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

Lecture No. 10-11.
Friction and lubrication.
Lubrication is the action of a lubricant that reduces wear, surface damage, and/or friction.
Lubrication is the supply of lubricant to the friction surface.
In liquids and gases, when the flow velocity of neighboring layers is not the same, internal friction arises. The forces needed to overcome it are very small. The external friction arising from the mutual sliding of two solid bodies is much greater than the internal friction in a fluid. The essence of lubrication is to replace the external friction of the parts with the internal friction of the lubricant.
10.1 Lubrication mechanisms
The main purpose of the lubricant is to create a positive gradient of mechanical properties in the contact. This makes it possible to localize the main processes accompanying friction in a thin layer of lubricant adjacent to the friction surfaces. Lubrication can reduce both the adhesive and deformation components of the coefficient of friction. First, with a sufficient thickness of the lubricating layer, the friction surfaces do not touch, i.e. f _d 0. Secondly, the coefficient of internal friction of lubricants is much lower than f _a of structural materials from which friction units are made.
Fluid lubrication - lubrication in which the separation of friction surfaces is carried out by a liquid lubricant. Varieties of liquid lubrication are hydrodynamic and hydrostatic lubrication. In the first case, the complete separation of the friction surfaces occurs due to the pressure arising in the fluid layer during the relative movement of the surfaces, in the second case, due to the injection of fluid under external pressure into the gap between the friction surfaces, which are in motion or at rest.
Gas lubrication occurs when the friction surfaces are separated by a gas flow. Depending on the method of creating pressure in the gas layer, gas-dynamic and gas-static lubrication are distinguished. The viscosity of gases is about 100 times less than that of liquids. Therefore, gas-lubricated friction pairs have very low friction coefficients.
With an increase in the load in the friction pair, the thickness of the lubricating layer decreases. At a certain critical thickness, the effect of the force field of the friction surface will be so strong that the physical properties of the lubricating layer will begin to differ from the initial properties of the lubricant. Boundary lubrication occurs when there is a lubricating layer on the friction surfaces, the properties of which differ from the bulk properties of the lubricant. Boundary layers result from the adsorption of polar lubricant molecules on friction surfaces. Boundary lubrication friction can be realized with both liquid and solid lubricants.
A further increase in the load may lead to the fact that individual microprotrusions on the friction surfaces will come into contact, while the remaining parts of the surfaces will be separated by a lubricating layer (Fig. 10.1). Such a lubrication mechanism is most often found in friction units.
Semi-fluid lubrication - lubrication in which boundary lubrication is partially carried out.
The strongest bond between the lubricating layer and the friction surface takes place during chemical adsorption. The rate of chemosorption is maximum on "clean", i.e. free from adsorption films, surface areas. This feature of chemosrption determines the property of boundary layers to "self-heal" in case of damage. The strength of the adsorption film depends on the physicochemical activity of the lubricant. To increase it, additives are introduced into lubricants - substances that give the lubricant new properties or enhance existing ones.