Whilst no single person is credited with inventing the earliest cranes, the simple block and tackle arrangement was used by the ancient Greeks. This involves a single pulley positioned above the man doing the lifting. Thus to lift an object, he has to pull down on a rope. The advantage of this method is that he can pull downwards, using his own weight to help him. However, extra effort is required to counteract the friction from the pulley bearing.
The first cranes to come close to resembling anything in use today were built during the 16th century. These made use of a drum, around which a rope was wound. As the drum turned, the rope would wrap around the drum, lifting the load. Conversely, rotating the drum in the other direction unwinds the rope, lowering the load. This system is now known as a winch. The cranes consisted of two or three wooden legs, which straddled the load. The lifting was then done by the winch or, for heavier loads, a treadmill.
Modern day cranes are of two basic types: bridge cranes and jib cranes. Whilst different in construction, both use a winch and steel rope, although the winch is now driven by an electric motor. The jib crane (sometimes known as a tower crane) is the type often seen on building sites. The tower is built of steel sections, with the bottom section concreted into the floor. At the top of the tower is a long boom, called a jib (hence the name `jib crane'). This swings horizontally, picks up the load, then swings to where the load has to be delivered. Many cranes can also tilt the jib towards the ground, thereby reducing or extending its reach. At the opposite end to the load is a counterweight. This prevents the weight of the load from pulling the crane over.
The bridge crane is the type often found in factories and warehouses. The frame is made of box section girders and is called a gantry. A track is fixed at high level, on which the gantry runs. The lifting gear can move along the gantry, which in turn can move along the tracks, enabling the crane to reach most areas of the warehouse or factory.
Both types are operated by a driver who sits in a cab. With the jib crane the cab is at the top of the tower, whereas with a bridge version it is mounted on the gantry. Use is also made of gears and pulleys to allow very heavy loads to be lifted with little strain on the motor.
