Luffing Jib Tower Crane

Overview

A luffing jib tower crane lifts and places loads on tall buildings where a flat-top or hammerhead crane cannot fit. Instead of a fixed horizontal jib, the Luffing Jib pivots at its foot and is raised or lowered by a rope system, so the working radius changes by changing the jib angle rather than by trolleying along the jib. The minimum radius can be brought in to a few metres by luffing the jib steep, which keeps the slewing envelope tight on a congested site and clear of neighbouring buildings and other cranes. The trade-off is capacity: load at a given radius is lower than a comparable saddle jib crane because the jib carries bending as well as the rope pull.

The crane stands on a Foundation and Base cast into a concrete block, climbs to height through a self-erection Climbing Cage, and rotates on a Slewing Unit. Above the slew bearing sit the Tower Top A-Frame, the jib, and the Counter-Jib and Counterweight that carries the machinery and ballast. The Electrical and Control System runs the three powered motions and enforces the load limits.

Foundation and base

The crane reacts very large overturning moments into the ground. The Cross Base Frame is a cruciform welded frame that spreads the moment to a cast footing. Four Foundation Anchor Set sets are embedded in the concrete pour, each holding a group of Anchor Bolt fixings located by a buried Anchor Frame. The base sits on the anchors through Fixing Angle corners, and Central Ballast Block slabs are stacked on the cross arms to hold the crane down in free-standing service. For taller configurations the mast is tied back to the structure with a Building Tie Frame.

Tower mast and climbing

The mast is built from stacked Mast Section units, each a square lattice of four Mast Corner Chord legs braced by Mast Diagonal Brace and Mast Horizontal Brace members, with a Access Ladder Segment inside for access. Sections splice together with a Mast Connecting Pin Set and seat on the base through the Mast Foot Section.

The crane raises its own height with the climbing cage. The Climbing Frame surrounds the mast and carries the whole upper works. A long-stroke Climbing Hydraulic Cylinder, fed by the Hydraulic Power Pack, jacks the cage up by one section height. A Support Pawl Set rests the cage on the mast between strokes while a new Mast Section is rolled in across the Climbing Work Platform and pinned. Repeating the cycle adds sections one at a time until the crane reaches working height.

Slewing and tower top

The upper works rotate on the Slewing Bearing Ring, a large-diameter bearing whose Slew Inner Race (Geared) carries internal teeth. Two Slew Drive Unit units, each a motor and gearbox ending in a Slew Pinion, mesh the ring and turn the Turntable Platform that carries everything above. A Slew Holding Brake holds position and damps swing. Above the turntable, the Tower Top A-Frame A-frame anchors the luffing rope through the Top Sheave Set and ties back to the counter-jib through the Pendant Anchor.

Luffing jib and luffing drive

The jib pivots on the turntable at the Jib Foot Section, which carries the Jib Foot Bearing and turns on the Jib Foot Pivot Pin. Lattice Jib Section units extend out to the Jib Head, which holds the Jib Head Load Sheave for the hoist rope. The jib angle is set by the Luffing Mechanism. A Luffing Winch spools the Luffing Wire Rope, reeved between the A-frame sheaves and the jib, so winding in raises the jib and reduces radius. The Pendant Guy Rope stays carry the back tension to the counter-jib, and every rope ends in a Rope End Socket. Because the jib can fall under its own weight, the luffing winch uses two independent Winch Service Brake sets.

Hoisting and counterweight

The Main Hoist Winch is the primary lifting machine. Its Hoist Drive Motor drives the drum through the Hoist Reduction Gearbox, and two Hoist Brake units hold the load. The Hoist Wire Rope runs over the jib head sheaves down to the Hook Block, where multiple Hook Block Sheave sheaves multiply the line pull and the Forged Load Hook carries the load on a swivel Hook Thrust Bearing closed by a Hook Safety Latch. The Counter-Jib and Counterweight behind the tower top carries the winches on its Machinery Deck and the Counterweight Block ballast that balances the moment. A luffing crane carries less counterweight than a hammerhead because raising the jib shortens the load arm and shifts the balance as the radius changes.

Control, sensing, and operation

The operator works from the Operator Cab through a Control Console of joysticks and the Operator Display Panel. Each motion has its own drive: the Hoist VFD, the Slew VFD, and the Luffing VFD, all coordinated by the PLC Controller inside the Main Control Cabinet. The Load Moment Indicator reads a Load Cell and a Jib Angle Sensor to compute load moment against the rated chart and trip the drives before an overload. The Anemometer at the jib head warns the operator as wind rises toward the in-service limit; above it, the crane is taken out of service and left to weathervane. Power crosses the rotating joint through the Slip Ring Assembly in the Power Distribution Unit unit, fed from the ground by the Festoon Trailing Cable.