The frictional force of the rolls on the material acts in the direction of material movement on the entry side of the neutral point, but on the exit side of the neutral point the frictional force of the rolls on the material acts in the reverse direction of the material movement as shown in Fig. 5.2. It is difficult to calculate the rolling forces exactly mathematically. However with some approximations it has been shown that
Stress distribution over length L, assuming arc over length L to be a straight line, will be as shown in Fig. 5.3.
Fig. 5.4 shows that variation of roll pressure from between the entry and exit of metal in the roller. It would be seen that the roll pressure increases continuously from the point of entry upto neutral point and thereafter it decreases.
The peak pressure at the neutral point is called the friction hill. As the coefficient of friction increases, the peak pressure also increases and neutral point is shifted towards entry side.
The force trying to separate the rolls can be calculated by integrating the vertical component of the force acting at the roll-strip interface. The driving torque is required to overcome the torque exerted on the roll by the interfacial friction force.
Rolling Load in Hot Rolling:
The rolling load depends on geometry of reduction, i.e., initial width and thickness of stock, work roll diameter and the degree of reduction, on the frictional conditions existing at the roll- stock interface and on the flow stress of the metal being rolled.
Further the flow stress of a metal at high temperatures is dependent on its composition, its microstructure and on the strain, strain rate, temperature and deformation mode, which for rolling implies the extent to which the reduction deviates from plane strain.
