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In layman's terms, friction is a force that resists one surface from sliding or rolling over another. Therefore, it can be said that friction only occurs when two surfaces are in relative motion, such as when a crankshaft is rotating in a journal bearing or when a ball bearing is rolling along its raceway.
A microscopic view of these surfaces in relative motion reminds us that each surface contains tiny, jagged asperities (rough and uneven surfaces), no matter how closely these surfaces are machined. Without some form of separation, such as that formed by a lubricant film, these surface asperities may seize upon contact. At the very least, some abrasion, adhesion and/or ploughing of these asperities will take place as the movement occurs.
A number of factors affect the frictional conditions at the interface between these two surfaces in relative motion. These factors are:
The challenge is to reduce the coefficient of friction as much as possible by either eliminating the factors that may have an adverse effect on the surface in relative motion or at the very least controlling those factors.
There are several ways to reduce friction:
It is important to understand how two metal surfaces within a machine interact with each other. All metal surfaces have some degree of surface roughness. Regardless of how smooth a surface may appear, each metal surface has high points and low valleys. The high points are called the surface asperities. When the two surfaces move past each other, it is the asperities on one surface that come in contact with the asperities on the other surface.
The number and height of the asperities on the surfaces dramatically affect the friction between the surfaces. It is the job of the lubricant to keep these asperities apart and prevent them from contacting each other, thus lowering or eliminating metal-to-metal contact and friction.
Without an adequate oil film to separate the metal surfaces, metal-to-metal contact occurs, asperities from the two surfaces weld together and then are ripped apart from each other by the motion of the surfaces sliding against each other, causing adhesion (smearing) and abrasion (cutting). In severe cases, the two pieces of metal may weld and seize to each other.
If the oil film is sufficient to keep the surfaces separated but is too high a viscosity for the speed of the moving surfaces, then some drag or internal resistance within the fluid (fluid friction) will occur. This can be thought of as friction caused by layers of oil being forced to slide past each other .