In hydraulic transmission system, hydraulic pumps are generally classified into gear pump, vane pump, plunger pump, screw pump and so on. According to the flow characteristics of hydraulic pump, hydraulic pump can be divided into quantitative pump and variable pump. For variable pump, according to the direction of oil transportation, it can be divided into unidirectional variable pump and bidirectional variable pump.

1. Pay attention to parallel pump and series pump. Gear pump and vane pump can also be made into several pumps connected together, and use the same drive shaft of the double pump or triple pump, which can also be connected into a multi-stage pump. When the flow rate of hydraulic system varies greatly in a working cycle, multiple pumps can be selected. Multi-pump usually has one oil suction port, multiple oil outlet, and the pressure oil of each outlet can supply oil to different executive components of the system, or it can supply a certain executive element together.

2. When selecting the model of hydraulic pump, we should also pay attention to other requirements of the system for hydraulic pump. For example, weight, price, service life and reliability, hydraulic pump installation mode, hydraulic pump and prime mover connection mode and hydraulic pump shaft extension form (flat key, spline) can bear a certain radial load, oil port connection form and so on.

3. Pumps and motors are general purpose. The pump as a motor is conditional. In some hydraulic systems, the same component (pump or motor) is required to run sometimes as a pump and sometimes as a motor. When selecting such components, it should be noted that, in principle, hydraulic motors can be used as pumps, but pumping as motors is conditional. For example, some gear pumps can only rotate unidirectional when they are used as motors. The hydraulic pump with one-way valve oil distribution can not be used as hydraulic motor and so on.