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Brake Drums are cylindrical brake system components that are shallow and drum-like in shape. They transfer friction into torque used to slow the vehicle. Drum brake components include the backing plate, brake drum, wheel cylinder, brake shoe and various springs and pins.
How Does it Work?
The brake drum fastens via a flange arranged perpendicular to the braking surface to the axle flange.
The inner surface of the drum rotates with the road wheel and at the exact rotational speed of the wheel.
When hydraulic or mechanical actuation of the brake shoes (friction) occurs, the shoes are pressed into contact with the braking surface that comprises the inner surface of the drum.
Friction material (shoes) contact with the drum creating interference that impedes the motion of the road wheel, thereby creating the braking action. The harder the brake pressure, the more braking power (and heat) result from the brake drum.
Braking force is transferred from the brake shoes through the drum and into the road wheel.
How is it Made?
Brake drums are almost invariably cast from a ferrous metal, either steel or iron.
A secondary machining process cuts a uniform mounting flange and cuts the braking surface. The braking surface is cut to be exactly perpendicular with the mounting flange.
Brake drums frequently have cast-in features such as fins along their outer surface to assist in cooling.
The finished brake drum is usually coated with an anti-rust chemical to prevent it from rusting or corroding during shipping and storage.
Why Does it Fail?
Brake drums most frequently fail due to wear.
Every brake drum has a cast-in number representing the largest acceptable measurement diameter. Drums may be “cut” on a special lathe to eliminate friction wear grooves and scoring. If a drum cannot be cut to a dimension smaller than the maximum, the drum must be replaced.
Excessive heat, that can be caused by operating the vehicle with the parking brake left on, can cause the drum to warp or crack. If your drums warp or crack and cannot be cut into specification, it will be necessary to obtain replacement brake drums.
Severe rust will compromise the strength of a brake drum and therefore any drum that is severely rusted should be replaced.
What are the Symptoms of Failure?
A brake drum with an inner diameter measurement along the friction surface that is greater than that value cast into the drum flange is failed.
A brake drum with a visible crack has failed and must not be reused.
What are the Implications of Failure?
Drums that are cut beyond the safety maximum diameter are thin, do not have adequate structural strength, and are unable to dissipate heat properly.Such a brake drum can fail without warning and are not safe.
Out-of-round brake drums that cannot be adequately machined reduce braking effectiveness and are not safe to drive on.