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hydraulic radial piston pumps and motors. Hydraulic pump. Hydraulic pumps are used in hydraulic drive systems and can be hydrostatic or hydrodynamic.

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hydraulic pump
Hydraulic pump
  • Hydraulic pumps are used in hydraulic drive systems and can be hydrostatic or hydrodynamic.
  • Hydrostatic pumps are positive displacement pumps while hydrodynamic pumps can be fixed displacement pumps, in which the displacement (flow through the pump per rotation of the pump) cannot be adjusted, or variable displacement pumps, which have a more complicated construction that allows the displacement to be adjusted.
radial piston pump
Radial piston pump
  • Construction
  • A radial piston pump is an element of the hydraulic. At this pump the working pistons are in radial direction and symmetrical arranged around the drive shaft - in contrast to the axial piston pump. The stroke of each piston is caused by an eccentric drive shaft or an external eccentric tappet (e. g. stroke ring).When filling the workspace of the pumping pistons from "inside" (e. g. over a hollow shaft) so it's called a inside impinged (but outside braced) radial piston pump (picture 1). If the workspace is filled from „outside“ it's called an outside impinged radial piston pump (but inside braced) (picture 2)


  • The general mode of operation will be explained at the movement of one pumping piston by means of picture 1:The outer ring for bracing of the pumping pistons is in eccentrical position to the hollow shaft in the center. This eccentricity determines the stroke of the pumping piston.The piston starts in the inner dead center (IDC) with suction process. After a rotation angle of 180° it's finished and the workspace of the piston is filled with the to moved medium. The piston is now in the outer dead center (ODC). From this point on the piston displaces the previously sucked medium in the pressure channel of the pump.


  • Due to the hydrostatically balanced parts it's possible to use the pump with various hydraulic fluids like mineral oil, biodegradable oil, HFA (oil in water), HFC (water-glycol), HFD (synthetic ester) or cutting emulsion.That implies the following main applications for a radial piston pump:
  • machine tools (e. g. displace of cutting emulsion, supply for hydraulic equipment like cylinders)
  • high pressure units (HPU) (e. g. for overload protection of presses)
  • test rigs
  • automotive sector (e. g. automatic transmission, hydraulic suspension control in upper-class cars)
  • plastic- and powder injection moulding
  • wind energy
hydraulic motor
Hydraulic motor
  • A hydraulic motor is a mechanical actuator that converts hydraulic pressure and flow into torque and angular displacement (rotation). The hydraulic motor is the rotary counterpart of the hydraulic cylinder. Conceptually, a hydraulic motor should be interchangeable with a hydraulic pump because it performs the opposite function - much as the conceptual DC electric motor is interchangeable with a DC electrical generator. However, most hydraulic pumps cannot be used as hydraulic motors because they cannot be backdriven. Also, a hydraulic motor is usually designed for the working pressure at both sides of the motor.
  • Hydraulic pumps, motors, and cylinders can be combined into hydraulic drive systems. One or more hydraulic pumps, coupled to one or more hydraulic motors, constitutes a hydraulic transmission.
radial piston motors
Radial piston motors
  • Radial piston motors are available in two basic types. Crankshaft type (e.g. Staffa or SAI motors) with a single cam and pistons pushing inwards. This type of motor is basically an old design but is one which has extremely high starting torque characteristics. They are available in displacements from 40cc/rev up to about 12 litres/rev but can sometimes be limited in power output. Crankshaft type Radial Piston Motors are capable of running at "creep" speeds and some can run seamlessly up to 1500 rpm whilst offering virtually constant output Torque chacteristics. This makes them still the most versatile design.
  • Multilobe cam ring type (e.g. Hägglunds or Poclain type) has a cam ring with multiple lobes and the piston rollers push outwardly against the cam ring. This produces a very smooth output with high starting torque but they are often limited in the upper speed range. This type of motor is available in a very wide range from about 1 litre/rev to 250 litres/rev. These motors are particularly good on low speed applications and can develop very high power.