Safety Study of OSHA's Experimental Variance on Self-Tripping of Power Presses, Contract No. 210-80-0034, Phase II - Volume 1. A Safety Engineering Assessment of the Light Curtain -- Power Press System.
Quality, safety, and reliability considerations of an electro optical sensing device (light curtain) for safe hand feeding of automatically activated mechanical power presses are discussed. Self tripping light curtains are designed to stop movement of the press slide when the operator's hand penetrates the curtain during loading, and to resume movement after withdrawal of the hand. Under specifications of OSHA regulation 1910.217, only light curtains without self tripping capability are permitted on mechanical power presses in the United States. A variance was granted to one company, based on better safety and higher performance of self tripping light curtains compared to two hand controls. Safety assessment of these devices can include safety of individual components and system integration of these components. Today's technology allows for construction of light curtains which would have a no accident probability of 99.9994 percent over 50 years for 100,000 presses. Safety of the light curtain component depends on design, construction, manufacturing quality, and conditions of use. It should be designed to minimize the possibility of its circumvention by machine operators. Other components discussed include two hand or palm buttons, clutch and brake, redundant or auxiliary brakes, solenoid air valve, brake monitors, additional guarding, and cam setting. Safety aspects of system integration include installation, inspection, and monitoring. Liability is also discussed.