The burnishing issue
DON MILLS, Ont. – It’s easy to assume that a truck’s brakes are never safer or more in compliance than when the truck is brand new, or immediately after a brake job. Ironically, brand new trucks and trucks with new foundation brake components can actually have more trouble with diminished brake performance and adjustment issues.
The problem: brake burnish.
In spite of the latest in machining technologies, it is practically impossible to match brand new brake drums and shoes to the perfect tolerance right out of the box. Invariably, high spots on either surface will mean the new components do not make a perfect contact the arc of the shoe. A wear-in period, or burnishing, has to take place before the imperfections are worked out so the components can perfectly meet. And until the brakes are properly burnished, there is the potential for brake performance and compliance problems.
“What happens is, if you pull your slack adjusters by hand to get your brake adjustment, you would probably be within the proper tolerances,” said Kevin Berry, maintenance manager for Harmac Transportation of Concord, Ont. “But when you do a 90 psi brake application, like (an inspector) calls for, only parts of the shoe will touch the drum. And as you apply force to the foundation brake components, the shoes and everything bend together, and that’s where you’ll get your over-stroke.”
Brake performance and compliance are especially sticky issues for Harmac, because the company specializes in bulk liquids, many of which are hazardous.
Berry said there are a couple of ways to address the issue of brake burnishing.
“We try to use the best linings and the best drums that the market has to offer,” he said. “We don’t use very many rebuilt shoes because sometimes the tables are crooked or sometimes the tables are bent.”
If you are sure of the brake components you are using, the next thing is to make sure you take the necessary time before the truck hits the road to minimize the problems that can potentially be caused by new brakes. “We jack the wheels up and adjust to the closest possible tolerance, then we will run the vehicle around the yard and do brake applications and try to get the measurement in as close as possible,” Berry said. “We try and do 15 or 20 brake applications around the yard. If you can get it out on the highway and do brake applications, it will run them in better and faster.”
Frank Haselden, vice-president of maintenance and compliance for TST Overland Express, echoes Berry’s recommendations. “You can do the perfect brake job, but you are still not done and complete until the brakes are worn in,” he cautions.
The Number-1 problem that can occur when new brake components are not properly burnished is overstroking, Berry said. That will affect the overall effectiveness of the braking system because it throws the foundation brake components out of balance. “Basically, you are going to have one or a few wheels doing more braking than the others.”
And not only that, but the truck will be out of compliance on brake stroke, Berry warns: “If you are well over on all your wheels, you are going to go to Truck Jail.”
How long it takes to properly burnish new brakes depends on a number of factors, Haselden said, “like how far, how fast and how heavy the truck runs after the brakes are installed.” The burnish period for components also varies from manufacturer to manufacturer, he added.
In some cases, said Berry, the foundation components marry so well out of the box that brake burnishing isn’t a big issue, but that is “fairly infrequent”. The mechanic installing the new brake parts will find out if they are well matched pretty quickly when they do the 90 psi brake application.
“If it strokes fine, then we let it go,” Berry said. “But if there is any problem with the stroke, then we try to burnish them in.”
Regardless of the quality of the brake job, it is ultimately up to the driver, in the pre-trip inspection, to decide whether or not the brakes are in proper adjustment. It is important then, Haselden said, that drivers know if the vehicle’s brakes are burnished or not, and of the problems that could occur during the brake burnishing period.
“Drivers have enough to think about, and the biggest thing is knowing they have the best brakes possible,” Haselden said. “And brakes that are burnished will function better than brakes that are not burnished.
Both men agree it is a mistake for drivers to assume everything is perfect just because a brake job has just been done on a truck. In fact, drivers should pay even more attention to their brakes and wheels until the new components are properly run-in. n
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