轴承的摩擦与润滑 机械类毕业设计外文翻译.doc

轴承的摩擦与润滑 -机械类毕业设计外文翻译 毕业设计论文外文翻译毕业设计论文题目 工具盒盖注塑模具设计 外文题目 Friction Lubrication of Bearing 译文题目 轴承的摩擦与润滑 系 部 机 械 系 专业班级机械设计制造及其自动化学生姓名 指导教师 外文文献原文 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 parts to movementFriction 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 necessaryThe 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 causes expansion Expansion may cause a bearing or sliding surface to fit tighter If a great enough pressure builds up because made from low temperature materials may meltThere are three types of friction which must be overcome in moving parts 1 starting 2 sliding and 3 rolling Starting friction 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 made 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 over 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 conditions 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 frictionThe friction 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 provides 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 soft alloys and copper and tin iolite bearings are used with both soft and 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 This type of bearing carries its own lubricant to the points where the pressures are the greatestMoving 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 under which two moving surfaces are to work determine the type of lubricant to be used and the system selected for distributing the lubricantOn 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 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 connecting rod and parts of the pistonA 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 quantityThere are numerous other systems of lubrication and a considerable 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 devicesAlthough one of the main 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 lubricationUnlubricated sliding Metals that have been carefully treated to remove all foreign materials 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 slidingFluid-film lubrication 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 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 employed is airBoundary lubrication A condition that lies between unlubricated sliding and fluid-film lubrication is 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 portion of lubrication phenomena and commonly occurs during the starting and stopping off machinesSolid 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 only such familiar forms as fats powders and gases even some metals commonly serve as sliding surfaces in some sophisticated machines Function of lubricantsAlthough a lubricant primarily controls friction and ordinarily does perform numerous other functions which vary with the application and 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 surfaces 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 nature of the lubricantWear 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 asperitiesTemperature control 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 corrosive 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 functionsLubrication are frequently used for purposes other than the reduction of friction Some of these applications are described belowPower transmission Lubricants are widely employed as hydraulic fluids in fluid transmission devicesInsulation In specialized applications such as transformers and switchgear lubricants with high dielectric constants acts as electrical insulators For imum insulating properties a lubricant must be kept free of contaminants and waterShock dampening Lubricants act as shock-dampening fluids in energy transferring devices such as shock absorbers and around machine parts such as gears that are subjected to high intermittent loadsSealing Lubricating grease frequently performs the special function of forming a seal to retain lubricants or to exclude contaminantsThe object of lubrication is to reduce friction wear and heating of machine pars which move relative 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 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 lifeThe understanding of boundary lubrication is normally attributed to hardy and doubleday who 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 the spectrum from hydrodynamic lubricationFive distinct of forms of lubrication that may be defined a hydrodynamic b hydrostatic c elastohydrodynamic d boundary e solid filmHydrodynamic lubrication means that the load-carrying surfaces of the bearing are separated by a relatively thick film 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 pulling 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 obtained 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 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 mechanicsWhen 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 bearing materials with low wear rates as well as small frictional coefficientsIn 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 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 heatingThe field of application for journal bearing s is immense The crankshaft and connecting rod bearings of 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 applications 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 inertial 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 揔nowledge Representation and Conversion of HybridExpert SystemsTransactions of the ASME v 120pp 468-4742 Koelsch James R 1999 揝oftware boosts mold design efficiencyMolding Systemsv57 n 3p 16-233 Lee Rong -Shean Chen Yuh-Min Lee Chang-Zou1997 揇evelopment of a concurrent molddesign system A knowledge-based approach Computer Integrated Manufacturing Systems v 10n 4 p 287-3074 Steadman Sally Pell Kynric M 1995 Expert systems in engineering d esign An application forinjection molding of plastic partsJournal of Intelligent Manufacturing v6 p 347-3535 Fernandez A Castany J Serraller F Javierre C 1997 揅ADCAE assistant for the design ofmolds and prototypes for injection of thermo plastics 揑nformation Technological v 8 p 117-1246 Douglas M Bryce 1997 扬lastic injection molding -Material selection and product design v 2pp 1-487 Douglas M Bryce 1997 扬lastic injection molding-Mold design fundamentals v2 pp 1-120 中文译文 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整理光滑而且能相互抵消这两个表面之间越不光滑由运动造成的启动摩擦最大静摩擦力就会越大因为通常来说在两个相互配合的部件之间其表面不平度没有固定的图形一旦运动部件运动起来便有了规律可循滑动就可以实现这一点两个运动部件之间的摩擦就叫做滑动摩擦启动摩擦通常都稍大于滑动摩擦转动摩擦一般发生在转动部件和设备上这些设备抵触极大的外作用力当然这种