What is a Gear?

A gear is a toothed wheel that engages with another gear or with a rack to transfer rotary motion.

Gears are used to change the speed or direction of motion.

The axis of revolution of the gear

Axis

The ratio of number of teeth to pitch diameter

The ratio of number of teeth to pitch diameter. These must match for gears to mesh

Circular Pitch

Diametral Pitch

Anatomy of a Gear

The diameter of the pitch circle. Use this to find the distance between two gear axis by dividing the sum of both by two

Affects how gears contact one another and how force is distributed along the tooth. Standard pressure angles are 14.5, 20 and 25 degrees

The jagged faces on the outside of the gear that transmit rotation to other gears

Pitch Diameter

Pressure Angle

Teeth

The circle that defines the “size” of the gear. Two gear pitch circles must be tangent to mesh with one another

Pitch Circle

Torque is a measure of how much a force acting on an object causes that object to rotate.

Torque

Torque = Radius * Force

Torque = Radius * Fsin(θ)

The gear ratio of a system is the ratio between the rotational speed of the input gear (driver) and the rotational speed of the output gear (load). Gear systems are designed to output desired speed or torque. A gear system with more torque can turn larger things.

Gear Ratio and Power Output

Gear Ratios

Calculating Gear Ratio

Calculate the number of teeth (N) on each gear to find the gear ratio (R). N1 refers to the input shaft gear and N2 refers to the output shaft gear. 

R = N2 / N1

Gear ratio (R) can also be calculated using the pitch diameter or even the radius with a similar equation. D1 is the input gear and D2 is the output gear. 



R = D2 / D1

There are two ways to calculate gear ratio. 

Gear Ratio

R = N2 / N1

N1 = input gear

N2 = output gear

Input Gear A: 8 teeth

This gear will spin at twice the speed of the output gear

Output Gear B: 16 teeth

This gear will spin at half the speed of Gear A but has double the torque

Gear Types

Spur Gear

These are the simplest type of gear. They take the form of a cylinder or disk and mesh together on a parallel axis.

Bevel Gear

These gears

Worm Gear

The gear travels along a worm, resembling the thread of a screw. Although they are not very efficient, worm drives torque output while reducing rotational speed. 

Helical Gear

These gears have angled teeth that form a curve which resembles a segment of a helix. They are more refined and run smoother than spur gears.