液(ye)(ye)(ye)(ye)動(dong)(dong)(dong)機(ji)(ji)負(fu)載。原(yuan)動(dong)(dong)(dong)機(ji)(ji)將機(ji)(ji)械(xie)能(neng)輸入(ru)液(ye)(ye)(ye)(ye)壓(ya)(ya)(ya)(ya)系統，由液(ye)(ye)(ye)(ye)壓(ya)(ya)(ya)(ya)動(dong)(dong)(dong)力(li)元件(jian)——液(ye)(ye)(ye)(ye)壓(ya)(ya)(ya)(ya)泵轉變為(wei)液(ye)(ye)(ye)(ye)壓(ya)(ya)(ya)(ya)能(neng)，通(tong)過(guo)控(kong)(kong)(kong)制元件(jian)——液(ye)(ye)(ye)(ye)壓(ya)(ya)(ya)(ya)閥調整(zheng)控(kong)(kong)(kong)制壓(ya)(ya)(ya)(ya)力(li)油的(de)(de)(de)方(fang)向(xiang)(xiang)、流量和(he)(he)(he)壓(ya)(ya)(ya)(ya)力(li)的(de)(de)(de)大(da)小，然后傳遞給(gei)執(zhi)行(xing)元件(jian)——液(ye)(ye)(ye)(ye)動(dong)(dong)(dong)機(ji)(ji)，使其按照一定的(de)(de)(de)方(fang)向(xiang)(xiang)、速度和(he)(he)(he)出(chu)(chu)力(li)帶動(dong)(dong)(dong)負(fu)荷運動(dong)(dong)(dong)和(he)(he)(he)工作，構成液(ye)(ye)(ye)(ye)壓(ya)(ya)(ya)(ya)回(hui)(hui)路(lu)(lu)(lu)。原(yuan)動(dong)(dong)(dong)機(ji)(ji)主要有(you)(you)交流電動(dong)(dong)(dong)機(ji)(ji)、直流電動(dong)(dong)(dong)機(ji)(ji)和(he)(he)(he)內(nei)燃機(ji)(ji)等(deng)。液(ye)(ye)(ye)(ye)壓(ya)(ya)(ya)(ya)閥、液(ye)(ye)(ye)(ye)壓(ya)(ya)(ya)(ya)泵和(he)(he)(he)液(ye)(ye)(ye)(ye)動(dong)(dong)(dong)機(ji)(ji)等(deng)互相配合構成三種(zhong)基本(ben)類型的(de)(de)(de)控(kong)(kong)(kong)制回(hui)(hui)路(lu)(lu)(lu)，即壓(ya)(ya)(ya)(ya)力(li)控(kong)(kong)(kong)制回(hui)(hui)路(lu)(lu)(lu)，方(fang)向(xiang)(xiang)控(kong)(kong)(kong)制回(hui)(hui)路(lu)(lu)(lu)和(he)(he)(he)速度控(kong)(kong)(kong)制回(hui)(hui)路(lu)(lu)(lu)。此外，還有(you)(you)由此派(pai)生出(chu)(chu)來的(de)(de)(de)位置(zhi)控(kong)(kong)(kong)制回(hui)(hui)路(lu)(lu)(lu)和(he)(he)(he)時(shi)間控(kong)(kong)(kong)制回(hui)(hui)路(lu)(lu)(lu)。有(you)(you)時(shi)，一個回(hui)(hui)路(lu)(lu)(lu)可同時(shi)兼有(you)(you)幾(ji)(ji)種(zhong)職(zhi)能(neng)。液(ye)(ye)(ye)(ye)壓(ya)(ya)(ya)(ya)回(hui)(hui)路(lu)(lu)(lu)可用以下幾(ji)(ji)種(zhong)表(biao)示(shi)方(fang)法。它能(neng)直觀地表(biao)示(shi)出(chu)(chu)各(ge)液(ye)(ye)(ye)(ye)壓(ya)(ya)(ya)(ya)元件(jian)的(de)(de)(de)形狀(zhuang)、位置(zhi)和(he)(he)(he)管路(lu)(lu)(lu)的(de)(de)(de)聯接走向(xiang)(xiang)，不能(neng)表(biao)示(shi)出(chu)(chu)元件(jian)的(de)(de)(de)內(nei)部(bu)結構和(he)(he)(he)液(ye)(ye)(ye)(ye)壓(ya)(ya)(ya)(ya)系統的(de)(de)(de)工作原(yuan)理。

一(yi)(yi)般僅(jin)用(yong)(yong)于裝配工(gong)作。它(ta)直(zhi)(zhi)接(jie)表(biao)現出各(ge)元件(jian)(jian)的(de)(de)內部結(jie)構(gou)和(he)(he)系(xi)統(tong)的(de)(de)工(gong)作原(yuan)理，便于理解(jie)和(he)(he)查找故障(zhang)，但(dan)因制圖(tu)較麻煩，一(yi)(yi)般僅(jin)用(yong)(yong)于教學。它(ta)用(yong)(yong)簡單的(de)(de)符(fu)號把復(fu)雜的(de)(de)液(ye)壓(ya)(ya)系(xi)統(tong)表(biao)現出來，它(ta)既(ji)能表(biao)現出各(ge)元件(jian)(jian)之間管路(lu)(lu)的(de)(de)聯(lian)接(jie)方法(fa)，又可以說明(ming)它(ta)的(de)(de)工(gong)作原(yuan)理，制圖(tu)也很簡單。但(dan)是事先必須對各(ge)種(zhong)(zhong)元件(jian)(jian)的(de)(de)符(fu)號，工(gong)作原(yuan)理和(he)(he)職能有充分的(de)(de)了(le)解(jie)，否則看不懂符(fu)號圖(tu)。這種(zhong)(zhong)方法(fa)被國內外廣泛(fan)應用(yong)(yong)。為了(le)特(te)別說明(ming)某元件(jian)(jian)的(de)(de)工(gong)作原(yuan)理或不便于用(yong)(yong)符(fu)號表(biao)示液(ye)壓(ya)(ya)元件(jian)(jian)時(shi)，可在符(fu)號圖(tu)中采用(yong)(yong)局部截面圖(tu)。液(ye)壓(ya)(ya)系(xi)統(tong)按照油(you)(you)(you)液(ye)的(de)(de)循環(huan)情況可分開(kai)式(shi)(shi)回(hui)(hui)路(lu)(lu)和(he)(he)閉式(shi)(shi)回(hui)(hui)路(lu)(lu)。開(kai)式(shi)(shi)回(hui)(hui)路(lu)(lu)中液(ye)動(dong)機(ji)的(de)(de)回(hui)(hui)油(you)(you)(you)流到一(yi)(yi)個大(da)氣壓(ya)(ya)條(tiao)件(jian)(jian)下的(de)(de)開(kai)式(shi)(shi)油(you)(you)(you)箱，液(ye)壓(ya)(ya)泵靠自吸能力將油(you)(you)(you)箱中的(de)(de)油(you)(you)(you)液(ye)輸(shu)入液(ye)壓(ya)(ya)工(gong)作系(xi)統(tong)。閉式(shi)(shi)回(hui)(hui)路(lu)(lu)中液(ye)動(dong)機(ji)的(de)(de)回(hui)(hui)油(you)(you)(you)直(zhi)(zhi)接(jie)輸(shu)入液(ye)壓(ya)(ya)泵的(de)(de)吸油(you)(you)(you)口。

形成(cheng)(cheng)封閉(bi)的(de)回(hui)(hui)路(lu)(lu)。開式(shi)回(hui)(hui)路(lu)(lu)結構簡單，油液(ye)(ye)散熱條件(jian)好，但是它的(de)油箱體積較大(da)，空(kong)氣(qi)與油液(ye)(ye)的(de)接觸機(ji)會較多(duo)，因(yin)而容易(yi)混(hun)入空(kong)氣(qi)，使系統工作(zuo)不夠(gou)穩定。開式(shi)回(hui)(hui)路(lu)(lu)要求液(ye)(ye)壓(ya)(ya)泵有較好的(de)自吸(xi)能(neng)力(li)，對于自吸(xi)能(neng)力(li)較差的(de)柱塞泵等(deng)，需設置輔助液(ye)(ye)壓(ya)(ya)泵。閉(bi)式(shi)回(hui)(hui)路(lu)(lu)比開式(shi)回(hui)(hui)路(lu)(lu)效(xiao)(xiao)率高。一(yi)般(ban)開式(shi)回(hui)(hui)路(lu)(lu)的(de)換向(xiang)、調速由閥或泵閥聯合控(kong)制，壓(ya)(ya)力(li)過高時，多(duo)余(yu)的(de)油液(ye)(ye)自溢(yi)流(liu)閥流(liu)回(hui)(hui)油箱，造成(cheng)(cheng)效(xiao)(xiao)率損(sun)失(shi)。而閉(bi)式(shi)回(hui)(hui)路(lu)(lu)一(yi)般(ban)采用雙向(xiang)變(bian)量(liang)(liang)泵，通過改變(bian)變(bian)量(liang)(liang)泵的(de)輸(shu)出油液(ye)(ye)的(de)方(fang)(fang)向(xiang)和流(liu)量(liang)(liang)，控(kong)制液(ye)(ye)動機(ji)的(de)運動方(fang)(fang)向(xiang)和速度，回(hui)(hui)路(lu)(lu)中壓(ya)(ya)力(li)的(de)高低取(qu)決于負載的(de)大(da)小，因(yin)而沒有過剩(sheng)的(de)壓(ya)(ya)力(li)和多(duo)余(yu)的(de)流(liu)量(liang)(liang)，效(xiao)(xiao)率較高。所(suo)以，它很適用于功率大(da)，換向(xiang)頻繁(fan)的(de)液(ye)(ye)壓(ya)(ya)系統。在(zai)閉(bi)式(shi)回(hui)(hui)路(lu)(lu)中，一(yi)個(ge)主液(ye)(ye)壓(ya)(ya)泵只能(neng)供給一(yi)個(ge)執行元件(jian)。

不適用(yong)于多負載的(de)(de)(de)(de)系(xi)統(tong)。另外，為(wei)了補充回路(lu)中(zhong)(zhong)的(de)(de)(de)(de)流量損失，往(wang)往(wang)要(yao)增設輔助泵(beng)或補油泵(beng)，因(yin)而(er)系(xi)統(tong)比較(jiao)復雜。閉式回路(lu)油箱體(ti)積小，結構(gou)緊湊，污物和(he)空氣都不容(rong)易侵入系(xi)統(tong)，因(yin)而(er)運轉平(ping)穩。由于依(yi)靠液(ye)(ye)(ye)(ye)壓(ya)(ya)泵(beng)改變油液(ye)(ye)(ye)(ye)流動(dong)方(fang)向，所以換向沖(chong)擊較(jiao)小。但它的(de)(de)(de)(de)散熱(re)條件較(jiao)差，油溫容(rong)易升高。液(ye)(ye)(ye)(ye)壓(ya)(ya)系(xi)統(tong)的(de)(de)(de)(de)控(kong)(kong)(kong)(kong)制(zhi)(zhi)(zhi)可分為(wei)兩在類：開環(huan)(huan)控(kong)(kong)(kong)(kong)制(zhi)(zhi)(zhi)和(he)閉環(huan)(huan)控(kong)(kong)(kong)(kong)制(zhi)(zhi)(zhi)。開環(huan)(huan)控(kong)(kong)(kong)(kong)制(zhi)(zhi)(zhi)中(zhong)(zhong)，控(kong)(kong)(kong)(kong)制(zhi)(zhi)(zhi)元件——液(ye)(ye)(ye)(ye)壓(ya)(ya)閥接(jie)到(dao)主令信(xin)號(hao)(hao)后，單方(fang)向控(kong)(kong)(kong)(kong)制(zhi)(zhi)(zhi)執(zhi)(zhi)行(xing)元件——液(ye)(ye)(ye)(ye)動(dong)機(ji)(ji)的(de)(de)(de)(de)動(dong)作。而(er)液(ye)(ye)(ye)(ye)壓(ya)(ya)閥的(de)(de)(de)(de)動(dong)作并不受液(ye)(ye)(ye)(ye)動(dong)機(ji)(ji)動(dong)作的(de)(de)(de)(de)影響(xiang)（液(ye)(ye)(ye)(ye)動(dong)機(ji)(ji)行(xing)程中(zhong)(zhong)某一瞬間發出的(de)(de)(de)(de)信(xin)號(hao)(hao)除外，如行(xing)程終點撞限位開關等(deng)）。它用(yong)于一般液(ye)(ye)(ye)(ye)壓(ya)(ya)系(xi)統(tong)。閉環(huan)(huan)控(kong)(kong)(kong)(kong)制(zhi)(zhi)(zhi)中(zhong)(zhong)，當控(kong)(kong)(kong)(kong)制(zhi)(zhi)(zhi)元件——液(ye)(ye)(ye)(ye)壓(ya)(ya)閥接(jie)到(dao)主令信(xin)號(hao)(hao)后，一方(fang)面控(kong)(kong)(kong)(kong)制(zhi)(zhi)(zhi)執(zhi)(zhi)行(xing)元件液(ye)(ye)(ye)(ye)動(dong)機(ji)(ji)的(de)(de)(de)(de)動(dong)作，一方(fang)面隨時(shi)又接(jie)受液(ye)(ye)(ye)(ye)動(dong)機(ji)(ji)發出的(de)(de)(de)(de)反(fan)饋指(zhi)令信(xin)號(hao)(hao)的(de)(de)(de)(de)控(kong)(kong)(kong)(kong)制(zhi)(zhi)(zhi)。

使(shi)(shi)液(ye)(ye)動(dong)(dong)(dong)(dong)(dong)機(ji)能夠(gou)嚴格(ge)準(zhun)確地(di)按照指(zhi)令信號(hao)工(gong)作(zuo)。伺服系統一般為閉環控(kong)制(zhi)。當(dang)液(ye)(ye)動(dong)(dong)(dong)(dong)(dong)機(ji)運動(dong)(dong)(dong)(dong)(dong)到(dao)一定的行程(cheng)(cheng)(cheng)或位置時，觸(chu)動(dong)(dong)(dong)(dong)(dong)電(dian)器(qi)元(yuan)件(jian)（限(xian)位開(kai)關）或液(ye)(ye)壓(ya)(ya)(ya)(ya)元(yuan)件(jian)（行程(cheng)(cheng)(cheng)閥、機(ji)動(dong)(dong)(dong)(dong)(dong)換向閥等(deng)(deng)），然后開(kai)始(shi)下(xia)一個動(dong)(dong)(dong)(dong)(dong)作(zuo)，或使(shi)(shi)另(ling)一個液(ye)(ye)動(dong)(dong)(dong)(dong)(dong)機(ji)開(kai)始(shi)動(dong)(dong)(dong)(dong)(dong)作(zuo)，這(zhe)就是行程(cheng)(cheng)(cheng)控(kong)制(zhi)。它(ta)能直接保證運動(dong)(dong)(dong)(dong)(dong)部(bu)件(jian)的運動(dong)(dong)(dong)(dong)(dong)位置和行程(cheng)(cheng)(cheng)長度，有效地(di)控(kong)制(zhi)幾個運動(dong)(dong)(dong)(dong)(dong)部(bu)件(jian)之間(jian)的動(dong)(dong)(dong)(dong)(dong)作(zuo)順序(xu)。當(dang)液(ye)(ye)壓(ya)(ya)(ya)(ya)回路(lu)中某一部(bu)位壓(ya)(ya)(ya)(ya)力(li)(li)(li)升高到(dao)一定值時，促使(shi)(shi)壓(ya)(ya)(ya)(ya)力(li)(li)(li)繼(ji)電(dian)器(qi)、順序(xu)閥等(deng)(deng)元(yuan)件(jian)開(kai)始(shi)動(dong)(dong)(dong)(dong)(dong)作(zuo)或發出信號(hao)，借以控(kong)制(zhi)回路(lu)中其(qi)它(ta)元(yuan)件(jian)的啟(qi)閉和工(gong)作(zuo)，這(zhe)種依(yi)靠液(ye)(ye)壓(ya)(ya)(ya)(ya)力(li)(li)(li)自動(dong)(dong)(dong)(dong)(dong)控(kong)制(zhi)回路(lu)動(dong)(dong)(dong)(dong)(dong)作(zuo)的方法(fa)叫壓(ya)(ya)(ya)(ya)力(li)(li)(li)控(kong)制(zhi)。采(cai)用壓(ya)(ya)(ya)(ya)力(li)(li)(li)控(kong)制(zhi)時應充分考(kao)慮：工(gong)作(zuo)時壓(ya)(ya)(ya)(ya)力(li)(li)(li)控(kong)制(zhi)元(yuan)件(jian)所(suo)在(zai)位置的液(ye)(ye)壓(ya)(ya)(ya)(ya)力(li)(li)(li)是否(fou)足以將它(ta)打開(kai)；在(zai)不(bu)工(gong)作(zuo)時是否(fou)會因(yin)液(ye)(ye)壓(ya)(ya)(ya)(ya)沖擊等(deng)(deng)原因(yin)使(shi)(shi)它(ta)產生誤動(dong)(dong)(dong)(dong)(dong)作(zuo)；在(zai)工(gong)作(zuo)時是否(fou)會因(yin)為壓(ya)(ya)(ya)(ya)力(li)(li)(li)波動(dong)(dong)(dong)(dong)(dong)使(shi)(shi)壓(ya)(ya)(ya)(ya)力(li)(li)(li)控(kong)制(zhi)閥復位失靈。

因此(ci)，壓(ya)力控(kong)(kong)(kong)制(zhi)不如(ru)行程控(kong)(kong)(kong)制(zhi)可(ke)靠。但(dan)(dan)有(you)時(shi)(shi)(shi)它(ta)比行程控(kong)(kong)(kong)制(zhi)方便。液壓(ya)裝置(zhi)(zhi)工作(zuo)時(shi)(shi)(shi)往往需要(yao)控(kong)(kong)(kong)制(zhi)某一(yi)液動機(ji)幾(ji)(ji)個(ge)不同動作(zuo)的(de)(de)時(shi)(shi)(shi)間(jian)間(jian)隔，或(huo)幾(ji)(ji)個(ge)液動機(ji)之間(jian)動作(zuo)時(shi)(shi)(shi)間(jian)的(de)(de)間(jian)隔，它(ta)可(ke)以通過時(shi)(shi)(shi)間(jian)繼電(dian)器或(huo)延時(shi)(shi)(shi)閥來(lai)完成，這叫時(shi)(shi)(shi)間(jian)控(kong)(kong)(kong)制(zhi)。它(ta)只能用(yong)于(yu)一(yi)般時(shi)(shi)(shi)間(jian)要(yao)求不十分(fen)精確的(de)(de)場合。有(you)些液壓(ya)系(xi)統的(de)(de)執行元件的(de)(de)動作(zuo)時(shi)(shi)(shi)間(jian)、速度、頻率、位(wei)置(zhi)(zhi)和力直接(jie)依靠電(dian)氣指(zhi)令信號控(kong)(kong)(kong)制(zhi)，它(ta)一(yi)般用(yong)于(yu)要(yao)求精密的(de)(de)數(shu)字控(kong)(kong)(kong)制(zhi)和電(dian)液伺(si)服系(xi)統，它(ta)能嚴格按照(zhao)預先編好(hao)的(de)(de)程序進行工作(zuo)。但(dan)(dan)它(ta)的(de)(de)電(dian)氣系(xi)統比較復雜(za)。液壓(ya)系(xi)統中直接(jie)用(yong)于(yu)驅動液動機(ji)工作(zuo)的(de)(de)回(hui)路(lu)(lu)稱(cheng)為(wei)(wei)主回(hui)路(lu)(lu)，用(yong)于(yu)控(kong)(kong)(kong)制(zhi)泵(beng)閥動作(zuo)的(de)(de)回(hui)路(lu)(lu)稱(cheng)為(wei)(wei)控(kong)(kong)(kong)制(zhi)回(hui)路(lu)(lu)，控(kong)(kong)(kong)制(zhi)回(hui)路(lu)(lu)可(ke)分(fen)為(wei)(wei)壓(ya)力油(you)(you)控(kong)(kong)(kong)制(zhi)和回(hui)油(you)(you)控(kong)(kong)(kong)制(zhi)，如(ru)液動換向(xiang)閥的(de)(de)換向(xiang)為(wei)(wei)壓(ya)力油(you)(you)控(kong)(kong)(kong)制(zhi)，溢流閥遙控(kong)(kong)(kong)卸荷(he)為(wei)(wei)回(hui)油(you)(you)控(kong)(kong)(kong)制(zhi)。

所謂(wei)主(zhu)動(dong)(dong)力(li)(li)(li)就是(shi)直接用(yong)(yong)于(yu)(yu)驅動(dong)(dong)液(ye)動(dong)(dong)機工作(zuo)的(de)(de)(de)主(zhu)要動(dong)(dong)力(li)(li)(li)，它是(shi)液(ye)壓(ya)(ya)(ya)系(xi)(xi)統(tong)(tong)消(xiao)耗(hao)能量動(dong)(dong)力(li)(li)(li)裝置。它一(yi)(yi)般與溢流閥配合使(shi)用(yong)(yong)，使(shi)系(xi)(xi)統(tong)(tong)壓(ya)(ya)(ya)力(li)(li)(li)保(bao)(bao)持一(yi)(yi)定，當時(shi)，溢流閥打開，使(shi)壓(ya)(ya)(ya)力(li)(li)(li)油(you)(you)流回油(you)(you)箱(xiang)，保(bao)(bao)護液(ye)壓(ya)(ya)(ya)泵(beng)不受侵害(hai)。有時(shi)可(ke)在液(ye)壓(ya)(ya)(ya)泵(beng)出口加一(yi)(yi)個單(dan)向(xiang)(xiang)閥，防止(zhi)油(you)(you)液(ye)反向(xiang)(xiang)流動(dong)(dong)。可(ke)避免壓(ya)(ya)(ya)力(li)(li)(li)沖擊或系(xi)(xi)統(tong)(tong)中(zhong)其它液(ye)壓(ya)(ya)(ya)泵(beng)輸(shu)出高壓(ya)(ya)(ya)油(you)(you)對它的(de)(de)(de)影響。停(ting)泵(beng)后，它還可(ke)以(yi)防止(zhi)因負載(zai)的(de)(de)(de)作(zuo)用(yong)(yong)使(shi)液(ye)壓(ya)(ya)(ya)泵(beng)反轉(zhuan)。另(ling)外，由(you)于(yu)(yu)油(you)(you)液(ye)始終保(bao)(bao)持在單(dan)向(xiang)(xiang)閥以(yi)后的(de)(de)(de)回路中(zhong)，防止(zhi)空(kong)氣混入，所以(yi)增加了重新啟(qi)動(dong)(dong)的(de)(de)(de)平(ping)穩性。它采用(yong)(yong)限(xian)壓(ya)(ya)(ya)式變量泵(beng)或雙(shuang)向(xiang)(xiang)變量泵(beng)時(shi)一(yi)(yi)般可(ke)不用(yong)(yong)溢流閥調整它的(de)(de)(de)壓(ya)(ya)(ya)力(li)(li)(li)，因為它可(ke)隨負載(zai)的(de)(de)(de)變化自(zi)動(dong)(dong)調整輸(shu)出的(de)(de)(de)流量和壓(ya)(ya)(ya)力(li)(li)(li)。但是(shi)它往往需(xu)設置安(an)全閥，當系(xi)(xi)統(tong)(tong)時(shi)，可(ke)以(yi)從安(an)全閥卸油(you)(you)，從而(er)保(bao)(bao)護液(ye)壓(ya)(ya)(ya)泵(beng)以(yi)及系(xi)(xi)統(tong)(tong)不受損害(hai)。

兩個(ge)液(ye)(ye)(ye)(ye)壓(ya)(ya)(ya)泵(beng)(beng)(beng)(beng)同軸運轉，將壓(ya)(ya)(ya)油口聯接起來同時(shi)(shi)輸(shu)入液(ye)(ye)(ye)(ye)壓(ya)(ya)(ya)系(xi)統，這(zhe)樣可以(yi)增大(da)流(liu)(liu)量，但(dan)不能增加壓(ya)(ya)(ya)力。這(zhe)種方(fang)法(fa)只能同時(shi)(shi)使(shi)用，不能分(fen)泵(beng)(beng)(beng)(beng)供(gong)油。有上液(ye)(ye)(ye)(ye)壓(ya)(ya)(ya)系(xi)統除了設置(zhi)(zhi)工(gong)作(zuo)液(ye)(ye)(ye)(ye)壓(ya)(ya)(ya)泵(beng)(beng)(beng)(beng)外，還要設置(zhi)(zhi)一(yi)個(ge)備用液(ye)(ye)(ye)(ye)壓(ya)(ya)(ya)泵(beng)(beng)(beng)(beng)，以(yi)防止液(ye)(ye)(ye)(ye)壓(ya)(ya)(ya)泵(beng)(beng)(beng)(beng)發生(sheng)故障時(shi)(shi)液(ye)(ye)(ye)(ye)壓(ya)(ya)(ya)系(xi)統無法(fa)工(gong)作(zuo)而造成嚴(yan)重，如飛機的液(ye)(ye)(ye)(ye)壓(ya)(ya)(ya)系(xi)統。回路中設置(zhi)(zhi)兩個(ge)單(dan)向閥(fa)，以(yi)防工(gong)作(zuo)液(ye)(ye)(ye)(ye)壓(ya)(ya)(ya)泵(beng)(beng)(beng)(beng)輸(shu)出(chu)的液(ye)(ye)(ye)(ye)壓(ya)(ya)(ya)油流(liu)(liu)入不工(gong)作(zuo)臺的液(ye)(ye)(ye)(ye)壓(ya)(ya)(ya)泵(beng)(beng)(beng)(beng)中，使(shi)其反向轉動。是指根據情況，有時(shi)(shi)單(dan)泵(beng)(beng)(beng)(beng)供(gong)油，有時(shi)(shi)雙泵(beng)(beng)(beng)(beng)供(gong)油或多泵(beng)(beng)(beng)(beng)供(gong)油。協(xie)同供(gong)油可以(yi)提高液(ye)(ye)(ye)(ye)壓(ya)(ya)(ya)系(xi)統的效率，減少功率消耗。它由(you)高壓(ya)(ya)(ya)小流(liu)(liu)量泵(beng)(beng)(beng)(beng)低壓(ya)(ya)(ya)大(da)流(liu)(liu)量泵(beng)(beng)(beng)(beng)溢流(liu)(liu)閥(fa)單(dan)向閥(fa)4和卸荷閥5組成。液動機快(kuai)速(su)運動時兩個(ge)泵同時供油。工作行(xing)程時系統壓力升(sheng)高，打開(kai)卸荷閥(fa)5，低壓泵卸(xie)荷。同時關閉單向閥4。

此(ci)時只(zhi)有高(gao)壓泵向系(xi)統供油，它的供油壓力可(ke)由溢流閥3調整。三個定量(liang)泵的油量(liang)分別為Q1<Q2<Q3(Q3>Q1＋Q2),每個(ge)液壓(ya)泵(beng)都有一個(ge)二位三通換(huan)向(xiang)閥(fa)控制它輸(shu)出壓(ya)力油(you)(you)的(de)流(liu)向(xiang)，換(huan)向(xiang)閥(fa)處于圖示(shi)位置時(shi)，液壓(ya)泵(beng)卸荷。換(huan)向(xiang)閥(fa)切換(huan)到另一個(ge)位置時(shi)，壓(ya)力油(you)(you)輸(shu)入液壓(ya)系統(tong)。三個(ge)泵(beng)通過不同的(de)組合，可以使(shi)液壓(ya)系統(tong)得到七(qi)種不同的(de)流(liu)量，即(ji)QQQQ1＋QQ1＋Q３、Q2＋Q３、Q1＋Q2＋Q３，使液動機獲得七種(zhong)不(bu)(bu)同的(de)(de)運(yun)動速度。兩個(ge)(ge)液壓(ya)(ya)(ya)泵(beng)(beng)串(chuan)聯可以提高(gao)工作壓(ya)(ya)(ya)力。它(ta)的(de)(de)總壓(ya)(ya)(ya)力等于兩個(ge)(ge)液壓(ya)(ya)(ya)泵(beng)(beng)的(de)(de)壓(ya)(ya)(ya)力之和。但它(ta)的(de)(de)流量不(bu)(bu)能增加，為小容量泵(beng)(beng)的(de)(de)流量。有些液壓(ya)(ya)(ya)泵(beng)(beng)自吸(xi)能力較差(cha)，如柱塞泵(beng)(beng)等。為了解(jie)決吸(xi)油問題，往往設置一個(ge)(ge)自吸(xi)能力好的(de)(de)液壓(ya)(ya)(ya)泵(beng)(beng)與它(ta)串(chuan)聯。