Interpretation ID: aiam1540
Wagner Division
Wagner Electric Corporation
11444 Lackland Road
St. Louis
MO 63141;
Dear Mr. Kourik: This responds to your March 12, 1974, request for interpretation of th volume requirements for service brake chambers in S5.1.2.1 and S5.2.1.2 of Standard No. 121, *Air Brake Systems*:; >>>S5.1.2 Total service reservoir volume shall be at least eight time the combined volume of all service brake chambers at maximum travel of the pistons or diaphragms.<<<; You also requested the addition of language equating brake chambe volume with brake chamber displacement, based on nominal effective area and rated stroke. 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 '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.; Your request to add explanatory language to the standard of th measurement technique is denied as unnecessary in view of this interpretation.; Sincerely, Robert L. Carter, Associate Administrator, Motor Vehicl Programs;