Worm gearing is used to transmit power between two non-intersecting shafts whose axes lie at right angles to each other. The drive of a worm gear is basically a screw, or worm, which may have a single- or multistart thread, and this engages with the wheel. A singlestart worm in one revolution will rotate the worm wheel one tooth and space, i.e. one pitch. The velocity ratio is high; for example, a 40 tooth wheel with a singlestart worm will have a velocity ratio of 40, and in mesh with a two-start thread the same wheel would give a velocity ratio of 20.
A worm-wheel with a single-start thread is shown
in Fig. 24.29. The lead angle of a single-start worm is low, and the worm is relatively inefficient, but there is little tendency for the wheel to drive the worm. To transmit power, multi-start thread forms are used. High mechanical advantages are obtained by the use of worm-gear drives.
Worm-gear drives have many applications, for example indexing mechanisms on machine tools, torque converters, and slow-motion drives.
Figure 24.30 shows typical cross-sections through part of a worm and wheel. Note the contour of the wheel, which is designed to give greater contact with the worm.
Recommendations for the representation of many types of gear assembly in sectional and simplified form are given in BS 8888.
Worm-gearing terms applied to a worm and part of a worm-wheel
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