We will marvel at an operating crane almost every time we see it. Sometimes such machines are so huge that they have the ability to carry tons of cargos and lift them up to the height of mid-air. When we see these behemoths, we harbor a complicated feeling—excitement mingled with fear. Because it is human nature that we will consider such situations that goods being lifted may fall down from the crane or the crane is in the wrong place. However, there is no denying that crane is an amazing mechanical system that causes humans' attention, no matter he is a child or an adult.    
Crane is a kind of machine for lifting goods, doing cycling and intermittent motion. A single working circle is as follows: The fetching device lifts the product from the extract ground, moves it to the designated place and then lay down it. Afterwards, do the reverse movement so that the fetching device can return to its original place for next cycle. Generally, a crane consists of a drum, a bunch of metal wire or a metal chain, designed for the elevation, deposition or horizontal movement of goods. Additionally, cranes are commonly employed in the transport industry for unloading the goods, building business for transporting building materials and manufacturing industry for assembling heavy equipment.
Cranes have a long history. The first crane was invented by the ancient Greeks, which was using people or animals as the source of power and was used for large construction projects. During the Middle Ages the port crane was used for loading and unloading of cargos, and for greater lifting weights and better order some port cranes were made in the stone towers. The earliest cranes were made of wood, but after the industrial revolution, cast iron and steel replaced the wood for making cranes. The first crane which used mechanical energy was a steam crane, enjoying great popularity during the 18th and 19th century. Modern cranes usually use internal combustion engines, electric motors and the hydraulic system which can provide much greater promotion force than before.

There are various types of cranes. Now we have everything that one expects to find, from smallest arm cranes used in the workshop to highest tower cranes used for constructing high buildings. However, small cranes are also used for the construction of skyscrapers, because they can make the construction more convenient in confined spaces of high-rise buildings. Moreover, floating cranes with a larger body are generally used to build oil rigs and salvage submerged vessels. Therefore, cranes fully deserve a title of good assistants of mankind.