Interpretation ID: nht80-4.2
DATE: 09/25/80
FROM: AUTHOR UNAVAILABLE; F. Berndt; NHTSA
TO: R. H. Madison
TITLE: FMVSS INTERPRETATION
TEXT: This responds to your June 25, 1980, letter asking whether a proposed air brake system that you submitted would comply with the requirements of Standard No. 121, Air Brake Systems.
The National Highway Traffic Safety Administration (NHTSA) does not issue advance approval of systems constructed in compliance with safety standards. It is the responsibility of manufacturers to assure that their vehicles or equipment comply with the requirements of the safety standards. It is frequently impossible for the agency to tell from diagrams and descriptions of devices whether they will comply with the standards. Compliance is based upon testing and observation of the entire vehicle or piece of equipment as it is installed on the vehicle. Without the benefits of such tests, NHTSA cannot state whether your system would comply with Standard No. 121.
Our engineering staff has reviewed your letter and offers the following information. First, the standard contains no requirements for tractor protection valve control pressures. However, control pressures are usually set so that the trailer brakes apply before the tractor brakes.
Second, you asked whether it is appropriate to require the release of parking brakes by pushing in both the tractor protection control valve and the park valve. The standard states that the parking brake control shall control the parking brakes of the vehicle and any vehicle it is designed to tow. The standard is silent regarding the release of those brakes.
SINCERELY,
R. H. MADISON Engineering Consultant
June 25, 1980
National Highway Transportation Safety Administration
Gentlemen:
The purpose of this letter is to request an interpretation related to compliance with FMVSS 121.
A portion of one arrangement of air brake systems is illustrated on the attached sketch. The tractor protection control valve is designed to have several features.
1. With the system fully pressurized, pulling the knob out will exhaust the tractor supply line pressure and apply the trailer emergency brakes.
2. In case of trailer breakaway, the valve knob will automatically pop out, trap approximately system pressure in the tow vehicle, exhaust the tractor supply line, and apply the trailer emergency brakes.
3. If the total system pressure drops at a relatively slow rate (less than breakaway type leakage), the valve knob will automatically pop out and exhaust the tractor supply line at a system pressure not lower than 20 psi nor higher than 45 psi. The valve can be manually pushed back in but, if the system is 20 psi or lower, the valve will exhaust the tractor supply line pressure (non over-ride function).
4. When the tow vehicle is operated without a trailer, the knob is pulled out so that the tractor supply pressure is zero.
The tractor protection valve functions as follows:
1. It has a straight through passage so that the tractor supply line pressure and the trailer supply line pressure are equal.
2. The tractor service port pressure is blocked when the supply port pressure is between 20 and 45 psi.
Question One Is:
If the tractor protection valve were designed so it automatically blocked the tractor service port when the supply port was between 5 and 10 psi (rather than 20 to 45) would this, by itself, cause the system to be in non compliance?
One rationale for considering that this system would comply is as follows.
Function Present Proposed a) Tractor protection control 45 to 45 to valve automatically exhausts 20 psi 20 psi the supply pressure to the tractor protection valve. b) Tractor protection valve 45 to 10 to automatically blocks 20 psi 5 psi the trailer control port. c) Manual override of the 20 psi 20 psi tractor protection control valve is not possible below.
In both cases, when system pressure is reduced to 20 psi, the trailer emergency brakes are applied and the trailer control pressure port is blocked. The automatic operation of the tractor protection valve at 45 to 20 psi is redundant because the supply pressure to it goes to zero at not lower than 20 psi by the non override function built into the tractor protection control valve. The only difference in system performance would be the small increment of time required for the tractor protection control valve to deplete the supply pressure to the tractor protection valve from 20 psi to a range between 10 and 5 psi. System protection would still be provided by the tractor protection control valve at a supply line pressure of 20 psi or less.
Question Two Is: With the system described above (automatic operation at 10-5 psi), would a further modification, by itself, cause non compliance?
With this second proposal the park valve would apply the tractor and trailer parking brakes in the normal manner. Release would require that both the park valve and the tractor protection control valve be pushed in.
R. H. Madison
FIG. I -- TYPICAL CURRENT SYSTEM
FIG. II - TYPICAL SYSTEM WITH PROPOSED TRACTOR PROTECTION VALVE
FIG. III - PROPOSED SYSTEM WITH PROPOSED TRACTOR PROTECTION VALVE
(Graphics omitted)