Crane

The crane is an invention of ancient origin that is used to move heavy weights in both the vertical and horizontal directions, to load and unload heavy objects, and to construct tall buildings. Cranes can move objects weighing up to several hundred tons, depending on their design capacity, and they can be powered by human or animal power, water power, steam, internal combustion engines (gasoline or diesel), or electric power. One common forerunner of the crane was the shaduf, prevalent in Egypt and India around 1500 B.C. Employed by a single person for lifting water, the shaduf consisted of a vertical support, a long, pivoting beam, and a counterweight.

The first true cranes, founded on the principles of levers and counterweights, used a pulley system fixed to a single mast or boom. Lifting power was provided by humans or draft animals operating a treadmill or large wheel. Eventually, a second mast and guy wires were added to increase the strength and stability of this early form of crane.

One of the most significant developments in crane design, which probably occurred during medieval times with the advent of Gothic architecture, was the jib crane, which features a pivoting horizontal arm (a jib) that projects outward from the top of the boom. The addition of hinged movement to the outermost section of the jib allows for even further versatility and movement.

Jib cranes are also known as derrick cranes. Derrick is the term originally applied to gallows structures when Englishman Godfrey Derrick was a well-known hangman. Today, the derrick is a large hoisting machine similar in most respects to the crane, except for its typically stationary foundation. Oil derricks, for example, are specialized steel towers used for raising and lowering equipment for drilling oil wells. One of the most powerful cranes, a barge derrick, is a double-boomed structure capable of lifting and moving ships weighing up to 3,000 tons (2,700 metric tons).

Other cranes with specialized uses include the cantilever crane featuring a suspended horizontal boom and used in shipyards; the overhead traveling crane, also called a bridge crane, that is guided by rails and a trolley-suspended pulley system and used for indoor work; the gantry crane is a specialized bridge crane that is suspended between legs and moves laterally on ground rails; and the tractor-mounted crawler crane, which is a hydraulic-powered crane with a telescoping boom. A simple example of a small-scale crane is the fork-lift truck. Like its much larger relatives, the fork-lift is limited not so much by the size of its hoisting apparatus as by the force of its rear counterweight.