轉(zhuǎn)塔式數(shù)控加工中心設(shè)計(jì)論文,塔式,數(shù)控加工中心,設(shè)計(jì),論文 河南科技大學(xué)畢業(yè)設(shè)計(jì)論文 SLEEVE BEARINGS BEARING MATERIALS The antifriction properties of rubbing pairs are considered in conjunction with the materials of the shaft and bearing, and the lubricant. Bearing materials are chosen for application in a pair with a steel or, much less frequently, cast iron shaft journal. Owing to the fact that the cost of shafts is, as a rule, higher than that of the bearing liners (especially in the case of crankshafts and other main shafts), their wear must be less than that of the liners. The higher the hardness of the shaft journals, the more reliable their performance. As a rule, journals are hardened. Journals running in high-speed bearings are hardened to a high hardness, 55 to 60 Rc (following carburizing). Integrated requirements, complying with the basic criteria of bearing performance, can be made to bearing materials. Thus, they must: (a) be antifrictional, a property characterized by the coefficient of friction in operation in a pair with the material of the shaft journal, and by the temperature at the working surface; (b) have high wear resistance; and (c) have high fatigue strength. Of importance in meeting these integrated requirements are the following basic properties of bearing materials: (a) thermal conductivity, to provide intensive heat disposal from the friction surfaces, and a low coefficient of linear expansion to avoid large changes in the clearances in the bearings during operation; (b) capacity for being run in readily, which reduces edge and local pressures resulting from elastic deformation and errors in manufacture; (c) good wettability with oil and the capacity for forming stable and rapidly restorable oil films on its surfaces; (d) good corrosion resistance; (e) low modulus of elasticity. Also of basic importance are processing properties, such as castability, machinability, etc. The most favorable structure for an antifriction material is one having a plastic matrix into which hard components are embedded. With respect to their chemical composition, antifriction bearing materials can be classified into the following large groups: (a) metallic materials: babbitts, bronzes, zinc-base alloys, aluminium-base alloys and antifriction cast irons; (b) sintered metal powders; (c) nonmetallic materials: plastics, wood laminates, rubbers, etc. Bearing Metals. Babbitts are high-quality bearing alloys that have been used for many years in engineering. They are based on tin or lead and are characterized by their low hardness (they are used for casting into a support or housing), good running-in properties, and relatively lower requirements as to the hardness of the shaft journals and to the state of the rubbing surfaces. High-tin babbitts, grades B83, B89, etc. are to be applied for high running speeds and high pressures. They allow operation at pressures up to p=200 kgf/cm2 and pv= 1000 kgf-m/cm2-s. To avoid burning-out, babbitts are used for temperatures up to 110°C only. Typical applications are in the bearings of steam turbines, and of powerful electric generators and motors. The use of high-tin babbitts leads to minimum wear of the shaft journals. Shortcomings of high-tin babbitts are their relatively low fatigue strength, which restricts their application in impact-type machinery and high-speed piston machines, and the impossibility of applying them in thin layers because of their large crystals. Soviet automobiles use bearing liners with a layer of babbitt, grade COC6-6 (88% lead, 6% tin and 6% antimony), and a cermet sublayer, sintered of a powder with 40% Ni and 60% Cu, on a steel backing. There is excellent cohesion between the layers because the sublayer is impregnated with the babbitt which forms, with the sintered metal, a substantially increased cohesion surface. On the other side, the sublayer diffuses into the steel backing. This grade of babbitt has an increased fatigue strength and, owing to the absence of hard components, its use leads to less shaft wear. Another advantage is that it lends itself to high-production manufacturing techniques (pressforming from strip stock). A calcium babbitt, grade BR2, is used in diesel engine manufacture. Among those long in use in the engineering industries are the tin-lead babbitts, grades B16 and BH, which have properties close to those of the high-tin babbitts (p<=150 kgf/cm2 and pv <=500kgf-m/cm2-s). Calcium babbitt, grade BK, finds application for medium duty. It contains no tin and is used, for instance, for babbitting the bearings of railway rolling stock. A less expensive babbitt, grade BC, is used for light duty applications. Bronzes. At high speeds and pressures (up to p=300 kgf/cm2) and, in particular, for variable loads typical of internal-combustion engines, a leaded bronze, grade Bp.C-30, is used. It has a higher fatigue strength than the high-tin babbitts. Leaded bronze makes higher requirements than the babbitts do to the hardness of the shaft journals (hardening is compulsory),to the surface finish of the journals and liners, and also to the lubricants since oxidized oils cause corrosion. Shaft wear is higher than that with babbitt liners. If the lead content is increased to 35%,the wear can be reduced. Leaded bronze is applied to strip stock from which the liners are pressformed or it is cast into the liner. Owing to the danger of corrosion, the use of leaded bronze has been curtailed in recent years. The working surface of the liners of critical bearings is coated with a thin running-in layer of an alloy of lead with tin, or of indium or tin. Universal tin bronzes of the Bp. OΦ10-1 type provide for efficient performance at high pressures and medium speeds. However, owing to the high tin content, their application is limited. This bronze is not to be recommended for use in bimetallic bearing liners. Secondary tin-zinc-lead bronzes (i.e. ones obtained by remelting), for instance, grade Bp. OЦC6-6-3, have found extensive application and have proved quite satisfactory for medium duty. Aluminium-iron bronzes are used to some extent for bearings subject to considerable pressures at low speeds and operating with a hardened shaft journal. In recent years, aluminium-base bearing alloys have come into use because they are more economical due to the low cost of the basic material. These alloys have low density, low corrosion resistance, high thermal conductivity, low modulus of elasticity and a high fatigue strength. The use of these alloys increased considerably after a technique was developed for applying them to a steel backing by a rolling process. Tinless aluminium bearing alloys have sufficiently high antifriction properties, but at high speeds their resistance to scoring is not very high; they are sensitive to dirt in the oil and have a high coefficient of linear expansion. The most widely used of these alloys in the USSR is grade ACM which is found in the bearings of tractor engines. No favorable results were obtained, however, when attempts were made to apply them in the bearings of automobile engines which run at higher speeds. The most promising bearing materials are the aluminium-tin antifriction alloys which have high antifriction properties and high fatigue strength. In use are alloys grade AO9-2 (9%tin, 2%copper, cast in and used as a monometal), AO9-2B (cast in and used as a bimetal), AO9-1(made by rolling and used as a bimetal). and AO20-1 (made by rolling and used as a bimetal).These alloys have optimal structure and are capable, in cases of oil starvation, of forming a protective film of tin on the journal. Alloys AO9-1 and AO9-2, for example, are being efficiently employed in the bearings of internal-combustion engines of diesel locomotives, ships and heavy-duty tractors. Of the zinc bearing alloys, the best known is grade ЦAM10-5(10% aluminium,5% copper and the remainder zinc),Owing to its satisfactory antifriction properties, abundancy of its constituent materials, low cost and simple manufacture, this alloy is widely used in place of type Б16 babbitts and bronzes. Shortcomings of this alloy are its poor running-in properties, and the consequently higher requirements made to the accuracy of the bearing surfaces, and the high coefficient of linear expansion. The highest permissible temperature of such bearings is 80°C. The alloy is used either for cast-in or for pressfitted liners. A new bearing alloy is grade ЦAM9-1.5;a technique has been developed for making bimetallic strip stock. Tests show that it has high wear resistance. Polymetallic multiple-layer bearings are being employed to an ever-increasing extent. Bearings are used, for example, in automobile engines that consist of a steel backing, a 0.25-mm layer of leaded bronze, serving as a compliant cushion with good thermal conductivity and fatigue strength, an extremely thin layer of nickel or a copper-zinc alloy which prevents diffusion of the tin, and, finally, a surface antifriction layer of a tin-lead alloy, 25 microns thick, which has excellent running-in properties. Antifriction cast iron (USSR Std GOST 1585-70) can be applied for low-speed moderately loaded bearings. The hardness of the shaft journals must be higher than that of the cast iron liners. The working surfaces should be carefully run in using a colloidal mixture of graphite in oil. The permissible pressures in these bearings drop sharply with an increase in running speed . Cast iron liners are especially sensitive to edge pressures, impact loads, poor lubricant, etc. Sulphocyaniding has proved to be an effective measure for improving their performance. Sintered Metal Powders (cermets). These materials, made of metal powders by compression moulding at high pressure and subsequent sintering at high temperature, are used in connection with their satisfactory operation with poor lubrication. These materials have a porous structure in which the pores occupy from 15 to 40% of the volume. In the finished bearing these pores are filled with oil (by special impregnation of the liners with hot oil). The most widely employed are the iron-graphite liners consisting of 1 to 3% graphite and the remainder, iron. Also used are bronzegraphite liners with 10% tin,1 to 4% graphite and the remainder, copper, but their properties differ only slightly from the much less expensive iron-graphite liners. Their primary field of application is dictated by their property of self-lubrication: these are mainly bearings that are difficult or impossible to lubricate reliably by ordinary means. At low pressures and speeds, sintered bearings can operate for long periods of time with their only lubrication being from the oil in the pores of the liners. Permissible running conditions for iron-graphite bearings having an average porosity of 20 to 25% and subject to a steady load are: v, m / s....... 0.5 1 1.5 2 2.5 3 3.5 4 p, kgf / cm2... 70 65 60 55 45 35 18 8 Nonmetallic Materials. Nometallic materials employed for the liners of bearings are: (a) plastics, (b) compressed wood (laminated wood), (c) various hardwoods (lignum vitae, boxwood, oak, etc.), (d) rubber, and (e) graphite materials. An essential feature of most nonmetallic bearing materials, due to their low thermal conductivity ,is that the best lubricant for them is water which ensures proper cooling. Oil or an emulsion is required only at low speeds and high pressures. When water is used as the lubricant, corrosion of the shaft in the bearings can be avoided by introducing a plastic lubricant (for instance, grease) into the bearing before stopping the machine, or by coating the shaft with stainless steel. Reasons for applying nonmetallic bearing materials are: (a) no chemical affinity with the shaft material, (b) good running-in properties, (c)products of wear are soft, and (d) the feasibility of lubricating with water or any other fluid which may be the working medium in the machine. The main fields of application of plastic bearing liners are: (1) bearings for which it is impossible to use a fluid lubricant and for which complete or partial self-lubrication is required(automobile suspensions, and the bearings of certain chemical and textile machines); (2) bearings lubricated by the working medium (submersible pumps and certain food-processing machines); (3) bearings of heavy low-speed machinery in which fluid friction running conditions are not always attainable. This may be due to frequent starts and stops, low speeds, or high local pressures resulting from elastic deformation or manufacturing errors. - VI -