A lift truck allegedly starting moving on its own, and the control system is a prime suspect
By Myron Boyajian, Contributing Writer
About three years ago, I was retained to help investigate a forklift that allegedly started moving on its own as the driver climbed into his seat. The lead mechanical engineer requested my expertise in electric forklift controls to help determine the cause of the accident.
The driver said that he drove forward and stopped in a narrow aisle between storage racks. He dismounted the truck to pull components from the storage rack to place on a pallet on the truck’s forks. As he climbed back into the driver’s seat, the truck suddenly moved backward, pinning him against the rack beams. His cries for help brought co-workers running. They disconnected the battery to stop the truck as it continued to back into the driver, who was hospitalized with leg fractures.
We went to the plant to perform tests to see if there was any factual basis for the driver’s claim. The seat switch tested OK; control power off with the seat empty and power on with seat occupied. There was another similar driver complaint against that forklift, so our tests focused on operation when getting into the seat with the key switch turned “on” and the direction switch set to “forward.” We raised the drive wheels off the floor to prevent accidents.
Repeatedly pushing down the seat cushion to close the seat switch, we observed that, at random times, the reverse direction contactors closed and the drive wheels accelerated in the reverse direction. This seemed to confirm the driver’s statement, but the task now was to determine if this behavior was due to issues of control design, control breakdown, or improper maintenance or adjustment.
The subject forklift was one of a small fleet of identical units brought over from another of this company’s plants, so we drove the subject and one of the sister trucks for comparison. I expected identical operating characteristics, but it wasn’t so. The sister truck simply coasted, power off, to a stop when the accelerator was released. However, on the subject truck, when the accelerator was lifted, the reverse direction contactors closed briefly and the subject truck decelerated forcefully!
With company-supplied schematics, I checked the circuit wiring, but soon found that the reference numbers on the schematics didn’t match the control part number in the subject vehicle, but did match the control part numbers in the sister truck.
The subject truck had a control system that was unique from the other trucks in the fleet, both electronically and operationally. It automatically “plugged” the drive motor when the accelerator was released. “Plug” is an industrial term that describes applying reverse power to a motor. Most electric forklifts simply coast with gradual slowing when the accelerator is released. However, in the subject forklift, the control actually reversed electric power to the drive motor to provide a slowing effect that is much like the slowing one feels in an automobile when the gas pedal is released. When the forklift slowed sufficiently, the slowing function ceased and normal operation resumed. This is the auto-plug function.
At the time of the accident, the driver stated that his forklift was parked with the direction switch in forward. When he began to climb aboard, he moved the seat, closing the seat switch, and the forklift moved in reverse, striking him. Except for being parked and the seat raised, this action mimicked automatic plugging, i.e., forward direction selected and reverse contactor closed, powering the drive motor in reverse! This seemed to indicate a control malfunction.
Despite repeated requests, I was never supplied with correct schematics or shop manuals to facilitate checking the control board for correct operation. I was repeatedly stonewalled by the control manufacturer and its franchised re-builder, leaving me with no way to check correct control operation. I surmised that the auto-plug feature was trouble-prone and was withdrawn from the market. This case made me very uneasy and I did not render an opinion report, and finally withdrew my services. I called the insurance adjuster about a year later and found that all parties had reached an unspecified settlement.
Myron J. Boyjian, P.E., firstname.lastname@example.org) is president of Engineering Consultants, a consulting service for forensic and design activities. The situation presented here is from his case files.