Floating Cup Pump (Part 5) — Theory Behind Out-of-Phase

Previously on iHydrostatics N21, the topics of why high efficiency and how better low speed behavior of Floating Cup were summarized. Based on the testing results and theoretical analysis, we can clearly explain the better performance theory. If you are interested in this post, please reply N21 in Wechat iHydrostatics to get this episode. Any questions are always welcomed, iEditor encourages you to leave your comments in iHydrostatics QQ Group: 426372753.


Out-of-Phase Design of Floating Cup [1]
From the picture above, the latest concept of floating cup is with out-of-phase design, which is the back to back kit has 15 degree phase offset. What benefit can we get from this design? What is the theory behind this out-of-phase?


1Out-of-Phase DesignActually, for the out-of-phase design of floating cup, two concpets were proposed from the very beginning to the current.
One concept is with giving the port plate planes an angular distance of 15 degrees along the main axis direction [2], while the piston on the both sides are aligned.


Off-setting the Port Plate by 15 Degrees [2]
The other concept is with the pistons on one side 15 degrees offset to the opposite side. In this way, the pistons are splitted into two and assembled separately.


Off-setting the Pistons by 15 Degrees [1]
In either design, the flow from the 12 pistons at one rotor side and the flow from the other side are joined in an internal manifold. If the ducts in both sides of the pump are identical and if the outlet of the pump is connected to the middle of the connecting duct, all odd harmonics in the 12 piston based flow from on side of the pump will be cancelled by the odd harmonics in the flow from the other side. [2]



The ducts Inside Floating Cup Pump [3]


2Theory BehindWhy do we need this out-of-phase design for floating cup? The main purpose for this is to reduce the flow pulsation and finally achieve the target of noise reduction.
Actually, the basic theory behind out-of-phase is very simple. We all know the output flow of piston pump is sine-wave like, if two sine-waves have a perfect phase offset, then the average of the flow shape is much smoother as shown below:


Tandem Pump Flow vs. Single Pump Flow
Theoretical calculation has also approved that half piston angular distance phase offset will significantly reduce the flow pulsation.


Theoretical Flow Pulsation Comparison between Single and Tandem [4]
While we also know that the real pump design and working conditions have much difference from theoretical assumptions. Will the real world also apply with this principle? Is it true that the half piston angular distance offset is the perfect phase offset? Simulation result from paper [5] has clearly approved this conclusion.



Output Flow Rate with Different Index Angle [5]

Index angle, which is the official name for phase offset, with different values were simulated to compare the influence to flow rate pulsation.
The simulation above was carried out at 280bar output pressure and 1500 rpm with a 9 piston tandem pump each side. 5 different index angles were simulated. [5]
From this simulation, the flow rate amplitudes are smaller with larger index angles. [5] And when index angle is 20 degrees, the amplitude is the smallest, which is exactly the half of single kit piston angular distance in a 9 piston pump.

REFERENCES[1] INNAS. Floating Cup Principle 2012. www.innas.com.
[2] G. E. M. Vael. Reducing Flow Pulsation with the Floating Cup Pump: Theoretical Analysis. Innas BV, Breda, The Netherlands.
[3] INNAS. Design Details. www.innas.com.
[4] Zhang Junhui. Study on Valve Plate Design and Insensitive Flow-Distribution Method of Axial Piston Pump. Ph.D thesis, Zhejiang University.
[5] Bing XU, Shao-gan YE. Effects of index angle on flow ripple of a tandem axial piston pump. Journal of Zehjiang University – SCIENCE A ISSN 1673-565X.

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