Interpretation ID: nht73-5.21

DATE: 09/19/73

FROM: AUTHOR UNAVAILABLE; R. B. Dyson; NHTSA

TO: Pundalik K. Kamath

TITLE: FMVSS INTERPRETATION

TEXT: This is in reply to your letters of August 7 and August 23, 1973, concerning the conformity to Standard No. 121 of certain features in air brake systems submitted to you by suppliers.

Your first question is whether the volume of a supply reservoir that lacks a check valve is to be included in arriving at the required reservoir volume of 12 times the full-travel service brake chamber volume. Our reply is that S5.1.2.1 includes the volume of all service and supply reservoirs, regardless of valving, and that the volume of the supply reservoir in question would therefore be included in the combined reservoir volume.

You next ask whether a system that has no isolated emergency reservoir can rely on its service reservoirs to provide air for the two emergency stops proposed as S5.7.3 by Docket No. 73-13, Notice 1 (38 F.R. 14963). Under this proposal, the stops would have to be accomplished with a single failure of a part (other than a common valve, manifold, brake fluid housing or brake chamber housing) designed to contain compressed air or brake fluid. If the system is designed so that no single failure, other than a valve, manifold, or housing failure, will cause a loss of air in both tanks, it would seem to be capable of meeting the proposed requirement even though it does not have an isolated emergency reservoir. However, if a single failure in a brake line would deplete the air in both service reservoirs, the system would be unable to meet S5.7.3 unless an emergency isolated reservoir were provided.

2

In answer to your last question, the emergency stops proposed in S5.7.3 would be conducted from an initial pressure in excess of the compressor cut-in pressure. The reservoirs would not be depleted by prior stops.

Yours truly,

August 7, 1973

Lawrence Schneider Chief Counsel National Highway Traffic Safety Administration

Enclosed are schematic copies of air brake systems provided to OTC by Berg Manufacturing Company and Bendix-Westinghouse Air Brake Division which illustrate air brake systems incorporating provision for secondary brakes per the Notice of Proposed Rulemaking, Docket 73-13, Notice 1.

Our concern is if these systems, which are essentially the same, will meet the proposed change to FMVSS 121 when effective, particularly paragraph S5.7.3 as proposed? While we see no reason they would not OTC would like an opinion from your office before we adopt either system.

We would like said opinion as soon as conveniently possible to assist in our brake program execution. Thank you.

OSHKOSH TRUCK CORPORATION

Pundalik K. Kamath Senior Safety Engineer

Enclosures

August 23, 1973

Lawrence Schneider Chief Counsel National Highway Traffic Safety Administration

With reference to my letter dated, August 7, 1973, regarding compliance of the air brake systems proposed by Berg Manufacturing Company and Bendix-Westinghouse Air Brake Division with FMVSS 121 per Proposed Rulemaking, Docket 73-13, Notice 1, particularly with respect to the following questions, I would like to receive clarification from you.

1. Is it correct to use the supply reservoir volume, which is not protected by a check valve, when calculating the total volume of twelve times full stroke chamber volume? Our assumption is that this is acceptable however, we would like confirmation.

2. These systems have no isolated emergency reservoir and rely on air pressure from one of the two service reservoir sources, assuming a failure in the other service reservoir source, for the minimum two stop capability in modulated emergency mode. This implies a single failure affecting only one service reservoir system, it is our understanding that this is acceptable practice per the proposed standard. Please confirm.

3. We understand that a modulated emergency stop condition implies a fully charged service reservoir, that is 100-110 psi in the functional reservoir, that no normal stops which temporarily drop this pressure to 80-90 psi immediately prior to the failure condition are considered. In the latter case two modulated emergency stops may not be obtainable due to the lowered pressure. Please confirm that the baseline condition assumes at least one service reservoir is normally charged to 100-110 psi before simulating failure.

We would appreciate your reply to these questions, as soon as conveniently possible.

OSHKOSH TRUCK CORPORATION

PUNDALIK K. KAMATH Senior Safety Engineer