Electric motors are rapidly supplanting steam engines for hoisting work in many localities, and their use is particularly desirable in congested districts where the smoke, cinders, and steam from the boilers of steam hoisting-engines would be objectionable. Care must be taken to avoid shocks and burns when electric power is used, and therefore all parts of the electrical equipment should be inspected frequently and thoroughly, to see that everything is in good condition. Never permit unauthorized persons to tamper in any way with electrical apparatus, and see that all repairs and adjustments are made by competent electricians.

An electric hoist should always be operated in strict conformity with the directions issued by the makers of the apparatus. In certain forms, for example, it is important to keep the controller handle on the last contact point, after the machine has been brought up to speed, while in other forms it is permissible to keep the handle at other points, according to the speed desired. In every case, however, the operator should avoid advancing the handle too rapidly in starting the motor, not only because there is danger of burning out the coils if this precaution is neglected, but also because a too-sudden start may cause severe mechanical strains to be thrown on various parts of the hoisting mechanism. A suitable fuse or automatic circuit-breaker is essential to the safe operation of an electric hoist, and one or other of these devices should be provided in every case, and adjusted to act at not more than 50 per cent. overload.

Solenoid brakes form a desirable feature of the equipment of an electric hoist, because they provide an additional safeguard and are well worth the extra expense of installation. These brakes are applied to the armature shaft and operate automatically, but gradually, when the current fails for any reason, and also when the hoist is stopped.

Install switches and fuses of the inclosed type whenever possible, and do not permit the use of copper wire in place of proper fuses. See that all switchboards are railed off so that no unauthorized persons can get at them, and place rubber mats or coverings of other good non-conducting material in front of the switchboards, for the operators to stand on.

The power is transmitted to the winding drums of electric hoists of some types by means of sprocket wheels and chains, and these should be entirely inclosed or otherwise effectively guarded, to prevent the hands or clothing of the operators from being caught.

27. Signaling Systems

In the operation of material hoists and derricks it is important to have a signaling system or method that will be positive and reliable under all conditions. Signaling by motions of the hand (see Fig. 21) may be satisfactory when the signalman can be plainly seen by the hoisting engineer, but relaying the signals from one man to another and thence to the hoisting engineer is likely to result in accidents, due to the failure to transmit the signals correctly. Signaling by means of blasts on a whistle is also unreliable and unsafe, as a rule, because the noise from riveting hammers and from other sources makes it difficult for the hoisting engineer to hear the whistle distinctly.

The usual practice on large building operations is to install a bell or gong near the hoisting engine or motor, and to run a signal cord from the bell up through the material-hoist shaftway or near it, or, in the case of a derrick, to extend the cord to a point on the erecting floor where the signalman can plainly see the derrick and observe its movements.

In giving the signals for the movements of the derrick it is customary in many cases to give one stroke of the bell to stop when the derrick is in motion. A single stroke of the bell also signifies that the engineer should start to hoist when the derrick is not moving; and two strokes of the bell are given when it is desired to lower the load.

This system may be replaced with advantage by the more modern one of using three different signals, two strokes of the bell signifying to hoist, three strokes to lower, and one to stop. This code has the advantage of being perfectly definite. It also avoids the danger of giving a starting signal by the accidental fouling of the signal rope, since one stroke of the bell, on this system, has no meaning to the engineer when the load is stationary.

Fig. 21. Manual Signals for Hoisting, as Used in the United States Reclamation Service.

A similar system of signals is used in connection with derrick work to indicate desired movements of the derrick boom, - that is, to direct the hoisting engineer to lower the boom or to swing it to the right or left. In such a case it is necessary to have two bells or gongs, and the tone or sound of these bells should be distinctly different, in order that there may be no misunderstanding on the part of the hoisting engineer.

In very noisy places bell signals may often be advantageously replaced or supplemented by a system of electric lamps, the signals being given by using lamps of different colors instead of by sounding a number of strokes on a bell.