A Double Screw-gear Hoist is a lifting machine that raises a load using two parallel lead screws driven in synchrony from a single input shaft through bevel gears or a chain. Unlike a single-screw jack, which tilts under off-centre loads, the twin-screw layout keeps a wide platen level even when the load shifts. The mechanism converts low-torque rotary input into high-force linear lift, with the screw threads providing self-locking holding so the load stays put without a brake. Typical mill-floor units handle 2 to 50 tonnes lifting plate, ladles, or moulds.
Double Screw-gear Hoist Interactive Calculator
Vary lift, screw turns, lead mismatch, and screw spacing to see the accumulated platen out-of-level error in a synchronized twin-screw hoist.
Equation Used
The calculator accumulates differential screw lead error. If one screw advances by DeltaL more per turn than the other, the platen level error after N turns is E = DeltaL x N. The displayed tilt angle uses the screw spacing S.
- Both screws are driven by the same cross shaft rotation.
- Lead mismatch is the differential travel per screw turn.
- Frame stretch, shaft twist, and nut backlash are neglected.
- Small tilt angles are reported from the screw spacing.
Operating Principle of the Double Screw-gear Hoist
The hoist runs two trapezoidal lead screws in fixed bearings at each end of a frame. A hand crank, electric motor, or lineshaft input turns a cross shaft, and a pair of bevel gears at each screw converts that rotation 90° down into the screw itself. A travelling nut threaded onto each screw carries one end of the lift platen — turn the input one way, both nuts climb together, reverse it and they descend. Because the input shaft mechanically links the two screws, they cannot drift out of step. That synchronisation is the whole point. Try lifting a 4 m wide foundry ladle with two independent screw jacks and you'll find one races ahead of the other within a few revolutions, the platen cocks, the nuts bind, and you're chasing a jam.
Thread geometry decides nearly everything about the machine. Acme or trapezoidal threads at a 14.5° to 15° flank angle give you self-locking behaviour as long as the lead angle stays below the friction angle of the nut-to-screw interface — roughly 5° for steel-on-bronze with grease. Drop a 30 mm diameter screw with a 6 mm lead onto a heavy load and let go of the crank; it will not back-drive. Push that same screw to a 12 mm lead for faster lift and you've crossed the self-locking threshold. The load now overhauls the screw, and you need a worm reducer or a brake on the input. Get the lead angle wrong by even a degree on the wrong side of that line and the entire safety story of the hoist changes.
Failure modes are predictable. Nut wear shows up as backlash — if you measure more than about 0.5 mm of axial play on a 40 mm screw, the bronze nut is worn past service limit and will gall under shock load. Bevel gear lash on the cross shaft shows up as one screw lagging the other by a fraction of a turn, which tilts the platen visibly over a long lift. Bent screws from side-loading, dry bearings on the cross shaft, and stripped keyways at the bevel pinion are the other usual suspects.
Key Components
- Lead Screws (×2): Trapezoidal or Acme thread, typically 30-100 mm major diameter for mill-floor duty. Lead is set between 6 and 12 mm depending on whether self-locking is required. Both screws must be cut to identical lead — a 0.05 mm/turn mismatch over a 1 m lift puts the platen 50 mm out of level.
- Travelling Nuts: Bronze (typically C93200 SAE 660) running on hardened steel screws. Backlash above 0.5 mm on a 40 mm screw means replace the nut. The nut carries the full axial load, so its thread engagement length is sized to keep contact stress under about 25 MPa for continuous duty.
- Cross Shaft: Common input shaft running across the frame, mechanically locking both screws to the same rotation. Usually steel, 25-50 mm diameter, supported on plain or rolling bearings. Any twist in this shaft under load shows up as platen tilt during lift.
- Bevel Gear Pairs: One pair at each screw, typically 1:1 or 2:1 ratio. Backlash should be held under 0.10 mm at the pitch line. Mitre gears are standard for 1:1; spiral bevels run quieter under motor drive.
- Input — Crank, Motor, or Worm Reducer: For hand operation, a 400 mm crank arm gives a worker enough leverage to lift 2-5 tonnes through the screw mechanical advantage. For powered units, a worm reducer ahead of the cross shaft gives both speed reduction and a backup against overhauling.
- Lift Platen or Cross-Head: The horizontal beam carrying the load, bolted to the two travelling nuts. Must be stiff enough that bending deflection under load doesn't cock the nuts on the screws — limit deflection to under 1/1000 of span.
- Frame and End Bearings: Cast or fabricated steel frame holding the screws parallel within about 0.5 mm/m. Thrust bearings at the bottom of each screw take the full lifting load — usually tapered roller or spherical roller depending on capacity.
Who Uses the Double Screw-gear Hoist
Double Screw-gear Hoists earn their place where a load is wide, heavy, and must stay level through the entire lift — the kind of job a single-screw jack or a chain hoist will fight. You see them on factory floors moving moulds, in foundries handling ladles, in stage rigging where two-point lift keeps a beam horizontal, and in any situation where the self-locking screw is doing double duty as the holding brake. They're slow — a few millimetres per second is normal — but they don't drop their loads when the power goes out, and that matters more than speed in most of the places they live.
- Iron Foundry: Tilting and raising ladle hoists in grey-iron pour bays — typical 5 to 20 tonne ladles lifted between melt furnace and pouring station, where the wide twin-screw stance keeps the ladle from cocking off a shaky single jack.
- Press Shops: Die-change platens on mechanical presses — lifting 8 to 15 tonne stamping dies into bolster plates on machines like Bliss or Schuler stamping presses, where the platen has to land flat to within a millimetre across 2 m of bolster.
- Theatre and Stage Rigging: Orchestra-pit lifts and trap-door platforms in venues like the Royal Opera House — two synchronised screws give a level platen rise and the self-locking thread means no hydraulic creep during a held position.
- Heavy Machine Tool Service: Setting and levelling large planer beds and horizontal boring machines — rebuilders use twin screw hoists to land 10-tonne beds onto foundation grouting without cocking the casting.
- Steel Mill Cooling Beds: Walking-beam style lifters between rougher and finisher stands, where the bed must rise level under uneven bar loading — driven through a common cross shaft so both ends index together.
- Hydroelectric and Lock Gates: Smaller sluice and tainter gate hoists on mill races and canal locks, where two screws raise a wide gate leaf evenly against water pressure.
The Formula Behind the Double Screw-gear Hoist
What you actually need to compute is the input torque required at the crank or motor shaft to lift a known load. The relationship ties load, screw geometry, and friction together. At the low end of the typical operating range — small leads around 6 mm and well-lubricated bronze nuts — efficiency runs 30-35% and the hoist feels heavy on the crank but locks itself solid. At the nominal range — 8-10 mm lead, fresh grease, steel on bronze — efficiency hits the sweet spot of around 40%. Push lead past 12 mm and efficiency climbs toward 50%, but you've lost self-locking and now need a brake. The formula below is per screw — multiply by 2 for total load shared across both, or solve at half load if the screws share evenly.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Tin | Input torque required at the cross shaft per screw | N·m | lbf·ft |
| F | Axial load carried by one screw (half the platen load if evenly shared) | N | lbf |
| L | Screw lead — axial distance per one revolution | m | in |
| η | Screw-and-nut efficiency (0.30-0.50 for trapezoidal steel-on-bronze) | dimensionless | dimensionless |
| n | Gear ratio between input crank/motor and the screw (1.0 if direct) | dimensionless | dimensionless |
Worked Example: Double Screw-gear Hoist in a die-change hoist in a stamping plant
A pressed-metal parts plant in Windsor Ontario is sizing a Double Screw-gear Hoist to swap 12 tonne progressive dies in and out of a 1500 ton Minster mechanical press. The platen sits on two 50 mm trapezoidal screws with an 8 mm lead, driven through 1:1 mitre bevels off a common cross shaft. They want to know how much torque the cross shaft has to deliver so they can spec the gearmotor.
Given
- Total load on platen = 12000 kg (≈ 117720 N)
- F (load per screw, evenly shared) = 58860 N
- L (screw lead) = 0.008 m
- η (efficiency, trapezoidal steel-on-bronze, well greased) = 0.40 —
- n (gear ratio at screw) = 1.0 —
Solution
Step 1 — split the platen load between the two screws. Both nuts ride a stiff cross-head, so we assume even share:
Step 2 — compute the nominal input torque per screw at η = 0.40, the realistic figure for fresh grease and a properly bedded bronze nut:
That's per screw. The cross shaft drives both, so total cross-shaft torque is 2 × 187.4 = 374.8 N·m. A crank at 400 mm radius would need roughly 940 N (210 lbf) of pull — too heavy for a single worker, which is why you'd put a gearmotor on this size of hoist, not a hand crank.
Step 3 — at the low end of the typical efficiency range, a worn nut or thin lubrication drops η to about 0.30:
That's a 33% jump in input torque for the same lifted load. If your gearmotor is sized at the nominal figure with no margin, a single dry-running maintenance cycle will stall it. At the high end of efficiency — a 12 mm lead screw running fresh, η climbs to 0.50 — torque drops to Thigh-eff = (58860 × 0.012) / (2π × 0.50 × 1.0) = 224.7 N·m per screw, but you've lost self-locking and now need a holding brake on the input.
Result
Nominal cross-shaft input torque is 374. 8 N·m total, or 187.4 N·m per screw, lifting the 12 tonne die at typical efficiency. In practice that means a 1.5 to 2.2 kW gearmotor running through a worm reducer to deliver the torque at a sensible 5-10 mm/s lift speed — slow enough to land the die softly into the bolster. Across the operating range the torque demand swings from about 225 N·m per screw on a fresh fast-lead screw up to 250 N·m per screw on a worn slow-lead screw, so size the drive for the worn case, not the nominal case. If the measured stall torque comes in noticeably higher than 250 N·m per screw, suspect (1) a bent screw caused by an off-centre die set, easy to confirm by running the platen empty and watching for cyclic crank-effort change once per screw revolution, (2) thrust bearings at the screw base running dry or contaminated with mill scale, which adds a constant drag torque, or (3) parallelism out of spec between the two screws — over 1 mm/m of misalignment forces the nuts to fight each other through the platen.
Choosing the Double Screw-gear Hoist: Pros and Cons
The Double Screw-gear Hoist is one of three obvious choices for a level lift of a heavy, wide load. The other two are twin hydraulic cylinders driven from a common pump, and a wire-rope hoist with two drums on a common shaft. Each has a place — and each has a job it's wrong for.
| Property | Double Screw-gear Hoist | Twin Hydraulic Cylinders | Twin-drum Wire Rope Hoist |
|---|---|---|---|
| Lift speed (typical) | 5-25 mm/s | 50-300 mm/s | 100-500 mm/s |
| Self-locking under load | Yes — screw thread holds load with power off | No — needs pilot-operated check valve | No — needs mechanical brake |
| Synchronisation between sides | Mechanical (cross shaft) — cannot drift | Hydraulic (flow divider or rephasing) — drifts over cycles | Mechanical (common shaft) — cannot drift |
| Typical load capacity per unit | 2-50 tonnes | 5-500 tonnes | 1-100 tonnes |
| Maintenance interval (heavy duty) | 6-12 months — re-grease nut, check backlash | 3-6 months — seal and fluid checks | 3 months — rope inspection, drum wear |
| Capital cost (relative) | Medium | High (pump, valves, plumbing) | Medium-low |
| Best application fit | Slow level lift, holding load, indoor mill floor | Fast lift, high tonnage, where leak risk is acceptable | Long travel, vertical hoisting, overhead crane work |
| Failure mode if neglected | Nut backlash growing, eventual thread strip | Seal leak, cylinder drift | Rope fatigue, sudden drop |
Frequently Asked Questions About Double Screw-gear Hoist
Common cross shaft does not guarantee equal lift if the bevel gears at each screw have unequal backlash. If one pair has 0.05 mm of lash and the other has 0.30 mm, the loose side lags the tight side by a fraction of a turn — on an 8 mm lead screw that's already 0.3 mm of platen tilt at one end before you've moved.
Check it by marking the cross shaft and both screws with chalk lines, lifting one full revolution, and measuring the difference. Re-shim the bevel housings to bring both pairs under 0.10 mm of pitch-line backlash. If lash is even but the platen still tilts, the screws themselves have different leads — measure with a depth mic over 100 mm of thread.
Self-locking is a static-friction phenomenon, and static friction degrades under vibration. If your hoist sits on a press-shop floor with stamping presses running nearby, the constant low-amplitude vibration converts static friction to kinetic friction at the thread interface, and the load walks down. This is well documented on screw jacks holding loads in noisy environments.
The fix is a holding brake on the input shaft or a worm reducer between input and cross shaft. Worm-and-wheel adds a second self-locking stage that doesn't unlock under vibration the way a single screw thread does.
Pick the screw hoist when (a) the load has to be held in position for hours or days without leak risk, (b) the machine sits indoors over product or workers where a hydraulic burst is unacceptable, or (c) lift speed below 25 mm/s is fine. Pick hydraulics when you need to clear a die in under 30 seconds or your duty cycle is more than a few lifts per hour — screws build heat at the nut and 50% duty is the practical ceiling.
For a die change at 1-2 swaps per shift, the screw hoist wins on cost and on no-leak holding. For a high-volume changeover line at 10+ swaps per shift, hydraulics earn their plumbing.
Measure axial backlash with a dial indicator on the platen while reversing the input by hand. On a 40-50 mm screw with 8 mm lead, fresh nut backlash sits around 0.10-0.20 mm. Service limit is around 0.5 mm. Past 0.8 mm you're into immediate replacement territory because the remaining thread engagement can't carry shock load without galling.
The other tell is metallic glitter in the grease when you wipe the screw down. Bronze fines in the grease mean the nut is shedding, and once shedding starts it accelerates because the shed particles act as abrasive.
Three usual causes. First, breakaway friction — the static friction coefficient at the thread is roughly 1.4× the running coefficient, so startup torque is 30-40% above running torque. Always size the motor for breakaway, not running.
Second, thrust bearing drag. A pre-loaded tapered roller thrust bearing under a 12 tonne load contributes 15-25 N·m of constant drag per screw that the textbook formula ignores. Third, an off-square platen — if the platen is bolted to nuts that aren't perfectly co-planar, the screws bend slightly under load and the bending adds friction at the nut bore. Check by loosening the platen-to-nut bolts, lifting empty, and re-tightening at mid-stroke.
Not on a fixed cross-shaft hoist — the mechanical link forces both screws to identical rotation. To get differential lift you have to break the cross shaft and drive each screw with its own gearmotor, then synchronise electronically with encoders. That gets you to a true tilt-capable platen, but you've lost the inherent mechanical sync that makes a Double Screw-gear Hoist trustworthy.
For ladle-tip duty, the better arrangement is a Double Screw-gear Hoist for the lift plus a separate trunnion tilt drive on the ladle itself. Two mechanisms each doing one job they're good at, instead of one mechanism trying to do both.
References & Further Reading
- Wikipedia contributors. Jackscrew. Wikipedia
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