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

Lecture number 8.
Wear of parts surfaces.
The types of wear are distinguished in accordance with the existing types of wear - mechanical (abrasive, fatigue), corrosion, etc.
Mechanical wear is the result of friction forces when one part slides over another. With this type of wear, abrasion (cutting) of the surface layer of the metal and distortion of the geometric dimensions of the jointly working parts occur. Wear of this type most often occurs during the operation of such common mates of parts as a shaft - a bearing, a frame - a table, a piston - a cylinder, etc. It also appears during rolling friction of surfaces, since this type of friction is inevitably accompanied - there is also sliding friction, but in such cases, wear is very small.
The degree and nature of mechanical wear of parts depend on many factors: the physical and mechanical properties of the upper layers of the metal; working conditions and the nature of the interaction of mating surfaces; pressure; relative speed of movement; conditions for lubrication of rubbing surfaces; the degree of roughness of the latter, etc. The most destructive effect on parts is abrasive wear, which is observed in cases where the rubbing surfaces are contaminated with small abrasive and metal particles. Usually, such particles fall on rubbing surfaces during the processing of cast billets on a machine, as a result of wear of the surfaces themselves, dust ingress, etc. They retain their cutting properties for a long time, form scratches, scuff marks on the surfaces of parts, and also, mixing with dirt, they act as an abrasive paste, as a result of which intensive rubbing and wear of mating surfaces occurs. The interaction of the surfaces of parts without relative movement causes metal crushing, which is typical for keyed, slotted, threaded and other connections.
Mechanical wear can also be caused by poor maintenance of the equipment, for example, irregularities in the supply of lubrication, poor repair and failure to meet its deadlines, power overload, etc.
In. during operation, many machine parts (shafts, gear teeth, connecting rods, springs, bearings) are subjected to long-term action of variable dynamic loads, which have a more negative effect on the strength properties of the part than static loads. Fatigue wear is the result of variable loads acting on a part, causing fatigue of the part material and its destruction. Shafts, springs and other parts are destroyed due to fatigue of the material in the cross section. In this case, a characteristic type of fracture is obtained with two zones - the zone of developing cracks and the zone along which the fracture occurred. The surface of the first zone is smooth, while the second one is shelled and sometimes granular.
Fatigue failure of the material of the part does not necessarily lead to its failure immediately. It is also possible the occurrence of fatigue cracks, peeling and other defects, which, however, are dangerous, as they cause accelerated wear of the part and mechanism. To prevent fatigue failure, it is important to choose the right cross-sectional shape of a newly manufactured or repaired part: it should not have sharp transitions from one size to another. It should also be remembered that a rough surface, the presence of scratches and scratches can cause fatigue cracks.
Seizure wear occurs as a result of sticking (“seizing”) of one surface to another. This phenomenon is observed with insufficient lubrication, as well as significant pressure, at which two mating surfaces approach each other so closely that molecular forces begin to act between them, leading to their seizure.
Corrosive wear is the result of wear of parts of machines and installations that are under the direct influence of water, air, chemicals, and temperature fluctuations. For example, if the air temperature in industrial premises is unstable, then every time it rises, the contained