<p>The valve plate/cylinder block interface in an axial piston pump is often subject to extreme pressures, which can cause wear of the valve plate and ultimately, failure of the pump. The purposes of this study were to: a) experimentally investigate the film thickness generated between a floating valve plate and cylinder block in situ using proximity probes, b) develop a model which can predict the motion, film thickness and pressures of the floating valve plate and corroborate with experimental results, c) investigate surface pockets to provide additional lubricant at the valve plate interface by measuring the flow velocities and cavitation areas in a thrust washer bearing, d) numerically investigate surface modifications of the floating valve plate to observe any changes in lubricant pressure, temperature, cavitation, or valve plate deformation. Two different test rigs were designed, developed and used to investigate the performance of axial piston pumps and surface pockets. The axial piston pump test rig (APTR) was designed to operate and measure the steady state conditions of an axial piston pump. The APTR utilizes three non-contact proximity probes to measure the valve plate motion and film thickness between the cylinder block at various speeds and pressures. A thrust washer test rig (TWTR) was developed to measure the cavitation areas and flow velocities of lubricant in a pocketed thrust washer using μPIV. Through a novel interpolation approach, the depths of the micro-particles in the bearing pocket were determined using an analytical model. Using this approach, the μPIV measured 2D velocity field was employed to develop a 3D velocity field, which illustrates the fluid motion inside a pocketed thrust bearing at various speeds and viscosities. A dynamic lubrication model was developed using the thermal Reynolds equation augmented with the JFO boundary condition and the energy equation to determine the pressure, cavitation regions and temperature of the lubricant at the valve plate cylinder block interface. The lubricating pressures were then coupled with the equations of motion of the floating valve plate to develop a dynamic lubrication model. The stiffness and damping coefficients of the floating valve plate system used in the dynamic lubrication model were determined using a parametric study. The elastic deformation of the valve plate was also considered using the influence coefficient matrix approach. The experimental and analytical motion of the valve plate were then corroborated and found to be in good agreement. 4 and 8 pocket designs were then added as surface modifications to the floating valve plate in the dynamic lubrication model. The addition of surface modifications improved the lubricating conditions at the valve plate/cylinder block interface and resulted in increased minimum film thicknesses and lowered lubricant temperatures at the same operating conditions.</p>

<p>This thesis investigates the effects of swashplate vibration and slipper surface geometry on the performance of the slipper-swashplate interface. The lubricating interfaces within a swashplate type axial piston machine are the most complicated part of the design process. These interfaces are supposed to provide support to the significant loads they experience during operation and to prevent continuous contact of the sliding surfaces. Therefore a proper slipper-swashplate interface design ensures full film lubrication during operation and provides sufficient load support while minimizing viscous and volumetric losses at the same time. The effects of two factors on the performance of the slipper-swashplate are examined during this work; swashplate vibration and slipper surface micro-geometry. An already existing model of the slipper-swashplate interface was used to carry out the results for this work however some modifications were made to the model to suit the needs of this research. Swashplate vibration is a phenomenon that has not been implemented in the model before, therefore its effects on the performance of the interface were analyzed. Thickness of the fluid film in the lubricating regime corresponds with its performance and is directly affected by the micro-geometry of the sliding interfaces. Therefore the effects of slipper surface micro-geometry is crucial to study in order to find the optimal slipper-swashplate interface design.</p>

Overall losses in swash plate type axial piston machines are mainly defined by three tribological interfaces. These are swash plate/slipper, piston/cylinder and cylinder block/valve plate. Within a research project, funded by the German Research Foundation, a combined approach of experimental research and simulation is chosen to acquire further knowledge on the cylinder block/valve plate contact. The experimental investigations focus on the friction torque within the contact and the measurement of the cylinder block movement in all six degrees of freedom. Simultaneously a simulation model is created focusing on the main physical effects. By considering the results of the experimental investigations significant physical effects for the simulation model are assessed. Within this paper a first comparison between experimental results and the simulation is presented, showing that for a qualitative match the implemented effects (mainly the fluid film, solid body movement, solid body contact, surface deformation) are sufficient to model the general behaviour of theinvestigated pump.

The cylinder block/valve plate interface is a critical design element of axial piston machines. In the past, extensive work has been done at Maha Fluid Power Research center to model this interface were a novel fluid structure thermal interaction model was developed which accounts for thermal and elasto-hydrodynamic effects and has been proven to give an accurate prediction of the fluid film thickness. This paper presents an in-depth investigation of the impact of the elastic deformation due to pressure and thermal loadings of the end case/housing on the performance of the cylinder block/valve plate interface. This research seeks to understand in a systematic manner the sensitivity of the cylinder block/valve plate interface to the structural design and material properties. A comparison between simulations results is done by utilizing different end case designs and material compositions, both in the valveplate and end case solids.

Markets show a clear trend towards an ever more extensive electronic networking in mobile and stationary applications. This requires a certain degree of electronic integration of hydraulic components such as axial piston pumps. Beside some wellknow approaches, the transmission of axial piston units still is relatively unexplored regarding electronification. Nonetheless there is a quite high potential to be optimized by electronic. In view of this fact, the present paper deals with the tribological contacts of pumps based on a demand driven hydrostatic relief. The contact areas at cylinder - distributor plate, cradle bearing and slipper - swash plate will be investigated in detail and it will be shown how the pump behavior can be improved considerably through a higher level of relief and a central remaining force ratio. The potential of optimization is to improve the efficiency, especially in partial loaded operation, power range, also for multi quadrant operation, precision and stability. A stable lubricating film for slow-speed running and for very high speeds at different pressures is ensured as well.

The present study investigates the f ormation of tribolayers on bronze CuSn12Ni2. Two different test rigs are used, of which one is a sliding bearing test rig in order to perform lubricated thrust bearing tests. Bronze CuSn12Ni2 is used for the sliding elements and the counter body is made of C45 steel. In addition to that, an axial piston pump test rig was used to determine t he transfera bility of the results to th e axial pist on pump. The test conditions are set up in a way t hat the tribological load s in the contacts are similar to each other. Changes in the subsurfa ce morphology and the chemical composition of the tribolayer were analysed using electron pro be micro a nalysis (EPMA), trans mission electron microscopy (TEM), energy dispersive X -ray spectro scopy (EDS) and X-ra y photoelectron spectroscopy (XPS). Focused ion beam (FIB) milling was used to prepare site -specific TE M foils fro m the wear track. The formation of a nano scale tribolayer was associat ed with red uced wear, which leads to low leak age in the a xial piston pump. This tribolayer is enriched with oxygen, sulfur and zinc, which is an effect of tribochemical reactions of environment molecules and surface molecules.

Axial piston machines of the swashplate type are commonly used in various hydraulic systems and with recent developments in displacement control, it is essential to maximize their efficiency further reducing operation costs as well as improving performance and reliability. This paper reports findings of a research study conducted for the piston-cylinder interface utilizing a novel fluid structure thermal interaction model considering solid body deformation due to thermal and pressure effects in order to accurately predict the transient fluid film within the gap. A large reduction in energy dissipation is possible due to reduced clearances allowable due to the surface shaping of the piston resulting in a reduction in leakage. From this study, it is shown that surface shaping of the piston in combination with a reduced clearance is not only beneficial by improving the efficiency of a machine, but also increases the reliability and the performance of the machine as the load support is enhanced.

<p>A mechanical model of an axial piston-type machine with a so-called wobble plate and Z-shaft mechanism is presented. The overall aim is to design and construct an oil-free piston expander demonstrator as a first step to realizing an advanced and compact small-scale steam engine system. The benefits of a small steam engine are negligible NOx emissions (due to continuous, low-temperature combustion), no gearbox needed, fuel flexibility (e.g., can run on biofuel and solar), high part-load efficiency, and low noise. Piston expanders, compared with turbines or clearance-sealed rotary displacement machines, have higher mechanical losses but lower leakage losses, much better part-load efficiency, and for many applications a more favourable (i.e., lower) speed. A piston expander is thus feasible for directly propelling small systems in the vehicular power range. An axial piston machine with minimized contact pressures and sliding velocities, and with properly selected construction materials for steam/water lubrication, should enable completely oil-free operation. An oil-free piston machine also has potential for other applications, for example, as a refrigerant (e.g., CO₂) expander in a low-temperature Rankine cycle or as a refrigerant compressor.</p><p> </p><p>An analytical rigid-body kinematics and inverse dynamics model of the machine is presented. The kinematical analysis generates the resulting motion of the integral parts of the machine, fully parameterized. Inverse dynamics is applied when the system motion is completely known, and the method yields required external and internal forces and torques. The analytical model made use of the “Sophia” plug-in developed by Lesser for the simple derivation of rotational matrices relating different coordinate systems and for vector differentiation. Numerical solutions were computed in MATLAB. The results indicate a large load bearing in the conical contact surface between the mechanism’s wobble plate and engine block. The lateral force between piston and cylinder is small compared with that of a comparable machine with a conventional crank mechanism.</p><p> </p><p>This study aims to predict contact loads and sliding velocities in the component interfaces. Such data are needed for bearing and component dimensioning and for selecting materials and coatings. Predicted contact loads together with contact geometries can also be used as input for tribological rig testing. Results from the model have been used to dimension the integral parts, bearings and materials of a physical demonstrator of the super-critical steam expander application as well as in component design and concept studies.</p>

Axial piston machines are widely used in the industry ranging from aerospace, agriculture, automotive, heavy machinery, etc. These applications require better pumps and motors to meet current market demands such as higher power density in hydraulic units, smarter pumps (diagnostics and prognostics), higher efficiencies, and compactness. The current state-of-the-art in pump design is mostly based on heuristic design approach with very limited use of numerical toolssince the invention of this positive displacement machine until the present time. The numerical tools being used do not capture the physical phenomena in the thin fluid film between the rotating group components. The work presented in this dissertation aims to demonstrate the feasibility of virtual prototyping utilizing a combination of in-house developed multi-domain models and to propose a novel computational based design methodology for axial piston machines. The methodology is an iterative process between the virtual components in 3D CAD models and the function evaluations for the design requirements utilizing the numerical models which provide an accurate prediction to the behavior of the mechanical components working together. To validate the proposed methodology a case study on a 24 cc/rev axial piston machine was carried out. The machine was built virtually, simulated,and optimized for desired performance. A physical prototype was built based on the case study and tested successfullyfor forty-five operating conditions.

<p>Condition Monitoring is an area that has seen substantial growth in the last few decades. The purpose for implementing condition monitoring in industry is to increase productivity, decrease maintenance costs and increase safety. Therefore, condition monitoring can be used not only for planning maintenance but also for allowing the selection of the most efficient equipment to minimize operating costs. </p><p>Hydraulic systems are widely used in industry, aerospace and agriculture and are becoming more complex in construction and in function. Reliability of the systems must be supported by an efficient maintenance scheme. Due to component wear or failure, some system parameters may change causing abnormal behaviour in each component or in the overall circuit. Research in this area has been substantial, and includes specialized studies on artificial fault simulation at the University of Saskatchewan. In this research, an axial pump was the focus of the study. In an axial piston pump, wear between the various faces of components can occur in many parts of the unit. As a consequence, leakage can occur in locations such as between the valve plate and barrel, the drive shaft and oil wiper, the control piston and piston guide, and the swash plate and slippers. In this study, wear (and hence leakage) between the pistons and cylinder bores in the barrel was of interest. Researchers at the University of Saskatchewan, as well as at other research institutions, have been involved in studies to detect wear in pumps using a variety of condition monitoring algorithms. However, to verify the reliability and indeed, limitations of some of the approaches, it is necessary to test the algorithms on systems with real leakage. To introduce actual wear in the piston of pumps can be very difficult and very expensive. Hence, introducing piston wear in an artificial manner would be of great benefit in the evaluation of various condition monitoring techniques.</p><p>Since leakage is a direct consequence of piston wear, it is logical to conclude that varying the leakage in some prescribed manner can be used to artificially simulate wear. A prime concern, therefore, is to be able to precisely understand the dynamic relationships between the wear and leakage and the effect it has on the output flow or pressure waveform from the pump.</p><p>Introducing an artificial leakage to simulate the wear of pistons is a complex task. The creation of an artificial leakage path was not simply a process of providing a resistive short to the tank at the outlet of the pump port as was done in other studies. The objective was to create a leakage environment that would simulate leakage from a single piston (or combination of several pistons thereof). The complexity of the flow and pressure ripple waveforms (which various condition monitoring algorithms did require) was such that a more comprehensive leakage behaviour had to be modeled and experimentally created. A pressure control servo valve with a very high frequency response was employed to divert the flow from the pump outlet with a prescribed waveform directly to the tank to simulate the piston leakage from the high pressure discharge chamber to the pump case drain chamber as the simulated worn piston made contact with the high pressure chamber. The control algorithm could mimic the action of a single worn piston at various degrees of wear. The experimental results indicated that the experimental system could successfully introduce artificial leakage into the pump which was quite consistent with a unit with a real worn piston. Comparisons of the pressure ripples from an actual faulty pump (worn piston) and the artificial faulty pump (artificial leakage) are presented.</p>