机械类毕业设计外文文献翻译.doc

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 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 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 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 melt.There 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 friction.The 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 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 under 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 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 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 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 devices.Although 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 sliding.Fluid-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 air.Boundary 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 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 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 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 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 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 insulators. For maximum insulating properties, a lubricant must be kept 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 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 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 life.The 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 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 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 mechanics.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 bearing 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. 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 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. 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，当然这种外力会导致部件的变形和性能的改变。在这种情况下，转动件的材料趋向于堆积并且强迫运动部件缓慢运动，这种改变就是通常所说的形变。可以使分子运动。当然，最终的结果是，这种额外的能量产生了热能，这是必需的。因为它可以保证运动部件的运动和克服摩擦力。由运动部件的表面不平度的楔入作用引起的摩擦可以被部分的克服，那就需要靠两表面之间的润滑。但是，即使是非常光滑的两个表面之间也可能需要一种物质，这种物质就是通常所说的润滑剂，它可以提供一个比较好的、比较薄的油膜。这个油膜使两个表面分离，并且组织运动部件的两个表面的相互潜入，以免产生热量使两表面膨胀，又引起更近的接触。减小摩擦的另一种方式是用不同的材料制造轴承和转动零件。可以拿黄铜轴承、铝合金和含油轴承合金做例子进行解释。也就是说用软的或硬的金属组成表面。含油轴承合金是软的。这样，当轴承在油中浸泡过以后，因为毛细管的作用，将由带到轴承的各个表面。这种类型的轴承把它的润滑剂带到应力最大的部位。对运动部件润滑以减小摩擦，应力和热量，最常用的是油、脂、还有合成剂。每一种润滑剂都有其各自不同的功能和用途。两个运动部件之间的运动情况决定了润滑剂的类型的选择。润滑剂的分布也决定了系统的选择。在低速度运动的部件，一个油沟足以将所需要的数量的润滑剂送到相互运动的表面。第二种通用的润滑方法是飞溅润滑系统，在每个周期内这个系统内一些零件经过润滑剂存储的位置，带起足够的润滑油，然后将其散布到所有的运动零件上。这种系统用于草坪修剪机中发动机的曲轴箱，对曲轴、连杆和活塞等零件进行润滑。在工业装置中，常用的有一种润滑系统是压力系统。这种系统中，一个机器上的一个泵，可以将润滑剂带到所有的轴承表面。并且以一种连续的固定的速度和数量。关于润滑，还有许多其他的系统，针对各种类型的润滑剂，对不同类型的运动零件是有效的。由于设备或装置的速度、压力和工作要求的提高，现代工业比以前任何时候都更注重选用适当的润滑剂。尽管润滑的主要目的之一是为了减小摩擦力，任何可以控制两个滑动表面之间摩擦和磨损的物质，不管是液体还是固体或气体，都可以归类于润滑剂。润滑的种类无润滑滑动。经过精心处理的、去除了所有外来物质的金属在相互滑动时会粘附或熔接到一起。当达不到这么高的纯净度时，吸附在表面的气体、水蒸气、氧化物和污染物就会降低摩擦力并减小粘附的趋势，但通常会产生严重的磨损，这种现象被称为“无润滑”摩擦或者叫做干摩擦。流体膜润滑。在滑动面之间引入一层流体膜，把滑动表面完