外嚙合齒輪泵的組成:前、后泵蓋，泵體，一對齒數、模數、齒形完全相同的漸開線外嚙合。
工作原理：
密封容積形成—齒輪、泵體內表面、前后泵蓋圍成齒輪退出嚙合,容積↑吸油
容積變化↓壓油
吸壓油口隔開—兩齒輪嚙合線及泵蓋外嚙合齒輪泵的流量計算
由于齒輪嚙合時，嚙合點的位置瞬時變化，其工作體積變化率不同，因此瞬時流量不均勻，即脈動。在計算瞬時流量時，必須準確計算積分，這很麻煩。近似計算法常用。
排量計算
假設：齒槽容積=輪齒體積
即相當于有效齒高和齒寬所構成的平面所掃過的環形體積，則V=πdhb=2πzm2b。
實際上，由于齒槽體積>輪齒體積，所以取出V=6.66zm2b流量計算
結論：
1齒輪泵的qt是齒輪幾何參數和轉速的函數
2∵轉速等于常數，流量等于常數 ∴定量泵
3理論流量與出口壓力無關瞬時流量
由于每對輪齒嚙合時嚙合點位置的變化會導致瞬時流量變化，因此會出現流量脈動
流動脈動結果：引起系統壓力脈動，產生振動和噪聲，影響傳動穩定性。
外嚙合齒輪泵結構要點：
困油及其消除措施
徑向作用力不平衡
泄漏困油現象產生原因：
由于為保證齒輪連續平穩運轉，又能夠使吸壓油口隔開，齒輪嚙合時的重合度必須大于1，所以有時會出現兩對輪齒同時嚙合的情況，故在齒向嚙合線間形成一個封閉容積
困油引起的結果：
a→b容積縮小p↑高壓油從一切可能泄漏的縫隙強行擠出，使軸和軸承受很大沖擊載荷，泵劇烈振動，同時無功損耗增大，油液發熱。b→c容積增大p↓形成局部真空，產生氣穴，引起振動、噪聲、汽蝕等
總之：由于困油現象，使泵工作性能不穩定，產生振動、噪聲等，直接影響泵的工作壽命。
消除困油的方法
原則：a→b密封容積減小，使之通壓油口，b→c密封容積增大，使之通吸油口，b密封容積最小，隔開吸壓油方法：在泵蓋（或軸承座）上開卸荷槽以消除困油，CB-B形泵將卸荷槽整個向吸油腔側平移一段距離，效果更好
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