The Brake Shop: Determining brake maintenance intervals

Everyone in the trucking industry recognizes the fact that brakes need to be maintained regularly. But how do you determine what the proper maintenance interval should be and what should be included in the inspection?

Truck Parts & Service spoke with several brake component manufacturers to get their expert advice on setting up brake maintenance schedules that help to ensure your customers’ trucks stop safely.

A good place to begin is with the guidelines set by the truck OEMs and the brake system component suppliers. These can be either mileage or time based.

Since these are suggested guidelines for typical applications, you must examine your customer’s specific application and adjust the interval as needed.

Joe Kay, engineering manager, foundation brake, ArvinMeritor, Inc., suggested looking at the environment in which the vehicle operates. For example, if the vehicle runs in an area with a lot of moisture-either rain or humidity-braking can be affected.

“Moisture will begin to work its way into the internal components of the braking system and once it does, corrosion can start,” he explained.

Duty cycle is another factor to consider. “The number of times the driver actually has to brake will have an effect on component wear and fatigue,” he added.

Even temperature can impact component life. “Higher temperatures will degrade parts faster,” Kay explained. Both ambient temperature and the temperature of the braking system components themselves must be considered.

“If the vehicle stops every quarter mile, the brakes tend to get hotter, which eventually wears components,” he said.

Randy Petresh, vice president, technical services, Haldex, said, “The factors that determine proper intervals for brake maintenance include many vehicle operational parameters in addition to driver practices and characteristics.”

Operational parameters include things like duty cycle, type of service, geographic location and environment, vehicle specifications, hardware and component selection, replacement part usage, brake balance and tractor-trailer compatibility, vehicle loading characteristics, etc.

According to Bob Pawluk, service engineer, air brakes, Bendix Commercial Vehicle Systems, terrain is a major contributor to brake life. “If your customer drives in a relatively flat part of the country, he probably uses a little bit of brake action. And if he is doing highway driving, he probably does even less braking than if he were on flat terrain in a city, for example, where he is stopping at every traffic light, every block or two.

“And then think about driving in San Francisco with all the severe hills. He is going to be using a lot more brakes there,” Pawluck said.

John Hawker, service engineer, Bendix Spicer Foundation Brake, believes that vehicle load is as much a factor in brake life as is terrain.

“The heavier the load, the more retardation that is needed to stop the vehicle. Put another way, more horsepower has to be generated for heavier loads because the large mass in motion coming to a stop is going to require more power to stop. When you ask a brake to create more power, that power creates more heat and more heat creates friction wear.”

Petresh concurred and added, “The heavier the vehicle, the faster the brake linings wear, the shorter the brake maintenance interval.”

He continued, “Improper loading practices and load distribution can further accelerate wear patterns as a result of some axles being overloaded while others are underloaded.”

Once you’ve set the proper maintenance interval, you need to make sure you perform a thorough inspection.

According to Kay, the first thing to check is the brake stroke. “The brake stroke tells you a lot about the condition of the brake and the slack adjuster.”

All braking system components should be visually inspected for signs of wear, cracks and looseness. Inspect the cam shafts and bushings for wear and clearance, making sure the cam shaft can rotate freely, Kay said.

Inspect linings and drums for wear and cracks. “Make sure the technician performing the inspection is capable of determining if he should allow the vehicle back into service based on the amount of lining wear,” he said.

“You do not want to send a vehicle out on the road that is going to wear through its lining and drum before it is due for its next scheduled maintenance.”

You also need to make sure the slack adjuster is functioning properly. Pawluck cautioned that automatic slack adjusters are not to be adjusted manually. “If you check the power stroke and it is out of specification for the chamber size, the technician needs to find out what is wrong in the braking system and fix that. Manually adjusting the slack adjuster will not fix the problem.”

Also check the air chambers and look at the springs and push rods to make sure there are no problems with corrosion. Finally, check the air lines and fittings for fatigue and wear.

Hawker explained that it is important to measure the brake release position as well as the free stroke. “If you don’t have the proper free stroke or clearance, the brakes are going to become hot, which creates even more wear.”

He also said to check the pressure stroke and power stroke. “This will give you an indication of the condition of the brakes, the actuators, the slack adjusters, the cam shafts and the bushings.”

If any of these measurements are outside of their specified parameters, the vehicle needs to be taken out of service immediately and repaired.

Proper component lubrication is one area that Hawker believes is misunderstood. “You must lubricate the S-cam and the slack adjuster. Apply new grease until all the dirty contaminated grease is purged from the system.

“If you don’t do this, and the slack adjuster contains water, rust or contaminants such as sand or chemicals, you are going to see premature failure.”

He said the technician should run his hand up and down the hoses, around the valves and fittings to check for leaks.

“An air leak reduces the amount of input to the brake, which will in turn require the brakes to take more time to bring the vehicle to a complete stop, reducing the life of the friction material.”

He added, “A proper brake inspection should start while building up air pressure. The technician should begin at the compressor and go down the line. He should make sure there are no leaks in the control circuit that goes to the governor.”

This means the brake’s air should build up, the governor should cut off the compressor and the air dryer should purge.

Once the air dryer has purged, the next step is to go to the primary and secondary tanks and perform a short drain. “Open the valves and see if water comes out. These are pneumatic systems, not hydraulic ones, so there should not be water in them,” Hawker explained.

“If you are getting water out of the tanks, check the vehicle’s maintenance records to see when the desiccant cartridge was replaced last.”

The technician also should consider going along the frame rails of the vehicle to check the lines for kinks and to see any indication that they are pierced or damaged in any way.

The last thing to check is the relay valve for leaks. “Generally a leak at the relay valve does not mean the valve is bad but rather indicates a leak downstream that is coming out at the first vent it reaches,” he said.

“The whole inspection process starts at the compressor and ends at the brake shoe, but it needs to be done in its entirety.”

Pawluk summed up the importance of regular brake maintenance: “Emergency repair dollars are greater than preventive maintenance dollars.”

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