Worm Gear Unit Double Stage

There are three different types of gears that can be used in a worm drive.

The first is a non-throated worm gear. It doesn't have a throat or groove machined around the circumference of either the worm or worm wheel.

The second is single-throated worm gear, in which the worm wheel is throated.

The final type is double-throated worm gear, which has both gears throated. This type of gearing can support the highest loading.

Direction of transmission

Unlike ordinary gear trains, the direction of transmission (input shaft vs output shaft) is not reversible when using large reduction ratios. This is due to the greater friction involved between the worm and worm-wheel and is especially prevalent when a single start (one spiral) worm is used. This can be an advantage when it is desired to eliminate any possibility of the output driving the input. If a multi-start worm (multiple spirals) is used then the ratio reduces accordingly and the braking effect of a worm and worm-gear may need to be discounted, as the gear may be able to drive the worm.

Worm gear configurations in which the gear cannot drive the worm are called self-locking. Whether a worm and gear are self-locking depends on the lead angle, the pressure angle, and the coefficient of friction.

Left hand and right-hand worm

A right-hand helical gear or right-hand worm is one in which the teeth twist clockwise as they recede from an observer looking along the axis. The designations, right hand, and left hand are the same as in the long-established practice for screw threads, both external and internal. Two external helical gears operating on parallel axes must be of the opposite hand. Internal helical gear and its pinion must be of the same hand.

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