Chapter 99 - Drum Brakes Flashcards
The loss of stopping power that occurs when excessive heat reduces the friction between the brake shoe/pad and friction surface
Brake Fade
Occurs when the brake drum gets so hot it expands away from the brake linings. Causes brake pedal to drop toward floor as additional fluid gets into hydraulic system
Mechanical Fade
Occurs when friction coefficient of brake lining material drops because of intense heat making it “slippery”. Pedal becomes hard and there is a noticeable loss of braking power
Lining Fade
Occurs under extended hard braking from high speeds where a thin layer of hot gases/dust particles build up between shoe linings and drum, acting as a lubricant. Pedal becomes hard and increased force is needed to apply appropriate stopping power
Gas Fade
Occurs when moisture is trapped between the shoes and drum, acting as a lubricant. Lowers brake efficiency until friction creates enough heat to evaporate the water
Water Fade
Occurs when friction assemblies of opposite sides of vehicle have different amounts of stopping power, caused by component failures, fade, mis-adjustment, etc.
Brake Pull
Foundation of brake drum that mounts to suspension or axle housing
Backing Plate
Stamped on the backing plate to align brake shoes to the center of friction surface inside brake drum
Shoe Support Pads or Shoe Contact Areas
The water barrier created when the outer lip of backing plate sits against a machined groove in the open edge of the brake drum
Labyrinth Seal
Prevents brake shoes from rotating with the drum when brakes are applied
Shoe Anchors
Curved metal piece on the outer portion of brake shoe that supports the friction material
Lining Table, Shoe Rim, or Platform
Small V- or U-shaped notches on ends of lining table that rest against support pads
Nibs
Metal piece of brake shoe welded under the lining table that actually actuates the shoe when force is applied
Shoe Web
Why does secondary/trailing brake shoe have more lining in a servo type brake drum?
Servo action causes the primary/leading shoe to hit the drum and then push the secondary shoe into the drum, since the shoe anchor is mounted on top, causing the trailing shoe to do most of the braking
Name the two ways brake shoe lining is assembled
Riveting
Bonding
Lining assembly that is reliable and operates more quietly, but can create stress cracks, has a reduced service life, and rivets can possibly damage drums if allowed to run thin
Riveted Linings
Lining assembly that uses high-temperature adhesive to glue unto the shoe. Can wear closer to the platform, allowing longer service life and have less cracking issues. However, adhesive can fail to excessive heat and more prone to making noise since there is no absorption flex
Bonded Linings
Retracts the shoes to unapplied position when pedal is released
Brake Shoe Return Springs
Keeps shoes securely against support pad on the backing plate when brake is unapplied, preventing noise vibration and wear.
Brake Shoe Hold Downs
Brake function where each brake shoe is applied individually, and the action of one shoe has no effect on the other
Non-servo Brake
Explain self-energizing brake action
When leading shoe presses against drum at the top, and attempts to rotate with the drum. Since it is anchored and can’t rotate, the drum pulls against it “energizing” the braking force
The most common drum brake design, where one action of one shoe plays upon the other. This makes it more powerful and equally able to hold both directions, though can lead to faster fade and is more susceptible to brake pull
Dual-servo Brake
Found in dual-servo brakes, that connects the two shoes from the bottom
Adjusting Link or Starwheel Adjuster
Types of automatic brake adjusters
Cable
Lever
Link
The part of a self adjusting drum brake that engages and turns the starwheel
Pawl
Prevents the pawl from overadjusting
Over-travel Spring
