GB1184919A - A Method of Lifting and Conveying Objects of MagnetizableMaterial, Preferably Iron Sheets and a Plant for Carrying the Method into Effect - Google Patents

Abstract

1,184,919. Feeding sheets. H. NIELSON & SON MASKINFABRIK A.S. 19 May, 1967 [4 Aug., 1966], No. 23483/67. Heading B8R. [Also in Division H2] In a method of lifting and conveying magnetizable iron sheets by at least one lifting magnet and in which the supply to the winding of the magnet(s) is cut off for a brief period to separate the uppermost sheet from a number of other underlying sheets adhering to it, the current being subsequently cut in for retention of the uppermost sheet, a voltage impulse induced in the winding of the lifting magnet(s) caused by the fall of one or more sheets is fed to an electronic unit which, after a determined first time interval again reinstates the magnet current. After another determined time interval the current may be again interrupted then reinstated a determined number of times to ensure that all the sheets, except the uppermost one, have been dropped prior to lifting magnet moving to the destination of the uppermost sheet. One or more lifting magnets 1 are energized from a three-phase supply R, S, T, through a rectifying unit including thyristors 5, 6, 7, control electrodes 8 of which are connected to a trigger circuit 10 which at determined intervals supplies firing pulses to the thyristors. After current through the magnets is interrupted and when one or more sheets begin to fall, the voltage induced in the magnet windings appears through a diode 16 across a transformer winding 15 and is fed through a secondary winding 53 to a control unit 11 which causes the thyristor firing pulses to be reinstated, then interrupted repeatedly for a fixed period within which all the sheets except the uppermost are shaken off. In operation the magnet is lowered and on contacting the uppermost sheet, a contact 18a disposed on the yoke of the magnet causes the hoisting mechanism to reverse and raise the magnet, and a contact 18b energizes a time measuring device (TH) which cuts out the hoisting after a determined time which determines the height to which the sheet stack is to be lifted before the lowermost sheets are to be shaken off. The shaking off circuit is then energized subsequent to which the uppermost sheet is transported to its destination. Where a number of magnets are employed a time delay is introduced before reinstatement of the supply as some magnets may release before others and give rise to voltage impulses which will cause the supply to be resumed too early. Some of the circuit functions are performed by logic elements.