Frictional materials: choice, requirements

Modern production equipment has a rather complex design. Frictional mechanisms transmit motion with the help of frictional force. These can be clutches, clamps, clamps and brakes.

To make the equipment durable, it worked without downtime, its materials are put forward with special requirements. 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.

general characteristics

Friction materials are integral parts of units and mechanisms that have the ability to absorb mechanical energy and dissipate it into the environment. In this case, all structural elements should not wear out quickly. For this, the materials presented have certain properties.

The coefficient of friction of friction materials should be 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.

That the substance fulfilling frictional functions, is not caught on working surfaces, it is endowed with sufficient adhesive qualities. The combination of such properties ensures the normal operation of equipment and systems.

Material properties

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 determined by a set of physicomechanical and thermostatic indicators. Such parameters change during the operation of the material. But their limiting value is taken into account in the selection process 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 indices, determined in dynamic conditions, thermal stability 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 system of brakes and clutches are usually 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.

Application area

There is a classification of friction materials, depending 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 mechanisms, 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 there is a liquid lubricant. 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.

Metallic materials

As already mentioned above, the main metal 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.

Non-metallic materials

Frictional clutch or brake materials are also made of non-metallic substances. They are created mainly on an asbestos basis (resin, rubber act as binding components).

The coefficient of friction remains sufficiently high to a temperature of 220 ° C. If the binder is a resin, the material is characterized by high abrasion resistance. But their coefficient of friction 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 creating the same fabric. 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 the strength in them.

Sintered materials

There is one more variety of the presented components 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 often made on a steel basis. In the process of welding, other components are included. Pre-pressed blanks consisting of powder mixtures are subjected to high-temperature heating.

Such materials are used most often in heavily loaded couplings and braking systems. Their high performance during operation are 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.

Steel-based materials for dry friction

The choice of material for different systems is based on the 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, were later 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.

Materials on the basis of bronze for dry friction

In the transfer and braking systems for various purposes, materials based on tin bronze have proven themselves. They wear the mating parts of cast iron or steel much less than iron-based friction materials.

The presented variety of materials is used even 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 are widely 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 silicon dioxide or silicon increases the coefficient of friction.

Operation in the conditions of liquid lubrication

The materials used in dry systems have a significant drawback. They are subject to rapid wear. If they get grease from the adjacent nodes, their effectiveness decreases drastically. Therefore, recently, materials that are intended for work in liquid oil are 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 increased production volumes of such products 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.

Improving properties

First of all, improvement requires wear resistance, which is possessed by 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, and also regulate the working conditions.

If the materials are used in conditions of dry friction, special attention is paid to their heat resistance and oxidation resistance. 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, while improving the quality of friction 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 improve the applied additional materials in the process of manufacturing various mobile, transfer devices. This will meet the growing consumer and operational requirements put forward for friction materials.