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Basic Gear Concepts

The major factors that influence the performance of your wind turbine are the vane design and choosing the ideal gear ratio. While structural integrity matters, it’s these two components that create a vast impact on your turbine’s output.

So, we will now make an attempt at understanding how the gears work and how different gear ratios produce different outputs.

What are gears?​

A gear is a circular machine part with “teeth” that usually fits with another gear. When two gears are meshed with each other and one of them turns, the other one also turns (refer fig 1).

[Figure 1: Two gears meshed together. By Jahobr.]


(Note that when one gear turns clockwise, the other one turns counter-clockwise. Image accessed from Wikipedia.)

This property makes them very useful as we can achieve the following:

  1. Changing speed: In wind turbines , even if the blade turns slow, the gears inside turn fast to generate enough electricity.
  2. Changing Force: In bicycles, when climbing up a hill, it’s easier to pedal with a lower gear as the change in gear changes the amount of force you have to apply. A car jack would be another example where we can lift a big car with little force due to gears.
  3. Changing Direction: When you reverse a car a different gear called the “reverse gear” is used that changes the rotation direction of your wheels.

Parts of a Gear​

[Figure 2: Parts of a gear. Image accessed from instructables.]

  1. Pitch circle: An imaginary circle that gives an idea about the size of a gear.
  2. Pitch Diameter: The diameter of the pitch circle.
  3. Axis: The axis along which a gear rotates.
  4. Circular Pitch: The distance between two teeth along the pitch circle (length of the arc).
  5. Pressure Angle: “The pressure angle of a gear is the angle between the line defining the radius of the pitch circle to the point where the pitch circle intersects a tooth, and the tangent line to that tooth at that point.” (instructables)

(Note: Two gears will mesh only if: The pitch circles of two gears are tangent to each other. They have the same pressure angle.)

Types of Gears​

[Figure 3:Image accessed from KHK Gears (https://khkgears.net/)]

Gear Ratios​

It is the ratio of the number of teeth on two gears meshed together. It determines how much or by what factor the speed, force etc. will change.

Suppose you have a big gear (input gear) and a small gear (output gear) with a gear ratio 3:1. Now, if the big gear spins once, the small gear has to spin more than once to keep up with it. Think of it as how you have to take more steps to keep up with a tall friend if you both want to cover the same distance.

So, if the big input gear spins once, the small input gear has to spin three times to keep up with it. This increases the speed but reduces the torque.

Time to Think!​

  1. How do you think the gear ratio is related to the number of teeth on the gears?
  2. If your driver gear was small and the driven gear was big, what would be the effect on the speed and torque?