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The Case of the Cobbled Contractor
Myron J. Boyajian, President, Engineering Consultants -- Design News, November 16, 2008
Safety equipment is often a quiet sentinel, waiting to protect life or property. Safety equipment, improperly called into action, can have just the opposite effect.
The Scene of the Crime
At a wholesale grocery warehouse, the operator of an order picker forklift sweated in the hot summer air among the top racks. Hustling back and forth, he rode up and down, stopping at various levels to pull boxes full of canned goods this time and cake mix the next time. He was relieved to see the last items on the order pull ticket were in an older section of the warehouse. Its lower roof line promised cooler (well, OK, less hot) air blowing about, so he already felt refreshed.
He raised the platform and released the lift control, but the platform kept rising! The overhead bars of the operator's cage struck a fire sprinkler, instantly cooling him uncomfortably as water from the smashed sprinkler gushed down. A fire alarm tripped and signaled the local fire department, while the fire zone system turned on close-by sprinklers. Overcoming his shock, the operator hit the emergency battery disconnect, but the damage was done. The watery cascade insured there would be no “Snap, Crackle and Pop” left in the cereal boxes in his order. The fire department arrived and the water was shut off. The shift supervisor reached into the truck to manually lower the lift, bringing the soaked operator and his soggy groceries to the floor. There were no personal injuries, but there was significant material loss.
The damaged groceries were insured, but the insurer sought compensation from the forklift manufacturer and a forklift service company who both argued the operator failed to make sure of overhead clearance. The insurer retained me to determine if there was a forklift malfunction and its cause.
The Investigation
On this truck, the operator's platform rode up and down with the forks and travel and lift/lower controls were connected remotely via electrical cables. The lift control energized a motor-driven pump that sent fluid through a check valve to the lift cylinder. The lift cylinder, via the mast, raised the operator's compartment and forks. When the motor-pump unit stopped, the cylinder remained pressurized at the desired extension. To lower, the operator's control energized the lowering solenoid valve letting fluid from the lift cylinder bypass the check valve to flow back to the reservoir.
At the inspection, the truck seemed to operate OK. The operator reported occasional malfunctions during lifting, but control was regained by rapidly operating the lift/lower lever. Service records showed that the subject forklift received a preventive maintenance check only a week before the accident. We all took turns operating the lift/lower control which continued to operate correctly, leading one to believe the operator had, indeed, slipped up while lifting. Then, the lift function did lock up, continuing to lift when the handle was released. One fellow hit the battery disconnect to stop the lift. I operated the plunger on the lowering solenoid to lower the lift.
The Smoking Gun
I insulated the contactor tips to prevent motor operation, put a voltmeter on the terminals of the contactor coil and operated the lift control. Coil voltage was present only when lifting was called for. The contactor operated when energized, but I could see rough, arc-damaged contact tips. The contactor was removed to a bench top for all to see. The badly worn tips had no silver alloy plating left and striations showed the tips had been filed, a definite no-no. Even worse, the arm carrying the movable contact had been bent to compensate for the badly worn tips. The result was no wiping action, a designed-in slight sliding motion that assured good contact on closure and a clean break on tip opening.
My opinion report cited the serviceman's failure to replace the worn tips and failure to follow the manufacturer's service bulletin that called for installation of an available, new-design contactor with more reliable double tips. The service shop settled with the warehouse's insurer.
