Stiffleg Derrick Crane

Overview

A stiffleg derrick is a fixed lifting machine built around a vertical Vertical Mast held upright by two rigid struts, the Stiffleg Strut members, set at roughly a right angle to each other. A hinged Lattice Boom pivots at the base of the mast and is raised or lowered by a topping rope. The whole assembly of mast and boom turns as one unit on a pivot at the mast foot, so a load picked up at one bearing can be slewed across an arc and set down somewhere else.

The defining feature is the pair of stifflegs. An older guy derrick uses several wire guy ropes anchored to the ground to keep its mast vertical. A stiffleg derrick replaces those ropes with two stiff compression members, the Stiffleg Tube struts, that run from the Mast Cap & Spider down to the ends of two horizontal Sill Beam members on the ground. The sills meet at a corner under the Mast Foot Socket, so the base forms an L. Because the struts are rigid, the crane needs no surrounding anchor field and can sit on a roof, a floor slab, or a quarry bench where there is no room to run guys.

Base and foundation

The Base & Sill Foundation carries every load down to the ground. Two heavy Sill Beam members lie at about 90 deg, bolted at their meeting corner to the Mast Foot Socket. Each sill spreads its load through a Sole Plate onto a concrete pad, and the outer ends are weighed down by Ballast Block kentledge and held by cast-in Foundation Anchor Bolt sets. When the boom lifts a load, the mast tries to overturn toward that load. One stiffleg goes into compression and pushes its sill end down, while the opposite sill end is held by ballast and anchor bolts against uplift. The right-angle layout means that wherever the boom points within the working arc, at least one of the two sills resists the overturning moment.

Mast and stifflegs

The Vertical Mast is a lattice column of stacked Mast Lattice Section modules, each built from four Mast Chord Angle corners braced by Lacing Bar diagonals. At the top, the Spider Casting gathers the stiffleg heads and the luffing bridle on hardened pins. At the bottom, the Mast Foot Pivot turns inside the foot socket on a Footstep Thrust Bearing thrust bearing that takes the full vertical weight. The two Stiffleg Strut struts run from the spider down to the sill ends, each ending in a Stiffleg Foot Casting casting pinned to the beam and a Stiffleg Head Casting casting pinned to the mast cap. The struts are sized for compression buckling rather than tension, which is why they are large tubes rather than ropes.

Why slew is limited to about 270 degrees

The two stifflegs occupy a quadrant of the circle around the mast. The boom can swing freely through the open part of the circle, but it cannot pass through the space taken by a strut, so a stiffleg derrick reaches a working arc of roughly 270 deg rather than a full turn. A tower crane or a guy derrick can slew the full 360 deg because nothing stands in the boom path; the stiffleg pays for its rigid, anchor-free base with that lost quadrant. A Slew Limit Cam trips a switch and stops rotation before the boom can foul a strut. Site layout therefore matters: the corner formed by the stifflegs is pointed away from the main lifting zone so the usable arc covers the work.

Boom and luffing

The Lattice Boom is a lattice jib whose length is set by the number of Boom Insert Section inserts between the Boom Butt Section and the Boom Head. It hinges on the Boom Foot Pivot at the mast base and is held up by the topping rope. Raising the boom shortens the radius and increases the safe load; lowering it reaches farther for a lighter load. The Luffing Winch pays its Topping Rope over the Bridle Block at the mast top and down to the boom head. A Boom Stay Pendant pendant steadies the boom against side sway. The luffing winch carries a Band Brake so the boom holds position with the motor off.

Hoist and slew drives

The Main Hoist System does the lifting. The Main Hoist Winch spools its Hoist Rope over the Load Sheave groups at the boom head and the Hook Block, so the line is reeved several falls to multiply capacity. The block ends in a forged Load Hook on a Hook Swivel Bearing so the load can turn without twisting the rope. A Hoist Disc Brake holds the drum when the load is suspended. The Slewing System turns the crane: a Slew Pinion driven by the Slew Drive Unit meshes with the Slew Bull Wheel ring at the mast foot, and a Slew Brake holds the crane against wind and load swing. Each motion runs through a Winch Drive Motor with its own holding brake.

Power, control, and use

Power comes either from electric drives fed through the Electrical System cabinet and a set of Variable Frequency Drive inverters, or from a Diesel Engine turning the Hydraulic Pump in the Power & Drive Pack pack. A Slip Ring Assembly carries power and signals across the slewing joint. The operator works from the Operator Cab, using the Control Joystick levers on the Control Console. A Crane PLC sequences the drives and enforces the Motion Limit Switch interlocks, and a Load Moment Indicator warns when the load and radius approach the rated limit. In steel erection the derrick sits on a finished floor and hoists beams and columns up to the next level, raising itself as the frame climbs. In quarries it is mounted on a bench to lift cut blocks of stone onto trucks, where its fixed base and high capacity at short radius suit the repetitive work.