FRICTION AND LUBRICATION - BASIC PRINCIPLE :

Friction is frequently defined as resistance to motion when one solid body slides or rolls over another. In a piece of machinery, friction is undesirable, as it wastes power and generates heat, but the main trouble is that excessive friction lead to wear and ultimately seizure of working parts.

When resistance to motion or the frictional force is high enough to prevent motion, it is termed as static friction. When motion commences and the moving body slides over the fixed body, the resistance to moton is known as sliding friction or kinetic friction. Sliding friction is always lower than static friction so that it takes less force to keep a body moving than to commence moving from rest.

It is convenient for design and comparison purpose to quote the coefficient of friction rather than the frictional force.

                                                     Frictional Force

Coefficient of friction (u) =  ........................

                                                                       Load

The three basic laws of friction between sliding and fixed bodies are defined as :

1.   The frictional force is independent of the area of the sliding surface.

2.   The frictional force is directly proportional to the load.

3.   The frictional force is independent of the speed of sliding.

Motion catagory of machines :-

1.     The slider or linear sliding motion, e.g. a piston in a cylinder.

The journal bearing, involving rotary motion but essentially a form of sliding motion.

2.    Rolling motion, e.g. a ball or roller in a circular race as in a ball or roller bearing or a train wheel rolling on a railway line.

3.   Gears, of various type which combine both rolling and sliding motion.

4.                     Cams, which also embrace both sliding and rolling motion.

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