The operation process of technical units, machines and individual element groups of equipment is inevitably accompanied by wear. The mutual mechanical effect of the parts on each other with varying degrees of intensity leads to abrasion of their surfaces and the destruction of the internal structure. Moreover, the environment in the form of erosion and cavitation often has a similar effect. As a result, there is a loss of operability of the equipment or at least a decrease in operational properties. The following reviews of powder friction and antifriction materials will help you figure out ways to minimize unwanted friction. Such materials are recommended for use both for industrial equipment and household appliances, as well as for building tools.
Differences between friction and anti-friction materials
Consideration of these materials in one context is due to the fact that their function is associated with a general characteristic of the operation of mechanisms - the coefficient of friction. But if anti-friction elements and additives are responsible for lowering this value, then friction - on the contrary, increase it. At the same time, for example, powder alloys with a high coefficient of friction provide wear resistance and mechanical strength of the target working group. To achieve such qualities, refractory oxides, boron carbides, silicon carbides, and others are introduced into the composition of friction raw materials. In contrast to antifriction elements, friction ones often also represent full-fledged functional organs in mechanisms. This, in particular, can be brakes and clutches.
Providing tasks to increase friction, they simultaneously perform specific technical tasks. At the same time, both friction and anti-friction materials undergo rigorous laboratory tests before use. The same brake alloys undergo full-scale and bench tests, during which the expediency of their application in practice is determined. The most technologically advanced friction materials made of polymers are today produced by different methods. So, for the mechanisms of the brake group, the pressing technique is used - pads, plates and sectors are made on the molds. Tape materials are produced by woven technique, and lining - by rolling.
Properties of antifriction materials
Parts with anti-friction function must meet a wide range of requirements that determine their basic working qualities. First of all, the material must be compatible with both the mating part and the working environment. In conditions of compatibility before and after running in, the material provides the necessary degree of friction reduction. Here it is necessary to note the running-in as such. This property determines the ability of the element to naturally correct the surface geometry to the optimal shape, which is suitable for a particular operation site. In other words, the excess structure with microroughness is erased from the part, after which the running-in will provide working conditions with minimal loads.
Wear resistance is also an important property that these materials possess. Anti-friction elements must have a structure that provides resistance to various types of wear. At the same time, the part should not be excessively rigid and hard, since in this case the risk of seizing will increase, which is undesirable for antifriction material. Moreover, technologists distinguish such a property as the absorption of solid particles. The fact is that friction to varying degrees can contribute to the release of small elements - often metal. In turn, the antifriction surface has the ability to “push” such particles into itself, eliminating them from the work area.
Metal anti-friction materials
Metal-based products make up the most extensive range of anti-friction group elements. Most of them are oriented towards operation in the liquid friction mode, that is, under conditions when the bearings are separated from the shafts by a thin oil layer. And yet, when the unit is stopped and started up, the so-called boundary friction regime inevitably arises, in which the oil film can be destroyed by high temperatures. The 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 babbits, brass and bronze alloys. Thanks to the soft structure, they quickly break in and for a long time retain the characteristics of an oil film. On the other hand, solid-state inclusions cause increased wear resistance during mechanical contacts with adjacent elements - for example, with the same shaft.
By babbits they mean an alloy, the basis of which is formed by lead or tin. Also, in order to improve individual qualities, alloying alloys can be added to the structure. Among the improved properties, corrosion resistance, hardness, toughness and strength can be noted. A change in one or another characteristic is determined by the alloying materials used. Antifriction babbits can be modified with cadmium, nickel, copper, antimony, etc. For example, 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 is represented by lead babbits. Affordability determines the specifics of the operation of this material - in the least responsible working functions. Compared to tin, the lead base provides babbits with less high mechanical resistance and low corrosion protection. True, even in such alloys it can not do without tin - its content can reach 18%. In addition, a copper component is also included in the composition, which prevents segregation processes - the uneven distribution of metals of different masses in the volume of the product.
The simplest lead materials with an antifriction property are characterized by a high degree of fragility, therefore, they are used in conditions with reduced dynamic loads. In particular, bearings for track machines, diesel locomotives and elements of heavy engineering constitute a target niche where such materials are used. Calcium-based antifriction alloys can be called a modification of lead alloys. In this case, such qualities as high density and low thermal conductivity are noted. Lead is also the basis, but in substantial proportions it is also supplemented by inclusions of sodium, calcium and antimony. As for the weaknesses of this material, oxidizability refers to them; therefore, it is not recommended to use it in chemically active media.
Speaking generally about babbits, it can be stated that this is far from the most effective solution to minimize friction, but in terms of the totality of qualities it turns out to be advantageous from the point of view of operation. These are materials whose antifriction properties can be leveled by reduced fatigue resistance, which worsens the performance of the element. However, in some cases, the lack of strength is compensated for by the inclusion of steel or cast iron bodies in the design.
Features of bronze anti-friction alloys
The physicochemical properties of bronze are organically combined with the requirements for antifriction alloys. This metal, in particular, provides sufficient specific pressure indicators, the possibility of operation under shock loads, a high speed of rotation of the bearing, etc. But also the choice of bronze for certain functions will depend on its brand. The same format for operating liners under shock loads is acceptable for the BrOS30 brand, but not recommended for BrAZh. There are also differences in the class of bronze materials in terms of 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. And again, one cannot talk about the monolithic structure of the alloy.
Bronze products may also include tin, brass, and lead. Moreover, if all of these metals can be used as the basis of babbitt, antifriction materials based on copper are used extremely rarely. In this case, the copper component more often acts as the same additive with a content ratio of 2-3%. Optimal tin-lead combinations of inclusions are considered. They provide sufficient indicators of the alloy as an antifriction component, although they lose to other compositions in terms of mechanical strength. Combined bronze materials are used in the manufacture of monolithic bearings for electric motors, turbines, compressor units and other units that operate at high pressure and low sliding speed.
Powder Friction Materials
Such materials are used in compositions intended for transmission and brake assemblies of caterpillar equipment, automobiles, machine tools, building mechanisms, etc. Finished products based on powder components are produced in the form of sector linings, discs and pads. At the same time, the starting materials for the antifriction type of powder alloys are formed by the same nomenclature as in the case with friction components — iron and copper are most often used, but other combinations exist.
For example, materials from aluminum and tin bronzes, which include graphite and lead, effectively manifest themselves under friction at a sliding speed of parts of about 50 m / s. By the way, when the bearings are operating at a speed of 5 m / s, metal powder products can be replaced with metal-plastic raw materials. This is an antifriction composite material with a flexible working structure and reduced strength. The most favorable in terms of use under high loads are considered materials from iron and copper. As additives, graphite, silicon oxide or barium is used. The operation of these elements is possible at a pressure of 300 MPa and a sliding speed of up to 60 m / s.
Powder anti-friction materials
Antifriction products are also made from powder raw materials. They are characterized by high wear resistance, reduced coefficient of friction and the ability to quickly run in to the shaft. Also, anti-friction powder materials have a number of advantages compared to friction-minimizing alloys. It is enough to note that their wear resistance is on average higher than that of the same babbits. The porous structure formed by powder metals makes it possible to effectively impregnate with lubricants.
Manufacturers have the ability to form final products in different forms. It can be frame or matrix parts with intermediate cavities filled with other softened raw materials. And, on the contrary, in some areas antifriction powder materials having a soft-body skeleton base are more in demand. In special cells, solid inclusions of different dispersion levels are provided. This quality is of great importance precisely from the point of view of the possibility of regulating the parameters that determine the friction intensity of parts.
Antifriction Polymer Materials
Modern polymer raw materials make it possible to obtain new technical and operational qualities for parts that reduce friction. Composite alloys and metal-plastic powders can be used as the basis. One of the main distinguishing properties of such materials is the ability to evenly distribute additives throughout the structure, which will subsequently serve as solid lubricants. Graphites, sulfides, plastics, and other compounds are noted in the list of such substances. The working properties of polymeric and antifriction materials are largely similar at the basic level without the use of modifiers: this is a low coefficient of friction, resistance to chemically active environments, and the possibility of operation in an aqueous environment. If we talk about unique qualities, then polymers can perform their tasks even without reinforcement with special grease.
The use of materials to protect against friction
Most anti-friction elements are initially designed for use in bearing groups. Among them are parts designed to increase wear resistance, and components that improve sliding. In machine and machine tools, such products are used in the manufacture of engines, pistons, couplers, turbines, etc. Here, the basis of consumables is made up of antifriction materials of sliding bearings, which are embedded in the structure of running and stationary equipment.
The construction industry also cannot do without an anti-friction function. With the help of such parts, engineering structures, installation structures and masonry materials are strengthened. In the construction of railways they are used in the installation of structural elements of rolling stock. The use of polymer-based antifriction materials, which find their place, for example, as a bonding structure for pulleys, gears, belt drives, etc., is also widespread.
Conclusion
The task of reducing friction, only at first glance, may seem secondary and often optional. Improvement of lubricating fluids really allows you to save some mechanisms from auxiliary technical elements that reduce wear of the main working group. The transition link from classic babbitt to a modified high-performance lubricant can be called antifriction polymer materials, which are characterized by a softer structure and versatility in terms of operating conditions. However, the operation of metal parts at high pressure and physical impact still requires the inclusion of solid-state anti-friction liners. Moreover, this class of materials not only does not become a thing of the past, but also develops by improving the characteristics of strength, hardness and mechanical stability.