In this paper, a new methodology is presented to estimate the lifetime impact of design changes, called Failure Likelihood Assessment (FLA). The discussion in this paper is on the fatigue lifetime prediction of axial piston units, especially after a design change. The demonstration object is an axial piston pump due to extreme environmental conditions and high specification demands, where the FLA is applied to a manufacturing change in an existing product and delivers an effect on the unit reliability. The resulted reliability imp rovement, if combined with typical calculation methods like Weibull analysis, delivers an increase in predicted lifetime considering the intended modification. As demonstration subje ct, a change of the manufacturing process of the cylinder block hub in an axial piston pump is used. The effect to the lifetime is predicted via the FLA-method and the results are calculated with test data and compared to theoretical results. The paper shows that the methodology delivers highly accurate results providing that the FLA is a powerful tool to analyze design changes as weil as new designs in regard to reliability and lifetime. The benefit for the user of this methodology will hence be more reliable products with optimized designs tobest fulfil customer's performance requirements.

Research work performed on an axial piston pump is shown in a holistic manner, analyzing each lubricating interface by linking their gap height and temperature behavior to the overall pump efficiency. The temperature field and dynamic fluid film height were measured in two of the three lubricating interfaces. This is the first time that the temperature fields and gap heights were simultaneously measured in two of the main three interfaces of an axial piston machine. For a deeper analysis of the measurement data, all gaps were simulated with a numerical tool which takes solid body deformation due to temperature and pressure loads into account. This unique combination of both extensive measurement data and sophisticated simulation resulted in novel trends that clarify the complex phenomena occurring in these hydrostatic fluid films.

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Different special brass (e.g. CuZn37Mn3Al2PbSi) and bronze alloys (e.g. CuPb15Sn) are well known for use in oil-hydraulics having in common to be alloyed with lead. The lead content of special brass alloys in this use ranges from 0.1 to 2.0 mass-%. Some bronze alloys provide even much higher contents of lead of 10 to 15 mass-%. Typically, lead is considered for improvement of machinability or castability. Beyond this purpose lead in brass and bronze alloys affects many more properties of manufacturing and application. During the shaping of the parts by means of hot or cold forming often the materials are strained close to their limits. Thanks to lead cracking is prevented during this process. Lead is also of great importance for the improvement of tribological systems. The surfaces of these systems are exposed to friction and wear. Lead is incorporated in the surface layers and supports the tribological system in their running-in process to achieve a steady state of friction and wear. Above all lead is unique because it forms no solid solution with copper or brass and forms no compounds with other typical copper alloying elements. The feasibility assessment of elements in order to substitute lead in brass or bronze alloys has to be done for each alloy and application individually. In oil-hydraulic applications as bushings, slippers or distributor plates, lead-free alloys must fulfil different profiles of requirements, depending on the conditions of manufacturing and application. The requirements do not only include mechanical strength, formability and thermal strength, but also fatigue strength, low friction and high wear resistance and lubricant compatibility. Consequently, the substitution of lead in brass and bronze alloys for application in oil-hydraulics is a challenging task. This does not only apply for the requirements for machining and forming, but particularly for the need of the new alloys to function under wear, friction and corrosion. Examples are given for how these challenges of new lead-free special brass alloys can be met in bushings (machining, friction properties), slippers (forming, strength) and distributor plates (fatigue strength) for axial piston pumps. Further on, new lead-free special brass alloys for contact with environmentally compatible lubricants are presented. All these examples show that there is not the one and only lead-free alloy for applications in oil-hydraulics. In fact, every application requires a different alloy which is composed and processed individually to meet the specific demands.

In hydraulic pumps, typically in axial piston pumps, reduction of pressure and flow ripples was attempted by providing relief grooves and pre-compression for noise reduction. Pre-compression is normally achieved by using the dead space between pump ports in the valve plate. Also valve plate profile modification is required, if system operating conditions such as pump output pressure and flowrate change, to maintain optimum operating conditions for reduced pressure/flow ripple. An earlier simulation study confirmed effectiveness of varying dead centre position to reduce pressure and flow ripples. A specifically designed mechanism, outlined in the earlier work, achieves this goal by varying the dead centre position of the pump swash plate. This study reports on the findings of the effect of varying dead centre position and groove configurations on forces and moments acting on the swash plate for various operating conditions. The simulation model cited in the earlier work was used in this study. This information is vital for the design of an actuating mechanism to vary dead centre position of a pump valve plate. These simulations were run using MATLAB/Simulink and S-functions. Results of this study are promising.

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.

<p>Emission regulations and global policies to tackle climate change have forced industries and businesses to take measures to curb their impact on the environment. According to the United Nations Environment Program 2022 report on emissions [1], the transportation sector contributes to one-quarter of all energy-related CO2 emissions, and it is set to double by 2050. A recent report [2] suggests that off-road vehicles and equipment account for three-quarters of particulate matter and one-quarter of the nitrogen oxides emitted from mobile transportation sources in the US. The major challenge in decarbonizing or electrifying off-road machines is that they come in a wide range of sizes, weights, and functions, creating barriers to bringing down costs through economies of scale. Fluid power systems which are ubiquitous in these machines have been electrified in a compact and efficient manner to break even the costs of electrification. </p> <p>In off-road applications, where actuation systems heavily depend on hydraulics, there is a high demand for novel systems based on electric prime movers that can enable zero-carbon emission vehicles. An appropriate combination of electric prime movers and hydraulic machines commonly known as electrohydraulic units (EHUs) can help leverage the benefits of both these technologies. The integration of these two technologies in a single casing shaftless EHU can further maximize compactness and reduce cost. However, to achieve such an integrated EHU there is no standard procedure or recommended guidelines for equipment manufacturers owing to the interdisciplinary nature of the problem. </p> <p>This study proposes a generic design methodology to design electrohydraulic units (EHUs). As a starting point, a survey study was undertaken to compare different combinations of electric and hydraulic machines when designing an EHU. The different combinations were investigated for different operating drive cycles for their performance as well as other factors such as power-to-weight, cost, and the possibility of variable displacement. An axial piston machine (APM) was selected as a hydraulic machine (HM) to be integrated with a permanent magnet synchronous motor (PMSM) as the electric machine. </p> <p>The design methodology is demonstrated for an integrated electrohydraulic architecture with the APM housed inside the core of the PMSM. Such an architecture not only makes the overall integration much more compact but also allows for better thermal management of the EM. In such an architecture, the EM governs the overall power density of the integration and the total mass of the integration owing to inherent torque density differences. An EM design optimization is adopted for a predefined HM architecture to design the proposed EHU integration. The design optimization is used to quantify the effect of key EHU design specifications on the EM size and performance. EHU specifications such as sizing torque, operating voltage, aspect ratio, cooling efficacy, number of poles, and power-to-weight ratio have been studied to draw generic trends. These generic trends in the design specifications are used to outline clear guidelines on the impact of each of the EHU specifications for future EHU designers.</p> <p>Using the generic design trends, the design methodology is extended to size the EHU based on typical operating demands using the HM variable displacement, EM overload capability, and the EM flux weakening operation. These sizing studies allow the designers to size the EHU for the specific drive cycle operating demands and avoid oversizing the EHU. The EM flux weakening mode of operation allows the EM to be sized for a peak power level lower than the corner power of operation. The EM overload operation allows a reduction in the sustainable sizing torque lower than that of the maximum torque demand. The variable displacement in the HM can be used for improving overall EHU efficiency when selecting a low voltage or using a compact EM as well as to reduce the EM sizing torque. Two operation algorithms are proposed to define the EHU operation using variable displacement. Additionally, the sizing of a single EHU for multiple applications is also demonstrated. Such multi-utility EHU sizing can promote mass production and improve the rate of electrification in off-road machines.</p> <p>Finally, a prototype-tested EHU design based on the sizing study is demonstrated and the design considerations in such a design process are discussed. The prototype of the integrated EHU with a fixed displacement APM was able to reach the full capability of the reference APM. Thermal considerations are made on the EM sizing, to ensure the reliability of the designed EHU.  A novel self-sustained EHU architecture using the HM working fluid as a cooling fluid for the EM was designed. This was achieved by proposing a three-port valveplate design to divert part of the delivery stroke to cool the EM. A lumped parameter HM model was used to optimize this third port for an EHU prototype.</p>

<p>  </p> <p>Cavitation is one of the most common causes of failures in axial piston machines. Due to the detrimental effects that cavitation has on unit performance, it is of important consideration both in the design of new units and in defining the operational limits of existing market products. The work in this thesis aimed to contribute to the current knowledge in both areas, with a focus on design considerations with respect to cavitation scalability, and on operating conditions by measuring cavitation severity under separate and combined inciting parameters. Though the application of unit scaling is common in industry for the design of pump families, there have been no comprehensive attempts to quantify whether cavitation in fluid power units may be adequately accounted for in published scaling laws. In this thesis, the scalability of cavitation phenomena was examined through a CFD scaling study performed using a modified version of the Full Cavitation Model.  Results indicate that linear scaling is consistent in maintaining volumetric efficiency performance within 1% across scaled units up to eight times larger or smaller than the baseline. However, the gas and vapor volume distributions vary significantly between scaled units, due largely to the linear non-scalability of fluid inertia and turbulent factors. Physical exchange between phases within a working fluid was shown to be time-dependent, such that the scaled-down unit exhibits bubble collapse rates up to 30% and 150% greater than the baseline and scaled-up units, respectfully. Considering these effects, the presented work demonstrates a potential for increased cavitation damage area when downscaling a unit and reduced risk in upscaling, despite the scaling law being a reliable indicator for volumetric efficiency. </p> <p>To define a more complete study of cavitation under a variety of operating conditions and inciting parameters, this a new experimental procedure and testing circuit was proposed with focus on repeatability by controlled pressure drops and preliminary quantification of inlet fluid quality. By measuring cavitation conditions under pressure starvation, incomplete filling, and combinations thereof, the direct effect of different inception methods on unit performance was shown to be readily identifiable. Through visualization of the inlet flow, reduction in inlet pressure levels was correlated to fluid cloudiness levels and bubble size, with transparency loss at 0.0 bar_g and transition from bubbly to plug flow at -0.4 bar_g. Incomplete filling-induced cavitation was also shown to be detectable by inlet flow conditions, with a distinct change in bubble coalescence rate when operating under shaft speeds greater than or equal to fill speed for a given inlet pressure. </p>