Digging Deeper on Gear Mechanism: Line of Action, Pressure Angle and Backlash Discussion

Understanding gear mechanism can be very challenging. This simple machine provides countless benefits and makes our lives comfortable and convenient, especially when gears are used within another machine to create some of the most useful every day appliances and devices that are around us. And this is why gears are extensively studied and researched so they can be improved for better uses.

In order to fully understand the action of gears and what makes them quite effective, we shall be digging deeper into 3 factors that affect this. These factors are the line of action, pressure angle and the backlash.

Line of Action

As you may already know, a gear is a toothed part of a rotating machine that meshes with another toothed part to create torque. Knowing the different parts of a gear is important to be able to fully understand how these parts affect its actions.

Now, when we talk about the line of action, it is the line connecting the initial and final points of contact in a driving gear. In the simplest of terms, when two gears mesh, the first point of contact between them, which is called the initial point and is always on the addendum circle of the driven gear, and the final point of contact which is always on the addendum circle of the driving gear, create a line. This tangent line, which goes through the pitch point, is the line of action.

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The line of action is also called the force line. It is quite significant in the action of gears because this is where force is applied (from the first gear) and acts on in order to drive the second gear into action.

Pressure Angle

Technically, the pressure angle is the angle formed between the line of action and the line tangent to the pitch point. Simply put, it refers to the angle through which forces are transmitted between meshing gears, as the line of action is the line where force acts on.

Here are three reasons why the pressure angle is an important parameter for gear specification:

  1. When the pressure angle is increased, the tooth strength improves.
  2. Increasing the pressure angle can eliminate interference because the base circle becomes smaller, making more of the tooth involute.
  3. Power transmission is improved when the pressure angle is increased, but it also increases wear and noise.

Now, the ideal pressure angle is 20° because it:

  • Reduces risk of undercuts
  • Reduces interference
  • Makes the tooth stronger
  • Increases load carrying capacity


The backlash is described as the clearance between two mating teeth. This design parameter is important because with an optimum backlash, which is measured in microns (ideally a very low number, if not zero), gears are able to run efficiently.

The clearance that defines the backlash allows gears to mesh without binding to each other. It also provides a space for lubricating oil to be applied, which helps prevent overheating and damage to the tooth.

Hopefully, this article helped you better understand how gears work.

If there is something about gears and its action that still concern you, don’t hesitate to send us at Kore Industries a message. We will be glad to assist you at no obligation, at all.

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