A Travelling Derrick is a rail-mounted crane that combines a vertical mast and pivoting boom with a wheeled base, letting the entire derrick roll along a fixed track while still hoisting and slewing loads. It solves the problem of moving heavy material across a long, narrow yard or mill bay without building a full overhead bridge crane. The mast supports the boom, the boom luffs and slews to position the load, and the trucks below carry the whole assembly down the rails. Yards routinely move 5 to 50 ton loads over runs of 100 metres or more this way.
Travelling Derrick Interactive Calculator
Vary the load radius, resisting moment, boom weight, boom center of gravity, and safety factor to see the safe hook load and tipping-moment balance.
Equation Used
The calculator applies the article safe-load relationship: subtract the boom self-weight moment from the resisting moment, then divide the remaining stabilizing moment by load radius and safety factor. Increasing radius directly lowers the allowable hook load.
- Imperial short tons and feet are used.
- Static side-view stability check only.
- Boom self-weight is represented as W_boom times r_cg.
- Wind, swing, rail shock, slew dynamics, and hoist tackle limits are not included.
How the Travelling Derrick Works
The Travelling Derrick is really three machines stacked on top of each other. At the bottom you have a wheeled base — usually a pair of trucks running on steel rails set 8 to 16 ft apart, driven by a small geared motor or hand-cranked through a worm reducer on lighter yard units. On top of that base sits the mast, a vertical column held upright by either guy ropes (guy derrick variant) or two rigid back-stays (stiffleg derrick variant). The boom pivots off the foot of the mast and is raised or lowered by the topping lift, while the load itself runs on a separate hoist line through a sheave at the boom tip. Slewing happens by rotating the mast and boom together relative to the base.
Why build it this way instead of a gantry crane? Because a gantry needs a top runway beam spanning the full width of the work area, and that's expensive once spans go past 40 ft or when you need clear airspace overhead for stacked lumber or hot ingots. The travelling derrick only needs ground-level rails. The trade-off is load radius — as the boom luffs out, the tipping moment grows fast, and a stiffleg derrick rated for 20 tons at 20 ft radius might only handle 8 tons at 40 ft. Get the radius wrong and the whole machine lifts a wheel off the rail. That's the classic failure mode and it's why every reputable derrick has a load radius indicator on the mast.
Tolerances matter on the rail gauge and on the back-stay anchor pins. Rails out of parallel by more than about 6 mm over a 10 m length will cause the leading truck to bind and skip under load. Worn back-stay pins introduce mast lean, and once the mast leans more than a degree or so off vertical the boom-foot bearing starts wearing oval — you'll hear it as a knock at the top of every lift. Topping-lift wire rope is the other common failure point; standard practice is to retire 6×19 IWRC rope once you see 6 broken wires in any one lay length.
Key Components
- Wheeled Base / Trucks: Two four-wheel trucks running on parallel steel rails, typically 8 to 16 ft gauge. The trucks carry the entire dead weight of the derrick plus the load reaction, and incorporate rail clamps to lock the unit in position before lifting. Wheel diameter is usually 12 to 18 in with axle bearings rated for the worst-case overturning load.
- Mast: Vertical column, traditionally timber or riveted steel lattice, taking the compressive load from the boom and the tension from the topping lift. Length runs from 20 ft on small yard units to 80 ft on heavy stiffleg derricks. The mast must stay within 1° of vertical under full load or the foot bearing wears unevenly.
- Boom: Pivots at the foot of the mast and luffs through an arc of roughly 15° to 75° above horizontal. Boom length sets the maximum load radius — longer boom means greater reach but lower capacity at the tip. A 60 ft boom on a 25-ton derrick typically rates 25 tons at 25 ft radius and 7 tons at 55 ft radius.
- Topping Lift: The wire rope and tackle that raises and lowers the boom. Usually a multi-part rope reeved through sheaves at the mast head and boom tip. Reeving ratios of 4:1 to 8:1 are common to bring line pull within hoist drum capacity. Inspect for broken wires every 200 lift cycles.
- Hoist Line and Load Block: Independent wire rope running over the boom-tip sheave to the load hook. Reeved into a load block (multiple sheaves) to multiply lifting force. A 4-part hoist line on a 5,000 lb drum pull lifts 18,000 lb after sheave friction losses of about 4% per sheave.
- Back-stays or Guy Lines: Resists the horizontal pull of the boom. A stiffleg derrick uses two rigid steel struts set at 90° to each other; a guy derrick uses six to eight wire ropes anchored to ground points. Stiffleg gives you 270° of slewing freedom; full guy derricks give 360° but need a much larger anchor footprint.
- Slewing Mechanism: Rotates the mast-and-boom assembly relative to the base. Bull wheel and pinion drive at the mast foot, hand-cranked or motorised. Slew speed is slow — typically 0.5 to 1 RPM — because momentum of a swinging load can easily overload the back-stays.
- Travel Drive: Geared motor on one truck driving through a chain or shaft to the wheels. Travel speed is deliberately slow, 5 to 15 m/min, because you do not move the derrick under load on most installations. Rail clamps must be released before travel and reapplied before lifting.
Who Uses the Travelling Derrick
Travelling Derricks belong to the era when material handling meant rails on the ground rather than beams overhead, and they survive today wherever the work area is long, narrow, and outdoor. You still see them in shipyards, lumber mills, stone quarries, and steel-stockholder yards. The reason is economic — for a 200 m long yard, a travelling derrick costs a fraction of a gantry crane of the same span and needs no overhead structure. The downside is that it cannot work directly above an obstruction, so you would not pick one for a congested factory floor.
- Lumber Yards: Weyerhaeuser yard derricks moving 40 ft Douglas fir timbers between sawmill outfeed and the drying stacks, typical lift 3 to 5 tons at 30 ft radius.
- Stone Quarries: Vermont Quarries Corp uses rail-mounted derricks to lift 8-ton marble blocks from the cut face onto flat cars for transport to the mill.
- Shipbuilding: Historical use at Harland & Wolff Belfast where travelling guy derricks ran along quayside rails to set hull plates during riveted construction up to the 1950s.
- Steel Stockholders: Cleveland-Cliffs scrap yard derricks shifting 10-ton bundled rebar and structural beams across stocking bays 150 m long.
- Heavy Construction: Stiffleg travelling derricks setting precast concrete bridge beams along a 300 ft launching gantry on highway projects.
- Foundries: Travelling derricks moving 5-ton sand moulds and pattern boxes between the moulding line and the pouring floor in iron foundries.
- Logging Operations: Pacific Northwest log-sorting yards using rail-mounted derricks to grade and stack saw logs by species and diameter.
The Formula Behind the Travelling Derrick
The number that decides whether a travelling derrick will or will not pick a given load at a given position is the safe load at radius. The boom acts as a lever — the load tries to tip the derrick about the leading rail or stiffleg, and the mast and back-stays resist that moment. At short radius the geometry favours you and capacity is limited by the hoist tackle. At long radius capacity drops fast because the overturning moment grows linearly with reach. The sweet spot on most yard derricks sits between 30% and 60% of maximum boom length — short enough to keep capacity high, long enough to avoid constant repositioning of the trucks.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Wsafe | Safe working load at the hook for a given radius | kg | lb |
| Mresist | Stabilising moment from derrick dead weight and back-stay anchorage | N·m | lb·ft |
| Wboom | Weight of boom alone | kg | lb |
| rcg | Horizontal distance from mast centreline to boom centre of gravity | m | ft |
| rload | Horizontal distance from mast centreline to load hook (load radius) | m | ft |
| FoS | Factor of safety, typically 1.5 to 2.0 against tipping | dimensionless | dimensionless |
Worked Example: Travelling Derrick in a granite quarry travelling stiffleg derrick
A dimensional granite quarry in Barre Vermont is sizing the safe lift chart for a rail-mounted stiffleg derrick that runs the length of a 180 m extraction trench. The derrick has a 60 ft boom and an 80,000 lb·ft stabilising moment from the stiffleg geometry. The boom itself weighs 4,000 lb with its centre of gravity at half its deployed horizontal reach. The crew wants to know what the derrick can safely lift at 20 ft, 35 ft, and 55 ft radius using a tipping factor of safety of 1.5.
Given
- Mresist = 80,000 lb·ft
- Wboom = 4,000 lb
- Boom length = 60 ft
- FoS = 1.5 dimensionless
Solution
Step 1 — at the nominal mid-range radius of 35 ft, the boom CG sits at roughly half the horizontal reach. Compute the boom self-moment:
Step 2 — apply the safe load formula at 35 ft radius:
That number tells you immediately the derrick is sized too light for 35 ft work — a single granite block weighs many times that. Real quarry derricks of this geometry would have Mresist in the 400,000 to 800,000 lb·ft range. Let's redo the math with Mresist = 600,000 lb·ft, which matches a real Barre-style stiffleg.
Step 3 — at the short end of the operating range, 20 ft radius:
That's a comfortable 9-ton lift — typical of pulling a freshly cut block straight up off the bench. Step 4 — at the nominal 35 ft radius:
Step 5 — at the long-radius end, 55 ft, where the operator wants to drop a block onto the flatcar two tracks over:
You see the capacity collapse from 9 tons to under 3 tons just by booming out. The sweet spot for production work is 25 to 40 ft radius — long enough to clear the cut, short enough to keep capacity above 5 tons.
Result
At 35 ft radius the realistic stiffleg derrick safely lifts about 10,100 lb — call it 5 tons of granite, which is the working size for most Barre dimensional blocks. The low-end 20 ft radius gives you 18,700 lb of headroom for the heaviest tail-end blocks, while the 55 ft long-reach drops to 5,940 lb, which only suits trim cuts and shop offcuts. If your measured tip capacity comes in 20% below this prediction, the usual culprits are: stiffleg anchor bolts working loose under cyclic load (you'll see paint cracking around the base plate), boom-foot pin wear pushing the effective pivot rearward by 50 to 100 mm, or mast lean greater than 1° from vertical caused by uneven settling of one rail tie. Each of those quietly eats into M<sub>resist</sub> without showing up on any gauge.
Choosing the Travelling Derrick: Pros and Cons
Picking a travelling derrick over the alternatives is mostly about yard geometry and budget. Compare on capacity per dollar, footprint, slewing freedom, and how much overhead structure you can tolerate.
| Property | Travelling Derrick | Overhead Bridge Crane | Mobile Crawler Crane |
|---|---|---|---|
| Typical capacity | 5 to 50 tons | 5 to 200 tons | 30 to 500 tons |
| Working radius / span | Up to 80 ft boom radius | Span of runway, 20 to 100 ft | Up to 200 ft boom |
| Travel speed under load | No travel under load (5-15 m/min unloaded) | 30 to 60 m/min | Variable, slow on tracks |
| Slew range | 270° (stiffleg) or 360° (guy) | N/A — linear travel only | 360° continuous |
| Capital cost | Low — rails plus derrick | High — full overhead structure | Very high but mobile |
| Setup permanence | Semi-permanent rail run | Permanent building feature | Fully mobile, no fixed install |
| Best application fit | Long narrow outdoor yards | Indoor production bays | Construction sites, varied locations |
| Maintenance interval | Quarterly rope and pin inspection | Annual runway and trolley service | Pre-shift OEM checks |
Frequently Asked Questions About Travelling Derrick
This is the classic warning that you have exceeded the stabilising moment for the current radius — but at a specific azimuth. Stiffleg derricks have asymmetric resistance: the two stifflegs are set at 90° to each other, so when you slew the boom into the quadrant directly opposite both stifflegs, Mresist is at its peak. When you slew toward the open quadrant between the stifflegs, resistance can drop by 30% or more, and a load that was safe at one heading becomes a tipping load 90° away.
Check your load chart for the worst-case azimuth, not the best case. If the chart only quotes one number, derate by 25% any time the boom passes through the open quadrant.
Stiffleg every time, unless you genuinely need 360° slewing. The stiffleg gives you 270° of working arc, which covers any practical yard task, and the rigid struts mean no guy-rope retensioning, no anchor pits, and a much smaller footprint outside the rails. Guy derricks need 6 to 8 ground anchors set at radii equal to or greater than the mast height — on a 60 ft mast that's a 60 ft radius anchor field, which often does not fit alongside the rails.
The only real case for a guy variant is when you need to lift directly behind the rail line, into the quadrant a stiffleg blocks.
On almost every installation the answer is no, and the load chart will say so explicitly. The rail wheels and trucks are sized for vertical load only — adding the dynamic horizontal loads of starting and stopping a moving derrick under load can shift the centre of pressure outside the rail base, plus any rail joint or alignment defect becomes a shock load through the boom tip.
The correct sequence is: lift, slew to centre, lower the load to a safe transport height or ground it, release rail clamps, travel, reset clamps, lift again. Any derrick rated for travel-under-load will say so on the nameplate and have a separate, much lower, capacity column for that mode.
Work backwards from the maximum boom moment. The topping lift has to support the boom weight plus the load times their respective radii, all referenced to the boom-foot pin. For a 60 ft boom carrying 10,000 lb at the tip, the moment about the foot is roughly 600,000 lb·ft. Resolved through the topping-lift geometry — typically a topping angle of 30 to 45° — the line tension can run 15,000 to 25,000 lb.
You then choose a reeving ratio (number of rope parts) that brings that load below your hoist drum's rated single-line pull, usually around 4,000 lb on a yard derrick. So 6 to 8 parts of line, with 4% friction loss per sheave, is standard. Undersize the reeving and the drum stalls before the boom lifts.
The calculation assumes Mresist is rigid and instantaneous — the real machine has flex and slack. Three common culprits eat capacity invisibly: stiffleg foundation settlement (a 10 mm drop on one stiffleg can shift the effective pivot by enough to halve apparent capacity), worn bull-wheel teeth letting the slewing assembly lash backward at lift initiation, and dynamic load multiplication from a snatched lift — yanking a stuck block can apply 2× the static load for a fraction of a second.
Diagnostic check: do a slow, smooth lift with a calibrated load and a digital inclinometer on the mast. If the mast leans more than 0.5° before the load clears the ground, you have geometry losses, not a strength shortfall.
Yes, more than people expect. A 60 ft boom presents roughly 30 sq ft of projected area when horizontal. At 30 mph wind, drag force on the boom alone is about 90 lb — but applied at 30 ft radius, that's 2,700 lb·ft of overturning moment from the boom alone, before the load even contributes. A flat granite slab or a sheet of plywood as the load can easily double that.
Standard practice is to stop lifts at 20 mph sustained wind and stow the boom (luffed up to near vertical) at 35 mph. Boom up means the wind moment arm collapses to a few feet rather than tens of feet.
References & Further Reading
- Wikipedia contributors. Derrick. Wikipedia
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