339. Electric Motors. Very often the cycle of operation of a hoist is of an intermittent character. The power required is at a maximum only a part of the time, even though the hoist may be operated practically continuously. From an economical point of view, these conditions give the electric motor-driven hoist special advantages, in that the electric hoist should always be ready, but using power only when in actual operation and then only in propor tion to the load handled. The ease with which a motor is moved, and the simplicity of the connection to the service supply, re quiring only two wires to be connected, are also in favor of the electric motor.
Fig. 136 shows a motor made by the Westinghouse Electric & Manufacturing Company, which is designed for the operation of cranes, hoists, or for intermittent service in which heavy starting torques and a wide speed variation are required. The frames are enclosed to guard against dirt and moisture, but are so designed that the working parts may be exposed for inspection or adjustment without dismantling. These motors are series-wound, and are designed for operating on direct-current circuits. The motor frames are of cast steel, nearly square in section and very compact. The frame is built in two parts, and so divided that the upper half of the field can be removed without disturbing the gear or shaft, making it easy to take out a pole-piece and field-coils, or to remove the armature. Fig. 137 shows the controller for this type of motor. These con trollers, when used for crane service, may be placed directly in the crane cage and operated by hand, or mounted on the resistance frames outside the cage, and operated by bell cranks and levers, so that the attendant may stand closer to the operating handles and away from the contacts and resistance.
Polyphase induction motors are being used to some extent for general hoisting and derrick work. These motors may be of the two-phase or three-phase type; but the latter are slightly more efficient. These motors are provided with resistances in the rotor circuit, and with external contacts for varying the same. Two capacities of resistance can be furnished: (a) Intermittent service, zero to full load ; and (b) Intermittent service, zero to half speed; and continuous service, half-speed to full speed. The controllers are of the drum type, similar to those used on street cars.
with either an electric motor or all engine, depending upon the general arrangement of the plant. These hoists are usually of the single-drum type.
341. Hoisting Buckets. In build ing construction, concrete is usually hoisted in automatic dumping buckets. The bucket is designed to slide up and down a light framework of timber, as shown in Fig. 138, and to dump auto matically when it reaches the proper place to dump. The (lumping of the buckets is accomplished by the bucket pitching forward at the point where the front guide in the hoisting tower is cut off. The bucket rights itself again automatically as soon as it be gins to descend. These buckets are often used for hoisting sand and stone as well as concrete. The capacity of these buckets varies from 10 cubic feet to 40 cubic feet. Fig. 139 shows a Ransome bucket which has been satis factorily used for this purpose.
342. Charging Mixers. The mixers are usually charged by means of wheel barrows, although other means are sometimes used. Fig. 140 shows the type of wheelbarrow generally used for this work. The capacity varies from 2 cubic feet to 4 cubic feet, the latter size being generally used, as with good runways, a man can handle four cubic feet of stone or sand in a well-constructed wheelbarrow.
In ordinary massive concrete construction, as foundations, piers, etc., where it is not necessary to hoist the concrete after it is mixed, the mixer is usually elevated so tha t the concrete can be discharged directly into wheelbarrows, carts, cars, or a chute from which the wheelbarrows or carts are filled. It is much better to discharge the concrete into a receiving chute than to discharge it directly into the conveyor. The chute ca.n be emp tied while the mixer is being charged and rotated; while, if the concrete is discharged directly into wheel barrows or carts, there must be sufficient wheelbarrows or carts waiting to receive the discharge, or the man charging the mixer will be idle while the mixer is being discharged. A greater objection is that if -the man in charge of the mixer finds that the charging men or conveying men are waiting, he is very apt to discharge the concrete before it is thoroughly mixed, in an effort to keep all the men busy. A platform is built at the elevation of the top of the hopper, through which the materials are fed to the mixer, Fig. 141. This is a rather expensive operation for mixing concrete, and should always be avoided when possible.