s a f e t y   &  o p e r a t i o n s

Service Free?

Low and no maintenance brakes must not lead to lax inspection practices.

JIM WINSOR
EXECUTIVE EDITOR

      The venerable S-cam air brake is over 70 years old and still the primary foundation brake of choice. Of course, the original design has seen countless improvements since its beginning and today's S-cam brakes continue to be the lowest original cost and the least expensive to maintain.
      A major part of the maintenance savings these days is attributable to the extended lining life design with 20% thicker brake blocks, plus S-cam and rollers and return springs and bushings that are nearly service-free. Even automatic slack adjusters require only periodic inspection and lube to make sure they are working properly. ArvinMeritor offers LX and MX brake options that, on line haul, require no periodic lubrication for three years or 500,000 miles. The MX option doesn't even require a lining replacement in the period.
      As in this example, foundation brake and brake lining suppliers have developed products that offer extended life and little or no maintenance. These have been a bonanza in helping fleets further reduce brake costs. The addition of premium wheel-ends with sealed hubs and pre-set wheel bearing adjustment is making brakes, at least in line-haul trucking operations, virtually service-free.
      Even in the more severe operations such as dump, logging — and especially refuse pickup and transit — brake lining life is triple what it was a decade ago.
      Because of this, in many operations where trailers are frequently parked for long periods at shippers' docks or simply aren't needed because of reduced freight demand, brakes simply aren't getting the inspection attention they really should.
      Often years can pass without the need for brake drum removal — the only way to see the entire brake lining/shoes and cams and rollers. Even when dust shields aren't used, or have been removed, lining inspection from the inside shows only the lining edge, edge cracks and any oil and grease accumulation.
      This is great ... up to a point. And that point is brake performance and safety. Year in and year out, statistics gathered from roadside inspections show brakes are number one on the defects list. Brakes are also the primary reason for a vehicle being placed out of service.

A THOROUGH BRAKE JOB
      Out-of-adjustment brakes continue to be an industry-wide problem. Bendix brake engineers remind us that one key to top brake performance — getting maximum mechanical output from an S-cam brake — is proper adjustment. Slack adjuster stroke should be as short as possible without brake shoes dragging.
      Automatic slack adjusters (ASA), required by NHTSA on all new air-braked vehicles, are not automatically maintenance-free. Nor should drivers assume their brakes will always be in perfect adjustment because their truck, tractor or trailer have ASAs. This is why brake adjustment should be part of pre-trip inspections and why brake adjustment, including ASA operation, should be checked during vehicle preventive maintenance service.
      It is increasingly important that when vehicles require brake relining that fleets insist on a thorough, quality job, whether done in your own shop or by outside vendors. Throwing cheap replacement linings on old shoes should never be accepted. For both safety and long life, brake experts strongly urge "doing it right." The following recommendations were recently issued as "Technical Tips" from Haldex Brake Systems.
      For starters, when disassembling brakes, inspect every part carefully looking for unusual wear. Wear patterns tell a lot about each brake as well as the entire brake system. Before pulling wheels, check for signs of end-play. End-play may be an indication of seal problems or excessive wheel bearing or cup wear.
      After pulling wheels, inspect all shoes for even wear or unusual wear patterns and compare wear pattern from brake to brake. If there is an uneven pattern, it could be a signal of excessively worn, failed or misadjusted components that should be corrected before brake reassembly.
      Thoroughly examine removed linings, especially for signs of heat-checking. If appearing on some linings and not others, this is evidence that wheels are not braking evenly and that the heat-checked brakes are doing a greater share of the work. On the other hand, if there's heat-checking all around, it may be a signal the vehicle has been overloaded or that wrong linings have been used, or they've been subjected to driver abuse. Haldex recommends looking across the lining surface to see if there are uneven patterns or excessive grooves in the linings. If there are, better look further for bent spiders or bellmouthed drums.
      Never reuse brake shoes that had oil-soaked linings on them. When relined, Haldex engineers say grease or oil can migrate to the friction material causing it to glaze or not be in friction balance with the other wheels. To maintain brake (torque) balance, the experts say never to do a one-wheel brake job unless that lining is less than 10% worn. And if it is replaced, make sure it is the exact same lining from the same manufacturer.

BRAKE DRUM WEAR PATTERNS
      Brake drum wear patterns also tell a story. Look closely for uneven wear patterns, glazing or heat-checking. Heat checks, cracks and blue spots mean excessive heat. Haldex says that heat-check lines over one inch are abnormal. Never reuse a drum when wear is over .080 in., or if there are several heat checks aligned across the surface or if there is evidence of hard spots on the surface.
      An important part of every brake job should include inspecting brake S-cams for bushing wear. A brake with a worn S-cam or S-cam bushing reduces the torque input and stroke and increases the chances that a brake is not doing its share of the braking. Repair or replace according to manufacturers' recommendations.
      Next, thoroughly go over the slack adjuster to make sure it's working properly. At the same time, inspect all clevis pins and slack adjuster bushings for wear. Any wear over .030 in., replace both, says Haldex. Never mix ASAs and manual adjusters and never mix different brands of ASAs on the same axle. When brakes are completely disassembled, take a close look at brake spiders, checking especially for squareness in the anchor-pin hole area.

SELECTING REPLACEMENT LININGS
      In selecting replacement brake linings, it's important to buy linings and shoes which will give the same friction (torque) performance as the removed linings. Any change to a more aggressive or less aggressive friction rating will change torque balance of the entire brake system. Changes should only be done on the advice of a competent brake service engineer.
      The Technology and Maintenance Council (TMC) of the American Trucking Assns. has a Recommended Practice (RP 628A) which is an excellent guideline to follow in selecting replacement linings (see story page 52 for details).
      Buying relined shoes without knowing if or how the shoes were checked for proper dimensions should be avoided. Shoes should be coined and painted or coated with rust-resistant materials. Severe brake rust and lining cracking — called rustjacking — is a serious problem is many areas of the U.S. and Canada where powerful snow/ice removal chemicals are used on highways. These can attack and destroy brakes, the worst cases being locally bought replacement linings and shoes which are improperly treated (see accompanying story and photos page 50).

REASSEMBLING BRAKES
      The first step in putting brakes back together is replacing all shoe-attaching parts: anchor pin; pin bushing; cam bushing, roller and return springs. For a first-class job which should not require any brake disassembly until the next brake overhaul, most brake manufacturers recommend all these parts be replaced with new. It's also good insurance to thoroughly examine S-cams, bearings and cups and replace them if at all marginal. All parts to be reused must be cleaned with solvent, then rinsed and dried.
      Haldex engineers point out that it's important to lubricate anchor pins and the bushing areas of the camshaft. It's even more important not to lubricate or allow any lubricant to get on the face of the roller which is the contact point with the S-cam. Lube only the roller's bearing area that contacts the shoe web. If replacing brake drums, never mix lightweight and heavy cast drums together. Brake experts say this will lead to uneven brake shoe wear and may affect performance.
      When everything is back together, manually adjust each slack adjuster. Haldex recommends .015 in. drum clearance for manual adjusters; .018 to .025 in. clearance with ASAs unless the manufacturer states differently.
      Finally, inspect all brake hoses going to brake chambers and replace any with cracks. Also inspect all spring brakes for center seal leakage. If any brake chambers need replacing, never mix sizes or long stroke types with standard ones. Haldex urges whenever relining tractor brakes, also replace the air compressor governor with cut-in/out of 105/120 psi because many governors get out of adjustment over time. When relining trailers, it's always a good idea to replace both glad hand seals.
      Haldex also recommends a road test to break in new linings before returning the vehicle to service. Sequence to follow: 10 brake snubs from 40 to 20 mph; 10 stops from 20 mph using moderate brake pressure; two stops from 20 mph with full brake application. Then re-check wheel-ends for excessive play and re-check slacks and brake chambers for proper and equal stroke.

Sidebars
Beware of Rustjacking
Selecting Replacement Linings