动力元件的作用是将原动机的机械能转换成液体的压力能，指液压系统中的油泵，它向整个液压系统提供动力。液压泵的结构形式一般有齿轮泵、叶片泵和柱塞泵。
The function of the power element is to convert the mechanical energy of the prime mover into the pressure energy of the liquid. It refers to the oil pump in the hydraulic system, which supplies power to the whole hydraulic system. The structure of hydraulic pump generally includes gear pump, vane pump and plunger pump.
执行元件(如液压缸和液压马达)的作用是将液体的压力能转换为机械能，驱动负载作直线往复运动或回转运动。
The function of actuator (such as hydraulic cylinder and hydraulic motor) is to convert the pressure energy of liquid into mechanical energy, and drive the load to make linear reciprocating motion or rotary motion.