A swing-boom crane is a lifting machine with a fixed-length horizontal or near-horizontal boom that pivots around a vertical mast or column to move loads through an arc. Workshops, loading docks, and small construction sites rely on them where a full mobile crane will not fit. The boom carries a hoist trolley or fixed hook, and an operator swings the load by hand, by a slew drive, or by a hydraulic ram. The result is repeatable lifts of 250 lbs to 5 tons inside a defined radius, often replacing a forklift in cramped bays.
Swing-boom Crane Interactive Calculator
Vary load, hoist weight, boom weight, boom length, and trolley radius to see the mast-base tipping moment update.
Equation Used
The calculator adds the moment from the lifted load and hoist at trolley radius R to the boom self-weight moment acting at half the boom length.
- Static vertical loading only; dynamic swing, impact, wind, and seismic factors are not included.
- Boom self-weight acts at the boom midpoint.
- Trolley radius should not exceed the physical boom length.
How the Swing-boom Crane Actually Works
The mechanism is simple in principle and unforgiving in practice. A vertical mast carries the entire bending moment of the boom plus the load, and the boom pivots around that mast through a slew bearing or a pair of thrust bearings stacked top and bottom. The hoist hangs from the boom tip, or rolls along it on a trolley, and the operator rotates the whole assembly through an arc — typically 180° for a wall-mounted jib, 270° to 360° for a pillar-mounted unit. The slewing radius defines the working envelope. Anything outside that arc you cannot reach without moving the base.
Why build it this way? Because a fixed boom is dramatically stiffer and cheaper than a luffing boom, and for repeat lifts inside a known radius you do not need the boom to change angle. The trade is that the load path is a circle, not a line. If the boom geometry is off — say the mast is out of plumb by more than 1° — you will see the load drift on its own when you let go of it, because gravity finds the low side of the arc. Same problem if the slew bearing has more than 0.5 mm of radial play: the boom tip will sag under load, the trolley will roll toward the tip, and the operator fights the crane on every lift.
Failure modes cluster around three things. First, the mast-to-floor connection — anchor bolts pulling out of concrete is the single most common catastrophic failure, and it almost always traces back to undersized chemical anchors or a slab too thin for the tipping moment. Second, the slew bearing wearing oval, which lets the boom drop a few millimetres per year until the trolley binds. Third, fatigue cracks at the boom-to-mast weld, which sees a fully reversing stress cycle every time you swing a load from one side to the other. Inspect that weld with dye penetrant every 12 months on a duty-cycle unit.
Key Components
- Mast (column): The vertical structural member that carries the full overturning moment from the boom and load. Sized as a heavy-wall structural tube or wide-flange section, typically 6 to 12 inch nominal diameter for capacities up to 2 tons. Plumb tolerance must be held to 0.5° or the load will self-swing.
- Boom: The horizontal arm that carries the hoist. Built as an I-beam (S-shape or W-shape) so the trolley wheels ride the lower flange. Length sets the slewing radius — common spans are 8 to 20 ft. Deflection at the tip under rated load should not exceed L/450, or roughly 1/2 inch on a 16 ft boom.
- Slew bearing or thrust bearing pair: Lets the boom rotate while transferring vertical and tipping loads into the mast. On smaller units a top-and-bottom tapered roller pair handles it; on heavier units a single slewing ring with integral gear teeth does the job. Radial play must stay under 0.5 mm or boom-tip sag becomes noticeable.
- Hoist and trolley: The lifting unit itself — chain hoist, wire rope hoist, or air hoist — mounted on a trolley that rolls along the lower boom flange. Capacity matches the crane rating, with a 5:1 design factor on the chain or rope. Trolley wheel flanges must clear the beam flange by 1 to 2 mm.
- Slew drive (powered units only): An electric or hydraulic motor driving a pinion that meshes with the slewing ring's gear teeth. Speeds are deliberately slow — 0.5 to 2 RPM at the boom — to keep load swing under control. A brake holds position when the motor is off.
- Base anchor system: Chemical anchors or cast-in J-bolts tying the mast base plate into a concrete foundation. The foundation must resist the tipping moment plus a 1.5× safety factor. Slab thickness of 8 to 12 inches with rebar mat is typical for a 1-ton, 12 ft boom unit.
Industries That Rely on the Swing-boom Crane
Swing-boom cranes show up wherever you need repeat lifts in a fixed footprint and cannot justify a bridge crane or run a mobile crane through the door. They are the workhorse of small fabrication shops, machine maintenance bays, and dockside loading stations. The mechanism scales from a 250 lb wall-mounted shop jib up to 10-ton pillar cranes feeding heavy machine tools, and the cost-per-lift over a 20-year service life beats almost anything else in that capacity range.
- Machine shops: Gorbel free-standing jib cranes feeding CNC mills and lathes, typically 1/2 ton to 2 ton capacity with 12 to 16 ft booms covering a single machine cell.
- Loading docks: Wall-mounted swing jibs at distribution facilities, often Spanco or David Round units rated 1,000 to 4,000 lbs, used to lift palletized goods from flatbed trucks where a forklift cannot back in.
- Boat yards and marinas: Pillar-mounted davit cranes lifting outboard motors, sailboat rigging, and small inboard engines — Garhauer and Forespar build the marine-grade versions in 500 to 1,500 lb capacities.
- Foundries and forging shops: Heavy pillar cranes with 3 to 5 ton capacity swinging hot castings from the furnace to the trim press. Whiting and Konecranes both supply duty-rated units for this service.
- Construction job sites: Trailer-mounted swing-boom cranes like the Venturo HT50KX setting HVAC rooftop units, generators, and concrete formwork on residential and light commercial sites.
- Wind turbine nacelle maintenance: Internal swing-boom service cranes mounted inside the nacelle of GE and Vestas turbines for swapping gearbox components and yaw motors at hub height.
The Formula Behind the Swing-boom Crane
The single most important calculation on a swing-boom crane is the tipping moment at the mast base, because that number drives anchor selection, foundation thickness, and whether the unit will stay upright on a free-standing pillar install. At the low end of the typical operating envelope — load close to the mast — the moment is small and the crane feels stable. Push the load out to the boom tip and the moment grows linearly with radius. The sweet spot is usually around 70% of full boom reach, where you keep operator control of the swing without driving the foundation design into a corner.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Mtip | Tipping moment at the mast base | N·m | lb·ft |
| Wload | Weight of the lifted load | N | lb |
| Whoist | Weight of the hoist and trolley | N | lb |
| Wboom | Weight of the boom itself | N | lb |
| R | Radius from mast centreline to load (trolley position) | m | ft |
| Lboom | Total boom length | m | ft |
Worked Example: Swing-boom Crane in a precast concrete plant pillar crane
A precast concrete plant in Lehigh Valley, Pennsylvania is sizing a free-standing pillar-mounted swing-boom crane to move 2,000 lb concrete formwork panels from the curing rack to the demoulding station. The boom is a 16 ft W8x21 wide-flange beam weighing 336 lbs. A Harrington 1-ton electric chain hoist with trolley adds 180 lbs. The mast is 10 inch schedule-80 pipe anchored to a 4 ft × 4 ft × 12 inch reinforced concrete pad. They need to know the tipping moment at three trolley positions to spec the chemical anchors.
Given
- Wload = 2000 lb
- Whoist = 180 lb
- Wboom = 336 lb
- Lboom = 16 ft
- Rnominal = 11 ft (70% of boom length)
Solution
Step 1 — compute the nominal tipping moment with the trolley at 11 ft, the design sweet spot at 70% of full reach:
That is the number the foundation engineer designs around. At this radius the crane operator has good visual control of the swing arc and the load does not feel like it is fighting back.
Step 2 — compute the low end of the typical operating range, with the trolley pulled in to 4 ft from the mast:
That is 43% of the nominal moment. The crane feels rock-solid here, anchor stresses are well below limit, but you have given up most of your useful working radius — this position is only sensible for parking the load while you reposition the receiving cart.
Step 3 — compute the high end, with the trolley run out to 15 ft (just shy of the boom-end stop):
That is 33% higher than the design point. The mast deflection roughly doubles versus the 11 ft case, and on a free-standing install you will feel the column flex visibly when the operator stops the swing. This is also where chemical anchor pull-out failures originate. Every doubling of radius effectively doubles the anchor tension.
Result
Nominal tipping moment is 26,668 lb·ft at the 11 ft trolley position. That translates to roughly 6,700 lbs of tension on the upwind anchor bolts of a 4-bolt pattern on a 24 inch bolt circle, which sets the chemical anchor at a Hilti HIT-RE 500 V3 with 5/8 inch threaded rod or larger. The low-end 11,408 lb·ft case lets the operator stage formwork without stressing the foundation, the 26,668 lb·ft nominal is your everyday lift, and the 35,388 lb·ft high-end case should be treated as the worst-case design load. If your measured anchor bolt elongation exceeds 0.5 mm under rated load, suspect (1) chemical anchor cure time cut short — they need a full 24 hours at 70°F before loading, (2) a slab thinner than the 12 inches specified (a borescope through one anchor hole tells you the truth), or (3) edge distance under the 6 inch minimum, which lets the anchor cone-fail through the slab edge instead of holding tension.
When to Use a Swing-boom Crane and When Not To
A swing-boom crane is not the right answer for every lifting problem. The decision usually comes down to working envelope, capacity, install cost, and how often you will actually lift. Here is how it stacks up against the two most common alternatives — an overhead bridge crane covering a full bay, and a workshop gantry crane you can roll where you need it.
| Property | Swing-boom crane | Overhead bridge crane | Portable gantry crane |
|---|---|---|---|
| Working envelope shape | Circular arc, 180° to 360° | Full rectangular bay coverage | Linear path along the gantry's wheel travel |
| Typical capacity range | 250 lb to 10 tons | 1 ton to 50+ tons | 500 lb to 5 tons |
| Slew or travel speed | 0.5 to 2 RPM (powered) or hand-swung | 20 to 100 ft/min bridge travel | Manual push, ~50 ft/min |
| Installed cost (1-ton class) | $3,500 to $9,000 incl. foundation | $25,000 to $60,000 incl. runway | $2,000 to $4,500 |
| Foundation/structural requirement | Reinforced slab + chemical anchors, or wall mount | Building columns sized for runway loads | None — rolls on existing floor |
| Setup and relocation effort | Permanent install, days to relocate | Permanent — runway is fixed | Minutes — wheel it where needed |
| Service life (hours of duty) | 20,000+ hrs at CMAA Class C | 30,000+ hrs at CMAA Class D | 10,000 hrs typical |
| Best application fit | Single workstation, repeat lifts inside fixed radius | Whole-bay material handling | Occasional lifts in changing locations |
Frequently Asked Questions About Swing-boom Crane
That wobble is mast deflection plus base-plate rocking, not anchor bolt slip. A 10 inch pipe mast on a 24 inch bolt circle deflects roughly 1/4 inch at the boom tip under a stop-event from 1 RPM with a 1-ton load — that motion amplifies along the 16 ft boom and reads as a visible wobble.
The fix is almost never bigger anchors. It is either a stiffer mast section (go to a 12 inch schedule-80 or a wide-flange column), a wider bolt circle, or a thicker base plate. A 1 inch base plate on a 24 inch bolt circle flexes; a 1-1/2 inch plate on a 30 inch circle does not. Slowing the slew brake's stopping ramp from 0.5 seconds to 1.5 seconds also kills the wobble without touching steel.
Three factors decide it. First, building structure — a wall mount needs a column or reinforced wall section that can take the reaction couple, which is typically 2 to 3 times the load weight at each anchor point. If your wall is metal stud or unreinforced block, you do not have a wall-mount option. Second, swing arc — a wall jib gives you 180° at best, a pillar crane gives you 360°. Third, headroom — a wall jib steals less overhead because there is no top mast bracket above the boom.
Rule of thumb: if you have a structural column or a poured concrete wall within the working radius, mount to it. If not, eat the foundation cost and go free-standing.
The mast is out of plumb. Gravity is finding the low side of the slew arc and pulling the boom there. On a free-standing pillar this almost always means the foundation pad settled unevenly after install, or the base plate was shimmed and the shims compressed under load.
Put a 4 ft machinist's level on the mast in two perpendicular planes. Anything more than 0.5° out of plumb will produce noticeable drift with a 1-ton load on a 16 ft boom. Re-shim with steel (not aluminum, which creeps) and re-grout the base plate. If the slab itself has tilted, you have a foundation problem and the crane needs to come down until the pad is corrected.
No, and this is the single most common field modification that ends in failure. The tipping moment scales linearly with radius — going from a 12 ft boom to a 16 ft boom at the same rated load increases the moment by 33%, and the anchor tension by roughly the same amount. Your existing chemical anchors were sized for the original moment with a specific safety factor, usually 1.5 to 2.0. Adding 33% load eats most of that margin.
If you need more reach, the correct path is to derate the capacity inversely with the radius increase, or redesign the foundation and mast as a system. A reputable manufacturer like Gorbel or Spanco will not sell you a longer boom for an existing mast without restamping the rated capacity.
That is almost certainly a damaged tooth on the slewing ring or pinion. A slew ring takes its highest local stress where the load passes most often — typically the position where you pick up loads — and a single chipped tooth will make the pinion lose mesh and stall the motor at that one angular position.
Pull the inspection cover and rotate the boom slowly by hand with the motor de-energized. Run a fingernail along the gear teeth at the stall position. You will feel the chipped tooth before you see it. Replacing one slewing ring on a 1-ton crane runs $800 to $2,000 in parts and a full day of labour, but ignoring it leads to total loss of slew control.
0.7 to 1.2 RPM at the boom. Faster than that and a 2-ton load on a 14 ft boom develops enough kinetic energy that the operator cannot stop it safely without a controlled brake ramp — and the load will pendulum on stop, which is dangerous around other workers. Slower than 0.5 RPM and the operator gets impatient and tries to push-assist by hand, which is a hand-injury risk.
The David Round and Konecranes catalog defaults for this capacity class land at 1 RPM nominal with a soft-start, soft-stop VFD on the slew motor. That number is not arbitrary — it is the result of decades of incident data on what speeds keep loads controllable.
References & Further Reading
- Wikipedia contributors. Jib (crane). Wikipedia
Building or designing a mechanism like this?
Explore the precision-engineered motion control hardware used by mechanical engineers, makers, and product designers.
