| Any of a diverse group of machines that lift and move heavy objects. Cranes differ from hoists, elevators, and other devices intended for vertical lifting, and from conveyors, which continuously lift or carry bulk materials such as grain or coal. Cranes have been widely used only since the introduction of steam engines, internal-combustion engines, and electric motors in the 19th century. They range in type and function from the largest derrick cranes to small, mobile truck cranes. Most derrick cranes can lift 5 – 250 tons (4.5 – 230 metric tons). Floating cranes, built on barges for constructing bridges or salvaging sunken objects, may be able to lift 3,000 tons (2,700 metric-tons). Small truck cranes are mounted on heavy, modified trucks; they make up in mobility and ease of transport what they lack in hoisting capacity.Mechanical device used primarily for raising and lowering heavy loads but occasionally for moving objects horizontally. It usually consists of a block and tackle-a combination of one or more fixed pulleys, a moving pulley with a hook or other similar means of attaching loads, and a rope (or cable) between them. Motive power for a hoist may be either manual or supplied by an electric motor. Electrically powered hoists, commonly mounted to the floor or wall, are used for varied lifting and hauling operations in factories and warehouses. |
