An open center system (OC-System), which is one of the major hydraulic architectures for excavators, has been improved in the world to reduce fuel consumption for global environment conservation and lower operating costs. However, the total system efficiency, including the internal combustion engine (ICE), has not been thoroughly considered. In contrast, a constant pressure system (CP-System) enabling the engine to be driven optimally is developed, but is not accepted in the industry owing to the complexity of the required components. Thus, in this research, a hybrid system combining an OC-System with a CP-System is proposed to improve the total system efficiency. An accumulator, which is used to provide flow rate to actuators, is essential for the new hybrid system, and it is vital to consider the nominal gas volume and pressure level for the accumulator in terms of energy savings and initial cost. Therefore, the influences of accumulator volume and pressure level are discussed in this paper.

The leakage characteristics of metallic seals have a significant influence on valve performance. The contact status along the sealing zone greatly affects sealing properties, but is still not fully understood in current studies. There are mainly two different models for the seal's contact mechanics: the multi-asperity model and the magnification-based model. This paper conducted a comparative analysis of leakage calculations for metallic seals of ball-seat valves based on the above two contact models. The test rig for measuring leakage values at different pressures has been constructed. The simulation with/without considering plastic effects is presented and verified by the experiment. The results indicate that the magnification-based model, with the consideration of plastic effects, can provide a better coincidence with experimental results.

The macroscopic geometry of ball seat valves is important for the quality of the seal. This works discusses the influence of different geometric properties on the contact area, the contact pressure and their relation to the leakage. The leakage is calculated using the results of finite element method (FEM) calculations and Persson’s percolation based method. The following properties of the seat are examined: the angle, the curvature and the eccentricity.

The numerically stable simulation of cavitation effects is mandatory for predicting the friction and wear behavior of translational hydraulic seals. This contribution provides a comparison of two different implementations of the Jakobsson-Floberg-Olsson (JFO) cavitation model, an investigation of their properties and possible options for their stabilization. These methods are tested and compared both within a simple divergent gap test case as well as within an EHL simulation of a rubber metal contact. Based on these comparisons and theoretical investigations, the strengths and weaknesses of the different methods are summarized and discussed with respect to an application in EHL simulations of translational hydraulic seals.

Metallic ball seat valves are an essential component in almost every hydraulic system in the form of check valves. Despite their wide usage in real‐life applications, the physical sealing mechanism is not yet fully understood and still debated. One promising method is the contact mechanics theory which has been developed by Persson et al. for rubber seals. It can predict the leakage of rubber seals successfully, but its application for metallic seals, where plastic deformation plays an import role, has only been developed recently and is not yet fully validated. This theory is combined with surface scans of investigated seats and simulations that are performed using the finite element method (FEM) for different geometric configurations. This combination allows an efficient calculation of the leakage of ball seat valves depending on their specific design and applied forces and pressures. In this work, the predictions by simulations according to Persson's theory are compared with experimental results. This comparison is done by analyzing the dependency of the leakage of ball seat valves in relation to applied normal forces and for different surfaces in contact. This validates the applicability of Persson's method in this field. The presented experimental method allows the measurement of the leakage of ball seat valves under realistic conditions.

In this paper, a simulation study is carried out for the development of concepts to optimize the tribological contact of valve plate and cylinder block in an axial piston machine in swash plate design.The valve plate/cylinder block contact is one of the three essential tribological contacts in axial piston machines.In a research project at the Institute for Fluid Power Drives and Systems (ifas), this contact is investigated by a specifically designed simulation tool.In addition, a test rig exists for the experimental investigation.With the results of simulation and experiment, it was shown before that the cylinder block is tilting to the high pressure side.Due to this movement, the gap height is not constant.In the area of minimum gap height, not only the fluid friction, but also the danger of solid body friction increases.Because of the higher friction losses in the area of minimum gap height, the temperature increase reduces the lifetime of the leaded coatings.In this paper, the results of the measurements as well as the simulation model are briefly summarized.It is followed by a simulation study of different possibilities to raise the gap height.Based on this pre-study, a first concept for the optimization of the tribological contact valve plate/cylinder block is presented and its applicability is discussed.

In the presented paper, the applicability of pressure-decay methods to determine the diffusivities of gases in hydraulic fluids is analysed.First, the method is described in detail and compared to other measurement methods.Secondly, the thermodynamics and the mass transfer process of the system are studied.This results in four different thermodynamic models of the gaseous phase in combination with two diffusion models.Thirdly, the influence of the models on the pressure-decay method is evaluated computationally by examining the diffusion process of air in water as all system parameters are available from literature.It is shown that ordinary pressure-decay methods are not applicable to gas mixtures like air and therefore a new method for calculating the diffusivities is suggested.

In the field of fluid power, accurate knowledge of fluid properties is vital for reasonable prediction of component behaviour and system performance. In general, these properties depend on the pressure and temperature levels that the respective medium is exposed to. The properties and their respective dependencies are not publicly accessible for many fluids commonly used in fluid power. If measured values – typically published in the form of mathematical fluid property models – are available at all, their quality is typically unknown. The paper aims to provide tools to objectively ascertain the quality of measured fluid properties. For this purpose, an equation is derived which establishes a relationship between the thermodynamic parameters of density, bulk modulus, heat capacity and thermal expansion coefficient. The presented equation is always satisfied by liquids as well as gases as long as they can be treated as a continuum. Based on this relationship, the degree of thermodynamic consistency of measured properties is evaluated: The less the equation is fulfilled by experimentally determined fluid properties, the more the measured values violate physical laws. The procedure of assessing the thermodynamic consistency is demonstrated by evaluating published fluid property models with the method outlined above. To aid engineers in judging which degree of thermodynamic inconsistency is acceptable, a cut-off value is suggested.

In hydraulic systems, a certain amount of air is always present as entrained air in the form of gaseous bubbles and as dissolved air. Large amounts of entrained air negatively affect the behavior of hydraulic systems by decreasing the bulk modulus of the fluid and increasing the risk of cavitation damage, e.g., cavitation erosion and the microdiesel effect. The diffusion-driven growth of entrained gaseous bubbles surrounded by a liquid phase is referred to as gas-cavitation, in which dissolved air from the solution diffuses into the bubble. In the proposed paper, the diffusion-driven growth of a single gas bubble immersed in a liquid is examined. To that end, a comprehensive literature review, a mathematical description of the problem as well as the numerical implementation is presented.

Axial piston machines are the most widely used type of hydraulic displacement machines and are characterized by their high reliability and efficiency. However, in order to ensure the high efficiency, the tribological contacts have to be precisely optimized. One of the three essential contacts in axial piston machines is the contact of valve plate and cylinder block, which is the subject of this paper. In a previous research project, a simulation model was built up specifically for the tribological contact of valve plate and cylinder block. A test rig was developed and installed for the validation of the simulation results. Both, the experimental and the simulation results show that the cylinder block tilts to the high-pressure side. It holds this preferred position nearly constantly for the different load situations over one revolution with four or five pistons pressurized with high pressure at the same time. The tilting increases the danger of solid body contact in the area of minimum gap height. In addition, it leads to temperature hot spots. Both effects necessitate the use of coatings as alternatives to the commonly used leaded alloys. This paper presents new design concepts for the optimization of the tribological contact of valve plate and cylinder block. Additional pressure pockets in the valve plate's high-pressure kidney generate a torque and thus reduce the tilt angle of the cylinder block. By implementing additional pressure pockets at the cylinder block an imbalance results, which prevents a constant preferred position. Both concepts have the aim to reduce the heat concentration and improving the overall behavior of the tribological contact. The development and comparison of these concepts are based on a numerical analysis.

ISO 4409 is the most important international standard for measuring the efficiency of hydraulic pumps and motors, the latest edition being 4409:2007. The standard describes methods for determining the steady-state performance in terms of overall efficiency. It also defines equations for calculating the volumetric efficiency of pumps and motors. The hydro-mechanical efficiency is only defined for motors, not for pumps. This paper analyses the efficiency and losses of pumps and motors in an alternative way. The preference is on loss analysis instead of efficiencies. Especially the effects of the bulk modulus are considered in a different and more inclusive manner. The new methodology results in a higher total loss for motor and a lower total loss for pumps than the current ISO 4409 standard. Furthermore, it results in significant changes of the hydro-mechanical and volumetric losses. The differences between the new methodology and ISO 4409 become larger for high load pressures. The new methodology demands knowledge about the minimum volume of the displacement chamber. The ratio between this volume and the full displacement of a single displacement chamber strongly influences the hydro-mechanical and volumetric losses of the pump or motor. The new methodology is valid for all positive displacement hydrostatic pumps and motors. The volumetric efficiency, as defined in ISO 4409, can still be used as a flow rate factor, but should not be regarded as an energy conversion efficiency. The importance of adopting the proposed methodology is further demonstrated by analyzing and comparing the measurement data about a fixed displacement pump and motor, showing the differences in the loss analysis by means of ISO 4409 and the new equations. The methodology, observations and validation results presented in this paper are significant and can pave the road for improving the current ISO 4409:2007 standard, which would ultimately benefit the industry.

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.