10.4 WORM GEARS

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Worm gears use a long helical screw that drives a larger helical gear.

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Basically we use a screw like gear (the worm) and a large cylindrical gear (worm gear) that is driven by the worm.

The worm gear curves to the shape of the worm to increase contact. Also note that worm gear is a helical gear.

The worm acts very much like a rack, except that it is threaded onto a cylindrical surface.

These gears find their best applications when a large gear ratio is needed in a compact space. The shafts typically intersect at a 90° angle - when this is the case the helix angle on both gears is the same.

The worm gear can be single enveloping or double enveloping.

- The single envelope is the parallel sided gear. This type of gear is more forgiving for position and alignment tolerance problems.

- The double enveloping gear has a curvature that increases the surface on contact between the gears. This can be very useful for power applications.

The following values are reasonable for finding the profile dimensions of the teeth,

Suggested pressure angles for given lead angles are listed below,

10.4.1 Harmonic Drives

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These are actually normal servo motors, but with an integrated harmonic gear. The harmonic gears are very compact, and as a result the overall size is reduced. These gears also allow very high gear ratios (eg, 100:1)

Harmonic drives are also gaining popularity with smaller manipulators. They use a rotating elliptical core that deforms a flexible section. The flexible section is in contact with an outer section for short periods of time, and as the ellipse rotates, there is a geared down rotation generated. This allows integral gears in motors

10.4.2 Practice Problems

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