What is a metric gear?
Metric means "of meter" or "in the metric system," and a metric gear is a gear that uses the ISO-standardized module (m) as its tooth size unit.
The module is the value obtained by dividing the circular pitch (p = πd / z) by the circumference (π), and its unit is mm. In practice, however, in many cases, the module is not marked with mm, but only a numerical value after m, such as m0.5, m3, or m5.
A pair of gears can only mesh if they have teeth with the same numerical value in the module.
Metric gears are currently used in many countries around the world as mainstream gears.
On the other hand, there are also gears called imperial gears (DP gears), which use diametral pitch (DP) as the tooth size unit.
Imperial gears are used in some countries, such as the United States, where the inch is used as the size unit.
Because of the difference in tooth size units, imperial gears and metric gears are not interchangeable and cannot mesh with each other.
Below is a conversion table between module (metric gears) and DP (imperial gears).
| Module (m) Metric Gears |
Circular Pitch (CP) CP Gears |
Diametral Pitch (DP) Imperial Gears |
|---|---|---|
| 0.39688 | 1.24682 | 64 |
| 0.5 | 1.57080 | 50.8 |
| 0.52917 | 1.66243 | 48 |
| 0.6 | 1.88496 | 42.33333 |
| 0.79375 | 2.49364 | 32 |
| 0.79577 | 2.5 | 31.91858 |
| 0.8 | 2.51327 | 31.75 |
| 1 | 3.14159 | 25.4 |
| 1.05833 | 3.32485 | 24 |
| 1.25 | 3.92699 | 20.32 |
| 1.27000 | 3.98982 | 20 |
| 1.5 | 4.71239 | 16.93333 |
| 1.59155 | 5 | 15.95929 |
| 1.58750 | 4.98728 | 16 |
| 2 | 6.28319 | 12.70 |
| 2.11667 | 6.64970 | 12 |
| 2.5 | 7.85398 | 10.16 |
| 2.54000 | 7.97965 | 10 |
| 3 | 9.42478 | 8.46667 |
| 3.17500 | 9.97456 | 8 |
| 3.18310 | 10 | 7.97965 |
| 4 | 12.56637 | 6.35 |
| 4.23333 | 13.29941 | 6 |
| 4.77465 | 15 | 5.31976 |
| 5 | 15.70796 | 5.08 |
| 5.08000 | 15.95929 | 5 |
| 6 | 18.84956 | 4.23333 |
| 6.35000 | 19.94911 | 4 |
| 6.36620 | 20 | 3.98982 |
| 8 | 25.13274 | 3.175 |
| 8.46667 | 26.59882 | 3 |
| 10 | 31.41593 | 2.54 |
Comparison of various pitch measurements