Tractor Braking Systems

How to avoid potential brake compatibility issues

Drum brakes, the old stand-by, have a key feature that the disc brake doesn’t. They are capable of “sharing” the braking workload with other brakes on the vehicle under high-energy conditions. This is also called “fade,” which can be a good thing.

When a drum brake becomes too hot, the drum expands and the resin of the drum brake lining surface will start to “burn” away. This causes the brake to fade - basically reduce its torque for a given brake pedal application.

This also causes other brakes on the vehicle to take over more of the braking workload, essentially causing a sharing of the work.

However, when a disc brake heats up, the rotor surface doesn’t move away from the lining. Rather, it moves toward the lining surface.

Not only that, but the disc brake lining or pad is primarily made up of metal particles, meaning it can withstand significantly higher temperatures and continues to produce the same friction forces.

Put these two different types of brakes on the drive and trailer axles of a combination vehicle, and put that vehicle in any situation where the brakes are heavily utilized, and the disc brakes will simply continue to take on workload until they are overloaded.


What are those situations, and what kind of trouble can you expect?

Let’s say you’ve got a tractor that has air discs on all axles and it’s pulling a fully loaded, older trailer down a mountain. If everything is well maintained, there is a good driver at the wheel, an engine retarder that’s operating well and the driver knows how to use it, you probably won’t have a problem.

However, add inferior brake linings, manual slacks or inoperative automatic slacks adjusters, or simple poor maintenance, and you will likely have problems.

What kind of problems? Well, rotor cracking, premature wear out - since wear increases exponentially with temperature - and even trailer bump, among others.

Just as serious is an all-disc braked trailer being pulled by an-all drum braked tractor. Any time the driver uses the hand valve as he goes down the mountain adds to the above problems.

With these mixed brake technology situations in mind, TMD Friction has paid special attention to the issue of compatibility and “work sharing” between brakes. To help address this issue, they have formulated special air disc brake pads that have a capability of work sharing with drum brakes at high temperatures.

Not only does this help with compatibility, but it ultimately improves brake life and reduces the potential for rotor cracking.

TMD also actively supports RP 628 and continues to list their linings so knowledgeable fleets at least have a chance of having properly balanced brake systems.

Jim Clark is director of engineering for TMD Friction. The company is one of the world’s leading manufacturers of brake friction materials to the automotive and commercial vehicle industry and has a leading position in the global replacement parts market. TMD markets and sells brake linings in North America under the Textar brand.

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