The gearbox designed with the use of a worm and worm wheel is considerably much smaller compared to one made from plain spur gears and it also has drive axes at about 90 degrees to each other. With a single start worm, for every 360 degrees turn on the worm, the worm gear motors will advance only one tooth of the gear. Therefore, one can conclude that regardless of the size of the worm, the gear ratio is particularly the size of the worm gear to 1”. If the DP or diametrical pitch of every gear is the same, then when it comes to the physical size of the 240 tooth gear to the 20 tooth gear, the worm arrangement is considered to be smaller in volume.
Types
There are typically three types of gears which can be used in the worm drive. The first are non-throated worm gears. These do not have groove or throat machined around the circumference of the worm wheel or worm. The second are the single throated worm hears in which the worm wheel is typically throated. The final type is the double throated worm gears which usually have both gears being throated. This kind of gearing can support the highest amount of loading. An enveloping worm has one or more teeth and increases in diameter coming from its middle portion and then towards both end. Double enveloping worm gearing also comprises enveloping worms which are mated with fully enveloping worm gears. This is also called the globoidal worm gearing.
Direction of transmission
Unlike the ordinary gear trains, the direction of the transmission is not really considered as reversible when using the larger reduction ratios because of the greater friction which is involved between the worm as well as the worm wheel, when usually a single start worm is being used. This can also be an advantage when one desires to eliminate the possibility of the output driving into the input. If a multi-start worm is being used then the ratio will reduce accordingly and the braking effect of the worm as well as the worm gear will have to be discounted as the gear can be able to drive the worm.
Worm gear configurations in which the gear cannot drive the worm are referred to as self locking. Whether a gear and worm is self-locking will depend on the lead angle, the coefficient friction as well as the pressure angle. However, it is also roughly correct to state that the worm and gear are both self locking if the tangent of the lead angle is deemed less than the coefficient of the friction.