Interpretation ID: nht79-3.32
DATE: 08/27/79
FROM: AUTHOR UNAVAILABLE; F. Berndt; NHTSA
TO: The Berg Manufacturing Company
TITLE: FMVSS INTERPRETATION
TEXT: This responds to your June 28, 1979, letter asking several questions about the compliance of your trailers with Standard No. 121, Air Brake Systems.
You first ask whether section S5.6.3 allows the use of service air to apply the parking brakes as long as a source of energy to apply the parking brakes is available at all times and is unaffected by any single failures in the service brake system. The answer to this question is yes. On August 9, 1979, the agency published in the Federal Register a notice amending section S5.6.3 of the standard to permit the type of parking brake system that you outlined in your letter.
In your second question, you ask whether your braking system complies with section S5.2.1.1. That section requires that a reservoir be provided that is capable of releasing the vehicle's parking brakes and that is unaffected by the loss of pressure in the service brake system. Your trailer has a tank and valve for each axle. You state that in the event of a service system failure including a failure of one of the tanks, that a reservoir is available such that when the supply line is pressurized to 55 psi, the vehicle's parking brakes will release. Assuming that your system operates as you have detailed in your letter, the agency concludes that it complies with S5.2.1.1 of the standard.
SINCERELY,
The Berg Manufacturing Company
June 28, 1979
Chief Counsel National Highway Traffic Safety Administration
Re: 49CFR 571.121
Dear Sir:
The purpose of this letter is to request an interpretation as to the compliance of an air brake system for trailers with FMVSS 121.
Figure 1 depicts a typical tandem axle trailer brake system. It has a tank and valve for each axle. In the event of a service system failure, including the failure of one of the air tanks, the parking brakes can be released by pressurizing the supply line to 55 psi. In operation, the system works as follows:
Charging the system. Tractor air pressure, from the supply (emergency) line enters the trailer valve. Below 55 psi, the supply line air goes directly to the emergency section of the spring brake chambers. Above 55 psi supply pressure, the air flows through the valve directly to the air tank.
Normal driving conditions. Air pressure in the trailer air tanks and valves maintains a normal pressure of 100 psi to 120 psi and is ready when it is necessary to apply the brakes of the trailer by the tractor brake valve through the control (service) line. The spring brakes are kept released, through a double check valve built into the valve, by either the tank pressure or the supply line pressure, whichever is greater.
Service braking. In normal brake applications, control air from the tractor actuates the relay piston in the trailer valve, which applies air pressure from the air tank to the service section of the spring brake chambers applying the brakes in a service mode.
Parking and/or emergency braking. When air presssure in the supply line drops below 45 psi, the emergency piston in the trailer valve closes the control line and applies pressure to the top of the relay piston which delivers tank air pressure to the service section of the spring brake chambers applying the brakes in an emergency mode. If the supply line pressure drops slowly, the emergency brakes apply slowly. If the pressure drops rapidly, the emergency brakes apply rapidly. It is possible to release the emergency brake application by re-establishing pressure in the supply line, which will exhaust the pressure applied to the brake chambers.
No system air pressure. With no air pressure in the system, the brake is applied by the spring in the spring brake chamber in a stable parking mode. No matter how the air pressure in the system decays, slowly or rapidly, the parking brake application never ceases as the spring force takes over as the air pressure decreases.
We have determined, by a careful review of the standard, that we clearly comply with the specific requirements of the standard except S5.6.3 and S5.2.1.1 with which we believe we comply with the intent. We have an interpretation from your office, dated March 14, 1977, that allows the use of service air to apply the parking brakes as long as a source of energy to apply the parking brakes is usable at all times and is unaffected by any single failure in the service brake system which clarifies that the system does comply with S5.6.3.
Careful reading of the various interpretations of which we are aware, leads us to believe that we do comply with S5.2.1.1. In the case of any single service system failure, including one of the air tanks, a protected reservoir is available and pressurization of the supply line to 55 psi will release the parking brakes.
We encourage any queries or comments and would be pleased to supply any further information you may require.
We look forward to receiving your reply to this letter.
Robert J. Crail Director of Engineering
cc: SERGIO CAMPANINI; DAVID SPILLER (Illegible Word) PRESSURE PROTECTED RELAY EMERGENCY VALVE.
FIGURE I
(Graphics omitted)