How does oil work?
By John S. Evans, B.Sc.
Friction and wear
Whenever friction is overcome, the dislocation of surface material generates heat and this frictional heat can be highly destructive to metal surfaces and cause wear to take place.
Additionally, when there is solid friction (as opposed to fluid friction) wear will also take place. Material is lost due to the cutting action of opposing asperities and to the shearing of microscopic welds. In extreme cases the combination of high frictional temperatures, welding and shearing can cause complete seizure of moving parts.
The harmful effects of friction cannot be overemphasised. The job of the engineer and particularly the lubrication engineer is to control friction: to increase it where it is needed and to decrease it where it will cause damage. Lubrication reduces friction by replacing solid friction with fluid friction.
Sliding and rolling friction
When one body slides over another, the force of resistance encountered at the points of contact is known as sliding friction. If a ball or cylinder were to roll over a metal surface, the relative velocity of the points of contact are actually zero and this results in rolling friction. It should, however, be noted that rolling friction is always accompanied by some sliding friction.
Two main types of rolling friction exist. The first is where large tangential forces are experienced as in the case of a car wheel in contact with the road which generates considerable sliding forces. The second is where minimal tangential forces are present as is the case with a ball or roller bearing. This is sometimes known as free rolling.
Studies show that elastic deformation of the roller and the surface occurs and this gives rise to the resistance to motion (friction) that is encountered. The energy returned to the system when the deformation returns to normal is less than the energy required to cause the deformation. This excess energy is lost as frictional heat.
Tests show that rolling friction is not influenced by the presence or absence of a lubricant. However, lubrication is still important in this situation because the elastic deformation of the surfaces introduces sliding friction which can be reduced by the introduction of a lubricant to convert solid friction into fluid friction.
