4 / 107
U.S. 888-725-9355
MeritorPartsOnline.com
3
Canada 800-387-3889
WHAT MAKES A GOOD BRAKE DRUM
There are a number of attributes that differentiate between mediocre and a good brake drum. The following highlights these and what
they mean to the end user.
1. A HEAT ABSORPTION DEVICE (HEAT SINK)
The braking system in a vehicle converts the vehicle energy of motion into heat energy by means of friction between the brake
lining and the brake surface of the brake drum when the vehicle’s brakes are applied.
a) The surrounding parts, such as brake lining, rim, tire, hub, bearings, and drum absorb heat by conduction and radiation.
b) The surrounding air is heated by convection.
Radiated heat to surrounding parts is small and heat dissipated by convection to the surrounding air is slow relative to the actual
time of braking. For these reasons nearly all the braking energy must be stored in the form of heat in the brake drum. This energy
is stored by heating the drum material to a higher temperature than the drum temperature prior to the brake application. A heavier
drum will store more energy than a light one will. A heavier drum is stronger and more fade resistant.
2. A CYLINDRICAL BRAKE LINING INTERFACE
The brake drum moves past a stationary brake lining. When (if) the brake surface does not move past the lining uniformly, the brake
drum has run out. This will overstress the brake drum, linings, and attaching hardware, causing variable heating of the brake drum
braking surface which results in heat checking, excessive ware of both the drum and lining, pulsing of the brake, and shortened life
of both the drum and lining. Also to keep the drum dimensionally stable under these conditions, it must be a relatively heavy brake
drum.
A heavy precise brake drum does not reduce the need for linings to fit the drum correctly. If the linings are not contoured to fit
properly, the brake will not produce proper breaking torque. Poorly contoured linings cause additional mechanical stress in the
drum, shoes and lining, and brake hardware. This additional stress hastens failure and greater wear.
Before linings are “burnished” or seated into the brake drum, they may rub on only a few high spots. These superheat the brake
drum braking surface in very small areas and can be seen on the brake dynamometer as fire bands. A light brake application will
cause this to occur whereas a snub brake application will force the lining against more of the braking surface of the brake drum
thus limiting or even eliminating these types of hot spots. If these hot spots are allowed to continue, martensite hot spots will occur,
and in turn drum distortion, and very early cracking can happen.
Lining that takes longer to burnish will be more likely to cause hot spotting. Many of the non-asbestos organic linings fall into this
category. Since martensite hot spotted brake drums cannot usually be successfully rebored, they usually must be scrapped.
3. HIGH STRENGTH, HEAT, AND WEAR RESISTANT BRAKE DRUM MATERIAL
A brake drum must be strong enough to resist the maximum mechanical stress caused by the expanding and self-energized brake
shoes at high temperatures. The brake drum’s stiffness or resistance to deflection is derived from the thick shell structure and the
stiffness of the gray cast iron. Higher tensile strength gray iron does not change the deflection of the brake drum significantly.
Although higher strength iron is more resistance to thermal stress caused by temperature gradients, as a “hot” brake surface and
a “cold” outside brake drum diameter. The occurring problem here is the “cold” outside diameter of the brake drum does not want
to expand to a bigger diameter, but the “hot” inside diameter of the brake drum does want to expand. The “hot” inner surface is
caused to compress or compresses plastically so that it fits within the “cold” outer surface. But as the “hot” inner surface cools,
cracks form because the metal was too compressed to return to its original position thus resulting in hairline surface cracks known
as heat checks.
4. HEAVY ROTATING PARTS THAT IF IMBALANCED CAN SIGNIFICANTLY AFFECT THE VEHICLES RIDE
The mass of a 16.5” x 7” brake drum is concentrated about 9 inches radially from the hub bearing centerline. Unless the brake
drum is balanced, it helps excite vibration in trucks and trailers. That is why it is important to use balanced brake drums on all
front and rear applications. Most brake drum balancing is done by welding weights to the outside of the drum, but Meritor pioneered
and patented a technique to balance brake drums without welding on balance weights. This is called Machine to Balance (MTB).
The industry typically requires front hub/drum or hub/rotor assemblies balanced to 20-in. oz. maximum and rear hub/drum or hub/
rotor assemblies balanced to 50-in. oz. maximum.