Frictional materials: choice, requirements
Modern production equipment has a rather complex design. Frictional mechanisms transmit motion by means of frictional force. These can be clutches, clamps, clamps and brakes.
To make the equipment durable, it worked withoutidle time, special requirements are put forward to its materials. They are constantly growing. After all, machinery and equipment are constantly being improved. Their capacities, working speeds, and also loads increase. Therefore, in the process of their operation, various friction materials are used. On their quality depends reliability, durability of equipment. In some cases, the safety and life of people depends on these elements of the system.
Friction materials are integral elementsnodes and mechanisms that have the ability to absorb mechanical energy and dissipate it into the environment. At the same time, all structural elements should not wear out quickly. For this, the materials presented have certain properties.
The coefficient of friction of friction materials should bebe stable and high. The wear index is also required to meet operational requirements. Such materials have good heat resistance and are not subject to mechanical influences.
For a substance that performs frictional functions,It was not attached to working surfaces, it is endowed with sufficient adhesive qualities. The combination of such properties ensures the normal operation of equipment and systems.
Friction materials have a certain set of properties. The main ones were listed above. These are official qualities. They determine the performance characteristics of each substance.
But all service characteristics are determineda set of physico-mechanical and thermostatic indicators. Such parameters change during the operation of the material. But their limiting value is taken into account in the selection of the friction substance.
There is a separation of properties into static,dynamic and experimental indicators. The first group of parameters includes the limit of compression, strength, bending and stretching. Also included here are the heat capacity, thermal conductivity and linear expansion of the material.
To the indicators, determined in dynamic conditions, heat resistance and heat resistance are considered. In the experimental situation, the coefficient of friction, wear resistance and stability are established.
Types of materials
Friction materials of the brake system andThe clutch is most often made on a copper or iron basis. The second group of substances is used in conditions of increased load, especially with dry friction. Copper materials are used for medium and light loads. And they are suitable for both dry friction, and with the use of lubricating fluids.
In modern production conditions, materials on a rubber and resin basis have been widely used. Various fillers of metal and non-metallic components can also be used.
There is a classification of friction materials independing on the area of their application. The first large group includes transfer devices. These are medium and lightly loaded mechanisms that operate without lubrication.
Further, the friction materials of the brake system, intended for medium and heavily loaded machinery, are distinguished. In these nodes, no lubrication is applied.
The third group includes substances used in the clutches of medium and heavily loaded units. They have oil.
Also separate a separate group of brake materials in which liquid lubricant is present. The main parameters of the mechanisms determine the choice of friction materials.
In the clutch, the load acts on the elements of the system for about 1 s, and in the brake - up to 30 s. This indicator determines the characteristics of the materials of the nodes.
As already mentioned above, the main metallic friction materials of the clutch system, brakes are iron and copper. Steel and cast iron are very popular today.
They are applicable in different mechanisms. For example, friction materials for brake shoes, the composition of which contains cast iron, is often used in railway systems. It does not warp, but sharply loses its sliding qualities at a temperature of 400 ° C.
Frictional clutch or brake materials are also made from non-metallic substances. They are created mainly on an asbestos basis (resin, rubber act as binding components).
The coefficient of friction remains sufficiently high up totemperature of 220 ° С. If the binder is a resin, the material is characterized by high abrasion resistance. But their friction coefficient is somewhat lower relative to other similar materials. A popular plastic material on this basis is retinax. It contains phenol-formaldehyde resin, asbestos, barite and other components. This substance is applicable to knots and brakes with heavy operating conditions. It retains its qualities even when heated to 1000 ° C. Therefore, retinax is applicable even in aircraft braking systems.
Asbestos materials are made by creatingof the same name. It is impregnated with asphalt, rubber or bakelite and pressed at high temperatures. Short asbestos fibers can also form non-woven patches. They add fine metal shavings. Sometimes brass wire is introduced to strengthen strength in them.
There is one more variety ofcomponents of systems. This is the sintered friction material of the brake system. What is this variety, will become clearer from the way they are made. They are most often made on a steel basis. In the process of welding with it, other components are included. Pre-pressed blanks consisting of powder mixtures are subjected to high temperature heating.
Such materials are used most often inheavy-loaded couplings and braking systems. Their high performance during operation is determined by two groups of components that make up the composition. The first materials provide a good coefficient of friction and durability, and the second - stability and a sufficient level of adhesion.
Materials on a steel basis for dry friction
Selection of material for different systemsis carried out on the basis of economic and technical feasibility of its manufacture and operation. Decades ago, such iron-based materials as PMK-8, MKV-50A, and also QMS were in demand. Frictional materials for brake pads, which worked in heavily loaded systems, later began to be made from PMK-11.
MKV-50A is a newer development. It is used in the manufacture of linings for disc brakes. He has an advantage over the group of PMK on the indicators of stability, wear resistance.
In modern production, materials of the SMK type have become more widespread. They increased the content of manganese. Also included in the composition are boron carbide and nitride, molybdenum disulphide and silicon carbide.
Bronze-based materials for dry friction
In the transmission and braking systems of variousThe materials are based on tin bronze. They wear the mating parts of cast iron or steel much less than iron-based friction materials.
The presented variety of materials is usedeven in the aviation industry. For special operating conditions, tin can be replaced with substances such as titanium, silicon, vanadium, arsenic. This prevents the formation of intercrystalline corrosion.
Materials based on tin bronze widelyare used in the automotive industry, as well as in the manufacture of agricultural machinery. They withstand heavy loads. The constituents of the alloy 5-10% tin provide increased strength. Lead and graphite play the role of a solid lubricant, and silica or silicon increases the coefficient of friction.
Operation in the conditions of liquid lubrication
Materials used in dry systems havea significant drawback. They are subject to rapid wear. When they get grease from the adjacent nodes, their effectiveness decreases drastically. Therefore, recently, materials that are intended for work in liquid oil have been increasingly distributed.
Such equipment is smoothly turned on, characterized by a high level of durability. It is easily cooled and simply sealed.
In foreign practice, recently growingproduction volumes of a product such as friction sheet material for brakes, couplings and other mechanisms based on asbestos. It is impregnated with resin. The composition includes molded elements with a high content of metallic fillers.
Most often for the lubricating medium used sintered materials, made of copper. To increase the frictional characteristics, non-metallic solid components are introduced into the composition.
First of all,wear resistance of friction materials. The economic and operational expediency of the components presented depends on this. In this case, technologists are developing ways to eliminate excessive heating on friction surfaces. To do this, improve the properties of the friction material itself, the design of the device, as well as regulate the working conditions.
If the materials are used in dry conditionsfriction, special attention is paid to their heat resistance and resistance to oxidation. Such substances are less prone to abrasive wear. But for systems with lubrication, the heat resistance is not so important. Therefore, more attention is paid to their strength.
Also, technologists in improving qualityFriction materials pay attention to their degree of oxidation. The smaller it is, the longer the components of the mechanisms are. Another direction is to reduce the porosity of the material.
Modern production should improveused additional materials in the process of manufacturing various mobile, transfer devices. This will meet the growing consumer and operational requirements put forward to friction materials.