Design News for Mechanical and Design Engineers

Personal injury can result from careless or incorrect use of equipment. When combined with improperly installed or improperly maintained equipment, the result can be serious — even deadly. This was such a case.

An attorney, who represented a building material supply yard, retained me to inspect a dump truck that allegedly lowered unintentionally, causing an injury. The injured person, a building trades contractor, planned to load supplies at the yard. With the contractor's only truck already on a job, the yard manager allowed the use of the yard's older, seldom-used truck to transport the contractor's purchases. The yard manager assigned a helper to aid the contractor by moving the truck into position to load materials. This truck was stored outside with the dump body raised to avoid rainwater pooling in it. The helper warmed the engine and then attempted to lower the dumper by using controls on a small center console in the truck cab. One knob engaged the hydraulic pump to the transmission power takeoff and another knob operated the hydraulic lift/lower system.

After several attempts to push and pull the knob marked “LIFT,” the dumper remained raised. When the contractor asked that the pump be disengaged from the power takeoff, the helper complained that the knob marked “DRIVE” seemed stiff and difficult to move. Wanting to look at the lift valve and cylinder, the contractor ordered hands off the controls. As the contractor leaned under the raised dumper, it lowered and trapped his head against the frame. Hearing the cries of the contractor, the helper ran for help. Other yard workers used a forklift to raise the dumper enough to free the man. The contractor's life was spared only because some lumber spanned the truck frame rails preventing the dumper from fully lowering. As it was, the injuries required serious reconstructive surgery including repairs to one eye socket, major cosmetic surgery and extensive dental repairs.

The injured man sued the truck manufacturer, the lift device manufacturer, the device installer, the pump and valve manufacturers and the manufacturer of the control cables.

Using manufacturers' performance specifications and installation procedures, I conducted tests of components and assemblies that comprised the lifting system. Dumper lift/lower speeds were correct. A prop-rod used to hold the dumper up for maintenance was in place, but was tight and difficult to position under the dumper. I confirmed the high forces needed to operate the Lift and Pump Drive controls. Also, when I pushed and pulled the control knobs, I observed a noticeable delay in control operation. I crawled under the truck, took photos and made a scale drawing of the arrangement of the lift and pump control cables and attached levers.

I compared the bend radius of the installed cables versus the manufacturer's specs. Both jacketed cables were jammed against a frame cross member. Each push-pull cable had a 4-inch or less bend radius, while the product data sheets specified a 12-inch minimum bend radius. When I pushed and pulled the control knobs, the cable jackets flexed and sprung enough to cause a delay in each control operation.

Finding the improper cable installation solved only half the mystery, leaving the question of why the dumper body lowered when the driver operated the pump drive control. I was puzzled until I saw the legends on the knobs didn't match the illustrations in the dumper installation manual. The knobs had been switched resulting in the pump drive knob operating the lift control valve, while the lift knob controlled the pump drive.