Interpretation ID: nht76-3.44

DATE: 01/22/76

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

TO: Wagner Electric Corporation

TITLE: FMVSS INTERPRETATION

TEXT: This responds to Wagner Electric Corporation's October 21, 1975, question whether a trailer would satisfy the requirements of S5.2.1.1 of Standard No. 121, Air Brake Systems, to provide a reservoir "that is unaffected by a loss of air pressure in the service brake system," if the reservoir provided is either of two service brake system reservoirs on the vehicle, equipped with a pressure protection valve directly adjacent to each reservoir. The drawings enclosed in your letter indicate that the "protected tank" that is normally provided, separate from the service brake system, would be eliminated and either of the service brake system reservoirs would be used to satisfy S5.2.1.1 in the event of a parking brake application.

Your interpretation of S5.2.1.1 is correct. That section calls for a reservoir of air as an energy source that is used to release the vehicle's parking brakes after an automatic or manual application. In requiring that this reservoir be "unaffected by a loss of air pressure in the service brake system," the NHTSA means that a single failure of the service brake system would not result in loss of this air supply. With the pressure protection valves located as described in your enclosures, it appears that the system would comply with Section S5.2.1.1.

This "single failure" requirement must be distinguished from the requirement of S5.6.3 that the energy source for application of the parking brake be "not affected by loss of air pressure or brake fluid pressure in the service brake system." The NHTSA has interpreted this requirement to require an uninterrupted energy source despite loss of all air pressure from the service brake system. We recognize that the language of the two passages is substantially identical, and should be changed for clarity.

In a recent proposal to revise the parking brake requirements of the standard (40 FR 56920, December 5, 1975), the NHTSA inadvertently failed to make this distinction clear in its newly-proposed definition of "parking brake system" and intends to publish a correction of the proposal in the near future.

Yours truly,

ATTACH.

WAGNER ELECTRIC CORPORATION

October 21, 1975

Chief Counsel -- National Highway Traffic Safety Administration

Re: 49 CFR 571.121

Dear Sir: The early or first generation systems for FMVSS 121 complying trailers have completed almost a year of evaluation. Serious consideration for more economical systems has naturally evolved in this period of time.

In comparing tractor air brake systems and trailer air brake systems it is apparent that a good level of safety is provided on two axle tractors which employ only service reservoirs, i.e., do not have an isolated reservoir for the parking brake system. This introduced the probability that a (Illegible Word) axle trailer could benefit from a similar system schematic.

Our review of FMVSS 121 indicates that the equipment requirements are minimal - in keeping with the NHTSA policy of issuing performance oriented requirements. The system to be discussed later in this document meets all of the equipment requirements and will satisfy the related performance criteria.

In addition, all of the benefits for commercial or non-regulated necessities are maintained.

For introductory purpose we have reproduced the section from FMVSS 121 that deals with trailer equipment requirements:

S5.2.1.1 A reservoir shall be provided that is unaffected by a loss of air pressure in the service brake system.

On single axle trailers utilizing one service reservoir, a separate reservoir or protected reservoir compartment for parking brake control is clearly required to meet S5.2.1.1.

Tandem axle trailers afford other opportunities for system considerations. The system shown on Figure 1 introduces a new set of operating parameters which will in fact eliminate the expense of adding a third (parking brake control) reservoir on tandem axle trailers. We interpret S5.2.1.1 to mean that a single loss of air pressure in the service brake system following pressurization of the reservoir system to 90 psi shall not prevent a single release of the parking brakes.

The system shown on Figure 1 provides a separate reservoir and anti-lock system for the service brakes on each axle. Failure of system A will not reduce the pressure in service reservoir B or vise versa. The presence of one service reservoir pressurized to 90 psi will permit a full release of the parking brakes when the supply line is pressurized to the maximum 45 to 48 psi. The supply line pressure will be limited to this value by the pressure protection valve [which has an integral check valve] in the supply line immediately adjacent to the failed service reservoir. The four parking chambers will then receive a pressure of approximately 66 psi as delivered by the amplifying relay valve which receives air from the intact service reservoir.

This evaluation is based on the venting of one trailer service reservoir to atmosphere after the system is charged to 90 psi. We consider this to be the most extreme service brake system failure which can occur on the trailer. Note that a pressure loss in both service reservoirs would require two service brake system failures. Functional requirements following multiple service system failures is not required in any other section of FMVSS 121 and is not required in S5.2.1.1. We, therefore, conclude that the system shown on Figure 1 meets the intended requirements of S5.2.1.1.

The proposed system provides an opportunity to reduce the cost of the brake system required on tandem axle trailers. A comparison of the proposed system, which is Figure 1, with the existing system as shown on Figure 2 will show that the modification entails the addition of two check valves, one tee and three short lines to the amplifying relay valve and permits the elimination of the large parking brake reservoir, reservoir drain cock, supply line to the reservoir, and the cost of installing the reservoir on the vehicle. All of the parts shown on the schematics are currently being supplied for production FMVSS 121 trailer systems. The cost reduction can not be accurately defined by Wagner, but we estimate that the saving could range from $ 20.00 to $ 30.00 per vehicle depending on variable labor costs, reservoir cost, and vehicle configuration.

We encourage any NHTSA comments or questions regarding the system shown on Figure 1 and the requirements of S5.2.1.1 of FMVSS 121. It has been customary for requests for interpretation to the Chief Counsel's office which seek system or product "endorsement" to be answered by stating that the requesting party should be able to make that determination themselves.

In the instant case, we have made the determination that the dual service - two reservoir brake system for tandem axle trailers (Figure 1) is functionally acceptable and in compliance, and are therefore reporting it as such.

The dual service - two reservoir brake system for tandem axle trailers will be entering production in the immediate future. We would appreciate an acknowledgement of receipt of this letter and are anxious to answer any questions or supply additional information.

If you have any concern for the performance of the dual service - two reservoir brake system for tandem axle trailers or its compliance with respect to FMVSS 121 an early response from you will avoid the possibility of economic loss to the depressed trailer industry if, for some reason, they would have to reconvert this more economical system to earlier configurations.

Very truly yours,

John W. Kourik -- Chief Engineer, Automotive Products

Attachment

(Graphics omitted)

FIGURE 1

DUAL SERVICE - TWO RESERVOIR BRAKE SYSTEM FOR TANDEM AXLE TRAILER

(Graphics omitted)

FIGURE 1

DUAL SERVICE - TWO RESERVOIR BRAKE SYSTEM FOR TANDEM AXLE TRAILER

(Graphics omitted) FIGURE 2

DUAL SERVICE - THREE RESERVOIR BRAKE SYSTEM FOR TANDEM AXLE TRAILER