Friction
'Dry' friction (as opposed to 'fluid' or 'lubricated' friction) is the force resisting lateral motion between two solid surfaces in contact.
It divides into:
- 'static' or 'initial' friction - the force experienced when there is no relative motion between the two surfaces
- 'kinetic' or 'dynamic' friction - the force experienced when the surfaces are moving relative to each other
The commonly used value is the coefficient of friction (COF), which describes the ratio of the frictional force between the surfaces of the two bodies, and the force pressing them together. Since it is a ratio, it is dimensionless without any unit.
Testing of the COF typically employs a friction table, where a sled of known weight is covered in one material and drawn at constant speed along a flat surface of a different, or same, material. The forces are measured and then divided by the mass of the sled to calculate the COF.
An accurate friction testing system must enable the peak force to be captured as the sled just starts to move - the static ('initial') friction, also known as the ‘stiction’ effect - and also detect 'kinetic' frictional properties, where the surface judders under dynamic ‘stick-slip’ during the moving phase of the test.
Coefficient of friction testing is typically used in the plastics and paper packaging industry, particularly for testing the starting and sliding friction of plastic film and sheeting, when assessing the slip properties generated by additives in plastic films. It is essential to know when setting the feed and running speeds on automatic glueing, filling and packaging lines.