Measurement Of Lubricant Film Thickness Research Articles

Accuracy-improved ultrasonic phase algorithm for measuring lubricant film thickness

The reflected ultrasonic waves are generally used for the determination of the lubricant film thickness, which consist of a directly reflected wave at the first solid-lubricant interface and the other reflected waves passing through the oil layer. The whole reflected waves are generally utilized in current available ultrasonic algorithms. However, the directly reflected wave is independent on the lubricant film, which can be ignored for improving the measurement accuracy. In the present work, an accuracy-improved ultrasonic phase algorithm is proposed based on the reflection coefficient phase of the other reflected waves passing through the oil layer, rather than that of the whole reflected waves. Both experimental and finite element analysis results show that the proposed new algorithm possesses a high sensitivity and a robust anti-noise interference ability, which effectively improves the measurement accuracy, especially for thin films. With comparison to the current available phase algorithm, the maximum measurement error of lubricant film thickness decreased from 45 % to 6 %.

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

An improved ultrasonic reflection coefficient phase model is proposed for lubricant film thickness measurement with consideration to the influence of the solid layers. In the model, the theoretical relation of the lubricant film thickness to ultrasonic reflection coefficient phase is established, which is used for effectively determining the lubricant film thickness. Furthermore, the proposed model is verified by both the finite element analysis and experiments. The results show that the lubricant film thickness ranges between 2 μm and 45 μm can be measured by using the proposed model, which covers both the spring model region and the blind region. In addition, at a small film thickness, the reflection coefficient phase is significantly affected by the acoustic impedances in both solid layers. However, the reflection coefficient phase is dominantly affected by the solid layer in the neighbourhood of the incidence source at a large film thickness.

Tribotronic control of an ionic boundary layer in operando extends the limits of lubrication

The effect of electric potential on the lubrication of a non-halogenated phosphonium orthoborate ionic liquid used as an additive in a biodegradable oil was studied. An in-house tribotronic system was built around an instrument designed to measure lubricant film thickness between a rolling steel ball and a rotating silica-coated glass disc. The application of an electric field between the steel ball and a set of customized counter-electrodes clearly induced changes in the thickness of the lubricant film: a marked decrease at negative potentials and an increase at positive potentials. Complementary neutron reflectivity studies demonstrated the intrinsic electroresponsivity of the adsorbate: this was performed on a gold-coated silicon block and made possible in the same lubricant system by deuterating the oil. The results indicate that the anions, acting as anchors for the adsorbed film on the steel surface, are instrumental in the formation of thick and robust lubricating ionic boundary films. The application of a high positive potential, outside the electrochemical window, resulted in an enormous boost to film thickness, implicating the formation of ionic multi-layers and demonstrating the plausibility of remote control of failing contacts in inaccessible machinery, such as offshore wind and wave power installations.

A Novel Set-Up for In Situ Measurement and Mapping of Lubricant Film Thickness in a Model Rolling Bearing Using Interferometry and Ratiometric Fluorescence Imaging

This paper describes a unique experimental set-up constructed for studies of lubricant behaviour in an operating rolling element bearing including in situ quantitative measurements of film thickness in and around the element-raceway contact. The set-up is based on a deep groove ball bearing in which the outer race is made of sapphire to allow full optical access to the zone in which the rolling elements are loaded against it. This allows direct imaging of lubricant films under both steady-state and transient conditions and at contact pressures and rotational speeds representative of those present in real rolling element bearings. Optical interferometry is used to measure thin EHL films inside the ball–raceway contacts while a specific laser induced fluorescence approach, referred to as ratiometric fluorescence, is implemented to observe the lubricant distribution and quantify its thickness ahead of the ball–raceway contact. Results are presented to validate the accuracy of the method and to investigate the influence of bulk lubricant viscosity and bearing speed on contact film thickness, inlet starvation and lubricant distribution around the ball–raceway contact. To the best of our knowledge, the work described here is the first to directly measure lubricant distribution and EHL film thickness in a ball–raceway contact in an operating radial rolling bearing.Graphical

Instrumented Engine Bearings for Lubricant Film Thickness Measurement

Instrumented Engine Bearings for Lubricant Film Thickness Measurement

The application of the principle of wave superposition in ultrasonic measurement of lubricant film thickness

The principle of resonance is often applied in the ultrasonic measurement of the thickness of lubricant films. The information data relating to the thickness of the lubricant film involves the acquisition of the resonance frequency of a continuous wave with the application of a continuum model. However, ultrasonic signals are often times, generated by piezoelectric elements but these exhibit some variations from the continuous wave because they are dispersed pulses that have a wide range of frequency bandwidths. In this paper, the authors have applied the principle of wave superposition for the measurement of lubricant film thickness. This principle exhibits the capability to generate the resonant mechanisms of the pulse waves. The implemented theoretical analysis in this research work, depicted that the principle of wave superposition may demonstrate more applicability than the continuum model under the condition of the partial inclusion of the reflected pulse waves. The application of this principle facilitated the analysis of the resonant mechanisms of the pulse wave from the phase variations of the reflected echoes. It followed the further investigations into the influence of both the number of the reflected echoes and the materials of the solid layers on the film resonance method. The results of these investigations were validated using experimental results.

On the Temperature and Lubricant Film Thickness Distribution in EHL Contacts with Arbitrary Entrainment

An understanding of mechanisms which are responsible for elastohydrodynamic lubricant film formation under high sliding conditions is necessary to increase durability of machine parts. This work combines thin-film colorimetric interferometry for lubricant film thickness measurement and infrared microscopy for in-depth temperature mapping through the contact. The results describe the effect of operating conditions such as speed, slide-to-roll ratio, ambient temperature, and sliding direction on lubricant film thickness and temperature distribution. Film thickness data shows how much the film shape is sensitive to operating conditions when thermal effects are significant, while the temperature profiles provides an explanation of this behavior.

Wide Range Measurement of Lubricant Film Thickness Based on Ultrasonic Reflection Coefficient Phase Spectrum

The ultrasonic technique is very effective in measuring lubricant film thickness in a noninvasive manner. To estimate the film thickness with reflection signals, two main ultrasonic models are often applied in cases of different film thicknesses; they are the spring model for thin films and the resonant model for thick films. However, when measuring oil film thicknesses distributed in a wide range, there is an inherent blind zone between these two models. This problem is especially prominent in online monitoring because the abrupt variation of film thickness is highly correlated with the occurrence of abnormal conditions. To address this issue, we further proposed a method using the phase spectrum of reflection coefficient which can cover a wide range of film thicknesses. The slight variation of reflection signal in the blind zone can then be identified and bridged the measurement gap between those two traditional models. A calibration rig was used to verify the theoretical analysis and the results indicated that the developed model is capable of providing reliable ultrasonic measurement of lubricant film thicknesses in a wide range.

Measurement of Lubricant Film Thickness in a Slider-on-Disc Contact

Measurement of Lubricant Film Thickness in a Slider-on-Disc Contact

The influence of lubricant degradation on measured piston ring film thickness in a fired gasoline reciprocating engine

Abstract A laser induced fluorescence system has been developed to visualise the oil film thickness between the piston ring and cylinder wall of a fired gasoline engine via a small optical window mounted in the cylinder wall. A fluorescent dye was added to the lubricant in the sump to allow the lubricant to fluoresce when absorbing laser radiation. The concentration of the dye did not disturb the lubricant chemistry or its performance. Degraded engine oil samples were used to investigate the influence of lubricant quality on ring pack lubricant film thickness measurements. The results show significant differences in the lubricant film thickness profiles for the ring pack when the lubricant degrades which will affect ring pack friction and ultimately fuel economy.

Continuous Lubricant Film Thickness Measurement Between Piston Ring and Cylinder Bore

Measurement of film thickness between piston ring and cylinder bore has been a challenge for decades; laser-induced fluorescence (LIF) method was used by several groups, and promising results are obtained for the investigation of lubricant film transport. In this study, blue light generated by a laser source is transmitted to a beam splitter by means of a fiber optic cable and combined with another fiber optic line, then transmitted to the piston ring and cylinder bore conjunction. The light causes the fluorescence dye present in the lubricant to emit light in a longer wavelength, i.e., green. Reflected light is recollected; blue wavelength components are filtered out using a narrow band pass optical filter, and only components in the florescence wavelength is transmitted to a photomultiplier tube. The photomultiplier produces a voltage proportional to instantaneous lubricant film thickness. Then, the photomultiplier signal is calibrated for lubricant film thickness using a laser textured cylinder bore with known geometries. Additional marks were etched on the liner for calibration. The LIF system is adapted to a piston ring and cylinder bore friction test system simulating engine conditions. Static piston ring and reciprocating liner configuration of the bench test system allow the collection of continuous lubricant film thickness data as a function of crank angle position. The developed system has potential to evaluate new designs, materials, and surface properties in a controlled and repeatable environment.

Measurement of nanometer-thick lubricating films using ellipsometric microscopy

Abstract A method based on vertical-objective-based ellipsometric microscopy (VEM) is presented for measuring lubricant film thickness in nanometer sliding gaps. It provides an image of nanometer-thick lubricating films in real time at high lateral and thickness resolutions without any special layers. The ellipsometric image is directly converted into the film image by using a piezo-stage displacement method combined with a rotating compensator ellipsometry method. The accuracy of thickness measurement is about 1 nm. The VEM-based method revealed that nanometric deformation of the sliding surfaces arises in nanometric gaps even if the load is low, which significantly affects the lubrication properties in small gaps. This method is useful for clarifying the lubrication phenomena in nanometric sliding gaps.

An Improved Interference Method for Measuring Lubricant Film Thickness Using Monochromatic Light

Optical interferometry is one of the most important methods used to measure lubricant film thickness. Based on the principle of relative optical interference intensity and using monochromatic light, this paper proposes an improved interference method to measure lubricant film thickness. First, a universal formula for calculating lubricant film thickness is deduced according to the basic principle of relative optical interference intensity. Then, based on the actual curve describing the relationship between the light intensity and film thickness, the accuracy of the interference method for monochromatic light is improved. The methods used to calibrate the inference order are also discussed. This paper uses the proposed method to measure the lubricant film thickness of base oil. The proposed method was validated through measurements and compared to the calculation results from the Hamrock–Dowson formula and another measurement method. Finally, a fitting formula for calculating the film thickness was derived for the light loading and high-velocity conditions.

The behaviour of lubricated EHD contacts subjected to vibrations

Machine components containing contacts working in elastohydrodynamic (EHD) conditions are often subjected to vibrations. These may be originated from the mechanism or machine the contact is part of, the surrounding environment and within the contact itself. The influence of vibrations upon the behaviour of elastohydrodynamic films has been studied experimentally in a number of papers, but a comprehensive study of the effect of the parameters of the oscillatory motion upon the film thickness has not been carried out yet. In this study the authors evaluate the effect of the frequency of the oscillatory motion upon the EHD film thickness. Optical interferometry is used to measure lubricant film thickness in a ball-on-flat disc arrangement. A high – speed camera records the interferometric images for later analysis and conversion into film thickness maps. The disc runs at a constant angular velocity while the ball is driven by the traction forces developed in the EHD film. In steady state conditions, this would ensure pure rolling conditions, however in the present investigation the ball is subjected to harmonic vibrations in a direction perpendicular to the plane of the film. The contact under study is lubricated by basic oils and the temperature is kept at a constant value of 60°C. The aim of this paper is to understand how vibrations influence the lubricant film formation.

光干渉による潤滑膜厚計測

Optical interference is one of the most powerful tools for lubrication studies, since it enables in-situ measurements of lubricant film thickness under practical conditions. After various improvements, optical interferometry has been developed to apply not only to thick (e.g., micrometer-scale) films in hydrodynamic lubrication regime but also to thin (e.g., nanometer-scale) films in boundary lubrication regime. This paper reviews the principles of measuring lubricant film thickness by optical interferometry and some technical tips on methods using white-light spectroscopy.

Elastohydrodynamic film thickness of soft EHL contacts using optical interferometry

Robust, chromium, semi-reflective coatings have been applied to transparent polymethylmethacrylate and polyurethane discs and this has enabled conventional, normal incidence optical interferometry to be used to measure lubricant film thickness in soft EHL conditions for the first time. High quality interferograms comparable to those obtained from coated glass discs are obtained. Measured film thickness has been compared with existing soft EHL film thickness equations obtained using computer modelling and revised central and minimum film thickness equations have been proposed. These film thickness measurements and measurement technique have applicability to our understanding of the performance and design of lubricated gears and bearings manufactured from polymeric materials.

Discussion on Surface Plasmon Resonance Technique in the Otto Configuration for Measurement of Lubricant Film Thickness

This study presents a novel method of in situ observation of lubricant films formed within contact interfaces, focusing on the use of the surface plasmon resonance (SPR) technique in the Otto configuration, i.e., the SPR-Otto system. First, a numerical analysis based on the multiple-beam interference theory showed that the SPR-Otto method proposed in this study exhibits high sensitivity for the measurement of lubricant film thickness, comparable to that of a well-known optical interferometry method. Additionally, the advantages and disadvantages of the SPR-Otto method were considered on the basis of the numerical results. Subsequently, a model experiment was performed using an SPR-Otto optical microscope newly developed in this study to demonstrate the predictions obtained in the numerical analysis.

Lubricant film thickness and friction force measurements in a laser surface textured reciprocating line contact simulating the piston ring–liner pairing

Applying surface texture to piston liners may provide an effective means of controlling friction and hence improving engine efficiency. However, little is understood about the mechanisms by which pockets affect friction, primarily because of a lack of reliable experimental measurements. To address this, the influence of surface texture on film thickness and friction force was measured simultaneously in a convergent-divergent bearing, under conditions that closely replicate an automotive piston ring-liner conjunction. Film thicknesses were measured using a modified version of the ultra-thin film optical interferometry approach, enabling film thicknesses <50nm to be measured under transient, mixed lubrication conditions. This involved using the out-of-contact curvature of the specimens in place of a spacer layer and analysing multiple interference fringes to avoid fringe ambiguity. Tests were performed on both a textured sample (with features oriented normal to the direction of sliding) and a non-textured reference sample, while angular velocity, applied normal load and lubricant temperature were controlled in order to study the effect of varying lubrication regime (as typically occurs in service). Results showed that the presence of surface pockets consistently enhances fluid film thickness in the mixed lubrication regime by approximately 20nm. Although this is only a modest increase, the effect on friction is pronounced (up to 41% under these conditions), due to the strong dependence of friction on film thickness in the mixed regime. Conversely, in the full film regime, texture caused a reduction in film thickness and hence increased friction force, compared with the non-textured reference. Both textured and non-textured friction values show nearly identical dependence on film thickness, (showing that, under these conditions, texture-induced friction reduction results entirely from the change in film thickness). These results are important in providing film thickness data to validate piston-ring lubrication models and also in helping to understand the effect of surface roughness on texture performance.

Influence of wetting and thermophysical properties of diamond-like carbon coatings on the frictional behavior in automobile gearboxes under elasto-hydrodynamic lubrication

Abstract With regard to the transportation sector, an efficient powertrain technology contributes to sustainable lowering of CO2 emissions. Due to the periodic or continuous operation in boundary and mixed friction, minimization of friction losses in automobile gearboxes offers massive potential in terms of efficiency improvement and saving fossil fuels. In close cooperation between the Surface Engineering Institute (IOT) and the Gear Research Center (FZG), the aim of this work was to reduce friction losses in powertrain by diamond-like carbon (DLC) coatings on highly loaded gears under severe rolling–sliding conditions. The zirconium based DLC coatings ZrCg (a:C-H/ZrCg) and nanocomposite ZrC (a-C:H/ZrC) were deposited by physical vapor deposition (PVD) at IOT. The industrial DLC coating DLC-REF1 served as reference. Application-related tribological tests of lubricated highly-loaded rolling–sliding contacts were performed in a twin-disc test-rig and a gear efficiency test-rig at FZG. Calculations and measurements of relative lubricant film thickness confirmed that the tribological model tests covered the entire friction regime from boundary and mixed friction to fluid friction (elasto-hydrodynamic lubrication, EHL). Despite complete separation of the coated surfaces, the Coefficient of Friction was reduced by 35% using ZrCg coated discs in the twin-disc test-rig. Practical investigations of DLC coated gears in the FZG gear efficiency test-rig revealed that compared to uncoated gears friction losses in EHL were reduced by up to 25% using the industrial reference DLC-REF1 and 39% using ZrCg, especially at higher loads and higher circumferential speeds. This yet widely unknown favorable effect of DLC coatings under EHL conditions was attributed to the thermophysical properties of DLC coatings and confirmed by simulations of real rolling–sliding contacts at FZG. Wetting analyses of tribological surfaces were analyzed determining the surface properties, interfacial tension and surface energy, of the DLC coatings and the gear oils by means of contact angle measurements. The adhesion energy was calculated from contact angle data. Correlation analyses revealed a clear impact of the interfacial tension and adhesion energy on the frictional behavior under boundary and fluid friction conditions. It was found that a higher adhesion energy (good wetting) contributes to a lower CoF under boundary and mixed friction conditions as well as in the fluid friction regime (EHL).

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

The lubricant film thickness in sliding bearings with a thin liner have previously been immeasurable due to the overlapping of ultrasonic signals making the use of the ultrasonic spring model impossible. In this paper, we firstly modeled the overlapped echoes in terms of superimposed Gaussian echoes corrupted by noise and used the EM algorithms to extract the interested echo. Then the extracted echo was used to obtain the lubricant film thickness by an ultrasonic spring model. A high precision calibrated rig which could form a series of known film thicknesses was set up and the performance of the method was shown to work well.