Equipment Type

Reliable, Low-Cost Braking Is a Matter of Discipline

Maintaining the foundation brake is largely a matter of avoiding shortcuts and taking the time to inspect wheel ends thoroughly

January 01, 2004

Brake maintenance means more than passing roadside DOT inspections. Peak braking performance is a requirement for safe, cost-effective operation. Maintaining peak performance requires that service technicians preserve brake balance, avoid shortcuts, and take the time to thoroughly inspect brakes.

The good news about brake balance is that most technicians intuitively do the things that matter most. They use the specified friction material on shoes. They don't mix and match components such as slack adjusters or air chambers. When replacing valves, they make sure the crack pressure of the new valve is correct. They check for consistency in adjustments. The good technicians observe these practices on each axle; the great ones observe them on every wheel end so that stopping power is evenly balanced at each wheel end.

Shortcuts can be tough to resist, though, especially in a busy shop. But shortcuts ultimately lead to more work. The worst shortcut is a one-wheel brake job. Servicing the brake at one wheel disrupts brake balance. It also ignores whatever underlying problem caused that single brake to wear prematurely. Few technicians will yield to the temptation to do a one-wheel brake job.

Re-use of wear parts is a more common problem. It takes only a few extra minutes to round up and install new anchor pins, rollers and bushings, and doing so ensures the new shoes will operate as they should. Springs should be replaced as well. Old, tired springs reduce brake performance. And because many automatic slack adjusters do their business as the shoes retract after a brake application, sluggish springs can confuse the slacks and result in incorrect adjustment.

Another shortcut is adjusting automatic slack adjusters (ASAs) manually. Slacks need to be adjusted when brakes are relined. In some cases, they may need manual adjustment after an extended downhill on a curving road. Under those conditions, the drums can overheat and expand. The ASAs compensate for the larger inside diameter of the drum, and the brakes will then be too tight when the drums cool and contract. Modern ASAs and good driving habits can eliminate this problem, however.

If the ASAs need adjustment other than at reline or after severe braking loads, the technician should take the time to diagnose the reason. The fault is often found in worn components, such as S-cam bushings.

Diagnostics are time consuming, but no more so than a roadside repair. Thorough diagnostics requires an understanding of the entire brake system and how all the parts interact. It also requires attention to the smallest details.

The first diagnostic is the one the DOT will perform if they pull you over: pushrod stroke. The amount of allowable stroke varies with air chamber volume, so make sure you know what size air chamber is on that wheel. Check the stroke at 90 to 100 psi of air pressure.

Before pulling the wheels, check each for endplay and oil-seal leakage. With the wheels off, closely inspect all the braking components.

Uneven wear on brake shoes may indicate failed or misadjusted components that will have to be repaired or replaced before reassembly. An uneven pattern of wear or excessive grooves may indicate the need to replace the drum. Uneven wear also results from a bent spider, a bell mouthed drum, or weak return springs. Check the wear difference between shoes on the front and rear axle. Unequal wear may indicate a timing imbalance or friction materials with different ratings.

Excessive heat checking of the friction material indicates unbalanced braking, use of the wrong lining, overloading or some combination of these. Never reuse grease- or oil-soaked shoes.

Check the shoe for rust jacking. This occurs when road chemicals get under the lining material, causing oxidation of the metal. Highway maintenance crews now fight ice by pre-treating roads in advance of storms. There's a lot of corrosive chemical on the roads in winter. Brake-shoe manufacturers fight this growing source of chemical contamination by designing shoes to resist rust jacking. If rust is a problem, check to see what solutions your vendors can offer as you shop for replacement shoes.

Check shoes carefully. Don't assume the shoe arc is correct; measure it. Make sure the slots and holes are not excessively worn.

Inspect the drum surface for even wear, glazing and heat checking. Heat checks, cracks and blue spots are indicators of excessive heat (hairline heat checks not over 1 inch are normal). Never reuse a drum if the wear is greater than .080 or if several heat checks are aligned across the braking surface. Never reuse a drum with hard spots, also called "martensite."

Check the S-cam for wear at the inner and outer bushing surfaces, S-cam head and spline areas. Wiggle the S-cam up and down and back and forth to check for wear.

During reassembly, make sure the S-cam rollers are properly lubricated. Do not get lubricant on the roller-to-cam contact, as this will wear a flat spot on the roller.

Brake inspections should be performed at least every 90 days—more often in severe duty cycles or demanding operating conditions.

Measure Elusive Wear
Measure Elusive Wear

 

Air S-cam brakes are remarkably reliable, considering the force they must withstand. Inspect them for wear—not only the obvious shoe and drum wear, but also hard-to-see wear in the slack adjuster, S-cam bushing, cam and roller—by measuring push-rod stroke. Learn how DOT safety inspectors measure pushrod stroke by going to this Eaton document online: www.roadranger.com/
csee/MungoBlobs/brsm0022-0800.pdf
.


Automatic doesn't mean maintenance free
"Automatic" doesn't mean "maintenance free"

Automatic slack adjusters (ASAs) should be greased with regular preventive maintenance on brakes. However, manual adjustment of the ASA should not be part of that PM. Except in unusual circumstances, the need for manual adjustment is a symptom of an underlying problem. Possibilities include excessive wear on other brake components or failure of the slack adjuster's clutch mechanism.


Reusing Parts Is False Economy
Reusing Parts Is False Economy

Tired springs and wear to the S-cam rollers, clevis pin, and bushings hamper brake performance and can lead to more frequent, more costly repairs in the future. If you're replacing the shoes, replace the hardware, too.


Inspect Shoes Carefully
Inspect Shoes Carefully

Inspect shoes for radial wear (around the circumference) and lateral wear (side to side), loose rivets or elongated holes, and rust jacking. Replacement shoes must have the correct friction material to preserve balance and braking performance. Properly burnishing new shoes is important; make sure drivers understand how. Don't assume relined shoes meet the spec; measure the arc and check the mounting points to confirm their suitability.


Analyze Heat Damage

 

 

Analyze Heat Damage

Analyze Heat Damage

Heat damage (discoloration, cracking) occurs when one brake is doing more than its share of the work. The imbalance may be because that brake is set too tight (insufficient clearance), or because the others are set too loose. Martensite (top) is a type of heat damage that forms during light braking applications. The eventual outcome is cracking.


Edge Markings Are Only a Guide
Edge Markings Are Only a Guide

A key consideration in maintaining brake balance is using the specified friction material on all wheels. In theory, edge markings tell you the coefficient of friction for the shoes. In practice, however, friction characteristics vary considerably. Shoes with the same rating from a single manufacturer can have measurable differences in performance. Even two blocks of friction material from the same shoe can have different characteristics.

It's hard to manufacture friction materials that have high homogeneity, that is, materials where the mix of resin and frictional fibers are consistent throughout. The best way to minimize variations is to:

  • Buy all shoes from just one manufacturer.
  • Buy all the shoes needed for one brake job at the same time from the same vendor to reduce the likelihood of inconsistencies between batches of friction materials.
  • Avoid one-wheel brake jobs, and consider relining all the brakes in an application—such as all four brake sets on a truck's tandems—at one time.

 


Acknowledgements
The following companies contributed information for this article: Arvin Meritor, Crewson Industries, Gunite, Haldex, and Meritor Wabco.

 

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