In industrial practice greases are mainly used as lubricants in hydrodynamic plain bearings and roller bearings. The use in hydrostatic bearings is avoided due to the difficult controllability. One reason is the complex non-Newtonian flow behavior of greases. The motivation for this paper is the use of greases to increase the efficiency of hydrostatic bearings. The assumption is that the so-called yield stress of consistent greases can lead to self-sealing behavior in the bearing under stationary operation conditions. Assuming a volume-flow-free operation of the bearing, a concept for the active control of the sealing gap height and thus the bearing stiffness was developed. The concept idea is the use of a second medium with Newtonian characteristic for pressure transfer. The grease and the pressure control fluid are structurally separated. The grease is induced in the shortest possible way into the bearing pocket to keep the pressure losses as low as possible. The results of test bench investigations indicate the feasibility of a gap height control with very high bearing stiffness under use of highly consistent greases and initiate further investigation on non-stationary operation.

Standardized parts like hydraulic accumulators are used in nearly every hydraulic system, in many cases even several. Therefore, even small changes in size and weight of accumulators can save considerable material costs. In mobile applications, hydraulic accumulators are used among others in hydro-pneumatic suspension systems. There is a strong focus on miniaturization and weight reduction, as the components always have to be transported with the vehicle. Energy density and energy content of conventional hydraulic accumulators cannot be maximized at the same time. This limitation can be overcome by adding a heat capacity with large surface into the gas volume of the accumulator. The heat capacity enlarges the isothermal frequency range and therefore enlarges the energy density of the accumulator at the given frequency and the given size. In this paper an experimental comparison of conventional hydraulic accumulators and accumulators with foam inserts shows, that at a specific frequency band, the stiffness of foam filled accumulators is significantly lower than of conventional accumulators. The energy density is about 11 % higher than in conventional accumulators. Consequently, a space reduction of about 18 % is possible.

The article presents the results of experimental tests of the liquid exchange level in a new design of a hydraulic cylinder in comparison with the classic one. Comparative results are presented, as well as the influence of the stroke range on the degree of liquid exchange. The results allow the empirical determination of the degree of real improvement of the process of liquid exchange in the cylinder during operation, and thus allow the initial determination of the reliability level increase in hydraulic cylinders. It should be emphasized that the presented results are a part of a research and implementation project carried out in cooperation between a university and an industrial partner. In addition, the presented test method of hydraulic cylinders is, in the opinion of the authors, an interesting proposition, which can be used by manufacturers of this type of components in the world.

In some industrial fields, such as aerospace, electro-hydrostatic actuators (EHAs) are increasingly used to replace conventional standard hydraulic actuators due to their better energy performance. Moreover, implementing different type or technology of actuators in redundant actuation systems working on the same moving part introduced some new challenges. This paper presents a force-tracking controller for an asymmetric electro-hydrostatic actuator that is submitted to an external motion generated by an external source. In this case, the rod displacement is considered as an external disturbance for the hydraulic cylinder, but it is assumed that this disturbance can be easily measured using sensors. The theoretical motivation of this work is discussed along and a variable gain state feedback control based on Linear Parameter Varying control (LPV) theory is proposed to achieve stability, disturbance rejection and tracking performance. The Linear Matrix Inequalities (LMI) framework is used to determine a control law including an augmented state feedback with an integral action that reduces trajectory-tracking errors. Simulation results of the control law are finally given to verify the global performance of this control design.

In the paper a new position signal filtering method with position prediction is presented along with test results using a simulation tool. The complete active heave compensation system performance with input signal filtering is also shown. The control system uses an input acceleration signal taken from the motion reference unit, which usually contains noise that is not acceptable for the position controller. Currently, a Kalman filter is used which is okay to use for certain conditions. The filter works similarly to how it is used for autonomous applications where two input positions are necessary, one from position sensors and another one taken from the model. The challenge is that there is no physical wave model available for the Kalman filter used for offshore position control and the waves are not predictable. It was found that a Kalman filter with a special signal prediction instead of the model input can be used. This position prediction helps to avoid system delays and the potential of missing the signal for a short period of the time.

Hydraulic devices play an essential role in mechanical engineering due to their high-power density, good controllability, flexible application and high robustness, which expose innovative methods of energy transmission. However, in applications where there is an increased risk of fire or explosion, the commonly used combustible mineral oils represent an unacceptable safety hazard. In such cases, fireresistant, water-based hydraulic fluids are in demand. A special feature of these liquids is their high cavitation tendency and the associated strong erosion wear. The aim of this research is to predict the cavitation behaviour of HFC and the subsequent erosion phenomena using numerical methods and to validate the results with experiments. Additionally, experimental results for HFC were compared with HLP. The findings help to implement further developments to decrease the erosive effect of cavitation in high-pressure differences in hydraulic components. For this purpose, flow geometries of typical hydraulic components, e.g. valve and pump, are used for experimental and numerical investigation. The large-eddy simulation (LES) turbulent modelling is used with Zwart-Gerber cavitation model. The cavitation aggressiveness is quantified by cavitation erosion indices according to Nohmi.

Hydraulic development engineers and tribology specialist still exert substantial effort, time and resources into finding a hydraulic fluid that would be near an ideal fluid. In addition to its basic physicochemical properties, it must meet a number of other requirements related to its practical use within hydraulic system and the materials used therein. Ionic Liquids, as novel lubricants, offer the solution in this regard. The paper gives an overview of the basic properties of selected and tested Ionic Liquids suitable for use as hydraulic fluids. The practically obtained data refer to the basic physico-chemical properties of Ionic Liquids and properties important for practical use within hydraulic system, e.g. compatibility with materials. The results are given in a comparison with common mineral oil based hydraulic oil.

Well working hydraulic systems need clean hydraulic oil. Therefore, the system must ensure the separation of molecular, gaseous, liquid and solid contaminations. The key element of the separation of contaminants is the hydraulic reservoir. Solid particles are a major source of maintenance costs and machine downtime. Thus, an Euler-Euler-Lagrange multiphase CFD model to predict the transport of solid particles in hydraulic reservoirs was developed. The CFD model identifies and predicts the particle accumulation areas and is used to train port-to-port transfer functions, which can be used in system models to simulate the long-term contamination levels of hydraulic systems. The experimental detection of dynamic particle contamination levels and particle accumulation areas validate and confirm the CFD and the system model. Both models in combination allow for parameter and design studies to improve the fluid management of hydraulic reservoirs.

The displacement of the variable displacement pumps can be adjusted by changing the swivel angle of the swash plate. In fact, the swivel angle oscillates because of the oscillating torque on the swash plate, which caused by the pressure fluctuation of the piston chamber. The swivel angle is most often considered as a constant value in previous studies. However, the oscillation of the swash plate leads to an additional movement of the piston, which has an impact on the pressure fluctuation and the flow ripple. In this study, an improved model of a self-supplied variable displacement pump is established. The swash plate oscillation under different operating conditions is presented. In order to investigate the effect of the swash plate oscillation on the pressure ripple, a comparison between the case of the fixed swash plate and the oscillated swash plate is conducted. Results show the pressure ripple with an oscillated swash plate shows a smaller pressure ripple. It also shows that the nine pistons and the control mechanism both affect the pressure ripple and flow fluctuation.

Axial piston pumps are universal displacement machines that are used in a vast variety of applications. Their high pressure resistance and ease of operation make them very popular, especially in mobile applications. Some applications require more robust pumps with an extended lifetime, particularly those that operate in remote environments such as marine type or mining operations. Especially new applications like displacement control have high demands on pumps such as through shaft operation (many pumps on one shaft), high dynamics and multi-quadrant operation. These demands create challenges in terms of lifetime expectancy and robustness for pump manufacturers and machine OEMs. Currently most axial piston pumps go through a run-in process. During this process the softer bronze parts shave off and change their shape according to the necessary one for the pumps’ proper operation. This process is highly dependent on the design of the parts and their manufacturing tolerances. In this paper the run-in process of the slippers of an axial piston pump was investigated by means of measurements of the gap height and wear profile as well as simulation. The measurements show a clear change of profile and gap heights for the first 120 h of the pumps operation. After that the gaps stabilize. The numerical simulations made with the program Caspar FSTI were coupled with contact wear models to output wear profiles. Different models will be introduced and compared with measurements. Both the amount of material removed and the performance of the pump before and after run-in will be discussed.