Machine Design

Power Trowel with Poorly Placed Kill Switch Injures Worker

A worker lost control of the walk-behind power trowel he was using to finish a concrete slab. The machine went on to injure another man on the work crew.

Witnesses said one of blades on the trowel used to apply a surface finish to curing concrete caught on an electrical conduit about 6 in. from the edge of the slab. The torque from the trowel’s 8-hp motor was enough to pull the handle of the trowel out of the operator’s hands.

Most trowels are equipped with an automatic-kill device, such as a lever the operator must squeeze against the machine’s handle to keep the machine running. If the operator releases the lever for any reason, the motor stops.

The trowel involved in this incident had a so-called fingertip throttle. The operator could adjust the throttle with his fingertips but was not required to hold it in place to keep the machine running. The tip of the throttle lever was about an inch from the right grip. The machine had an on-off kill switch, but it was 10 to 12 in. from the left grip, on the handle stem.

Once the operator lost contact with the machine, there was no way to turn it off. Witnesses said it “went wild,” spinning around until it worked itself free from the conduit and moved down a nearby trench where it came to rest.

During the machine’s uncontrolled movement, it hit the other worker, giving him a herniated disk that required surgery and time off to recuperate.

An information placard on the handle stem gave lubrication instructions, but did not provide any warnings about the dangers of operating the machine.

A better design that ensured the machine would stop if the operator lost contact with it could have prevented this accident.

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

© 2010 Penton Media, Inc.

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