In order to take care of your customer’s braking system, it’s vital that you understand each system component. Following are definitions of some terms and conditions that braking components may experience.
If you’re familiar with the information below, it should be easier for you to maintain your customer’s braking system and diagnose problems more quickly and efficiently.
Actuator
This is a device that physically initiates the mechanical motion of a braking system component.
Air Disc Brakes
These are air-actuated brakes, which upon application, employ a caliper to clamp two brake pads against a rotor. Air discs, compared to drum-type brakes, have superior ability to resist fade.
Automatic Slack Adjuster
This is a mechanism that adjusts the brakes automatically. Every time the brake is applied, a take-up mechanism in the actuating lever compensates for wear on the brake lining to maintain brake balance at each wheel.
Bell-Mouthed Drum
When a drum has a larger diameter at the open end than at the flange, it is considered bell-mouthed and it prevents full contact with the lining.
Blue Drum
Friction surface is blued resulting from high temperatures which, in turn, may be caused by weak return springs or an out-of-balance drum.
Brake Balance
Balance is achieved when all brakes on all axles do their even share of the work when braking the vehicle.
Brake Chamber
This is a device, inside which a diaphragm converts air pressure to mechanical force, via a push rod, for brake actuation.
Brake Drag
Brake fade occurs when one or more of the brakes fails to release immediately or completely after a driver removes his foot from the brake pedal.
Brake Fade
Reduction or elimination of a vehicle’s braking ability that usually occurs when heat is generated by continuous or repeated use of the brakes.
Brake Lining
The friction material that wears against the inside of the brake drum, to slow and stop the truckis called the lining; its effectiveness in doing this depends on the brake surface area, surface temperature and the brake lining material itself.
Brake Pads
These are the steel plates to which friction material is attached.
Brake Shoe
This is the part of the brake to which the brake lining is riveted.
Caliper
In an air disc brake system, it is the clamping device containing friction material. When actuated, the caliper applies braking force to both sides of the rotor.
Cam Brakes
Also called S-cam brakes, these are a type of brake in which the brake lining is pressed against the drum cam action.
An S-shaped cam is rotated when air pressure in the brake chamber moves the lever that rotates the cam.
As the cam rotates, it pushes a pair of rollers that force the brake shoes into contact with the brake drum. The friction created by this contact creates the torque necessary to slow and stop the vehicle.
Clevis Pin
This is the pin connecting the arm of a slack adjuster lever to a brake chamber push rod yoke.
Cracked Drum
This is a brake drum that is cracked all the way through by excessive heat and buildup or resurfacing of a drum beyond the manufacturer’s limit.
Drum
This is a component upon which brake lining material is applied in order to slow or stop a vehicle.
Dual Braking System
A dual braking system includes primary and secondary air systems which are designed to provide a backup in case one brake system fails.
Foundation Brake System
The components that collectively supply the retardation force for vehicle stopping, including brake chambers, slack adjusters, brake drums and brake linings are known as the foundation brake system.
Friction Material
Material attached to the brake shoe that wears against the inside of the brake drum to slow and stop the vehicle.
Heat-Damaged Drums
Heat-damaged drums are heat-cracked (with hairline cracks from thermal cycling) or hot-spotted (raised spots on the friction surface).
Hydraulic Disc Brakes
Hydraulically-actuated brakes which, upon application, employ a caliper to clamp two brake pads against a rotor.
Hydraulic Drum Brakes
These are defined as a type of brake in which the brake lining is pressed against the drum via hydraulic action.
Out-Of-Round Drum
A brake drum is out-of-round if it is dimensionally inaccurate which reduces braking effectiveness; the drum could be oval or bumpy.
Push Rod
A push rod is a rod, protruding from a brake chamber, which is connected to the arm of a slack adjuster via a clevis pin.
Retarder
A retarder is an auxiliary braking device such as an engine brake, exhaust brake, hydraulic retarder or electric retarder.
Rotor
The rotor is a component that turns about its own axis to create friction in conjunction with friction pads.
Slack Adjuster
A slack adjuster is a lever, connecting the brake chamber push rod with the foundation brake camshaft, which transmits torque to rotate the brake camshaft when the brake treadle is depressed, and it also provides a means of adjusting the clearance between brake shoes and the drum to compensate for lining wear.
Spring Brake
A parking or emergency brake that does not require air pressure to release the brakes. It works automatically in the event of a sudden loss of air pressure, and it can be activated manually by a dash-mounted parking brake control.
It is released only when air pressure comes back into the system. Some of these units are parking and emergency brakes only, and some have a tandem chamber that incorporates a service brake chamber as well as a spring actuator.
Stopping Distance
Stopping distance is the distance traveled by a vehicle on a road between the initial application of the brake and a full stop.
Stroke
This refers to the total distance traveled by a brake chamber push rod or slack adjuster arm during brake application.
Torque Balance
This is a condition achieved when individual brakes exert an equal amount of braking force. The brakes are considered to be proportionally appropriate, but not necessarily equal.
Editor’s Note: Truck Parts & Service wishes to thank ArvinMeritor, Inc. for the information as well as some of the photographs used in this article.
