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

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

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

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