Interpretation ID: aiam2721
Kelsey-Hayes Company
Romulus
MI 48174;
Dear Mr. Thompson: This responds to Kelsey-Hayes' September 2, 1977, request to kno 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 momentarily.; A burnish prior to the parking brake test is not permitted by Standar 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 state '[t]he 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 b the initial rush of air pressure into the brake chamber, or by other anomolies (sic). 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, Joseph J. Levin, Jr., Chief Counsel