外啮合齿轮泵的组成:前、后泵盖，泵体，一对齿数、模数、齿形完全相同的渐开线外啮合。
工作原理：
密封容积形成—齿轮、泵体内表面、前后泵盖围成齿轮退出啮合,容积↑吸油
容积变化↓压油
吸压油口隔开—两齿轮啮合线及泵盖外啮合齿轮泵的流量计算
由于齿轮啮合时，啮合点的位置瞬时变化，其工作体积变化率不同，因此瞬时流量不均匀，即脉动。在计算瞬时流量时，必须准确计算积分，这很麻烦。近似计算法常用。
排量计算
假设：齿槽容积=轮齿体积
即相当于有效齿高和齿宽所构成的平面所扫过的环形体积，则V=πdhb=2πzm2b。
实际上，由于齿槽体积>轮齿体积，所以取出V=6.66zm2b流量计算
结论：
1齿轮泵的qt是齿轮几何参数和转速的函数
2∵转速等于常数，流量等于常数 ∴定量泵
3理论流量与出口压力无关瞬时流量
由于每对轮齿啮合时啮合点位置的变化会导致瞬时流量变化，因此会出现流量脉动
流动脉动结果：引起系统压力脉动，产生振动和噪声，影响传动稳定性。
外啮合齿轮泵结构要点：
困油及其消除措施
径向作用力不平衡
泄漏困油现象产生原因：
由于为保证齿轮连续平稳运转，又能够使吸压油口隔开，齿轮啮合时的重合度必须大于1，所以有时会出现两对轮齿同时啮合的情况，故在齿向啮合线间形成一个封闭容积
困油引起的结果：
a→b容积缩小p↑高压油从一切可能泄漏的缝隙强行挤出，使轴和轴承受很大冲击载荷，泵剧烈振动，同时无功损耗增大，油液发热。b→c容积增大p↓形成局部真空，产生气穴，引起振动、噪声、汽蚀等
总之：由于困油现象，使泵工作性能不稳定，产生振动、噪声等，直接影响泵的工作寿命。
消除困油的方法
原则：a→b密封容积减小，使之通压油口，b→c密封容积增大，使之通吸油口，b密封容积最小，隔开吸压油方法：在泵盖（或轴承座）上开卸荷槽以消除困油，CB-B形泵将卸荷槽整个向吸油腔侧平移一段距离，效果更好
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