Worm Drive

HOW TO RECOGNISE IT

A worm drive is identified by a screw-threaded worm on one shaft meshing with a toothed wheel on a shaft at right angles to it — here the threaded shaft below turning the large wheel above. One full turn of the worm advances the wheel by a single tooth, which is why the pair gives a very large reduction in one step and normally cannot be driven backwards.

MISTAKEN FOR

• Rack and Pinion
• Bevel Gear Pair
• Chain and Sprocket