Lift trucks can transport, lift, and lower loads of ALL kinds. In an electric forklift, the operator controls these functions with transistors, thyristors, and other solid-state components. The operator in this story used a multifunction "joystick" to control travel speed, travel direction, and load elevation.
Scene of the Crime
At a Seattle plant where she worked, the driver was using the stand-up, end-control vehicle to move loads to and from storage racks. Skillfully gliding down the narrow aisles, the driver looked like Bobby Fischer moving chess pieces in a speed match at a high-school tournament. She was delighted with the smooth operation of her lift truck in sharp contrast to her anger and frustration of the prior week when that same machine repeatedly jerked and stopped.
By that Friday her cranky machine was sidelined and her shift supervisor called the dealer for service. Anxious to please this major service and sales account, the technicians quickly got the vehicle operational, even though it was a different brand than the one they sold. The technicians said that the vehicle was usable, and they promised to return the following week to "fine-tune" it.
On Monday, the supervisor gave the OK to drive the forklift. During a run, the driver placed a load in the racks and accelerated forward. At that time, another vehicle entered the aisle. Our driver stopped and then backed down the aisle. As she did so, her machine shot backward at full speed and crashed through a cement-block wall. The falling cement blocks pushed into the open driver's compartment, crushing her legs. The driver retained an attorney who filed a lawsuit naming the forklift manufacturer and service company. I was retained to determine the cause of the accident.
At the inspection, the potentiometer and switches in the multi-function hand control operated smoothly and in the correct sequence, and a thyristor used to control drive motor speed was not short-circuited. There was significant pitting on the tips of the directional and the bypass contactors.
Power contactors control heavy electrical currents: one for a hydraulic pump motor for lifting and tilting loads, and two contactors to control drive motor direction. Drive motor speed is controlled by a thyristor pulsed at varying frequency and pulse width to obtain a varying average voltage. Due to certain circuit switching events, the thyristor cannot achieve 100 percent on-time, so a thyristor-bypass contactor applies full voltage to obtain full motor speed.
Thyristors can sometimes lock up at full conduction. Ordinarily, no harm occurs because a safety circuit monitors the thyristor during pulsing. If not pulsing when called for, the circuit opens the direction contactors, breaking the circuit. This circuit also monitors the thyristor and bypass contactor. At start-up, the circuit blocks contactor closure if the thyristor is internally short-circuited, or the contactor tips are welded closed.
I concluded that the pitted bypass contactor tips had welded during the previous forward travel movement. Upon reversing, the directional contactor tips welded and the truck shot back at top speed. The mystery was why the safety circuit failed to detect the welded bypass contactor and block the contactors to prevent movement.
Records showed that the service technicians found that a faulty safety circuit caused the nuisance tripping. They expedited putting the vehicle back into service by removing the safety circuit module that could have detected stuck bypass contactor tips.
Without available parts, they planned to return to install a new safety circuit and to replace all contactor tips. They did not warn the customer against using the vehicle, nor did they red-tag itóbut indicated that it was OK to use. Because these technicians failed to sideline the vehicle, or to warn of its condition, it became an accident waiting to happen.
I concluded that product design was not an issue, but that the technicians failed to follow the procedures described in ASME/ANSI and OSHA regulations, and the service manuals. This case settled before trial.