Patent GB-1466367-A (GB)
Title: Circuit for enabling safe actuation of a machine
IPC: F16P3/20
Filed: 1975-10-07
Abstract: 1466367 Machinery safety devices GENERAL ELECTRIC CO 7 Oct 1975 [7 Oct 1974] 41017/75 Heading F2J A two-handed safety operating device for a press (not shown) comprises two separate manual push-button switches 10, 12 which have to be closed simultaneously to actuate a main contactor or relay 16 to start the press, which relay 16 has a coil 18 and a normally open contact 20. As shown, each switch 10, 12 has three terminals 32, 34, 36 and 40, 42, 44 the latter terminals 36, 44 of which are permanently connected across a capacitor 14. The capacitor 14 is charged by a voltage step-down transformer 22 having primary and secondary windings 24, 26, a polarizing rectifier 28 and a current limiting resistor 30, the transformer 22 serving to reduce the voltage magnitude of an A.C. power source, controlled by a switch 50, to prevent damage to the capacitor 14 and relay coil 16, and provide a safe low voltage level across the manual switches 10, 12. The push-button conductive poles 38 and 46 of switches 10, 12 are permanently respectively connected to terminals 36 and 44 and are normally (i.e. when the pushbutton poles 38, 46 are not hand operated viz. depressed) in electrical contact with respective terminals 32, 40 which are connected across the secondary winding 26 via the resistor 30 and rectifier 28. The primary winding 24 of the transformer 22 is connected across the source of A.C. power via a protective line fuse 48 and the main A.C. power switch 50, a pilot lamp 52 being connected across the primary winding 24 to indicate whether or not power is applied to the circuit. The normally open contact 20 of the relay 16 is connected across a further source of A.C. power by a solenoid coil 54 which controls the enabling of the operation of the press. In operation, and with the capacitor 14 in a discharged state, closure of the main power switch 50 lights the pilot lamp 52 and via the transformer 22 applies a reduced voltage across the secondary winding 26 thereof to charge the capacitor 14. By simultaneous manual operation of the two push-button switches 10, 12 the respective switch poles 38, 46 are moved i.e. depressed to respectively engage terminals 34, 42 to discharge the capacitor 14 through the coil 18 of relay 16 whereby the normally open contact 20 of relay 16 is closed to apply A.C. power directly across the solenoid 54 to start the press. When the push-button switches 10, 12 are released, the switch poles 38, 46 again engage terminals 32, 40 to recharge the capacitor 14 ready for re-starting the press at the end of its operating cycle. Thus in order to perform a series of repetitive press operations both switches 10, 12 must be repeatedly jointly depressed and released, the arrangement avoiding misuse by the operator, i.e. by short-circuiting one or other switch 10, 12 to save time, by ensuring that unless both switches are jointly operated the press cannot be started since if only one switch 10 or 12 is operated when the other switch 12 or 10 is short-circuited, i.e. held down permanently by for example adhesive tape, then the capacitor 14 cannot be recharged after a single operation of the press.