08Brake Inspections Tech Tip130007032019

“While cracks across the braking surface can be caused from improper installation, it is typically due to a thermal or mechanical overloading of the braking surface,” he says. “This can be due to various factors including: improper installation; inferior friction material; dragging brake, inadequate drum for the specific application; or parking brakes set while drum is overheated.” While mostly normal, there are times, according to Kay, when those small heat cracks may be signaling a problem. “With heat checks it’s okay to continue using the drum as they sometimes wear off over time or they do not grow,” Kay says. “It is important to note that heat checks would be expected to cover the drum 360 degrees and over the width of the shoe contact. This symptom indicates the shoe is in full contact when the brake is applied. “If the drum shows localized heat checking, for example in a certain section of the drum, then there may be a dimensional problem that is forcing contact in only one location,” he continues. “Sometimes the thermal stress will continue fatiguing the drum and larger cracks will develop. Once a larger crack forms then the drum must be replaced.” Beyond heat cracks look for drum discoloration as a sign of overheating. “If the brake surface has a blue color to it, this is typically a symptom of overheating,” Kay says. “Dragging brakes or simply over-using a brake can lead to excessive temperatures and as a result the drum turns blue from the oxidation.” Drum discoloration from heat damage may also result from martensite formation which leads to leopard spotting, Sturdy adds. Beating the drum about best practices Since it’s typically more time- and cost-effective to replace old drums with new ones, most shops have largely abandoned the process of turning drums. Penske, however, never took up the practice. “We have never turned drums and don’t see this as cost effective,” Hasinec says. “Turning drums creates additional down time for the vehicle. Turning drums has become somewhat rare and even finding a supplier that can do so can be a challenge. I believe most fleets have this same train of thought.” Pfost agrees. “The down time required to send a drum to be turned can be costly,” he says. “And if a drum is turned too far, it may still be in spec but by the time you wear out a new pair of shoes, it will no longer be in spec.” Sturdy says that the challenging aspects of drum turning have more shops opting for new drums over turning. “With the skills required to properly setup and align a brake drum on the pilot and mounting flange we are finding more shops are opting for replacement vs. turning,” he says. “Some sites that have consistent and proper maintenance programs we find them utilizing drums without having them turned, merely going from first lining to the second.” For those times when turning makes sense be sure to take the time to determine if the drum is a sound candidate. “Most manufacturers do not recommend resurfacing or turning a brake drum,” Capps says. “However, if turning is necessary, allow for at least .040 inches under the maximum diameter cast into the brake drum. For example, if the maximum diameter cast into the drum is 16.620 in., the maximum rebore limit should be 16.580 in.” When considering a drum for turning, be sure to select proper measurement points. “This should be measured at the worst location (any grooves or out of round for instance) to make sure sufficient material is there to remove,” Sturdy says. “Check the diameter of the braking surface at two points 90 degrees apart at the inner brake surface area and two on the outer brake surface to check for any roundness or taper issues. Checking in these four locations will help verify the drum diameter as well as any shape issues in the braking zone.” Brakes are only as good as their hardware, which means all related components also should be regularly inspected along with brake drums.