Gear Sizing: Module vs Pitch
The size of gear teeth is defined by two related parameters: module (used in metric systems) and diametral pitch (used in imperial systems). Understanding these is essential for gear design.
What Is Module?
Module (m) is the ratio of the pitch diameter to the number of teeth, expressed in millimeters:
m = d / N
Where d is the pitch diameter (mm) and N is the number of teeth. A larger module means larger teeth.
What Is Diametral Pitch?
Diametral pitch (P) is the number of teeth per inch of pitch diameter:
P = N / d
A higher diametral pitch means smaller teeth. Module and diametral pitch are inversely related: m = 25.4 / P.
Standard Module Values
ISO and most international standards specify preferred module values:
- Series 1 (preferred): 1, 1.25, 1.5, 2, 2.5, 3, 4, 5, 6, 8, 10, 12, 16, 20, 25, 32, 40, 50
- Series 2: 1.125, 1.375, 1.75, 2.25, 2.75, 3.5, 4.5, 5.5, 7, 9, 11, 14, 18, 22, 28, 36, 45
Always use standard values when possible — non-standard modules require custom tooling.
Circular Pitch
Circular pitch (p) is the distance between corresponding points on adjacent teeth, measured along the pitch circle:
p = π × m
This is useful for rack gears and when measuring tooth spacing directly.
How to Choose Module
Module selection depends on the load, speed, and space constraints:
- Small module (0.5-2): Instruments, watches, small mechanisms, light loads
- Medium module (2-6): General machinery, power tools, automotive accessories
- Large module (6-25+): Heavy machinery, mining equipment, marine drives