Worms and wormwheels – introduction
An interesting type of gears are worms and wormwheels. They are different in several ways from any other gears. Firstly the form of the worm and the form of the wormwheel are completely different and yet they can only be used as a pair.
One of the great merits of a worm and wormwheel is that it is possible to achieve a larger ratio between input and output than would be possible using spur occupying the same sort of space. The second most important feature is that it is, usually, only possible for the worm to drive the wormwheel. Turning the wormwheel is not possible.
The ratio of a worm/wormwheel pair is completely set by the number of teeth on the the wormwheel.
Whereas on other type of gears the centers on axes are set by the number of teeth on the gears, with worms and wormwheels it is possible for the diameter of the worm to be varied without changing the ratio between worm and wormwheel.
Two examples of worm/wormwheel gearing occur in the dividing head and the rotary table. In both cases the high ratio is an advantage. In the dividing head the ratio is usually 40:1 in industrial dividing heads though sometimes 90:1 in amateur ones. On a rotary table the gear often increases as the rotary table gets bigger. On a 12 inch one it might be 120:1.
Both of these devices only work the way they do because whatever force is applied to the workpiece being held by the device the wormwheel will not rotate unless the worm drives it.
Whereas most gears are designed so the surfaces of two gears in contact never slide over each other on a pair of worm and wormwheel it is inherent in the design that the surfaces do slide. This means the materials chosen for these has to allow for this. The most common arrangement is that the worm is made of steel and the wormwheel is made of phosphor bronze.