Axial pumps with cam-driven commutation units — so-called PWK pumps — emerged as a result of a research project conducted in the Department of Hydraulics and Pneumatics at the Gdansk University of Technology. As for all axial hydraulic piston pumps, several cylinder chambers are positioned around the rotating shaft of an axial pump with cam-driven commutation units — called PWK pumps. The rotation of the shaft and the attached swash plate leads to movement of the pistons that alternately decreases and increases the fluid volume of the chambers. A window — which is part of the control sleeve or commutating bushing — connects the chamber between the pistons with the low-pressure and highpressure intake and outtake channels.

Figure 1. Variable displacement pump with four chambers (visible).
The axial piston pumps with constant displacement show a very good performance with a working pressure of up to 55 MPa, an overall efficiency of 94%, and good power density. In constant displacement pumps, the pistons always travel the same distance inside the cylinder chamber, resulting in a constant amount of output fluid.

To control the fluid amount, variable displacement pumps are used. Usually, the piston displacement of these pumps is controlled with a complicated hydraulic servomechanism. Theoretical analysis shows that the displacement of the PWK pump can be controlled by a low-energy actuator. This is the major advantage of the new developed pumps because it reduces the pump’s cost and dimension significantly, thereby simplifying the entire process chain.

Figure 2. CAD models of a pump with two chambers (left) and seven chambers (right). The left model is a symmetric half model. All movable parts are shown in red.
To steer the displacement of the pump, and thereby the amount of fluid that is put through, the angular position of the control cam in relation to the pump’s shaft has to be controlled. As both parts are rotating fast, a special planetary gearbox that ensures precise control over the displacement in the operating range was developed. This can be driven by a stepping motor.

First prototypes of the variable displacement PWK pumps have been built and tested, and showed good performance. Unfortunately, harmful pressure peaks that could lead to pump damage were observed. With the help of a compensation chamber, these peaks could be significantly reduced. To find the optimal layout of the pump and the compensation chamber, experiments and numerical simulation were used.

Pressure Peaks

First tests of the control mechanism for the variable displacement pump confirmed the feasibility of the concept. However, harmful pressure peaks in the cylinder chambers were observed when the cylinder chamber is disconnected from the inlet and the outlet channel. The pressure reaches very high values — 20 MPa above the average pressure in the chamber — for a short time.

These peaks are influenced by many different factors. The most important ones are the displacement adjustment, the rotational speed, and leakage. When the displacement of the pump is decreased, the pressure peaks increase. This is due to the higher velocity of the pistons at the moment when the chamber is disconnected from the intake and outtake channels. A higher rotational speed of the shaft also leads to higher pressure peak values; mainly, because it reduces the effect of leakage. Oil leakage reduces the amount of fluid to be compressed and thereby decreases pressure peak value. For pumps with high rotation numbers, the piston movement is fast, shortening the time during which the fluid is compressed and reducing the influence of leakage. This increases the pressure peak values.

Figure 3. Abaqus model of the wall of the compensation chamber with boundary conditions.
The compressibility of the hydraulic oil is directly connected to the occurring leakage and thereby also plays an important role in the investigation of pressure peaks. Leakage is influenced significantly by the pressure of the fluid. The small gaps in the pump through which small amounts of fluid can leave the pump cycle get considerably larger during pressure increase. This aggravates the leakage, which in turn, influences the pressure, resulting in a very complicated feedback.