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The Case of the Wacky Walkie
Myron J. Boyajian, Contributing Editor -- Design News, September 24, 2007
The handy, electric-powered walkie has two fork-like projections supported by load rollers that are slid under a pallet and raised by a motor-pump unit. A tiller arm steers the vertically pivoted motor/transaxle unit that powers the elastomer-covered, pressed-on drive wheel.
I was retained to inspect a walkie that injured its operator's hand and forearm. Medical and rehabilitation costs were paid, but my client, the comp carrier, hoped to recover its costs. Accident causes could include negligent design, assembly, maintenance or operator misuse.
The Scene of the Crime
During the morning trailer-loading operation at a Chicago-area, big-box food chain warehouse, the walkie driver alleged he picked up a load and then accelerated up the dockboard into the trailer. He slowed to position the pallet next to the trailer wall and then accelerated to snug his pallet against the pallet ahead. He said that as he did so, the steering tiller arm violently swung to his right, wrenching his right arm and slamming his hand against the trailer's wall. He was treated for a strained arm muscle and mashed fingers.
The Investigation
For my inspection, a spotting tractor backed the subject trailer against the accident site dock. As at the time of the accident, the trailer was supported with jackstands and the tractor moved away. I measured and noted trailer length, width, floor condition, slope and height of the trailer floor, dock incline and dock height. I had already reviewed driver and witness statements, loading records and walkie service records.
I then visually and operationally checked the subject walkie. Unloaded, on level ground, operation seemed normal except for a noticeable wiggling motion. With a test load equal to the load at the time of the accident, acceleration using first and second speed of the resistor control was relatively smooth. However, when I rapidly twisted the speed control handle to engage top speed from a standstill, there was a twisting or turning effect on the steering tiller. Third speed applied full battery voltage to the drive motor without time delay.
Driving forward with the forks leading, the tiller jerked to the right. Driving in reverse with the forks trailing, the tiller snapped to the left. OK, I mused, if this condition prevailed at the time of the accident, could the jerky tiller have slammed the driver's hand against the trailer wall? Even if the jerky tiller didn't seem uncontrollable, what could cause this crazy behavior?
I drove the loaded walkie into the trailer. From the level dock floor, I traveled up the dockboard into the trailer. I stopped and re-started as though positioning the load. The combined slope of the dock plus the trailer on jackstands was about 5 percent, requiring full throttle to move the loaded walkie forward. As I hit third speed, the tiller arm snapped to the right as the truck began to move. After several repeats, I backed the walkie out of the trailer for further inspection.
The Smoking Gun
This walkie received regular maintenance and appeared to be in good order. A new drive tire and load wheels were installed the night before the accident. However, the new drive tire was offset by an inch to the right side as viewed with the forks leading. In a moment, the picture became clear. Normally, the press-on tire is centered on the drive wheel and steering is unaffected by motor torque. With the offset tire, full battery voltage (third speed) caused the drive motor to deliver enough torque to jerk the steering tiller.
The loaded weight of the walkie was 5,100 lb. The force at the drive wheel/floor interface needed to climb a 5 percent grade (2.86 degrees) equaled the force to overcome gravity (5,100 lb x sin 2.86 degrees = 254.5 lb) plus the rolling resistance ([5,100 lb x 50 lb/ton x cosine 2.86 degrees] ÷ 2,000 lb/ton = 127.3 lb) = 381.8 lb. That force on the 1-inch offset tire produced a torque of 381.8 inch-lb about the tiller pivot. This torque caused a sideways force just over 13 lb at the handle of the 2-ft-long tiller. A force gauge confirmed the calculation. Jerky? Yes. Uncontrollable? Possibly, but I didn't think so.
I concluded my inspection and prepared my report citing the offset tire and evident force on the tiller handle. I opined that the force, while not proper, was not uncontrollable and that the driver's injury could have been due to driving error resulting in his hand striking a fixed object. I later learned that a nominal settlement with the maintenance company was reached. The drive tire was later centered properly and the walkie returned to service.
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