Failure Analysis
Signs of Unusual Wear
(DISCLAIMER: Before beginning, please note that not all brake complaints and problems can be solved by only brake shoe inspection. Common problems also occur as a result of high temperature issues, such as air, mechanical or friction imbalances. Any imbalance in one or a combination of all three will appear as a friction material issue. The necessity for inspecting and replacing all worn parts has become more critical with the advent of automatic slack adjusters and ABS.)
When brake drums are removed, the brake lining should be inspected for signs of unusual or atypical wear. Common problems may arise as a result of ignoring hardware upkeep and s-cam bushings. These problems include but are not limited to: Bottom brake shoe worn more than the top shoe; Lead shoe blocks primary contact, with very little contact on the anchor block; Excessively noisy brakes.
Scenario #1
|
Problem
|
Noisy Brake |
|
Signs
|
Cam block, glazed Anchor block shows no contact Roller seat on lead shoe shows wear on high side |
|
Potential Issue
|
Standard lining may be placed in an extremely oversized drum |
Scenario #2
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Problem
|
High Temperatures |
|
Signs
|
Resin Bleaching Blistering Continual Cracking, rivet-to-rivet the full width of the shoe |
|
Potential Issue
|
Over-adjusted brake; Improper downhill braking procedure; Improper friction application |
Scenario #3
|
Problem
|
Bent spider; Bent S-Cam tube |
|
Signs
|
Tapered wear along the side of the lining |
|
Potential Issue
|
Poor Lining-to-Drum contact |
Scenario #4
|
Problem
|
Blistering and flaking of the brake lining |
|
Signs
|
Rust, obvious flaking of brake lining with 75% of the lining left |
|
Potential Issue
|
Improper friction application |
Scenario #5
|
Problem
|
Excessive center wear on brake lining |
|
Signs
|
|
|
Potential Issue
|
Friction torque level is insufficient for vehicle application |
Scenario #6
|
Problem
|
Lining separation from brake shoe table |
|
Signs
|
Cracking at rivet |
|
Potential Issue
|
Oversized drum; Previously lined on a stretch core |
Scenario #7
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Problem
|
“Craters” in brake lining |
|
Signs
|
Cracks or gauges missing in brake lining |
|
Potential Issue
|
Oil-soaked brake lining |
Scenario #8
|
Problem
|
Poor stopping power/ Noisy brake |
|
Signs
|
Grooved wear pattern |
|
Potential Issue
|
Brake shoes that are placed in a grooved drum may take longer to seat and make good contact, or else make no contact at all |
