ATLANTA, Ga. – The Technology and Maintenance Council of the American Trucking Associations (TMC) is looking to establish recommended maintenance practices to address excessive stroke in S-cam brakes. The result will provide a diagnostic and troubleshooting guide to help technicians repair the actual cause of a brake taken out of service because of excessive pushrod travel.
Ever since automatic brake adjusters (ABA) – otherwise known as automatic slack adjusters – were first required by law in 1994, many drivers and brake technicians still respond to an over-stroking S-cam brake by manually readjusting the device rather than determining the root cause of the problem. By design, the adjuster will self-adjust based on the application stroke or the return stroke, depending on the model. Unfortunately, a manually readjusted ABA will return to an over-stroke condition in just a few brake applications if some other physical problem exists with the foundation brake, the parking brake, or any other component in the wheel-end brake assembly.
“All too often, the root cause of the excessive applied stroke condition is not diagnosed properly,” said Meritor’s Glen Cram, chairman of the council’s task force looking at the issue. “It became clear during our research that no publication out there in the industry really gives the technician a clear-cut way to determine how to diagnose this problem.”
When complete, the recommended practice will take technicians through an eight-step process — from confirming that an over-stroke condition actually exists (not just a faulty stroke measurement), to removing the wheel and inspecting the lining, drum, and foundation brake. It will help to spot issues such as excessive axial rotation of the cam and faulty return springs. The document’s structure will begin with the least-invasive steps and most likely causes. The final step will discuss how to test the adjuster’s function.
The risk in manually adjusting ABAs, except for those that are first installed ore reinstalled after work on a wheel end, lies in the potential to damage the adjuster or wear out the internal adjustment mechanism.
In 2003, the U.S. National Transportation Safety Board reported that a manually adjusted automatic brake adjuster led directly to a fatal collision between a dump truck and a passenger car in Glen Rock, Pa.
“The National Transportation Safety Board determines that the probable cause of this accident was the lack of oversight by [the company], which resulted in an untrained driver improperly operating an overloaded, air brake-equipped vehicle with inadequately maintained brakes. Contributing to the accident was the misdiagnosis of the truck’s underlying brake problems by mechanics involved with the truck’s maintenance; also contributing was a lack of readily available and accurate information about automatic slack adjusters and inadequate warnings about the safety problems caused by manually adjusting them,” it concluded.
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