A swinging derrick crane is a fixed-base lifting machine with a vertical mast and an inclined boom that pivots at the mast foot, allowing the load to swing through an arc by rotating the mast. The mast is the central component — it carries all vertical and overturning loads down to the foundation while the boom angles up from its base. The design solves the problem of lifting heavy loads in confined sites where mobile cranes cannot enter or set up. Guy and stiffleg derricks routinely handle 5 to 250 tons within radii of 20 to 90 ft on jobs from steel erection to quarry work.
Swinging Derrick Crane Interactive Calculator
Vary load, working radius, boom length, and allowable moment to see derrick load moment, utilization, capacity, and overload.
Equation Used
The derrick load moment is the lifted load W multiplied by the working radius R. Compare that moment with the allowable rated moment to estimate utilization; as radius increases, the allowable load drops in direct proportion.
- Static lifted load; dynamic effects, wind, shock, and side loading are excluded.
- Working radius is the horizontal distance from mast centerline to hook load.
- Allowable moment is taken from the crane load chart or engineering rating.
- Boom length is used for the visual angle check and does not change the moment formula.
Inside the Swinging Derrick Crane
A swinging derrick has three motion axes you control independently. The hoist line raises and lowers the load. The topping lift (also called the boom hoist) raises and lowers the boom angle, which changes the working radius. The slewing system rotates the mast around its vertical axis, swinging the entire boom and load through an arc. Combine those three and you can place a load anywhere inside a hemispherical work envelope set by boom length and the maximum allowable boom angle.
The mast stays vertical because it's restrained at the top — either by six or more guy wires anchored to the ground (a guy derrick) or by two rigid stifflegs framed back to a sill (a stiffleg derrick). The boom pivots on a heel pin at the mast foot. When you pull the topping lift, the boom climbs; when you pay it out, the boom falls and radius increases. The load chart drops fast as radius grows, because load moment = load × radius, and the mast and its supports have a fixed moment capacity. Push past it and the mast either lifts a stiffleg off its sill or yanks a guy wire's anchor out of the ground.
Tolerances matter more than people think. The bull wheel at the mast base must run true within about 1/8 in over its full circumference or the slew drive will bind under load. Guy wire pretension on a guy derrick should sit at roughly 10% of breaking strength — too loose and the mast walks under load, too tight and you preload the foundation. If you notice the boom drifting down with the topping lift locked, the boom hoist pawl or brake band has worn and the load is back-driving the drum. That's the failure mode that drops loads.
Key Components
- Mast: The vertical column that carries all reaction loads to the foundation. Typical heritage masts run 40 to 100 ft of timber or fabricated steel; the slenderness ratio (length to least radius of gyration) must stay below 120 to avoid buckling under combined axial and bending load.
- Boom: The inclined member that carries the hoist sheave at its tip. Boom length sets maximum working radius and minimum lift height. Heel pin clearance must hold to within 1/16 in side play or the boom will cock under off-centre loads and gall the pin.
- Topping Lift (Boom Hoist): A multi-part wire rope tackle from the mast cap to the boom tip that controls boom angle. Mechanical advantage of 4:1 to 8:1 is typical; a worn brake band on the boom hoist drum is the single most common cause of uncontrolled boom drop.
- Hoist Line and Fall Block: The working line from the hoist drum, over the boom-tip sheave, down to the load hook. Reeve count (parts of line) sets the load capacity of the rope itself — a 4-part fall on 3/4 in IWRC rope handles around 12 tons before you hit the 5:1 design factor on the rope.
- Bull Wheel and Slewing Drive: The horizontal toothed wheel at the mast base that the swing drive engages. Bull wheels typically run 6 to 12 ft diameter; tooth backlash above 1/4 in causes load swing during slew start and stop.
- Guys or Stifflegs: The lateral restraint for the mast top. A guy derrick uses 6 to 8 wire-rope guys at roughly 45° from vertical; a stiffleg derrick uses two rigid struts at 90° to each other, framing back to a counterweighted sill.
- Sill and Foundation: The base structure carrying compression from the mast and tension from the stifflegs or back guys. Anchor bolt pull-out capacity must exceed the maximum stiffleg tension by a factor of 3 — a typical failure mode is concrete cone failure under back-leg tension during a max-radius lift.
Industries That Rely on the Swinging Derrick Crane
Swinging derricks earn their place wherever a mobile crane can't reach, can't fit, or can't sit. They're also the right tool for repetitive lifts inside a fixed work envelope, where setup time for a mobile crane outweighs the lift itself. The mechanism survives in granite quarries, shipyards, structural steel erection on tight urban sites, and heritage restoration where a period-correct derrick is required.
- Granite Quarrying: Stiffleg derricks at the Rock of Ages quarry in Graniteville Vermont lift 30-ton dimension stone blocks from the quarry floor to the rim hauler, swinging through a 270° arc.
- Structural Steel Erection: Guy derricks self-jumped up the floors of the Empire State Building during 1930-31 construction, setting steel columns and beams ahead of the floor deck.
- Shipyard and Drydock: Roof-mounted stiffleg derricks at Bath Iron Works in Maine handle hull plate and machinery into drydock positions where mobile cranes can't reach over the dock wall.
- Bridge Construction and Demolition: Floating derrick barges like the Chesapeake 1000 use a swinging-boom derrick rig to lift bridge segments and salvage wreckage in confined waterways.
- Heritage and Monument Work: Restored guy derricks set obelisks and large monument stones at historic cemeteries and memorial sites where modern mobile cranes would damage protected ground.
- Industrial Plant Maintenance: Permanent stiffleg derricks at hydroelectric dams pull turbine runners and generator rotors during overhaul cycles inside the powerhouse roof envelope.
The Formula Behind the Swinging Derrick Crane
The most useful number on a derrick is the load moment — load weight multiplied by working radius. Every component on the machine is sized against this single value, and your load chart is just load moment capacity divided by the radius you're operating at. At a short radius the boom is steep and the mast sees mostly vertical load, so capacity is high. At long radius the boom is flat, the mast sees heavy overturning moment, and capacity drops fast. The design sweet spot for most stiffleg derricks sits around 60° boom angle — high enough to keep load moment manageable, flat enough to give useful reach.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Mload | Load moment about the mast base | kN·m | ft·lb |
| W | Load weight at the hook (including rigging) | kN | lb |
| R | Horizontal working radius from mast centreline to hook | m | ft |
| Lboom | Boom length, heel pin to tip sheave | m | ft |
| θ | Boom angle measured from horizontal | degrees | degrees |
Worked Example: Swinging Derrick Crane in a hydroelectric powerhouse rotor lift
A hydroelectric utility in northern Quebec is using a permanent roof-mounted stiffleg derrick inside a 1955-vintage powerhouse to extract a 14,000 lb generator rotor for rewinding. The derrick has a 50 ft boom, a rated load moment capacity of 500,000 ft·lb, and the rotor pickup point sits 22 ft horizontally from the mast centreline. The crew needs to know whether this lift fits the envelope and what the boom angle works out to.
Given
- W = 14,000 lb
- Lboom = 50 ft
- R = 22 ft
- Mcapacity = 500,000 ft·lb
Solution
Step 1 — compute load moment at the nominal 22 ft pickup radius:
That sits at 62% of the 500,000 ft·lb capacity. Comfortable margin, no headaches.
Step 2 — solve for the required boom angle:
That's right in the sweet spot — boom steep enough to keep mast moment manageable, flat enough that the rotor clears the generator pit lip.
Step 3 — check the low end of the working range. If the rotor needed to be set down at 35 ft radius (boom dropped to about 46°):
That's 98% of capacity. You'd be one rigging error or one wind gust away from overload. Most operators won't lift past 90% of chart in a permanent derrick.
Step 4 — check the high end (boom up). At 12 ft radius the boom angle climbs to 76°:
Only 34% of capacity. The derrick barely notices the load, but you've lost reach — the rotor can't clear the generator pit at that radius. Geometry beats capacity here.
Result
At the nominal 22 ft pickup radius the load moment is 308,000 ft·lb, comfortably inside the 500,000 ft·lb capacity at a boom angle of 64°. Compare that to the 35 ft set-down radius — load moment jumps to 490,000 ft·lb, 98% of capacity, which is the upper limit any sane rigger will work to. At 12 ft radius the derrick is loafing at 34% but can't reach where the work needs to happen. If the topping lift gauge reads boom angle but the load won't clear an obstacle as predicted, the most likely causes are: (1) heel pin bushing wear letting the boom heel shift 2-3 in forward of the mast centreline so your effective radius is greater than the gauge reads, (2) hoist line stretch under load adding 4-6 in of vertical drop at full extension on a 4-part fall, or (3) a miscalibrated boom angle indicator — the original mechanical pendulum types drift 2-4° as the pivot bearing wears.
When to Use a Swinging Derrick Crane and When Not To
A swinging derrick isn't always the right answer. Mobile cranes are faster to deploy, tower cranes reach higher, and overhead bridge cranes give you full XY coverage inside a building. The derrick wins on heavy lifts in confined sites, on permanent installations with repetitive lift paths, and on jobs where you need to self-erect the crane from the load it's lifting. Compare on the dimensions that actually drive your decision.
| Property | Swinging Derrick Crane | Mobile Hydraulic Crane | Tower Crane |
|---|---|---|---|
| Maximum load capacity | 5 to 250 tons | 5 to 1,200 tons | 2 to 64 tons typical jib |
| Working radius | 20 to 90 ft | 10 to 350 ft | 30 to 250 ft |
| Setup time | Days to weeks (semi-permanent) | 1 to 4 hours on site | 1 to 2 weeks erection |
| Footprint required | Mast base + guy radius or sill area only | Full crane carrier + outrigger spread | Foundation pad + slewing clearance |
| Capital cost | $30K to $300K (often custom-built) | $500K to $5M plus rental | $300K to $2M plus erection |
| Slewing speed | 0.2 to 1 RPM | 1 to 3 RPM | 0.5 to 1 RPM |
| Best application fit | Confined sites, repetitive heavy lifts, quarries, shipyards | General construction, road work, short-duration lifts | High-rise construction, long-duration projects |
| Reliability and complexity | Mechanically simple, decades of service | Hydraulic system maintenance heavy | Electrical and structural inspections frequent |
Frequently Asked Questions About Swinging Derrick Crane
Rated load on a stiffleg derrick is set with the load lined up on the bisector between the two stifflegs. Slew the load 90° so it's directly opposite one stiffleg, and that stiffleg sees nearly all the back-tension while the other goes slack. The slack one will physically lift off its sill if the counterweight on that leg isn't sufficient.
Most older derricks list a reduced capacity in the 'over the leg' quadrant for this reason — sometimes 60-70% of bisector capacity. If your load chart doesn't break out quadrants, treat 65% of rated as the real number when slewing past either stiffleg.
Guy derricks give you 360° clear slew because there are no stifflegs in the way, and they self-jump up a building floor by floor — that's why they built every pre-1960 American skyscraper. The cost is 6 to 8 anchor points spread roughly equal to mast height in every direction, plus retensioning the guys after every jump.
Stiffleg derricks limit you to roughly 270° of useful slew because the back leg quadrant is restricted, but they need no guy anchors and they handle eccentric loads better because the stifflegs take both tension and compression. Pick stiffleg for permanent installations and confined sites; pick guy when you need full slew or vertical reuse on a tall structure.
This is almost always boom deflection under load, not instrument error. A 50 ft lattice boom under a 10-ton hook load deflects 2-4 in at the tip from elastic bending alone, plus another 1-2 in from heel pin and pivot slop. At a flat boom angle that vertical droop projects forward as added radius.
The fix is to use a calibrated boom-tip pendant indicator rather than the heel-mounted angle gauge, or simply add a 5% radius margin to your load chart numbers when working at boom angles below 50°. Old hands working heritage derricks just chalk-mark the actual landing radius and ignore the gauge.
Two causes, usually combined. First, bull wheel tooth backlash — if there's more than 1/4 in of free play between the slew pinion and bull wheel, every direction reversal lets the load build momentum before the gear engages. Second, hoist line angle: any time the hoist line isn't perfectly vertical at slew start, the horizontal component pulls the load through an arc that the slew has to chase.
Diagnostic check: with the load just clear of the ground, sight up the hoist line. If it's leaning more than 2° off vertical from the boom tip, the load isn't rigged directly under the tip sheave and the slew will fight that geometry the whole arc.
Only up to the limit of the boom-tip sheave and the upper-block sheave count. Adding parts of line raises the rope's load capacity and reduces drum line speed, but it doesn't change the derrick's structural moment capacity at all — the mast, boom, and stifflegs see the same load regardless of how many parts you reeve.
If your load chart says 14 tons at 22 ft and you reeve 8 parts instead of 4, you can lift 14 tons more slowly with less rope tension, but you cannot lift 28 tons. The structure is the limit, not the rope. The only ways to genuinely increase capacity are reducing radius, upgrading the structural members, or adding counterweight on a stiffleg sill.
For one-off lifts, no — rent the mobile and go home. The derrick wins on three specific job profiles: confined sites where the mobile carrier physically won't fit (urban infill, inside existing buildings, quarry pits), repetitive permanent-installation lifts where setup amortization makes a fixed derrick cheaper over years (powerhouse maintenance, shipyard plate handling, foundry pour stations), and heritage or memorial work where regulators require period-correct equipment.
Rule of thumb: if you'll do more than 30 lifts a year inside the same envelope and a mobile can't stay parked there, run the numbers on a permanent stiffleg. The capital cost pays back in 4-6 years versus repeated mobile rental.
References & Further Reading
- Wikipedia contributors. Derrick. Wikipedia
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