Interpretation ID: 77-4.18
TYPE: INTERPRETATION-NHTSA
DATE: 10/17/77
FROM: AUTHOR UNAVAILABLE; J. J. Levin, Jr.; NHTSA
TO: Crane Carrier Company
TITLE: FMVSS INTERPRETATION
TEXT: This responds to Crane Carrier Corporation's June 8, 1977, question whether the maximum time limits specified by S5.1.1 of Standard No. 121, Air Brake Systems, for build-up of brake system air pressure from 85 to 100 psi includes the time taken to build up air pressure in an accessory reservoir (for an air starter) that is replenished only when the truck is started.
The answer to your question is no. Section S5.1.1 is a performance requirement that assures that repeated use of the brakes during vehicle operation will not deplete the available air supply because of insufficient air compressor capacity. The purpose of this requirement only indirectly relates to the initial air pressure build-up that occurs when the vehicle is first started.
The agency's existing laboratory procedure for compliance testing provides for fully charging the air brake system (and any accessory reservoirs which charge automatically in the process) before the test is begun. The engine is shut off while brake system air pressure is reduced to a level that permits a subsequent build-up for measurement purposes. In order to properly test vehicles with air starters, the agency is modifying its procedure to keep the engine running throughout the test, so that the air starter reservoir remains fully charged throughout the measurement period.
SINCERELY,
HEAVY DUTY TRUCK MANUFACTURERS ASSOCIATION
June 20, 1977 Frank Berndt Office of the Chief Counsel National Highway Traffic Safety Administration
Pursuant to our telephone conversation last Friday, I am pleased to enclose the inquiry we discussed. You suggested that interpretive rulings should be in writing, and I am pleased to respond.
F. MURRAY CALLAHAN General Counsel
CRANE CARRIER COMPANY
June 8, 1977
Heavy Duty Truck Manufacturers Assoc.
Attention: F. Murray Callahan, General Counsel Subject: Compliance with Section S5.1.1 of MVSS 121, when vehicles are equipped with air starters.
We are seeking an interpretation of Section S5.1.1 due to the following condition occurring on vehicles equipped with air starters. These vehicles require a separate large volume (17,787 C.I.) starter reservoir isolated from the trucks air brake system by means of a pressure protection and check valve which maintains a minimum of 75 psi air pressure in the service brake system. However, after initial start up of truck, which could use up to approximately 50% of starter reservoir capacity, and the truck brake system is built up to 75 psi, the protection valve between the two systems opens, and at this point the total system capacity is equal to the brake reservoir volume plus that of the air start reservoir. When this occurs, it is impossible for us to comply with the time limit specified in Section S5.1.1 due to the extremely large combined volume of the two systems.
What we seek interpretation of is if the standard will allow: (1) air start reservoir to be completely refilled as soon as engine is started, (2) draining of air in the service brake reservoirs and then, (3) replenishing the air in the brake reservoir in the time limit specified. This seems to us to satisfy the standard since the standard is only trying to insure of a large enough air compressor to replenish the service brake reservoirs and once the air start system is filled it will have no effect on the brake system operation.
If you have any questions concerning this matter, please contact me.
Ray Sizemore Engineer
cc: KEN LAWRENCE