The Mechanical Handling and Storing of Material

412 THE MECHANICAL HANDLING OE MATERIAL be said that they possess lasting efficiency; and the price, size, current consumption, and lifting capacity are in reasonable proportion to their own weight. They have, in fact, become satisfactory and reliable tools. The makers, however, have not been content to rest upon the satisfactory results thus far obtained with the type S. magnets, but have been constantly at work on improvements both in the direction of dependability and in increased efficiency in the handling of material. (unquestionably a successful lifting magnet must withstand more severe abuse and rough handling than any other type of electrical apparatus. In operation they are suspended from the hook of a crane, frequently not equipped with a lowering brake, so that it may be dropped at a high speed upon the material to be lifted j then it is swung when in use against cars, boxes, piles of pig iron, etc., and must be capable of with- standing the blows and shocks which result. It must operate under all weather conditions, and its insulation must withstand the voltage much higher than line voltage due to the inductive kick which occurs when the circuit of the magnet is opened. Its winding must not be injured by the heat which is generated within it, and preferably should also withstand external heat when the magnet is called upon to handle hot material. The accompanying illustration, Fig. 585. is a section through a typical S. A. magnet, and shows the arrangement of the parts, which by experi- ence have been found to meet the severe and exacting require- ments of the service. a is an annular casting of special electrical steel, forming the body or framework of the magnet; it is heavily ribbed on both the upper and outer surface, the ribs serving as cooling sur- faces. They are so disposed add to the cross section of the Fig. 585. Section through Lifting Magnet. as to stiffen the magnet case, and at the same time to magnetic surface. To this frame rings for supporting the magnet on the crane are also secured. B is the core of the magnet which is surrounded by the winding c. This winding is composed of a series of coils, each wound with a conductor in the form of a copper ribbon or strap, the turns of which are insulated with asbestos ribbon. Neighbouring coils are mechanically separated and electrically insulated by non-combustible insulating discs d. 1 he coils are wound upon a heavy brass form e, which resembles a spool with one head removed. This form serves to support the coils during the process of winding, and ensures coils of uniform and perfect shape, since there is no danger of springing or otherwise distorting the winding. After the last or uppermost coil is wound and the outer disc of insulation in place, the entire winding is rigidly clamped to the forms by means of radial straps f, which are bolted and locked in place, thus making the winding and the brass form which carries it a rigid unit. The completed winding is then dried in a heated chamber under a vacuum, and when the initial drying process is completed, the winding is impregnated with a plastic insulating compound—first under influence of a vacuum and then under air pressure, and finally is again dried under a vacuum. This results in a winding not only fireproof, but what is of still greater importance the winding is thoroughly waterproofed. The completed winding is then placed in the magnet case.