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Fig 4.4 Scheme of liquid lubrication of the plain bearing.
1 - shaft, 2 - bearing, 3 - liquid lubricant, h=const - gap between shaft and bearing
4.5 Rolling friction
When rolling, the velocities of the contacting bodies at the points of contact are the same in magnitude and direction. Rolling friction is realized in rolling bearings, without which it is impossible to imagine any more or less complex machine. This circumstance is due to the fact that when rolling, a low coefficient of friction is realized.
,(4.3)
where M is the moment of the rolling resistance force relative to the point of contact A, M \u003d FR, where F is the rolling resistance force, R is the radius of the cylinder or ball, N is the normal load (Fig. 4.5).

Figure 4.5 Scheme of the action of forces during the rolling of a cylinder or ball
When an elastic roller rolls on an elastic base, the rolling resistance is due to three reasons:
1) hysteresis losses,
2) microslip in the contact zone,
3) adhesion of contacting bodies.
Hysteresis is the delay in changing a physical quantity characterizing the state of a substance from a change in another physical quantity that determines external conditions. Hysteresis losses are caused by the fact that real solids are not perfectly elastic. Under the influence of stress in a solid body, a deformation occurs, which in real bodies is not directly proportional to the stress. The physical nature of the elastic hysteresis is related to the imperfection of the crystal structure of technical materials. Loading causes the movement of defects in the crystal lattice in the material, which leads to energy dissipation during sample deformation.
Rolling microslip occurs as follows. A contact area is created in the contact of the elastic base, near which the sample material is compressed, and the base is stretched (Fig. 3.6).


Lecture number 5.
COEFFICIENT OF FRICTION
SLIDING FRICTION
Sliding friction is the most common type of friction. The main characteristic of sliding friction is friction force .
The friction force is the force of resistance to the relative movement of one body over the surface of another under the action of an external force, tangentially directed to the common boundary between the two bodies.
Since the contact surface is discrete, the interaction of rubbing bodies is carried out on sections of the actual contact area isolated from each other, and the friction force is the resultant of all elementary forces of resistance to movement on these spots.
(5.1)
where ΔF _i - elementary friction force on the i-th discrete contact.
The contact diagram of rubbing bodies during sliding friction is shown in Fig. 5.1. The friction force is directed opposite to the direction of the relative motion of the contacting bodies.
preliminary displacement takes place .