Here are some important points in related to gear technology, which can be pretty useful while designing transmission system which require gears. You can Google and find the definition terms unknown to you.
1. More is the pressure angle, more stronger the gears teeth. 14.5' pressure is mostly used in USA, as they use inches system and calculations become easy for them. 20' is common in Europe and most of the countries in the world. While automobile companies use the pressure angle of 25'.
2. More is the contact ratio between meshed gears, more is the smoothness. Normal Contact ratio is normally 1.6, and is not preferred to use below 1.4 or else very precise arrangement is required between gears centers.
3. Base Circle become smaller than root circle in case of 20' pressure angle gear as the number of teeth increases above 41, which is theoretically proved fact. For 25' pressure angle it is 26 teeth, and for 14' pressure angle gears it is 78.
4. 14' pressure angle have more curvy involute profile of teeth, while the 20' pressure angle teeth are pointy on tip circle.
5. While two gears mesh, there is two type of motion between teeth- sliding and rolling. For good power transmission sliding motion should be minimum, which depends on various factors like teeth finish, lubrication, number of teeth etc. On the pitch circle, the sliding motion is zero, it has only pure rolling motion. As you move further from pitch circle in both radial directions along teeth, the sliding increases.
