Machine Design

Poor Hydraulic-Mechanism Adjustment Led to Worker Injury

A refuse-collection worker was operating his truck’s lift mechanism to dump a 50-gallon recycling cart when the lift broke. The cart fell to the ground and hit the worker’s knee.

The mechanism lifts large refuse containers up the side of the truck and tilts them. The roof of the truck opens to let container contents fall into the cargo area.

About five months after workers started using the truck, the lift mechanism broke down. A technician from the manufacturer adjusted the throttle advance from 900 to 1,300 rpm, sped up the machine’s cycle time to 15 sec, and replaced broken lever arms on the passenger side of the cab and a damaged hydraulic cylinder on the roof.

Two weeks after the service visit, the lift failed catastrophically, dropping the full recycling cart that injured the worker. The lift controls were outside the truck cab, so the worker was standing near the lift when it failed.

A second technician repaired the lift mechanism. The immediate cause of the failure: Two of the mechanism’s four eye bolts, which had connected lifting lever arms to hydraulic cylinders, had sheared off.

The technician replaced all the eye bolts and added jam nuts. He also reset the cycle time — which was now too fast — to 12 sec and bumped hydraulic-line pressure down from 1,800 to 1,200 psi. According to the machine’s operating and maintenance manuals, a pressure relief valve should limit system pressure to 1,000 psi.

These maintenance visits pointed to a combination of incorrect or missing adjustments at the factory, incompetent maintenance, a mechanism that quickly drifted out of adjustment, or a combination of all three. There was no documentation that component settings had been checked at the factory, nor were there instructions on how users could check or readjust the machine after delivery.

The manufacturer should have double-checked adjustments when the truck went out the door, better trained its maintenance personnel, and provided thorough documentation. In addition, the mechanism should have been designed to operate safely under reasonably foreseeable errors in adjustment. Warnings on the equipment could have alerted operators to hazards related to incorrect adjustment.

The placement of the lift controls near the lift was another hazard that contributed to the accident. Placing the controls in a safer location could have protected the worker from the lift mechanism’s failure. Failing that physical protection, the mechanism should have carried warnings alerting the operator that carts or components could fall and cause injury.

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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