A swing derrick is a mast-and-boom lifting machine where a vertical mast carries a pivoting boom that swings horizontally through an arc to place loads. The boom rotates about the mast's vertical axis using a bull wheel or slewing ring at the base, while a topping lift controls boom angle and a load line raises the hook. Crews use it on confined sites where mobile cranes can't set up. A typical guy derrick handles 5 to 50 tons and swings 270° to 360°, which is why steel erectors used them on early skyscrapers like the Empire State Building.
Swing Derrick Interactive Calculator
Vary boom geometry, overturning moment, and swing cycle to see working radius, lift capacity, and swing speed.
Equation Used
The derrick working radius is the horizontal projection of the boom: R = L cos(theta). The allowable load is the available overturning moment divided by that radius, so a longer radius reduces capacity. Swing speed and hook plan speed use the selected swing arc and cycle time.
- Boom angle theta is measured from horizontal.
- Allowable overturning moment already includes the required safety factor.
- Static capacity only; wind, shock loading, boom self-weight, and dynamic hoist effects are not included.
- Plan-view swing speed assumes constant motion through the selected arc.
How the Swing Derrick Works
A swing derrick has three motions: hoist the load, luff the boom (raise or lower the boom angle), and swing the boom around the mast. The mast stays vertical, anchored either by guy wires running to ground anchors (a guy derrick) or by two rigid legs braced back to a sill (a stiffleg derrick). The boom heel pins to the foot of the mast and the boom point carries the load line sheave. A topping lift line runs from the boom point up to the mast head and back down to a winch — that line controls how steep the boom sits, which sets the working radius. The load line runs over the boom point sheave and down to the hook.
The swing motion is what gives the derrick its name. A bull wheel — a large horizontal gear or grooved drum at the mast base — engages a swing line or a pinion drive. Pull the swing line one way and the whole boom assembly rotates about the mast. On a guy derrick the swing arc is limited by the guy wire spacing, typically 6 guys at 60° spacing giving roughly 280° of usable swing. On a stiffleg the legs block roughly 90° of arc, leaving 270° clear. If the bull wheel teeth wear or the swing line stretches, the boom will hunt and overshoot the target landing — you'll see the load drift past the set point and have to reverse swing to land it. That's the most common operator complaint on older rigs.
Get the topping lift geometry wrong and you cap the rated load. The boom angle directly sets the load radius, and the rated capacity drops as a square-law function of radius — double the radius and you cut capacity to roughly a quarter. Boom angles below about 30° from horizontal also push the topping lift line tension above the load line tension, which is why the topping lift sheaves and drum are sized heavier than the hoist drum. If your topping lift drum is overheating during long boom-down cycles, the boom is being worked at too flat an angle for the load.
Key Components
- Mast: The vertical column that carries the boom and reacts the overturning moment from the load. On a guy derrick the mast is a single timber or steel tube 40 to 90 ft tall, held plumb by 6 to 8 guy wires at 45° to ground anchors. The mast must stay within 1° of plumb under no-load conditions or the swing torque will be uneven around the arc.
- Boom: The inclined member pinned at the mast foot that carries the hook at its tip. Boom length runs 30 to 100 ft on most derricks. The boom heel pin must have less than 1.5 mm radial slop or the boom point will wander under load and the load line will lay-up unevenly on the hoist drum.
- Bull wheel: The large horizontal gear or grooved sheave at the mast base that converts swing-line pull into rotation of the entire boom. Diameters run 4 to 10 ft. The bull wheel and its swing line must be aligned within ±3 mm of concentric to the mast axis or the swing motion will pulse — operators feel it as a stutter through the swing levers.
- Topping lift: The wire rope system between the mast head and the boom point that raises and lowers the boom angle. Typically a 4-part or 6-part reeving on derricks above 20 ton capacity. The topping lift winch must hold load with a brake rated to at least 1.5× the maximum static line pull.
- Load line and hoist drum: The main hoist rope running over the boom point sheave to the hook block. Drum capacity must hold full boom-length rope plus 5 dead wraps. Load line speed runs 50 to 200 ft/min on construction derricks.
- Guys or stifflegs: The mast restraint system. Guy derricks use 6 to 8 wire rope guys pretensioned to roughly 10% of breaking strength. Stiffleg derricks use two rigid steel legs at 90° to each other, ballasted with concrete or steel sill weights. Stifflegs are quicker to set up but limit swing arc to 270°.
Real-World Applications of the Swing Derrick
Swing derricks live on jobs where a mobile crane physically can't get to the lift point or where the lift duty is repetitive enough to justify a fixed rig. You'll see them on stone quarries, steel erection of tall buildings before tower cranes existed, ship-loading wharves, and heritage restoration jobs where an authentic period rig is part of the scope. The mast-and-boom geometry gives you a long horizontal reach with a small footprint, which is the whole reason they survived alongside hydraulic cranes for niche work.
- Steel erection (historical): American Bridge Company used guy derricks to climb and erect the structural steel of the Empire State Building in 1930-1931, leapfrogging the derrick up the frame floor by floor.
- Granite and stone quarrying: Rock of Ages quarry in Barre, Vermont uses stiffleg derricks with 60 to 100 ft booms to lift 20-ton dimensional granite blocks out of the quarry pit.
- Shipbuilding and marine: Wharf-mounted swing derricks at the Brooklyn Navy Yard handled gun turret components and plate steel during WWII destroyer construction.
- Heritage construction: National Park Service crews use period-correct guy derricks on masonry restoration of structures like the Washington Monument, where a modern crane footprint isn't permitted on the lawn.
- Bridge construction: Roebling's crews used guy derricks mounted on the towers of the Brooklyn Bridge during cable spinning and stiffening truss erection in the 1880s.
- Power plant and industrial maintenance: Stiffleg derricks bolted to the roof of older coal-fired generating stations lift turbine casing sections during overhauls where there's no overhead crane bay above the unit.
The Formula Behind the Swing Derrick
The capacity of a swing derrick is set by the overturning moment the mast and its anchorage can resist, divided by the working radius. Working radius is what the practitioner actually controls in the field — it's set by boom length and boom angle. At the low end of the typical operating range, the boom sits steep (around 75° from horizontal) and the radius is short, so the rig can lift its full rated capacity. The sweet spot is around 60° boom angle, where you get useful reach without giving up too much capacity. At the high end, the boom drops below 40° from horizontal, the radius gets long, and capacity drops sharply — that's where operators get into trouble.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Wmax | Maximum safe load at the hook for the given boom geometry | kN | lbf or tons |
| Mcap | Rated overturning moment capacity of the derrick (set by mast, anchorage, and structural design) | kN·m | ft·lbf |
| R | Working radius from mast centreline to hook centreline | m | ft |
| Lboom | Boom length from heel pin to boom point sheave | m | ft |
| θ | Boom angle from horizontal | degrees | degrees |
Worked Example: Swing Derrick in a shipyard stiffleg derrick lifting hull plate
A shipyard maintenance crew at a Great Lakes drydock in Sturgeon Bay, Wisconsin is using an existing roof-mounted stiffleg derrick to lift 3/4 inch hull plate sections from a flatbed onto the drydock platform. The derrick has a 60 ft boom, a rated overturning moment capacity of 360,000 ft·lbf, and the crew chief wants to know the safe load at boom angles of 75°, 60°, and 40°. Each plate weighs 4,200 lbs and the rigging adds another 300 lbs.
Given
- Lboom = 60 ft
- Mcap = 360,000 ft·lbf
- Plate + rigging = 4,500 lbs
Solution
Step 1 — at the steep low-end angle of 75°, compute the working radius:
Step 2 — divide the moment capacity by that radius to get the safe load:
At 75° the rig has plenty of margin — the 4,500 lb pick is using only about 19% of available capacity. The boom is almost vertical, the load tracks tight to the mast, and the operator has a comfortable swing.
Step 3 — at the nominal sweet-spot angle of 60°:
This is the working sweet spot. The hook reaches 30 ft out from the mast — enough to clear the truck bed and land on the drydock platform — with capacity at 12,000 lbs, still 2.7× the actual pick weight. Operators get useful reach without flirting with the capacity envelope.
Step 4 — at the flat high-end angle of 40°:
At 40° the hook reaches 46 ft but capacity has dropped to 7,830 lbs. The 4,500 lb plate is still safe, but you've burned through most of the margin, and the topping lift line tension has climbed sharply — at this angle the topping lift carries roughly 1.5× the load line tension. Drop the boom another 5° and most stiffleg derricks of this size hit their structural limit.
Result
At the 60° nominal boom angle the safe load is 12,000 lbs, which gives the crew a comfortable 2. 7× margin on the 4,500 lb plate pick. Comparing the three points: 75° gives 23,200 lbs capacity at 15.5 ft reach (overkill on capacity, short on reach), 60° gives 12,000 lbs at 30 ft (the working sweet spot), and 40° gives 7,830 lbs at 46 ft (long reach, thin margin). If your measured load on the line gauge reads higher than the predicted hook load, the most common causes are: (1) topping lift line slack letting the boom settle 3 to 5° lower than indicated, putting you at a longer radius than you think, (2) bull wheel swing-line stretch causing dynamic side-loading on the boom during swing, or (3) load chart referencing a different mast configuration — guy derrick capacities don't transfer to stiffleg ratings even on the same boom.
When to Use a Swing Derrick and When Not To
A swing derrick competes against the mobile crane, the tower crane, and the gin pole on construction lift duties. Each one wins on different jobs. The derrick wins where the lift is repetitive, the footprint is tight, and the rig can stay set up for weeks or months.
| Property | Swing Derrick (guy or stiffleg) | Mobile Hydraulic Crane | Gin Pole |
|---|---|---|---|
| Typical load capacity | 5 to 250 tons | 10 to 1,000+ tons | 1 to 20 tons |
| Swing arc | 270° to 360° | 360° continuous | 0° (fixed direction) |
| Setup time | 1 to 5 days (guy) or 4 to 12 hours (stiffleg) | 30 minutes to 2 hours | 2 to 4 hours |
| Footprint | Mast base only — guys reach out 30 to 60 ft | Outrigger pad 25 to 40 ft square | Mast base plus 3 guy anchors |
| Lift speed (load line) | 50 to 200 ft/min | 200 to 600 ft/min | 30 to 100 ft/min |
| Capital cost (rig only) | $40k to $300k for new build | $400k to $5M+ | $5k to $30k |
| Best application fit | Repetitive lifts in confined sites, quarry work, heritage restoration | One-off lifts, road-accessible sites, short duration | Single-axis pick like erecting a single column or pole |
| Operator skill required | High — independent control of swing, luff, and hoist | Moderate — single joystick coordination | Low to moderate |
Frequently Asked Questions About Swing Derrick
Swing inertia is the answer. The boom plus load is a long-radius rotating mass, and once you've accelerated it the kinetic energy has to go somewhere. If the swing brake clamps hard, the energy translates into boom deflection — the boom flexes forward, springs back, and the load oscillates over the landing.
The fix is technique, not hardware. Operators on production stiffleg rigs feather the swing line off about 15° before the target, let drag and bearing friction decelerate the boom, and apply the brake only for the last few degrees. If the bull wheel bearing is dry or the swing line has stretched out of round, this technique stops working and you'll see consistent overshoot regardless of operator input — that's a maintenance flag, not a training problem.
Three questions decide it: how much swing arc do you need, how much ground anchorage can you get, and how long is the rig sitting there. Guy derricks need 6 to 8 ground anchors at 30 to 60 ft radius from the mast — if your site is hemmed in by adjacent buildings or property lines, you can't develop those anchors.
Stifflegs need almost no ground anchorage but they consume 90° of your swing arc with the legs themselves, and they need ballast (concrete blocks or sill steel) to hold down the leg ends. Rule of thumb: guy derrick if you have open ground around the mast and need full 360° work, stiffleg if you're on a roof, a barge, or a paved site with no anchor possibilities and you can live with a 270° arc.
Two things are usually compounding. First, the load line reeving — if you're running a 4-part block, the line tension at the drum is roughly the hook load divided by 4, but only if the sheaves are clean and turning freely. A seized sheave converts that 4-part advantage into something closer to 2.5-part, and the drum sees nearly double the expected tension.
Second, dynamic loading during hoist acceleration. A snap-pick — where the hook tightens onto a load already under tension from an outrigger or a stuck plate — adds an impact factor of 1.3 to 1.8× the static weight. Slow the hoist start, free the load before lifting, and check every sheave on the load line for free rotation. If the discrepancy stays above 20% after that, the load cell calibration is off.
The crossover sits around 30° from horizontal on most derricks, depending on the geometry between the mast head sheave and the boom point. Below 30° the topping lift is doing more work holding the boom up against gravity than the load line is doing holding the load up.
This matters because the topping lift drum, brake, and reeving have to be sized for that crossover condition. If you're running a derrick at flat boom angles to maximise reach and the topping lift drum brake is overheating or slipping, the rig is being worked outside the geometry it was designed for. Either rerate it for a steeper minimum angle or upgrade the topping lift drum.
Mast walk happens when the guy tensions aren't balanced around the arc. As the boom swings, the load reaction at the mast head transfers from one guy to the next, and if one guy is tighter than its neighbours that guy gets overloaded while the opposite guy goes slack. The mast head pulls toward the slack side, and over a full swing rotation the mast traces a small circle — typically 25 to 75 mm diameter at the head.
Set guy tensions with a tensiometer, not by feel. All guys should read within ±10% of each other at no-load condition. If you can't get them balanced, one of the ground anchors is creeping — check anchor pin bearing on the soil and re-pin if needed.
No, and this is one of the most common ways crews get into trouble with older rigs. The rated overturning moment depends on the mast cross-section, the guy or stiffleg geometry, the bull wheel bearing rating, and the original design factor — none of which transfer between manufacturers even when the visible boom and mast dimensions look identical.
A 1940s American Hoist guy derrick and a 1960s Clyde Iron guy derrick with the same 80 ft boom can have rated capacities differing by 40%. Always work to the chart for the specific rig serial number, and if the chart is missing, get a structural engineer to rerate the derrick from first principles before it picks anything heavier than a test load.
References & Further Reading
- Wikipedia contributors. Derrick. Wikipedia
Building or designing a mechanism like this?
Explore the precision-engineered motion control hardware used by mechanical engineers, makers, and product designers.
