Antifriction materials: overview, properties, application

The process of operation of technical units, machines and individual element groups of equipment is inevitably accompanied by wear. The mutual mechanical action of the parts on each other with varying degrees of intensity leads to abrasion of their surfaces and destruction of the internal structure. In addition, such an environment is often affected by the environment in the form of erosion and cavitation. As a result, there is a loss in working capacity of the equipment or at least a decrease in performance. The following reviews of powder friction and antifriction materials will help you to understand the ways that minimize unwanted friction. Such materials are recommended for use for industrial equipment, and household appliances, as well as for construction tools.

Differences in friction and antifriction materials

Consideration of these materials in one context is due to the fact that their function is related to the overall performance of the mechanisms - the coefficient of friction. But if the antifriction elements and additives are responsible for lowering the given value, then the frictional elements, on the contrary, increase it. In this case, for example, powder alloys with an increased coefficient of friction provide resistance to wear and mechanical strength of the target working group. To achieve such qualities, refractory oxides, carbides of boron, silicon, etc. are introduced into the composition of the friction raw material. Unlike antifriction elements, friction elements often represent also full functional organs in the mechanisms. This, in particular, can be brakes and couplings.

Providing tasks of increasing friction, they simultaneously fulfill specific technical tasks. At the same time, both frictional and antifriction materials are thoroughly tested before use. The same alloys for brakes undergo full-scale and bench tests, during which the expediency of their application in practice is determined. The most technologically advanced friction materials from polymers are now manufactured by different methods. So, for the mechanisms of the brake group, the pressing technique is used - the molds produce pads, plates and sectors. Tape materials are produced by woven technique, and lining - by rolling.

Properties of antifriction materials

Parts with antifriction function must comply with a wide range of requirements that determine their basic performance. First of all, the material must be compatible with both the mating part and the working medium. In the conditions of compatibility before and after the run-in, the material provides the necessary degree of friction reduction. Here it should be noted that the workability as such. This property determines the ability of the element to naturally adjust the geometry of the surface to the optimum shape that is suitable for the particular site of operation. In other words, an extra structure with microroughnesses is erased from the part, after which the run-in will ensure the working conditions with minimal loads.

Wear resistance is also an important property that these materials possess. Antifriction elements should have a structure that provides resistance to different types of wear. At the same time, the part should not be excessively rigid and rigid, since in this case the risk of seizing increases, which is undesirable for antifriction material. Moreover, technologists distinguish such property, as absorption of solid particles. The fact is that friction to various degrees can contribute to the release of small elements - often metallic. In turn, the antifriction surface has the ability to "push" into such particles, eliminating them from the working area.

Metal antifriction materials

Products on a metal basis constitute the most extensive range of elements of the antifriction group. Most of them are oriented to operation in the mode of liquid friction, that is, in conditions when the bearings are separated from the shafts by a thin oil layer. And still, when the unit is stopped and started, the so-called boundary friction regime inevitably arises, in which the oil film can be destroyed under the influence of high temperatures. Metal parts used in bearing groups can be divided into two types: elements with a soft structure and solid inserts and alloys with a rigid base and soft inserts. If we talk about the first group, then as antifriction materials you can use babbitts, brass and bronze alloys. Due to their soft structure, they are quickly worked on and retain the characteristics of the oil film for a long time. On the other hand, solid state inclusions cause an increased wear resistance at mechanical contacts with adjacent elements - for example, with the same shaft.

By babbits means an alloy, the basis of which forms lead or tin. Also for the sake of improving individual qualities, alloying alloys can be added to the structure. Among the properties that can be raised are corrosion resistance, hardness, toughness and strength. The change in this or that characteristic is determined by the use of alloying materials. Antifrictional babbits can be modified by cadmium, nickel, copper, antimony, etc. For example, the standard babbit contains about 80% tin or lead, 10% antimony, and the rest is copper and cadmium.

Lead alloys as a means of minimizing friction

The initial level of antifriction alloys are lead babbits. Price accessibility determines the specifics of the operation of this material - in the least important operational functions. The lead base, in comparison with tin, provides babbitts with less mechanical resistance and lower corrosion protection. True, even in such alloys, tin can not be dispensed with - its content can reach 18%. In addition, a copper component is added to the composition, which prevents segregation processes-the uneven distribution of different metals in the volume of the product.

The simplest lead materials with an antifriction property are characterized by a high degree of brittleness, therefore they are used in conditions with reduced dynamic loads. In particular, bearings for track machines, diesel locomotives and heavy engineering elements constitute the target niche where such materials are used. Antifriction alloys with the use of calcium can be called a modification of lead alloys. In this case, such qualities as high density and low thermal conductivity are noted. The basis is also lead, but in significant proportions it is also supplemented with inclusions of sodium, calcium and antimony. As for the weak points of this material, it includes oxidizability, therefore in chemically active media it is not recommended to use it.

Speaking in general about babbits, it can be stated that this is far from being the most effective solution for minimizing friction, but in terms of the totality of the qualities it proves to be beneficial from the point of view of exploitation. These are materials whose antifriction properties can be leveled by reduced fatigue resistance, which worsen the performance of the element. Nevertheless, in some cases, the lack of strength is compensated by the inclusion of steel or cast-iron casings in the structure.

Features of bronze antifriction alloys

Physico-chemical properties of bronze are organically combined with the requirements for antifriction alloys. This metal, in particular, provides sufficient indicators of specific pressure, the ability to operate under shock loads, high speed of rotation of the bearing, etc. But also the choice of bronze for these or other functions will depend on its brand. The same format for the operation of the liners under impact loads is acceptable for the brand BrO30, but is not recommended for BRAJ. There are also differences in the class of bronze materials by mechanical properties. This group of qualities will depend on the nature of the interface with the hardened shafts and on the use of the trunnion, which may have additional hardening. Again, one can not speak of the monolithic structure of the alloy.

Bronze ware can also include tin, brass, lead. In this case, if all listed metals can be used as a basis for babbitt, antifriction materials based on copper are used extremely rarely. In this case, the copper component often acts as the same additive with a coefficient of 2-3%. Optimal are tin-lead combinations of inclusions. They provide sufficient parameters of the alloy as an antifriction component, although they lose to other compositions with respect to mechanical strength. Combined bronze materials are used in the manufacture of monolithic bearings for electric motors, turbines, compressor plants and other units that operate at high pressure and low sliding speed.

Powder friction materials

Such materials are used in the formulations intended for transmission and brake units of tracked vehicles, cars, machine tools, construction machinery, etc. Finished products based on powder components are manufactured in the form of sector linings, discs and pads. At the same time, the raw materials for the antifriction type of powder alloys are formed by the same nomenclature as in the case of friction components, most often iron and copper, but there are other combinations.

For example, materials of aluminum and tin bronzes, which include graphite and lead, are effective under friction conditions at a slip speed of parts of the order of 50 m / s. By the way, when bearings operate at a speed of 5 m / s, metal powder products can be replaced with metal plastic raw materials. This is already an antifriction composite with a flexible working structure and reduced strength. The most advantageous in terms of use in conditions of high loads are materials from iron and copper. As additives, graphite, silicon oxide or barium is used. The work of these elements is possible at a pressure of 300 MPa and a sliding speed of up to 60 m / s.

Powdered antifriction materials

From powdered raw materials are produced and antifriction products. They are characterized by high wear resistance, reduced coefficient of friction and the ability to quickly run to the shaft. Also, antifriction powder materials have a number of advantages over alloys that minimize friction. Suffice it to say that the indices of their wear resistance are on average higher than those of the same babbitts. The porous structure formed by powdered metals enables effective impregnation with lubricants.

Manufacturers have the ability to form final products in different forms. These can be frame or matrix parts with intermediate cavities filled with other softened raw materials. And, on the contrary, in some spheres antifriction powder materials with a soft-skinned framework are more in demand. In special honeycombs, solid inclusions of different levels of dispersion are provided. This quality is of great importance precisely from the point of view of the possibility of regulating the parameters that determine the intensity of the friction of the parts.

Antifriction polymeric materials

Modern polymer raw materials make it possible to obtain new technical and operational qualities for friction-reducing parts. As a basis, composite alloys and metal-plastic powders can be used. One of the main distinctive properties of such materials is the ability to evenly distribute the additives throughout the structure, which in the future will perform the function of solid lubrication. In the list of such substances, graphites, sulfides, plastics and other compounds are noted. The working properties of polymer and antifriction materials largely converge at a basic level without the use of modifiers: this is a small coefficient of friction, and resistance to chemically active media, and the possibility of operation in an aquatic environment. If we talk about unique qualities, then polymers can perform their tasks even without reinforcement with special grease.

Application of materials for protection against friction

Most of the antifriction elements are initially calculated for use in bearing groups. Among them are parts designed to increase wear resistance, and components that improve glide. In machine and machine tool building, such products are used in the manufacture of engines, pistons, hitch assemblies, turbines, etc. Here, the basis of the consumables is antifriction materials of sliding bearings that are introduced into the structure of running and stationary equipment.

The construction industry also does not do without antifriction function. With the help of such details, engineering structures, erection structures and masonry materials are strengthened. In the construction of railways they are used for the assembly of structural elements of rolling stock. It is also common to use antifriction materials on a polymer basis, which find their place, for example, as a connecting structure of pulleys, gears, belt gears, etc.

Conclusion

The problem of reducing friction only at first glance may seem secondary and often unnecessary. The improvement of lubricating fluids really allows to get rid of some mechanisms from auxiliary technical elements that reduce the wear and tear of the main working group. The transition from classic Babbit to modified high-performance lubricant can be called antifriction polymeric materials, characterized by a softer structure and versatility in terms of working conditions. Nevertheless, the work of metal parts at high pressure and physical impact still requires the inclusion of solid antifriction liners. And this class of materials is not just not a thing of the past, but also develops due to the improvement of the characteristics of strength, hardness and mechanical stability.