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Experimental Study of the Effect of Thickener on the Film Thickness in the Contacts of a Grease-Lubricated Ball Bearing at Low Speed
(Taylor & Francis Group, LLC., 2025-01-02) Okál, Michal; Košťál, David; Osara, Jude A.; Lugt, Piet; Křupka, Ivan; Hartl, Martin
The level of starvation in axially loaded grease lubricated ball bearings can be well described by the product of the base oil viscosity, the half contact width and the rotational speed (gbu). This paper highlights the effect of the thickener as another parameter for estimating the film thickness in a ball bearing. Film thickness measurements were done with an optical simulator of the ballon- disc configuration and a real ball bearing using the capacitance method. Three greases with the same base oil but different thickener types were selected for the experiments. Two greases followed the gbu model but the grease with alicyclic di-urea thickener produced a higher thickness than the model prediction, which is most likely caused by the formation of residual layers on the contact surfaces. The layer formation occurs in the first few hours of operation reaching a thickness of about 500–550 nm. Other greases also form layers, but they are comparable to prior observations, and they follow the model. The evolution of the normalized film thickness versus gbu in the case of alicyclic-di urea grease shows the same trend as the other grease types but with overall higher values.
A Uniaxial Hysteretic Superelastic Constitutive Model Applied to Additive Manufactured Lattices
(John Wiley & Sons, 2024-11-29) Schasching, Marius; Červinek, Ondřej; Koutný, Daniel; Pettermann, Heinz; Todt, Melanie
Lattice materials with superelastic properties offer great potential for engineering applications, as they are able to undergo large deformations while ensuring the reversibility of the deformations due to stress-induced phase transformation. Adequate prediction of the mechanical response of lattice materials requires models that properly capture the deformation mechanisms of the internal architecture and the material response of the parent material. To analyze large-scale lattices by means of the finite element method, numerical efficiency becomes crucial. For this purpose, we propose a simple approach relying on beam-based modeling in combination with a uniaxial superelastic constitutive material model. The latter is based on polynomial functions, which make it easy to take customer-based material data into account being especially important for additive manufactured materials. To verify our constitutive model, a comparison with a well-established standard model is performed. The capabilities of the beam-based model to predict the mechanical response of lattice materials are evaluated by the comparison to high-fidelity models using continuum elements. We show that beam-based modeling is able to capture the governing deformation mechanisms of the investigated lattices and that our constitutive model is able to capture the smooth stress–strain response of the experimental data that are not available to the standard model.
Case Study: Correlations Between Curve Squeal, Weather Conditions, and Traction in a Tram Loop
(Faculty of Engineering, University of Kragujevac, Serbia, 2024-12-15) Valena, Martin; Omasta, Milan; Klapka, Milan; Galas, Radovan; Navrátil, Václav; Křupka, Ivan; Hartl, Martin
This study explores the relationship between the coefficient of traction (CoT) and squeal noise parameters on a tram line loop, focusing on the influence of weather conditions. An automatic noise module was placed near a tram loop known for noise complaints. This module distinguishes between squeal and flange noise, recording their duration, root mean square (RMS) sound pressure level, and maximum sound pressure level when a threshold in the appropriate frequency band is exceeded. Concurrently, weather conditions were monitored, and the CoT on the rail was measured using a BUT rail tribometer. The findings reveal a notable correlation between the CoT and the duration of squeal noise, while the association with sound pressure levels was less pronounced. An increase in CoT was observed with rising relative humidity, which may be attributed to increasing temperature throughout a sunny April day, while absolute humidity remained almost constant. Furthermore, noise parameters rose with higher relative humidity and showed an inverse relationship with temperature. These findings suggest that weather conditions, particularly relative humidity and temperature, influence both the CoT and noise parameters on tram lines.
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.

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