Interpretation ID: nht78-2.49
DATE: 01/05/78
FROM: AUTHOR UNAVAILABLE; J. J. Levin, Jr.; NHTSA
TO: Kelsey-Hayes Company
TITLE: FMVSS INTERPRETATION
TEXT: This responds to Kelsey-Hayes' September 2, 1977, request to know whether Standard No. 121, Air Brake Systems, allows a burnish of the brake friction elements immediately prior to the parking brake tests of S5.6, and whether the service line air pressure limitation of 100 p.s.i. specified in S5.4.2.1 can be exceeded monentarily.
A burnish prior to the parking brake test is not permitted by Standard No. 121, except that S6.1.8 specifies burnish of the parking brake friction elements before testing in those cases where the parking brake system does not utilize the service brake friction elements. The National Highway Traffic Safety Administration (NHTSA) would consider a provision to deal with the condition of the brake linings prior to the parking brake test upon receipt of technical data showing justification for such an action.
Section S5.4.2.1 of the standard specifies a procedure that states "[the] service line air pressure shall not exceed 100 p.s.i. during any deceleration." You describe a condition in which the service line pressure exceeds 100 p.s.i. for a short period (typically 0.1 seconds), which might be traced to characteristics of the friction material, brake mechanism, dynamometer, or instrumentation.
The NHTSA recognizes that peak pressure may be momentarily increased by the initial rush of air pressure into the brake chamber, or by other anomolies. In the case of such momentary pressure increases, the NHTSA interprets S5.4.2.1 to mean that the source of air pressure for applying the brake must never exceed 100 p.s.i. Thus, it would be permissible to experience momentary pressures above 100 p.s.i. in the service line as long as the pressure source never exceeds that level. Sustained periods of pressure above 100 p.s.i. would not be permissible.
SINCERELY,
KELSEY-HAYES COMPANY
September 2, 1977
Joseph J. Levin, Jr. National Highway Traffic Safety Administration
RE: Request for Interpretation FMVSS-121, Air Brake Systems @ 5.3.1 Stopping Distance-Trucks and Buses
Kelsey-Hayes Company, a domestic manufacturer of motor vehicle equipment including antilock systems, requests an interpretation of the above referenced section of Standard 121.
It is the nature of newly manufactured braking systems to have residual coatings of various materials deposited upon the brake components together with surface irregularities as a result of the manufacturing process.
FMVSS-121, as well as FMVSS-105, allow for this by providing burnish stops to remove foreign substances and to condition brake linings at the outset of the testing sequence. In addition to the initial burnish, some test procedures including FMVSS-105 specify burnishing at specific intervals during the test.
Reburnishes at specific intervals are provided in recognition of the fact that a rapid series of test stops will generate high temperatures and cause surface glazing on the brake lining material. To accurately gauge the effectiveness of the braking system, FMVSS-105 allows the brakes to be reburnished before the parking brake test. While perhaps not intended by FMVSS-105, the snub burnish procedure has the effect of paralleling actual vehicle use as it is normal to expect a driver to apply the service brakes in a routine fashion prior to parking the vehicle. No identical language appears in FMVSS-121, although Table 1-Stopping Sequence does provide for a burnish in its initial step.
The authors of FMVSS-105 recognized the abnormal conditions which develop during the harsh testing sequence. Further, they recognized that lining surface glazing will impair the holding characteristics of a braking system, perhaps to the extent that an otherwise completely adequate design will fail this standard for an unintended reason.
It is with these thoughts in mind that Kelsey-Hayes Company respectfully requests an interpretation by the NHTSA as to whether step 1 of Table 1 of FMVSS-121 will allow a burnish of the parking brakes immediately prior to step 5-parking brake test.
Thank you for your consideration.
David M. Thompson