Views: 0 Author: Site Editor Publish Time: 2023-02-09 Origin: Site
A crane is a machine,usually equipped with hoisting ropes,wire ropes or chains and pulleys,which is used both to raise and lower materials and to move them horizontally.It is mainly used for lifting heavy objects and transporting them to other places.The device uses one or more simple machines to create a mechanical advantage that moves loads beyond a human's normal ability.Cranes are commonly used for loading and unloading goods in transportation,construction for moving materials,and assembly and manufacturing of heavy equipment.The first known crane was the shaduf,a water-lifting device that was invented in ancient Mesopotamia (modern-day Iraq) and then appeared in ancient Egyptian technology.Construction cranes later appeared in ancient Greece and were powered by humans or animals (such as donkeys) and used to construct buildings.Larger cranes were later developed in the Roman Empire, using human treaded wheels,which could lift heavier weights.During the High Middle Ages,harbor cranes were introduced to load and unload ships and assist in construction-some built in stone towers for added strength and stability.
The earliest cranes were built of wood, but with the advent of the Industrial Revolution,cast iron,steel and steel took over.For many centuries electricity was provided by the physical exertion of man or animal,although the cranes in watermills and windmills could be driven by harnessed natural power. The earliest mechanical power was provided by steam engines,the first steam cranes were introduced in the 18th or 19th century, and many remained in use until the late 20th century.Modern cranes typically use internal combustion engines or electric motors and hydraulic systems to provide greater lifting capacity than previously possible, although hand cranes are still used where providing power is not economical.There are many different types of cranes, each suitable for a specific purpose.Sizes range from the smallest jib cranes,used in workshops,to the tallest tower cranes,used in the construction of high-rise buildings.Micro cranes are also used in the construction of tall buildings,facilitating construction by reaching tight spaces.Large floating cranes are generally used to build oil drilling platforms and salvage sunken ships.Some lifting machinery does not strictly meet the above definition of cranes, but is generally referred to as cranes, such as stackers and truck cranes.
Structure and location
A medieval treadwheel was a large wooden wheel that turned on a central axis with a tread wide enough for two workers to walk abreast.While the earlier "compass arm" wheel spokes were driven directly to the central axle, the more advanced "button arm" type features an arm arranged as a chord to the rim,offering the possibility of using a thinner axle,And provide thereby obtain greater mechanical advantage.Contrary to commonly held belief,cranes on medieval building sites were neither placed on the extremely light scaffolding that was used at the time,nor on the thin walls of Gothic churches that could not support the weight of the crane and the load.Instead, the crane is placed on the ground, usually inside the building, during the initial stages of construction.When the new floors were completed and the roof's massive tie beams joined the walls, the cranes were dismantled and reassembled on the roof beams,moving from bay to bay during vault construction.The cranes thus "grew" and "wandered" with the buildings, with the result that all extant construction cranes in England today are found in church towers above the vaults and below the roofs,where they remained after construction there so that materials can be transported to high places for repairs.Less commonly,medieval illuminations also show cranes mounted on the outside of a wall,the machine's brackets fastened to wood.