Measuring Lubricant Film Thickness with Image Analysis

Ultrasonic measurement of lubricant film thickness in sliding Bearings with overlapped echoes

This paper reports direct measurements of lubricant film thickness in a radially-loaded grease-lubricated model rolling bearing. These measurements are performed on a recently developed set-up, termed the Sapphire Bearing Rig, which allows for direct observation and measurement of film thickness in a single ball-outer ring contact in a rotating bearing using optical interferometry. Measurements are made with a selection of greases covering different base oils and thickener types. The results show that starvation occurs as the bearing speed increases. This is confirmed by observations of the inlet meniscus. The bearing speed at which starvation occurs is determined by grease properties, particularly the base oil viscosity. When the bearing operates for a prolonged time at a constant speed, as is usually the case in practice, the initial film thickness is seen to decay in the first few hours of operation, to eventually reach a steady state value that is lower than that predicted by elastohydrodynamic lubrication (EHL) theory. For conditions tested here, this steady-state film thickness was largely independent of bearing speed, which suggests that it is mainly governed by the level of lubricant supply in the inlet. Such film behavior clearly cannot be predicted using the standard EHL theory. Stopping the bearing for a sufficiently long time leads to a temporary recovery of the film thickness upon restart. These measurements provide new insights into grease lubrication mechanisms of rolling bearings and can aid the development of new tools for improved prediction of grease film thickness in rolling bearings.

Ultrasonic measurement for lubricant film thickness with consideration to the effect of the solid materials

Analysis of Film Thickness Effect in Slow-Speed Lightly-Loaded Elastrohydrodynamic Contacts. Part III. Experimental Measurement of Lubricant Film Thickness.

In this paper, we describe the behavior of ultrathin liquid lubricant films for contact sliders in hard disk drives. In the experiments, the ultrathin liquid lubricant film behavior is investigated using Zdol and cyclotriphosphazene-terminated PFPE lubricant which have different end groups as a function of lubricant film thickness. The disks are examined with a scanning microellipsometer before and after contact slider experiments. It is found that the lubricant film thickness profiles almost do not change, when the lubricant film thickness is less than one monolayer. It can also be observed that lubricant film thickness instability due to dewetting occurs as a result of slider-disk contacts for the tested lubricants and the films undergo spontaneous redistributions, resulting in significantly nonuniform film thickness profiles, when the lubricant film thickness is thicker than one monolayer. In addition, it is found that the observed behavior of ultrathin liquid lubricant films for cyclotriphosphazine-terminated PFPE lubricant contrasts markedly with that for Zdol. The difference between cyclotriphosphazene-terminated PFPE lubricant and Zdol is only the functional end group. Therefore, it may be concluded that their unstable lubricant behavior depends on the chemical structure of functional end groups.

Measuring of distance as presented at the universities is usually limited to dimension down to microns. This orders of distance are sufficient for common engineer’s application. However dimension are a lot smaller when engineers deal with the tribology research. One of the most important progresses in the field of tribology in last century is without doubts the elastohydrodynamic lubrication (EHL). This lubrication regime can be found for example in rolling bearings or gears. Common thickness of the lubricant film in the EHD contact is in order of tens or hundreds of nanometers. Optical method which enables measurements of such distances was developed by Gohar and Cameron in 1960s. This method is based on interference of monochromatic light and provides possibility of obtaining lubricant film thickness from known wavelength of used light and its intensity in the lubricated contact. Students can create central film thickness prediction with use of the analytical formulae of the Hamrock and Dowson. This prediction is then confirmed by hands-on experiment in laboratory. Contact simulator with microscope and red light is used to record sequence of the interferograms. These interferograms are then processed in Matlab® software in order to obtain film thickness.

PurposeThe purpose of this paper is to study starvation in grease-lubricated finite line contacts and to understand film-forming mechanisms of grease-lubricated finite line contacts.Design/methodology/approachA multiple-contact optical elastohydrodynamic (EHL) test rig is constructed to investigate the influences of lubricant properties on film thickness and lubrication conditions at different working conditions. The film thickness is calculated according to the relative light intensity principle. The degree of starvation is evaluated by the air–oil meniscus distance and the corresponding film thickness.FindingsThe experimental results show that for greases with high-viscosity base oil, the high-frequency fluctuation of film thickness is observed in low-speed operating conditions. Reducing the viscosity of the base oil and improving running speed can weaken the fluctuation of film thickness. The degree of starvation increases with increasing base oil viscosity, rolling speed and the crown drop. In addition, reducing the replenishment time by reducing the gap between the rollers also can increase the degree of starvation.Originality/valueStarvation is often to occur in finite line contacts, such as roller bearings and gears; there are still limited finite line contact EHL test rigs, much less multiple-contact optical test rigs. Therefore, the present work is undertaken to construct the multiple-contact test rig and to evaluate the mechanism of starvation in finite line contacts.

Most of finite element softwares for numerical simulation of bulk metal forming solve the mechanical behaviour at the tool‐workpiece interface with Amonton’s law, the constant friction law or any combination of both. These classical friction laws are suited for boundary and mixed lubrication regimes but can’t model thick or thin film lubrication conditions. In this work, two new constitutive laws are discussed and developed for their implementation in a proprietary finite element software for bulk forming modeling. The first formulation considers the flow of a low viscosity fluid between the roughnesses of the workpiece and die surfaces. The actual asperity geometries are modeled with triangular periodic functions. The pressure distribution as well as the corresponding friction stress inside the tool‐workpiece interface are analyzed for lubricant film thickness lower than the roughness magnitude. The second formulation deals with the case of thick film lubricants of temperature dependent viscosity. The Couette and the Poiseuille flows are associated for the evaluation of the friction stress for a linear distribution of the temperature in the thickness of the tool‐workpiece interface. These new interface constitutive laws are introduced in interface elements with the fluid mass balance and the film thickness changes. They are coupled with the plastic wave theory for mixed lubrication regime. The upsetting of a cylinder between two flat rough dies and the well known ring test are presented as first applications.

In the multi-effect evaporator (MED), the seawater falls on the outside of the tube, and the heat is transmitted from the steam flowing inside the tube. As a result, part of seawater evaporates and vapor is generated. Vapor flows in either co-current direction, cross direction or combination of both. The vapor flow exerts external force on falling film and affects film hydrodynamics, heat and mass transfer. This study focuses on analyzing the effects of co-current vapor flow on falling film distribution and hydrodynamics. A two-dimensional (2D) computational fluid dynamics model is developed and validated, in which sea water enters at 65 °C, which represents the maximum limit of current MED plant, and falls on a tube of 25.4 mm diameter from a 2 mm orifice. First, the liquid load is varied between 0.02 and 0.05 kg/(m·s), and its effects on the film thickness and the wetting time are calculated, presented and discussed. Afterwards, the impact of co-current vapor flow on film distribution, wetting time, minimum thickness and its location are quantified and examined by increasing the vapor velocity up to 6 m/s with 2 m/s intervals. It is found that the vapor flow significantly affects the film distribution on the upper half of the tube. The average film thickness and wetting time is reduced by 14.1% and 18.5%, respectively for the vapor velocity of 6 m/s compared to the no vapor flow, for 0.02 kg/(m·s). In addition, the minimum film thickness decreases by 23.1% for 0.02 kg/(m·s). The high decrement in film thickness can lead to dry patches and scale deposition in case of evaporation.

Recently developed colorimetric interferometry technique was used for the study of both minimum and central film thicknesses for a wide range of operating parameters. Over 300 film thickness maps were obtained for the combination of four values of the materials parameter G, five values of the load parameter W and many values of the speed parameter U. The use of a spacer layer extended the range of film thickness measurement down to 5 nm. An excellent agreement was found between experimental values and data obtained from numerical solution presented by Venner and ten Napel, especially for thin lubrication films. An increase in a speed exponent with increasing material parameter G was observed for both central and minimum film thicknesses. The minimum film thickness and, thereby, the ratio between central and minimum film thickness was confirmed to be of a stronger dependence on material and load dimensionless parameters than Hamrock and Dowson equations predict. Presented at the 54th Annual Meeting Las Vegas, Nevada May 23–27, 1999

Understanding the dynamics of evaporating thin liquid films is of practical and fundamental interest. Practically, this understanding is crucial for tuning bubble stability, while fundamentally thin films are an excellent platform to study the characteristics of evaporation-driven two-dimensional (2D) flows. Here, we experimentally study, across a wide range of volatile species concentrations (c0), the spatial and temporal dynamics of film thickness profiles [h(r, θ, t)] over bubbles in binary liquid mixtures subjected to evaporation-induced Marangoni flows. Initially, we probe the spatial structure and show that the spatial symmetry of the film thickness profiles is non-monotonic functions of volatile species concentration with profiles being axisymmetric for both very low (∼1%) and very high (∼90%) concentrations. The temporal evolution of the film thickness fluctuations reveals a similar non-monotonic dependence between the species concentration and the spatial prevalence of fluctuation stochasticity. At a tested intermediate species concentration of 50%, we observe a complete breakdown in spatial symmetry and obtain film thickness fluctuations that are chaotic everywhere in space with spatially invariant fluctuation statistics and rapidly decaying spatial correlation. The observed non-monotonic behavior is a result of the system sensitivity to ambient perturbations scaling as Δγc0(1 − c0)/μ, where Δγ is the difference in equilibrium surface tension between the two species in the mixture and μ is the dynamic viscosity. These insights along with the reported experimental setup serve as an excellent platform to further investigate evaporation-driven 2D chaotic flows.

In this article, the problem of predicting the film thickness in so-called narrow or slender elliptic contacts is revisited. In particular, the variation of the minimum film and central film thickness and their ratio with load and ellipticity are studied. It is shown that the minimum film tends to decrease linearly with increasing ellipticity. The central film thickness, until a certain threshold, decreases very slowly, and then decreases linearly too, with a slope independent of the Hertzian pressure. The ratio central to minimum film thickness is not a constant and varies strongly with ellipticity and load conditions. In this article, it is shown that the ratio of central to minimum film thickness is a linear function of a single nondimensional parameter which has the physical meaning of the ratio of the length of the inlet pressure sweep to the relative width of the contact. Results for narrow contacts published in the engineering literature when represented as a function of this parameter exhibit the same trend. The behaviour is the consequence of the same unifying mechanism that, in the last decade, was discovered to determine aspects of behaviour in elasto-hydrodynamically lubricated (EHL) contacts under time varying conditions, such as surface waviness deformation. For these problems, the single parameter dependence facilitated the development of simple engineering tools (e.g. to predict the roughness deformation). The observation in this article that the ratio central to minimum film thickness is also governed by this parameter opens new perspectives to develop a simple computational engineering tool to predict central and minimum film thickness in EHL contacts.

The Influence of Slide/Roll Ratio on the Film Thickness of an EHD Contact Operating Within the Mixed Lubrication Regime

This paper describes the effect of end-group functionality and molecular weight of ultra-thin liquid lubricant films on contact slider dynamics in hard disk drives. In the experiments, the contact slider dynamics as well as ultra-thin liquid lubricants behavior are investigated using three kinds of lubricants which have different end-groups and molecular weight as a function of lubricant film thickness. The dynamics of a contact slider is mainly monitored using Acoustic Emission (AE). The disks are also examined with a scanning micro-ellipsometer before and after contact slider experiments. It is found that the lubricant film thickness instability due to de-wetting occurs as a result of slider-disk contacts, when the lubricant film thickness is thicker than one monolayer. Their unstable lubricant behavior depends on the chemical structure of functional end-groups and molecular weight. In addition, it is also found that the AE RMS values, which indicate the contact slider dynamics, are almost equivalent, independent of the end-groups and molecular weight for the lubricants, when the lubricant film thickness is around one monolayer. The molecular weight, however, affects the contact slider dynamics, when the lubricant film thickness is less than one monolayer. In other words, the AE RMS values increase remarkably as the molecular weight for the lubricant increases. When the lubricant film thickness is more than one monolayer, the AE RMS values decrease because of the effect of mobile lubricant layer, while the lubricant de-wetting instability affects the contact slider dynamics. Therefore, it would be concluded that the lubricant film thickness should be designed to be around one monolayer thickness region in order to achieve contact recording for future head-disk interface.

Based on the Reynolds equation considering the temperature, the effection of non-newtonian and the magnetic field, the elastohydrodynamic lubrication(EHL) analysis of water-based ferrofluid journal bearing were carried out. The influence of roughness peak height and wavelength on the journal bearing surface with cosine roughness model were analysed. The results reveal that the pressure and film thickness of water-based ferrofluid wave apparently with roughness fluctuation; The fluctuation range of pressure and film thickness increase with the increase of roughness peak height, the minimal film thickness decreases; The pressure and film thickness fluctuation range become more and more sparse with the increase of roughness wavelength, the minimal film thickness increases.