The Crane.—The crane consists of four parts, the mast, jib, sleepers, and guys or stays. The mast, which rises vertically, is connected at its base to the platform on which the engine stands; and at the top, to the guys by a pivot which allows of rotation in a horizontal plane. It may be of iron, balk timber of oak or pitch pine, or in two pieces of the same, strutted and braced. The jib, which may be built of the same materials as the mast, is fastened to the lower end of the mast by a joint which allows of rotary movement in a vertical plane. The steel rope or chain which supports the weight runs from the drum placed near the engine and over the top of the mast and jib. Wheels are placed at these points to lessen friction.
The combination of movement allowed by the pivot of the mast and joints of the jib, enables the load to be carried to any point commanded by the effective length of the jib, except that it cannot be placed behind the guys. Jibs are used up to 70 feet in length. To prevent slewing under wind pressure, jibs over 50 feet long should be fitted with wind brakes, especially on exposed situations.
The crane will stand the greatest strain when the jib is most upright, and, reversely, less strain as it approaches the horizontal. It is a good rule, and one which works for safety, not to allow the top end of the jib to reach a lower level than the top of the mast, whatever the weight of the load carried may be.
Cranes are made suitable for derrick staging to carry a weight of 7 tons. If the boiler is attached to the rotating platform of the crane, it helps to counterbalance the load.
Cranes, while offering the readiest means of dealing with heavy weights, do not give the best results when used for placing material in its final position on the building. The vibration of the engine, the swaying of the supporting rope from the jib, and the unevenness of lowering under the band brake, prevent that steadiness of the material which is necessary for good fixing.
Fig. 105
[Fig. 105] shows a small building crane; it is worked by manual power, and is very suitable for light work. The illustration shows the general method of construction, but there are other patterns which give greater power.
The crane is fitted with two hoisting ropes which are wound on the drum at A. One rope rises while the other descends. The ropes pass through the arms B, and when the catch C rises against the slot, it lifts the arm up. The base of the jib to which it is connected then rises in the sliding groove and swings inward, carrying the load well over the platform where it is to be deposited. When the new load begins to rise, the jib swings outward and downward, the rope paying out as required. By this means the jibs are in use alternately for lifting.
Pulleys.—The pulley ([fig. 106]) is a circular iron disc which revolves freely on an axle fitted into an iron box. The circumferential edge is grooved to receive the rope or chain which passes round it.