How to select Gear? A Quick Overview

This article will help you in selecting the right gears for your project, covering all the essential parameters.

What is a Gear?

Gears are mechanical devices that rotate and use ‘teeth’ to transmit torque between two axes. A transmission is a set of two or more collaborating gears that can generate a mechanical advantage by altering speed, torque, or rotation direction.

Teeth, also known as cogs, are used by gears to engage and convey rotational motion. The drive is a gear that has its movement supplied by the rotation of its shaft. The equivalent gear is pushed to turn in a ratio by interlocked teeth. Torque is improved when the drive rotates a larger gear; rotational speed is increased when the drive turns a small gear. A gear does not always have to turn another gear, as in the case of a rack. 

Different Types of Gear Materials Used

 Gear composition varies by application and affects the gear’s service, rotation speed, accuracy and more.

  • Cast iron provides durability and ease of manufacture.
  • Alloy steel provides superior durability and corrosion resistance. Minerals may be added to the alloy to further harden the gear.
  • Cast steel provides easier fabrication, strong working loads and vibration resistance.
  • Carbon steels are inexpensive and strong, but are susceptible to corrosion.
  • Aluminum is used when low gear inertia with some resiliency is required.
  • Brass is inexpensive, easy to mold and corrosion resistant.
  • Copper is easily shaped, conductive and corrosion resistant. The gear’s strength would increase if bronzed.
  • Plastic is inexpensive, corrosion resistant, operationally quiet and can overcome missing teeth or misalignment. Plastic is less robust than metal and is vulnerable to temperature changes and chemical corrosion. Acetal, delrin, nylon, and polycarbonate plastics are common.
  • Other material types like wood may be suitable for individual applications.



  • 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):

A gear’s size is described by the Module. A pair of gears can mesh correctly only if Module of both gears are same. So, based on your Gear ratio requirement, choose gears with the same Module (M) to mesh together with a different number of teeth.

2. Teeth

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

Since gear “A” has 20 teeth and “B” has 40 teeth, “B” will travel through ONE complete turn/rotation for TWO complete turns/rotations of Drive gear “A,” resulting in a 2:1 ratio. If we examine the situation differently and treat “A” as a driven gear and “B” as a drive gear, the ratio becomes 1:2, implying that rotational speed at “A” is doubled. Similarly, the speed reduction/increment of the Driven Gear is determined by various tooth ratios.

 3. Internal Diameter (ID)

Internal diameter, also known as gear bore diameter, is the distance between the shaft of the drive motor and the gear. If your motor/application shaft is 6 mm in diameter, for example, choose a gear with a 6 mm internal diameter (ID)

4. Pitch Circle Diameter (PCD):

When choosing gears, PCD is also crucial to consider because it determines the distance between the Drive and Driven Shafts. A pair of gears should mesh so that their reference circles, also known as PCDs, are in contact, with the centre distance (a) equal to half the sum 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.

Other Factors To Be Considered While Choosing Gear

When selecting a gear the workload and environment of the gear set should be considered.

  • Power, velocity and torque consistency and output peaks of the gear drive so the gear meets mechanical requirements.
  • Inertia of the gear through acceleration and deceleration. Heavier gears can be harder to stop or reverse.
  • Precision requirement of gear, including gear pitch, shaft diameter, pressure angle and tooth layout.
  • Handedness (left or right teeth angles) depending on the drive angle.
  • Gear lubrication requirements: Some gears require lubrication for smooth, temperate operation.
  • Mounting requirements: Application may limit the gear’s shaft positioning.
  • Noise limitation: Commercial applications may value a smooth, quietly meshing gear.
  • Corrosive environments:Gears exposed to weather or chemicals should be especially hardened or protected.
  • Temperature exposure: Some gears may warp or become brittle in the face of extreme temperatures.
  • Vibration and shock resistance: Heavy machine loads or backlash, the deliberate surplus space in the circular pitch, may jostle gearing.
  • Operation disruption resistance: It may be necessary for some gear sets to function despite missing teeth or misalignment.

Hope this blog helps you to understand  how to select the right gears for your project, by considering all essential parameters and different gear materials.We ,MATHAELECTRONICS  will come back with more informative blogs.

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