Tractor Parking Brake Failure

Tractor Trailer

R. Scott King, BSME, Principal Automotive / Mechanical Engineer

A warehouse worker was seriously injured when an industrial tractor he was standing behind rolled suddenly backward, striking and pinning him between the tractor and an adjacent structure. A subsequent OSHA investigation revealed that, consistent with previous employee complaints, the tractor suffered from a malfunctioning brake system. OSHA concluded that the incident was primarily the result of ineffective maintenance; however, subsequent civil litigation identified the scope of the parties involved in the lift’s maintenance, as well as additional factors in the incident.

Co-workers had previously reported problems with the tractor’s brakes. Believing the problem was related to the parking brake adjustment, shift managers provided instruction on how to adjust the parking brake handle to make the brake hold better. The tractor was equipped with an Orscheln-style parking brake, a simple mechanical lever with a user-adjustable handle that removes excess “slack” in the parking brake cables, which in turn, improves the brake’s holding capability. The instruction was consistent with the manufacturer’s recommended remedy to the worker’s complaints. However, that remedy was, according to the tractor’s user manual, predicated on routine tractor inspection and periodic maintenance. Because the problem persisted, the tractor owner utilized an independent service provider to diagnose why.

“Adjust Parking Brake” appeared on the ensuing service invoice and a post-incident forensic examination affirmed that the brake was indeed adjusted; however, the latter identified what the former did not. Specifically, engineers discovered that the friction element of the parking brake mechanism had worn beyond its service limits and required replacement. Worn as it was, the brake was simply incapable of restraining the tractor no matter how “tight” the parking brake handle was adjusted.

R. Scott King, BSME, Principal Automotive / Mechanical Engineer with DJS Associates, Inc., can be reached via email at experts@forensicDJS.com or via phone at 215-659-2010.

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