Milan Omasta

Finite element analysis for the evaluation of the structural behaviour, of a prosthesis for trans-tibial amputees

The finite element analysis (FEA) has been identified as a useful tool for the stress and strain behaviour determination in lower limb prosthetics. The residual limb and prosthetic socket interface was the main subject of interest in previous studies. This paper focuses on the finite element analysis for the evaluation of structural behaviour of the Sure-flex™ prosthetic foot and other load-bearing components. A prosthetic socket was not included in the FEA. An approach for the finite element modelling including foot analysis, reverse engineering and material property testing was used. The foot analysis incorporated ground reaction forces measurement, motion analysis and strain gauge analysis. For the material model determination, non-destructive laboratory testing and its FE simulation was used. A new, realistic way of load application is presented along with a detailed investigation of stress distribution in the load-bearing components of the prosthesis. A novel approach for numerical and experimental agreement determination was introduced. This showed differences in the strain on the pylon between the experimental and the numerical model within 30% for the anteroposterior bending and up to 25% for the compression. The highest von Mises stresses were found on the foot-pylon connecting component at toe off. Peak stress of 216MPa occurred on the posterior adjusting screw and maximum stress of 156MPa was found at the neck of the male pyramid.

Effect of Surface Velocity Directions on Elastohydrodynamic Film Shape

This article is focused on the effects of the angle between lubricant entrainment velocity and sliding velocity on elastohydrodynamic film thickness distribution. Thin-film colorimetric interferometry was used to evaluate the film thickness distribution in smooth glass–steel contacts to provide basic data on the effects of the slide–roll ratio and the direction of sliding with respect to entrainment velocity. It was observed that as the sliding perpendicular to the entrainment velocity increased, the overall film thickness was reduced and asymmetry of the film profile with respect to the direction of the entrainment velocity increased. The asymmetry of the film profile with respect to the direction of the entrainment velocity increased with the entrainment speed or the overall film thickness. When the speed of the glass disk was larger than that of the steel ball, a dimple was formed even if there was a difference in direction between the entrainment and sliding velocities. A part of the dimple was exhausted from the elastohydrodynamic lubrication (EHL) conjunction as the angle between the entrainment and sliding velocities approached 90°.

Theoretical and Experimental Investigations on EHL Point Contacts with Different Entrainment Velocity Directions

Film thickness prediction plays an important role in evaluating the performance and durability of machine elements under elastohydrodynamic lubrication (EHL). However, current formulae may not be appropriate for general conditions occurring in real contacts. This study investigates the effect of different lubricant entrainment velocity angles on film thickness distribution. For this purpose, a steady-state isothermal EHL model is used under a wide range of parameter sets including varying sum velocity, contact pressure, and sum velocity angle. Considerable differences in the trend of the central film thickness with respect to the lubricant entrainment velocity angle for low and high loaded contacts are shown. The results are compared with experimental measurements by means of an optical ball-on-disc tribometer and a twin-disc machine using capacitance method. Good agreement between numerical results and experimental measurements was found.

Effect of Sliding Direction on EHL Film Shape Under High Sliding Conditions

ABSTRACT It is well known that a sliding speed influences a lubricant film thickness of elastohydrodynamic rolling–sliding contacts significantly. The effect of sliding is described quite well for unidirectional rolling and sliding; however, there are a limited number of papers dealing with sliding in different directions. This study describes how the sliding direction influences elastohydrodynamic film shape under high sliding conditions. An optical ball-on-disc tribometer together with thin-film colorimetric interferometry method is used for a film thickness measurement. The results show that the sliding direction influences lubricant film shape and the effect is connected with dimple phenomena. The temperature–viscosity wedge effect is discussed as a possible mechanism. The results are important for a film thickness prediction under high sliding conditions and provide experimental evidence for an extension of elastohydrodynamic lubrication (EHL) theory.

The Effect of Friction Modifier on the Wheel-Rail Contact

One of the key factors affecting the rolling-sliding contact is traction, which is expressed by the adhesion coefficient. It is a major characteristic for safety, efficiency, reliability and maintenance costs of rail transportation. The adhesion is influenced by many factors such as contaminants, environmental and operating conditions. The friction modifier (FM) is applied on the top of rail in order to control the adhesion coefficient in required range (0.3–0.35). This range is often called an intermediate level of friction. This study has two aims. The first aim is to introduce tribological aspects of FM. The second aim is to investigate the influence of the FM on the adhesion coefficient. The experiments were carried out on the ball-on-disc apparatus.

Effect of high slide-roll ratio on thermal elastohydrodynamic lubrication in line contacts with surface waviness

In this study, the effect of high slide-roll ratio (2.0 ≤ S ≤ ∞) on thermal elastohydrodynamic lubrication in line contacts is explored numerically. The surface dimple produced by the heat transportation of the surface is explained. Furthermore, the deformation of the surface waviness under zero entrainment velocity is analyzed. The results show that, under high slide-roll ratios, similar to those under rolling or rolling–sliding conditions, the deformation of the surface waviness depends on the slide-roll ratio and the wavelength of the harmonic waviness.

Top-of-Rail Lubricants: Potential Risks and Benefits

Top-of-rail (TOR) lubricants represent modern approach for friction modification between wheel and rail. The main goal of this study was to investigate potential risks and benefits associated with the application of these products, especially in terms of adhesion, wear, and noise. For this purpose, both laboratory and field experiments were carried out.

Foreword: 2018 International Symposium on Rail Infrastructure Systems Engineering (i-RISE 2018)

Social and economic growth, security and sustainability around the globe are at risk of being compromised due to aging and failing railway infrastructure systems. […]

Film formation in EHL contacts with oil-impregnated sintered materials

The purpose of this study is to investigate lubricant film-forming capability of oil-impregnated sintered material in highly loaded non-conformal contacts. This self-lubrication mechanism is well described in lightly loaded conformal contacts such as journal bearings; however, only a little has been published about the application to highly loaded contacts under elastohydrodynamic lubrication regime (EHL).,Thin film colorimetric interferometry is used to describe the effect of different operating conditions on lubricant film formation in line contacts.,Under fully flooded conditions, the effect of porous structure can be mainly traced back to the different elastic properties. When the contact is lubricated only by oil bleeding from the oil-impregnated sintered material, starvation is likely to occur. It is indicated that lubricant film thickness is mainly governed by oil bleeding capacity. The relationship between oil starvation parameters corresponds well with classic starved EHL theory.,To show practical, relevant limitations of the considered self-lubrication system, time tests were conducted. The findings indicate that EHL contact with oil-impregnated sintered material may provide about 40 per cent of fully flooded film thickness.,For the first time, the paper presents results on the EHL film-forming capability of oil-impregnated sintered material by measuring the lubricant film thickness directly. The present paper identifies the phenomena involved, which is necessary for the understanding of the behavior of this complex tribological system.

The Experimental Determination of the Grease Amount to Effective Wear Reduction in the Wheel-Rail Contact

The wear mechanisms within the wheel flange and rail gauge contact affect safety and operating costs of railway transport. This contact occurs when a rail vehicle reaches a curved track. A low coefficient of friction (COF) is required. This COF and wear rate are influenced by operating conditions (load, velocity and temperature), lubrication and geometry (i.e. the track curvature radius). The damage to the contact surfaces can be decreased by using on-board lubrication systems. Grease is applied to the first wheelset of a driving car by a nozzle. The rate of wear reduction depends on the grease amount and its application intervals introduced by the on-board lubrication system into the contact. In this paper, the full-scale laboratory apparatus has been used to study friction, wear and lubricant film distribution under pure sliding conditions in the wheel flange contact. The grease amount and its appropriate application intervals were found.