Coefficient of friction due to rolling (μr) is generally smaller than that caused by sliding action. Therefore wherever possible rolling friction compared to sliding friction is desired. μr is defined as the force required to maintain steady rolling, divided by the load carried by the roller.
Rolling friction coefficients often depend on hardness of contacting solids. On increasing hardness, elastic deformation under load decreases. Therefore, hysteresis loss and so the value of μr decreases. For hard smooth steel rollers, the coefficient of rolling friction ranges between 0.01 and 0.001. A roller or sphere made of soft material. when rolled over other soft surface, generates a higher level of rolling friction.
Sources of Rolling Friction :
It is important to know the source of rolling friction, so that proper actions may be implemented to control the rolling friction. Let us consider a hard steel ball which rolls over a softer rubber such as shown in Fig. 2.20. As it rolls along, the ball displaces rubber elasto-plastically around and ahead of it. The force required to display rubber is almost equal to the observed rolling friction. Thus, the rolling friction is essentially a measure of the force required to deform other material. With a very bouncy rubber rolling friction will be lesser compared to a very soggy rubber.
The main contributions to friction in rolling contacts are:
1. Micro-slip effect within the contact area.
2. Elastic hysteresis of the contacting materials
3. Plastic deformation of the materials, and
4. Adhesion effects in the contact
It is important to note that lubricant cannot reduce deformation of surface, therefore, lubricants have very little effect(except reduction in adhesion effects) on the rolling friction.