Interpretation ID: nht79-2.27
DATE: 10/30/79
FROM: AUTHOR UNAVAILABLE; F. Berndt; NHTSA
TO: Bendix Corporation
TITLE: FMVSS INTERPRETATION
TEXT:
October 30, 1979 NOA-30
Mr. R. W. Hildebrandt Group Director-Engineering Bendix Corporation 901 Cleveland Street Elyria, Ohio 44035
Dear Mr. Hildebrandt:
This responds to your September 20, 1979, letter asking the National Highway Traffic Safety Administration (NHTSA) to give you written confirmation that your tandem axle trailer brake system complies with Standard No. 121, Air Brake Systems.
The NHTSA does not provide advance determinations of compliance with the agency's safety standards. It is the manufacturer's responsibility to test for and certify the compliance of its vehicles or equipment. The agency cannot always tell by diagrams and word descriptions whether a system will or will not comply with applicable safety standards. Compliance of a braking system, such as yours, can only be determined when tested on a completed vehicle.
Our technical staff has reviewed the diagrams and the letter that you submitted. In their opinion, your system appears to comply with the requirements of the standard. However, this is merely an opinion based upon your submission and does not bind the agency in any way should your device fail any compliance tests conducted by the agency.
Sincerely,
Frank Berndt Chief Counsel
Office of Chief Counsel National Highway Traffic Safety Adm. 400 Seventh Street, S.W.
Washington, D.C. 20590
Sept. 20, 1979
Subject: Request for Confirmation - FMVSS 121
Gentlemen:
A customer of The Bendix Corporation, Heavy Vehicle Systems Group (Bendix), has requested that Bendix obtain written confirmation from the National Highway Traffic Safety Administration (NHTSA) that the two reservoir tandem axle trailer brake system (Bendix System) shown and described herein, meets the requirements of Section S5.2.1.1 and S5.2.1.2 of Federal Motor Vehicle Safety Standard 121 - Air Brake Systems. Previous Bendix studies concluded that based on the interpretation given by the NHTSA letter of January 22, 1976, to Wagner Electric Corporation (copy attached) the Bendix System is in compliance with these reservoir requirements.
The design objectives for the Bendix System were safety, performance, reliability, simplicity, and economy of cost and space. Two years of production installations of this Bendix System have proven that the design objectives have been met.
The Bendix System (Figure 1), utilizes two service reservoirs for normal service braking and reserves sufficient air pressure in one or the other to provide the required spring brake release in the event of a failure to a reservoir.
The major component of the Bendix System is the SR-4 Spring Brake Control Valve (Figure 2) whose operational functions are as described herein.
SYSTEM CHARGING
Trailer supply line air pressure enters the SR-4 valve at the trailer supply port, actuates the control piston, opens the spring brake inlet/exhaust valve and flows into the cavity under the pressure protection piston. When the air builds to a pressure of aproximately 55 psi, the pressure protection inlet valve opens and the air pressure flows past Check Valve "A" and into the rear service reservoir and PR-3 Pressure Protection Valve and then past Check Valve "B" and through the open spring brake inlet/exhaust valve and into the spring brake units. Air pressure is prevented from passing into the front service reservoir by Check Valve "C". When the pressure in the PR-3 valve reaches approximately 60 si the PR-3 inlet valve opens and the air pressure flows past the PR-3 check valve and into the front service reservoir. Both reservoirs and all the spring brake units can now be charged to full system pressure and parking brakes will be released.
NORMAL OPERATION
Service Brakes
Trailer service brakes are controlled by application and release of pressure in the trailer service line connected to the control port of the relay valve. Air pressure consumed from the front reservoir is replenished from the rear reservoir and the trailer supply line via the open PR-3 Pressure Protection Valve. The combined volume of front and rear reservoirs is at least 8 times the volume of all the service chambers at maximum travel.
Since the rear reservoir is in communication with the front reservoir via the open PR-3 Pressure Protection Valve during normal operation (system supply pressure in excess of 60 psi) that total service reservoir volume of the system is in compliance with the requirements of S5.2.1.2.
Parking Brakes
Trailer parking brakes are controlled by application and release of air pressure in the trailer supply line. Loss of supply line pressure due to trailer breakaway, leakage or operation of tractor valving causes the pressure protection valve to close and deactuates the control piston, which closes the spring brake inlet/exhaust valve and vents the air pressure in the spring brake units via the SR-4 exhaust port; thereby causing application of all trailer parking brakes. Reapplication of air pressure in the tractor supply line reactuates the control piston, opens the spring brake inlet/exhaust valve and pressurizes the spring brake units with the air pressure contained in both reservoirs; thereby releasing all trailer parking brakes.
SERVICE FAILURES
Failure of Rear Reservoir System
With a failure in the rear reservoir full air pressure is retained in front reservoir due to the check valve in the PR-3 Pressure Protection Valve and Check Valve "B". Pressure in trailer supply line is maintained at approximately 55 psi due to closing of the pressure protection inlet valve. The reduced trailer supply line pressure actuates the low pressure warning on the tractor alerting the driver to a system failure. The trailer parking brakes do not automatically apply because the trailer supply pressure keeps the control piston actuated and front reservoir pressure is maintained in the spring brake units. In this failure mode, trailer braking is provided by service brake applications on both axles using the air pressure retained in the front reservoir, or by a manual parking brake application on both axles, releaseable by the air pressure retained in the front reservoir.
Failure of Front Reservoir System
With a failure in the front reservoir, approximately 55 psi air pressure is retained in rear reservoir due to closing of the PR-3 Pressure Protection Valve and Check Valve 'C'. Pressure in trailer supply line is maintained at approximately 55 psi due to closing of the pressure protection inlet valve. The reduced trailer supply line pressure actuates the low pressure warning on the tractor alerting the driver of a system failure. The trailer parking brakes do not automatically apply because the trailer supply pressure keeps the control piston actuated and full pressure is maintained in the spring brake units. In this failure mode, trailer braking is provided by a manual parking brake application on both axles, releaseable by the air pressure retained in the rear reservoir. Air pressure in rear reservoir is rechargeable to approximately 60 psi from the trailer supply line pressure.
With respect to the foregoing service failures in the front and rear reservoir systems, the failure modes depict system functions after the reservoirs have been pressurized to 90 psi air pressure and the respective reservoir pressure has been vented to atmosphere to simulate an extreme failure in a service brake system.
In summary, the Bendix Two Reservoir Tandem Axle Trailer Brake System, with its four wheel non-automatic "back-up" braking on tandem axle trailers under conditions of a trailer service system failure, complies with the noted reservoir requirements of FMVSS 121 and provides the safety, performance and economy of cost and space objectives necessary for acceptance by the industry.
Bendix hereby respectfully requests from the NHTSA written confirmation similar to that issued to the Wagner Electric Corporation that the Bendix System complies with the requirements of Sections S5.2.1.1 and S5.2.1.2 of FMVSS 121. We would be pleased to discuss this system in more detail at your convenience.
Very truly yours,
R. W. Hildebrandt Group Director-Engineering
RWH:ep
Attachments