An overhead crane is a crane which is permanently fixed in place overhead for the purpose of manipulating classically large and heavy objects which cannot be moved easily by hand. Steel mills usually have overhead cranes to handle the steel as it is fabricated into parts, and these cranes are also used at ports all over the world, to bring objects on and off ships. These cranes tend to be quite large, very expensive, and capable of moving huge volumes of material.
The construction of an overhead crane relies on a rail or beam which is permanently fixed in place on a support structure. This can be accomplished by building a crane right into a structure, or by constructing a platform which holds the beam in place. Some manufacturers also make mobile overhead cranes which are pulled by large vehicles, allowing for more flexibility.
On this type of crane, the mechanism which operates the crane is mounted on a trolley which runs along the rail. People can raise and lower items by running cable or rope through the trolley-mounted mechanism, and objects can be moved horizontally along the rail. Unlike more flexible jib cranes, which have swinging booms to allow people to move objects in multiple directions, an overhead crane only goes back and forth.
This back and forth motion is usually all that is needed. On a container ship, for example, the ship can be positioned near the crane, and the operator can send the mechanism back and forth along the trolley to shuttle goods from the ship to a flatbed truck or train, or the other way around. By being fixed in place, the crane is much more stable, and capable of carrying very heavy loads, a distinct advantage when handling the heavy weights involved in shipping, steel working, and other heavy industries.
As a design concept, the overhead crane dates to the 1870s, when several people developed versions of this crane design for an assortment of applications. In addition to the huge scale versions used in industrial applications, smaller overhead crane styles can also be found for use in home workshops and small businesses where a crane might be useful. A woodworker, for example, might appreciate the availability of an overhead crane to haul lumber, finished projects, and heavy tools between a workshop and a loading area. Enterprising craftspeople can build their own, assuming that they have a workshop or structure with exposed beams upon which rails can be mounted.