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After deciding on which type of hydraulic drive to use, one can proceed to selecting the type of hydraulic pump. Gear pumps, vane pumps, screw pumps, and piston pumps are described in detail in terms of hydraulic pump classification and performance; their characteristics vary, as do their prices. We should determine the type of hydraulic pump based on the host type and operating conditions, power level, and the performance requirements of the system; at the same time, factors such as fixed-displacement or variable-displacement design, type of prime mover, speed, volumetric efficiency, overall efficiency, self-priming capability, and noise levels also need to be taken into consideration. It is necessary to carefully evaluate whether to use a fixed-displacement pump or a variable-displacement pump. Positive displacement pumps are simple and inexpensive, while variable displacement pumps are complex and costly, but they can save energy. Fixed-displacement pumps and variable-displacement pumps are suitable for different applications. Leaf pumps, axial piston pumps, and radial piston pumps come in both fixed-displacement and variable-displacement types. There are various forms of variable mechanisms for variable displacement pumps (especially axial variable displacement piston pumps). Control methods include manual control, internal pressure control, external pressure control, solenoid valve control, sequence valve control, electromagnetic proportional valve control, servo valve control, etc. Control outcomes include proportional variable, constant pressure variable, constant current variable, constant torque variable, constant power variable, load-sensing variable, etc. Not all variable displacement pumps have all the aforementioned variable and control methods. The choice of variable method should meet the requirements of the system; in practical use, it is necessary to understand the static and dynamic characteristics as well as the usage methods of these variable methods. Pumps of different types from various manufacturers have distinct characteristics regarding their variable mechanism; therefore, it is necessary to carefully consult the product catalog or user manual when making a selection. Hydraulic pumps can also be designed as multiple pumps – either of the same type or of different types – connected in parallel, using a single drive shaft; or they can be constructed as multi-stage pumps with the oil circuits connected in series. When the flow rate in a hydraulic system varies greatly over one working cycle, a multi-pump system can be used. A multi-pump usually has one oil suction port and multiple oil discharge ports; the pressurized oil from each discharge port can be supplied to different actuators in the system, or it can all be combined to supply a single actuator. When selecting the type and design of a hydraulic pump, other requirements of the system regarding the pump must also be taken into account, such as weight, price, service life, and reliability; the method of installing the pump; the way in which the pump is driven and how it is connected to the prime mover; the type of shaft extension used (cylindrical, conical with external threads, involute splines, etc.); whether the pump can withstand certain radial loads; and the type of connections for the oil inlets (threads, flanges, etc.). The choice of hydraulic pump type determines future operational efficiency; therefore, when deciding on a specific model and intending to make a purchase, it is important to select the right hydraulic pump supplier. One should avoid choosing a model only to end up purchasing the wrong product or a low-quality one – such a situation is quite frustrating for anyone. For more information, please visit: Rexroth hydraulic pumps