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

Lecture No. 18
Ways to increase the resistance to wear of the working surfaces of machine parts.
18.1 Service life of rubbing machine parts
The service life of machines is determined by the wear of its parts - a change in the size, shape, mass or condition of their surfaces due to wear, i.e., residual deformation from permanent loads or due to destruction of the surface layer during friction. The failure of parts and working bodies of machines under normal operating conditions is a consequence of physical wear of various types: fatigue failure, creep of materials, mechanical wear, corrosion, erosion, cavitation, aging of the material, etc. The most common cause of failure of parts and mechanisms of machines is not breakage, but wear and damage to working surfaces in friction units.
The wear rate of equipment parts depends on many factors: the conditions and mode of their operation; the material from which they are made; the nature of the lubrication of rubbing surfaces; specific effort and sliding speed; temperature in the interface zone; environmental conditions (dustiness, etc.).
The amount of wear is characterized by established units of length, volume, mass, etc. Wear is determined by changing the gaps between the mating surfaces of parts, the appearance of leaks in seals, reducing the accuracy of processing the product, etc. Wear can be normal and emergency . Normal , or natural, is the wear that occurs during the correct, but long-term operation of the machine, i.e., as a result of using a given resource of its work. Emergency , or progressive, is wear that occurs within a short time and reaches such proportions that further operation of the machine becomes impossible. At certain values of changes resulting from wear, wear limit sets in, causing a sharp deterioration in the performance of individual parts, mechanisms and the machine as a whole, which necessitates its repair.
Particularly great is the wear of parts and working bodies of machine tools that are operated in abrasive and aggressive environments, and parts of transport machines operating in dirt and dust. For example, the resource of crushing plates averages 4 ... 6 months, the teeth of buckets of excavators, scrapers and bulldozers, screws - 6 ... 8 months. On a crusher weighing 6 tons, when crushing highly abrasive rocks, 17 tons of crushing plates are consumed per year. The duration of the aviation piston engine before repair is 600 ... 1000 hours. The facts of extremely short service life (less than 1000 hours) of cylinder bushings of powerful marine diesel engines are noted. There are cases of failure as a result of cavitation destruction of large pumps and propellers in less than a year of operation. Diesel engines installed on powerful dump trucks require major repairs after 1500...2000 hours of operation, i.e. after -6...8 months. Onboard gears of tractors work before replacement no more than 2000...2500 hours, the service life of transport transmissions before repair is 2500...3500 hours. During the season of tractor operation on sandy soils, two or three sets of caterpillars have to be replaced, which on average costs 50% of the cost of a new tractor. The plowshare of a tractor plow, on average, processes only 15 ... 20 hectares of soil until it is completely worn out ; shares, not to mention the fact that on heavy soils the cutting edge of the share requires repair after 4 ... 6 hectares of plow operation. The annual need for spare links in the drive chains of agricultural machines is about 100 million pieces.
Wire ropes of hoisting machines, especially machines at metallurgical enterprises, have a short duration of work. The average service life of blast-furnace hoist ropes is 3-4 months, pouring cranes of open-hearth shops 6-7 months, while pouring 3-4 heats per day.