Helical gears transfer motion at higher speeds and higher torque capacities. Helical gears accomplish this because of their twisted tooth helical design that naturally provides more tooth overlap than spur gears.
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For example, if a spur gear set for a particular ratio to provide 1.5 teeth in contact at all times in operation, an equivalent ratio helical set may provide 3 teeth in contact at all times in operation. It is this additional tooth contact that gives helical gears their advantage.
Helical gears not only can transfer motion across parallel shafts if they are the opposite hand, they can also transfer motion across perpendicular shafts if they are the same hand. Some applications require that helical gears transfer motion between shafts that are anywhere between 0 and 90° and also between 90° and 180°. These type of helicals are called angular helicals. There are two main types of helical gears: transverse helical gears and normal helical gears.
Transverse Helical Gears
Transverse helical gears have a whole number pitch from tooth to tooth, on the pitch line, along the axis of rotation of the helical gear. Most gears in stock gear catalogs are transverse helicals. Transverse helicals cannot be cut with the same gear tools that you would use to cut spur gears.
One advantage of using transverse helicals is that they have standard pitch diameters in the plane of rotation (as do spur gears). Due to this, if your ratio is available in a stock gear catalog, you can readily replace a set of spur gears with a stock set of transverse helical gears without a center distance or pitch diameter modification. Higher speeds and higher torque capacities can then be achieved. Two disadvantages of transverse helicals are that the gear tools can be expensive and are not as readily available as spur gear tools (which are used to create normal helical gears).
Spur Gears or Normal Helicals
Normal helical gears have a whole number pitch from tooth to tooth, on the pitch line, perpendicular to the tooth itself. For the most part, you won’t find a single normal helical gear in a stock gear catalog. But 90% of the gearing that we manufacture are normal helical gears. Due to the fact that normal helical gears are cut with spur gear tools, the cost and availability of the tooling is usually not an issue. Spur gears do not have standard pitch diameters in the plane of rotation but with a twist of the helix angle just about any pitch diameter may be achieved. Usually gear engineers, when designing a machine or gearbox, need to fill a particular center distance with a particular ratio. If larger pitch diameters are desired, we steepen the angle of the set. If smaller pitch diameters are desired, we can lessen the angle of the set.
Helical gears are designed with fine pitch teeth, as well as coarse pitch teeth, depending upon the load carrying requirements of the application. At Gearmakers, we have a vast inventory of helical gear tooling. Some examples include, diametral pitch, module pitch, circular pitch, etc.
Helical gears can be made out of most any material including steels, preheat treat steels, hardened steels, stainless steels, delrin, nylon, uhmw, other plastics, brass & bronzes, non-metallics (phenolics), wood, and other materials.