Boom
The main extending or articulated arm of a crane that places the hook over the load, available in telescopic, lattice, knuckle, and folding configurations.
The boom is the structural arm of a crane that supports the lifting hook and positions it over the load. Boom type is one of the primary ways crane categories are distinguished. The four main types are telescopic, lattice, knuckle (articulated), and folding (self-erecting).
A telescopic boom is a series of nested steel tubes that extend hydraulically. It's compact when retracted (good for road transit) and quick to deploy. Telescopic booms are standard on all-terrain, rough-terrain, truck-mounted, and many pick-and-carry cranes. Maximum lengths are typically 30 to 80 metres for road-going machines.
A lattice boom is a bolted or pinned steel-truss structure, much lighter per metre than a telescopic boom and capable of much greater length and capacity. Lattice booms are used on crawler cranes and the largest tower cranes. They must be assembled on site from transported sections.
A knuckle boom is an articulated arm with a hydraulic elbow joint partway along its length, used on truck-mounted loader cranes. The folding action lets the crane work in tight spaces.
A folding boom (also called a self-erecting boom) is found on self-erecting tower cranes and some compact mobile cranes — the boom unfolds hydraulically into operating position when the crane is set up.
When comparing cranes, boom length and maximum extended capacity are the two most important boom specs. A jib (sometimes called a fly jib) is a smaller secondary boom mounted at the head of the main boom for extra reach; it can be fixed or luffing (pivotable).