4. Cranes
Cranes are material handling equipments designed for lifting and moving heavy loads. Some of the important types of cranes are bridge cranes, gantry cranes and jib cranes. These are discussed as follows.
4.1 Bridge crane
Fig. 4.6.6 Bridge crane
It consists of one or two horizontal beams supported between fixed rails on either end as shown in Fig. 4.6.6. The hoist moves along the length of the bridge, and the bridge moves along the rails. The x- and y-axes movements are provided by the above said movements and the hoist provides motion in the z-axis direction. In the bridge crane, vertical lifting is due to the hoist and horizontal movement of the material is due to the rail system. They are generally used in heavy machinery fabrication, steel mills, and power-generating stations.
4.2 Gantry crane
Fig. 4.6.7 Gantry crane
These types of cranes have one or two vertical legs which support the horizontal bridge. The bridge of the gantry crane has one or more hoists that help in vertical lifting as shwon in Fig. 4.6.7. Gantries are available in a variety of sizes. A double gantry crane has two legs. Other types of gantry cranes are half gantries and cantilever gantries. In a half gantry crane, there is a single leg on one end of the bridge, and the other end is supported by a rail mounted on the wall or other structural member. In a cantilever gantry crane the bridge extends beyond the length of support legs.
4.3 Jib crane
Fig. 4.6.8 Jib crane
It consists of a hoist mounted on a horizontal cantilever beam which is supported by a vertical column as shown in Fig. 4.6.8. The horizontal beam is pivoted about the vertical axis formed by the column to provide a horizontal sweep for the crane. The beam acts as a track for the hoist trolley to provide radial travel along the length of the beam. The horizontal sweep of a jib crane is circular or semicircular. The hoist provides vertical lifting and lowering movements.