The first question asked when ordering a gear is usually "what is the module?". Module is the figure that defines tooth size and decides whether two gears will run together. Two meshing gears must always have the same module.
What does module mean?
Module (m) is the pitch diameter (d) divided by the number of teeth (z):
m = d / z
Module is given in millimetres and comes in standard steps: 0.5 – 0.75 – 1 – 1.25 – 1.5 – 2 – 2.5 – 3 – 4 – 5 – 6 – 8 – 10. The larger the module, the larger the teeth.
How to find the module of the gear you have
Pitch diameter cannot be measured directly, but the outside diameter can. On a standard spur gear:
Outside diameter = m × (z + 2)
which gives
m = outside diameter / (z + 2)
- Count the teeth (z). Mark the first tooth with a pen so you do not lose count.
- Measure the outside diameter with a caliper. On a gear with an even number of teeth, measure across two opposite tooth tips.
- Divide the outside diameter by (tooth count + 2).
- Round the result to the nearest standard module.
Example
A gear with 24 teeth and a measured outside diameter of 51.8 mm: 51.8 / (24 + 2) = 1.99. The nearest standard value is 2, so the gear is m = 2.
Gears with an odd number of teeth
With an odd tooth count no two tooth tips sit exactly opposite each other, so a caliper reads slightly less than the true outside diameter. Measure from the bore edge to a tooth tip instead, add the bore radius and double the result.
Watch out for worn gears
On a gear that has run for years the tooth tips wear down, so the measured outside diameter comes out smaller than it should be. If your result falls between two standard modules, measure the mating gear as well to confirm.
Inch-based gears (DP)
Machines of American origin use diametral pitch (DP) instead of module. DP is the number of teeth per inch of pitch diameter, and it relates to module as m = 25.4 / DP. If your calculated module matches no standard value, the gear is probably DP based.
If you are not sure
Not being able to calculate the module is not a problem. Leave the technical fields of the quote form empty and add a photo of the gear next to a ruler, or send the old part as a sample. Our engineers will work out the dimensions.