Maintaining brake maintenance

“Fleet managers need to calculate the amount of wear they’re experiencing versus the number of miles driven,” said Joe Kay, Meritor’s director of engineering and brakes for North America. “Once that’s determined, they can predict when brake lining will reach the worn-out condition. This evaluation is necessary for friction materials as well as the drum and rotors.”

Regardless of the drum or ADB spec, brake maintenance is of the utmost importance to ensure that your trucks are operating safely and that you’re pulling the most efficiency out of your brakes.

“The brake lining thickness must be checked at regular intervals, in relation to vehicle use, during maintenance intervals, as well as in the context of applicable local laws and regulations,” said Jon Morrison, WABCO’s president for the Americas. “Burned, glazed or oil contaminated brake linings must be replaced immediately. Always replace all brake linings by axle, using a new retaining system for brake linings and pressure plates. To avoid damaging the brake disc, replace the brake linings no later than at the point when they reach the wear limit at their weakest spot. The residual lining thickness must not be allowed to become less than 2 mm above the backing.”

A comprehensive preventative maintenance program includes two critical components—maintaining good records and understanding your operating conditions.

“Dynamics like average length of haul, application, terrain, driver patterns and tractor-trailer equipment combinations are important factors to consider when determining intervals at which brake linings should be replaced,” Hendrickson’s Wittlinger explained.

“S-Cam drum brake friction material is typically very easy to inspect for lining thickness while the wheel is still on the vehicle,” Stemco’s Reis explained. “Friction material manufacturers incorporate lining wear-indicating features, which make a visual inspection rather straightforward. In some cases, dust shields may need to be removed to make an inspection. ADB friction material can be inspected by looking through the caliper-loading opening, but some applications may require removal of the wheel for an accurate inspection.”

Kevin Pfost, coordinator of technical services for Bendix Spicer Foundation Brake, recommended that fleets should:

• Measure at the thinnest part of the friction. If the lining is close to its minimum thickness of ¼ in. or less, it should be replaced;
• Inspect for cracking in the friction block material. Examples would be:
• Any radial cracking on the edge of the friction block;
• Cracks from the top of the friction running down to the shoe table; and
• Evidence of rust jacking; or
• Consult TMC RP-627A (Brake Lining Structural Defects).

Additionally, Haldex’s Petresh recommended checking for the following: Differential wear between two shoes, both top to bottom and side to side, examining tapered conditions across the shoe and whether one end is wearing more than the other; glazing issues; contact pattern between lining and drum; lining cracking (which is watched very closely by state agencies); and oil seal leaks.

Don’t cut the slack

According to Commercial Vehicle Safety Alliance (CVSA), out-of-adjustment brakes and brake system violations combine to represent half of all out-of-service violations issued for commercial vehicles on the road. To measure the power stroke of the brake chamber, Accuride recommended the following:

• Be sure to block the vehicles wheels.
• Make sure air pressure is in the normal operating range.
• Release the spring brakes.
• Choose one of the following methods for measurement:
  • Method 1: Mark the pushrod either at the brake chamber or at a chosen fixed reference point. Be sure that marks are narrow and precise.
  • Method 2: Measure the released position of the pushrod from the mounting side of the brake chamber out to the center of the large ½-in. clevis pin.
• Raise or lower the air pressure by running the engine or pumping the brake pedal until both the primary and the secondary pressure gauges read 90-100 PSI.
• Shut the engine off.
• Apply the brakes at the brake pedal and hold using an assistant.
• Determine the applied pushrod stroke. Use the method that corresponds with the choice from above.
  • Method 1: Measure the distance from the mark on the pushrod to the brake chamber or the fixed reference point.
  • Method 2: Measure the applied position of the pushrod from the mounting side of the brake chamber out to the center of the large ½-in. clevis pin.
• The effective (power) stroke is the difference when subtracting your first measurement from the second measurement.
• When the applied effective (power) stroke is longer than the adjustment limit, the brake is out of adjustment.

The brake adjustment limit charts are available in Automatic Slack Adjuster manufacturer’s service manuals, TMC’s RP609 and CVSA’s Out of Service Handbook.