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

Figure 10.1 Scheme of semi-fluid lubrication.
1 and 2 - friction surfaces, 3 - boundary layers, 4 - lubricant microvolumes, 5 - contact areas
The wear of the boundary layers occurs much more slowly than the wear of metallic solids. Lubrication causes cooling of friction units by removing the heat of friction from the contact areas with the help of a lubricating layer. Therefore, the heat capacity and thermal conductivity are important parameters of the lubricant, characterizing its ability to reduce the thermal tension of friction pairs.
10.2 Lubricants
The purpose of lubricants is to reduce friction and (or) wear.
The ratio of the cost of lubricants and the cost of lubricating equipment is usually from 1:5 to 1:15. Therefore, the use of better quality lubricants in industry, although more expensive, is almost always economically justified.
Lubricants are divided into liquid, plastic and solid.
Liquid lubricants are primarily oils (petroleum, vegetable, animal and synthetic), water-based compositions and liquid metals.
The most important physical characteristics of liquid materials are viscosity - the property to resist flow, as well as its dependence on temperature and pressure. Sometimes the term lubricity is used to characterize oils , based on the feeling of the degree of friction reduction when rubbing the lubricant with your fingers.
Petroleum oils are oils refined in the usual way based on petroleum feedstocks. Of all lubricants, they are the most widely used. Depending on the purpose, the following types of petroleum oils are distinguished.
Industrial oil is used to lubricate machine tools and industrial equipment.
Transmission - for mechanical transmissions.
Motor - for piston internal combustion engines.
Cylinder - for piston steam rings.
Turbine - for lubrication of steam and gas turbines, turbocompressors.
Aviation - for aviation piston, turboprop, turbojet engines.
This division is conditional, for example, motor oils are often used to lubricate industrial equipment.
Vegetable oils are obtained from seeds or fruits of plants by pressing or extraction. They are composed of glycerides (esters of glycerol) and fatty acids. The lubricating properties of vegetable oils are better than those of petroleum oils. However, they are less stable due to rapid oxidation at high temperatures. The presence of active radicals in vegetable oils determines their high adhesion to metals. Therefore, they are used as additives for petroleum oils.
Animal oils are obtained from the adipose tissue of animals. They played a big role at the dawn of the development of technology, but after the advent of petroleum oils, they lost their significance. They are used as additives and raw materials for the food, pharmaceutical, and perfume industries.
Synthetic oils are made by synthesis from the simplest unsaturated hydrocarbons or other compounds. They are inferior to petroleum oils in terms of lubricating characteristics, but surpass them in their ability to operate at extremely high and low temperatures. Therefore, they are used in the technology of the North, power equipment, in jet aviation and space engineering, and in the automotive industry.
The main groups of synthetic oils: esters, glycols, silicones, fluoride carbons.
Esters are used for lubrication of aircraft instruments at operating temperatures from -60 to +120, and with antioxidant additives - up to +200 ⁰ C.
Glycols have a narrower operating temperature range (from -60 to +100 ⁰ C) and are prone to oxidation. Their advantage is a small dependence of viscosity on temperature.
Silicones have silicon atoms in their structure. Their viscosity depends little on temperature from -60 to +250 ⁰ C. Silicone oils are heat-resistant, but have a relatively low lubricity.
Fluorocarbon oils are thermally stable, superior to other synthetic oils. Their disadvantages are the strong dependence of viscosity on temperature and high cost. Optimal areas of application are friction units operated at high loads and temperatures.