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

4.2 Friction theories
At different times, deformation, molecular, energy and molecular-mechanical theories of friction were put forward and are used to some extent for calculations.
deformation friction theories explain friction as a result of deformation of a certain volume of the friction body when the protrusions of the counterbody are introduced into it. D. Leslie (1801) believed that friction arises due to a deformation wave running in front of each ledge that has penetrated into a deformed body. L. Gumbel (1921) considered friction as a resistance to the displacement of material by irregularities that have penetrated into it. Tribologist G.I. Epifanov (1959) developed views on friction as a process of shear formation in the surface layer.
Molecular friction theories explain friction as the result of the molecular interaction of two solids. The best known are the theories of Desaguliers (1734), Hardy (1919), Kuznetsov (1926), Tomlinson (1929), Bartenev (1954).
Energy friction theory was proposed by G. Fleischer in 1968. and describes wear using energy laws and relationships. It is assumed that there is a relationship between the friction energy and the volume of worn material.
Founder of molecular mechanical theory of friction I.V. Kragelsky saw in friction an effective molecular-mechanical nature.
On the one hand, attractive forces act on the sites of actual contact. They appear even at distances ten times greater than the interatomic distance in crystal lattices. These forces cause adhesive interaction of bodies in the areas of actual contact. Adhesion forces are directly proportional to the actual contact area. The normal load N affects these forces indirectly, through the actual contact area, which grows slightly with increasing N.
The mutual displacement of the contacting bodies is accompanied by the breaking of adhesive bonds, which requires energy expenditure. Friction especially increases with strong adhesion, when the bonds between the rubbing bodies are broken not in the areas of actual contact, but at a certain depth of the material.
Seizure is a local connection of two bodies due to adhesion, which occurs during friction. As a result of setting, the operating mode of the friction unit is created, leading to seizing and catastrophic wear of the contacting materials. Seizure does not correspond to normal modes of friction with minimal wear. Therefore, an important factor in friction is the strength of the bond between the surface layer and the underlying layers of the material .
Gradient mechanical properties - a vector characterizing the change in the mechanical properties of an element of a friction pair along the normal to the friction surface. With a positive gradient of mechanical characteristics, the shear force of the material on the friction surface ( _o ) is less than in the depth of the material ( ), i.e. _o < , with negative - _o > .
The second feature of friction is the mechanical deformation of the friction material by microroughnesses present on the friction surface of the counterbody.