CASPAR image


The prediction of pump and motor performance for a given design of a displacement machine requires a simulation model that describes the flow of a compressible and viscous fluid from the ports through the valve plate to the displacement chamber. It must further consider the gap flow through the lubricating gaps that seal the displacement chamber. The change of pressure in the displacement chamber resulting from the basic working process of the displacement machine causes fluctuating forces and moments leading to oscillating micro motion of moveable parts of the rotating group. The simulation program CASPAR, which has been developed at the Institute for Aircraft Systems is based on a non-isothermal gap flow model considering the change of gap heights due to micro motion of parts and due to surface deformations for the connected gaps of a swash plate axial piston machines. The program allows the calculation of real flow ripples at both ports, further the calculation of the instantaneous cylinder pressure, the internal and external volumetric losses, viscous friction forces, gap heights, oscillating forces and moments exerted on the swash plate. The program represents a powerful design tool for this kind of displacement machines. CASPAR is a stand alone tool developed using the C++ programming language. Models implemented and solved in CASPAR consider the time dependent change of gap heights due to oscillating forces, the interaction between machine parts, the dependency on design and operating parameters and the energy dissipation within the gaps. The updated relase of CASPAR includes the consideration of elasto-hydrodynamic effects due to surface deformation of parts forming the gaps. The mathematical description of the fluid flow from the ports to the displacement chamber and through the sealing and bearing gaps leads to a system of partial and ordinary differential equations. A new numerical method based on iterative coupling of separate solvers for fluid/solid domains has been developed to solve this transient nonlinear system consisting of the Reynolds equation and the energy equation for fluid domain, the equation of elasticity for the solid domain and the determination of gap heights by solving the motion equation of the multibody system of the rotating group. The initial boundary conditions such as instantaneous cylinder pressure are obtained by solving the fluid flow from displacement chamber to the ports.


The change of pressure in the displacement chamber of a displacement machine is greatly influenced by the smallest cross-section of the fluid flow which is formed by the valve plate and the rotating cylinder block. For simulation calculations, it is important to know the exact size of the flow passage opening to the high and low pressure side, depending on the angle of rotation. Because of the complex geometric sectioning, an analytical description of the cross-section is not possible. In the past the cross-section was measured and interpolated manually to obtain the area profile. Using a 3D-model of the valve plate AVAS is able to compute the smallest cross-section of the fluid for a complete revolution of the cylinder block automatically. In the singlestep mode every calculated passage area can be visualized. AVAS uses Unigraphics based routines to determine the smallest cross-section into the estimated flow direction. The program is written in C++ and uses the UG/Open++ interface to start as an internal application in Unigraphics. The 3D-model of the valve plate can be imported from any other CAD-System by the STEP interface.

AVAS image
SUEZ image


SUEZ allows an automatic design of valve plate openings by reading a corresponding opening area file. SUEZ is based on the 3D CAD System Unigraphics. The pilot control notches of the valve plate are assumed to be manufactured by ball end milling. The cross section area, the length and the angle of the notches can be manipulated to obtain the desired instantaneous cylinder pressure for given operating parameters. The combination of the simulation tools CASPAR, AVAS and SUEZ allows optimizing swash plate axial piston machines in a very cost effective way. This method can also be used for other displacement machines.


One of the main reasons for use of power split drives in many application is the possibility to have a continuously variable transmission with simultaneously high efficiency in a wide range of operating parameters. This requires the consideration of real loss behaviour of all parts of the transmission. Due to the strong dependence of losses of displacement machines on operating parameters the integration of precise loss models is necessary. The PSDD software tool allows the calculation of system parameters including power losses in the whole range of operation for any kind of power split drive structure. This gives the design engineer a very good support during the design process and helps the design engineer to find an optimal structure of the power split drive using a hydrostatic transmission, thus the highest possible efficiency in the required operating parameter range can be achieved. The tool has libraries for hydrostatic components, gears, clutches and planetary gear sets. These libraries can be extended and completed by the user in a very easy way. The PSDD software tool is built in a modular way on the Matlab and Simulink platform.

AVAS image