Classification of gears

Gears can be classified according to the relative positions of their shaft axes as follows :

Parallel shafts :

In the parallel shaft, the manner of contact and uniform rotary motion between two parallel shafts is equivalent to the rolling of two cylinders. 

The following are the main types of gears to join parallel shafts :

• Spur gears :

In this type of gear, the teeth are straight and parallel to the axes and thus are not subjected to the axial trust due to load. 

• Spur rack and pinion :

Spur rack is a special case of a spur gear where it is made of infinite diameter so that a pitch surface is a plane. 

The spur rack and pinion combination convert rotary motion into translatory motion or vice-versa. 

This type of gear usually used in a lathe in which rack transmits motion to the saddle.

• Helical gears or Helical spur gears :

In this type of gear, teeth are curved, each being helical in shape. Two mating gears have the same helix angle but have teeth of opposite hands. 

In the helical gear, at the beginning of engagement contact occurs only at the point of the leading edge of the curved teeth. As the gear rotates, the contact extends along with a diagonal line across the teeth. Thus, the load application is gradual which results in low impact stresses and it can be used for higher velocities than the spur gears. 

• Double helical gears or Herringbone gears :

A double-helical gear is equivalent to a pair of helical gears secured together, one having a right-hand helix an the other a left-hand helix. 

Axial thrust which occurs in case of single helical gear is eliminated in double helical gears. 

If the left and the right inclinations of a double helical gear meet at a common apex and there is no groove in between, the gear is known as herringbone gear. 

Intersecting shafts :

Kinematically, the motion between two intersecting shafts is equivalent to the rolling of two cones, assuming no slipping. This type of gear is known as bevel gears.

• Straight bevel gears :

The teeth are straight, radial to the point of intersection of the shaft axes vary in cross-section throughout their length. 

Usually, they are used to connect shafts at right angles which runs at low speeds. Gears of the same size and connecting two shafts at right angles to each other are known as mitre gears. 

• Spiral bevel gears :

When the teeth of bevel gears are inclined at an angle to the face of the bevel, they are known as spiral bevels or helical bevels. 

They are smoother in action and quieter than straight tooth bevels as there is gradual load application and low impact stresses. 

• Zerol bevel gears :

Spiral bevel gears with curved teeth but with a zero degree spiral angle are known as zerol bevel gears. 

They are quieter in action than the straight bevel type as the teeth are curved. 

Skew shafts :

In case of parallel and intersecting shafts, a uniform rotary motion is possible by pure rolling contact. But in case of skew shafts, this is not possible.

• Crossed helical gears :

The use of crossed helical or spiral gears is limited to light loads. By a suitable choice of helix angle for the mating gears, the two shafts can be set at any angle. 

• Worm gears :

Worm gear is special case of spiral gear in which the larger wheel, usually has a hollow or concave shape such that a portion of the pitch diameter of the other gear is enveloped on it. 

The smaller of the two wheels is called the worm which also has a larger spiral angle. 

• Hypoid gears :

Hypoid gears are approximations of hyerboloids though they look like spiral gears. 

The hypoid pinion is larger and stronger than a spiral bevel pinion. 

A hypoid pair has a quite and smooth action.