08Brake Inspections Tech Tip130007032019

Sturdy adds that grooving can also be caused by aggressive linings, improper brake setup and improper burnishing. Since brakes are only as good as their hardware be sure to take a close look when the drum is removed. “Look for damage to components, cracks loose or missing fasteners, corrosion, etc.,” Capps says. “Refer to the manufacturer’s maintenance manuals for specific dimensional inspection points. Also refer to the manual to determine where to lubricate and where not to lubricate.” Excessive lubrication can make its way to the drum’s braking surface and shoes, get in the way of effective braking and lead to premature wear, adds Kevin Pfost, coordinator of technical services, Bendix Spicer Foundation Brake (BSFB). Pfost says that while grooving can be caused by foreign material like sand and dirt entering the drum it also may be the result of excessively hard brake shoes. Picking high-quality shoes to match the duty cycle can help avoid trouble down the road. “If the drum is wearing faster than expected, then there may be a friction couple incompatibility or that particular wheel end is doing more work than other wheel ends,” says Joseph Kay director of brake engineering, Meritor. Wear patterns like grooving, however, aren’t always indicative of a problem. “A groove on the outer edge of the drum is quite common as the drum is typically slightly larger in width than the shoe,” says Mike Hasinec, vice president of maintenance, Penske Truck Leasing. “Deep grooves throughout the drum surface typically indicate the brake lining has worn down to the rivets. Pitting can be an indication of debris getting in the drum — backing plates help prevent that. Some very small cracks inside the drum surface are normal. In rare cases, you may see the drum worn more towards the outside versus the inside. An alignment problem or even a bent or twisted brake spider can cause this.” The heat is on Since it takes plenty of friction to slow down an 80,000-lb. truck it’s not hard imagining how hot brakes can get and the impact that heat can have on drums. Gray iron is the standard material used in brake drum manufacturing. However, according to Kay, variance in composition can affect thermal characteristics of the drum. The lower the thermal properties, the higher the thermal stress to the drum. Costly elements like molybdenum combined with gray iron can help increase strength but the heat will always be present along with its effects which, if left unchecked, can result in drum failure. “The main function of a brake drum or rotor is to convert energy to achieve a vehicle deceleration,” Kay says. “The drum does this by converting the rubbing friction into thermal energy (heat) and then release the thermal energy to the atmosphere. The drum must have excellent material properties to manage the energy quickly and efficiently. “For example, an emergency stop with a fully-loaded commercial truck from 60 mph will happen under six seconds, therefore if the energy cannot convert quickly the result will be a longer stopping distance.” Obviously, no one wants a longer stopping distance, which is a vital reminder of the importance of choosing the right brake components and conducting routine and thorough inspections that include looking for signs of overheating. “The drum has to handle a lot of variation in temperature,” Hasinec says. “During braking the drum experiences expansion and contraction, which causes many of the smaller cracks you see on the inner drum surface. These smaller cracks are generated from the heat of the braking event and are normal. A lot of heat and a panic stop usually causes larger cracks that can actually be seen from the outer edge of the drum. This is a drum that you discard, as it is usually cracked all the way to the outer shell.” According to Pfost other factors also can lead to abnormal heat in the drum, such as a dragging brake and a duty cycle that’s too severe for the drum. Capps agrees. Heat cracks and drum discoloration are both signs of brake drum overheating.