Mechanical devices are often used in human activities. The reliability of moving parts in any mechanism is ensured by reducing friction and deformation. For this, special materials called antifriction are used. Their main purpose is to reduce the coefficient of friction, facilitating the sliding of the moving surfaces of mechanisms. This article will discuss the anti-friction properties of various materials used for these purposes.
Types of friction
Friction occurs when bodies move that are in contact with each other. There are two main types:
- Dry - the surfaces of solids are in direct contact. It is observed in belt and friction drives.
- Liquid - when there is an oil layer of liquid between the parts of the mechanisms and the bodies do not touch. Found in thrust bearings, bearings.
And also distinguishintermediate types of friction: semi-dry and semi-liquid.
Regarding the movement of bodies, the following types of friction are noted:
- rest - occurs when the relative rest of the bodies;
- slip - manifests itself with the relative movement of mechanisms;
- rolling - external friction when rolling bodies.
Depending on the type of friction, a material with certain antifriction properties is selected for the surfaces of bodies.
Types of materials used to reduce friction
All anti-friction materials providing a low coefficient of friction are divided into:
- Metal - three-metal alloys are used, which contain copper (babbits). Designed to work in liquid friction mode.
- Powder - based on iron and copper with the addition of graphite and sulfides. Used in plain bearings.
- Self-lubricating sintered - materials of various combinations of iron with graphite, copper and bronze are used for manufacturing. They are used for the production of plain bearings at low speeds and in the absence of shock loads. Good anti-friction properties allow them to be installed in places where lubrication is difficult.
- With solid lubricating components - as a thin layer on the surface of parts, particles of a solid lubricant consisting of chlorides, metal oxides, fluorides, plastics are applied. Products operate at higher sliding speeds.
- Non-metallic - made fromplastics: thermoplastic and thermosetting. Used for propeller bearings, rolling mills.
- Metal-polymer - consist of heterogeneous components. They are divided into matrix, dispersed and layered. Used for the production of plain bearings, gears and sprockets.
- Minerals - use natural (agate) and artificial (corundum). They make small wear-resistant bearings for tachometers, clocks, gyroscopes.
Each material finds its application to produce parts according to its anti-friction properties.
Low friction alloys
From such alloys, friction bearing shells are made, so they must have:
- Low coefficient of friction compared to the shaft material, which is most often hardened steel.
- Good thermal conductivity.
- Corrosion resistance.
- Small hardness.
- Property that allows grease to be retained.
To meet the listed properties, the structure of the alloy should include metals with antifriction properties, which have increased softness and ductility of the base. And it is already interspersed with solid particles consisting of chemical compounds. In this case, the shaft is quickly run in to the bearing, small grooves appear on it from solid particles, which are filled with grease and through which wear products are removed. Based on tin, lead, copper, cadmium, bismuth,and inclusions are made from antimony and copper alloys.
Use of bronze alloys in friction units
Bronze is an alloy of copper with various metals, which may include tin, aluminum, silicon, lead, beryllium and many other additives. Depending on the percentage of one or another element included in its composition, bronze is called tin, aluminum, lead. Bronzes are widely used in the manufacture of products that are used with increased friction. The best bronzes are considered to be anti-friction properties, made on the basis of tin.
Tin-phosphorus have shown themselves especially well, from which bearing bearings are made, operating under significant load and at high speed. The only drawback is their high cost, so they are being replaced by aluminum and lead bronzes. When working in an aggressive environment, aluminum bronze is often used for bearing bushings. They, in addition to resistance to friction, have good corrosion resistance. Lead provides a low coefficient of friction. These materials are used to make bearing shells for high pressure and high speed operation.
Anti-friction alloys: composition and properties
In industry, for the manufacture of rubbing parts of mechanisms, various alloys are used with a small coefficient of friction:
- Brass is an alloy whose main components are copper and zinc. It may include components in the formaluminum, tin, lead, manganese and other elements. In terms of strength and low coefficient of friction, it is inferior to bronze and is used in the manufacture of plain bearings operating at low speeds.
- Babbit are complex alloys with different composition and physical properties, but consisting of one soft base: tin or lead with hard additions of alkali metal alloys, copper or antimony. Due to the soft base, the bearings are well run in to the shaft, and hard additives increase wear resistance. The high anti-friction qualities of babbitt, but lower strength than bronze and cast iron, makes it possible to use them only for applying a thin layer to the surface of products.
Properties of oils
In order to guarantee the reliability and efficiency of rubbing parts, reducing sliding friction, lubricating oils are used. They are all classified by:
- origin;
- method of receipt;
- assigned.
Lubricating oils perform the following functions:
- reduce friction between parts in contact;
- reduce wear and prevent scuffing;
- provide heat dissipation from rubbing parts;
- protect against corrosion.
The anti-friction properties of oils lie in their ability to reduce the amount of energy for friction. Viscosity is the main indicator of these properties and is determined by the carbon and fractional composition. To improve the quality of oils, variousantifriction additives to increase power, extend the operation of the unit, reduce loads. They enhance the properties of oils, increasing the timing of the replacement of the lubricant composition. Antifriction additives contribute to the creation of a protective layer during the interaction of parts, level their surfaces and smooth out friction. By creating an oily, durable film, they reduce wear on parts.
Anti-friction properties of epoxy polymers
Epoxy polymers are viscous liquids that harden when various organic substances are added to them. They have high mechanical strength and are used to bond concrete, metal, glass and wood. Due to these properties, they are used for the production of metal-polymer parts, producing bushings, rollers, gears, bearings and couplings.
Fillers give epoxy polymer products high anti-friction characteristics. Parts can run without lubrication if water wetting is used. Coatings are weather and chemical resistant.
Non-metallic anti-friction materials
For plain bearings, two types of plastics are often used:
- Thermosetting - these include textolite, which is used to manufacture bearings for rolling mills, propellers and hydraulic machines. Parts are heavy duty, water lubricated and cooled.
- Thermoplastic - polyamides are widely used: fluoroplastic, nylon, anid. Benefits are highanti-friction properties of materials, corrosion resistance and good wear resistance under high loads and sliding speeds.
To reduce friction between parts, various fillers are introduced in the form of solid lubricants, which, when used on the surface, create a structure of liquid crystals. It is worth noting that fluoroplast has a very low coefficient of friction, but poor heat dissipation and fluidity under load are considered a disadvantage, so it is used in conjunction with other materials.
Conclusion
Anti-friction materials are suitable for the manufacture of liners and bearings, which are easily replaced when worn. The raw material for the product must have a higher coefficient of friction, i.e., when the parts come into contact, the hard-to-replace part of the mechanism remains undamaged. This happens only when the material of the valuable part is endowed with excellent anti-friction properties over the analogue.
