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譯文題目： Friction And Lubrication
摩擦與潤(rùn)滑
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20xx年 2月 27日
Friction And ?Lubrication
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.?
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.?
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?wedgingaction.?To?produce?motion?in?these?parts,?the?wedgeshaped?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.?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?ofbearing?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?.?
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.?
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?.?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.? ?
Sealing.?Lubricating?grease?frequently?performs?the?special?function?of?forming?a?seal?to?retain?lubricants?or?to??exclude?contaminants.?
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??.
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?.
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?assistrelative?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.
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.
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.?
摩擦與潤(rùn)滑
現(xiàn)在看來(lái)，有很多這種情況，許多學(xué)生在被問(wèn)到關(guān)于摩擦的問(wèn)題時(shí)，往往都沒(méi)引起足夠的重視，甚至是忽視它。實(shí)際上，摩擦從某種程度上說(shuō)，存在于任何兩個(gè)相接觸并有相對(duì)運(yùn)動(dòng)趨勢(shì)的部件之間。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(duì)運(yùn)動(dòng)趨勢(shì)。
摩擦是有害的還是有益的，關(guān)鍵在于它作用在什么地方。對(duì)于像螺絲和鉚釘這些依靠摩擦固定緊固體和機(jī)械部件的緊固結(jié)構(gòu)來(lái)說(shuō)，摩擦是必要的。?
在一個(gè)機(jī)器中，運(yùn)動(dòng)部件的摩擦是有害的，因?yàn)樗档土藱C(jī)械對(duì)能量的充分利用。由它引起的熱能是一種浪費(fèi)的能量，因?yàn)樗蛔鋈魏喂?。而且，它還需要更大的動(dòng)力來(lái)克服這種不斷增大的摩擦。熱能是有破壞性的，因?yàn)樗a(chǎn)生了膨脹。而膨脹可以使得軸承或滑動(dòng)表面之間的配合更緊密。
有三種摩擦是在運(yùn)動(dòng)中必須被克服的：（?1）啟動(dòng)摩擦（?2）滑動(dòng)摩擦（?3）轉(zhuǎn)動(dòng)摩擦。?啟動(dòng)摩擦是兩個(gè)固體之間產(chǎn)生的傾向于抵抗其相對(duì)運(yùn)動(dòng)趨勢(shì)的摩擦。當(dāng)兩個(gè)固體處于靜止?fàn)顟B(tài)時(shí)，這兩個(gè)零件表面的不規(guī)則處傾向于相互嵌入，形成楔入作用。為了使這些部件“動(dòng)”起來(lái)，這些固定表面的楔形峰和谷必須制作光滑，而且能相互抵消。這兩個(gè)表面之間越粗糙，由運(yùn)動(dòng)造成的啟動(dòng)摩擦（最大靜摩擦力）就會(huì)越大。?
因?yàn)?，通常?lái)說(shuō)，在兩個(gè)相互配合的部件之間，其表面不規(guī)則沒(méi)有固定的圖形，當(dāng)這些部件在運(yùn)動(dòng)時(shí)，不規(guī)則處不能相互配合。兩個(gè)表面之間產(chǎn)生的摩擦就是我們所知的滑動(dòng)摩擦。由計(jì)算可知，啟動(dòng)摩擦通常都稍大于滑動(dòng)摩擦。?
當(dāng)輥裝置承受引起機(jī)構(gòu)形變或變形的巨大壓力時(shí)，轉(zhuǎn)動(dòng)摩擦就產(chǎn)生了。在這些情況下，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)，這種引起分子運(yùn)動(dòng)的形狀的改變就是通常所說(shuō)的形變。當(dāng)然，最終的結(jié)果是，這種必須的保證運(yùn)動(dòng)部件運(yùn)動(dòng)和克服摩擦力的額外的能量產(chǎn)生了熱能。?
由于運(yùn)動(dòng)部件表面不規(guī)則的楔入作用而引起的摩擦可以通過(guò)兩表面之間的潤(rùn)滑被部分的克服。但是，即使是非常光滑的兩個(gè)表面之間也可能會(huì)使用一種材料去減少摩擦。這種材料通常是一種潤(rùn)滑劑，它可以提供一個(gè)比較好的、比較薄的油膜。這個(gè)油膜使兩個(gè)表面分離，并且阻止運(yùn)動(dòng)部件的兩個(gè)表面之間的凝聚力使它們更近的接觸并產(chǎn)生熱量。?
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(dòng)零件。這就解釋了為什么黃銅軸承、鋁合金和含油軸承合金都使用軟的或硬的金屬組成表面。含油軸承合金有較多孔洞。因此，當(dāng)軸承在油中浸泡過(guò)以后，因?yàn)槊?xì)管的作用，將油帶到軸承的各個(gè)表面。這種類(lèi)型的軸承把它的潤(rùn)滑劑帶到應(yīng)力最大的部位。?
對(duì)運(yùn)動(dòng)部件潤(rùn)滑以減小摩擦，應(yīng)力和熱量。最常用的潤(rùn)滑劑是油、脂、還有合成劑。每一種潤(rùn)滑劑都有其各自不同的功能和用途。兩個(gè)運(yùn)動(dòng)部件之間的運(yùn)動(dòng)情況決定了潤(rùn)滑劑的類(lèi)型的選擇，并且系統(tǒng)也被選擇來(lái)適應(yīng)這種潤(rùn)滑劑。?
在承受最小壓力低速度運(yùn)動(dòng)的部件中，一個(gè)油溝足以將所需要的數(shù)量的潤(rùn)滑劑送到相互運(yùn)動(dòng)的表面。?
第二種通用的潤(rùn)滑方法是飛濺潤(rùn)滑系統(tǒng)，在每個(gè)周期內(nèi)這個(gè)系統(tǒng)內(nèi)一些零件經(jīng)過(guò)潤(rùn)滑劑存儲(chǔ)的位置，帶起足夠的潤(rùn)滑油，然后將其散布到所有的運(yùn)動(dòng)零件上。這種系統(tǒng)用于草坪修剪機(jī)中發(fā)動(dòng)機(jī)的曲軸箱，對(duì)曲軸、連桿和活塞等零件進(jìn)行潤(rùn)滑。?
在工業(yè)裝置中，常用的有一種潤(rùn)滑系統(tǒng)是壓力系統(tǒng)。這種系統(tǒng)中，一個(gè)機(jī)器上的一個(gè)泵，可以將潤(rùn)滑劑以一種連續(xù)固定的速度和數(shù)量帶到所有的軸承表面。?
關(guān)于潤(rùn)滑，還有許多其他的系統(tǒng)。有可觀數(shù)量的潤(rùn)滑劑可被運(yùn)用在任意一種給定的操作狀況中。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?rùn)滑劑。?
盡管潤(rùn)滑的主要目的之一是為了減小摩擦力，任何可以控制兩個(gè)滑動(dòng)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(lèi)于潤(rùn)滑劑。?
潤(rùn)滑的種類(lèi)
無(wú)潤(rùn)滑滑動(dòng)。經(jīng)過(guò)精心處理的、去除了所有外來(lái)物質(zhì)的金屬在相互滑動(dòng)時(shí)會(huì)粘附或熔接到一起。當(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(shì)，但通常會(huì)產(chǎn)生嚴(yán)重的磨損，這種現(xiàn)象被稱(chēng)為“無(wú)潤(rùn)滑”摩擦或者叫做干摩擦。?
流體膜潤(rùn)滑。在滑動(dòng)面之間引入一層流體膜，把滑動(dòng)表面完全隔離開(kāi)，就產(chǎn)生了流體膜潤(rùn)滑。這種流體可能是有意引入的。例如汽車(chē)主軸承中的潤(rùn)滑油；也可能是無(wú)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(lèi)的液體，它可以是氣體。最常用的氣體是空氣。?
邊界潤(rùn)滑。處于無(wú)潤(rùn)滑滑動(dòng)和流體膜潤(rùn)滑之間的潤(rùn)滑被稱(chēng)為邊界潤(rùn)滑。它可以被定為這樣一種潤(rùn)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(rùn)滑劑中的其他性質(zhì)。邊界潤(rùn)滑包括大部分潤(rùn)滑現(xiàn)象，通常在機(jī)器的啟動(dòng)和停止時(shí)出現(xiàn)。?
固體潤(rùn)滑。當(dāng)普通潤(rùn)滑劑沒(méi)有足夠的承受能力或者不能在溫度極限下工作時(shí)，石墨和二硫化鉬這一類(lèi)固體潤(rùn)滑劑得廣泛運(yùn)用。但是潤(rùn)滑劑并不是只有油脂，粉末和氣體這幾種熟悉的形式。在一些靜密的儀器中，甚至一些金屬一般也能也能作為滑動(dòng)表面。
潤(rùn)滑劑的作用
盡管潤(rùn)滑劑根本上是為了控制摩擦，但一般也起著許多其他的作用。
摩擦控制。滑動(dòng)表面可用潤(rùn)滑劑的數(shù)量和特征對(duì)其產(chǎn)生的摩擦有著深遠(yuǎn)的影響。舉個(gè)例子，無(wú)視像熱量和壓力這樣的相關(guān)因素，只考慮在潤(rùn)滑劑作用下相同表面的摩擦。在液膜狀態(tài)下，摩擦是存在的。在一個(gè)大的粘度范圍內(nèi)，從而能夠滿(mǎn)足廣泛的功能要求。在邊界潤(rùn)滑條件下，粘度對(duì)摩擦的影響變得顯著低于化學(xué)性質(zhì)的潤(rùn)滑劑。
磨損控制。磨損通過(guò)磨蝕、腐蝕和固體與固體的接觸作用于潤(rùn)滑表面。壓力通過(guò)提供一個(gè)膜來(lái)增加滑動(dòng)表面之間的距離，從而減少磨料污染物和表面粗糙的破壞。
溫度控制。潤(rùn)滑劑協(xié)助控制腐蝕的表面。當(dāng)機(jī)器閑置時(shí)，潤(rùn)滑劑可作為防腐劑使用。當(dāng)機(jī)械在使用中時(shí)，潤(rùn)滑劑通過(guò)具有保護(hù)膜的、含有中和腐蝕性材料的添加劑的涂層潤(rùn)滑部分來(lái)控制腐蝕。潤(rùn)滑劑控制腐蝕的能力與保留在金屬表面的潤(rùn)滑膜厚度和潤(rùn)滑劑的化學(xué)成分直接相關(guān)。
其它作用
潤(rùn)滑除了用于減少摩擦還經(jīng)常用于其它目的。其中的一些應(yīng)用如下所述。
能量傳輸。潤(rùn)滑油被廣泛用作液壓流體在流體傳輸設(shè)備中使用。
絕緣。在如變壓器、開(kāi)關(guān)設(shè)備這些專(zhuān)門(mén)的應(yīng)用中，具有高階電常數(shù)的潤(rùn)滑油廠用作電器絕緣體。為了保持最大的絕緣特性，潤(rùn)滑劑必須遠(yuǎn)離污染物和水。
密封。潤(rùn)滑脂經(jīng)常被用作一些特殊的功能，形成一個(gè)密封空間從而保存潤(rùn)滑劑或者排除污染物。
液體動(dòng)力潤(rùn)滑意味著軸承的承載面有著相對(duì)厚的潤(rùn)滑膜，可以防止金屬接觸，獲得在潤(rùn)滑壓力下可以接受的良好穩(wěn)定性。
靜壓潤(rùn)滑是通過(guò)引入潤(rùn)滑劑，有時(shí)是空氣或水，到壓力足夠高能夠用較厚的潤(rùn)滑膜去分離表面的承載區(qū)域。所以，它不需要像液體動(dòng)力潤(rùn)滑那樣，從一個(gè)表面運(yùn)動(dòng)到另一個(gè)表面。
對(duì)于邊界潤(rùn)滑的理解主要?dú)w功于哈布和道布爾迪，他們發(fā)現(xiàn)依附于表面的極度薄的潤(rùn)滑膜通常已經(jīng)足夠用于相對(duì)滑動(dòng)。他們得出的結(jié)論是，在這種情況下流體的化學(xué)成分是很重要的，他們介紹了術(shù)語(yǔ)“邊界潤(rùn)滑”。
非穩(wěn)態(tài)彈流潤(rùn)滑現(xiàn)象發(fā)生在當(dāng)潤(rùn)滑劑在兩個(gè)表面之間滾動(dòng)接觸時(shí)，如齒輪配合和滾動(dòng)軸承。數(shù)學(xué)解釋要求赫茲接觸應(yīng)力和流體力學(xué)的理論。
潤(rùn)滑的目的是為了減少機(jī)械相對(duì)運(yùn)動(dòng)部分產(chǎn)生的摩擦，磨損和熱量。潤(rùn)滑劑是在移動(dòng)表面之間實(shí)現(xiàn)這些目的的任何物質(zhì)。大多數(shù)潤(rùn)滑劑是液體（如礦物油、有機(jī)硅液體和水），但它們也可能是用于干燥軸承的固體、用于滾動(dòng)軸承的油脂、或者是用于氣體軸承的氣體（如空氣）。潤(rùn)滑劑和潤(rùn)滑表面之間的物理和化學(xué)作用必須理解為為了使機(jī)器有令人滿(mǎn)意的工作狀態(tài)。