The Mechanical Handling and Storing of Material

that of magnets under 2 ft., or eight to Fig. 587. The Witton-Kramer Magnet. DISCHARGING BY MEANS OF SKIPS AND GRABS 415 immersed. Non-inductive resistances are employed to take up the induction of the coil when breaking the circuit. . . The wiring necessary for operating lifting magnets consists of a twin cable, which is led over a system of pulleys provided with a counterweight to keep the cable taut. In the case of jib cranes this cable is connected to the supply in the crane cabin, but for travelling cranes it is usual to install two trolley wires alongside the bridge and to provide special collectors to collect the current. A spring drum may sometimes be employed upon which ths cable is wound and unwound as the crane is in use. The method of working a magnet crane is similar to that of any electric crane, with the addition only of an extra controller for exciting the magnet when lowered on to the material to bs lifted, and for releasing the load at the destination. The holding capacity of magnets depends largely on the nature of the material which is to be handled, such as the magnetic quality of the load, the temperature of the material (which should not be above black heat), the shape of the material, and the.size of the pieces. For general guidance it may be assumed that a magnet is capable of lifting a solid piece of steel with machined surface of not less diameter than the magnet itself, having approximately a weight of fifteen times twelve times the weight of magnets from 2 to 3 ft. in diameter, and five to six times the weight of magnets from 3 to 5 ft. diameter. For such materials as sand-cast pigs, or heavy scrap iron, the lifting capacity decreases considerably, as the material does not conform to the surface of the magnet. When handling iron or steel plates the load depends on their size; sometimes up to six plates deep can be lifted according to the thickness and weight of the plates and the type of magnet. During the times the plates are pended on the magnet it is possible, by pass- ing the current through resistances, to drop one _ Lifting magnets are made round, oval, square, or oblong, and the surface of the magnet should also have a shape as near as possible to that of the objects generally lifted by it; for instance, if used for lifting iron pipes the lifting surface of the magnet should be hollowed out to fit, and by the same rule if used for a “ scull cracker the surface should be concave. Where long or bulky objects have to be lifted, a number of smaller magnets are suspended from girders or frames in order to cover a larger surface of the objects being dealt with. Lifting magnets are not only used for handling iron and steel in all its forms in the ordinary way of manufacture and shipment, as well as in engineeiing works, but thej are also great labour-saving appliances for breaking up cast-iron scrap, as the same magnet will not only handle it before and after it is broken, but will also lift and drop the ‘‘skull cracker ” with which the scrap is broken up. They can also be used for pile driving 01 to take the place of any other mechanical clutches. Lifting magnets are only constructed for direct current and are best driven from tie mains without the intervention of cut-outs. In some cases it is advisable to install an additional accumulator battery to work in parallel with the dynamo, so that should the latter break down, the accumulator will hold the load. sus- plate at a time and still hold the remainder.