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The Case of the Lethargic Linkage
Myron J. Boyajian, President, Engineering Consultants -- Design News, February 24, 2008
Operator misuse or carelessness does indeed cause many industrial accidents, but even the best trained and motivated operator may not be able to avoid an accident when operating badly designed or poorly maintained equipment.
The Scene of the Crime
I was retained by an attorney for a forklift operator whose forklift failed to stop while traveling in reverse and collided with a steel storage rack, injuring the driver. The stand-up, end-control forklift had a rear-facing open compartment. In fear his feet were going to be injured in a collision, the driver attempted to lift his foot from the point of impact, but actually stuck out his foot. His raised foot made a poor bumper and several bones were broken and his kneecap was fractured in the collision.
The Investigation
Based on the driver's and witness statements, the driver's attorney filed suit alleging poor maintenance and failure to replace or adjust the forklift's brakes with consequent loss of stopping power. The suit also named the forklift manufacturer, alleging negligent design of the brake system. The defendants in this matter accused the driver of negligent and careless driving, citing his failure to keep his extremities within the operator's compartment resulted in his severe injuries.
I traveled to the plant where the truck had been used and then placed in storage after the accident. I learned an outside service company routinely maintained the forklift, lubing and adjusting it along with the 10 other forklifts in the fleet. I drove the truck, taking special note of braking power. When I released the floor-mounted brake pedal, the truck slowed but did not stop with authority. This test seemed to back up the driver's version of events leading up to the accident. The brake pads appeared OK and the brake disc was clean, so I had to look elsewhere for the cause of the seemingly anemic brake performance.
In order to check brake performance, I set up a measured test course where the forklift was operated. With relatively close confines of the racks and aisles, I found the truck could only reach 5 mph. At this speed, the truck took 12 ft to come to a stop. The forklift standard; ASME B56.1, Section 7.15, Brake Performance, Section 7.15.5, Test Method; uses an equation, s = (3.34 v2)/D to predict a stopping distance (s), based on the speed of the forklift (v2) and required braking drawbar drag (D) as a percent of the gross vehicle weight. For a stand-up rider, the required drag is 15 percent. At 5 mph, this forklift should have stopped in only a bit over 5½ft. With brakes of such poor performance, it became clear how this collision occurred.
The Smoking Gun
With a shop mechanic's assistance we opened all the panels and removed the floorboards to expose all the brake system components. I could only gasp as I noted the compartment under the floorboard so jam-packed with plastic palletizing sheeting, bits of broken pallet wood and cardboard debris, it looked like the inside of a recluse's shack. With concurrence of all parties we recorded the removal of the debris, piece by piece. I noted the debris packed around the brake linkage between the brake pedal and hydraulic master cylinder was restricting the linkage movement. As the debris was cleared, the linkage motion was freed.
I again set up my test driving course and taking care not to alter the brake adjustment, I repeated the stopping distance test. The stopping distance measured less than 5 ft, confirming debris had jammed or restricted the brake linkage, extending stopping distance. I studied the forklift structure to be sure no shrouds or panels were missing or altered in some way that allowed the debris to get under the floorboard. I concluded the truck design lacked critically placed shields thereby allowing the drive tire to pick up and throw debris and waste into the compartment as the truck was driven back and forth and steered left and right.
My report, backed by test results, included my opinion that there was negligent design that allowed debris to accumulate and jam the brake linkage and that the servicing company failed to check brake performance and failed to detect poor brake operation. I learned this case settled before trial.
