GB819898A - Improvements relating to magnetic holding devices - Google Patents

Abstract

819,898. Magnets. NEILL & CO. (SHEFFIELD) Ltd., J., and BOWER, W. L. June 2, 1958, No. 17531/58. Addition to 758,183. Class 35. [Also in Group XXIII] The magnetic work-holding device of the parent Specification employing a magnetic circuit extending between magnetically-separated polepieces and including a permanent magnet and an electromagnetic coil through which a direct current is passed to magnetize the permanent magnet so that a workpiece can be held on its operative surface, is modified by the use for demagnetizing the permanent magnet of a direct current which is passed through the coil in the opposite direction to the energizing current and is automatically interrupted when it reaches a predetermined value. The current is passed in one direction or the other through the coil by selective operation of push-buttons MAG, DEMAG controlling contactors C1, C2 which are interlocked to operate alternatively and actuate switches in the circuit from an alternating power supply and rectifier RECT to the coil. During magnetization of the magnet, a timing relay TR is connected in the circuit to control the length of time the current passes through the coil to energize the magnet. When the push-button DEMAG is operated, the switches in the circuit are actuated to reverse the direction of current through the coil which, because of its high inductance, restricts the rate of growth of the current which, upon reaching a predetermined value, operates an adjustable relay OL to interrupt the circuit. The circuit is so arranged that the push-buttons can only be operated alternately to energize and de-energize the magnet.