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Thrust 1 - Efficiency

Title

1B.1: New Material Combinations and Surface Shapes for the Main Tribological-Systems of Piston Machines

Project Leader

Prof. Monika Ivantysynova (Purdue)

Statement of Project Goals

The goal of this project is to discover the impact of material combinations and advanced surface shaping on the reduction of energy dissipation and the increase of load carrying ability of the lubricating gaps of axial piston machines. While studying the role of material properties in combination with gap micro geometry through a fully-coupled fluid-structure-thermal and multi-body dynamics simulation model for the piston cylinder interface a better understanding of the complex physical phenomena characterizing lubricating gaps performance will be generated and used to propose new design solutions. The research will also extend the new piston-cylinder model to the other two main interfaces of axial piston machines - the slipper/swash plate interface and the cylinder block valve plate interface. This will allow studying advanced material combinations and unique surface shapes for the main tribological systems of axial piston machines.

Project's Role in Support of the Strategic Plan

The project primarily addresses the efficiency barrier by providing a deeper understanding of axial piston machines lubricating gaps behavior through the analysis of the impact of novel material properties and structured surface designs, finding new ways to drastically reduce energy dissipation. The design optimization will be carried out using a computer model based approach which couples together for the first time the main machine lubricating gaps considering the main physical effects. Piston pumps form the heart of energy saving displacement controlled hydraulic systems and hydraulic hybrids. Both new system concepts have been proposed and developed in the CCEFP to drastically reduce energy consumption of current hydraulic systems in the transportation sector and other applications. After replacing throttling valves, the pumps and motors represent the main source of losses of these new hydraulic systems. The reduction of power loss of pumps and motors will also help to increase system pressure and to increase compactness of fluid power systems. The low efficiency and the lack of compactness are barriers for a breakthrough of hydraulic hybrids into automotive transmissions. 

Description and Explanation of Research Approach

Primary Problem and Challenge Statement
Swash plate type axial piston machines are widely used today in industry. The hydraulic systems in which these machines are placed require the units to operate under a wide range of operating conditions, necessitated by system performance requirements.  Unfortunately, at the present time, there is only limited range of operating conditions where these machines are highly efficient. The sealing and bearing gaps separating the movable parts of the rotating group (piston, slipper, and cylinder block) form the most critical design element of piston machines. These gaps, as illustrated in Fig. 1, determine the achievable machine performance (speed, pressure, and maximum swash plate angle) and overall efficiency. The energy dissipated in these sealing and bearing gaps is very significant and in fact represents up to 90% of entire machine loss at low swash plate angle and up to 60% at maximum swash plate angles. The main problem is to gain an understanding of the physical effects taking place in the gaps and find ways to reduce the energy dissipated by introducing new design features. The development of physical models which can be used to predict the energy dissipated in the gaps is fundamental to propose better gap designs, including novel material combinations and shaped surfaces. These innovative designs will lead to better machine performance and increased efficiency especially at low swash plate angles. 

Lubricating gaps in axial piston machine

Figure 1: Lubricating gaps in an axial piston machine

The physical effects occurring in the gap are complicated and interact with each other. To suggest improvements in gap design either through material combinations or surface shaping, the physical effects influencing the fluid film behavior must be accurately captured by the model. These effects include:

  • Dynamic pressure in the fluid due to the hydrostatic and hydrodynamic effects
  • Micro and macro motion due to dynamic loading and machine kinematics
  • Deformation of the solid bodies due to the pressure load from the fluid
  • Heat generation in the fluid due to the viscous shearing
  • Heating of the solid bodies due to heat transfer from the fluid
  • Thermal deformation of the solid bodies due to their heating

Each of these aforementioned physical effects must be captured by the pump model, and moreover the interaction between effects must be captured as well. This creates a very complicated transient load fluid-structure-thermal solid body dynamics problem. As complicated as the problem may be, it is imperative to that the physical effects are captured to predict the effect new design modifications will have.

State of the Art
Due to very small gap heights the flow in lubricating gaps of piston machines can be assumed to be laminar. By assuming an incompressible Newtonian fluid, neglecting inertia forces and the change of pressure with the gap height as well as the derivative of fluid velocity in direction of gap length and breadth, and assuming an ideal roughness of surfaces, the Reynolds equation can be used for description of laminar flow in narrow gaps. Many computer programs and solution techniques for studying hydrostatic pumps lubricating gaps have been developed such as Dowd and Barwell (1974); Harris et al. (1993) and Tanaka et al. (1999). These works neglect the effects of significant physical phenomena influencing lubricating gap performance.

The program CASPAR, developed by Wieczorek and Ivantysynova (2002), is based on a non-isothermal gap flow model and considers the dynamic secondary motion of all moveable parts of a swash plate axial piston machine.  This program, however, did not represent a fully coupled model considering simultaneously the impact of non negligible physical phenomena on the lubricating gap performance, i.e. surface elastic deformations and heat transfer. One of these phenomena is known as elastohydrodynamic lubrication and, in fact, the local surface elastic deformation of mechanical parts subjected to high operating pressures strongly influences the behavior and ultimately the efficiency of machines that are designed to operate with extremely low gap heights. Among all the various kinds of fluid machines, the field of hydraulic axial piston machines recently started to include EHD effects in different numerical models, introducing with a progressively higher level of detail the complex problem of fluid-structure interaction. Huang and Ivantysynova (2006) proposed models for the lubricating gaps of axial piston machines considering the EHD phenomenon.  This work included the piston/cylinder interface and a special test rig was built, as presented by Ivantysynova et al. (2005), to measure the pressure and temperature fields in the tribological pair and validate the numerical results. However, none of these latter numerical studies proposed a complete integrated finite element model of the solid parts, communicating with a laminar gap flow model based on finite volumes. In fact, the use of external FEM software to predict the surface elastic deformation of the solid parts due to pressure loading introduced significant limitations to the flexibility and accuracy of the model. Furthermore, none of the studies included heat transfer phenomena trough the solid parts and their impact on the fluid boundary conditions.

References

Pelosi, M. and Ivantysynova, M. 2010. A Simulation Study on the Impact of Material Properties on Piston/Cylinder Lubricating Gap Performance. Proc. of the 6th FPNI PhD Symposium, West Lafayette, USA, pp 373 - 386.

Pelosi, M., Zecchi, M., and Ivantysynova, M. 2010. A fully-coupled thermo-elastic model for the rotating kit of axial piston machines. Bath ASME Symposium on Fluid Power and Motion Control. pp 217 - 234.

Pelosi, M. and Ivantysynova, M. 2009a. A Novel Fluid-structure Interaction Model for Lubricating Gaps of Piston Machines. Proceedings of the Fifth Fluid Structure Interaction Conference, eds. C.A. Brebbia, WIT Press, Southampton, pp.13-24, 2009

Pelosi, M. and Ivantysynova, M. 2008. A New Fluid-Structure Interaction Model for the Slipper- Swashplate Interface. Proc. of the 5th FPNI PhD Symposium, Cracow, Poland, pp 219 - 236.

Pelosi, M. and Ivantysynova, M. 2009b. A Novel Thermal Model for the Piston/Cylinder Interface of Piston Machines. Bath ASME Symposium on Fluid Power and Motion Control (FPMC2009), [DSCC2009- 2782].

Ivantysynova, M. and Baker, J. 2009. Power Loss in the Lubricating Gap Between Cylinder Block and Valve Plate of Swash Plate Type Axial Piston Machines. International Journal of Fluid Power, Vol. 10, No. 2, pp. 29 - 43

Baker, J. and Ivantysynova, M. 2009b. Advanced surface design for reducing power losses in axial piston machines. Proceedings 11th Scandinavian International Conference on Fluid Power, SICFP'09, June 2-4, 2009, Linköping, Sweden

Tanaka, K., Kyogoku and K. Nakahara, T. 1999. Lubrication characteristics on sliding surfaces between piston and cylinder in a piston pump and motor (Effects of running-in, profile of piston top and stiffness). JSME International Journal, Series C (Mechanical Systems, Machine Elements and Manufacturing), 42(4), pp. 1031-1040.

Wieczorek, U. and Ivantysynova, M. 2000. CASPAR - A Computer Aided Design Tool for Axial Piston Machines. Bath Workshop on Power transmission and Motion Control PTMC, Bath, UK , pp. 113-126.

Wieczorek, U. and Ivantysynova, M. 2002. Computer aided optimization of bearing and sealing gaps in hydrostatic machines - the simulation tool CASPAR. International Journal of Fluid Power. Vol. 3 (2002) No. 1, pp. 7-20.

Olems, L. 2000. Investigation of the temperature behaviour of the piston cylinder assembly in axial piston pumps. International Journal of Fluid Power, Vol. 1 (2000), No.1, pp. 27-38

Huang C. and Ivantysynova M. 2006. An advanced gap flow model considering piston micro motion and elastohydrodynamic effect. 4th FPNI Phd Symposium, Sarasota, Florida.

Ivantysynova, M., Huang, C. and Behr, R. 2005. Measurements of elastohydrodynamic pressure field in the gap between piston and cylinder. Bath Workshop on Power Transmission and Motion Control PTMC 2005, Bath, UK, pp. 451 - 465.Ivantysynova, M. and Huang, C. 2005. Thermal Analysis in Axial Piston Machines using CASPAR. Proc. of the Sixth International Conference on Fluid Power Transmission and Control, Hangzhou, China, pp. 573-578.

Ivantysynova, M., Huang, C. and Behr, R. 2005. Measurements of elastohydrodynamic pressure field in the gap between piston and cylinder. Bath Workshop on Power Transmission and Motion Control PTMC 2005, Bath, UK, pp. 451 - 465 - Best paper award.

Huang, C. and Ivantysynova, M. 2006. An advanced gap flow model considering piston micro motion and elastohydrodynamic effects. Proc. 4th FPNI PhD symposium Sarasota, Fl, USA, pp. 181-196.

Ivantysynova, M., Huang, C. and Japing, A. 2006. Determination of gap surface temperature distribution in axial piston machines. ASME International Mechanical Engineering Congress, Chicago, USA, IMECE2006-15249.

Ivantysynova, M. and Huang, C. 2002. Investigation of the gap flow in displacement machines considering the elastohydrodynamic effect. 5th JFPSI International Symposium on Fluid Power. Nara, Japan. pp. 219-229.

Huang, C. and Ivantysynova, M. 2003. A new approach to predict the load carrying ability of the gap between valve plate and cylinder block. Bath Workshop on Power transmission and Motion Control PTMC 2003, Bath, UK.

Ivantysynova, M. 2003. Prediction of pump and motor performance by computer simulation. 1st International Conference on Computational Methods in Fluid Power Technology. Melbourne November 2003, Australia, pp. 505 - 522

Ivantysynova, M. and Lasaar, R. 2004. An investigation into Micro- and macro geometric design of piston/cylinder assembly of swash plate machines. International Journal of Fluid Power, Vol. 5 (2004), No.1, pp. 23 - 36.

Ivantysynova, M. 2004. EHD-based simulation model for connected tribosystems of displacement machines. (in German). Tribologie und Schmierungstechnik. No 51 2004 No 5, pp. 18 - 24.