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Truck Brake Lining Performance Characterization: Introduction The National Highway Traffic Safety Administration (NHTSA) reported (see: http://www-nrd.nhtsa.dot.gov/pdf/nrd-30/NCSA/TSF2004/809907.pdf) that in 2004, 416,000 large trucks were involved in traffic crashes in the U.S., that a total of 5,190 people died (12% of all the traffic fatalities reported in 2004) and an additional 116,000 were injured in those crashes. One out of eight traffic fatalities in 2004 resulted from a collision involving a large truck. In a widely cited study by Jones and Stein (Jones I. and H. Stein, Defective Equipment and Tractor-Trailer Crash Involvement. Accident Analysis and Prevention 21:469-81, 1989), it was reported that brake defects were quite common and were found in 56% of the tractor-trailers involved in crashes. In the more recent Large Truck Crash Causation Study sponsored by DOT, it was concluded that 29.4% of all large truck crashes involved brake failure, brakes out of adjustment, or other brake related issues. Compounding the brake defects issue even more is the fact that although original equipment (OE) brakes must comply with federal motor vehicle safety standards which specify maximum stopping distances according to vehicle weight, loading, pedal effort (with and without power assistance) and brake condition (green and burnished linings), there are no federal performance standards for aftermarket (AM) brake linings. It is typically assumed that replacement AM brake linings perform the same as or better than the OE brake linings on a vehicle. Unfortunately, there is currently no methodology or rating system available that can assure OE-equivalent brake performance from AM brake linings. NHTSA Brake Performance Characterization Project – Research Summary The Oak Ridge National Laboratory’s Center for Transportation Analysis (CTA) has, for the past five years, conducted a significant amount of heavy truck research under its Heavy truck Safety Program. This research has included:
Because of this strong emphasis in heavy truck safety, the National Transportation Research Center, Inc. (NTRCI) selected CTA to conduct a National Highway Traffic Safety Administration (NHTSA) study of brake performance of OE and AM brake linings. An important and unique distinction of this research was that brake performance would be evaluated within three distinctive research domains; laboratory, test-track, and field test. In addition, ORNL was to look at the performance in these domains to see if a correlation exists between them. Please note that with regard to these three domains, face validity increases from laboratory to field-test, but so does the complexity in controlling the study variables. Objective/Scope The NHTSA Brake Performance Correlation Project involved the study of four OE and four AM brake linings. In discussions with NHTSA, the vehicle platforms of interest were heavy single-unit trucks that offered significant braking duty cycles. The trucks selected were a class-7 Single Axle Dump Truck, a Class-8 Tandem-Axle Dump Truck, a Class-8 Tri-Axle Dump Truck, and a Class-8 Tandem-Axle Refuse Hauler. The trucks are shown in Figures 1 through 4.
Partnerships were established with local trucking firms in order to have access to these vehicles during the test-track and field-test portions of the study. Partners included:
Within the laboratory, friction performance testing was performed on multiple samples of each of the eight brake linings within ORNL’s Sub-Scale Brake Testing System (SSBT – see Figure 5), and on a Chase Friction Test Machine at Link (a subcontractor to ORNL). Brake material wear was also studied for the eight brake lining materials.
In the test-track track studies, focus was put onto stopping performance. One of the test trucks (Class-7 Single-Axle dump truck) traveled to the Transportation Research Center (TRC) in East Liberty Ohio for brake testing based on the Federal Motor Vehicle Safety Standard (FMVSS) – 121 brake testing regimen. The other three vehicles traveled to Laurens Proving Grounds (LPG) in Laurens, South Carolina for more simple straight-line stopping tests (based again on FMVSS-121). Prior to test-track testing, each of the four test vehicles was outfitted with new tires, drums and initially with OE brake linings, and subsequently with AM brake linings. Stopping tests were conducted multiple times at a constant braking pressure. Brake wear data was not gathered because of the limited number of braking events. In the field test, focus was again put onto stopping performance. Each of the four test vehicles and four “sister” vehicles were each outfitted with the same set of materials that were tested on the trucks during the test-track studies. For each of these eight field test vehicles, wear data and stopping performance was gathered at the start of the field test, and near the end of brake life. The test vehicle would engage in their normal vocational activities, and the test would continue for up to 12 months. Interestingly, wear data from the partner fleets indicated a significantly short wear-out period. The refuse hauler claimed to wear out brakes every six-to-eight weeks. Data from the field were dramatically different from these expectations – the actual wear-out times were much longer. One explanation was that during the testing, all wheel-ends were brand-new, and the maintenance and calibration of the brakes were closely monitored. This suggests that attention to the quality of the wheel-ends could add considerable life to braking systems. For the field testing, however, this finding resulted in fewer than expected life-cycles and less data than had been expected. Results The project provided a unique opportunity to study the same OE and AM brake linings in multiple testing domains (a laboratory setting, a test-track and a field test setting). The focus of the laboratory testing was on the friction properties of the selected brake linings. The focus of the test-track efforts was to look at stopping performance in a controlled environment. The focus of the field tests was to look at stopping performance and wear over an extended period with the test vehicles engaged in their normal vocational activities. The SSBT tests were conducted under low-speed/low-load (Low-PV) conditions as well as high-speed/high-load (High-PV). For the Low-PV cases, results compared well to that typically reported by industry for friction materials. The High-PV results, because of thermally induced fade (i.e., reduced frictional properties at elevated temperatures), did not compare as well. Regarding SSBT-based wear tests, a good correlation was found to exist between mass loss (wear) and the friction coefficient. For data generated in the SSBT-based friction and wear tests, there was, however, no evident trend that OE brake linings were necessarily any better than AM linings. Similar results were obtained from the Chase Tests. Test-track testing involved stopping data for the same brake materials as addressed in the laboratory-based friction testing. For the stopping data generated from the test-track tests, the OE brake linings were shown to provide higher deceleration rates than the AM brake linings (measurements were taken from the same truck, with the same payload, with the same driver, at the same test track, almost at the same time). The field tests involved the least controlled of the test environments. Because the vehicles were engaged in their normal vocational activities, the loads of the vehicles varied. In addition, even though the test vehicle drivers were professional drivers, there was significant variation in the amount and steadiness of the applied braking pressure. Without appropriately “correcting” the data to account for these phenomenon, results would be meaningless. ORNL developed a means for correcting collected data due to treadle pressure variations, and variations in the truck weight during testing. For the stopping data generated from the field tests, the OE brake linings were shown to provide higher deceleration rates than the AM brake linings (measurements were taken from sister trucks, with the same driver). Wear analysis for the field test data indicated that the AM brake linings exhibited more wear than the OE brake linings. Interestingly, some of the measurements were possibly confounded due to a “curing” or “swelling” effect as the new brake lining was used. Comparison of results between testing domains was complex. In comparing SSBT friction test results to Chase test results, it was found that there was a fair-to good correlation of relative rankings in friction coefficient between the Chase and SSBT test results. The AM linings, however, ranked in more widely different order between these tests. Comparison of SSBT/Chase results to the test-track results showed a positive correlation for special cases of the SSBT (i.e., Low-PV) and Chase tests. Comparison of SSBT/Chase results to the field test results was similar in that moderate correlations existed for certain special cases. Overall, however, correlations involving laboratory-generated data were found to be weak. Comparison of the test-track results to the field test results involved separate comparisons between the test-track results from TRC and from LPG. For the TRC data, there was an exceptionally strong correlation between the test track results and the field test. In contrast, data from LPG produced no correlation with the field data. It should be noted that the TRC tests were done with a strong adherence to FMVSS-121 protocols. Similar adherence at LPG were experienced. Overall, this project has resulted in a significant database of information related to friction, wear and stopping performance for multiple OE and AM brake linings in multiple testing domains. Correlation of laboratory results involving friction performance with more real-world testing involving stopping distances and deceleration rates were weak. Better correlation was seen between test-track results and field-testing. Issues with such comparisons involved consistency of brake-pressure and vehicle weight. Future research should involve more controlled, real-world testing, possibly utilizing a Performance-Based Brake Tester. It should also focus on the performance of low-cost brake lining imports that may exhibit significant differences between their performance, and the performance of OE brake linings. For more information on this project or other Heavy Truck Safety Program research, please contact Bill Knee, Group Leader for the Transportation Technology Group (946-1300, kneehe@ornl.gov). Submitted by: Bill Knee, Transportation Technology Group |
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