John R. Yannaccone, P.E., Senior Mechanical Engineer
Case Synopsis: One morning, a wheelchair-bound student was being picked up by a school bus equipped with a commercial wheelchair lift. As the lift containing the wheelchair-bound student was being raised up to the bus, the lift continued to run and then began to fold to a stowed position. The attendant inside the bus was able to get the student safely into the bus as the lift was folding. The driver, who was operating the lift, caught her arm in the mechanism while attempting to prevent the student from falling as the lift folded.
Expert Analysis: Inspection of the wheelchair lift revealed it was in good condition, but did have some damage from extrication of the driver’s arm. The electrical components of the lift were inspected and tested, which failed to show any problems that would explain why the lift continued to run when the operator released the controller. Additional inspection of the solenoid, which provides power to the motor for the hydraulic pump, showed evidence of arcing and possible welding on the contacts. Welded/sticking of this contact would continue to supply power to the pump motor, and cause the lift to continue to run.
The wheelchair lift was designed with a safety system to prevent it from folding with an occupant on the lift. The system used an adjustable pressure relief valve, which limited the system pressure such that it was insufficient to fold an occupied lift. There was also a requirement that the lift be periodically tested with a fifty pound weight on the platform to verify proper pressure relief setting. When the system was tested as part of the investigation into this incident, the pressure relief valve was adjusted such that the lift would still fold with 220 pounds on the platform. This indicated that the pressure relief valve setting had been changed since it was last tested prior to the incident. This alteration of the pressure relief setting resulted in the system, designed to prevent folding of the lift in the event of an electrical malfunction, being incapable of stopping the lift when the solenoid contacts stuck closed.
Result: Prior to trial, a settlement was reached with all parties.
John R. Yannaccone, P.E., Senior Mechanical Engineer with DJS Associates, Inc., can be reached via email at experts@forensicDJS.com or via phone at 215-659-2010.
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