The historical double screw-gear hoist is a chain hoist with two worm screws driving the two toothed flanges of one load-chain wheel. The load hangs from the chain running over that wheel.
In Alfred Box’s 1879 arrangement, one hand-chain wheel drives two worm-shaft pinions, and both worms turn the same hoisting wheel. The animation includes a side section and a separate view of the hand-wheel gearing.
Double Screw-gear Hoist Interactive Calculator
Explore Box’s two-worm chain hoist. Change the shared wheel ratio, chain pitch, hand-wheel speed and suspended force.
Equation Used
- Two equal single-start worms drive one shared wheel.
- Hand-wheel spur gear 48 teeth; both worm pinions 24 teeth.
- Twelve chain pitches feed per load-wheel turn.
- Hand-chain radius is 160 mm.
- Chain uses a mean pitch path; polygonal fluctuations are not simulated.
- Two raising turns and two lowering turns form the displayed cycle.
Box’s 1879 patent: two worms drive opposite flanges of one chain wheel. One shared reduction, not two stages in series.
Two worms, one lifting chain wheel
Box’s patent shows a combined chain sheave and hoisting wheel, A, with a toothed flange on each side of the chain groove. Two parallel worm shafts, B, run below the wheel, one beneath each flange. The load chain passes between the worms.
The hand-chain wheel carries a spur gear, G, on its inner face. G meshes with two pinions, H, one on each worm shaft. The worms therefore act together on one output wheel. Their reductions are not multiplied as two successive worm stages.
The front diagram shows one worm in side elevation; the other lies behind it. The lower diagram looks along both worm shafts and shows their common spur driver. Both views follow the same shaft angles.
The illustrated load hangs directly from one end of the chain. There is no lower moving pulley in this version. Turning the hand wheel in one direction raises that end; reversing it lowers the load in the ideal kinematic model.
What the controls change
Worm-wheel tooth count changes the reduction between the hand wheel and load wheel. Both wheel flanges use the same count. Load-chain pitch changes the chain feed per wheel revolution.
Hand-wheel rpm controls the animation time and calculated speeds. Suspended load changes the ideal hand-chain pull. The geometry and speed ratio are independent of the selected load force.
The model uses a 48-tooth hand-wheel gear, two 24-tooth worm-shaft pinions, single-start worms, a 12-pitch chain wheel and a 160 mm hand-chain radius. These values explain the mechanism; they are not dimensions or a capacity rating stated in the patent.
Reduction, chain feed and ideal effort
The 48-tooth gear drives each 24-tooth pinion twice as fast as the hand wheel. For hand-wheel speed N, each worm turns at 2N. A single-start worm advances its Z-tooth wheel by one tooth per worm revolution, giving load-wheel speed 2N/Z and total reduction Z/2.
A 12-pitch load wheel advances a mean chain length of 12p per revolution. Mean lift per hand-wheel turn is therefore h=24p/Z. Mean hook speed is hN/60. These formulas count both worms as one shared stage, not two reductions in series.
A hand-wheel revolution draws 2πR of hand chain, with R=160 mm. The ideal mechanical advantage is 2πR/h, and ideal hand-chain pull for suspended force W is Wh/(2πR).
The animation alternates two hand-wheel turns raising the load with two turns lowering it. Current rise equals h times the accumulated raising turns. It uses a continuous mean pitch path for the chain: small polygonal speed fluctuations, actual pocket profiles, elastic stretch and joint clearances are not resolved.
Example: a 32-tooth worm wheel
With Z=32 and p=12 mm, the reduction is 16:1. Each hand-wheel revolution feeds 9 mm of load chain. At 12 rpm the two worms each turn at 24 rpm, the output wheel turns at 0.75 rpm and the hook moves at a mean 1.8 mm/s.
For a 100 N suspended load and 160 mm hand-chain radius, ideal mechanical advantage is 111.70:1 and ideal pull is 0.895 N. Real worm gearing requires more effort because of losses; this ideal result does not establish an actual hoist’s efficiency or lifting capacity.
Two hand-wheel turns raise the displayed hook by 18 mm. The animation then reverses the hand wheel and worms to return the hook to its starting height.
Scope of the model
The two worms act on opposite flanges of the same wheel. This can distribute the transmitted effort across two meshes, but this calculator does not assume perfectly equal load sharing or calculate tooth stresses.
The worm teeth are shown as sectional indications; they are not manufacturing tooth profiles. The hand-wheel spur gears use involute outlines and the correct angular relationship. Both shafts and the chain groove are separated in the explanatory views.
Ideal lifting effort is an energy calculation, not a safe working load. The model does not calculate worm friction, a brake, self-locking, hook strength, chain strength or holding ability. Worm gearing must not be assumed to hold a suspended load merely because it has a high ratio.
Double screw-gear hoist questions
Are there two lifting screws?
No. In this historical mechanism the screws are worms. They turn one chain wheel rather than translating two lifting nuts.
Do two worms square the reduction?
No. Both worms drive the same output wheel in parallel. They are not cascaded stages.
Why is only one worm visible in the upper view?
The other is behind it. The lower view looks along the worm shafts and shows both driven pinions and their common hand-wheel gear.
Is the calculated hand pull a product rating?
No. It is a lossless mean-force calculation. Actual effort, allowable load and load-holding provisions require the design or manufacturer’s data.
Why does the animation reverse?
It demonstrates two hand-wheel turns raising the load followed by two turns lowering it, with the hook and chain connected throughout.
Primary references
Alfred Box, US patent 218,223, “Improvement in Hoisting-Pulleys and Worm-Gear Apparatus,” August 5, 1879: drawings and description of the two parallel worms, flanged chain wheel, common driving cog wheel and two worm pinions.
Smithsonian National Museum of American History, Box Chain Hoist patent model, dated August 5, 1879.
Hiscox, Mechanical Movements, Powers, Devices and Appliances, figure 1415, printed page 340, identifies a Box double screw-gear chain hoist. The patent describes the two worms, shared chain wheel and common hand-wheel gearing illustrated here.
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