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.


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)。
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)椴荒苡盟鋈魏问虑?。還有，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。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。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(huì)損壞甚至融化。
在運(yùn)動(dòng)部件之間會(huì)發(fā)生很多摩擦，如
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)生的傾向于組織其相對(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(lái)。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(gè)表面之間越不光滑，由運(yùn)動(dòng)造成的啟動(dòng)摩擦（最大靜摩擦力）就會(huì)越大。