COEFFICIENT OF FRICTION
Definition
The coefficient of friction is the ratio between the friction force and the force perpendicular to the contact surfaces of two bodies in relative motion.
Calculation
If a body rests on a flat surface whose incline increases, it eventually slides. The angle reached at this moment is the angle of friction. The tangent of this angle gives the coefficient of sliding friction, also called the dynamic coefficient of friction.
Static and dynamic friction
The static coefficient of friction applies just before sliding begins. It is generally higher than the dynamic coefficient, which applies once motion has begun.
Rolling friction
The coefficient of rolling friction is notably lower than that of sliding friction. This is why rolling bearings reduce friction losses more than plain bearings.
Tooth profile
The tooth profile of a gear is designed to favor rolling contact rather than sliding contact between the teeth. This design limits friction losses throughout the meshing, an important consideration in a watch movement where the transmitted torque remains very low.
Importance in watchmaking
A watch movement contains many pivots rotating in bearings. Reducing the coefficient of friction at these points limits wear and energy losses, thereby improving power reserve and accuracy. Jewel bearings, lubrication, and epilame treatments all contribute to this goal.
See also
For the bearings themselves, see Bearings (jewels) and Ruby / Jewel. For lubrication, see Lubricants.
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