Overview of gear types

Spur gear

The spur gear, also known as the cylindrical gear, is the most common of the gears. The toothing lies on the circumference of the cylindrical disc. In order for a combination to result in a spur gear, the axes of the spur gear must be parallel to the counterpart.

The following gearing profiles are possible:

• Axially parallel gearing
• Helical gearing
• Various curved gearing
• Double helical gearing: undercut & herringbone gearing

Gear rack

The gear rack is a linear machine element with serrations. The backlash is limited by the length of the rack, which is why a back and forth movement usually takes place when a gear rack is used. Together with a spur gear, the combination is called a rack gear.

Several gear racks can be lined up to form a toothed rail or gear track.

Elliptical gear

An elliptical gear consists of non-circular wheels and has an uneven transmission ratio. While gear drives are usually made up of round gears, an elliptical gear is, as mentioned, a non-circular gear. When a normal gear rotates, the connected wheel rotates evenly. With elliptical gears, the connected gears must be identical to each other so that the constant centre distance is taken into account. If an elliptical gear is combined with a spur gear, one gear must be mounted on a movable oscillating axis or run over a chain or toothed belt connecting the two gears.

Examples of use

• Chain drive for bicycles
• Weaving machines

Bevel gear
The axes of bevel gears are usually at an angle of 90° to each other and intersect. The tips of two coupled bevel gears come together. The toothed lateral surface lies on a truncated cone and the teeth usually run straight to the surface line. In hypoid gears, they are curved, while the tooth height profile generally corresponds to an octoid.

Worm wheel and worm

A worm gear consists of a worm, which is helical, and a worm wheel. The teeth on the worm are similar to a thread, whereby the teeth wrap around the cylinder like a screw. One turn is equivalent to one tooth. In the simplest case, the worm wheel can be a spur gear with helical teeth.

Due to the lines to be achieved, the tooth flanks of the worm wheel are hollow so that there is not just point contact. The worm gear has a relatively high transmission ratio and is also the quietest gear drive.

A possible modification is the globoid worm, which is adapted to the round shape of the worm wheel. In this case, the cylinder is waisted and not cylindrical.