Gear Drive
The pitch circle diameter is where the gear speed is measured.
This is the line of action. When gears are meshed, the point of contact is on this line.
A power source or motor makes the drive gear turn, transferring motion, speed and force to the driven gear.
This is the driven gear. The teeth of the drive gear mesh with the teeth of the driven gear and push them in the opposite direction.
If the drive gear is larger than the driven gear, we see that speed increases, but the rotational force, or torque, decreases.
Pressure angle is the angle at which the pressure from the tooth of one gear is passed onto the tooth of another gear.
The module is the size of the gear, calculated from the ratio of the gear diameter to the number of teeth a gear has. For two gears to mesh properly, they must have the same module.
Test it out
Tap the diagram for more information and click on the buttons to change a component and see what happens!
 Increase Driving Power Switch Driving Gear Add Idle Gear
Example calculations
In order to work with Gear Drives, mechanical engineers need to understand and work with a variety of calculations. Here are just a few:
Centre Distance (C):
$C = \frac{PCD_a + PCD_e}{2}$
Module (m):
$m = \frac{PCD}{T}$
Circular Pitch (CP):
$CP = \pi \times m$
Gear Ratio (R):
$R = \frac{T_2}{T_1}$
You can also calculate gear ratio using angular velocity, gear speed or diameter:
$R = \frac{\omega_1}{\omega_2} = \frac{n_1}{n_2} = \frac{d_2}{d_1} = \frac{T_2}{T_1}$
 Pressure Angle $\alpha$ Centre Distance $C$ Circular Pitch $CP$ Pitch Circle Diameter $PCD$ Dedendum (Root) Diameter $D_e$ Addendum (Tip) Diameter $D_a$ Module $m$ Teeth $T$
Gear drives are used in machines of all kinds to change the speed, force, rotational direction or movement of machine parts. In cars, they are used in the gearbox to help control the amount of power and speed transferred from the engine to the wheels.
Low gears provide the most pulling power, but also the lowest speed, for when the car needs to do things like go up a hill. Higher gears have more speed, but less power.
Depending on their intended purpose, gears vary in size and shape. If you go from a big gear with many teeth, to a smaller gear with less teeth, you're increasing the speed of the machine part you're transferring power to. If you turn the gears in the opposite direction, and transfer power from the small gear, to the larger one, you're decreasing speed, but increasing the force.
In order to do this, gears turn opposite directions, and "mesh" with interlocking teeth. Because of this, there is no slippage, and gears can withstand high strain torque. They're less likely to fail, but if they break, they're not repairable. They're also expensive to make, and need constant lubrication.
Practice Questions
$m = \frac{PCD}{T}$