Floating Cup Pump (Part 3) — How to Float

Previously on iHydrostatics N19, more detailed design principle and assembly process of floating cup pump have been illustrated. If you are interested in this post, please reply N19 in Wechat iHydrostatics to get the previous episode. Any questions are always welcomed, iEditor encourages you to leave your comments in iHydrostatics QQ Group: 426372753.


Cross Section of Floating Cup Machine [1]
We have known what floating cup is, how it differs from slipper type and bent axis type machine, what kinds of parts are designed and how they are assembled together. Whole outline view of FC is established, from now on, iEditor will show you the theories behind this outline. In this post, we will discuss the theory of “Floating”, you’ll see the secret how to suppor this “Floating”.


1Why “Floating” Is NecessaryAs we know, in slipper type and bent axis type pumps, the cylinders are all part of a solid barrel: they can only move together with the barrel in the radial direction. The both pump pistons have a spherical joint at the opposite end of the piston sealing as shown below. [2]


Kinematical Principle of Current Axial Piston Machine [2]
In the floating cup principle, the pistons are locked onto the central rotor and their movement is confined to the rotation of the rotor and shaft. [2] Since so, there is no spherical joint degree of freedom for the pistons. Meanwhile because of the tilting angle of barrel, the piston is inclined to the cylinder cup. In order to accommodate the elliptical movement trace of cylinder cup, the cup has to have a degree of freedom to move on the barrel plate.

Above all, principle of floating cup was proposed as shown below.


Cross Section View of Floating Cup Principle [3]
Consequently the rotor takes all loads acting on the pistons. This creates the possibility to convert the hydraulic pressure forces directly into a driving torque on the drive shaft. [2]


Radial Force on the Piston to Create the Torque [4]


2Design for “Floating”Although the cylinder cup can be allowed to float on barrel plate, it also needs to be prevented from tipping.


Non-Tipping and Tipping Cup [5]
How to confine the “floating level” is related to the design principle. During the development of floating cup, two proposals have been proposed. In the first prototype of the floating cup machine, the cup was held down to the barrel with a holding plug and a positive hydrostatic balance force. In this form-closed construction, a minimal gap had to be allowed because of production tolerances. The seal land of the cup was balanced in such a way that when pressurized, the cup was hydrostatically pressed to the barrel. When the cup was connected to the low-pressure kidney however, it tilted off the barrel at high speeds until it was caught by the plug. [6]


First Prototype with Holding Plug [6]
An alternative construction has been proposed as well: the hold spring. The spring presses the cup on the barrel plate whether it is pressurized or not and there is no need for a hydrostatic force anymore. This force-closed construction is less sensitive to production tolerances than the form-closed plug alternative. [6]



Alternative Prototype with Holding Spring [5]


3Theory behind “Floating”We have talked a lot about the advantages of floating cup, but dear readers, you should know every advantage has its own theory behind itself.
– Much less contact force between piston and cylinder cup
As we know, for slipper type and bent axis type machine, the contact force between piston and cylinder is very high because either the cylinder or piston is driven to rotate by the other. High contact force causes high friction, wear and failure.
While for floating cup, cylinder cup is connected with barrel, piston is fixed on rotor. And barrel and rotor are separately driven by shaft, in this way no torque was transmitted between piston and cup. The only contact force is to drive cup to move a little bit on the barrel plate because of elliptical trace.


Contact between Piston and Cylinder Cup [7]
– Balanced Pressure Force Acting on the Cup
From the picture above, we can see that the contact line in the cup is perpendicularity to the cup inner bore surface no matter the tilting angle of the barrel or the position of the piston. Since so, the pressure force acting on the pump is naturally balanced radially and axially.


Balanced Force on Cylinder Cup [4]
– Balanced Pressure Force Acting on the Piston
Because the floating cup machine is back to back configured, so the axial force is balanced. For the radial force, since the pressure area is different at the top and bottom, so a radial resultant force is generated. This torque generated by this force is finally transmitted to the rotor through the fixed connection between piston and rotor.


Pressure Force Acting on Piston [2]






REFERENCES[1] O+P Zeitschrift Fluidtechnik 48 (2004).
[2] Peter A. J. Achten. Designing the Impossible Pump. Innas BV, Breda, The Netherlands.
[3] Peter A. J. Achten. Efficiency and Low Speed Behavior of the Floating Cup Pump. SAE 2004-01-2053.
[4] INNAS. Efficiency and Start-up Torque. www.innas.com.
[5] INNAS. Floating Cup Design Details. www.innas.com
[6] T. L. van den Brink, Peter A. J. Achten. Containing the Cup in the Floating Cup Axial Piston Machine. www.innas.com
[7] Peter A. J. Achten. Design and Testing of an Axial Piston Pump Based on the Floating Cup Principle. SICFP’03, May 7-9 2003.

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[3]. This post reprint should be noted with “iHydrostatics Original”.