Cranes
Cranes are devices used to lift weighty or bulky materials, most commonly for the purpose of constructing or dismantling buildings and heavy equipment. A standard crane consists of a tower to which is affixed a freely moving arm with a heavy metal hook attached. Cranes are often placed atop motor vehicles to allow them mobility.Crane Booms
The largest and most obvious component of a crane is the boom, the arm which performs the actual lifting. The boom is itself raised and lowered by means of a hydraulic ram arm, which is attached to the underside of the boom and applies the pressure to raise the boom when it is filled with hydraulic oil.
Because oil is so atomically dense, it cannot be compacted into a smaller form no matter how much force is exerted on it. This means that force, when applied to a dense liquid substance such as oil, will simply cause the fluid's level to rise. Thus, the force exerted on oil within the crane's tubing raises in turn the arm, the boom, and the load itself. Such hydraulic pressure enables the crane to bear massive loads.
Other Parts of a Crane
The hook of a crane is secured by heavy steel cables, each able to bear thousands of pounds of weight, and these are controlled by a winch near the base of the crane. More cables allow more weight to be borne.
A horizontally laid gear (the rotex gear) placed beneath the boom enables the crane to swivel 360 degrees, and a lattice framework attached to the boom (called the jib) allows the arm of the crane to telescope outwards when greater length is required.
At the base of the crane, outriggers are commonly used to provide additional stability. These consist of extendable beams with feet which are planted firmly to the ground, assisting the entire unit to remain steady. During the lifting process, additional counterweights are frequently attached to the base to provide further support.
Types of Cranes
The most common type of crane is the mobile crane described above. Other cranes include:
Gantry cranes - massive rail-based cranes supported by cross-beams which are used to unload freight from ports
Telescopic cranes - consists of numerous interlocking tubes which can unfold to great lengths for tasks requiring such precision
Tower cranes - the tallest of all cranes, in which most of the long boom arm extends horizontally over one side of the tower (somewhat resembling an upside-down letter L), requiring the short part of the arm to be heavily counterweighted by concrete blocks in order to maintain the arm's balance. Tower cranes are most commonly used to construct skyscrapers and other tall buildings.