Hard Braking

I maintain a refuse fleet, and I'm trying to improve our brake maintenance practices. But I'm wondering, is this a losing battle? — Reader

In severe-duty brake systems the "duty cycle" is the culprit that causes more concerns when it comes to performance and operation safety. While over-the-road big rigs replace their brake friction material every 500,000 miles, refuse trucks replace theirs in some instances every five weeks!

While any brake system is extremely important and must be maintained properly, severe-duty brakes have many practices that are recommended to ensure that they can perform much more efficiently. Closer tolerances in the parts installed and inspected are important to ensure that the more rapid wear of these components can be judged and replaced at a more intensive rate. With severe-duty vehicles, a part within specification at the time of brake service may not be able to maintain that specification until the end of the new life being installed.

Many severe-duty vehicles are under-specified when it comes to air brake compressors. With the more severe-duty cycle, they work much harder than the over-the-road vehicle compressors. Because of this overwork, the compressors wear faster and can generate oil within the air brake system that is not good for the air that is being used for braking. In these instances a compressor that delivers much more cubic feet per minute would allow the compressor to work less and last longer.

In many severe-duty vehicles the weight capacity can and will be taxed to its maximum ability when in use. The brakes are designed to require all axles and wheel end locations to share in the proper braking of the vehicle. There is a set timing of the brakes that must be accomplished, and this is done by means of valving (that has what is known as split timings) that make the larger rear brakes work first and the smaller front brakes work less, but at maximum to balance the total braking effort.

Each axle is factored into the equation of heat and weight to ensure that the maximum temperature for the friction material is not exceeded during braking. Each axle and each wheel end must share the load to ensure proper performance. The system must also function in the proper temperature range to ensure safe and quiet operation.

The contact amount of the friction material to the drum at each wheel is extremely important. Ensuring that the maximum amount of contact is made—80 percent or more—is of the utmost importance to make sure that the temperatures are not exceeded during braking. In transit bus applications the industry often uses a brake lathe that is designed to give as close to 100 percent touch as possible to ensure proper heats can be maintained.

New brake drums are always the rule in severe-duty service. Re-use of a brake drum is not recommended as the amount of brake shoe contact is greatly reduced, and this in turn causes the friction material to heat much more than is allowed. Brake drums are replaced in pairs, and used drums, even though within specifications, are not an option for severe-duty vehicles.

At each wheel end there is a brake spider that aligns all of the components and positions the brake shoes and friction material in the proper location. This is important to ensure that the right amount of friction material is allowed to touch the brake drum and stay within the heat range required during operation.

Brake drums for severe-duty vehicle applications are not the same as for over-the-road vehicles. The increased thickness of the drum allows more ability to handle higher heats and function well at the severe-duty level needed. It is recommended to replace them in pairs to ensure braking balance on an axle for efficiency.

When performing brake service to any severe-duty vehicle, it is recommended to use the TMC (Technology and Maintenance Council) recommended practices RP1503 as your guide. This procedure was created specifically for severe-duty vehicles.

After performing brake service on any severe-duty vehicle, it is important to verify that the brake system is functioning in the proper heat range and that all wheels are working together to provide effective performance. Driving the vehicle, "burnishing the brakes," and verifying the correct temperatures is the only way to do this effectively.