Replacement Brake Linings
Choose them with care, or risk poor brake balance.
JIM WINSOR
EXECUTIVE EDITOR
Make sure you buy replacement brake linings with the same friction ratings as the old linings or you may end up with poor brake balance. Use TMC's Recommended Practice 628A as a guide when selecting linings.
Manufacturers of new trucks, tractors and trailers must certify their brake systems (which obviously includes linings) comply with Federal Motor Vehicle Standard (FMVSS) 121, better known as the air brake standard requiring antilock brakes.Vehicle engineers select brake components and linings that will give a combination of good performance, fade resistance and good life. The linings must pass a rigorous brake dynamometer test spelled out in FMVSS 121 in order to be sold on new vehicles.
Believe it or not, there is no comparable requirement for replacement brake linings. There are some that are equal to what the manufacturer put on the vehicle. There are some that are sub par; and there are still others which, frankly, are marginal at best. The challenge for fleet maintenance personnel is to buy linings which perform as closely as possible to the linings being replaced. This includes friction ratings and the aggressiveness of the lining — meaning the amount of brake torque generated at a given amount of air pressure applied.
One of the many chances for error can happen any time a mechanic or parts chaser goes to a parts house with an old set of brake shoes and tells the counterman: "Give me a cheap set of linings like these." There's no telling what he'll be buying or whether the linings will have different brake torque characteristics. It can significantly throw off the brake balance — especially between tractor and trailer — or affect stopping distances if the replacement lining has lower torque values.
For this reason the American Trucking Assns. (ATA) petitioned NHTSA over 25 years ago for a rule requiring aftermarket linings to be tested and certified to the FMVSS 121 standard for new brakes. So far a NHTSA rule has not been forthcoming. That's why 10 or so years ago the Technology and Maintenance Council (TMC) of ATA developed a Recommended Practice (RP) for 16 1/2 x 7-in. S-cam air brake replacement linings. It's strictly voluntary and lining manufacturers may list their linings by gross axle weight ratings, brake chamber size (type 24 or 30) and brake torque values at 40-psi pressure.
There are lining families for axles in the 17-20,000 lb GAWR range and most recently, in the 22-23,000 lb GAWR range. These cover the most popular drive and trailer axles in single-axle ratings and as used in tandems from 34,000 lb to 46,000 lb. In RP628A, linings are listed by brand name and lining name or number in ascending torque ratings from lowest to highest.The higher the torque value, the more aggressive the brake lining.
Lining ratings are submitted to the Performance Review Institute which is administered through the SAE. Manufacturers' lining submitted for listing in RP628A are selected at random for FMVSS 121 dynamometer testing. If lining ratings show up differently than as submitted, the Brake Lining Review Committee can reject the lining and/or require the lining supplier to run new dyno tests, TMC helps to fund the PRI spot testing to keep lining manufacturers "honest."
Every fall, the Performance Review Institute releases its updated list of lining ratings. Linings on the list must be requalified every five years or they're dropped. The newest list was just recently released by TMC and is published in its membership newsletter TMC Fleet Advisor Vol. 16, No.10. You can contact TMC headquarters at 2200 Mill Rd., Alexandria, VA 22314 for a copy or email tmc@trucking.org. Phone is: (703) 838-1763.
Or if you prefer, I'll send you a copy of the new list. You can reach me by email at jwinsor@truckinginfo.com. Mail me at Heavy Duty Trucking, 175 Strafford Ave., Suite # 1, Wayne, PA 19087 or fax me at (610) 687-7860. The list is a page long. I'll be glad to send you a copy.
