Cranes lift and lower loads with a hoist that is attached to a trolley. The trolley moves back and forth along a bridge. The bridge moves along a runway. Some cranes – such as a jib crane – don’t have a bridge but still have a hoist attached to a trolley and can move a load in a horizontal direction.
A bridge crane is defined as a crane, fixed or able to move along track(s) having at least one primarily horizontal girder and equipped with at least one hoisting mechanism. There are four main types of overheads cranes - top running, under running, single girder and double girder.