A crab winch is a hand-cranked lifting and pulling device that uses a pair of spur-gear reductions between the crank handle and the rope drum to multiply operator force. Turn the handle, the pinion drives a larger gear on a layshaft, that layshaft drives a second pinion meshing with the drum gear, and a pawl-and-ratchet locks the load between strokes. The double reduction lets one person lift loads of 500 to 2000 kg using 10 to 20 kg of hand effort, which is why theatres, shipyards and quarry workshops still keep them on the floor.
How the Crab Winch Actually Works
The crab winch — sometimes called a hand crab or double-purchase winch — is one of the oldest force multipliers still in working service. Two cast or fabricated side plates hold three parallel shafts: the crank shaft, the layshaft, and the drum shaft. A small pinion on the crank shaft meshes with a large spur gear on the layshaft. A second small pinion on the layshaft meshes with the large gear cast onto, or keyed to, the rope drum. Multiply the two ratios together and you get the total mechanical advantage — typically 20:1 to 60:1 depending on the build.
Why two stages and not one? Because a single-stage reduction big enough to give 40:1 would need a gear roughly the diameter of a dinner plate meshing with a tiny pinion, and the tooth-bending stress on that pinion goes through the roof. Splitting the ratio across two stages keeps each pinion at a sensible 12 to 16 teeth and keeps tooth root stress inside what plain cast iron or mild steel can handle for thousands of cycles. You will see this on a Tirfor TU-32 style hand winch and on traditional brass-bound theatre crabs alike.
If the gear mesh centre distance drifts more than about 0.3 mm from spec, you get backlash that lets the drum spin back a few degrees each time you release the handle to take another bite — and that is what overloads the pawl. The pawl-and-ratchet is the safety-critical part. The ratchet wheel sits on the layshaft or drum shaft, and a spring-loaded pawl drops into each tooth as the drum rotates lifting-side. If the pawl tip is worn flat, if the spring has lost tension, or if the ratchet teeth are case-hardened only on the leading face and the operator has been running the winch in reverse to pay out, you get a pawl that skips under load. That is the classic crab winch failure — the load free-falls one ratchet pitch before the next tooth catches, and the handle whips backwards hard enough to break a wrist.
Key Components
- Crank handle and pinion: The operator's input. Handle radius is typically 300 to 400 mm to keep input torque comfortable at 10 to 15 kg of hand force. The pinion has 10 to 16 teeth, module 3 to 5, and is keyed to the crank shaft with a parallel key — never a grub screw alone.
- Layshaft with large gear and second pinion: The intermediate stage. The large gear takes the first reduction (typically 4:1 to 6:1) and the second pinion delivers the second reduction into the drum gear. Layshaft bearings must be shimmed to keep gear face misalignment under 0.1 mm per 100 mm — anything more and you wear one end of the tooth flank long before the other.
- Drum gear and rope drum: The output. Drum gear has 60 to 100 teeth. The drum itself is grooved for the rope diameter — a 6 mm wire rope wants a groove pitch of 6.5 to 7 mm, not a flat barrel, otherwise the rope crosses over itself on the second layer and crushes.
- Pawl and ratchet: The non-return safety. Ratchet teeth are usually 8 to 12 around the wheel. The pawl pivot must sit so that the line of action through the pawl tip passes behind the pivot — that geometry is what forces the pawl harder into the tooth as load increases. Get that geometry wrong and the pawl kicks itself out under load.
- Side plates and tie rods: The structural frame. Plates are usually 6 to 10 mm steel. Tie rods hold the plates parallel under gear separating force, which on a 2 tonne winch can hit 8 to 10 kN per shaft. Loose tie rods let the plates flex, the gears walk apart, and the mesh degrades within hours of use.
- Wire rope or chain: 6 mm to 10 mm wire rope is standard for 500 to 2000 kg working load. Rope must be terminated to the drum with a properly sized wedge socket or two wire-rope clips with the saddle on the live side — never reverse the saddle.
Who Uses the Crab Winch
Crab winches survive in industries where electricity is inconvenient, intermittent duty makes a powered hoist overkill, or the load profile is so varied that a hand-feel-controllable lift beats a fixed-speed motor. They are still specified new today for theatre flying, shipboard rigging, mine workshops, and heritage restoration work. Where you need precise inching of a heavy object — lowering a sculpture onto a plinth, easing a boat into chocks, holding a flown scenic piece at a cue mark — the crab winch beats most powered alternatives because the operator's hand is the speed controller and the pawl is the brake.
- Theatre rigging: Donmar Warehouse and similar drama houses use brass-bound timber-cheek hand crabs for counterweight-assisted scenic flying, typically rated 250 to 500 kg with a 40:1 reduction.
- Shipyard and marine: Lewmar and Harken still produce sailing-yacht winches that share the crab winch's double-spur-gear topology, used for sheet and halyard tensioning on yachts above 12 m.
- Heritage railway: The Severn Valley Railway workshop uses pedestal-mounted crab winches to lift locomotive cab components — typically 800 kg loads — without bringing in a powered crane.
- Mining and quarry workshops: Pit-bottom maintenance bays use crab winches to handle pump and motor swaps where flameproof electrical gear would be required for any powered alternative.
- Construction and building services: Lifting plant rooms still use wall-mounted crab winches rated 500 kg to feed boilers, cooling towers, and tank sections through restricted hatches in retrofits where a chain hoist will not fit.
- Museum and conservation: The Mary Rose Trust used hand crab winches during the long lift conservation work on hull fragments where vibration from powered equipment was unacceptable.
The Formula Behind the Crab Winch
What you need from a crab winch is the relationship between hand effort at the crank and load lifted at the drum. The formula below is the ideal mechanical advantage — efficiency losses come off the top. At the low end of the typical range, with a single-stage 8:1 winch, one operator pulling 15 kg lifts about 100 kg before efficiency losses, which is barely enough to justify the gearbox. At the high end, a double-reduction 60:1 winch lets that same 15 kg of hand effort hold 750 kg of static load — but the crank turns 60 times for every drum turn, so a 1 m lift takes 60 m of hand travel. The sweet spot for theatre and workshop work sits around 30:1 to 40:1, which lifts 450 to 600 kg per 15 kg input and keeps lift speed practical.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Fload | Load lifted at the rope | N | lbf |
| Fhand | Tangential force applied at the crank handle | N | lbf |
| Rhandle | Crank handle radius | m | in |
| rdrum | Effective rope drum radius (drum + ½ rope diameter) | m | in |
| N1 / n1 | First-stage gear ratio (large gear teeth / pinion teeth) | — | — |
| N2 / n2 | Second-stage gear ratio (drum gear teeth / layshaft pinion teeth) | — | — |
| η | Combined efficiency (typically 0.70 to 0.85 for two-stage spur) | — | — |
Crab Winch Interactive Calculator
Vary the two pinion and gear tooth counts to see stage ratios, total mechanical advantage, and drum speed reduction.
Equation Used
The calculator multiplies the two spur-gear reductions: the first large gear divided by the crank pinion, then the drum gear divided by the layshaft pinion. The result is the ideal mechanical advantage and the reciprocal is the drum speed relative to crank speed.
- Ideal spur-gear train; friction and bearing losses are not included.
- Mechanical advantage is based on gear tooth ratios only.
- Rope drum radius, cable layering, and pawl impact loads are not included.
Worked Example: Crab Winch in a heritage crab winch rebuild for a brewery cellar lift
A craft brewery in Tasmania is rebuilding a wall-mounted hand crab winch to lift 200 L barrels — about 270 kg loaded — from the cellar up a 3.8 m hatch into the loading bay. The existing winch has a 350 mm crank, a 75 mm radius drum (including 6 mm rope), a first-stage ratio of 5:1 (60T / 12T), and a second-stage ratio of 6:1 (72T / 12T). Combined efficiency is estimated at 0.78. The operator can sustain 12 kg of hand force comfortably for the 30-second lift cycle. They want to know whether the winch is sized correctly, what hand force is needed, and what happens at the edges of the operating range.
Given
- mload = 270 kg
- Rhandle = 0.350 m
- rdrum = 0.075 m
- N1/n1 = 5 —
- N2/n2 = 6 —
- η = 0.78 —
Solution
Step 1 — convert the load to a force at the rope:
Step 2 — compute the total ideal mechanical advantage from handle to rope, including the handle-to-drum lever ratio:
Step 3 — apply efficiency and solve for the hand force needed at nominal 270 kg load:
That is the nominal answer — about 2.5 kg of hand effort, comfortable for any adult operator. Now the range. At the low end of the brewery's expected loads — an empty 50 L keg-rack at roughly 60 kg — required hand force drops to about 0.55 kg, so light the operator will overshoot the lift speed and risk cracking the rack against the hatch frame. At the high end — a worst-case 400 kg pallet of full kegs — required hand force climbs to:
Still comfortable. The winch has plenty of margin. The real constraint at the high end is not operator effort — it is the pawl-and-ratchet rated load and the wire rope's working load limit, both of which need checking against the 400 kg case before the brewery commits to that as the duty load.
Result
Nominal hand force at the 270 kg design load is about 24 N, or 2. 5 kg — well inside what a single operator can sustain for a 30-second lift. Across the working range the operator feels almost no difference between an empty 60 kg load (0.55 kg at the handle) and a 400 kg overload case (3.7 kg at the handle), which is exactly why the double-reduction crab winch suits cellar work — the operator's hand stays the speed controller, not a strength contest. If the measured hand force comes out higher than predicted, the three usual culprits are: (1) gear efficiency below the assumed 0.78, almost always from dried-out grease in the layshaft bearings turning rolling friction into sliding friction, (2) rope build-up on the drum exceeding the assumed 75 mm radius — once you are on the second layer, r<sub>drum</sub> climbs to 87 mm and MA drops by 14%, and (3) crank handle bent or shortened from a previous repair, dropping R<sub>handle</sub> below 350 mm without anyone documenting the change.
When to Use a Crab Winch and When Not To
The crab winch competes against electric chain hoists and manual lever hoists (Tirfor-style come-alongs) for the same workshop and rigging jobs. Each wins on different axes — the choice usually comes down to duty cycle, operator-feel requirements, and whether the site has reliable power.
| Property | Crab winch | Electric chain hoist | Lever hoist (come-along) |
|---|---|---|---|
| Typical load capacity | 250–2000 kg | 250–10,000 kg | 750–6000 kg |
| Lift speed | 1–4 m/min (operator-controlled) | 4–8 m/min (fixed) | 0.5–1.5 m/min |
| Mechanical advantage | 20:1 to 60:1 | N/A (motor-driven) | 30:1 to 50:1 |
| Operator effort at rated load | 10–20 kg at handle | Push-button only | 20–35 kg at lever |
| Power requirement | None | 230 V or 400 V mains | None |
| Capital cost (500 kg class) | £400–£900 | £1200–£2500 | £250–£600 |
| Service life | 20–50+ years with pawl/grease care | 10–20 years | 10–20 years |
| Best application fit | Inching, theatre cues, intermittent lifts | Repetitive production lifts | Field rigging, recovery, remote sites |
| Failure mode under abuse | Pawl skip / handle kickback | Motor brake failure / drift | Lever bend / chain jam |
Frequently Asked Questions About Crab Winch
That creak is almost always the side plates flexing against the tie rods, not the gears. Gear separating force on a 1 tonne crab winch can hit 6 to 8 kN per shaft, and if the tie rods have backed off even a quarter turn the plates deflect outward as load comes on, then snap back as load releases — that is the creak.
Check tie rod torque first. Then check that the layshaft does not have axial play greater than 0.2 mm. Persistent creaking under steady load (no pumping) usually points to a hairline crack at a tie rod boss, which is a scrap-the-frame condition.
Always go double-stage for ratios above about 15:1. A single-stage 40:1 needs either a huge bull gear or a tiny pinion, and the pinion path will fail by tooth-root fatigue inside a season of theatre use. Splitting it across two stages of 8:1 and 5:1 puts each pinion at a sensible 12 to 15 teeth with module 3 or 4, and the load on any single tooth flank drops by roughly the square root of the split ratio.
For 600 kg theatre work specifically, a 40:1 to 50:1 total ratio with a 350 mm handle gives an operator about 5 to 6 kg of hand effort at the cue, which is the sweet spot for repeatable, controllable scenic moves.
That is almost certainly pawl geometry, not pawl wear. The pawl-and-ratchet only blocks the drum after the handle has been released and the load has settled back one ratchet tooth. If the kickback is violent, the load is back-driving through the gear train against the operator's hand because the pawl pivot is not catching fast enough — usually because the pawl spring is weak or the pawl pivot has worn oval and the pawl is sitting too high above the ratchet wheel.
Quick diagnostic: with the winch unloaded, lift the pawl and let it drop. It should land with an audible click and sit fully seated in a tooth root. If it bounces or sits with the tip on a tooth flank, replace the spring and check the pivot pin for ovality.
Because the effective drum radius just grew. On a 150 mm diameter drum with 8 mm rope, the first layer rolls at r = 79 mm (drum + half rope). The second layer rolls at r = 87 mm. That 10% increase in drum radius reduces mechanical advantage by 10%, so the hand force you feel goes up by the same 10%.
If you cannot avoid multiple layers — most cellar and theatre winches do reach two layers — size the winch using the outer-layer radius, not the bare drum radius. A 600 kg load that needs 4 kg of hand force on layer one will need 4.4 kg on layer two and 4.9 kg on layer three.
Add a separate locking device. The pawl-and-ratchet on a crab winch is a non-return device, not a sustained holding brake. The pawl tip and the ratchet tooth flank are in line contact, and over hours of static load that line contact creeps — particularly if the gear teeth above the ratchet are loaded, because thermal cycling in the workshop alternately tightens and loosens the gear mesh against the ratchet.
For loads suspended over 30 minutes — flown scenery between performances, a hull fragment in conservation — fit a load-rated wire rope clamp or independent rigging stop directly on the load-side rope. Treat the winch pawl as a temporary lock during operation only.
This catches a lot of restorers. Original cast-iron-on-steel plain bearings, when properly oil-bathed, run at lower coefficient of friction than dry or grease-packed bronze — somewhere around 0.05 versus 0.10 to 0.12 for bronze with grease. If you replaced the original oil-bath setup with grease-lubricated bronze, you have roughly doubled the bearing friction.
Two fixes: either restore the original oil-bath system (felt wick or ring oiler) or swap the bronze bushes for caged needle rollers on the layshaft, which will get you back to around 0.04 friction coefficient and restore the original feel of the winch.
References & Further Reading
- Wikipedia contributors. Winch. Wikipedia
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