Crane is a machine that lifts, transports, and sets down heavy objects. Cranes are used in such workplaces as factories, shipyards, and construction sites. A crane picks up a load using an attachment, such as a hook, bucket, or platform. On most cranes, the attachment is connected to a block and tackle, a system of pulleys and a cable. The end of the cable winds around a winch (rotating drum). Turning the winch raises or lowers the load.
There are many types of cranes, each of which has its own use. Crawler mounted cranes and wheel mounted cranes are used on construction sites to lift heavy objects and to move materials and equipment. Both types use a boom (long arm) and a block and tackle to raise and lower the load. A crawler mounted crane moves on continuous tracks. It moves equally well over hard, soft, and uneven surfaces. Heavy weights called counterweights on the back of the crane help prevent tipping and increase the amount of weight the crane can lift. Wheel mounted cranes include those attached to trucks or special wheeled vehicles. They can get from one place to another quickly. They move well only on flat, hard surfaces. They use outriggers, long arms that extend from the crane to the ground, to prevent tipping.
A tower crane is typically fixed in place. It is shaped like a “T.” The vertical section is made up of segments that form a steel framework or support tower called a mast. An arm called a jib forms the horizontal section. One side of the jib holds the load, and the other holds a counterweight. Tower cranes are often built temporarily in the elevator shafts of tall buildings under construction. They extend above the top of the building and move heavy objects, tools, and materials from the ground to the construction area. As the building grows, some tower cranes add sections to the mast to increase their height. Others use a telescoping mechanism. Some tower cranes are fixed to the ground and use cables called guy lines for support. Others are mounted on railroad tracks so they can move. Such cranes are used at docks to unload freighters.
Different attachments allow cranes to perform different types of work. Cranes with hook attachments can lift objects. Magnet attachments are used to move scrap metal. Bucket attachments allow cranes to move wet concrete. Using a shovel attachment, a crane can dig in an area not accessible to other digging machines, such as the bottom of a body of water.
See also Block and tackle.
