BRAKES: THE IMPORTANCE OF A BALANCED SYSTEM AND PROPER ADJUSTMENT

BRAKES: THE IMPORTANCE OF A BALANCED SYSTEM AND PROPER ADJUSTMENT

APPARATUS: THE SHOPS

This column is provided through the cooperation and coordination of the Apparatus Maintenance Section of the International Association of Fire Chiefs. The information contained herein is, however, the sole opinion of the author and not the Apparatus Maintenance Section unless specifically referenced.

Brake balance is the proper distribution of the total brake load between all the braking axles on a vehicle. When break balance is attained, braking performance is optimized, allowing the vehicle to stop with stability and in the minimum distance. A brake system that is out of balance causes some brakes to work harder than others, resulting in elevated temperatures, uneven wear, poor braking, wheel skids, and, often, loss of control of the vehicle.

An essential element of balanced braking is torque balance between the brakes on each axle. If one axle has a greater braking torque disproportionate to the weight exerted on that axle, it will override the others, creating an unbalanced braking system that generates the problems mentioned above. Proper brake torque balance between the axles means equal distribution of braking effort. There are two key factors involved: brakes designed for the specific weight carried on the axle, and the maintenance and adjustment of the brake system.

Brake temperature is a direct indication of the amount of work being done and an important clue to whether a system is in balance. A wide variation in temperature is evidence of unbalanced brakes. Abnormally high temperatures lead to brake fade and rapid lining wear and cause damage to other brake components, including the brake drum.

To achieve brake balance, the engineer must develop a system that takes into consideration axle weight, single or duel wheel axles, brake chamber size, braking surface, brake block friction rating, etc.

To maintain a balanced system, all brakes should be relined on a vehicle at the same time, using quality linings of the same formula from the same manufacturer as the original vehicle. Brake drums, too, should be identical on every wheel because they also absorb the tremendous heat and energy’ generated while braking. Drums should be of the same weight, size, and design to maintain brake balance. Additionally, proper adjustment must be maintained. If there are deviations, you run the risk of interrupting the balance of the braking system.

A recent study evaluating the impact of adjustment on the braking performance of heavy-duty’ trucks with air brakes concluded that the level of adjustment has a very’ pronounced effect on air brake system performance, particularly when the brakes are hot. The study was performed for the US. Department of Transportation, using two trucks and an inertia-type brake dynamometer to measure the energy’ required to bring the vehicle to a stop. It also looked at the status of adjustment on actual vehicles and summarized available accident data that identified improper brake adjustment as a causal or related factor.

Another finding is that adjustment level must be closely controlled to avoid serious degradation in air brake system performance. Under normal operating conditions, the distance required to stop a truck can double before brake adjustment is required by current maintenance standards. Furthermore, the study points out that a significant number of trucks on the road have brakes out of adjustment; since in the last 21 years approximately 30 percent of accidents involving heavy trucks have been caused by brakes out of adjustment, improving the adjustment situation would have a positive impact on safety

The study shows that the level of brake actuator stroke set by vehicle manufacturers as the point for readjustment allows a 23 to 35 percent increase in stopping distance when the brakes are cold and more than a 75 percent increase when the brakes are hot. To improve the adjustment situation, the study recommends using reliable automatic adjusters that maintain minimum stroke levels and recommends that awareness of the importance of brake adjustment be increased.

Keep in mind that fire apparatus basically performs three functions en route to an emergency: It accelerates, brakes, and corners—over and over again. A balanced braking system adjusted properly for maximum performance goes a long way toward a safe response.

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