Scored Drum Surface/Bell-mouthed Drum/Concave Drum/Convex Drum/Hard Spots On The Drum/Threaded Drum Surface/Heat Checks/Cracked Drum/Out-Of-Round Drum
Brake drums act as a heat sink. They absorb heat and dissipate it into the air. As drums wear from normal use or are machined, their cooling surface area is reduced and their operating temperatures increase. Structural strength also reduces. This leads to overdistortion, which causes some of the drum conditions covered here.
Scored Drum Surface
A scored drum surface shows a scored drum surface. The most common cause of this condition is buildup of brake dust and dirt between the brake lining and drum. A glazed brake lining, hardened by high heat or in some cases by very hard inferior grade brake lining, can also groove the drum surface. Excessive lining wear that exposes the rivet head or shoe steel will score the drum surface. If the grooves are not too deep, the drum can be turned.
Example of a scored brake drum. Courtesy of Wagner Brake Products.
Bell-mouthed drum shows a distortion due to extreme heat and braking pressure. It occurs mostly on wide drums and is caused by poor support at the outside of the drum. Full drum-to-lining contact cannot be achieved and fading can be expected. Drums must be turned.
Example of a bell-mouhed brake drum. Courtesy of Wagner Brake Products.
A concave drum exhibits an excessive wear pattern in the center area of the drum brake surface. Extreme braking pressure can distort the shoe platform so braking pressure is concentrated at the center of the drum.
Examples of concave and convex brake drums. Courtesy of Wagner Brake Products.
A convex drum exhibits excessive wear at the closed end of the drum. It is the result of excessive heat or an oversized drum, which allows the open end of the drum to distort.
Hard Spots On The Drum
This condition in the cast-iron surface, sometimes called chisel spots or islands of steel, results from a change in metallurgy caused by braking heat. Chatter, pulling, rapid wear, hard pedal, and noise occur. These spots can be removed by grinding. However, only the raised surfaces are removed, and they can reappear when heat is applied. The drum must be replaced.
Threaded Drum Surface
An extremely sharp or chipped tool bit or a lathe that turns too fast can result in a threaded drum surface. This condition can cause a snapping sound during brake application as the shoes ride outward on the thread, then snap back. To avoid this, recondition drums using a rounded tool and proper lathe speed. Check the edge of the drum surface around the mounting flange side for tool marks indicating a previous rebore. If the drum has been rebored, it might have worn too thin for use. Check the diameter.
Heat checks are visible, unlike hard spots that do not appear until the machining of the drum. Extreme operating temperatures are the major cause. The drum might also show a bluish/gold tint, which is a sign of high temperatures. Hardened carbide lathe bits or special grinding attachments are available through lathe manufacturers to service these conditions. Excessive damage by heat checks or hard spots requires drum replacement.
Example of a heat checked and over-heated brake drum. Courtesy of Wagner Brake Products.
Cracks in the cast-iron drum are caused by excessive stress. They can be anywhere but usually are in the vicinity of the bolt circle or at the outside of the flange. Fine cracks in the drums are often hard to see and, unfortunately, often do not show up until after machining. Nevertheless, should any cracks appear, no matter how small, the drum must be replaced.
Drums with eccentric distortion might appear fine to the eye but can cause pulling, grabbing, and pedal vibration or pulsation. An out-of-round or egg-shaped condition is often caused by heating and cooling during normal brake operation. Out-of-round drums can be detected before the drum is removed by adjusting the brake to a light drag and feeling the rotation of the drum by hand. After removing the drum, gauge it to determine the amount of eccentric distortion. Drums with this defect should be machined or replaced.
Measure the inside diameter of the drum in several spots to determine out-of-roundness.