What is Hydraulic Pump? (Function, Types, Application＆Selection Method)

Figure 1: Hydraulic system diagram of loader.

The basic hydraulic system components include oil tank, hydraulic pump, control valve, oil cylinder, connecting pipe and relief valve. So what is a hydraulic pump? What is the role of a pump?
A hydraulic pump is the power component of the hydraulic system. It is driven by an engine or an electric motor (so called electric hydraulic pump), sucks oil from the hydraulic oil tank, makes it be pressure oil and discharges it, and then sends it to the actuator.

1. What is Function of Hydraulic Pump?

The function of the hydraulic pump is to provide pressure oil to the hydraulic system. The hydraulic pump is an energy conversion device that converts the mechanical energy input by the prime mover (such as an electric motor) into liquid pressure energy. It is a power element in the hydraulic transmission system, and is an important part of this system.

2. Types of Hydraulic Pumps

Hydraulic pumps can be divided to the following 4 types according the different structures,
(1) Hydraulic gear pump (external gear pump and internal gear pump)
(2) Hydraulic vane pump (single-acting vane pump and double-acting vane pump)
(3) Hydraulic piston pump (axial piston pump and radial piston pump)
(4) Hydraulic screw pump (single screw pump, twin screw pump, three screw pump and five screw pump)

According to the different displacement, it can be divided into: variable displacement pump and fixed displacement pump. The output flow of the pump can be adjusted according to the needs which is called a variable displacement pump, and its flow can not be adjusted called a fixed displacement pump.

Table 1: Types of hydraulic motor.

3. Where are Hydraulic Pumps Used?

3.1 Gear Pump

Hydraulic gear pump features simple structure, low price, reliable work, convenient maintenance, good adaptability to the impact load, and small inertia of the rotating part.
But its bearings bear a large load and wear quickly. Compared with the vane pump and the piston pump, its efficiency is the lowest.
Hydraulic gear pumps are mostly used in machine tools, construction machinery, mining machinery, and agricultural machinery.

Figure 2: Hydraulic gear pump diagram.

3.2 Vane Pump

Hydraulic vane pump has compact structure, small size, stable operation, uniform flow, low pulsation and noise, and long service life. Its efficiency is generally higher than that of the gear pump, and the price is lower than that of the piston pump.

Hydraulic vane pumps with small and medium flow are often used in throttling regulation systems, and those with large flow are generally only used in non-regulated hydraulic systems in order to avoid excessive power loss.
Vane pumps are mostly used in hydraulic systems of machine tools, hydraulic presses, vehicles, construction machinery and plastic injection machines.

Figure 3: Hydraulic vane pump diagram.

3.3 Piston Pump

Hydraulic piston pump also has compact structure, long service life, low noise, high pressure, large flow rate, and large power ratio per unit weight. It can realize the adjustment of flow rate and the change of flow direction easily.

However, it has complex structure, high price, and high requirements on material and processing accuracy, and the cleanliness of the oil.
Piston pumps, especially axial piston pumps, are widely used in high-power hydraulic systems that require high pressure, large flow and need to be adjusted.

Figure 4: Hydraulic piston pump diagram.

3.4 Screw Pump

Hydraulic screw pump is essentially a gear pump, which is characterized by simple structure, light weight, small pulsation of flow and pressure, uniform delivery, no turbulence, no agitation, and few bubbles produced, reliable operation, and low noise. It has higher stable operation than gear pump and vane pump and high volumetric efficiency and suction lift.

But its processing is more difficult and its flow cannot be changed. It is suitable for hydraulic transmission systems of machine tools or precision machinery. Generally, two-screw or three-screw pumps are used, and vertical and horizontal installation methods are available. In general, marine screw pumps adopt a vertical installation method.

Figure 5: Hydraulic screw pump diagram.

Piston pumps are generally used when gear pumps and vane pumps cannot meet the requirements. The application of the screw pump is not as common as the other 3 above.

4. Introduction to Performance Parameters of Hydraulic Pumps

1. Hydraulic pump pressure

The working pressure of the hydraulic pump refers to the oil pressure of the pump outputting (or inputting) during actual work, which is determined by the external load.
The rated pressure refers to the highest pressure that the pump can run continuously according to the test standard under normal working conditions. Its value is limited by the pump life. If the pump speed exceeds the rated pressure, its life will be shorter than the designed. When the working pressure of a pump is greater than the rated pressure, it is called overload pressure.

2. Speed

The working speed refers to the actual rotation speed of the pump when it is working.
The rated speed refers to the highest speed that the pump can run normally for a long time under the rated pressure. If the pump speed exceeds the rated speed, it will cause insufficient oil absorption, generation of vibration and loud noise, cavitation damage of pump parts and short service life.
The minimum stable speed refers to the minimum speed allowed for the normal operation of the hydraulic pump motor. At this speed, the hydraulic pump motor does not appear crawling phenomenon.

3. Displacement, flow rate

Displacement refers to the volume of liquid discharged (or input) obtained by the volume change of the sealed chamber for each revolution of the pump (or motor). The common unit is ml/r (ml/revolution). That the displacement can be changed by adjustment is called a variable pump (or variable motor), while that it can not be changed is called a quantitative pump (or quantitative motor).

Actual flow refers to the flow at the outlet (or at the inlet) when the pump (or motor) is working. Due to the internal leakage of the pump itself, the actual flow rate is less than the theoretical flow rate. In order to achieve the specified speed and compensate for the leakage, the input actual flow rate must be greater than the theoretical flow rate.

Figure 6: Ultra-high pressure electric hydraulic oil pump.

4. Efficiency

Volumetric efficiency, for hydraulic pumps, refers to the ratio of its actual flow to the theoretical flow. For hydraulic motors, it refers to the ratio of its theoretical flow to actual flow.
Mechanical efficiency, for hydraulic pumps, refers to the ratio of its theoretical torque to the actual input torque. The actual output torque of the hydraulic motor is the torque after the theoretical torque overcomes the friction, so its mechanical efficiency is the ratio of the actual output torque to the theoretical torque.
Overall efficiency refers to the ratio of the output power of the pump (or motor) to the input power. The overall efficiency is equal to the product of the volumetric efficiency and the mechanical efficiency.

5. Selection of Hydraulic Pump

Select the hydraulic pump according to the working pressure:
Hydraulic piston pump up to 31.5mpa;
Hydraulic vane pump 6.3mpa, high pressure up to 31.5mpa;
Hydraulic gear pump 2.5mpa, high pressure up to 25mpa.

Select the hydraulic pump according to whether the variable displacement is required; if the variable is required, you can choose single-purpose vane pump, axial piston pump and radial piston pump.

Select the pump according to the environment. The gear pump has the best anti-pollution ability.

Select pumps according to noise. Low-noise hydraulic pumps include internal gear pump, double-acting vane pump and screw pump.

Select the pump according to the efficiency. The total power of the axial piston pump is the highest. Among hydraulic pumps with the same structure, the one with the larger displacement has highest efficiency, and among those with the same displacement, the axial piston pump has the highest total efficiency under the rated operation.

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