<div><p>Water is inflammable, non-toxic, environmentally friendly---
desirable traits, for a hydraulic fluid. However, its extremely low viscosity
diminishes the load-bearing and sealing capacity of lubricating interfaces.
Case in point: axial piston machines of swash plate design are compact, highly
efficient positive displacement machines at the heart of hydraulic systems in
forestry, construction, aerospace, and agricultural equipment, as well as
industrial applications (presses, etc.); however, the three main lubricating
interfaces decisive to the performance of such units in terms of both component
life and efficiency are challenged by the use of water as working fluid.
Especially during high-pressure operation, this low-viscosity lubricant can
cause the these interfaces to fail in carrying the imposed load, resulting in
severe wear, or even pump failure. The piston-cylinder interface is
particularly challenging to design for water because it stands under obligation
to carry the heavy side load that acts on the pistons of these machines, which
increases with operating pressure. Furthermore, the architecture of axial
piston machines of swash plate design does not allow this interface to be
hydrostatically balanced.</p>
<p> </p>
<p>Through the development of a methodology that separates the
fluid pressure fields of the three main lubricating interfaces of axial piston
machines into their hydrostatic and hydrodynamic components, the present work
enables a direct comparison of these interfaces in terms of how they support
load. A case study of a 75 cc unit running on hydraulic oil conducted via this
methodology at three different operating conditions (low pressure/low speed,
low pressure/high speed, and high pressure/low speed) demonstrates that in the
piston-cylinder interface, the force from hydrostatic pressure reaches such
high magnitudes over the high-pressure stroke that less than half of it is
needed to counter the load. The excess force from hydrostatic pressure then
becomes the load. Consequentially, hydrodynamic pressure must counter a force
from hydrostatic pressure that exceeds the original load. In the other two
interfaces, by contrast, over half the load is being carried by hydrostatic pressure,
thus significantly diminishing the amount of hydrodynamic pressure the
interfaces are required to generate in order to achieve full load support.
Moreover, nearly all of the moment on the piston is countered by hydrodynamic
pressure, while less than half of the moment on the block is countered by
hydrodynamic pressure, and the moment on the slipper is negligible by
comparison.</p>
<p> </p>
<p>While this case study only investigates one pump, it shows
how critical hydrodynamic pressure can be to load support in the
piston-cylinder interface. The use of a low-viscosity fluid, e.g. water,
reduces the hydrodynamic pressure that is generated in this interface, which,
at challenging operating conditions, can lead to metal-to-metal contact. However,
the performance of the interface can be improved via micro surface shaping,
i.e. by giving the surface of the piston, or the bore that it moves through, a
shape on the order of microns in height. The aim of present work is to pursue
design trends leading to surface shapes that will enable this interface to
function at higher pressures than currently achievable. </p>
<p> </p>
<p>This pursuit takes the form of systematic virtual design
studies, an optimization procedure, and an algorithm developed specifically for
tailoring the bore surfaces through which the pistons travel to piston tilt and
deformation. From this emerges not only a set of design trends corresponding to
the dimensions of two particularly powerful types of micro surface shaping, but
also a profound insight into the behavior of the water-lubricated
piston-cylinder interface fluid film, and how that behavior can be manipulated
by changing the component surfaces that constitute its borders. Furthermore, in
collaboration with Danfoss High Pressure Pumps, a physical prototype of a 444
cc axial piston pump with surface shaping generated via the aforementioned
algorithm has been constructed and tested, achieving a total pump efficiency
roughly 3% higher than that achievable by the commercial unit that the geometry
of the prototype is based on.</p><br></div>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/13954325 |
| Date | 01 March 2021 |
| Creators | Meike H Ernst (10135868) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/thesis/Enabling_High-Pressure_Operation_with_Water_for_the_Piston-Cylinder_Interface_In_Axial_Piston_Machines/13954325 |
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