The reliability and durability of many structural elements and technological machines largely depends on the performance characteristics of friction units. Plain bearings, thrust bearings, seals and other similar units are currently made of various materials that differ in their mechanical and physicochemical characteristics, cost and are selected based on the operating conditions. Along with traditional materials that have already won their niche in the manufacture of Composite Bearings (babbitts, bronzes, zinc-aluminum alloys, cast iron, steel, diamond, sometimes cadmium or silver, etc.), leading global and domestic manufacturers of bearings use a variety of composite and powder materials. Self-lubricating materials allow you to abandon the expensive oil supply system and are especially in demand there, where the supply of lubricant is difficult or contraindicated (for example, in the food industry). Plain bearings made of such materials are also advantageous for use in aggressive corrosive environments or at temperatures exceeding the operating temperatures for existing types of greases.
Obviously, for the same food industry, composite bearing materials are quite applicable, consisting of a supporting base of fiberglass impregnated with epoxy resin. Such a reinforcing base contains lubricating elements made of Teflon or polyester.
How does bearing material affect performance?
One of the materials of this class is Duralon. Composite bearing material made from this are said to withstand up to 25 million cycles at 4000 psi (27.58 MPa) or 1 million cycles at 20,000 psi (137.0 MPa). Duralon is used in pneumatic cylinders of industrial plants, winches and pulleys working on marine vessels during loading and unloading, belt conveyors, etc. Working temperatures of products vary from cryogenic to 160 ° C.
Similar materials are offered by other manufacturers and may differ in design features. For example, Teflon (polytetrafluoroethylene) is a rather soft, plastic material that is reinforced not only with epoxy resin fabrics, but also with metal or ceramic powders. This significantly increases its strength, which is also facilitated by the use of a stainless steel or bronze substrate.
Along with PTFE, graphite is widely used to create a lubricating layer in self-lubricating bearings. Naturally, the physical and mechanical properties of graphite do not allow it to be used in the form of monolithic parts, therefore, in most cases, the flakes of this material are bonded with heat-shrinkable resins or carbon. There is also a class of materials that are graphite impregnated metals. The use of metals as a support base is associated not only with an increase in the mechanical properties of the product, but also with the temperature component. It is these materials (Dryslide, Graphalloy) that effectively remove heat from the friction unit.
One of these materials is Graphalloy, which can be of various bases: babbitts, silver, bronze, copper and iron. At the same time, the range of working loads ranges from 5000 to 6000 psi (34-41 MPa), and the maximum loads reach 130-170 MPa.
Also a promising class of self-lubricating materials for plain bearings is molybdenum disulfide, which can be bonded with metals or epoxy resin. Quite often, Mo2S crystals are used in plastic bearings. In this case, the bearing support can be nylon, acetal or polyamide. In the operation of bearings made of these materials, it is very important to cool the friction unit, which can be done by dipping into water or other liquid or gas (ammonia, propane, etc.). Such cooling contributes to the formation of a hydrodynamic film in the friction unit, which reduces shock loads and reduces the negative effect of transient processes that occur when reaching operating speeds of rotation.
In addition to self-lubricating composite bearings, plain bearings created by powder metallurgy methods are widely used. The presence of porosity in the structure of such products makes it possible to pre-impregnate the bearings with various greases. During the operation of the bearing, the oil trapped in the pores under the influence of temperature begins to escape outward and forms a lubricating whip in the friction unit. After stopping the mechanism, the oil is absorbed back into the pores and held inside until the next working cycle. Sintering methods allow varying the porosity value up to 35%, which contributes to the preservation of the self-lubricating effect for 3000-5000 hours. The wear resistance of sliding bearings manufactured by powder metallurgy methods is several times higher than that of traditional metal products.