Types of gears



Gears can be classified:

  1. According to the relative position of their axes
  2. According to the type of contact between the surface of the gear
Figure 1: Types of gears

Parallel shaft
  • Depending upon the teeth of the equivalent cylinder, i.e., straight or helix 
  • We have following parallel shaft gears
  • Spur gears: straight teeth parallel to the axes of the wheel
  • Helical gears: teeth are curved and inclined to the shaft axis. Two mating gears have the same helix angle but have teeth of the opposite hand
  • Herringbone gears: A double-helical gear is equivalent to a pair of helical gears attached together, one has a right-hand helix and other a left-hand helix




Figure 2: Spur gear


Figure 3: Helical gear

Figure 4: Herringbone gear

Intersecting gear

  • Two non-parallel or intersecting shafts can be connected by means of bevel gears. 
  • Straight bevel gears: teeth are straight, radial to the point of intersection of the shaft axes, and vary in cross-section throughout their length. Used to connect shafts at the right angle
  • Mitre gears: when two bevel gears of the same size connect two shafts at a right angle
  • Spiral bevel gears: teeth of bevel gear are inclined at an angle to the face of the bevel 

Figure 5: Straight bevel gear

Figure 6: Spiral bevel gear

Non-parallel, non-intersecting shafts

Spiral gears or crossed-helical gears: have a helix cut on their periphery in such a manner that they have two non-parallel axes
Worm gears: Consists of a worm and worm-wheel
Hypoid gears: similar to spiral bevel gear but the pinion is larger and stronger than a spiral pinion
Figure 7: Spiral gear


Figure 8: Worm and Worm wheel


Figure 9: Hypoid gear


Internal and external gears

External gears:

The gears of the two shafts mesh externally with each other. The larger wheel is called the spur wheel while the smaller wheel is called a pinion. The rotation of the two wheels is in the opposite direction.

Internal gears: 

The gears of the two shafts mesh internally with each other. The larger wheel is called the annular wheel while the smaller wheel is called a pinion. The rotation of the two wheels is in the same direction.
Figure 10: External and Internal gears


Rack and pinion


The rack is a special case of spur gear; it is made of an indefinite diameter so that a pitch surface is a plane. Usually, the rack is a straight line gear while a pinion is a circular wheel. The rack and pinion converts rotatory motion into translational motion
Figure 11. Rack and Pinion


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