Auxiliary electrical equipment for motor-operated cranes

This is one of a series of papers on the selection of electrical apparatus for cranes and deals with the brakes, overload protective panels and limit switches. Other papers of the series cover the requirements as to motors and control. A magnetic friction brake is needed for every crane hoist, in addition to what may be provided in the way of dynamic braking or mechanical brakes, but its required characteristics are very definitely affected thereby. In selecting a magnetic brake the character of the service must be well understood, and the part that it plays therein, and it must possess an adequate energy dissipating capacity. A definite formula for such selection is given. The paper discusses the various service requirements and describes the several available types of magnet brakes and their particular fields of application. Limit switches, while occasionally employed to limit the travel of the trolley or bridge motions of a crane, are universally used to limit the upward travel of the hook block. Hoist limit switches may be geared to the machine or directly operated by the block. The former do not take into account the stretching of cables and require complete readjustment when new cables are installed. Switches operated by the hook block are the surest. The paper describes the various forms of geared and direct-operated limit switches and points out their relative advantages. Considerations of safety often demand that the operation of the switch not only disconnects the motor but simultaneously closes a dynamic braking circuit for quick stop. The history is given of the evolution of the modern crane overload protective equipment and devices. Fuses and railway type circuit breakers have been tried, but up-to-date equipment employs overload relays in each motor circuit operating in conjunction with magnetic switches. In the interests of safety it is possible to cut off all power instantly by tripping a switch that causes the magnetic switches to open. Safety locks are provided that permit of locking the entire crane equipment against operation. The best results are secured by properly inspected time-element overload relays in each motor circuit and a common-return instantaneous relay.