Selecting Replacement Linings
Whenever possible, try to buy replacement brake linings with the same friction rating as the ones taken off. Changing friction levels without guidance from a brake engineer is risky because it can throw off brake balance, the amount of "work" done by each brake.
To make replacement lining selection easier and to help fleets select quality products which will give good braking performance, the Technology and Maintenance Council (TMC) of the American Trucking Assns. developed Recommended Practice 628A as a guide for selecting linings by their torque ratings.
Manufacturers of new trucks, tractors and trailers must certify their brake systems (which obviously include linings) comply with FMVSS 121, better known as the air brake standard requiring antilock brakes. Linings must pass rigorous dynamometer tests spelled out in FMVSS 121 in order to be sold on new vehicles. Believe it or not, there is no comparable test requirement for replacement brake linings. As you might assume, there are some replacement linings which are equal or better than those put on new vehicles. There are some that are sub par; and there are still others that are marginal at best.
The challenge for fleet maintenance managers is to buy replacement linings that will perform as closely as possible to the ones being replaced. This includes friction ratings, meaning the amount of brake torque generated at a given amount of applied air pressure. ATA petitioned NHTSA over 25 years ago for a rule requiring aftermarket linings meet the same test requirements as new ones. Nothing so far. This is why TMC developed its RP for 16<+>1/<->2 x 7-in. S-cam air brake replacement linings. It's strictly voluntary and suppliers can list their linings by axle gross weight ratings, brake chamber size and brake torque values at 40-psi pressure and certify they meet the FMVSS 121 requirement.
The list shows lining families for axles in the 17,000-20,000-lb. GAWR range and others in the 22,000-23,000-lb GAWR range. These cover the most popular drive and trailer axles in single-axle ratings and as used in tandems from 34,000 to 46,000 lb. In RP628A, linings are listed by brand name and lining name or number in ascending torque values. The higher the torque value, the more aggressive the lining.
Lining ratings are submitted to the Performance Review Institute which is administered through SAE. Its Brake Lining Performance Review Committee compiles the aftermarket lining list that the manufacturers state meet the FMVSS 121 lining requirements.
This list is updated monthly. Every year, linings are bought at random on the open market and submitted to the Review Committee which then has them tested on independent brake dynomometers to the FMVSS 121 standards. Results are compared with the data submitted by the manufacturers. If the data doesn't match, lining manufacturers are asked to re-do their tests. If the data still doesn't match, linings are dropped from the list. This has happened on several occasions. Linings must be requalified every five years or they are dropped from the list.
If you want the current Performance Review Institute lining list, download it from www.pri.sae.org/NADCAP/brakerep.pdf. This is the list that meets the criteria covered in TMC's RP628A. If you want the entire RP, contact TMC at: phone (703) 838-1763; fax (703) 684-4328; mail to: TMC, 2200 Mill Rd., Alexandria, VA 22314-5388. E-mail at: tmc@trucking.org. There is a charge for the RP.
Starting point should be to identify, if possible, what linings came on your truck, tractor or trailer by brand and part number. Using the vehicle VIN, dealers can electronically access the vehicle specs to get that information which will include the lining friction data. Goal should be to buy replacement linings as close as possible to that friction level.
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