Types of Gears: Spur gears, helical gears, bevel gears and worm gears are types of gears and information on gear types are summarized below.

External Spur Gears:
External Spur Gears

Spur gears are used to transmit rotary motion between parallel shafts. Spur gears are usually cylindrical in shape, and the teeth are straight and parallel to the axis of rotation. Loads on gear tooth produce no axial thrust. Shafts rotate in opposite directions.

The spur gear is the simplest and most common type of gears. Excellent at moderate speeds but there may be noise problems if used in high speed applications.

Internal Spur Gears:
Internal Spur Gears

Similar to external spur gears. The teeth of the internal spur gear are formed in the inner surface of the gear. The mating gear is an external spur gear. It is a compact arrangement to transmit motion between shafts which are rotating in same direction.

Rack and Pinion Gears:
Rack and Pinion Gears

Rack is a straight gear and can be imagined as a spur gear with infinitely large pitch diameter. A rack and pinion is a gear set which translate rotational movement (from pinion) to a translational movement (to rack).

Helical Gears :
Helical Gears

Helical gears are used to transmit motion between parallel shafts. Teeth of helical gears are cut inclined to the axis of rotation. The helix angle is the same on each gear, but one gear must have a right-hand helix and the other a left-hand helix. Helical gears produce both radial and thrust loads on the shaft bearings. Helical gears have higher load carrying capacity and quieter than spur gears.

Double Helical Gears (Herringbone):
Double Helical Gears

Double helical gear is a type of gear which is identical to two opposite helix angled helical gear assembled side by side on the same shaft. Each of the helical gears create opposite thrust loads and thus cancels each other.

Crossed Helical Gears:
Double Helical Gears

In this configuration, motion is transmitted between non-parallel shafts via meshed helical gears. Crossed helical gears carry very small loads as the gears wear in. This type of gears is not advised for use in the transmission power.

Straight Bevel Gears:
Straight Bevel Gears

Straight bevel gears are used to transmit motion between intersecting axes. Bevel gears have teeth formed on conical surfaces. They are used to connect two shafts on intersecting axes. End thrust tends to separate the gears. Straight bevel gears offer easy design and manufacturing and good results in service. However they are noisy at the higher velocities. Straight bevel gears are suggested for peripheral speeds up to 1000 ft/min (5 m/s) where maximum smoothness and quietness are not of prime importance.

Spiral Bevel Gears:
Spiral Bevel Gears

Spiral bevel gears have spiral-shaped teeth on which contact begins at one end of the tooth and advances smoothly to the other end. They mesh similar to straight bevel gears but as the result of gradual teeth engagement, the motion will be transmitted more smoothly than by straight bevel or zerol bevel gears. This reduces noise and vibration especially at high speeds. The spiral angle affects the direction of the thrust loads created but no affect on quietness of operation. Spiral bevel gears are suggested for peripheral speeds up to 8000 ft/min (40 m/s). This type of gears is most often used in vehicle differentials.

Zerol Bevel Gears:
Zerol Bevel Gears

Zerol bevel gears have curved teeth with a zero spiral angle. Teeth have same orientation on the gear face as straight bevel gears. Zerol bevel gears are used for peripheral speeds up to 8000 ft/min (40 m/s) and work quieter than straight bevel gears. The permissible axial thrust loads for Zerol bevel gears are not as much as the spiral bevel gear.

Hypoid Gears:
Hypoid Gears

Hypoid gears are similar to spiral bevel gears except that the axes of the gears are non-parallel and non-intersecting. The distance between the axes of gears is named as offset. The offset permits higher ratios of reduction for hypoid gears than other type of bevel gears. Hypoid gears are suggested for peripheral speeds up to 8000 ft/min (40 m/s).

Worm Gears:
Worm Gears

Worm gears transmit motion between shafts at 90° angle though other angles can be used. They are used when large speed reduction ratio is required. Worm gears consist of a worm and worm wheel. The worm is the screw like part and worm wheel is similar to a section of nut. A tooth of worm resembles an ACME screw thread. Worm gears are used for power transmission, but the higher the ratio the lower the efficiency. Worm-gear meshes have a much lower mechanical efficiency than other gearing systems.