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FINAL REPORT
t-N. St^rt
Mar 31 m?
on
ANALYSIS OF THE FEASIBILITY OF REPLACING ASBESTOS IN AUTOMOBILE AND TRUCK BRAKES
Prepared for the
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^Environmental Protection Agency
March 6, 1987
by The American Society of Mechanical Engineers
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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 3 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. (Skip) Fletcher Associate Director Texas Engrg. Experiment Station The Texas A&M University System 301 Engineering Research Center i College Station, TX 77843 (409) 845-7270
Members
Mr. Arnold E. Anderson Ford Motor Company 20000 Rotunda Drive P.a. Box 2053 Dearborn, MI 8121-205*
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Mr. Robert Nelson
Manager, Technical Service ABEX Corporation 300T West Big Beaver/Suite 710 Troy, MI 48084
Mr. John Fabian Automotive Engrg. Dept. American Automobile Assoc. 8111 Gatehouse'Road Falls Church, VA 22047
Dr. Serge Gratch 32475 Bingham Road Birmingham, MI 48018
Mr. Jerry McCullough Head, Development Section Lyndon B. Johnson Space Center Houston, TX 77058
Dr. Michael J. Rabins Assoc. Dean-Graduate Engrg. Programs Wayne State University 731 Science Library Oetroit, MI 48202
Mr. Richard Radi inski Vehicle Stability & Control Branch U.S. Dept, of Transportation P.0. Box 37 East Liberty, OH 43319
Dr. Ernest Rabinowicz Prof, of Mechanical Engrg. Mass. Institute of Technology Cambridge, MA 02139
Draft Report Prepared by:
Mr. Scott Barber Mr. Jeff Hadden
Mr. Joseph Hoess Mr. Keith Dufrane
Battelle Columbus Division 505 King Avenue
Columbus, Ohio 43201
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TABLE OF CONTENTS
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EXECUTIVE SUMMARY AND CONCLUSIONS ...................................... . .......................... I
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 . ...................................................................15
* 2.4 Influence of BrakeDesign onBrake Effectiveness............................. 15
2.5 Influence of Brake Lining-Brake Drum Pressure Distribution onBrakeEffectiveness ................................................... 20
2.6 Issues of Friction-Induced Vibrationand Brake Noise .... 22
, 2.6.1 Stick Slip................................. /.............................
24
2.6.2 Negative Slope of the FrictionVelocity Curve .... 24
2.6.3 Positive Mechanical Feedback ............................................... 25
2.7 Comparison of 4-Wheel Disc Brake Systems With Front Wheel Disc-Rear Wheel Drum System.................................. . 25
3. EVALUATION OF FRICTION MATERIAL PERFORMANCE .......................................... 27
3.1 Section Summary.............................................................................................27
3.2 General Evaluation Criteria ........................................................... . 28
3.2.1 Fade Resistance........................................................................... 29
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 ................................................................... 33
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TABLE OF CONTENTS (Continued! *5
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3.2.6 Wet Friction . . ....................................................................... 33
3.2.7 Moisture Sensitivity ...................................... ........................ 35
3.2.8 Lining Wear Rate.......................................... ............................ 35
3.2.9 Friction Material Qualification ................. ...... 37
3.3, Laboratory and Vehicle Fric|Jon Material Evaluation . . . . 37
3.3.1 Friction Assessment Screening Test (FAST) ...... 39 I
3.3.2 Friction Materials Test Machine (FMTM) ......................... 39
3.3.3 Girling Scale Dynamometer ....................................................... 40
3.3.4 Full Brake Inertia Dynamometer................................. . . 41
3.4 Correlation of Laboratory Test Results With Vehicle Test Results ................................................................... 43
3.5 Federal Braking Requirements and Other Brake Tests ................. v-
3.5.1 Federal Motor Vehicle Safety Standard 105 ..... .
44 44
3.5.1.1 Stopping Distance Requirements ..................... . 45
3.5.1.2 Parking Brake Requirements ......... 45
3.5.2 Federal Motor Vehicle Safety Standard 121 ................. 47
3.5.2.1 Vehicle Braking Experiments . ......................... 47
3.5.2.2 Parking Brake Test .................................................. 48
3.5.2.3 Dynamometer Testing for FMVSS 121 . . . . . 48
3.5.3 Federal Motor Vehicle Safety Standard 135 ..... . 48
3.5.3.1 Front Brake Biasing . . . . ............................. 48
3.5.3.2 Control Forces for Brake Application . . . . 49
3.5.3.3 Parking Brake Performance ................................. 49
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TABLE OF CONTENTS (Continued)
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3.5.4 SAE Recommended Practices for Evaluating Brake Systems and Friction Materials .............................. 49
4. PERFORMANCE ATTRIBUTES OF FRICTION MATERIALS NOW IN USE ................. 52
4.1 Section Summary........................................................................................ 52
4.2 Introduction toFrictionMaterial Formulations ........................... 53
4.3 Asbestos-Based Friction- Materials . . ..................................
54 '
4.4 Non-Asbestos FrictionMaterials .... ............................
56
4.4.1 Semimetallic Friction Materials ......................................
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4.4.2 Non-Asbestos Organic Friction Materials .....................
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4.4.3 Sintered Metallic Friction Materials .............................. 59
4.4.4 Carbon-Carbon Friction Materials .............................. . . 60
' 4.5 Aftermarket Vehicle Considerations .................................................. 60
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4.5.1 Dr*um Brlkes.........................................................................................60
4.5.2 Disc Brakes..............................................................
61
4.5.3 Substitution ..................................... .... ......... 61
4.6 Direct Comparison of OEM and Aftermarket
Brake Linings
........................................................................... * 66
4.6.1 Dynamometer Test Data....................................................................66
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 . ...................................... 73
5.1 Section Summary....................................................................................... 73
5.2 Brake System Trends in Passenger Cars, Trucks, and Equipment..................................... ................................. ... 74
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TABLE OF CONTENTS (Continued)
5.2.1 Passenger Cars ...... . 1.............................................. '74 5.2.2 On-Highway Trucks ....................................................................... 78
5.2.2.1 Light Trucks ............................................................... 78 5.2.2.2 Medium Trucks ........................................................... 79 5.2.2.3 Heavy Trucks .... ..................... ...... 80 5.2.3 Off-Highway Trucks |nd Equipment...................................... 81 5.3 Current European and Japanese Experience in Brake jr, Design and Friction Material Selection (Automobiles and Light Trucks) ... .................................................. ..... 81 6. SUMMARY OF INDUSTRY RESPONSES TO PROPOSED 8AN ..................................... 85 6.1 Section Summary . . . . ....................................................................... 85
( 6.2 Concerns of Respondents to Issues of BrakePerformance ... 85
6.3 Concern erf Respondents to Issues of Friction Material Availability ........................................................................... 88
6.4 Concerns of Respondents to Feasibility of the Proposed Phase-Down Schedule ............................................................... 89
7. REFERENCES................................................................................................................. 91
LIST OF FI6URES
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 Figure 4. Effect of Lining Pressure Distribution on Brake
Effectiveness for a Leading Shoe Drum Brake...............................21
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LIST OF FIGURES (Continued)
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Figure 5. Fade Characteristics of Good and Poor Brake Friction Materials ....................................... .................................... 31
Figure 6. Fade Recovery Characteristics of Good and Poor Brake Friction Materials ................................................................... 31
Figure 7. Delayed Fade Characteristics of Friction Materials .... 32
Figure 8. Speed Versus Braking Performance Characteristics for Good and Poor Friction Materials ........... 32
Figure 9. Friction Stability Characteristics of Both Good and Poor Friction Materials .............................. ....... 34
Figure 10. Wet Braking Performance for Good and Poor Performance Braking Materials ....................................................... 34
Figure 11. Effect of Moisture Sensitivity on Braking Performance.............................
36
Figure 12.'Wear Performance for Braking Materials......................
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.Figure 13. Schematic of Inertia Dynamometer..................................................... 42
Figure 14. Comparison of Dynamometer Test Results for Four Different Friction Materials, 300 F After Burnish, 12 x 3 Duo Servo Drum Brake.......................................................... 69
Figure 15. Vehicle Test Results Comparing the Performance of Aftermarket Friction Materials . . ........................................... 71
LIST OF TABLES
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 ...............................................................
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Table 3. Machines For Friction Material Evaluation (Reference 3) ............................. .............................................................. 38
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LIST OF TABLES fContinued)
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Table 4. Test and Performance Criteria Specified Under FMVSS 105......................................................................
46
Table 5. Selected SAE Vehicle Brake Test Codes.......................................... 51
Table 6. Producers of Brake Linings for Light and Medium Vehicles..................................................................................... 63
Table 7. Producers of Brake Linings for Heavy Vehicles ....... 65
Table 8. New Vehicle Sales and Vehicle Registration For 1987 (Ref. 15) .... J.............................................................. 75
Table 9. List of 1984 American Automobiles Equipped ts With 4-Wheel Disc Brakes (Ref. 14).............................................. 77
Table 10. List of 1984 European Automobiles Equipped With 4-Wheel Disc Brakes (Ref. 14)
82
Table 11. List of 1984 Japanese Automobiles Equipped With
( * 4-Wheel Disc Brakes (Ref. 10) ...... .................................. 84
Table 12. List of Respondent's to- Federal Register Notice Regarding Proposed EPAAction . .. .................................................. 86
Table 13. Summary of Respondent's Comments to Proposed EPA Plan....................
87
LIST OF APPENDICES
Appendix A.
Vehicle and Dynamometer Test Procedures for Qualifying Friction Materials and Brake Systems Under FMVSS 105 and FMVSS 121 . ...................................... . . A-l
Appendix B. Concerns of Respondents to Issues of Brake Performance..................... ..................................... . . . . B-l
Appendix C. Concerns of Respondents to Issues of Friction Material Availability . . . .............................................. . . C-l
Appendix D. Concerns of Respondents Regarding Feasibility of Proposed Phase-Qown Schedule ......................................
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EXECUTIVE SUMMARY AND CONCLUSIONS
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 brakes and friction mate rial s was assembled by the American Society of Meehanical Engineers (ASME). iThe panel addressed technological Issues associated with the removal of asbestos from friction materials, specifically:
(1) Identification of substitute brakes and systems, (2) Influence of the ban on motor vehicle safety standards, - (3) Problems associated with replacing asbestos friction mate
rials in aftermarket vehicles, a'nd (4) Pae?e 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.
Conclusions
The panel has made the following observations and conclusions:
(1) Vehicle controllability during braking is strongly affected by friction material performance. Vehicle controllability during braking is affected by both driver and brake system performance. Generally, vehicle front-to-rear braking ratios are adjusted on a per vehicle basis to provide optimal braking. Too much braking on an axle results in premature wheel lock-up and this can lead to loss of steering control {front lock-up] or spin out (rear lock-up). While the effectiveness of disc brakes is directly
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proportional to pad-disc*friction levels, the effectiveness of drum brakes can be greatly affected by changes in lining-drum"friction. Therefore, unqualified substitution of friction materials in either vehicle disc pads or drum linings may have an adverse effect on vehicle brake balance and controllability. 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, and brake effectiveness can be affected by the frictjon properties of the friction material. For these reasons, automobill and truck manufacturers have strongly opposed the immediate ban of asbestos in aftermarket friction materials for older vehicles. fa Information supporting this conclusion is presented in Chapter 2 of this report.
(2) Vehicle manufacturers submit braking systems and friction products to mraerous levels of qualIfIcation tests. The qualification of original equipment braking systems is regu lated under Fiberal Motor Vehicle Safety Standards (FMVSS) 105 and 121. For many original equipment manufacturers, these requirements represent minimal standards of performance, and supplemental qualifi cations are usually satisfied due to customer demand. Satisfactory compliance with FMVSS 105 and 121 requires vehicle and inertial dyn amometer test facilities to evaluate brake system performance on a per vehicle basis. Results obtained using scaled-down laboratory apparatus, such as friction material testing machines (FMTM) or fric tion assessment screening test (FAST) machines, have been shown to correlate poorly with vehicle test results. No simple bench-top tests are available to evaluate the performance characteristics of friction materials. The performance of aftermarket friction materials is not regulated by law.
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(3) Qualified, non-asbestos friction materials are not yet available for all current applications. Non-asbestos friction material technology is advancing rapidly. Several new asbestos-free car, light truck, and heavy vehicle brake systems have been released for OEM applications in the past 3 years. Hore new vehicle brake systems will be released with non-asbestos friction materials for the 1988 model year. Original equipment vehicle manufacturers in the United States and Europe are rapidly moving toward asbestos-free brake systems for all new cars, light trucks, and heavy trucks. Adequate non-asbestos friction material formulations presently are not available for all vehicle systems, but with present progress continuing, most new pas
te senger cars should 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-.1 inings may not*be developed in time for incorpora& tion liy 1992. New classes of non-asbestos friction materials have unique com pilations which may have new and unexpected failure mechanisms. In many cases, unique failure modes are revealed only through extensive vehicle testing.
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(4) The substitution of unqualifled non-asbestos friction materials In the
aftermarket poses the largest potential safety Issue. A large number of older vehicles in the United States have brake
systems designed with asbestos friction products. The use of different materials in place of proven asbestos materials could result in a loss of vehicle controllability during braking.
Currently, there are no required performance evaluation 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.
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Non-asbestos frictio| 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. Mandating aftermarket non-asbestos friction materials for vehicles that were originally equipped with asbestos-based linings would lead to a potentially serious customer safety risk, unless stringent friction material qualification specification tests are Included.
(5) 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 sonie time. Non-asbestos drum brake materials for automobile drum brakes
b have been difficult to qualify in the past. For this reason , several European automakers use 4-wheel disc brakes in conjunction with semimetallic pad materials as a means of eliminating psbestos. Because of differences between qualification standards and driver preference in the United States and Europe, American and Japanese automakers are utilizing r^ar drumbrakes. Suitable non-asbestos materials are not available for all of these applicatlons, and industry-wide substi tution of non-asbestos materials in all existing brake designs would require considerable developraent. It is unreal 1st1c to assume that all automakers will redesign all passenger car and truck braking systems around disc brakes in order to utilize semimetal 11c materials.
(6) Industry gave mixed response to the proposed EPA action. 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 manufacturers were optimistic about the future availability of asbestos-free materials, they were unanimous in their opposition to a proposed phase-down 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 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.
Friction product manufacturers gave mixed response to the issues. One manufacturer indicated that for many applications, suitable substi tutes are available, although for many vehicle applications no quali fied materials exist. Another manufacturer indicated that suitable substitutes were produced by their company, although no FMVSS 105 or FMVSS 121 qualification capabilities existed at their facility.
Recommended Future Work
Industry trends for tfop next/5 years appear to be directed toward the elimination of asbestos in^new ^passenger cars. For these applications, the use qualified non-asbestos fricBorrma ter falsi may not present a vehicle safety problem over the 1ife of the friction product. However, the use of unqualified non-asbestqs materials for the aftermarket still 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 States
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 trucksJ that will require aftermarket friction materials. For example, it would be useful to determine how many passenger cars in the 3000-3500 lb 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 drurn-front wheel disc brakes could be determined. Thus, vehicle populations would
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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 materials are fitted with duo-servo rear drum 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.
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Task 2. Conduct Dynamometer and Vehicle Qualification 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 ^ould draw upon the results of Task 1 by limiting tiii 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 aftermarket applications and (2) what range of effectiveness currently exists among suppliers of aftermarket nbn-asbestos materials.
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