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1、1 / 10 Friction , Lubrication of BearingIn many of the problem thus far , the student has been asked to disregard or neglect friction . Actually , friction is present to some degree whenever two parts are in contact and move on each other. The term friction refers to the resistance of two or more pa
2、rts to movement. Friction is harmful or valuable depending upon where it occurs. friction is necessary for fastening devices such as screws and rivets which depend upon friction to hold the fastener and the parts together. Belt drivers, brakes, and tires are additional applications where friction is
3、 necessary. The friction of moving parts in a machine is harmful because it reduces the mechanical advantage of the device. The heat produced by friction is lost energy because no work takes place. Also , greater power is required to overcome the increased friction. Heat is destructive in that it ca
4、uses expansion. Expansion maycause a bearing or sliding surface to fit tighter. If a great enough pressure builds up because made from low temperature materials may melt. There are three types of friction which must be overcome in moving parts: (1)starting, (2)sliding, and(3)rolling. Starting fricti
5、on is the friction between two solids that tend to resist movement. When two parts are at a state of rest, the surface irregularities of both parts tend to interlock and form a wedging action. To produce motion in these parts, the wedge-shaped peaks and valleys of the stationary surfaces must be mad
6、e to slide out and over each other. The rougher the two surfaces, the greater is starting friction resulting from their movement . Since there is usually no fixed pattern between the peaks and valleys of two mating parts, the irregularities do not interlock once the parts are in motion but slide ove
7、r each other. The friction of the two surfaces is known as sliding friction. As shown in figure ,starting friction is always greater than sliding friction . Rolling friction occurs when roller devces are subjected to tremendous stress which cause the parts to change shape or deform. Under these cond
8、itions, the material in front of a roller tends to pile up and forces the object to roll slightly uphill. This changing of shape , known as deformation, causes a movement of molecules. As a result ,heat is produced from the added energy required to keep the parts turning and overcome friction. The f
9、riction caused by the wedging action of surface irregularities can be overcome partly by the precision machining of the surfaces. However, even these smooth surfaces may require the use of a substance between them to reduce the friction still more. This substance is usually a lubricant which provide
10、s a fine, thin oil film. The film keeps the surfaces apart and prevents the cohesive forces of the surfaces from coming in close contact and producing heat . Another way to reduce friction is to use different materials for the bearing surfaces and rotating parts. This explains why bronze bearings, s
11、oft alloys, and copper and tin iolite bearings are used with both soft and 精选学习资料 - - - - - - - - - 名师归纳总结 - - - - - - -第 1 页，共 10 页2 / 10 hardened steel shaft. The iolite bearing is porous. Thus, when the bearing is dipped in oil, capillary action carries the oil through the spaces of the bearing.
12、This type of bearing carries its own lubricant to the points where the pressures are the greatest. Moving parts are lubricated to reduce friction, wear, and heat. The most commonly used lubricants are oils, greases, and graphite compounds. Each lubricant serves a different purpose. The conditions un
13、der which two moving surfaces are to work determine the type of lubricant to be used and the system selected for distributing the lubricant. On slow moving parts with a minimum of pressure, an oil groove is usually sufficient to distribute the required quantity of lubricant to the surfaces moving on
14、 each other . A second common method of lubrication is the splash system in which parts moving in a reservoir of lubricant pick up sufficient oil which is then distributed to all moving parts during each cycle. This system is used in the crankcase of lawn-mower engines to lubricate the crankshaft, c
15、onnecting rod ,and parts of the piston. A lubrication system commonly used in industrial plants is the pressure system. In this system, a pump on a machine carries the lubricant to all of the bearing surfaces at a constant rate and quantity. There are numerous other systems of lubrication and a cons
16、iderable number of lubricants available for any given set of operating conditions. Modern industry pays greater attention to the use of the proper lubricants than at previous time because of the increased speeds, pressures, and operating demands placed on equipment and devices. Although one of the m
17、ain purposes of lubrication is reduce friction, any substance-liquid , solid , or gaseous-capable of controlling friction and wear between sliding surfaces can be classed as a lubricant. Varieties of lubrication Unlubricated sliding. Metals that have been carefully treated to remove all foreign mate
18、rials seize and weld to one another when slid together. In the absence of such a high degree of cleanliness, adsorbed gases, water vapor ,oxides, and contaminants reduce frictio9n and the tendency to seize but usually result in severe wear 。this is called “unlubricated ”or dry sliding.Fluid-film lub
19、rication. Interposing a fluid film that completely separates the sliding surfaces results in fluid-film lubrication. The fluid may be introduced intentionally as the oil in the main bearing of an automobile, or unintentionally, as in the case of water between a smooth tuber tire and a wet pavement.
20、Although the fluid is usually a liquid such as oil, water, and a wide range of other materials, it may also be a gas. The gas most commonly employ ed is air. Boundary lubrication. A condition that lies between unlubricated sliding and fluid-film lubrication is 精选学习资料 - - - - - - - - - 名师归纳总结 - - - -
21、 - - -第 2 页，共 10 页3 / 10 referred to as boundary lubrication, also defined as that condition of lubrication in which the friction between surfaces is determined by the properties of the surfaces and properties of the lubricant other than viscosity. Boundary lubrication encompasses a significant port
22、ion of lubrication phenomena and commonly occurs during the starting and stopping off machines. Solid lubrication. Solid such as graphite and molybdenum disulfide are widely used when normal lubricants do not possess sufficient resistance to load or temperature extremes. But lubricants need not take
23、 only such familiar forms as fats, powders, and gases。 even some metals commonly serve as sliding surfaces in some sophisticated machines. Function of lubricants Although a lubricant primarily controls friction and ordinarily does perform numerous other functions, which vary with the application and
24、 usually are interrelated . Friction control. The amount and character of the lubricant made available to sliding surfaces have a profound effect upon the friction that is encountered. For example, disregarding such related factors as heat and wear but considering friction alone between the same sur
25、faces with on lubricant. Under fluid-film conditions, friction is encountered. In a great range of viscosities and thus can satisfy a broad spectrum of functional requirements. Under boundary lubrication conditions , the effect of viscosity on friction becomes less significant than the chemical natu
26、re of the lubricant. Wear control. wear occurs on lubricated surfaces by abrasion, corrosion ,and solid-to-solid contact wear by providing a film that increases the distance between the sliding surfaces ,thereby lessening the damage by abrasive contaminants and surface asperities. Temperature contro
27、l. Lubricants assist in controlling corrosion of the surfaces themselves is twofold. When machinery is idle, the lubricant acts as a preservative. When machinery is in use, the lubricant controls corrosion by coating lubricated parts with a protective film that may contain additives to neutralize co
28、rrosive materials. The ability of a lubricant to control corrosion is directly relatly to the thickness of the lubricant film remaining on the metal surfaces and the chermical composition of the lubricant. Other functions Lubrication are frequently used for purposes other than the reduction of frict
29、ion. Some of these applications are described below. Power transmission. Lubricants are widely employed as hydraulic fluids in fluid transmission devices. Insulation. In specialized applications such as transformers and switchgear , lubricants with high dielectric constants acts as electrical insula
30、tors. For maximum insulating properties, a lubricant must be kept 精选学习资料 - - - - - - - - - 名师归纳总结 - - - - - - -第 3 页，共 10 页4 / 10 free of contaminants and water. Shock dampening. Lubricants act as shock-dampening fluids in energy transferring devices such as shock absorbers and around machine parts
31、such as gears that are subjected to high intermittent loads. Sealing. Lubricating grease frequently performs the special function of forming a seal to retain lubricants or to exclude contaminants. The object of lubrication is to reduce friction ,wear , and heating of machine pars which move relative
32、 to each other. A lubricant is any substance which, when inserted between the moving surfaces, accomplishes these purposes. Most lubricants are liquids(such as mineral oil, silicone fluids, and water),but they may be solid for use in dry bearings, greases for use in rolling element bearing, or gases
33、(such as air) for use in gas bearings. The physical and chemical interaction between the lubricant and lubricating surfaces must be understood in order to provide the machine elements with satisfactory life. The understanding of boundary lubrication is normally attributed to hardy and doubleday , wh
34、o found the extrememly thin films adhering to surfaces were often sufficient to assist relative sliding. They concluded that under such circumstances the chemical composition of fluid is important, and they introduced the term “boundary lubrication”. Boundary lubrication is at the opposite end of th
35、e spectrum from hydrodynamic lubrication. Five distinct of forms of lubrication that may be defined :(a) hydrodynamic。 (b)hydrostatic。(c)elastohydrodynamic (d)boundary。 (e)solid film. Hydrodynamic lubrication means that the load-carrying surfaces of the bearing are separated by a relatively thick fi
36、lm of lubricant, so as to prevent metal contact, and that the stability thus obtained can be explained by the laws of the lubricant under pressure ,though it may be。 but it does require the existence of an adequate supply at all times. The film pressure is created by the moving surfaces itself pulli
37、ng the lubricant under pressure, though it maybe. The film pressure is created by the moving surface to creat the pressure necessary to separate the surfaces against the load on the bearing . hydrodynamic lubrication is also called full film ,or fluid lubrication . Hydrostatic lubrication is obtaine
38、d by introducing the lubricant ,which is sometime air or water ,into the load-bearing area at a pressure high enough to separate the surface with a relatively thick film of lubricant. So ,unlike hydrodynanmic lubrication, motion of one surface relative to another is not required . Elasohydrodynamic
39、lubrication is the phenomenon that occurs when a lubricant is introduced between surfaces which are in rolling contact, such as mating gears or rolling bearings. The mathematical explanation requires the hertzian theory of contact stress and fluid mechanics. 精选学习资料 - - - - - - - - - 名师归纳总结 - - - - -
40、 - -第 4 页，共 10 页5 / 10 When bearing must be operated at exetreme temperatures, a solid film lubricant such as graphite or molybdenum disulfide must be use used because the ordinary mineral oils are not satisfactory. Must research is currently being carried out in an effort, too, to find composite be
41、aring materials with low wear rates as well as small frictional coefficients. In a journal bearing, a shaft rotates or oscillates within the bearing , and the relative motion is sliding . in an antifriction bearing, the main relative motion is rolling . a follower may either roll or slide on the cam
42、. Gear teeth mate with each other by a combination of rolling and sliding . pistions slide within their cylinders. All these applications require lubrication to reduce friction ,wear, and heating. The field of application for journal bearing s is immense. The crankshaft and connecting rod bearings o
43、f an automotive engine must poerate for thousands of miles at high temperatures and under varying load conditions . the journal bearings used in the steam turbines of power generating station is said to have reliabilities approaching 100 percent. At the other extreme there are thousands of applicati
44、ons in which the loads are light and the service relatively unimportant. a simple ,easily installed bearing is required ,suing little or no lubrication. In such cases an antifriction bearing might be a poor answer because because of the cost, the close ,the radial space required ,or the increased in
45、ertial effects. Recent metallurgy developments in bearing materials , combined with increased knowledge of the lubrication process, now make it possible to design journal bearings with satisfactory lives and very good reliabilities. 参考文献：1. Chambers T. L., Parkinson A. R., 1998, “Knowledge Represent
46、ation and Conversion of HybridExpert Systems. ” Transactions of the ASME, v 120,pp 468-474 2. Koelsch, James R., 1999, “Software boosts mold design efficiency“ Molding Systems,v57, n 3,p 16-23. 3. Lee, Rong-Shean, Chen, Yuh-Min, Lee, Chang-Zou,1997 “Development of a concurrent molddesign system: A k
47、nowledge-based approach ”, Computer Integrated Manufacturing Systems, v 10,n 4, p 287-307 4. Steadman Sally, Pell Kynric M, 1995, “ Expert systems in engineering design: An application forinjection molding of plastic parts“ Journal of Intelligent Manufacturing, v6, p 347-353. 5. Fernandez A., Castan
48、y J., Serraller F., Javierre C., 1997, “CAD/CAE assistant for the design ofmolds and prototypes for injection of thermoplastics “Information Technological, v 8, p 117 -124. 6. Douglas M Bryce, 1997, “Plastic injection molding -Material selection and product design”, v 2,pp 精选学习资料 - - - - - - - - - 名
49、师归纳总结 - - - - - - -第 5 页，共 10 页6 / 10 1-48. 7. Douglas M Bryce, 1997, “Plastic injection molding-Mold design fundamentals”, v2, pp 1-120 轴承的摩擦与润滑现在看来，有很多这种情况，许多学生在被问到关于摩擦的问题时，往往都没引起足够的重视，甚至是忽视它。实际上，摩擦从某种程度上说，存在于任何两个相接触并有相对运动趋势的部件之间。而摩擦这个词，本身就意味着，两个或两个以上部件的阻止相对运动趋势。在一个机器中，运动部件的摩擦是有害的，因为它降低了机械对能量的充分利用
50、。由它引起的热能是一种浪费的能量。因为不能用它做任何事情。还有，它还需要更大的动力来克服这种不断增大的摩擦。热能是有破坏性的。因为它产生了膨胀。而膨胀可以使得轴承或滑动表面之间的配合更紧密。如果因为膨胀导致了一个足够大的积压力，那么，这个轴承就可能会卡死或密封死。另外，随着温度的升高，如果不是耐高温材料制造的轴承，就可能会损坏甚至融化。在运动部件之间会发生很多摩擦，如1.启动摩擦2.滑动摩擦3.转动摩擦。启动摩擦是两个固体之间产生的倾向于组织其相对运动趋势的摩擦。当两个固体处于静止状态时，这两个零件表面的不平度倾向于相互嵌入，形成楔入作用，为了使这些部件“动”起来。这些静止部件的凹谷和尖峰必须