Modern production equipment has a rather complex structure. Friction mechanisms transmit motion using friction. It can be couplings, clamps, openings and brakes.
In order for the equipment to be durable, work without downtime, 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. The reliability and durability of the equipment depend on their quality. In some cases, the safety and life of people depends on these elements of the system.
general characteristics
Friction materials are integral elements of assemblies and mechanisms that have the ability to absorb mechanical energy and disperse it into the environment. Moreover, all structural elements should not wear out quickly. For this, the materials presented have certain properties.
The friction coefficient of friction materials must be stable and high. Wear resistance is also required to meet operational requirements. Such materials have good heat resistance and are not subject to mechanical stress.
So that a substance that performs frictional functions does not stick to working surfaces, it is endowed with sufficient adhesive qualities. The combination of these 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 service qualities. They determine the operational 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 ultimate value is taken into account in the process of choosing a 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 tension. Also included are heat capacity, thermal conductivity, and linear expansion of the material.
The indicators determined in dynamic conditions include thermal stability and heat resistance. In the experimental environment, the coefficient of friction, wear resistance and stability are established.
Types of materials
Friction materials of the brake and clutch systems are most often made on a copper or iron basis. The second group of substances is used under conditions of increased load, especially with dry friction. Copper materials are used for medium and light loads. Moreover, they are suitable both for dry friction, and with the use of lubricating fluids.
In modern production conditions, rubber and resin-based materials are widely used. Various fillers of metallic and non-metallic components may also be used.
Application area
There is a classification of friction materials depending on their area of application. The first large group includes transmission devices. These are medium and lightly loaded mechanisms that operate without lubrication.
Further, friction materials of the brake system are designed for medium and heavily loaded mechanisms. These units do not use grease.
The third group includes substances used in the coupling of medium and heavily loaded nodes. They have oil in them.
Brake materials in which liquid lubricant is present are also distinguished as a separate group. 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 nodes.
Metal materials
As 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 pads, the composition of which contains cast iron, are often used in railway systems. It does not warp, but dramatically loses its sliding qualities at temperatures from 400 ° C.
Non-metallic materials
Clutch or brake friction materials are also made from non-metallic materials. They are created mainly on an asbestos basis (resin, rubber act as binding components).
The friction coefficient remains high enough to a temperature of 220 ° C. If the binder is resin, the material is characterized by high wear resistance. But their coefficient of friction is slightly 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 for knots and brake mechanisms with severe operational conditions. It retains its qualities even when heated to 1000 ° C. Therefore, retinax is applicable even in aircraft brake systems.
Asbestos materials are made by creating the fabric of the same name. It is impregnated with asphalt, rubber or bakelite and pressed at high temperatures. Short asbestos fibers can also form non-woven linings. They add small metal shavings. Sometimes, to enhance strength, brass wire is introduced into them.
Sintered materials
There is another variation of the presented system components. These are sintered friction materials of the brake system. That this variety will become clearer from the method of their manufacture. They are most often made on a steel basis. In the process of welding, other components are sintered with it. Pre-pressed preforms consisting of powder mixtures are subjected to high temperature heating.
Such materials are most often used in heavily loaded clutches and brake 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 wear resistance, and the second - stability and a sufficient level of adhesion.
Steel based materials for dry friction
The choice of material for various systems is based on the economic and technical feasibility of its manufacture and operation. Decades ago, iron-based materials such as FMK-8, MKV-50A, and also SMK were in demand. Friction materials for brake pads that worked in heavily loaded systems were later made from FMK-11.
MKV-50A is a newer development. It is used in the manufacture of linings for disc brakes. It has an advantage over the PMK group in terms of stability and wear resistance.
In modern production, materials like SMK are more widely used. They have increased manganese content. Also included are boron carbide and nitride, molybdenum disulfide and silicon carbide.
Bronze-based materials for dry friction
In transmission and brake systems for various purposes, materials based on tin bronze have proven themselves well. They wear out less mating parts from cast iron or steel than friction materials based on iron.
The presented variety of materials is used even in the aviation industry. For special operating conditions, tin can be replaced by substances such as titanium, silicon, vanadium, arsenic. This prevents the formation of intergranular 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. Included in the composition of the alloy 5-10% tin provide increased strength. Lead and graphite play the role of a solid lubricant, while silicon dioxide or silicon increase the coefficient of friction.
Oil lubricated operation
The materials used in dry systems have a significant drawback. They are subject to rapid wear. If grease from nearby nodes gets into them, their effectiveness sharply decreases. Therefore, in recent years, materials designed to work in liquid oil have become more widespread.
Such equipment starts up smoothly and is characterized by a high level of wear resistance. It is easy to cool and easy to seal.
In foreign practice, production volumes of a product such as sheet friction material for brakes, couplings, and other asbestos-based mechanisms have recently been growing. It is impregnated with resin. The composition includes molded elements with a high content of metal fillers.
Most often, sintered materials based on copper are used for the lubricating medium. To increase frictional characteristics, non-metallic solid components are introduced into the composition.
Property Improvement
Above all, the wear resistance required by friction materials requires improvement. The economic and operational feasibility of the presented components depends on this. In this case, technologists are developing ways to eliminate excessive heat on rubbing surfaces. To do this, improve the properties of the friction material itself, the design of the device, and also regulate the working conditions.
If materials are used in dry friction conditions, special attention is paid to their heat resistance and oxidation stability. Such substances are less susceptible to abrasive wear. But for lubricated systems, 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 oxidation state. The smaller it is, the more durable the components of the mechanisms. Another direction is to reduce the porosity of the material.
Modern production should improve the applied additional materials in the manufacturing process of various mobile, transmission devices. This will meet the growing consumer and operational requirements for friction materials.