Worm Gears & Worm Sets
One reason to use a worm set is because they provide a means to have a large reduction in ratio in relatively small compact design. Worm sets are one of the most inefficient types of gears sets due to the heat and friction created from the sliding action of the worm against the worm gear in operation.
In most cases, the worm is the driver; the worm gear is driven. Worms can have multiple threads (multiple starts). Most worms that you find in stock gear catalogs have single, double & quad threads. Years ago, triple thread worms used to be more common in stock gear catalogs but have been phased out over the years as a stock item. Single thread worms can also be used as a “one direction locking device” because the worm can turn the worm gear but the worm gear cannot turn the worm. This is due in short because of the slight lead angle of single start worms, which usually anywhere from 3-5 degrees. If you try spin the worm gear against the worm it more or less just slams against the side of the thread of the worm, instead of turning the worm.
We have produced worms up to 14 starts and everywhere in between.As the number of starts increaes, so does the lead angle. It is important to note that most worms use axial pitches, meaning that the pitches are whole numbers from tooth to tooth, on the pitchline, along the axis of the gear (similar to transverse helicals).
This makes it easy to measure the pitch of a worm with simple calipers. Likewise, it is important to note that gear gages do not provide an accurate means to measure worms because the thickness of the gear gage. Because the gear gages are thick and because the thread of the worm is on a lead angle, the gear gage touches the thread in different places on the worm and does not allow the gage to fully seat in the thread for precise identification.
On a side note, when you hear the words “axial pitch” and “transverse pitch”, think of them as the same. The only difference is that “axial pitch” applies to worms and worm gears; where as, “transverse pitch” applies to helical gears.
Worm Gear Materials
Materials for Worms can include: steels, hardened steels, stainless steels, plastics, brass & bronzes, etc. Most of the time worms are usually made from some type of steel. To aid in efficiency and longevity many worms are made from hardened steel with ground threads.
Most often than not, when a worm is hardened… it is also will need to have the thread/s ground. This is because, unlike spur gears, a worm thread is cut across the grain structure of the steel and when the worm is heat treated and quenched, much distortion occurs. In order to operate efficiently with the mating worm gear, this distortion needs to be removed by thread grinding.