Petr Sperka

Study of film formation in bovine serum lubricated contacts under rolling/sliding conditions

The aim of this study is to perform a detailed experimental analysis of lubricant film thickness of bovine serum within the contact between the artificial metal and ceramic heads (balls) and the glass disc to analyse the effect of proteins on film formation under various rolling/sliding conditions. Lubricant film observation of bovine serum solutions was carried out using an optical test rig. Chromatic interferograms were recorded with a high-speed CMOS digital camera and evaluated with thin film colorimetric interferometry. Film thickness was studied as a function of time. Under pure rolling conditions, film thickness increases with time as well as with rolling distance for all mean speeds and for both materials of the balls; however the metal ball always forms a thicker lubricating film in comparison to the ceramic ball. Under rolling/sliding conditions, when the disc is faster than the ball, the formation of lubricant film thickness is different compared to pure rolling conditions. At first, film thickness increases rapidly with a rolling/sliding distance for all mean speeds. When maximum film thickness is reached, then this effect is lost and film thickness starts to fall and finally, at the end of the measurement, film thickness drops down to a few nanometres. For the metal ball, maximum values of central film thicknesses are proportional to the mean speed; however this is not observed with the ceramic ball. An absolutely different formation of bovine serum film thickness is observed when the ball is faster than the disc. Under this condition, the protein layer is very thin for both materials of balls, and central film thickness reaches only about a few nanometres. Local protein spots are formed in a very small area of the contact zone and reach the thickness between 20 and 25 nm for the metal ball and 5 nm for the ceramic ball. From the performed experiments under rolling/sliding conditions, it is obvious that the formation of lubricant film thickness is markedly dependent on kinematic conditions acting in the contact, especially on the positive and negative slide-to-roll ratio and the mean speed. In addition, the material of the artificial head has a certain influence on the formation of bovine serum lubricating film.

Evidence of Plug Flow in Rolling–Sliding Elastohydrodynamic Contact

The existence of shear bands, boundary slippage or shear localization was observed several times by various researchers. At the same time, the concept of limiting shear stress is almost exclusively used for traction modeling without clear explanation of physical phenomena that it is associated with. Despite it, the classic linear distribution of speed through-film profile is often considered in current numerical analyses and in our thinking about experimental results. It seems there is a small effort to unify our approaches to meet general concept that can explain a wide range of phenomena. This paper presents experimental results that point out on irregular distribution of through-film speed profile. The observations show an existence of cohesive core of entrainment speed surrounded by two shear zones located closely to the surfaces. This result represents experimental evidence for plug flow mechanism in highly loaded rolling–sliding elastohydrodynamic contact. Possible mechanisms of shear localization are discussed.

Prediction of Real Rough Surface Deformation in Pure Rolling EHL Contact: Comparison with Experiment

The ability to predict the in-contact deformation of surface topography is very important for the design of machine components with respect to minimizing the friction and wear of rubbing surfaces. In this study the amplitude attenuation principle is verified as a simple tool for this purpose. Measured lubricant film profiles are compared with prediction based on this principle. From the results obtained it appears that the amplitude attenuation principle provides reasonable estimation of the deformation of rubbing surfaces that can be used for the prediction of in-contact behavior of surface roughness. Good agreement was obtained under pure rolling conditions, which provides a good initial point for the other studies under rolling/sliding conditions where the wear of rubbing surfaces is of key importance.

Experimental Study of Roughness Effect in a Rolling–Sliding EHL Contact. Part I: Roughness Deformation

ABSTRACT This article presents experimental results of artificial roughness behavior inside elastohydrodynamic contacts. An optical tribometer with a high-speed camera was used to measure the film thickness distribution inside the contact. The results are compared to the amplitude attenuation theory including a description of harmonic components modification. Part I of the article deals with roughness deformation. The effects of central film thickness and slide-to-roll ratio on the roughness feature deformation were investigated for two lubricants. It was found that roughness is independent of sliding magnitude and position inside a highly loaded zone. The results showed a good correlation with Hookes approximate amplitude attenuation model. However, it reaches a limiting deformation under thin film conditions. Part II of the article deals with complementary effect connected with surface roughness under rolling–sliding conditions.

Experimental Study of Roughness Effect in a Rolling–Sliding EHL Contact. Part II: Complementary Effects

ABSTRACT Part I of the article deals with roughness deformation. Part II presents experimental results of complementary effects under rolling–sliding. An optical tribometer with a high-speed camera was employed to measure roughness passage through the contact. The entire film profile of complementary effects was evaluated by combining several interferograms. The film profiles were analyzed by means of Fourier transform and related to the initial roughness profile. Achieved amplitude ratios of various harmonic components were compared with existing theoretical data from amplitude attenuation theory. This approach allows a general description of roughness effects in elastohydrodynamic contacts. It was found that it is not necessary to consider the amplitude decay rate and in-contact wavelength modification of the complementary effect for the studied case. Part of the measured amplitude ratios showed an agreement with theory; however, the other part showed a significant discrepancy. An explanation based on different wavelength scaling at the contact inlet and short wavelength limitation was suggested. A relation to lubricant flow and necessary future developments are discussed.

Lubricant Rupture Ratio at Elastohydrodynamically Lubricated Contact Outlet

Elastohydrodynamically lubricated (EHL) contacts rarely exist as single contacts. Multiple contacts or single contacts subjected to the repeated over-rolling represent more often the case in practical applications. A typical example is the rolling element bearing. A lubricant rupture mechanism at each contact outlet determines the lubricant availability to the succeeding contact. This work presents a quantitative description of the lubricant film thickness rupture in EHL contact outlet with the use of the fluorescent microscopy. A rupture ratio of the film thickness between two diverging surfaces exiting the contact was measured for both pure rolling and rolling–sliding conditions. The influence of variation of several parameters such as lubricant properties, rolling speed or rolling element ellipticity to the lubricant rupture ratio was investigated. Understanding of the physical phenomena of the lubricant rupture extends further possibilities in both experimental and theoretical researches of the starved EHL.

Quantitative elastohydrodynamic film forming for a gear oil with complex shear-thinning

Perhaps the most thorough characterization of the elevated pressure properties of any commercial EHL lubricant is presented here for a gear oil. Compressibility, thermal conductivity, and low-shear viscosity were measured. Of particular interest is the shear dependence of viscosity, measured across four decades of stress, which shows two transitions each with a specific value of power-law exponent. An attempt to capture a suspected third transition at very high stress resulted in mechanical degradation of the liquid in the viscometer. Numerical simulations of a point contact between a steel ball and a glass disc showed good agreement over a range of slide-to-roll ratio for the measured central thickness. The agreement for the minimum thickness was excellent. A new result is that shear-thinning of a higher molecular weight component that occurs from 3 to 200 kPa had little effect on the film thickness and could therefore be neglected in a film thickness calculation.

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.

Occurrence of High Pressure Spike in Unidirectional Start–Stop–Start Point Contacts

The transient film thickness and pressure distributions in point elastohydrodynamic lubrication (EHL) contacts during start–stop–start motion are discussed based on experimental and numerical analyses. When the machine element starts to move after the stopping, where the oil is entrapped between two surfaces, the pressure at the exit area increases very much. The pressure increase depends markedly on the overall film thickness before the stopping of the motion, but is hardly controlled by the acceleration after the stopping. It can be considered that this phenomenon affects the rolling contact fatigue damage.

Thermal Elastohydrodynamic Lubrication of Ceramic Materials

ABSTRACT The effect of thermal conductivity on point elastohydrodynamic lubrication (EHL) contacts was discussed with representative engineering ceramics and steel by a non-Newtonian thermal EHL analysis. Through this investigation, fundamentals of EHL characteristics of contact surfaces composed of different thermal conductivities were generally revealed and a combination of optimum ceramic materials has been proposed. Furthermore, when the contacting materials are the same, it has been pointed out that the equivalent elastic modulus should be rather small and the thermal conductivity of the contact material should be high to obtain a thick overall film thickness and low film pressure.