overview of crane

A crane is a mechanical lifting device equipped with a winder, wire ropes and sheaves that can be used both to lift and lower materials and to move them horizontally. It uses one or more simple machines to create mechanical advantage and thus move loads beyond the normal capability of a human. Cranes are commonly employed in the transport industry for the loading and unloading of freight; in the construction industry for the movement of materials; and in the manufacturing industry for the assembling of heavy equipment.

The first cranes were invented by the Ancient Greeks and were powered by men or beasts-of-burden, such as donkeys. These cranes were used for the construction of tall buildings. Larger cranes were later developed, employing the use of human treadwheels, permitting the lifting of heavier weights. In the High Middle Ages, harbour cranes were introduced to load and unload ships and assist with their construction – some were built into stone towers for extra strength and stability. The earliest cranes were constructed from wood, but cast iron and steel took over with the coming of the Industrial Revolution.

Cranes exist in an enormous variety of forms – each tailored to a specific use. Sizes range from the smallest jib cranes, used inside workshops, to the tallest tower cranes, used for constructing high buildings, and the largest floating cranes, used to build oil rigs and salvage sunken ships.

General characteristics of cranes:

• Used to move loads over variable (horizontal and vertical) paths within a restricted area


• Used when there is insufficient (or intermittent) flow volume such that the use of a conveyor cannot be justified


• Provide more flexibility in movement than conveyors


• Provide less flexibility in movement than industrial trucks


• Loads handled are more varied with respect to their shape and weight than those handled by a conveyor


• Most cranes utilize hoists for vertical movement, although manipulators can be used if precise positioning of the load is required
  

Lifting capacity:

Cranes illustrate the use of one or more simple machines to create mechanical advantage.

The lever. A balance crane contains a horizontal beam (the lever) pivoted about a point called the fulcrum. The principle of the lever allows a heavy load attached to the shorter end of the beam to be lifted by a smaller force applied in the opposite direction to the longer end of the beam. The ratio of the load's weight to the applied force is equal to the ratio of the lengths of the longer arm and the shorter arm, and is called the mechanical advantage.

The pulley. A jib crane contains a tilted strut (the jib) that supports a fixed pulley block. Cables are wrapped multiple times round the fixed block and round another block attached to the load. When the free end of the cable is pulled by hand or by a winding machine, the pulley system delivers a force to the load that is equal to the applied force multiplied by the number of lengths of cable passing between the two blocks. This number is the mechanical advantage.

The hydraulic cylinder. This can be used directly to lift the load (as with a HIAB), or indirectly to move the jib or beam that carries another lifting device.