• Preface
  • gear pump
  • vane pump
  • plunger pump
  • Introduction to calculation examples hydropump
  • Calculating hydropump volume flow
  • Calculating required hydraulic pump power
  • Calculate the required power of the drive motor

The hydraulic pump (1) draws oil from the reservoir (2) and pumps the oil into the system. After the oil enters the return line via the control slide, the pressure relief valve or the cylinder, the oil flows back to the reservoir without pressure.

The hydropump in the picture is driven by an electric motor, which regulates a mechanical power in the form of a torque and a speed. The hydropump converts this to a hydraulic power. The pump delivery / volume flow depends on the speed and the stroke volume of the hydropump.

Hydropumps almost all work according to the displacement principle. The versions can be divided into:

  • gear pumps;
  • vane pumps;
  • plunger pumps.

The following paragraphs elaborate on this.

System overview hydraulic system

Gear Pump:
The gear pump is used in hydraulic systems with a low working pressure of up to 140 to 180 bar. Due to its simplicity, low cost price and reliable properties, the gear pump is one of the widely used hydraulic pumps that we find in hydraulic applications.

In the gear pump with external gears there are two gears that move in opposite directions of each other. One of the gears is externally driven and takes the other gear with it.

  • suction side: the tines spread apart on the left side. Due to the increase in volume in the cavities, an underpressure of approx. 0,1 to 0,2 bar is created, as a result of which oil is drawn in. The gear wheels transport the oil to the discharge side via their outer circumference;
  • pressure side: here the teeth interlock. The oil in the pressure pipe is displaced into the system.
    The pressure on the discharge side depends on the resistance encountered by the oil in the hydraulic circuit.
Gear pump external gears

The internal gear pump contains a crescent-shaped attachment. The inner (blue) gear is externally driven, taking along the outer (purple) ring with internal teeth in the indicated direction of rotation. As with the pump with external teeth, a negative pressure is created as soon as the space between the teeth increases. The pump thus draws oil from the reservoir. When the gears are twisted together, there is displacement of the oil to the system. The sickle-shaped attachment separates the suction and discharge sides.

With this type of hydropump a pressure of up to 300 bar can be reached. The pump has an even flow and produces very little noise.

Gear pump internal gears

Gear pumps always have a fixed displacement. The yield is constant at a constant drive speed. On the outer circumference of the gears, the tooth heads run close to the pump housing and provide the radial seal. In the center of the pump, where the gears mesh, there is also a certain seal between the tooth flanks and the bearing plate. A small amount of oil will always leak between the sealing surfaces.

The gear pump can be found in the following areas of application:

  • vehicle technology (including automatic transmission);
  • mechanical engineering;
  • agricultural hydraulics;
  • aircraft hydraulics.

vane pump:
The vane pump has a rotor with radially placed vanes. On the suction side (blue), the volume increases, which creates a negative pressure and oil is drawn in. On the pressure side (red), the volume decreases, overpressure is created and the oil is pressed into the pipe.

The rotor is placed eccentrically with respect to the impact ring, which allows the flow to be regulated:

  • In the image below we see on the left a pump with a yield of 0 cm³ per revolution. The pump then no longer supplies oil;
  • The image on the right shows the adjusted impact ring, achieving maximum yield.
Vane pump zero flow
Vane pump maximum flow

We find the vane pump in the following areas of application:

  • agricultural and road construction machines;
  • machine tools;
  • aviation hydraulics;
  • mobile hydraulics.

plunger pump:
The axial piston pump can be found in systems in which higher pressures occur (>250 bar) and greater powers are transferred because the efficiency of this type of hydropump is high. We distinguish the plunger pumps into radial and axial plunger pumps.

Axial plunger pump:
The input shaft of the axial piston pump drives a tilt plate. The tilting plate is at a certain angle and converts the rotary movement of the input shaft into a reciprocating movement of the plungers. The pump is equipped with suction ports and discharge valves, so that the direction of rotation of the input shaft has no influence on the direction of flow of the hydraulic oil.

By adjusting the angle at which the tilting plate is located, the stroke of the plungers can be influenced. The more inclined the tilting plate is, the greater the stroke of the plungers and the more oil is displaced. We encounter this technique at the air conditioning compressors.

The pictures below show the axial piston pump.

Axial plunger pump (1)
Axial plunger pump (2)

Radial plunger pump:
Radial plunger pumps are mainly found in heavy drives in ships, such as dredging installations, winch drives and agitators and in machine construction. These pumps have a short installation length, are suitable for high working pressures (700 bar) and deliver a high torque at a low speed.

The radial plunger pump in the following figure contains five radially arranged plungers in star shape relative to the drive shaft. Because the ring is eccentric, a radial plunger movement is created. A distribution disc that rotates with the drive shaft ensures that each cylinder is connected to the suction or pressure line at the right time.

Radial plunger pump

Introduction to hydropump calculation examples:
In order for the piston to move with the correct force and speed, the hydropump must provide sufficient pressure and fluid flow that is large enough. The greater the load that the cylinder has to operate, the higher the demands are placed on the hydropump.

Below are three paragraphs in which we will calculate the volume flow, the required pressure and the required power, taking into account the efficiency of the hydropump in the accompanying diagram.

  • pump stroke volume (V) = 15 cm³ / rev;
  • pump speed (n) = 1200 rpm;
  • system pressure: 50 bar.
Hydraulic diagram

Calculating the hydropump volume flow:
The amount of hydraulic oil displaced by a hydropump depends on the speed and displacement of the pump. The data is listed in the paragraph above.

In the formula, we convert RPM to seconds by dividing the number by 60. In the last step, we convert cubic meters per second to liters per minute by multiplying the answer by 60.000.

gear pump

Calculating required hydraulic pump power:
The hydropump must provide hydraulic power to transport fluid to the cylinder and move the piston. 

With the data from the paragraph “Introduction to hydropump calculation examples” and the answer from the previous paragraph, we can calculate the required power of the hydropump. For clarity, they are listed again here:

  • pump stroke volume (V) = 15 cm³ / rev;
  • pump speed (n) = 1200 rpm;
  • system pressure: 50 bar;
  • volume flow: 18 liters per minute.

We convert the system pressure of 50 bar to Pascal and the volume flow to cubic meters per second. We write this in scientific notation.

Calculate the required power of the drive motor:
The pump shaft (input shaft) supplies the mechanical power, which usually comes from an electric motor or combustion engine. The hydraulic motor converts the mechanical power into hydraulic power. Losses occur at all times when converting energy. The drive motor must therefore provide more power in order for the hydropump to deliver its required power.

In this example we assume a return of 90%.

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