Pump Types

Different types of hydraulic pumps and how they work

External Gear Pump

The external gear pump consists of two intermeshing gears. One of the two gears is driven by a shaft. The second gear wheel is pulled along via the toothing.
When the wheels engage, the tooth gap is closed by the opposite tooth. Negative pressure occurs when a tooth emerges from the gap. This free space sucks in oil. With the further rotary movement, the liquid is transported via the tooth gaps to the pressure side.

Hydraulics - External Gear Pump - how it works

Hydraulics - External Gear Pump - how it works

Characteristics:

  • robust, small, loud,
  • Squeezing oil due to the meshing of the gears
  • Pressure up to approx. 350 bar, the higher the pressure, the greater the pulsation.
  • is one of the constant pumps

Gear pump with Internal Theeth

Pump usually with two gear wheels, of which the outer (ring gear) is internally toothed. The sprocket running inside is smaller. The fluid is conveyed in the spaces between the tooth gaps of the two gear wheels.

Internal gear pumps have very low flow pulsation due to a large meshing length and backlash-free running of the gearing and therefore operate quietly.

Internal Gear Pump - how it works

Internal Gear Pump - how it works

With axial backlash compensation they achieve medium pressures up to approx. 160 bar and with combined axial and radial backlash compensation on pinion and ring gear 300 to 320 bar. Their displacement volume is not adjustable (fixed displacement pump).

Axial Piston Pump

In these pumps, the displacement bodies are not arranged radially, but parallel to the axis of rotation of a cylinder drum. The conversion of the drive movement from the engine to the stroke movement of the pistons differs depending on the design. There are two principles:

Swashplate Principle:

Axial Piston Pump (Swashplate Principle) - how it works

Axial Piston Pump (Swashplate Principle) - how it works

In a cylinder barrel there are up to eleven pistons which are mounted on a fixed inclined plane. The barrel sits firmly on the drive shaft and receives a rotating movement from it. The pistons slide on the inclined plane and perform a double stroke per revolution of the drum.

During the suction phase, the pistons move out of the barrel. Liquid flows into the piston chamber via the control plate with control slots. As the rotation continues, the pistons on the opposite side move back into the drum. The liquid is thus displaced to the pressure port via a further control slot.

How can the displacement be changed?

The stroke volume increases with increasing inclination (swivel angle). For example, if the disc is perpendicular to the drive shaft (swivel angle = 0°), the pistons do not perform a stroke and there is therefore no conveying. If the swivel is in the opposite direction, the conveying and suction directions are reversed ( = reversing operation).

Bent Axis Design:

Axial Piston Pump (Bent Axis Design) - how it works

Axial Piston Pump (Bent Axis Design) - how it works

In contrast to the swash plate design, the stroke movement of the pistons is achieved by the inclined position of the cylinder drum to the drive shaft.

Features of axial pumps:

  • Pressures up to approx. 400 bar
  • Large flow rates up to 3000 l/min at 1500 min-1
  • Can be offered adjustable

Radial Piston Pump

Radial piston pump with external pressure - how it works

Radial piston pump with external pressure - how it works

In contrast to the axial piston pump, the working pistons of the Radial Piston Pump are arranged radially and star-shaped to the drive shaft. The conveying or lifting movement of each individual working piston/displacer is caused by an eccentric located on the pump shaft or an external eccentric.

There are two types of construction: A radial piston pump pressurized from the outside when the working chamber is filled from "the outside" and the radial piston pump pressurized from "the inside" when the cylinders are filled from the inside (via a hollow shaft).

Radial pumps are also available as variable displacement pumps. The eccentricity can be changed so that different piston strokes result.

Characteristics:

  • Operating pressure of 700 bar and more
  • Flow rates relatively small - approx. 30 l/min at 1500 min-1

Rotary Vane Pumps

Rotary Vane Pump - how it works

Rotary Vane Pump - how it works

Vane pumps have a cylindrical rotor, which is provided with slots. In the slots there are blades which can move radially to the rotor. The rotor with the movable blades rotates in a housing with a cylindrical bore. Along the circumference of the rotor, a gap of different width results. This enlarges and widens the chambers as the rotor rotates.

The suction process ends when the largest cell volume is reached. As the rotor continues to rotate, the chambers become smaller again.  The liquid is displaced via the pressure port P. The liquid is then forced out of the pressure port.

Vane pumps can be designed as variable displacement pumps.

For a better understanding watch the following animation:

Rotary Vane Pump - how it works

Rotary Vane Pump - how it works

Screw Pumps

Screw pumps consist of two, three or more rotating, intermeshing screw spindles. The spindles are designed with right-hand or left-hand thread. The oil is pushed forward in the axial direction towards the pressure side without pulsation and with a constant displacement space.

This means that screw pumps operate almost pulsation-free compared to gear and piston pumps. The thread form represents a favorable hydraulic load and therefore low operating noise and smooth running even with large volume flows.

Screw pump - how it works

Screw pump - how it works

However, volume adjustment is not possible. The pressure range is mainly in the lower range (below 60 bar) with a delivery volume of up to several thousand l/min. However, screw pumps up to 200 bar and small delivery volumes are also built, e.g. for lifts.

Further applications of screw pumps:

  • Circulation of oil in hydraulic systems
  • Filling and emptying of tanks
  • Conveying in stationary or mobile oil supply systems
  • Power hydraulics (presses, machine tools, rollers in processing machines, injection moulding machines, tilting devices, elevators, variable pitch propellers, hydraulic winches)
  • Food (chocolate, syrup, vegetable oils)
  • Refinery and petrochemical industry
  • And so on