Pressure Losses in Hydraulic Manifolds

Hydraulic manifolds are used to realize compact circuit layouts, but may introduce a high pressure drop in the system. In fact, their design is optimized more for achieving minimum size and weight than for reducing pressure losses.

This work studies the pressure losses in hydraulic manifolds using different methods: Computational Fluid Dynamic (CFD) analysis, semi-empirical formulation derived from the scientific literature, when available, and experimental characterization.

The purpose is to obtain the pressure losses when the channels’ connections within the manifold are not ascribable to the few classic cases studied in the literature, in particular for 90° bends (elbows) with expansion/contraction and offset intersection of channels.

The comparison of the results obtained allows for drawing some guidelines for the design of the manifold channels and to discuss the reliability of CFD analysis as a tool to improve the design of a hydraulic manifold. We focus first on simple geometries, considering the intersection between the channels of the manifold with one or two 90° elbows. This way, we can compare our results with the ones already published in the literature, validate the analysis, and then apply the same analysis method to test cases not previously discussed.

Results from CFD simulations show that the virtual analysis can depict the correct trend of pressure drop in all the different geometries analysed following, at a “certain distance”, the experimental results. The difference between experimental and CFD results normally increases with the flow rate and the pressure drop. The gap is, however, quite high and always with an overestimation of CFD simulations compared with the experimental results.

Going into the detail of manifold design rules, some considerations can be highlighted regarding the possibility to introduce an elbow with a moderate expansion in the manifold (no great changes with reverse flow, i.e. through a contraction), the use of offset intersections and the ‘shape’ to prefer (and the one to avoid) when two consecutive elbows are necessary in the manifold passage, always related to the relative distance between the elbows.

The next step of this research is to analyse what happens with more complex but realistic connections on a manifold block, still comparing the CFD analysis and the experimental results, and deepening the study of aeration/cavitation occurrence.

Modeling of Hydrostatic Bearings for Servo-Cylinders

Hydraulic servo cylinders are widely used in versatile industrial applications such as machine tools, test rigs for any kind of components, industrial robots, autonomous manufacturing systems and special applications in laboratories. In general, they are typically used whenever a smooth movement with low friction, fast dynamic response and possible radial forces are required.

Hydrostatic journal bearings with pockets are used on servo cylinders’ rod ends in order to guarantee the bearing of high loads, reduce friction, remove wear and allow smooth and controllable displacement of the actuator.

The design and manufacturing of these elements is challenging since the good operation relies on the very small tolerances required to bear the load on the cylinder and to reduce leakages.

Lubricated interfaces in fluid power components are one of the most critical issues to be carefully designed for assessing a smooth behaviour and good efficiencies in a system. Moreover, they are fundamental to analyse the complex phenomena determining the positive displacement pumps and motors efficiency and also fault behaviour.

In this work, a 2D fluid dynamic model of hydrostatic journal bearing is presented. The model is composed by a system of equations, created by SmartFluidPower, written in Modelica language and entirely solved in OpenModelica environment. This work concerns the first part of a research activity in which a virtual model and test tool for hydrostatic journal bearing with pockets are created.

The model proposed has the aim to explore the extreme and critical operating conditions of the servo-cylinder and to help and/or improve the design phase: the results show a significant influence of eccentricity and manufacturing tolerances and, therefore, an accurate choice of the design parameters must be followed to look for the best configuration.

With the help of an industrial partner the numerical model is tested and validated: at the end, a virtual design tool is created for industrial designers to help and guide their work.

Besides the specific results obtained regarding the design of the bearings, the work also demonstrates a different use of OpenModelica environment: a pure equations solver of which results are used to create an industrial virtual tool that helps the designer to simulate different configurations and look for the optimal solution.

Model of vane pumps developed in OpenModelica

research paper

In this paper, a 0D fluid dynamic model of a vane pump used to refill tanks with fuel is presented. The model is entirely developed in OpenModelica environment, where SmartFluidPower have created specific libraries of elements suitable for the physical modelling of fluid power components and systems.

Among the different approaches, the zero-dimension (0D) fluid-dynamic modelling of positive displacement machines is suitable to study many aspects as the fluid borne noise related to the flow rate and pressure irregularity and the dynamic behaviour of the variable displacement control. Well before the realization of a physical prototype, the model is useful to give indications on the impact of the design on the pressure transients inside the volume chambers and pressure and flow ripples.

The model of the vane pump is described together with the main design features that can be analysed in terms of their influence on the pump behaviour. The model has been created in a “parametric fashion”: this means that it can easily be adapted to analyse different design modifications, for example the number of vanes, the suction and delivery flow areas, the stator internal profile.

Overall, this approach in modelling allows to link the geometrical features of the machine with its dynamic behaviour and for this reason is particularly useful in guiding the design.

Besides the specific results obtained regarding the design of the pump, the paper also demonstrates the use of OpenModelica language and environment, and its efficacy, into the applications of fluid power modelling and simulation.