O’zbekistоn respublikasi оliy va o’rta maxsus ta`lim vazirligi

Figure 4.2 The structure of a multimolecular film according to A.S. Akhmatov.
1 – friction surface, 2 – adsorbed molecules, 3 – layers of molecules coordinating with the adsorbed layer
The friction mode in boundary lubrication can be implemented using solid lubricants. For example, on the friction surfaces of cast iron parts, the graphite contained in it can be smeared, forming a boundary layer. Fluoroplast has a similar effect when porous bearing materials are impregnated with it.
4.4 Oil lubricated friction
With liquid lubrication, the friction surfaces are separated by a layer of lubricating fluid, which carries the normal load applied to the friction unit. The minimum layer thickness exceeds the sum of the maximum roughness heights on both surfaces. Therefore, the friction of solid bodies is replaced by the internal friction of fluid layers. The force of internal friction of liquids obeys Newton's law and they are called Newtonian :
,(4.1)
where is the dynamic viscosity of the fluid, d / dx is the fluid shear rate gradient along the normal to the direction of movement (Fig. 3.3), S is the shear area.
If the pressure in the fluid is constant, d /dx= /h, where is the highest fluid velocity in the gap h. Then
,(4.2)
Oils are generally Newtonian fluids whose viscosity is independent of shear rate. Oils with additives, especially at low temperatures, are non- Newtonian fluids : their viscosity decreases as temperature increases and pressure decreases.

Figure 4.3 Velocity distribution in the fluid layer (1) separating the friction surfaces (2 and 3)
The foundations of the theory of friction in liquid lubrication were laid by Nikolai Pavlovich Petrov. He considered the friction of the shaft in the bearing at a constant clearance (Fig. 4.4).
It follows from it that the moment of friction increases with an increase in the speed of rotation, an increase in the radius of the shaft and the length of the bearing, and with a decrease in the clearance in it. Petrov's formula is valid at high shaft speeds, when the bearing operates in hydrodynamic lubrication mode.