ÚK-odbor tribologie

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Performance and stability comparison of hydrostatic bearing pad geometry optimization approaches
(Springer Nature, 2025-04-30) Michalec, Michal; Foltýn, Jan; Svoboda, Petr; Křupka, Ivan; Hartl, Martin
Hydrostatic bearings are commonly used across a range of applications, yet their reliance on externally pressurized lubricants presents significant energy consumption challenges. This research aims to experimentally assess various approaches for optimizing the geometry of hydrostatic bearing pads. Utilizing a two-pad hydrostatic tester equipped with online diagnostics, we analyzed optimized multi-recess pads developed through both analytical methods and computational fluid dynamics (CFD). Our results demonstrate that the CFD method achieves a substantially greater film thickness recess pressure compared to the analytical method under similar experimental conditions. Additionally, the CFD approach reduces pumping power losses by 14%. However, this improvement in performance is accompanied by a reduction in film stiffness and an increased sensitivity to eccentric overload or misalignment, as highlighted in our findings. While the adoption of CFD-optimized geometries offers significant potential for lowering energy consumption, maintaining precise alignment especially in large-scale applications remains essential. In summary, our study suggests that employing CFD optimization can effectively reduce the service costs associated with hydrostatic bearings, but optimal outcomes necessitate careful alignment considerations.
Uncertainty analysis of hydrostatic bearing working conditions with experimental, CFD, and analytical approach
(SPRINGER HEIDELBERG, 2025-05-02) Foltýn, Jan; Maccioni, Lorenzo; Michalec, Michal; Concli, Franco; Svoboda, Petr
The design and real-time control of hydrostatic bearings (HS) require precise models capable of accurately predicting bearing behaviour under diverse operational conditions. Traditional analytical models have been found to be inadequate to simultaneously estimating critical parameters, including carrying capacity, recess pressure, film thickness, and flow rate. To overcome these limitations, Computational Fluid Dynamics (CFD) has emerged as a powerful tool in recent years. However, the accuracy of operational data used to calibrate the numerical and analytical models significantly influences the propagation of uncertainty. This study focusses on an experimental campaign and the development of a CFD model within the OpenFOAM® environment. Numerical and analytical models were calibrated using various input parameters, such as flow rate and recess pressure, to replicate experimental conditions while accounting for extreme operational scenarios and the inherent uncertainties in the experimental data. The results indicate that although average CFD predictions exhibit consistent errors in estimating operational parameters, the uncertainty ranges of the experimental and numerical data overlap under the conditions examined. On the contrary, analytical predictions show notable discrepancies, even when measurement uncertainties are considered. In particular, recess pressure emerged as the most effective input parameter to accurately estimating carrying capacity. These findings highlight the critical importance of incorporating measurement uncertainties into the calibration of numerical and analytical models for HS bearings, offering valuable information for their precise design and effective real-time control. Moreover, this paper demonstrates how CFD enables the consideration of misalignments measured during experimentation, a factor that is not accounted for in current analytical models.
Decarburization and its effects on the properties of plasma nitrided AISI 4140 steel: A Review
(MDPI, 2025-05-10) Stojanović, Željko; Gligorijević, Bojan; Prvulović, Slavica; But, Adrian; Svoboda, Petr; Piteľ, Ján; Vencl, Aleksandar
Despite the surge in plasma nitriding research, few reviews thoroughly examine how surface decarburization—occurring both before and during the treatment—affects AISI 4140 and similar steel grades. This review addresses that shortfall by providing an investigation into the decarburization process and its consequences. It compiles essential findings from prior studies, demonstrating instances of decarburization in both plasma-free and plasma-containing environments. Furthermore, the analysis explores strategies to prevent decarburization and assesses its significant impact on the steel's microstructure, hardness, corrosion resistance, and wear properties in surface and near-surface regions. Moreover, the study proposes directions for future research, emphasizing the necessity for a more detailed understanding of the decarburization mechanisms and their influence on the properties of plasma-nitrided steels.
Pad Alignment Methods and Their Impact on Large Hydrostatic Bearing Precision
(MDPI, 2024-08-13) Foltýn, Jan; Hurník, Jakub; Michalec, Michal; Svoboda, Petr; Křupka, Ivan; Hartl, Martin
Hydrostatic bearings are frequently used for moving large structures smoothly and precisely. In such applications, difficulties with manufacturing, transportation, and assembly arise. The safety and precision of the entire supported structure depend on the accuracy of the hydrostatic bearing alignment. There are several suitable methods for its alignment, yet it is not clear which method can achieve the highest bearing precision. This study provides a comparative experimental assessment of the three methods. The measurements were performed on a hydrostatic bearing test rig with independent positioning of the pads. Conventional measuring devices, a pressure distribution alignment method, and an optical method, OCMM (optical coordinate measuring machine), were compared. The accuracy of the selected methods and the difficulty of the aligning process were included in the comparison. The OCMM method was able to achieve an accuracy 1.6 times higher relative to the pressure method and 6 times higher compared to conventional measuring devices. This method is versatile and can be applied for a wide range of bearing sizes.
Investigation of the tribological performance of ionic liquids in non-conformal EHL contacts under electric field activation
(Springer Nature, 2020-02-26) Michalec, Michal; Svoboda, Petr; Křupka, Ivan; Hartl, Martin; Vencl, Aleksandar
Real-time external alteration of the internal properties of lubricants is highly desirable in all mechanical systems. However, fabricating a suitable and effective smart lubricant is a long-lasting experimental process. In this study, the film thickness and frictional response of ionic liquid-lubricated non-conformal contacts to an electric field excitation under elastohydrodynamic conditions were examined. Film thickness was evaluated using a "ball-on-disc" optical tribometer with an electric circuit. Friction tests were carried on a mini traction machine (MTM) tribometer with a "ball-on-disc" rotation module and an electric circuit for contact area excitation. The results demonstrate that there is a difference in the behaviour of the ionic liquid during electric field excitation at the evaluated film thicknesses. The results of evaluated film thicknesses demonstrate that there is a difference in the behaviour of the ionic liquid during electric field excitation. Therefore, the ionic liquids could be a new basis for the smart lubrication of mechanical components. Moreover, the proposed experimental approach can be used to identify electrosensitive fluids.

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