Press with Two-hand Control
Industrial accident: unexpected operation of a pneumatic-hydraulic (pneumohydraulic) press equipped with two-hand control interlock. The issues involved design, maintenance, and modifications to the press in regard to Stewart Ergonomics two-hand anti-tiedown control, including aspects of OSHA control reliability.
Initial onsite testing of the complete press failed to duplicate the circumstances of the incident. Possible cause(s) of the incident were only revealed during subsequent forensic testing of the Stewart Ergonomics electronic control unit as a separate entity. That analysis revealed an unexpected "latched" control mode in which the two-hand controller emitted a run signal even when both hands were not in the safe position.
That control mode, which was not known during the onsite testing, would allow the controller to power the down stroke ram without safe hands. This would occur because the Stewart Ergonomics would latch "ON" during the final phase of a power stroke in a manner that allowed the operator to remove hands from the unit. Therefore, a failure on the press control components that retracted the press ram before the cycle was complete produced an unsafe condition of which the operator was unaware. Because the Stewart Ergonomics controller was still latched, the control system design enabled the ram to perform a second unexpected down stroke that may have contributed to the incident.
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