| With very few exeptions, wheels and gears fit on the axles without being able to turn or sliding on them: | |
 |
A wheel cannot turn relative to the axle. It also cannot slide on it: the friction is too high. This makes some mechanisms difficult to construct. For example, it is not readily possible to make a two-hand clockwork, since one of the hands needs to be fixed to a wheel that turns at a different speed of the one carrying the other hand, while both need to use the same axle. |
| But with gears there is another, more subtle problem: | |
 |
When two gears mesh, there is necessarily a difference in angular position corresponding to at least half a tooth. Since the axles are in a fixed position to the wheels, this angle is also transmitted to the axles.
If other objects fixed to the axles should remain in symmetric positions, then this requirement cannot be met: it can only be approximated by using large gears to reduce the difference. With gears of 40 teeth, the difference is 360°/40/2 = 4.5° which is acceptably small for many purposes. For gears of 8 teeth, it is 22.5°, not acceptable at all. |
 |
There are a few pieces that allow you to change angles in increments of 22.5 degrees (1/16 of a full circle) |
