Interpretation ID: aiam1601
Director of Engineering
Power Controls Division
Midland-Ross Corporation
490 So. Chestnut Street
Owosso
MI 48867;
Dear Mr. Denholm: This responds to Midland-Ross' February 8, 1974, petition for a amendment of S5.1.2.1 and S5.2.1.2 of Standard No. 121, *Air brake systems*, to establish separate air reservoir volume requirements for several brake chamber types generally available in the air brake component market.; The standard presently requires air reservoir volumes to be a multipl of the vehicle's brake chamber volumes. Midland-Ross also requested that S5.1.2.2 and S5.2.1.3 be amended to require that a reservoir withstand hydrostatic pressure five times greater than stated on its label without rupture, or permanent circumferential deformation exceeding one percent. The standard presently requires that an air reservoir withstand internal hydrostatic pressure of five times the vehicle compressor cutout pressure or 500 pounds, whichever is greater. The petition also requests modifications of the trailer test rig, which were made in a recent amendment of the standard (29 FR 17563, May 17, 1974).; You suggested that our requirement for air reservoir volume as multiple of brake chamber volume will encourage installation of smaller equipment and thereby create a safety problem. We cannot agree, in view of the standard's stopping distance requirements which in effect mandate the installation of high performance components. Indications to date are that manufacturers have in fact not reduced brake chamber volumes. A certain degree of chamber stroke standardization may occur, which the NHTSA views as favorable. For these reasons your request is denied.; With regard to the air reservoir pressure requirements of S5.1.2.2 an S5.2.1.3, you argued that a reservoir manufacturer is unable to establish the required strength of his product because he cannot control the compressor cutout pressure of the vehicle on which the reservoir is installed. It should be understood that the standard is not an equipment standard with which Midland-Ross must comply, but a vehicle standard with which the vehicle manufacturer must comply. We have determined that the reservoir should be designed to manage the pressures to which it might be exposed on the vehicle on which it is installed. The vehicle manufacturer is able to establish a compressor cutout pressure (on powered vehicles) and, based on that value, order the appropriate reservoir to meet the requirement. It is evident that commercial considerations will standardize compressor cutout pressures on a reasonable range of available reservoir strengths. Midland-Ross as a manufacturer of reservoirs is free to establish a range of reservoir strengths, and label the reservoirs as described in your petition. For the reasons cited, however, your petition to mandate this is denied.; We agree the requirement that a reservoir 'withstand' a certai pressure can be further specified, and we are considering a proposal to do this in the future. At this time the NHTSA has adopted the SAE Standard No. J10a, which specifies that there be no rupture or permanent circumferential deformation exceeding one percent.; Sincerely, James B. Gregory, Administrator