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挖掘機液壓油液的物理性質

2019/5/14

液(ye)(ye)(ye)體(ti)(ti)重度(du)為(wei)(wei)(wei)γ，則(ze)兩(liang)截(jie)面(mian)單位(wei)時間流(liu)過的(de)(de)液(ye)(ye)(ye)體(ti)(ti)的(de)(de)壓力能(neng)為(wei)(wei)(wei)p1Q、p2Q，動能(neng)為(wei)(wei)(wei)γQv12/2g，γQv22/2g，位(wei)能(neng)為(wei)(wei)(wei)γQhγQh2。此式為(wei)(wei)(wei)伯努利方(fang)程式。通常液(ye)(ye)(ye)體(ti)(ti)的(de)(de)位(wei)能(neng)可(ke)忽略不計，由此可(ke)見，當F1＞F2時，v1＜v2，p1＞p2。由于液(ye)(ye)(ye)體(ti)(ti)粘性和壓縮性，在運(yun)動中要消耗一部分(fen)能(neng)量(liang)(liang)ΔW，由于液(ye)(ye)(ye)體(ti)(ti)動量(liang)(liang)而產(chan)生的(de)(de)力稱為(wei)(wei)(wei)液(ye)(ye)(ye)動力。當液(ye)(ye)(ye)體(ti)(ti)向(xiang)(xiang)一個方(fang)向(xiang)(xiang)流(liu)動時，同樣會存在方(fang)向(xiang)(xiang)相反(fan)的(de)(de)反(fan)作用(yong)液(ye)(ye)(ye)動力。液(ye)(ye)(ye)體(ti)(ti)的(de)(de)流(liu)態可(ke)用(yong)雷諾(nuo)數Re來(lai)判別。Re＜Re臨界時，流(liu)體(ti)(ti)為(wei)(wei)(wei)層流(liu)；當Re＞Re臨界時，流(liu)體(ti)(ti)為(wei)(wei)(wei)紊流(liu)。水力半(ban)徑(jing)r等于液(ye)(ye)(ye)流(liu)有效截(jie)面(mian)積F與濕周長度(du)W（有效截(jie)面(mian)的(de)(de)周界長度(du)）的(de)(de)比值。圓形(xing)r＝d/4（d為(wei)(wei)(wei)直徑(jing)）矩形(xing)r＝bh/2(b+h)（h為(wei)(wei)(wei)高度(du)。
同心圓環r＝(d1-d2)/4d1為(wei)外圓直(zhi)徑(jing)，液(ye)體為(wei)紊流(liu)(liu)(liu)(liu)且內壁(bi)(bi)光滑時，當(dang)(dang)(dang)Re＜105時，λ＝0.3164Re-0.25；當(dang)(dang)(dang)105＜Re＜3×106時，λ＝0.0032＋0.221Re-0.237。液(ye)壓系(xi)(xi)(xi)統(tong)的(de)總壓力(li)(li)損(sun)失(shi)等于系(xi)(xi)(xi)統(tong)中全部沿(yan)程壓力(li)(li)損(sun)失(shi)和局部壓力(li)(li)損(sun)失(shi)的(de)總和。壓力(li)(li)損(sun)失(shi)與液(ye)流(liu)(liu)(liu)(liu)速(su)度(du)(du)的(de)平(ping)方成正比，因此液(ye)流(liu)(liu)(liu)(liu)速(su)度(du)(du)是影響壓力(li)(li)損(sun)失(shi)的(de)最(zui)重要因素。為(wei)減少系(xi)(xi)(xi)統(tong)的(de)壓力(li)(li)損(sun)失(shi)，液(ye)體在管(guan)路中的(de)流(liu)(liu)(liu)(liu)速(su)不應過(guo)高。壓力(li)(li)高，管(guan)路短(duan)，油液(ye)粘度(du)(du)低時可取大值(zhi)，反之應取小(xiao)(xiao)(xiao)值(zhi)。1.薄壁(bi)(bi)小(xiao)(xiao)(xiao)孔(kong)流(liu)(liu)(liu)(liu)量(liang)的(de)計算(suan)所(suo)謂薄壁(bi)(bi)小(xiao)(xiao)(xiao)孔(kong)，是指(zhi)孔(kong)的(de)長(chang)度(du)(du)與直(zhi)徑(jing)比值(zhi)l/d≤0.5，式中：Cα—流(liu)(liu)(liu)(liu)量(liang)系(xi)(xi)(xi)數(shu)當(dang)(dang)(dang)D/d≥7時，Cα＝0.6~0.62，當(dang)(dang)(dang)D/d＜7時，2.細(xi)長(chang)小(xiao)(xiao)(xiao)孔(kong)流(liu)(liu)(liu)(liu)量(liang)計算(suan)所(suo)謂細(xi)長(chang)小(xiao)(xiao)(xiao)孔(kong)是指(zhi)小(xiao)(xiao)(xiao)孔(kong)長(chang)度(du)(du)遠(yuan)(yuan)遠(yuan)(yuan)大于直(zhi)徑(jing)的(de)小(xiao)(xiao)(xiao)孔(kong)。
一(yi)般(ban)l＞20d以上。一(yi)個(ge)(ge)平(ping)面(mian)固定，另一(yi)個(ge)(ge)平(ping)面(mian)運(yun)(yun)動(dong)(dong)(dong)，并仍保(bao)持兩面(mian)構成(cheng)的(de)間(jian)隙(xi)長度l時(shi)(shi)(shi)(shi)，當平(ping)面(mian)運(yun)(yun)動(dong)(dong)(dong)方向(xiang)與液流方向(xiang)一(yi)致時(shi)(shi)(shi)(shi)取(qu)正(zheng)(zheng)號(hao)，方向(xiang)相反(fan)時(shi)(shi)(shi)(shi)取(qu)負(fu)號(hao)。環形縫隙(xi)的(de)一(yi)個(ge)(ge)表面(mian)運(yun)(yun)動(dong)(dong)(dong)并保(bao)持原(yuan)縫隙(xi)長度l時(shi)(shi)(shi)(shi)，當運(yun)(yun)動(dong)(dong)(dong)表面(mian)的(de)運(yun)(yun)動(dong)(dong)(dong)方向(xiang)與液流方向(xiang)一(yi)致時(shi)(shi)(shi)(shi)取(qu)正(zheng)(zheng)號(hao)，相反(fan)時(shi)(shi)(shi)(shi)取(qu)負(fu)號(hao)。環形縫隙(xi)中(zhong)，內環為圓錐(zhui)環時(shi)(shi)(shi)(shi)，圓環形平(ping)面(mian)縫隙(xi)中(zhong)液體呈放射(she)狀的(de)流動(dong)(dong)(dong)（圖2-17），以上兩公式適用(yong)于縫隙(xi)長度l較(jiao)長的(de)情況(kuang)，當l很小時(shi)(shi)(shi)(shi)，必(bi)須(xu)充(chong)分考慮(lv)助流區的(de)影(ying)響(xiang)。1)液流突然截(jie)斷或(huo)換向(xiang)引(yin)起(qi)液壓(ya)沖(chong)擊。2)運(yun)(yun)動(dong)(dong)(dong)的(de)工(gong)作部件制動(dong)(dong)(dong)或(huo)換缶時(shi)(shi)(shi)(shi)因慣性(xing)力(li)引(yin)起(qi)液壓(ya)沖(chong)擊。3)由于液壓(ya)元件的(de)滯后動(dong)(dong)(dong)作，使系統(tong)壓(ya)力(li)不(bu)能及時(shi)(shi)(shi)(shi)調正(zheng)(zheng)，引(yin)起(qi)液壓(ya)沖(chong)擊。2.液流通道迅速(su)關(guan)閉產(chan)生的(de)壓(ya)力(li)沖(chong)擊這種液壓(ya)沖(chong)擊與沖(chong)擊波在導(dao)管內傳播的(de)速(su)度有關(guan)。
沖(chong)(chong)擊(ji)(ji)波傳(chuan)播(bo)(bo)的(de)(de)速度(du)越快，沖(chong)(chong)擊(ji)(ji)壓力越大(da)。同(tong)時(shi)(shi)(shi)(shi)(shi)還因通(tong)道(dao)的(de)(de)關(guan)閉速度(du)不同(tong)而有所(suo)差(cha)別。這種沖(chong)(chong)擊(ji)(ji)包(bao)含(han)兩種情況，即完(wan)(wan)全(quan)(quan)沖(chong)(chong)擊(ji)(ji)和非(fei)完(wan)(wan)全(quan)(quan)沖(chong)(chong)擊(ji)(ji)。假設(she)通(tong)道(dao)關(guan)閉的(de)(de)時(shi)(shi)(shi)(shi)(shi)間為t，沖(chong)(chong)擊(ji)(ji)波從起(qi)始(shi)點(dian)開始(shi)再反(fan)射回起(qi)始(shi)點(dian)的(de)(de)時(shi)(shi)(shi)(shi)(shi)間為T，那末t＜T時(shi)(shi)(shi)(shi)(shi)為完(wan)(wan)全(quan)(quan)沖(chong)(chong)擊(ji)(ji)，此時(shi)(shi)(shi)(shi)(shi)液流(liu)動(dong)能(neng)(neng)全(quan)(quan)部轉(zhuan)為液壓能(neng)(neng)；t＞T時(shi)(shi)(shi)(shi)(shi)為非(fei)完(wan)(wan)全(quan)(quan)沖(chong)(chong)擊(ji)(ji)，此時(shi)(shi)(shi)(shi)(shi)只(zhi)有部分液流(liu)動(dong)能(neng)(neng)轉(zhuan)為液壓能(neng)(neng)。完(wan)(wan)全(quan)(quan)沖(chong)(chong)擊(ji)(ji)時(shi)(shi)(shi)(shi)(shi)(t＜T)，非(fei)完(wan)(wan)全(quan)(quan)沖(chong)(chong)擊(ji)(ji)時(shi)(shi)(shi)(shi)(shi)(t＜T)，由(you)此可(ke)知，1)延長通(tong)道(dao)關(guan)閉時(shi)(shi)(shi)(shi)(shi)間，如用先導閥減(jian)緩換(huan)向(xiang)(xiang)閥的(de)(de)換(huan)向(xiang)(xiang)速度(du)。2)降低通(tong)道(dao)關(guan)閉前的(de)(de)液流(liu)速度(du)，如在(zai)滑閥端部開緩沖(chong)(chong)槽等。3)縮短沖(chong)(chong)擊(ji)(ji)波傳(chuan)播(bo)(bo)反(fan)射的(de)(de)時(shi)(shi)(shi)(shi)(shi)間，如縮短導管(guan)距離，或在(zai)距通(tong)道(dao)關(guan)閉部位(wei)較近的(de)(de)位(wei)置設(she)置蓄能(neng)(neng)器(qi)。4)降低沖(chong)(chong)擊(ji)(ji)波傳(chuan)播(bo)(bo)速度(du)，如采用較大(da)的(de)(de)導管(guan)直徑(jing)，采用彈性系數(shu)較大(da)的(de)(de)導管(guan)材(cai)料。
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