Machine Design

From the Safety Files: Poor dock leveler maintenance leads to injury

A driver, having backed his truck up to the loading dock, set up the dockleveler, a ramp with an extension lip to match the dock’s height to the truck’s. As the lip was extending, it hit the parked truck. The mechanical lip-extender continued deploying the lip after the impact, and a springlike tension built up until the lip jumped loose and pinched the worker’s hand.

The dockleveler’s designers intended that it be operated from inside the dock. A worker pulls a chain at the inside end of the ramp to release a spring that elevates the ramp to its full height. Once the ramp reaches the top of its travel, the worker uses his body weight to lower the ramp to the right height for unloading the truck. Fully elevating the ramp also engages the lip-extension mechanism so the lip of the dockleveler extends from a vertical storage position to a horizontal one as the ramp is relowered.

In this case, poor lubrication caused the dockleveler to malfunction frequently. Spring forces in the elevation mechanism weren’t enough to overcome both the weight of the ramp and the additional friction, so the ramp never reached the height needed to engage the lip-extender.

The truck drivers’ work-around was to use a hooked metal pole to pull the release chain from the truck side of the dock. Then they manually lifted the front edge of the ramp to engage the lip-extension mechanism. The easiest place to grip the ramp’s front edge is by the lip-hinge gap.

Because it is reasonably foreseeable that a truck and dockleveler could touch one another during normal operation, a safety fuse or shear pin should have been built into the mechanism to let the lip extender disengage if it encountered an obstacle. In fact, the dockleveler’s manufacturer sold an aftermarket “safety-enhancement kit” to address the problem, which it knew had caused hand injuries like those of the unfortunate driver.

The company maintaining the dockleveler did not know the kit was available. However, because drivers had repeatedly complained about the dockleveler problems, the company should have investigated the unit’s operation, maintenance procedures, and potential improvements. Management should have tagged the dockleveler out of service until the problems were resolved.

This month’s safety violation comes from the files of Lanny Berke, a registered professional engineer and Certified Safety Professional involved in forensic engineering since 1972. Got a safety violation to share? Send your images and explanations to [email protected].

Edited by Jessica Shapiro

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