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The Case of the Errant End-Loader
Myron J. Boyajian, President, Engineering Consultants -- Design News, November 22, 2004
A southern coastal city was experiencing turmoil in its sanitation department. A large end-loader had "jumped into gear," crushing a worker. He suffered broken bones and internal injuries. His suit named the loader manufacturer, the parking brake and other loader component suppliers, and an outside service contractor. I was retained to inspect the loader and reconstruct the accident.
The loader had four tires and a front-mounted, 4-cubic-yard bucket. Diesel power came through a three-speed/forward-reverse, hydraulic transmission. Its output passed through a spring-applied, air-released, drum parking brake. The parking brake applied automatically with air supply loss, or by a control knob.
Evidence suggested this scenario: Unable to start the engine, the driver called the plaintiff, a maintenance man. The plaintiff brought a battery cart, squeezed in the space between the dead loader and another loader, and hooked up jumper cables. After starting the engine, the driver set both the parking brake and a hand-throttle to a fast idle to charge the battery. While disconnecting the battery cables, the loader jumped forward and pinched the plaintiff against the adjacent loader. Only the impact of bucket against bucket stopped the loader and prevented a fatality.
The usual cast of characters, including experts, lawyers and, yes, some loafing city workers, assembled for the inspection. The city sanitation department provided us with a mechanic. At my request, the mechanic turned the keyswitch to the start position and declared the machine unable to start. The regular loader driver who was watching the inspection, stepped over to tell us he always had to "jiggle" the shift lever to find the neutral position before starting. With this cue, the mechanic proceeded to fire the engine. He set the throttle for a fast idle and, after a few moments, the shift lever, a chunky steel bar that stuck out from the instrument panel, visibly vibrated and dropped downward, engaging forward speed and causing the loader engine to jump forward.
Service information and inspection revealed about a half-dozen linkage and pivot points between the shift lever and the transmission. With the neutral-start switch up on the shift lever, linkage wear-and-tear allowed the transmission shift spool to be moved out of its neutral detent while the driver fished around to find the neutral position on the maladjusted switch. Vibration from the fast-idling engine caused the heavy shift lever to drop, slipping the transmission into forward gear. Over time, the regular loader driver just accommodated this peculiarity and never reported it for repairs. So, okay, that explained the loader jumping into gear. But why did the loader move with parking brake applied?
The mechanic then tore down the parking brake. The spring unit was in good order, but we found a linkage pin in the drum brake assembly out of position. This left only one of the two internal brake shoes working. The brake could hold the loader in second and third speed, but allowed loader movement in first gear above idle speed. I deduced that the brake was never assembled correctly during manufacture as the friction lining on one brake shoe was virtually brand new.
My opinion report stated that the outside service contractor failed to follow procedures to check parking brake performance and make the necessary corrections. The manufacturer had failed to both install the parking brake correctly and to check its performance, and had produced a negligently designed gear-shift linkage and neutral-sensing system. The engineers who designed this loader failed to grasp the function of a neutral start switch—it must be designed to sense transmission neutral, not shift lever neutral.
Before trial, the service contractor and other component manufacturers settled with the plaintiff. Called to testify at an arbitration conference with the manufacturer, I used illustrations with transparent overlays to compare the existing linkage and neutral switch on the shift lever and its wear points to a neutral switch on the transmission spool. Settlement quickly followed this conference.
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Reach Myron J. Boyajian, P.E., at ntesla@ieee.org. Cases presented here are drawn from his actual files.
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