Gears Part I: Backlash & Ratios

Gear 101

In this series, you’ll learn about the different types of Gears, optimal uses of each, as well as some advantages and disadvantages of using the different kinds of gear. So, let’s start with a basic question. What are gears?

Gears are toothed wheels that, when engaged with another gear, transfer rotary motion from the driving gear to the gear being driven. Different gear sizes can change the direction of rotation, torque, and speed of the system. The pressure angle is the angle between the tooth profile and radial line at the pitch circle. The pitch circle is a circle through the pitch point centered at the gear axis. In this series we will focus on 5 types of gears:

  • Spur
  • Helical
  • Bevel
  • Rack
  • Worm

Before you begin selecting your gears, you will need to decide whether you are using a metric gear, which uses metric gear module, or inch gear, which uses pitch. to ensure proper gear sizing and compatibility. The gear module can be found by dividing the pitch diameter, or diameter of the pitch circle, by the number of teeth the gear has. It is important to make sure the gears you are using have the same module, otherwise they will not work together. Two other important gear features to keep in mind when selecting gears are backlash and gear ratio.

Backlash and Gear Ratio

Backlash, also known as play, is the smallest amount a space between two non-driving teeth when the driving teeth are in contact. Backlash is not desirable in applications where rotation direction is changing. When the gears change direction, the backlash leads to an increased force on each tooth that can lead to unwanted wear and tear.

Backlash - Gears


The gear ratio is also important to consider when choosing the gears for your system. The larger the ratio between the gears, the greater the difference in gear speed and torque.

Gear Ratio - Gears


Gear ratio is found by dividing the number of teeth in the gear being driven by the number of teeth in the driving gear. In this example, if Gear A was driving the system, the gear ratio would be 4/7. If Gear B was driving the system, the gear ratio would be 7/4.

Hopefully, this introduction to gears gave you a helpful foundation to better understanding Gears and how what’s important when selecting them. In the meantime, keep an eye out for part 2 of this series! Can’t wait until part 2 of this series, or have specific questions you would like to have answered? Comment below or visit us at or email me at

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One Comment

  1. 5 Common Gear Types | MISUMI USA Blog

    October 31, 2014 at 8:33 am

    […] the introduction to our gears series we wrote about Backlash and Gear Ratios. Now that you have more background information on gears we can break down the five gear types and […]


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