In hydraulics, threaded plugs are used to close various manufacturing holes and other fluid channels. They are preloaded to ensure sufficient sealing force. Since the range of recommended thread and underhead friction coefficients for preloaded threaded connections in the literature is very wide, they are not suitable for accurate determination of the preload–torque relationships of plug–valve connections. In the study, two non-standard plugs with metric threads were equipped with strain gauges and repeatedly tightened three times in valve housings under lubricated and unlubricated conditions. The preload and tightening torque were measured. (1) Although the plug–valve connections had a similar geometry with the same surface roughness of the contacting surfaces, the average overall friction coefficient (uniform thread and underhead friction coefficient) and torque coefficient differed between the two connections in the unlubricated and lubricated conditions by 16% and 18%, respectively. This indicates that even small geometrical differences can have a considerable influence on these coefficients. The overall friction and torque coefficients were between 8% and 17% higher in the unlubricated condition than in the lubricated condition (not statistically proven). (2) The overall friction and torque coefficients decreased with repeated tightening under lubricated conditions. This influence decreased with the number of tightening repetitions. (3) Consideration of the minimum and maximum thread and underhead friction coefficients given in VDI 2230 would lead to an error in the estimated preload of −15% to +86%. In conclusion, for accurate determination of the preload–torque relationship of the plug–valve connections, measurements considering repeated tightening are crucial. These should be performed for each type and size of plug–valve connection separately. To minimize the repeated tightening influence, it is recommended to re-tighten the connections several times before leaving production.

Simulation of positive displacement machines requires highly skilled engineers, use of advanced simulation tools and advanced simulation approach. The paper presents recent activities and progress on simulation of positive displacement machines – in particular, the axial piston pump and the radial hydraulic motor. Despite that these machines have been designed and produced for decades, there are still (design) features and phenomena not being investigated in detail or never being simulated. The simulation advancements mainly refer to the application of complicated kinematic motion, fluid properties, physics to consider as well as mesh and numerical algorithm techniques. In this paper, the focus is on modelling of advanced fluid material properties. Numerical approach has been performed by means of CFD within the environment of Siemens Simcenter Star-CCM+. The anticipation of cavitation has been possible by implementation of existing “full cavitation model”.

Upfront simulation involves activities in the early design phase. The goal is to identify potential problems early, optimize designs, and predict performance when changes are cheaper and easier to implement. Democratization of simulation is understood as making simulation tools accessible to non-simulation experts within a company, organization, or community. This means removing barriers that limit product managers, designers, or even non-experts from running simulations and making decisions. The paper presents current status regarding upfront simulations and their democratization within company Poclain. The practical cases demonstrate the abilities brought through the upfront (or front-loading) simulations. It is also explained what activities have been made to democratize (deploy) simulation tools among the group of design engineers. The power of upfront simulations is also demonstrated on real case example, performed in real time (in-situ) at the conference. It emphasizes its ease of use, the importance of understanding the product behavior as well as the gain in time to market.

Preloaded threaded plugs that limit axial movement of spools in hydraulic valves are repeatedly loaded by spool impacts, which affects their fatigue strength. For proper dimensioning of these plugs, their loadings must be known. The aims of the study are to: (a) determine the spool impact force ($F_{impact}$) on the plug head, (b) determine its influence on the force in the plug neck (FIPN) and (c) give design recommendations and practical considerations for plug–valve connections. Modified versions of the plugs were produced and equipped with strain gauges. The plugs were tightened into valves and loaded with spool impacts at three different pilot pressures. Meanwhile, $F_{impact}$, FIPN, pilot pressure, and spool displacement were measured. As the FIPN was measured in the modified plug–valve connections, which had different stiffnesses from the original connections, the FIPN in the original plug–valve connections was calculated by considering the stiffness ratios of the original connections obtained from finite element analyses. The results show that the spool impact force and the resulting additional FIPN increase linearly with pilot pressure. To achieve smaller amplitudes of the FIPN and, consequently, higher fatigue strength, the tensioned parts of the plugs should have lower stiffness and the clamping parts higher stiffness.