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外文文獻
Friction Lubrication of Bearing
In 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 lubrication
Unlubricated 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 lubricants
Although 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 functions
Lubrication 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.
軸承的摩擦與潤滑
現(xiàn)在看來，有很多這種情況，許多學(xué)生在被問到關(guān)于摩擦的問題時，往往都沒引起足夠的重視，甚至是忽視它。實際上，摩擦從某種程度上說，存在于任何兩個相接觸并有相對運動趨勢的部件之間。而摩擦這個詞，本身就意味著，兩個或兩個以上部件的阻止相對運動趨勢。
在一個機器中，運動部件的摩擦是有害的，因為它降低了機械對能量的充分利用。由它引起的熱能是一種浪費的能量。因為不能用它做任何事情。還有，它還需要更大的動力來克服這種不斷增大的摩擦。熱能是有破壞性的。因為它產(chǎn)生了膨脹。而膨脹可以使得軸承或滑動表面之間的配合更緊密。如果因為膨脹導(dǎo)致了一個足夠大的積壓力，那么，這個軸承就可能會卡死或密封死。另外，隨著溫度的升高，如果不是耐高溫材料制造的軸承，就可能會損壞甚至融化。
在運動部件之間會發(fā)生很多摩擦，如
1.啟動摩擦
2.滑動摩擦
3.轉(zhuǎn)動摩擦。
啟動摩擦是兩個固體之間產(chǎn)生的傾向于組織其相對運動趨勢的摩擦。當兩個固體處于靜止狀態(tài)時，這兩個零件表面的不平度傾向于相互嵌入，形成楔入作用，為了使這些部件“動”起來。這些靜止部件的凹谷和尖峰必須整理光滑，而且能相互抵消。這兩個表面之間越不光滑，由運動造成的啟動摩擦（最大靜摩擦力）就會越大。
因為，通常來說，在兩個相互配合的部件之間，其表面不平度沒有固定的圖形。一旦運動部件運動起來，便有了規(guī)律可循，滑動就可以實現(xiàn)這一點。兩個運動部件之間的摩擦就叫做滑動摩擦。啟動摩擦通常都稍大于滑動摩擦。
轉(zhuǎn)動摩擦一般發(fā)生在轉(zhuǎn)動部件和設(shè)備上，這些設(shè)備“抵觸”極大的外作用力，當然這種外力會導(dǎo)致部件的變形和性能的改變。在這種情況下，轉(zhuǎn)動件的材料趨向于堆積并且強迫運動部件緩慢運動，這種改變就是通常所說的形變?？梢允狗肿舆\動。當然，最終的結(jié)果是，這種額外的能量產(chǎn)生了熱能，這是必需的。因為它可以保證運動部件的運動和克服摩擦力。
由運動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，那就需要靠兩表面之間的潤滑。但是，即使是非常光滑的兩個表面之間也可能需要一種物質(zhì)，這種物質(zhì)就是通常所說的潤滑劑，它可以提供一個比較好的、比較薄的油膜。這個油膜使兩個表面分離，并且組織運動部件的兩個表面的相互潛入，以免產(chǎn)生熱量使兩表面膨脹，又引起更近的接觸。
減小摩擦的另一種方式是用不同的材料制造軸承和轉(zhuǎn)動零件?？梢阅命S銅軸承、鋁合金和含油軸承合金做例子進行解釋。也就是說用軟的或硬的金屬組成表面。含油軸承合金是軟的。這樣，當軸承在油中浸泡過以后，因為毛細管的作用，將由帶到軸承的各個表面。這種類型的軸承把它的潤滑劑帶到應(yīng)力最大的部位。
對運動部件潤滑以減小摩擦，應(yīng)力和熱量，最常用的是油、脂、還有合成劑。每一種潤滑劑都有其各自不同的功能和用途。兩個運動部件之間的運動情況決定了潤滑劑的類型的選擇。潤滑劑的分布也決定了系統(tǒng)的選擇。
在低速度運動的部件，一個油溝足以將所需要的數(shù)量的潤滑劑送到相互運動的表面。
第二種通用的潤滑方法是飛濺潤滑系統(tǒng)，在每個周期內(nèi)這個系統(tǒng)內(nèi)一些零件經(jīng)過潤滑劑存儲的位置，帶起足夠的潤滑油，然后將其散布到所有的運動零件上。這種系統(tǒng)用于草坪修剪機中發(fā)動機的曲軸箱，對曲軸、連桿和活塞等零件進行潤滑。
在工業(yè)裝置中，常用的有一種潤滑系統(tǒng)是壓力系統(tǒng)。這種系統(tǒng)中，一個機器上的一個泵，可以將潤滑劑帶到所有的軸承表面。并且以一種連續(xù)的固定的速度和數(shù)量。
關(guān)于潤滑，還有許多其他的系統(tǒng)，針對各種類型的潤滑劑，對不同類型的運動零件是有效的。由于設(shè)備或裝置的速度、壓力和工作要求的提高，現(xiàn)代工業(yè)比以前任何時候都更注重選用適當?shù)臐櫥瑒?
盡管潤滑的主要目的之一是為了減小摩擦力，任何可以控制兩個滑動表面之間摩擦和磨損的物質(zhì)，不管是液體還是固體或氣體，都可以歸類于潤滑劑。
潤滑的種類
無潤滑滑動:經(jīng)過精心處理的、去除了所有外來物質(zhì)的金屬在相互滑動時會粘附或熔接到一起。當達不到這么高的純凈度時，吸附在表面的氣體、水蒸氣、氧化物和污染物就會降低摩擦力并減小粘附的趨勢，但通常會產(chǎn)生嚴重的磨損，這種現(xiàn)象被稱為“無潤滑”摩擦或者叫做干摩擦。
流體膜潤滑:在滑動面之間引入一層流體膜，把滑動表面完全隔離開，就產(chǎn)生了流體膜潤滑。這種流體可能是有意引入的。例如汽車主軸承中的潤滑油；也可能是無意中引入的，例如在光滑的橡膠輪胎和潮濕的路面之間的水。盡管流體通常是油、水和其他很多種類的液體，它可以是氣體。最常用的氣體是空氣。
為了把零件隔離開，潤滑膜中的壓力必須和作用在滑動面上的負荷保持平衡。如果潤滑膜中的壓力是由外源提供的，這種系統(tǒng)稱為流體靜壓潤滑。如果滑動表面之間的壓力是由于滑動面本身的形狀和運動所共同產(chǎn)生的，這種系統(tǒng)就稱為流體動壓力潤滑。
邊界潤滑:處于無潤滑滑動和流體膜潤滑之間的潤滑被稱為邊界潤滑。它可以被定為這樣一種潤滑狀態(tài)，在這種狀態(tài)中，表面之間的摩擦力取決于表面的性質(zhì)和潤滑劑中的其他性質(zhì)。邊界潤滑包括大部分潤滑現(xiàn)象，通常在機器的啟動和停止時出現(xiàn)。
固體潤滑。當普通潤滑劑沒有足夠的承受能力或者不能在溫度極限下工作時，石墨和二硫化鉬這一類固體潤滑劑得到廣泛應(yīng)用。但潤滑劑不僅僅以脂肪、粉末和油脂這樣一些為人們所熟悉的形態(tài)出現(xiàn)，在一些精密的機器中，金屬也通常作為滑動面。
潤滑劑的作用
盡管潤滑劑主要是用來控制摩擦和磨損的，它們能夠而且通常也確實起到許多其他的作用，這些作用隨其用途不同而不同，但通常相互之間是有關(guān)系的。
控制摩擦力。 滑動面之間潤滑劑的數(shù)量和性質(zhì)對所產(chǎn)生的摩擦力有很大的影響。例如，不考慮熱和磨損這些相關(guān)因素，只考慮兩個油膜潤滑表面見的摩擦力，它能比兩個同樣表面，但沒有潤滑時小200倍。在流體潤滑狀況時，摩擦力與流體黏度成正比。一些諸如石油衍生物這類潤滑劑，可以有很多黏度，因此能夠滿足范圍寬廣的功能要求。在邊界潤滑狀態(tài)，潤滑劑黏度對摩擦力的影響不象其化學(xué)性質(zhì)的影響那么顯著。
磨損控制。磨蝕、腐蝕與固體和固體之間的接觸就會造成磨損。適當?shù)臐櫥瑒⒛軒椭朔鲜鎏岬降囊恍┠p現(xiàn)象。潤滑劑通過潤滑膜來增加滑動面之間的距離，從而減輕磨料污染物和表面不平度造成的損傷，因此，減輕了磨損和由固體與固體之間接觸造成的磨損。
控制溫度。潤滑劑通過減小摩擦和將產(chǎn)生的熱量帶走來降低溫度。其效果取決于潤滑劑的用量和外部冷卻措施。冷卻劑的種類也會在較小的程度上影響表面的溫度。
控制腐蝕。潤滑劑在控制表面腐蝕方面有雙重作用。當機器閑置不工作時，潤滑劑起到防腐劑的作用。當機器工作時，潤滑劑通過給被潤滑零件涂上一層可能含有添加劑，能使腐蝕性材料中和的保護膜來控制腐蝕。潤滑劑控制腐蝕的能力與潤滑劑保留在金屬表面的潤滑膜的厚度和潤滑劑的化學(xué)成分有直接的關(guān)系。
其他作用
除了減小摩擦外，潤滑劑還經(jīng)常有其他的用途。其中的一些用途如下所述。
傳遞動力。潤滑劑被廣泛用來作為液壓傳動中的工作液體。
絕緣。在象變壓器和配電裝置這些特殊用途中，具有很高介電常數(shù)的潤滑劑起電絕緣材料的作用。為了獲得最高絕緣性能，潤滑劑中不能含有任何雜質(zhì)和水分。
減振。在象減振器這樣的能量傳遞裝置中和在承受很高的間隙載荷的齒輪這樣的機器零件的周圍，潤滑劑被作為減振液使用。
密封。潤滑脂通常還有一個特殊作用，就是形成密封層以防止?jié)櫥瑒┩鉃a和污染物進入。
潤滑的目的就是為了，減小摩擦力，降低能量損耗，減少機器的熱量產(chǎn)生。熱量就是因為表面的相互間的相對運動造成的。潤滑劑可以是任何一種物質(zhì)，這樣的物質(zhì)被填充到發(fā)生相對運動的兩個表面之間，實現(xiàn)這一目的。大部分的潤滑劑是液體，比如說，油，脂，合成劑等。但它們有時也可能是固體，用在干軸承上，有的用在旋轉(zhuǎn)基體的軸承上，或者也可能是氣體，如空氣等，它是用在空氣軸承上。在潤滑劑和潤滑表面之間這種化學(xué)的和物質(zhì)的相互滲入作用，就是為了提供給機器一個良好的工作狀態(tài)。
對潤滑劑邊界的理解，往往是比較硬的，而且是流動的、非常薄的一層帖附在被潤滑的表面。這些表面通常是要發(fā)生相對滑動。有些人推斷，按這種理解，液體的這種化學(xué)合成是十分重要的，它們提出了這樣的詞“邊界潤滑”，邊界潤滑是和流體潤滑相對的另一種潤滑。
關(guān)于潤滑的五種不同的潤滑形式主要有：
（1）無潤滑潤滑劑。
（2）流體膜潤滑。
（3）干潤滑。
（4）邊界潤滑。
（5）固體潤滑。
無潤滑潤滑劑是指軸承的工作表面被一種相對比較厚的液體潤滑劑分隔開，于是阻止了金屬表面的直接接觸，這樣得到的這種穩(wěn)定性就可以用一種理論來解釋：潤滑液在外壓力下工作的理論，盡管這只是一種可能。但確實需要在任何時候都得提供的足夠充分。這種擠壓力是運動表面本身施加給潤滑劑而產(chǎn)生的，當然這仍然是一種可能。這種由運動表面產(chǎn)生的擠壓力產(chǎn)生了必要的壓力來分隔工作表面來抵抗加在軸承上的載荷。所以，這種潤滑也可以被叫做液體潤滑。
還有一種潤滑方式，那是一種特別的潤滑劑，它有時是空氣或水，當加在軸承上的外載荷足夠高時，它就會以一種比較厚的狀態(tài)分隔開相互相對運動的工作表面。所以，不象上面的那種潤滑方式，并不需要兩種工作表面一定發(fā)生相對運動。
第三種潤滑方式是一種現(xiàn)象，這種現(xiàn)象是，一種潤滑劑是用在發(fā)生相對轉(zhuǎn)動的工作表面之間。比如說齒輪或者是滾動軸承。從數(shù)學(xué)上的解釋就需要接觸壓力和流體機械的理論。
當軸承不得不在較高的溫度下工作的時候，固體潤滑劑例如合成物等，必須被使用，因為通常使用的潤滑油在這種情況下都不能工作。目前，在這方面的研究正在實施，為了尋找到合成軸承的材料，并且有低損耗和小的熱量產(chǎn)生的性能。
在有的軸承上，搖桿旋轉(zhuǎn)或在軸承上轉(zhuǎn)動，相對運動就是滑動。在一個自鎖的軸承裝置中，這種相對運動就是轉(zhuǎn)動。其他的裝置也可能是旋轉(zhuǎn)或滑動。齒輪的齒嚙合是轉(zhuǎn)動與相對滑動的合成。活塞是相對于剛體的滑動，所有的這些應(yīng)用都需要潤滑劑來減小摩擦，降低能耗，減少熱量的產(chǎn)生。
在有些軸承的應(yīng)用領(lǐng)域是不太成熟的。有些有連接桿的軸承，比如說汽車發(fā)動機上的，必須在幾千度高的高溫下和各種不同性質(zhì)的載荷下工作。這種軸承用在汽輪發(fā)動設(shè)備上可以說是穩(wěn)定性接近100%。還有另一種極端的情況，在有些軸承有幾千種應(yīng)用，應(yīng)對各種不同的載荷。其他的輔助設(shè)施就相對不重要了。需要的是一個簡單的、容易安裝的軸承。需要很少的甚至是不需要潤滑劑。在這種情況下，有的軸承并不是最好的選擇，因為成本和相近的公差。最近在軸承材料上的研究已有了一定的突破。隨著對潤滑的研究的知識的積累，設(shè)計出有良好工作狀況和較高的穩(wěn)定性的軸承已不是很遙遠了。
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