This study comprises testing of RoHS-compliant axial sliding bearing materials, including bronzes, brasses,
thermally sprayed coatings and PVD coatings, in a pin-on-disc tribometer and bench testing in an axial piston
pump. The aim was to compare and benchmark these materials against commonly utilized leaded bronze with
respect to durability and tribological mechanisms and to derive principles for axial sliding bearing material
suitability in hydrostatic components. By evaluating the test results, some fundamental understanding was gained
about characteristics which materials must exhibit to achieve sufficient tribological performance and durability in
hydrostatic components including, but not limited to resistance against friction-induced material transformation
and sufficient ductility to withstand pressure-induced part deflection.

Axial piston pumps are universal displacement machines that are used in a vast variety of applications. Their
high pressure resistance and ease of operation make them very popular, especially in mobile applications and
aerospace. The lifetime of axial piston pumps is depending on the design of the rotating kit, the application and
its overall robustness to external loads. The fluid film between the moving parts is responsible for bearing the
loads and sealing the displacement chambers. Its design is the most complicated part for a pump designer. All
pumps to this date have been designed in a trial and error process, which is not only costly, but doesn’t yield an
optimum in terms of efficiency and robustness. This paper aims to investigate the influence the fluid film has on
the lifetime of the pump. From the three main lubricating interfaces of an axial piston pump, two - cylinder block
/ valve plate and slipper / swash plate - were analysed in terms of temperature for both interfaces and gap height
for slipper only by means of measurements and simulation. The simulation tool that was used for the analysis is
called Caspar FSTI, developed by Purdue University. It is capable of calculating the resulting gap height
between all three main interfaces resulting from force balance calculations between external and fluid film forces
due to the pressure distribution in the gap and taking both deformation due to temperature and pressure into
account. The simulation will be used to understand the behaviour of the pump while the measurements will give
a deeper look into the actual steady state conditions as well as transient behaviour of the parts including wear in.

The rotational speed of slipper type, axial piston pumps and motors is limited. One of the most important reasons
for this limitation is the barrel tipping torque, which is (amongst others) affected by the centrifugal forces of the
pistons. The force of the barrel spring is needed to overcome the tipping of the barrel, and thus preventing the
malfunction of the pump or motor. The hydrostatic pressure can create an additional hydrostatic force, pushing
the barrel to the port plate, and thereby preventing the barrel to tip. But, at low operating pressures, the
hydrostatic force is insufficient, and the tipping torque can only be counteracted by the central barrel spring. Due
to the limited strength of this spring, the barrel will tip above a certain operating speed. At that point, the face
seal of the barrel will no longer make a full contact with the port plate, and the pump or motor cannot any longer
be operated, due to excessive leakage and wear.
Floating cup (FC) pumps and motors have the advantage that the pistons are press-fitted into the central rotor.
Therefore, the pistons can’t create anymore any tipping torque load on the barrel. But, unlike in conventional
axial piston designs, in FC-machines, the cylinders are no longer integrated and machined inside the cylinder
block or barrel. Instead, they are separated and have become cup-like cylinders which are floating on, and
supported by the remaining barrel plate. Being isolated from the barrel itself, these ‘cups’ will create another
tipping torque load on the barrel. The weight of the cups is however small, much smaller than the weight of the
pistons in an equally sized slipper type machine. Furthermore, the centrifugal tipping torque is reduced by the
short cup-stroke in the floating cup machine.
Nevertheless, it is desirable to further reduce the mass of the cups, not only to increase the maximum operating
speed, but also to reduce the force of the central barrel spring, which would further increase the overall
efficiency. The best way to reduce the mass of the cups, is by means of a reduction of the wall thickness of the
cups. It needs to be considered that the cups expand when being pressurized. Therefore, in order to maintain a
tight sealing between the cups and the pistons, the piston crown needs to expand as well, thereby following the
radial expansion of the cup. Consequently, the wall thickness of the piston crown needs to be reduced as well.
The question is whether these conditions can be met when the wall thickness of the piston crown and cup wall is
reduced. In this paper, the effects of a 50% reduction of the wall thickness are investigated and described. The
deformation is calculated by means of FEM analysis of the piston and the cup.

During flight, passenger comfort is affected by noise emissions from various aircraft systems. Apart from jet engines one of the main sources of noise within the fuselage is the power control unit (PCU) for high-lift actuation. In preparation for take-off and landing this hydraulic motor is responsible for the extension and retraction of the slats and flaps. Along with the increase in operating pressure from 206bar (3,000psi) to 345bar (5,000psi) noise and vibration induced by fluid power systems became more striking. Consequently the aim of the BMWI founded research project “Move On” was to reduce the emissions of Liebherr’s power control unit. The results of these research activities are presented within this paper. It is shown how the noise emissions could be reduced in a secondary controlled hydraulic motor by means of a valve-plate and structure optimization. In addition the results of a noise measurement campaign, conducted by Airbus on an A350, are presented.