Cranes are machines that use levers and/or pulleys to lift significant weights. A crane one passes on the road may look like a fairly modern invention, but these machines have actually been used for at least the past 2000 years, if not longer. The Romans used cranes to build huge monuments. Medieval churches were constructed with them. Also, the Egyptians may have used them to create pyramids. The modern version can be either simple or complex, and cranes vary based on their application.
A relatively simple crane is the mobile crane. A telescopic boom (arm) or steel truss mounts its movable platform. Either pulleys or levers raise the boom. Generally a hook suspends from the boom. The platform of a mobile crane can either have traditional wheels, wheels designed for railroad tracks, or a caterpillar track, which is useful for navigating unpaved and uneven surfaces. Mobiles can be used for demolition or earthmoving by replacing the hook with an appropriate tool, such as a wrecking ball or bucket. Telescopic cranes, with a series of hydraulic tubes fit together to form the boom, can also be mobile.
Truck mounted and rough terrain cranes are both essentially mobile as well. The truck-mounted crane generally has outriggers to increase its stability. Rough terrain cranes tend to have a base that resembles the bottom of a 4-wheel drive vehicle. Outriggers also stabilize these cranes. They tend to be used in rough terrain, as the name suggests, and are frequently used to pick up and transport materials.
Loader cranes have hydraulic powered booms fitted onto trailers. They load goods onto the trailer and the jointed sections of the boom are folded down when not in use. The loader may also be considered telescopic, as one section of the boom, in some designs, may telescope for ease of use.
Stacker cranes are most frequently seen in automated warehouses where they tend to follow an automatic retrieval system. For example, in huge automated freezers, these cranes, equipped with forklift apparatus, can work by remote, stacking or obtaining foods as needed. This retrieval system makes it possible to keep workers out of the cold.
Gantry cranes are most often found in ports and railroads, where they unload and move huge containers off of ships and trains. The bases are huge crossbeams which run on rails, so lifted containers can be moved from one location to another. The portainer is a special type of gantry that lifts materials on and off ships.
Floating cranes mounted on barges or pontoons are also essential to the shipping industry. Situated in water, they are used to construct ports, salvage ships or build bridges. Like portainers, floating cranes also can unload ships. They are able to handle very heavy loads and awkwardly shaped containers.
Tower cranes, conversely, do not generally have a moveable base. These are often the tallest cranes, and have to be assembled piece by piece. The base looks like a long ladder, and the boom is perpendicular to the base. Tower cranes are used to construct tall buildings, and in the case of skyscrapers, the tower crane is often assembled and affixed inside the building itself during construction.
All cranes represent a meeting of simple machines, used for the purpose of reducing workload. However simple they may seem, they are instrumental in many aspects of industry. They can dig, move, create, or destroy, depending on their type. Cranes exemplify that sometimes the oldest ideas are the best ones.
See http://www.wisegeek.com/what-are-the-different-types-of-cranes.htm
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