Crane, a hoisting machine widely used for handling materials at building construction sites, factories, warehouses, and shipyards. Cranes move heavy loads by means of ropes or cables that ride over pulleys. Nearly all of them move loads both vertically and horizontally. Machines that can only raise and lower loads are called hoists, or winches. Small cranes are operated by a hand crank, most large cranes are powered by electric motors or diesel engines. Originally large cranes were powered by steam engines.
Structure of Cranes. Among the principal components of cranes is the jib, a horizontal or inclined beam with structural supports to prevent it from bending. The jib supports either a fixed pulley system or a movable pulley system that can travel along its entire length. The jib is usually attached to a vertical pillar, or column, about which it can be rotated. In some cases a wall of a building can serve as the pillar. The jib is then attached to the wall with brackets. The pulley system of ropes or cables are wound onto a drum, or cylinder, that is turned by a hand crank or a motor. Some cranes are mounted on stationary supports, while others are mounted on movable bases. Special cranes may have a variety of additional features.
Types of Cranes. There is such a variety of cranes that they are not classified in one system. Some are classified by shape, others by the function they perform, and still others by their form of mobility. Cranes are also classified according to the greatest load they can move; on this basis they vary in size from 1 ton to over 250 tons.
Derrick. A derrick is a special type of crane in which the distance from the end of the jib to the pillar can be changed. A guy derrick consists essentially of a pivoted pillar that is braced by guy ropes, and a jib that is attached at the base of the pillar. The inclination of the jib is controlled by a tackle that connects the top ends of the pillar and jib. The hoisting rope extends from the end of the jib, over the top of the pillar, to the base of the pillar, where it is fastened to a drum. A tower derrick is similar to a guy derrick except that its jib is movable and may be secured at different heights on the tower, or pillar. Tower derricks are used mostly in building construction.
Pillar Crane. This type of crane has a self-supported column, or pillar, that can be turned on its vertical axis. A horizontal or inclinable jib is attached to the pillar. The load is raised or lowered from the end of the jib.
Portable Jib Crane. The portable jib crane is small and is attached to the columns or walls of a building by means of simple adapters, or brackets that are placed at the required locations. Portable jib cranes are particularly useful in various areas of shops and warehouses.
Cantilever Crane. This type of crane has a jib that extends outward from a supporting column that generally cannot be rotated. The jib is usually counterbalanced. Cantilever cranes are sometimes simply called jib cranes. A balance crane is a cantilever crane in which the jib extends on two sides of the column. The load is supported on one end of the jib, and a movable counterweight is supported by the jib on the opposite side of the column.
Traveling Cranes. There are several types of traveling cranes. One of the most common is the traveling jib crane. Its pulley system is suspended from a trolley that moves along the length of the horizontal jib. Traveling bridge cranes consist essentially of two elevated tracks that are bridged by a girder that travels along the tracks on wheels. The hoisting unit can be moved back and forth along the girder bridge. Gantry cranes are traveling cranes that are designed especially for use outdoors or where an overhead runway is not practical. Gantry cranes have a bridge that is supported at both ends by vertical columns, or legs. The legs are mounted on wheels or rollers so that the entire structure can move along rails or tracks.
Truck Crane. This type of crane is mounted on a truck that moves on large wheels or on tractor caterpillar treads. Cranes that travel on caterpillar treads are often called crawler cranes. Truck cranes are quite versatile because they are self-propelled and can be equipped with accessory booms, or jibs, to extend to about 150 feet (46 meters) high. Locomotive cranes are similar to truck cranes, but are designed to travel on standard gauge tracks.
Floating Crane. Hoisting and transporting cargo to and from ships are often done with floating cranes, which are mounted on pontoons or barges. Floating cranes are also used for water work such as driving piles and raising sunken vessels and other objects.
Cable Crane. Materials such as earth and rocks are often lifted and transported by cable cranes. The load is carried in buckets attached to a cable that moves between two towers.
Joseph Datsko, University of Michigan
Source: "Crane." Encyclopedia Americana. Grolier Online http://ea.grolier.com/cgi-bin/article?assetid=0112310-00
(accessed August 13, 2007).