A jib crane is a type of crane that uses a mounted arm to lift, move, and lower material. The arm, mounted either perpendicular to or at an acute angle upwards from a pillar or wall, may rotate along its central axis through a limited arc or a full circle. A jib crane is often used in industrial settings, like warehouses and docks, to load and unload shipping containers.
The history of the jib crane dates back to Ancient Greece, where the concept of using a mechanical arm to lift objects first came into wide use. The Romans later embraced the technology and applied it to the construction of roads, aqueducts, and other engineering projects. Cranes were refined, but remained largely identical in nature to classical designs until the industrial revolution and the introduction of the steam engine in the 19th century. With ever larger construction projects on land and sea, as well as the growing demands from increasingly complex factories and shipping companies, cranes too developed in size and intricacy during this time.
The fundamental basis of the crane stems from the concept of mechanical advantage. Mechanical advantage is the idea that a machine, such as a pulley, can multiply the force being applied to it by a certain factor. This is why it is possible, for instance, for a single man using a pulley to lift the same heavy crate that would take a team of men to lift directly. Cranes incorporate a variety of pulleys with other simple machines, such as gears and levers, to greatly increase lifting capacity.
In a modern jib crane, strong metal cabling is wrapped around the ends of the jib strut, with the hoist end usually connected to a hook or electromagnet, and the other end connected to a winch. When the winch is activated, the pulleys deliver a lifting force equal to the actual force being applied by the winch, multiplied by the number of cable lengths wrapped around the pulleys. In many jib cranes, the hoist may be able to move outwards and inwards along the length of the jib, offering even greater flexibility of motion.
Jib cranes are not limited to fixed locations, and may be mounted on a mobile chassis to be used at temporary work sites or in military operations. Such cranes generally sacrifice lifting capacity for mobility. Outriggers are essential for a mobile jib crane to maintain stability under load, unlike stationary ones that are firmly anchored in place.
