The worm and worm wheel experiment determines the velocity ratio, mechanical advantage and efficiency of a worm-gear lifting machine, which achieves a very large speed reduction in a single, compact stage. A small effort at the worm handle lifts a large load on the worm-wheel drum — the mechanism used in hoists, lifts and self-locking drives.
Aim of the experiment
To determine the velocity ratio, mechanical advantage and efficiency of a worm and worm wheel, and to verify the law of the machine.
Theory
In this machine, an effort P applied at a wheel or handle turns a worm (a screw), which meshes with a toothed worm wheel. The load W hangs from a drum on the worm-wheel axle. For each revolution of the worm, the worm wheel advances by the number of worm starts:
- Velocity Ratio: VR = (R × T) / (r × n), where R is the effort-wheel radius, T the number of teeth on the worm wheel, r the load-drum radius, and n the number of starts of the worm (1 for a single-start worm).
- Mechanical Advantage: MA = W / P
- Efficiency: η = (MA / VR) × 100%
The very high velocity ratio gives a large mechanical advantage, but efficiency is low — often well below 50% — which makes most worm drives self-locking: the load cannot drive the worm backwards. The effort-vs-load graph again follows the law of the machine, P = mW + C.
Apparatus required
- Worm and worm wheel apparatus with load drum and effort wheel
- Weights and hangers for load and effort
- Steel rule to measure the drum and effort-wheel radii; tooth count of the worm wheel
Procedure
- Note the number of teeth T on the worm wheel, the worm starts n, and the effort and load radii; calculate the velocity ratio.
- Apply a known load and increase the effort until the load rises steadily; record the effort.
- Repeat for several loads and tabulate W and P.
- Compute MA and efficiency for each load; plot effort vs load for the law of the machine.
Observations and result
The velocity ratio is high, giving a large mechanical advantage, while efficiency is modest and typically below 50% — confirming the drive is self-locking. The effort-vs-load plot is a straight line.
Applications
Worm gears drive hoists, lifts, conveyor drives and steering gears wherever a large, compact speed reduction and a self-locking action are needed. Compare it with the screw jack and winch crab experiments, and the gear analysis in the epicyclic gear train experiment. See related terms in the engineering lab glossary.
Frequently asked questions
What is the velocity ratio of a worm and worm wheel?
VR = (R × T) / (r × n), where R and r are the effort-wheel and load-drum radii, T the worm-wheel teeth, and n the number of worm starts.
Is a worm and worm wheel self-locking?
Usually yes — because efficiency is well below 50%, the load cannot turn the worm backwards, so the drive holds its load.
Why does a worm drive have a high mechanical advantage but low efficiency?
The large velocity ratio gives a big mechanical advantage, but the high sliding friction between worm and wheel dissipates energy, keeping efficiency low.
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