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The Effect of Hardness and Surface Roughness of Elastomer on Friction Between Shoe Soles and Particle-Contaminated Floors
(SPRINGER/PLENUM PUBLISHERS, 2025-07-19) Žůrek, Michal; Markusík, David; Rebenda, David; Kalina, Lukáš; Hartl, Martin; Vrbka, Martin
Nearly one third of workplace injuries results from slip- and trip-induced falls. Solid particles are among the most common floor contaminants in both occupational and outdoor environments, reducing shoe-floor friction and increasing slip risk. This study investigates how rubber hardness and surface roughness affect the frictional behaviour of shoe soles on smooth, particle-contaminated floors. Coefficient of friction (COF) measurements and post-test surface wear analyses were conducted using nitrile rubbers with hardness between 57.9 and 84.0 ShA and varied surface roughness. Samples were slid against smooth epoxy flooring in a pin-on-plate test simulating the heel-strike phase of walking. The floor was either clean or uniformly covered with corundum particles (40-50 mu m, 120-140 mu m, or 280-315 mu m). On clean floors, increasing rubber hardness and roughness significantly decreased COF (p < 0.0001) due to reduced real contact area. Under contaminated conditions, softer and rougher rubbers yielded higher COF values (p < 0.0001). Higher COF correlated with greater floor wear, showing long scratches and grooves, suggesting slip occurs mainly at the particle-floor interface. Rubber hardness and surface roughness primarily influence the strength of the particle-elastomer interface; greater particle-elastomer strength suppresses particle rolling and thereby leads to an increase in COF. These findings indicate that, on particle-contaminated smooth floors, slip resistance is governed more by particle-floor interactions than by rubber adhesion. Increasing outsole roughness and reducing hardness can help mitigate the adverse effects of particle rolling within the contact area and improve the frictional performance of the outsole.
Tribological behaviour of additively manufactured Ti6Al4V with controlled surface structure: An application in small joint implants
(ELSEVIER SCI LTD, 2025-05-27) Odehnal, Lukáš; Ranuša, Matúš; Malý, Martin; Křupka, Ivan; Koutný, Daniel; Hartl, Martin; Vrbka, Martin
This study conducts a tribological analysis of Ti6Al4V samples with and without surface structures, manufactured by 3D printing, to assess their suitability as friction pairs in joint replacements. The behaviour was analysed using a pin-on-plate tribometer under conditions simulating those typically observed in vivo, with a glass counterpart enabling optical observation. The grid structure outperformed other samples: while exhibiting comparable friction to a homogeneous surface, it was the only one capable of retaining albumin in the contact area and restoring the lubrication film after unloading. However, findings suggest further refinements are needed before the structure can be considered for application. Preliminary results indicate a need for greater proximity and regularity in structures to enhance performance.
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.

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