Hydraulic Pumps：Types and Pros ＆ Cons

Figure 1: Rexroth hydraulic piston pump.

A hydraulic pump is the power component of the hydraulic system, which mainly plays the role of pressurizing the liquid, and is widely used in the metallurgical industry, the power sector, and offensive and defensive construction. What types of hydraulic pumps are there? What are the advantages and disadvantages of each?

1. Types of Hydraulic Pumps

There are many ways to classify the hydraulic pump:
According to the structure, it is divided into gear pump, vane pump and piston pump;
According to the pressure, it is divided into low-pressure pump, medium-pressure pump, high-pressure pump and ultra-high-pressure pump;
According to the displacement, it is divided into quantitative pump and variable pump;
According to the direction of the oil, it is divided into one-way pump and two-way pump.

The advantages and disadvantages of three common hydraulic pumps are introduced below, including gear hydraulic pumps, piston hydraulic pumps, and rotary vane pumps.

Figure 2: Graphical symbols of hydraulic motors.

2. Advantages and Disadvantages of 3 Common Hydraulic Pumps

Table 1: 3 common hydraulic pumps.

2.1 Gear Hydraulic Pump

Hydraulic gear pumps are the most commonly used pumps in hydraulic systems, with very few moving parts, through the meshing and separation of two gear teeth to change the working volume.
The hydraulic gear pump is a fixed displacement pump and a one-way pump. These pumps have a constant flow rate, and their working pressure is generally between 50 and 210 bar. The operating speed of the gear pump is the highest of all pumps, up to 3000-6000rpm.

There are two types of hydraulic gear pumps: external gear pumps and internal gear pumps.

External gear pump:
In an external gear pump, only one gear, i.e. the drive gear, is connected to the drive. The other gear, i.e. the driven gear, rotates in the opposite direction, so the teeth of the rotating gear engage with each other.

There are also double external gear pumps, which combine two gear pumps driven by the same coupling shaft. The advantage of a double external gear pump is that it can power two independent hydraulic circuits, or provide more flow to one circuit.

Internal gear pump:
There is an eccentric internal gear and an external gear, also called a crown gear. When the external gear rotates, a volume is created between the gear profile and the external gear teeth attached to the gearbox wall. When the two gears disengage, the medium is sucked away.
Internal gear pumps operate very quietly and provide continuous flow.

Figure 3: Internal gear pump diagram.

Advantages of gear pumps:
1. Simple structure, good manufacturability and low cost.
2. Compared with other types of hydraulic pumps with the same flow rate, the structure is compact and the volume is small.
3. Good self-priming performance. It can reliably achieve self-priming no matter at high or low speed or even in manual conditions.
4. Large speed range. Because the transmission part of the pump and the gears are basically balanced, there will be no large inertial force generated at high speed.
5. The dirt in the oil has no serious influence on its work, and it is not easy to be stuck.

Disadvantages of gear pumps:

1. Low working pressure.
2. Low volumetric efficiency.
3. Large leakage and the existence of radial unbalanced force.
4. High noise. Because the meshing of gear teeth causes large output flow pulsation of the gear pump, it results in large pressure pulsation to make pipes, valves, etc., generate vibration.

2.2 Piston Hydraulic Pump

Hydraulic piston pumps are operated by pistons that move back and forth continuously and can handle very large flows at high pressure. They have good sealing performance, and can operate at high pressure or ultra-high pressure with low fluid leakage. Piston pumps are divided into radial piston pumps and axial piston pumps according to the arrangement of the pistons.

The radial piston pump has a large external size and uses a distribution shaft for distributing oil. The distribution shaft bears unbalanced force, so the pump working pressure is limited. The radial piston pump can be a variable displacement pump or a bidirectional pump.

The axial piston pump features small size, easy control, small leakage and high efficiency. It can be either a fixed displacement pump or a variable displacement pump. There are two types of axial piston pump, swash plate type and bent axis type, both of which change the working volume of the piston through the change of the included angle, thereby changing the displacement. Subject to the constraints of the variable mechanism, they are generally one-way pumps.

Figure 4: 3D drawing of an axial piston pump.

Advantages of hydraulic piston pump:
1. Providing the best overall volumetric efficiency.
2. Providing the highest pressure.
3. Reliable performance
4. High power density.
5. Two models available: variable displacement and fixed displacement.

Disadvantages of hydraulic piston pump:
1. The most expensive hydraulic pump.
2. More complex structure and large number of parts.
3. Poor self-priming.
4. The manufacturing process requirements are higher and the cost is more expensive.
5. It is more sensitive to oil pollution, and has higher requirements for filtration precision and use and maintenance.

2.3 Rotary Vane Pump

Vane type hydraulic pump has rectangular vanes fit in radial grooves in the rotor. The hydraulic vane pump has a large displacement and a high working pressure, belonging to a medium and low-pressure pump. Vane pumps are divided into single-acting vane pumps and double-acting vane pumps.

The double-acting vane pump is a fixed displacement pump and a one-way pump; the single-acting vane pump is a variable displacement pump, and it can change the eccentric direction of the rotor and the stator to change the direction of oil in and out. Therefore, the single-acting pump is also a bidirectional pump.

Advantage of hydraulic vane pump:
1. Usually providing better volumetric efficiency than gear pumps.
2.Uniform flow, stable operation and low noise. It can produce less noise while maintaining high speeds (up to 3000 rpm).
3. The radial hydraulic pressure on the rotor is balanced with each other. The bearing has a long service life and good durability.
4. The volumetric efficiency is high, up to more than 95%.
5. High working pressure. At present, the working pressure of the double-acting vane pump is 6.86~10.3mpa, sometimes up to 20.6mpa.
6. Compact structure, small size and large displacement.
7. Two models available: variable displacement and fixed displacement to choose from.
8. When the hydraulic pump is variable displacement type, the flow can be reduced if necessary, thereby reducing energy consumption.

Figure 5: Working principle diagram of single acting vane pump.

1—Drive shaft; 2—Rotor; 3—Stator; 4—Rectangular vane; 5—Shell; 6—Distribution plate

Disadvantage of hydraulic vane pump:
1. Rotary vane pumps are more expensive than gear pumps (and cheaper than piston pumps).
2. They are also fragile because the blades are subject to bending stress and discharge pressure.
3. The vanes are easy to be stuck, with poor working reliability, and they are sensitive to oil pollution, so the working environment is required to be clean, and the oil must be strictly filtered.
4. The structure is more complicated than that of the gear pump, and the requirements on manufacturing precision of the parts are higher.

5. The reliable speed of oil suction is required to be in the range of 8.3-25r/s. If the rotation speed is lower than 8.3rls, due to insufficient centrifugal force, the vanes cannot be closely attached to the inner surface of the stator, and a well-sealed closed volume cannot be formed, so that it cannot absorb oil.
If the rotation speed is too high and the oil suction speed is too fast, cavitation will occur. As thus, the oil will not be sucked, or the oil suction will be discontinuous.

The above is a detailed explanation of the advantages and disadvantages of the three common hydraulic pumps, including hydraulic gear pumps, hydraulic piston pumps, and hydraulic vane pumps.

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