【正文】 caused by the wedging action of surface irregularities can be overe 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 ing 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 monly used lubricants are oils, greases, and graphite pounds. 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 mon 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 lawnmower engines to lubricate the crankshaft, connecting rod ,and parts of the piston. A lubrication system monly 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 3 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 substanceliquid , solid , or gaseouscapable 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。(c)elastohydrodynamic (d)boundary。而摩擦這個(gè)詞，本身就意味著，兩個(gè)或 兩個(gè)以上部件的阻止相對運(yùn)動(dòng)趨勢。還有，它還需要更大的動(dòng)力來克服這種不斷增大的摩擦。如果因?yàn)榕蛎泴?dǎo)致了一個(gè)足夠大的積壓力，那么，這個(gè)軸承就可能會(huì)卡死或密封死。當(dāng)兩個(gè)固體處于靜止?fàn)顟B(tài)時(shí)，這兩個(gè)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(dòng)”起來。一旦運(yùn)動(dòng)部件運(yùn)動(dòng)起來，便有了規(guī)律可循，滑動(dòng)就可以實(shí)現(xiàn)這一點(diǎn)。在這種情況下，轉(zhuǎn)動(dòng)件的材料趨向于堆積并且強(qiáng)迫運(yùn)動(dòng)部件緩慢運(yùn)動(dòng)，這種改變就是通常所說的形變。 8 由運(yùn)動(dòng)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?？梢阅命S銅軸承、鋁合金和含油軸承合金做例子進(jìn)行解釋。這種類型的軸承把它的潤滑劑帶到應(yīng)力最大的部位。潤滑劑的分布也決定了系統(tǒng)的選擇。 在工業(yè)裝置中，常用的有一種潤滑系統(tǒng)是壓力系統(tǒng)。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(shí)候都更注重選用適當(dāng)?shù)臐櫥瑒．?dāng)達(dá)不到這么高的純凈度時(shí)，吸附在表面的氣體、水蒸氣、氧化物和污染物就會(huì)降低摩擦力并減小粘附的趨勢，但通常會(huì)產(chǎn) 生9 嚴(yán)重的磨損，這種現(xiàn)象被稱為“無潤滑”摩擦或者叫做干摩擦。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。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。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)。但潤滑劑不僅僅以脂肪、粉末和油脂這樣一