Interpretation ID: nht74-1.50
DATE: 07/11/74
FROM: AUTHOR UNAVAILABLE; Armstrong for R. L. Carter; NHTSA
TO: Bendix Corporation
TITLE: FMVSS INTERPRETATION
TEXT: This responds to your May 10, 1974, request for interpetation of the volume requirements for service brake chambers in S5.2.1.2 and S5.1.2.1 of Standard No. 121, Air Brake System:
S5.1.2.1 The combined volume of all service reservoirs and supply reservoirs shall be at least twelve times the combined volume of all service brake chambers at maximum travel of the pistons or diaphragms.
You also requested that reservoir volume be based on manufacturer "rated volume" based on the designed volume of the reservoir.
In testing for compliance with S5.1.2.1 and S5.2.1.2, the NHTSA will accept a manufacturer's published "rated volume" of the brake chamber with the piston or diaphragm at maximum travel. This means that the manufacturer may specify the full stroke of the piston or diaphragm and compute the "rated volume" based on the designed volume of the chamber and the full stroke he has established. This volume may be somewhat larger than "nominal brake chamber displacement" which does not necessarily account for the void ahead of the relaxed diaphragm or piston, the so-called "prefill volume." This volume must be included because it must be pressurized along with the displaced volume.
In the absence of manufacturer's published ratings, the NHTSA will measure the brake chamber volume with the push rod at maximum stroke.
With regard to air reservoir volumes, the NHTSA will determine the volume of reservoirs by actual measurement. As a practical matter air reservoirs are simple structures whose volumes are relatively easy to measure.
Sincerely yours,
ATTACH.
Bendix
Heavy Vehicle Systems Group
Chief Counsel -- National Highway Traffic Safety Administration
May 10, 1974
Subject: Title 49 CFR, Part 571, Motor Vehicle Safety Standard No. 121, Air Brake Systems
Dear Sir:
Concern has arisen in the motor vehicle industry relative to the matter of brake chamber and reservoir volumes as set forth in Section 5.1.2.1, of the subject standard which provides:
"The combined volume of all service reservoirs and supply reservoirs shall be at least twelve times the combined volume of all service brake chambers at maximum travel of the pistons or diaphragms."
The question posed is: How will the sizing be verified for compliance purposes? Hence, an interpretation of Section 5.1.2.1 is requested.
Historically, the Industry has used the "nominal" or rated full stroke of brake actuators, in determining available stroke. In practice, it was understood that the nominal or rated full stroke is all that can be counted on to provide braking. (The recommended readjustment stroke followed by the Industry has been from 3/8 to 3/4 inch less than the "rated full stroke" depending upon actuator size.) Further, SAE recommended Practice J813, "Automotive Air Brake Reservoir Volume" does not go into any detail relating to strokes, tolerances, etc. of the various components involved in the actuation of a foundation brake.
The design of a braking system requires that a component, such as an actuator, which is called upon to give a specified stroke must have its own dimensions and tolerances so designated that its "minimum" travel provisions are those required for the system. To use the actuator's maximum stroke, taking into account tolerance stack up, as the basis for sizing a reservoir, appears to be unreasonable and would not increase the safety of the vehicle. (The maximum possible stroke is, of course, not available in every actuator and thus cannot be counted on for braking.)
It is therefore recommended that NHTSA give favorable consideration to an interpretation of Section 5.1.2.1 to provide that the brake chamber volume be specified at the advertised nominal or rated full stroke of the brake chamber. This volume can be determined by a manufacturer and precludes the necessity of adding additional reservoir volume to compensate for theoretical tolerances that in all likelihood will never become cumulative in the system.
An analogous situation exists with reservoirs. Because of the profusion of sizes and special configurations (different sizes and quantities of ferrules, and a variety of lengths and diameters) the minimum volume determined by physically filling several reservoirs of each configuration to measure the exact volume does not appear to add any reasonable degree of accuracy or safety.
Therefore, it is also recommended that the reservoir volumes be certified based on a "rated volume" produced by calculation using the actual nominal Engineering drawing dimensions. Ferrules would be ignored in the calculations as being insignificant and at any rate generally a "plus" for volume.
We solicit your comments on these matters and would be happy to discuss our recommendations with you if you feel that additional information is necessary.
Very truly yours,
R. W. Hildebrandt -- Group Director of Engineering