Types of Friction

In general the friction is of the following two types :

1.    Static friction, and
2.    Dynamic friction.

While two bodies in contact do not move relative to each other but contain tendency to move, the friction is termed static. Alternatively if relative motion takes place the bodies still experience friction force. It is termed dynamic friction.
Two types of relative motion among contacting bodies might be easily indentified. These are sliding and rolling motions. In that order the terms sliding friction & rolling friction are utilized.

The friction may also be classified as

1.    Friction between unlubricated surfaces, and
2.    Friction between lubricated surfaces.

The friction experienced among two dry & unlubricated surfaces in contact is also known as dry or solid friction. It is because of the surface roughness. The dry or solid friction includes the sliding friction & rolling friction as described earlier.

At this stage it might be pointed out that all of surfaces, no matter how smooth they are, have irregularities at microscopic or molecular level. In such circumstances when two surfaces are in contact just peaks (or asperities) shall come in contact. Such asperities shall deform plastically because of concentrated loads. Such plastics deformation hinders the relative motion and is the cause of friction. The peaks might also insert into valleys thereby making relative motion complicated and causing friction. This is the explanation of sliding and rolling friction. For decreasing this friction, oil film (lubricant) is introduced between surfaces of contact. If thin film of lubricant is just adequate to fill the valleys, they shall reduce load concentration on asperities and result in less or no plastic deformation. This shall reduce the force of friction. The situation is termed boundary lubrication.

In yet another situation a thick layer of lubricant between rubbing surfaces may be established. This thick layer shall not permit contact among asperities and the surfaces will in fact float over oil film. The relative motion in this case shall be hindered by the viscosity (a term not yet defined) of lubricant rather than any contact between two slides surfaces. This shall effectively decrease the force of friction considerably.

 (a) Dry Lubrication    (b) Boundary Lubrication    (c) Fluid Lubrication