How to select the Gears



Pitch Circle Diameter (PCD) is also known as Reference Circle.




  • ØH: Hub Diameter
  • ØB: Bore Diameter or Internal Diameter (ID)
  • W: Total Width of Gear
  • FW: Face Width
  • ØPCD: Pitch Circle Diameter or Reference Circle
  • ØOD: Outside Diameter

To select Appropriate Gears for your Application, you need to consider the following four Parameters

1. Module (M):

The Module describes the size of a gear. A pair of gears can mesh correctly only if Module of both gears are same. So select the gear with the same Module (M) to be mesh together with the Different number of teeth according to your Gear ratio requirement.



2. Teeth

 Decide the Number of teeth according to your speed Reduction or Increment requirements.

Here in Above Example, Because gear “A” has 20 teeth and “B” has 40 teeth,  “B” will travel through ONE complete Turns/Rotation for TWO complete Turn/Rotation of Drive gear “A”,  this would give a ratio of 2:1. as we considered A as Drive Gear.

If we consider the things in another way and reverse the setup and gears as  “A” as a driven gear and “B” as a drive gear then the ratio would become 1:2, i.e. rotational speed will be doubled at “A”.

Likewise, Various teeth ratio decides the speed reduction/Increment of Driven Gear.

 3. Internal Diameter (ID)

Internal Diameter is also known as Bore diameter of gear where you connect the shaft of Drive motor to the gear. For example for if your Motor/Application shaft is of 6 mm Diameter, then select the gear with same 6 mm Internal Diameter (ID)

4. Pitch Circle Diameter (PCD):

PCD is also important to be considered while selecting Gears because it decides the distance between Drive and Driven Shaft. A pair of gears should mesh in such a way that their reference circles which are also known as PCD are in contact, the centre distance (a) is half the sum total of their reference diameters.

Center distance (a)
a = (d1+d2) / 2

So, “a” will be the distance between two Shafts on which Drive or Driven Gears to be mounted.