Machinedesign Com Green Palm Buttons
Machinedesign Com Green Palm Buttons
Machinedesign Com Green Palm Buttons
Machinedesign Com Green Palm Buttons
Machinedesign Com Green Palm Buttons

Inadequate Punch-Press Safety Measures Cost Operator Her Fingers

Sept. 8, 2009
Poor placement and maladjustment of safety interlocks cost a press operator six fingers.

A punch-press operator cycled the press once. When she reached in to reposition the workpiece according to standard procedure, the press cycled again, crushing fingers on both her hands.

In normal operation, the operator depresses a palm button on each side of the unit to initiate the press cycle. Releasing both buttons and pressing them again initiates the next cycle. Releasing either button at any point in the cycle activates the “antitie-down” mechanism, an air cylinder that disconnects the press-trip linkage and stops the press.

Two years earlier, the press was converted from its original foot-pedal control to the palm-button design. The foot-pedal control was also a “single- trip” mechanism requiring the operator to depress the pedal to cycle the press, release the pedal, and redepress it for the next cycle.

After the press was converted, an OSHA citation found the palm buttons were too close to the press platens. Officials determined that, based on press speed and reaction time, the palm buttons needed to be 115 in. away from the platens to ensure the press stopped before the operator’s hands could reach into it. The buttons were only 15 in. away.

Investigators examined the “single-trip” mechanism that should have stopped the press and found several problems. Bent and misaligned linkages were keeping the mechanism from releasing so the press cycled continuously when the palm buttons were depressed. In addition, the air cylinder was failing to activate the antitie-down safety mechanism because of a blocked exhaust port.

The press had no warnings alerting the operator to required maintenance on the single-trip mechanism or the danger of misalignment. Operator manuals for the press, single-trip mechanism, and antitie-down mechanism were also absent.

The single-trip-system literature called for a supply of clean, filtered, compressed air. The unfiltered air used in this case may have contained atomized oil, moisture, rust, or dirt that clogged the exhaust port. The installation instructions did not detail the hazards of using an unfiltered air source, nor did they specify preventive maintenance to keep the exhaust port from clogging.

OSHA and independent investigators concluded that the press design was inherently unsafe. The single-trip mechanism should have been protected from misalignment, and the machine should have incorporated point-of-operation guarding. Safety mechanisms must have built-in redundancy and shut down the machine when there is a fault anywhere in the system.

The press owners were planning to convert the press to an automatic-feed system with operator-side guarding. Conversion was put off for several weeks because of a delay in the automatic-feed system’s delivery. Unfortunately, the upgrade did not arrive in time to save the operator’s fingers.

Edited by Jessica Shapiro

This month’s safety violation comes from the files of Lanny Berke, a registered professional engineer and Certified Safety Professional involved in forensic engineering since 1972. Got a safety violation to share? Send your images and explanations to [email protected].

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