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

Fig3.5. The structure of the surface layer of a ground carbon steel part.
Defects in the form of microscopic cracks and voids are concentrated in the surface layer . Naturally, the destruction of friction surfaces begins in the places of defects.


3.2 Contacting parts
Contacting - mechanical contact of physical bodies. The heterogeneous structure of the surface of the parts determines the complexity of mechanical interactions in the contact zone.
Contact area
When two parts approach each other, the highest ridges touch first. They deform elastically or plastically. elastic deformation is characterized by complete restoration of the protrusion shape after the load is removed. With increasing stress, the deformation becomes plastic , i.e. persists after unloading. The deformation of the contacting protrusions is accompanied by a further convergence of the parts, as a result of which
lower protrusions come into contact. Under the action of the load, they are flattened and embedded into each other. Then there is a deformation of the waves on which the protrusions are located.
Due to the fact that real bodies have a wavy, rough surface, their contact is discrete. This led to the need to introduce the concepts of nominal, contour and actual contact areas.
Under the nominal contact area A _a understand the area that bodies would touch if their surfaces had an ideally smooth geometric shape.
When flat bodies come into contact, A _a is determined by their geometric dimensions.
Contour area And _c contact - the area of the collapse of the protrusions, forming a waviness, which are the areas of true touch. Areas A _c are detected when checking the quality of the mating of parts by painting - these are touch spots.
The actual area of contact (touch) FPC And _r - is determined by the sum of the areas of contact of individual protrusions of microroughnesses, forming the roughness of the surfaces of the mating bodies. The areas of actual contact are grouped in zones at the tops of the deformed waves.
Schematically, the actual A _r , contour A _c , nominal (i.e. geometric) A _a of the contact area are shown in fig. 3.6.