nht74-4.5 | NHTSA

DATE: 06/20/74

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

TO: Wagner Electric Corporation

TITLE: FMVSS INTERPRETATION

TEXT: This responds to your May 7, 1974, request to know whether check valves or equivalent devices must be placed immediately adjacent to or within each service reservoir in a trailer air brake system to comply with S4.2.1.5 of Standard No. 121, Air brake systems, and whether the standard intends each axle subsystem to have a separate service reservoir and check valve system, with particular regard to arrangements for liftable axle systems. S5.2.1.5 states:

S5.2.1.5 Each service reservoir shall be protected against loss of air pressure due to failure or leakage in the system between the service reservoir and its source of air pressure by check valves or equivalent devices.

The answer to both of these questions is no. In an April 3, 1974, letter to Great Dane Trailers, Inc., we interpreted S5.2.1.5 to permit location of the check valve at the isolated reservoir, permitting that valve to also guard the service reservoirs on that axle system. The language of S5.2.1.5 is unclear in this regard, and may be amended in the future. In response to your specific question, it is permissible to use a single check valve to protect more than one reservoir in a subsystem.

Standard No. 121 does not require a separate service reservoir check valve for each axle system in a tandem axle. As you point out this could decrease total vehicle reliability, and abuse in this area could lead to amendment of the provision. In answer to your fourth question, a single check value could be utilized to protect the air reservoir or reservoirs required for a liftable axle system or subsystem.

Yours truly,

WAGNER ELECTRIC CORPORATION

May 7, 1974

National Highway Traffic Safety Administration

Attention Lawrence R. Schneider, Chief Counsel

Re: Docket 74-10; Notice 1 49 CFR 571.121

SUBJECT: Request for Interpretation

As a supplier to the motor vehicle industry of air brake equipment designed to meet the requirements of FMVSS-121, we find ourselves faced with a constant responsibility to provide this equipment in the most economical form. Competitive influences and customer confidences demand this response from the equipment suppliers. The possible combination of several components essential to the FMVSS-121 trailer air brake system and related economies hinges on the degree of latitude allowed in the meaning intended for Section 5.2.1.5. We realize, however, that economy should not prevail if, in fact, there is any significant reduction in overall safety or system protection.

In designing and constructing air brake systems to meet the requirements of FMVSS-121, we have considered several approaches to the means of complying with Section S5.2.1.5, particularly with regard to the definition of the system connecting the trailer service reservoir with its source of air pressure. For convenience in reference, we reprint S5.2.1.5 below with the word "system", which is specifically in question, underlined.

It has been difficult for us to assess how much of the trailer air brake system is considered by NHTSA to be included in the wording . . . between the service reservoir and its source of air pressure . . .", particularly when the ultimate source of air pressure to charge said reservoir is located on another vehicle, the self-propelled towing vehicle, in the form of its air compressor.

In the case of a vehicle application where it is advantageous to use several reservoirs in a separate but connected subsystem, the possibility is presented of using a single check valve conveniently placed to protect the subsystem.

Question #1: Is it permissible to use a single check valve to protect a plurality of reservoirs in a subsystem?

One interpretation is that all of the air lines existing between the point of coupling to the towing vehicle (the supply air line coupler) and the service air reservoir are subject to failure or leakage, and that the protecting check valve or equivalent device is to be placed immediately adjacent to or within the reservoir.

This location of the check value or equivalent device would give the required protection for all failures or leakages occurring between the air reservoir and any source of air pressure notwithstanding a failure in the check value itself.

We have interpreted the "source of air pressure" for a service reservoir to be the last connection to that reservoir. This precludes the use of tubing or hose between some remotely-located device and the reservoir port.

Question #2: Must the check valve or equivalent device be either (1) immediately adjacent to [coupled at the reservoir port] or (2) within the service reservoir?

Any alternative location allowed for the protective device gives rise to either a need for redefinition of the intent of S5.2.1.5 or an interpretation of the meaning of the one word "system" as used in this particular section, hence our request for clarification.

We have read several preamble statements in 121-related Dockets (73-13, Notice 1; 73-13, Notice 3; 74-10, Notice 1) which emphasize the Administration's opinion that axle-by-axle systems, i.e. separate systems for each axle of a truck, enhance the total vehicle reliability. Does this same safety philosophy apply to trailers and to the service reservoirs on trailer axles? To paraphrase this, we submit

Question #3: Is it intended for each axle subsystem to have separate service reservoir and check valve provisions?

If the answer to Question #3 is negative, there is a further possibility that a separate definition is required to differentiate between (1) protection for a single-axle trailer and (2) protection for tandem or multiple-axle trailers.

To extend this idea further for installations on multiple-axle trailers where one or more "liftable" axles are employed, an additional question arises.

Question #4: May a single check valve be utilized to protect the air reservoir or reservoirs required for the "liftable" axle system or subsystem?

We have asked for interpretations rather than for rule changes so that these matters will not give cause for delaying the effective date for FMVSS-121 Air Brake Systems.

John W. Kourik

Chief Engineer, Automotive Products

Interpretation ID: nht74-4.5