providing materials for aftermarket heavy vehicle applications. However, both Scan-Pac and DuPont lack the facilities to conduct vehicle or dynamometer tests required for FHVSS certification.
6.4 Concerns of Respondents to Feasibility
of the Proposed Phase-Down Schedule
.
The comments of the respondents to this issue are summarized in
Appendix 0. The automobile industry, both domestic manufacturer and impor
ters, voiced unanimous concern over the proposed timetable for asbestos
phase-out. The following were major points made by this industry:
(1) The years 1981, 1982, and 1983 were years of severe recession
in the automobile industry. Therefore asbestos-usage quotas
based on asbestos usage during these years are excessively
restrictive.
-
(2) The proposed rating is unfair to auto importers, who voluntarily
restricted imports during 1981, 1982, and 1983.
(3) Option 1, which allows a five-year lead time to develop and
qualify new asbestos-free materials, is preferable than the
proposed staggered phase-down effort.
(4) If FHVSS 135 is adopted, more lead time may be required, depend
ing upon exact provisions of FHVSS 135.
Truck and equipment manufacturers, whose products rely upon drum
brake materials, were opposed to the proposed immediate reduction in allow
able asbestos in friction materials. Navistar, Freightliner, and Wagner
supported a plan in which most asbestos use was banned after a 5-year lead
time. Modifications of this provision would allow the use of asbestos in
88
w.w
:f..v>. t
materials for which no replacement material was qualified. John Deere requested an exemption for all materials used inside a machine housing, such as clutches and sealed axle brake units used on John Deere tractors.
The friction product manufacturers gave mixed reports concerning the proposed phase-down schedule. Allied/Bendix Indicated that reliable
alternatives for original equipment materials will be available within 5 years. DuPont and Scan-Pac, on the other hand. Indicated that acceptable substitute materials now exist. Both companies lack the facilities to qualify materials and systems under Federal safety standards.
The National Resources Defense Council (NRDC) supports an Immediate ban on all asbestos in friction products, both new and aftermarket. They contend that viable substitutes are available today.
89
7. REFERENCES
(1) Newcomb, T. P., and Spurr, R. T., Braking of Road Vehicles, Robert Bentley, Inc., Cambridge, Massachusetts 1969.
(2) Starks, H. 0., "Loss of Directional Control in Accidents Involving Commercial Vehicles", 1963 Symposium on Control of Vehicles (Inst. Hech. Engrs., London) 8.
(3) Anderson, A., "Wear of Brake Materials", from Wear Control Handbook. M. B. Peterson, W. A. Winer, Editors, ASME, New Vork, New Vork.
(4) Preston, 0. 0., and Forthofer, R. J., "Correlation of Vehicle, Oynamometer, and Other Laboratory Tests for Brake Friction Materials", SAE Paper No. 710250, Warrendale, Pennsylvania.
(5) Tsang, P., Jacko, M., and Rhee, K., "Comparison of Chase and Inertial Brake Dynamometer Testing of Automotive Friction Materials", 1985 Wear of Materials Conference, ASME, 1985, New York.
(6) Burkman, A. J., and Highley, F. H., "Laboratory Evaluation of 8rake Lining Materials", Paper NO. 670510, Warrendale, Pennsylvania.
(7) Preston, J. D., "Inertia Dynamometer Evaluation'of Brake Lining Mate rials", SAE Paper No. 730192, Warrendale, Pennsylvania.
(8) Steis, 0. E., "Inertia Brake Dynamometer Testing Techniques for FMVSS 121", SAE Paper No. 751010, Warrendale, Pennsylvania.
(9) Anderson, A., Ford Motor Company (personal communication).
(10) "Substitution Analysis for Asbestos Brake Linings for On-Road Vehicles", prepared by ICF, Inc. for Ms. Amy Moll, Environmental Protection Agency, Washington, D.C.
(11) Data provided by A. Anderson, Ford Motor Company
(12) Flick, M. A., Radllnski, R. W., and Kirkbride, R. L., "The Effect of Aftermarket Linings on Braking Efficiency", SAE paper 870267, February 1987, Warrendale, Pennsylvania.
(13) Fobian, J., AAA (American Automobile Association) (personal communication).
(14) Ward's Automotive Yearbook, 1985, Ward's Communication's, Detroit, Michigan.
(15) Motor Vehicle Manufacturers Associate (MVMA) Motor Vehicle Fact and Figures, 1985.
(16) Automotive News, 1986 Market Oata Book Issue, April 30, 1986.
90
(17) Aoki, K., "Japanese Track Brake Philosophy", Worldwide Braking Trends: Medium and Heavy Duty Trucks, SAE 1984, Warrendale, Pennsylvania.
(18) Thoms, E., "European Brake Philosophy", Worldwide Braking Trends: Medium and Heavy Duty Trucks, SAE 1984, Warrendale, Pennsylvania.
(19) RothVegel, K., and Denzler, U., "European Brake Philosophies for Commercial Vehicles in the Range From 6 to 16 Tons FVS", Worldwide Braking Trends: Medium and Heavy Duty Trucks, SAE 1984, Warrendale, Pennsylvania.
91
V
i
APPENDIX A Vehicle and Dynamometer Test Procedures for''Qualifying Friction Materials and Brake
--SylTemVTfnierFl^nM"aT(fTHVSS','in--
93
*
TABLE A-l. SUMMARY OF TEST PROCEDURES FOR .FMVSS 105
Sequence
Procedure
Stopping Distance Per Vehicle Group, ft
~E S C D
1 Instrumentation Check
2 First (preburnish) Effectiveness test
Check Instrumentation by making not more than 10 stops from 30 m.ph at a deceleration of not more than 10 ft/sec/sec
Make 6 stops from 30 mpht then . make 6 stops from 60 mph
57 65 69 88 216 242 267 388
3 Burnish brake linings (at 6WIR)
Procedure 1: Vehicles with 6VHR of 10.000 lbs or less
Make 200 stops from 40 mph at a deceleration rate of 12 ft/sec/sec. Use approximately 1 mile rest inter val in between brake applications
Procedure 2: Vehicles with 6VWR of over lO.oM Tbs
Make 500 "snubs* at 10 ft/sec/sec (maintain brake temperatures less than 500* F)
4 Second effectiveness test
Hake 6 stops from 30 mph, then make 6 stops from 60 mph, then make 4 stops from 80 mph (GVWR > 10.000 lbs)
54 57 57 81 204 216 194 388
5 First reburnish
Make 35 stops from 40 mph at a deceleration rate of 12 ft/sec
6 Test parking brake
Measure force required to actuate hand brake lever
Position vehicle on Incline and observe position-holding ability
Note: Unless otherwise specified, brakes on hottest axle are between 150-200 F at start of test stops
95
TABLE A-l. SUMMARY OF TEST PROCEDURES FOR FHVSS 105 (Continued)
No. Sequence
Procedure
Stopping Distance Per Vehicle 6roup. ft *1 B C B-
7 Third effectiveness test
8 Partial failure
Empty and loaded conditions evaluated under 20 percent and (light trucks and vehicles over 10.000 lbs) and 30 percent grades (passenger cars)
Make 6 stops from 60 mph under lightly-loaded conditions
Under lightly-loaded conditions. disable either the front brakes or rear brakes
Make A stops from 60 mph Restore disabled half, disable
2nd half, repeat stops Repeat procedure for vehicles
at GVWR
194 216 242 388 456 517 517 613
9 Inoperative brake power and power assist units
Disable power assist Make 4 stops from 60 mph at GVWR only
variable requirements
10 First fade and recovery (at GVWR)
.
baseline fade recovery
Procedure 1 - Vehicles With GVWR 10.000 or Less
. Make 3 stops from 30 mph at 10 ft/sec
/sec deceleration and measure control force readings Hake 5 stops from 60 mph at 15 ft/sec /sec deceleration, followed by 5 stops at maximum attainable deceleration between 5 ft/sec/sec and 15 ft/sec/sec at 0.4 mile intervals Drive 1 mile at 30 mph to prepare for recovery tests Hake 5 stops from 30 mph at 10 ft/sec
/sec deceleartion at 1 mile intervals
Note: Unless otherwise specified, brakes on hottest axle are between 150*200 F at start of test stops
96
TABLE A-l. SUHHARY OF TEST PROCEDURES FOR FHVSS 105 (Continued)
No. Sequence
Procedure
Stopping Distance Per Vehicle Group, ft
6 5"
Procedure 2: Vehicle With 6VWR ot Hore than 10.000 Lbs ~
.
baseline fade recovery
While In neutral, make 3 "snubs*
from 40 mph to 20 mph at 10 ft/sec
/sec deceleration
While In neutral, make 10 "snubs"
from 40 to 20 mph at 10 ft/sec
/sec deceleration at 30 second Intervals
Orlve 1.5 miles at 40 mph to
prepare for recovery tests
.
While In neutral, make 5 "snubs"
from 40 mph to 20 mph at 1.5 mile
intervals
11 Second reburnish
Hake 35 stops from 40 mph at a deceleration rate of 12 ft/sec/sec
12 Second fade and recovery
(at GVWR)
Repeat procedure for first fade
and recovery, except increase fade
.
stops to 15 for vehicles < 10,000 lbs
GVWR and to 20 for vehicles -
> 10,000 GVWR
13 Third reburnish
Hake 35 stops from 40 mph at a deceleration rate of 12 ft/sec/sec
14 Fourth effectiveness test (at GVWR)
Hake 6 stops from 30 mph, then make 6 stops from 60 mph
Hake 4 stops from 80 mph (vehicles with GVWR < 10,000 lbs)
Hake 4 stops from 95 or 100 mph (passenger cars only)
57 65 65 88 216 242 267 388
Note: Unless otherwise specified, brakes on hottest axle are between 150-200 F at start of test stops
97
TABLE A'l. SUMMARY OP TEST PROCEDURES FOR FHVSS 105 (Continued)
No. Sequence
Procedure
Stopping Distance Per Vehicle Group, ft
A B CD
15 Water recovery (at GVWR)
Make 3 stops from 30 mph at 10 ft/sec/sec deceleration
Orlve at 5 mph for 2 minutes
through a 6-Inch deep water trough Leave water trough, accelerate to 30 mph Make 5 stops from 30 mph at a deceleration of 10 ft/sec/sec
16 Spike stops (at GVWR)
Hake 10 successive stops from 30 mph using a control force of 200 lbs applied In not more than 0.08 sec.
Make 6 stops from 60 mph
216 242 267 388
Vehicle Group Description: A - passenger cars C - vehicles with 8000 < 10,000
B - non-passenger cars with GVWR of less than 8000 lbs
D - vehicles with GVWR > 10,000. (school buses over 10.000 lbs must meet all requirements. All other
vehicles over 10 K have to meet partial failure (8) and Inoperative brake (9) requirements only.
Mote; Unless otherwise specified, brakes on hottest'axle are between ISO-200 F at start of test stops
98
TABLE A-2. SUMMARY OF ROAO TEST PROCEDURES FOR FMVSS 121 (all vehicles except "non-school* buses are
exempt from these requirements)
Sequence
Procedure
Burnish brake linings Road tests - effectiveness
Make 500 brake applications at 10 ft/s/s deceleration, 1 mile intervals or 500 F brake temperatures
Readjust brakes if necessary
Under 6VWR and unloaded, make following stops:
From 20 mph with service brakes* From 60 mph with service brakes From 20 mph with emergency
brakes
From 60 mph with emergency brakes
These stops to be conducted on dry and wet pavement
99
TABLE A-3. SUMMARY OF DYNAMOMETER TEST PROCEDURES FOR FMVSS 121 (tractors, trucks, buses trailers)
Sequence
Burnish brake linings (all vehicles)
Brake effectiveness/ retardation factor (trailers only)
Fade and recovery (all vehicles)
Procedure
Evaluation Criteria
Make 200 stops from 40 mph at 10 ft/sec/sec
Make 200 additional stops from 40 mph at 10 ft/sec/sec
Make 7 consecutive stops from 50 mph with increasing brake air chamber pressure
Decelerate from 50 mph to 15 mph at 9 ft/sec/sec at 72 second intervals deceleration rate; repeat 10 times Hot stop from 20 mph at 14 ft/sec/sec Make 20 consecutive stops from 30 mph at 12 ft/sec/sec deceleration rate. (at 60 second Intervals)
Brake temperatures, T 315 F < T < 385 F
Brake temperatures, T 450 < T < 550
Calculated brake retardation force force at each air chamber pressure
Service air pressure must'be below 100 psi
No pressure limit
. Service air pressure must be below 85 psi and above 20 psi
APPENDIX B CONCERNS OF RESPONDENTS TO ISSUES OF BRAKE PERFORMANCE
101
I
Automobile Manufacturers
Mitsubishi
Replacement parts should be exempt.
Must evaluate and consider: - Adoption of larger brake booster - Increase of pad area - Adoption of ventilated disc brakes - Adoption of larger brakes
FMVSS 135 proposed by NHTSA is considered to be more stringent than FMVSS 105.
"Judder" is currently a problem for clutches.
Chrysler
'
Aftermarket materials for older vehicles should be exempt for safety reasons.
General Motors
Aftermarket is serious concern for safety reasons. Therefore, aftermarket for older vehicles should be exempt.
Essential that EPA collaborate with NHTSA on this rulemaking.
Considering safety, we are obligated to continue production of parts for servicing vehicles throughout their useful life.
Example of System Changes That Might Occur with a Lining Change
Assume that a replacement lining has 20 percent less friction than the original:
A larger wheel cylinder would be specified to achieve the required torque output.
A larger wheel cylinder results in more fluid displacement and may require a master cylinder change to obtain the proper pedal travel.
Larger master cylinder may make pedal effort too high necessitating a booster change.
Testing of the vehicle may indicate insufficient parking brake capability due to the reduced friction necessitating a park brake cable change.
Increased cable forces may result in parking brake strut buckling during abuse tests, necessitating a foundation brake redesign.
103
Mercedes-Benz
Exemption (indispensible) for replacement parts.
FHVSS-105 Is in the process of being reconsidered by NHTSA; impact may cause further delays.
A safe braking system is a well balanced design. Influencing one of its parameters alone, e.g. by replacing asbestos in friction materials, necessi tates reconsideration of the complete system. Immediate availability of substitutes is not given for all applications.
Ford
Complex issues must be researched. Including:
- Thermomechanical stability (hot-spotting and banding)
- Brake fluid displacement
.
- Fluid boil (semi-metallie and metallic linings)
* - "Morning sickness" (effects due to moisture)
- Different friction levels.
Exemption for parts replacement for older vehicles.
Austin-Rover Group .
Unrealistic lead times.
Friction materials have a major effect on safety.
Exempt replacement parts for vehicles produced before rule goes into effect.
Toyota
Oppose applying rule to replacement parts for earlier models.
.
National Automobile Dealers Association
Shouldn't rush brake development work.
OSHA standards cover danger to maintenance people.
AMC .
FMVSS-135; more stringent braking requires are currently under study.
104
Motor Vehicles Manufacturers Association
The safety and effectiveness of motor vehicle braking will be reduced.
The cost in terms of deaths and dollars from motor vehicle braking losses should be included with the potential impact assessment.
Problems with new friction materials: - Rotor scoring - Noise . - Lining cracks
. - Absence of consistent and reliable braking performance data
Safety must not be compromised.
Truck and Equipment Manufacturers
Wagner
CAFE requirements - Resulted in significant redesign of vehicle and brakes - But current vehicle population of pre-CAFE must be . operated safely
There are presently no data on brake performance from actual vehicle
tests using:
.
- Semi-metallic disk brake friction materials
- New substitutes
Safety - health effects do not consider the safety related risks associated with possible Inferior braking performance.
No or little available test data.
Navistar
Need both fleet tests and dynamometer tests to qualify friction mate rials. - Dynamometer tests run per FMVSS-121 - Fleet tests run per IH Fleet Test Phase
Qualification takes about 48.months
Freiqhtliner Technical problems to be calculated further before non-asbestos linings can be used universally:
Wear rate - At extremely high temperature.
105
Speed sensitivity - Generally less sensitivity at low speeds than at high speeds.
Static friction characteristics - Much lower and can create difficulty in complying with Federal HVSS-121 for parking.
Durability - Some are more susceptible to cracking and crumbling.
Low speed noise and chatter
Heat Conduction and corrosion resistance - needs further evaluation.
Rockwell International
Brake tests used to qualify materials: Our internal test requirements Requirements of our brake customers
. FHVSS-121 - highway, air SAE 0-1152 - off-highway vehicle SAE J-1473 - off-highway vehicle Specific industry standards for - Agriculture
' - Rail - Military specifications
Off-highway heavy duty applications: Unacceptable durability and performance problems Higher friction requirements for off-highway vehicles Several non-asbestos lining characteristics require more
development, including: - Low unburnished performance - Low after-burnish performance - Greater speed sensitivity at speeds *30 mph
Changes in FMVSS's can have a monumental impact when attempting conversion to new brake lining materials. .
John Deere -
Agricultural, industrial and garden tractors have much longer lives than traditional consumer products. Therefore service is demanded for several decades. Major concerns are: - Identical friction coefficient, fixed design - High safety risks, risk/risk concept - Low replacement part volume - Sizeable inventory of old agricultural tractors routinely provide service parts for machines build 20 to 35 years ago.
106
Friction Product Manufacturers
Allie'd/Bendix
Direct substitution of new materials for asbestos fibers resulted in - Poor friction levels - Friction instability - Increased noise - Front-to-rear vehicle brake imbalance
Substitute formulations need to be evaluated using - Vehicle tests (J843d) . - Inertia dynamometer tests
Scan-Pac
Industrial friction products - for non-asbestos friction products per formance is the same as, or better, than asbestos friction products.
Now use SAE J661a to evaluate materials: - Coefficient of friction versus - Wear test at 900 F
107
. s
APPENDIX C CONCERNS OF RESPONDENTS TO ISSUES OF
FRICTION MATERIAL AVAILABILITY
109
Automobile Manufacturers
General Motors
OK for front disc brakes (replacements available).
No substitute for rear drum brakes.
Especially difficult problem for heavy-duty trucks.
-
Chrysler
Currently, all but one vehicle that Chrysler manufactures has semimetallic front disc brake pads. However, even these require an asbes tos-containlng underlayer material for cushioning affect.
Brake design criteria:
- FMVSS-105 - Lining life of 50,000 miles - Noise free operation
.
Tests -
-
' Controlled laboratory tests narrow field of candidates
Field tests
Forcing the use of asbestos-free materials will compromise motor vehicle safety.
Mitsubishi
Replacement parts should be exempt. New materials may require a design change of related parts.
No substitute as yet for rear drum brake linings.
Honda
Should address any vehicle manufactured after the effective date. Not ,, applicable to replacement parts.
Retrofitting would require replacing other parts and is not cost-effective and reduces safety.
Volkswagen
Will have asbestos-free brake linings across its entire product line beginning in 1987 model year.
Mercedes-Benz
Rear brake pads and parking brake linings are asbestos free on all Mercedes-Benz passenger cars.
Ill
New 300-series are currently equipped with asbestos-free brake pads on all wheels.
Currently have asbestos-free clutch linings on all cars.
.
The other asbestos containing systems would have to be redesigned so as to accommodate non-asbestos friction materials. This process of redesigning
and testing is extremely time consuming and has not yet led to results that satisfy our criteria. The problems encountered are basically the following:
The brake factor level as compared to customarily approved linings is either too high or too low.
The brake factor scatters--in a non-permissible range--as a function of miles travelled, speed and temperature.
Hi wear of pads and discs.
Insufficient mechanical strength.
High thermal conductivity.
Low corrosion resistance.
These problems are less significant for rear brakes because the requirements regarding rear brakes are different from those for front brakes.
Ford
No practical substitutes for - Duo-servo car and light truck drum brake linings - Engine manifold gaskets
Motor vehicle safety must not be compromised. Even a slight increase in traffic fatalities would eliminate any benefits.
American Motors Corporation
Allow the use of asbestos in replacement parts for existing vehicles.
Truck and Equipment Manufacturers
Wagner
Considerably more progress made in manufacturing non-asbestos pads for disc brakes than for strip linings for drum brakes.
Some manufacturers of strip friction for drum brakes who restrict sale of non-asbestos material to OEM customer. No aftermarket sales.
Drum brakes have changed to non-self energizing (need higher coeffi-
112
Allied/Bendix
Bendix has, in fact, been successful in developing asbestos-free products for select applications but not all applications.
Aftermarket is of most concern.
Automobile front disc brakes is a very severe application for front
wheel drive cars.
.
Rear disc brakes - may be able to use a cost-effective, asbestos-free organic disc pad.
Non-asbestos organic drum brake linings first Installed on US 1984 light trucks and then on 1986 passenger cars.
Some of this technology has recently been transferred to use on medium size trucks.
Friction Materials Standards Institute .
Semi-metallic disc brake pads are now on front brakes of passenger cars and light trucks most have them.
Aircraft brakes are non-asbestos.
Non-asbestos developed for: - Clutch facings in heavy duty area - Passenger cars
The industry is working on substitutes for - Light, medium and heavy truck drum segments - Truck/trailer blocks
Scan-Pac
Manufacture non-asbestos friction materials for industrial application.
Non-asbestos products since 1977: - Lawn and garden tractors
- Snowmobiles - Snow throwers - Industrial clutches - Electric brakes - Hoists/winches/cranes/shovels
.
Asbestos-free products perform as well as, and in most cases better than, asbestos products.
Federal Express fleet of 390 IVECO vans have shown good results; last 4x as long.
113
25 vehicles - Heavy duty truck fleet.(18 wheels) better performance than asbestos lining, 60 percent longer life.
School bus fleets/transit buses.
DuPont Viable substitutes in friction products are currently available:
> Automotive disk and drum brake linings - Truck brake blocks - Industrial brakes (reinforced with aramid) and other fibers 75 percent of new cars have non-asbestos brake linings. Clutch, manual transmission in use in Europe (non-asbestos); Automatic transmission In use in U.S. DuPont believes substitutes are available for gaskets also.
Environmental Lobbyists National Resources Defense Council Semi-metallic disk brakes are in use
- In U.S., (GM) - In Europe, Saab, Volvo and Jaguar. DuPont - aramid-fiber products. In Sweden is it illegal to replace brakes on vehicles i_f an asbestosfree brake is certified as satisfactory by the brake or vehicle manu facturer--two- thirds of new cars sold in Sweden are equipped with asbes tos-free brakes.
114
APPENDIX D
CONCERNS OF RESPONDENTS REGARDING FEASIBILITY OF PROPOSER PNASe-tiOW-SCHEQULE
Automobile Manufacturers
Automobile Importers of America
Proposed ruling is unfair to importers . - 1981-1983 baseline permit--import quotas were in effect and poor economic conditions prevailed - Very likely that domestic manufacturer will not be required to meet the 70%, phase-down in the first year - New entrants into U.S. market who have no baseline - European braking standards based on braking balance and skid avoidance rather than stopping distance as in U.S. European customers accept accelerated brake wear and noise.
AIA proposes - Allow unlimited use of asbestos for a fixed period - Then impose ban for those uses where adequate substitute
materials are available - Important to continue use of asbestos for replacement parts
of previously manufactured vehicles
Subaru
.
Support a 5 year lead time required before banning friction products.
Include extension clause.
Replacement parts for the car produced before effective model year must be excluded from this regulation.
Honda
.
Set up timetable based on model year and must consider necessary lead time.
Gaskets should be exempted because exposure of asbestos products is very unlikely.
Mitsubishi
Lead-time too short.
5 years is more reasonable.
Need exemption provision if acceptable replacements are not available
after 5 years.
.
If FMVSS-135 is adopted, more lead time will be required.
Exemption past 5 years may be required for clutch facings and gaskets.
117
Nissan
Need longer lead time: - Light-duty vehicle, front brakes -- at least 5 years - Light-duty vehicle, rear brakes -- at least 6 years
- Light-duty trucks -- impossible to say . - Clutch linings -- at least 6 years
Rule should not apply to replacement parts.
Rule should only apply to major vehicles redesigned {as new models are introduced).
EPA should devise a flexibleexemption procedure.
Chrysler
Believe they can comply with time table of Alternative 2 except for trucks..
Truck brakes will be asbestos-free at end of 10-year period specified in Alternative 2.
EPA should publish guidelines forexemptions.
Exempt aftermarket.
Proposed schedule:
Product Category Construction products,clothing Passenger car brakes; all transmission
applications All other cases in motorvehicles
Time 2 years
7 years 12 years
Modification of Alternative 2 represents most practical approach.
General Motors
Brake testing process can require up to 5 years.
Exempt aftermarket replacement for older vehicles.
In 1979, GM made goal to eliminate asbestos from brakes by 1985. - Largely met on front disc brakes - Not met on rear drum brakes due to: - squeal - wear
- friction stability
70 percent reduction Is too severe after first year.
118
Recommend that a ban not take effect until after a period 10 years; and that the EPA permanently exempt aftermarket brake linings.
Volkswagen
Beginning In 1987 model year, no asbestos brake linings on VW.
Proposes Fixed Time Limits for new vehicles.
Product
Clutch
.
Transmission
Gaskets
Front disc brake pads
(light duty)
Rear drum brake lining
(light duty)
Medium and heavy duty brakes
[Ik
A years 4 years 4 years
4 years
6 years 10 years
Exemption.allowed if non-asbestos substitutes not available. .
No permits needed.
Exempt replacement parts.
Mercedes
Ban only after 5 years.
Including possibility of exceptions:
.
- Indispenslble with respect to replacement parts
Gaskets, particularly cylinder head gaskets, may need even longer lead
time.
.
Alternative 1 is preferable over Alternative 2.
If FMVSS-105 is revised, this might result in further delays.
ord
Introduce changes during model redesign.
Ford supports reasonable fixed time table for removal of asbestos pro ducts, coupled with a procedure for exempting where no reasonable alter natives exist.
Exemption for parts replacement.
Austin
Anticipates lead time of 5 years to remove asbestos from its vehicles.
119
Unrealistic lead times.
Alternatives 1 or 2 would be more equitable.
Exempt spare parts for earlier models.
Toyota
Opposed to applying rule for replacement parts to earlier models.
Against proposed phase-down over 10 years.
Prefer staged ban. - Friction products--5 year lead time
- Gaskets--10 year lead time - Insulators--5 year lead time
National Automobile Dealers Association
.
EPA should consider a simple, streamlined model-year-based ban on new vehicle parts.
Avoid regulating replacement parts.
EPA should consider - Ban beginning in a given model year for new vehicles - No regulation of replacement parts
EPA and manufacturer should determine year(s) of ban on different products. '
American Motors Corporation
Recommends - More realistic timetable - develop guidelines for exemption
EPA should ban asbestos use in specific products, such as disc brakes, rather than a product category, such as friction material.
.
Develop guidelines for exemptions.
Lead time needs can only be adequately addressed under a product-specific regulation. This is major reason AMC opposes the phase-down scheme proposed by EPA.
Motor Vehicles Manufacturers Association
Schedule for implementation must be contingent upon the availability of suitable substitute materials so that safety and braking performance are not compromised.
Lead times proposed are unrealistic.
MVMA supports "an exemption process for essential uses without substitutes".
120
NVMA supports "an exemption process for essential uses without substi
tutes".
EPA should propose exemption procedure.
Exemptions for aftermarket required.
".
HVMA requests that EPA allow at least two full years between promulgation and the effective date of any regulatory action.
Individual member companies will provide additional comments on this matter of timing.
Truck and Equipment Manufacturers
Wagner
Phase-down proposed in Option 3 10 year period represents too great a highway safety risk.
Brake strip friction material must be exempted from Option 3, and 5 year options 1 and 2.
Recommend EPA establish mechanism for exemption procedure.
Request (1) an exemption on drum brake strip lining, and (2) extension of 5-year ban option for disc brake pads used on older vehicles. .
Navistar
3-1/2 to 4 years lead time to develop and incorporate new material into any product.
One-year not enough lead time for brake linings..
Exemption required for replacement parts.
Navistar supports Alternative 1 (5 years) if certain modifications are
Year
Percent
~r ' 30
7 24
8 18
9 12
10 6
11 0
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Alternative 2 - comments same as for Alternative 1.
Recommends - Lead time of 5 years plus 5 year phase-down.
Frelghtliner
Oppose EPA's primary proposal to immediately begin to phase out asbestos as impractical and practically impossible.
Support EPA's alternate proposal number 1 which provides 5 years before asbestos friction products would be banned.
Requests EPA proceed with Alternative 1. Allows friction materials to remain on the market for 5 years--adequate time for all commercial vehicle applications.
Rockwell
Proposed schedule for phase-down:
.
- Within a reasonable period, non-asbestos will replace asbestos
in most of the heavy duty or highway truck and trailer market.
- Definitive time constraints cannot be expected to phase out or ban asbestos linings for heavy-duty off-highway vehicles
with severe brake requirements and a few very heavy duty ohhighway truck applications until higher friction, non-asbestos qualification materials are developed.
John Deere
A first-year reduction of 70 percent Is too fast to reallocate asbestos for critical remaining uses. - Host reductions contemplated by the regulations took place during the base years - Life cycle tests require 6 months of lab tests plus 2 years of
field tests
The regulation does not consider the necessity of providing quality
service parts for repair of existing machinery.
.
Recommendation - Exemptions be provided for asbestos used Inside a machine
' housing
- 70 percent reduction be changed as follows (for new parts)
Year
1 2 3 10
Percent 40 SO 70 100
Replacement parts over long period of time.
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Friction Parts Manufacturers
DuPont
Europe is ahead of U.S. in conversion of disc brake and manual clutches to substitutes.
Viable substitutes are currently available for any application in friction
products and gaskets.
-
- Substitutes are technically equivalent or superior
- Initial cost is generally higher, but this is offset by
increased value in use.
Allied/Bendix
Believe safe reliable alternatives will be developed for all original equipment friction materials within 5 years of the effective date of regulation.
10 years for aftermarket;
New model vehicles is "best time" to introduce these products.
Friction Materials Standards Institute ~
Need time to develop and test satisfactory products.
Particular problem for aftermarket.
Highway safety in paramount.
An Immediate ban cannot be supported because of the Importance of safe braking systems on the nation's highway.
Original Quality
Will take many years to develop a racing substitute at temperatures between 350 C and 1000 C.
5 to 10 years or even longer for current racing disc brake pads.
We favor a disc pad ban In 10 years.
Scan Pac
They already have acceptable material for industrial and automotive use--light through heavy trucks, buses, off-road, construction, farms, etc.
123
i
Environmental Lobbyists National Resources Defense Council Ban on the use of asbestos in the brakes of all new cars and trucks and
in all replacement brakes in existing motorized vehicles. Replacements are available today.
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