液压泵的定义：依靠密闭工作容积的改变实现吸、压液体，从而将机械能转换为液压能的机器。
常用液压泵的种类：
1、按流量是否可调节可分为：变量泵和定量泵。
输出流量可以根据需要来调节的'称为变量泵，流量不能调节的称为定量泵。
2、按液压系统中常用的泵结构分为：齿轮泵、叶片泵和柱塞泵3种。
齿轮泵：体积较小，结构较简单，对油的清洁度要求不严，价格较便宜；但泵轴受不平衡力，磨损严重，泄漏较大。
叶片泵：分为双作用叶片泵和单作用叶片泵。这种泵流量均匀、运转平稳、噪音小、作压力和容积效率比齿轮泵高、结构比齿轮泵复杂。
柱塞泵：容积效率高、泄漏小、可在高压下工作、大多用於大功率液压系统；但结构复杂，材料和加工精度要求高、价格贵、对油的清洁度要求高。
Hydraulic pump definition: rely on the change of sealed working volume to achieve suction, pressure liquid, so as to convert mechanical energy into hydraulic energy machine.
Types of commonly used hydraulic pumps:
1, according to whether the flow can be adjusted can be divided into: variable pump and quantitative pump.
The output flow can be adjusted according to the need 'is called variable pump, the flow can not be adjusted is called fixed pump.
2. According to the pump structure commonly used in the hydraulic system, it is divided into gear pump, vane pump and plunger pump.
Gear pump: small volume, simple structure, the cleanliness of oil requirements are not strict, the price is cheaper; But the pump shaft by unbalanced force, serious wear, large leakage.
Vane pump: divided into double - acting vane pump and single - acting vane pump. This pump flow uniform, smooth operation, low noise, pressure and volume efficiency than gear pump, structure than gear pump complex.
Plunger pump: high volume efficiency, small leakage, can work under high pressure, mostly used for high-power hydraulic system; But the structure is complex, the material and the processing precision requirements are high, the price is expensive, and the oil cleanliness requirements are high.