The slip and creep in the belt or rope drives is a common phenomenon, in the transmission of movement or power between two axes. The effect of sliding is to reduce the speed ratio of the drive. In the precision machine, in which a defined speed ratio is important (as in the clock mechanism, special purpose machines ... etc), the only positive impulse is by gears or cogwheels.
Friction Wheels:
Kinematically, the motion and power transmitted by the gears is equivalent to that transmitted by the friction wheels or discs in contact with sufficient friction between them. To understand the motion transmitted by two gearwheels, consider the two discs placed together as shown in the previous figure
When one disc is rotated, the other disc will rotate as long as the tangential force exerted by the drive disc does not exceed the maximum frictional resistance between the two discs. But when the tangential force exceeds the frictional resistance, there will be slippage between the two discs. Therefore, the friction drive is not positive a drive, beyond a certain limit.
The gears are machine elements that transmit the movement by means of teeth that are successively coupled. Gear teeth act as small levers. The gears are highly efficient (almost 95%) due to the mainly rolling contact between the teeth, so the transmitted motion is considered positive.
The gears essentially allow positive engagement between the teeth, so that high forces can be transmitted while still undergoing essentially rolling contact. Gears do not rely on friction and do better when friction is minimized.