While more and more passenger vehicles have adopted disc brakes in recent years, most semi-trucks contain old-fashioned yet highly reliable drum brakes. As the name suggests, drum brakes consist of a round drum, commonly made out of either cast iron or aluminum lined with cast iron. Inside reside a pair of brake shoes that press outward when applying the brakes.
Over time, the heat and friction generated by the braking process can give rise to a number of problems for the brake drums. Many such problems have a negative impact on your truck's braking ability, thus putting you and your cargo at serious risk. This article seeks to increase your knowledge of drum brake mechanics by discussing two of the most commonly experienced problems.
Scoring involves the formation of scratches or gouge marks on the surface of the brake drum. Professional technicians quantify scoring more technically as any depressions whose depth exceeds 0.10 inches and whose width exceeds 0.030 inches. Such gouges most commonly occur as the result of hard debris stuck to the brake shoe, such that it scratches the revolving brake drum.
Scoring may also stem from rivets or other metal parts of the brake shoe assembly that have worked loose and now contact the brake drum. Regardless of the source, the sooner you catch the problem, the better. A professional can often remove light scoring using a fine emery cloth. Deeper scoring, however, may require drum resurfacing.
Only a mechanic with the requisite experience should resurface your brake drum. Resurfacing involves removing a small layer of metal from the entire face of the brake drum. Even removal creates a smooth, level surface. However, a drum can only be resurfaced so many times before reaching what technicians refer to as the maximum wear limit.
As its name implies, the maximum wear limit governs how much material may be removed from the drum surface before creating problems. Manufacturers express maximum wear limit as a certain diameter. If a resurfaced brake drum exceeds this diameter, the entire drum must be replaced in order to ensure proper braking.
Martensite involves the formation of raised black spots on the inside surface of the drum. Also known as hot spots, martensite forms more readily on inferior quality brake drums. Such drums tend to be made of lower quality metals and don't always meet the most stringent dimensional specifications. As a result, the inside of the drum may contain patches of metal that sit at a slightly higher level. Such spots experience more friction and hence higher temperatures.
Intense braking exposes such raised spots to temperatures above 1,400 degrees Fahrenheit. Over time, this intense process of heating and cooling causes those spots to become super hardened relative to the metal around them. Even higher quality drums may develop martensite if brakes do not remain properly balanced.
If caught in the early stages, resurfacing can sometimes correct martensite. Yet once martensite has reached its fully hardened state, you can't do much about it except have a new drum installed. Those who continue to drive with martensite-affected brake drums risk developing more serious problems such as drum distortion and/or cracking.
When the surfaces of brake drums suffer damage, your brake shoes have a harder time generating the friction necessary to stop your truck. For that reason, have your brake drums periodically inspected and serviced. Regular servicing ultimately saves you money by preventing minor problems from growing worse.
For more information about how to protect your brake drums from damage, please contact our team of truck brake pros at Clutch & Brake Xchange Inc.