Interpretation ID: aiam1545
Heavy Vehicle Systems Group
Bendix Corporation
901 Cleveland Street
Elyria
OH 44035;
Dear Mr. Hilderbrandt: This responds to your May 10, 1974, request for interpetation (sic) o the volume requirements for service brake chambers in S5.2.1.2 and S5.1.2.1 of Standard No. 121, *Air Brake Systems:*; >>>S5.1.2.1 The combined volume of all service reservoirs and suppl 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 manufacture '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 wil 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 'pre-fill 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 wil measure the brake chamber volume with the push rod at maximum stroke.; With regard to air reservoir volumes, the NHTSA will determine th volume of reservoirs by actual measurement. As a practical matter, air reservoirs are simple structures whose volumes are relatively easy to measure.; Sincerely yours, Robert L. Carter, Associate Administrator, Moto Vehicle Programs;