Measurement Of Lubricant Film Thickness Research Articles

An extended ultrasonic time-of-flight method for measuring lubricant film thickness

An extended ultrasonic time-of-flight (TOF) method is proposed for measuring the thickness of lubricant film. Based on the spectrum analysis method of superimposed pulses, the extended TOF determines the travel time of ultrasonic wave and subsequently the film thickness in frequency domain. The method extends the measurable range of the original TOF method to the lubricant films of superimposed reflected pulses, which usually processed by the resonance model. The extended TOF gives another interpretation to the resonance phenomenon described by the resonance model. In theory and practice, the extended TOF integrates the resonance model and the original TOF method into one approach. This solved the problem that film thickness monitoring program needs to alter between time and frequency domains according to the changing of film thickness. A precision experimental apparatus was designed for verifying the correctness of the method. The lubricant films of different thickness were measured repeatedly and the measuring accuracy was analyzed.

Ultrasonic measurement of lubricant film thickness in sliding bearings with thin liners

When conducting ultrasonic measurements of the lubricant film thickness in sliding bearings with thin liners, the ultrasonic pulse reflected from the bearing liner–lubricant film interface will superimpose on the pulse reflected from the bearing substrate–liner interface. The thickness information of the lubricant film is contained in the reflected pulse from the liner–lubricant film interface. In this case, the film thickness could not be obtained directly from the superimposed reflected signals. The thin liner indicates that the thickness of the bearing liner is less than half the ultrasonic pulse width. Based on the spectrum analysis method of superimposed signals, a new method is proposed to measure the lubricant film thickness in sliding bearings with thin liners. The frequency-domain amplitude ratio between the echo component containing thickness information and the steady echo component from the bearing substrate–liner interface is extracted from the superimposed signal. The reflection coefficient of the liner–lubricant film interface is obtained by this amplitude ratio to determine the film thickness. The lubricant films of different thicknesses in a thin-liner thrust pad were measured in a high-precision experimental apparatus. The measurement results were compared with the known film thickness set by the experimental apparatus. In the thinner film region, the measurement results agreed well with the set film thickness. In the thicker film region, the mean values of the multiple measurement results represented the film thickness. The experimental results show that the method can be used to measure the lubricant film thickness in sliding bearings with thin liners.

A novel FSI–thermal coupled TEHD model and experimental validation through indirect film thickness measurements for the lubricating interface in external gear machines

A novel fluid–structure interaction–thermal coupled model for the lateral lubricating gaps between gears and lateral bushes in external gear machines (EGM) is presented in this study. The model is the first of the lateral lubricating interface in EGMs capable of considering heat transfer, thermal effects in the fluid film and solid components as well as thermo-elastic deformation of the solids enabling prediction of the film thickness in EGM lateral gaps considering thermoelastohydrodynamic lubrication. Model validation is performed using lateral gap leakage measurement which is dependent on absolute film thickness. A novel relative lubricant film thickness measurement method in gear machines using capacitive sensors is also presented, and used to validate the spatial film thickness predictions from the model.

The effect of transient motion on Isoviscous-EHL films in compliant, point, contacts

Laser induced fluorescence was employed to measure lubricant film thickness in a compliant, point, contact during transient motion. Two types of transient sliding motion were investigated: start-up and sudden halting. The effects of acceleration rate and sliding speed on film formation and breakdown were studied.A clear relationship between start-up acceleration and the period of the film formation phase was observed. During sudden halting motion entrapment of fluid occurred in the centre of the contact. This trapped fluid was squeezed out of the contact over several seconds. The size of this entrapment was dependent on the initial sliding speed. The findings are compared to similar results for hard, point, contacts and the implications discussed.

Transient Tribo-Dynamics of Thermo-Elastic Compliant High-Performance Piston Skirts

Advanced piston technology for motorsport applications is driven through development of lightweight pistons with preferentially compliant short partial skirts. The preferential compliance is achieved through structural stiffening, such that a greater entrainment wedge is achieved at the skirt’s bottom edge through thermo-elastic deformation, whilst better conforming contact geometry at the top of the skirt. In practice, the combination of some of these conditions is intended to improve the load-carrying capacity and reduce friction. The approach is fundamental to the underlying ethos of race and high-performance engine technology. Contact loads of the order of 5 kN and contact kinematics in the range 0–35 m/s result in harsh transient tribological conditions. Therefore, piston design requires detailed transient analysis, which integrates piston dynamics, thermo-elastic distortion and transient elastohydrodynamics. The paper provides such a detailed analysis as well as verification of the same using non-invasive ultrasonic-assisted lubricant film thickness measurement from a fired engine under normal operating conditions, an approach not hitherto reported in literature. Good agreement is noted between measured film thickness and predictions.

Lubrication Properties of Polyalphaolefin and Polysiloxane Lubricants: Molecular Structure–Tribology Relationships

This study investigates the rheological properties, elastohydrodynamic film thickness, and friction coefficients of several commercially available polyalphaolefin (PAO) and polydimethylsiloxane (PDMS)-based lubricants to assess relationships between molecular structure and lubricant performance. Molecular structures and masses were determined by nuclear magnetic resonance spectroscopy and gel permeation chromatography, respectively. Density and viscosity are measured from 303 to 398 K, while elastohydrodynamic lubricant film thickness and friction measurements were made at temperatures, loads, and speeds that are representative of boundary, mixed, and full-film lubrication regimes. The results show that PDMS-based lubricants are thermally and oxidatively more stable than PAOs, while the viscosity of PDMS-based lubricants is generally less temperature sensitive than PAOs, except for highly branched polysiloxanes. In particular, this study provides quantitative insight into the use of PDMS-based lubricants to obtain low friction through the entire lubrication regime (boundary to full film) by optimal tuning of the molecular mass and chain branching.

Lubrication of Microelectromechanical Devices Using Liquids of Different Viscosities

Lubrication of contacting and sliding surfaces in MEMS (microelectromechanical systems) is particularly challenging because of the predominance of surface forces at the microscale. The current paper explores the possibility of using liquid lubrication in this application. Measurements of friction and lubricant film thickness have been made for liquid lubricants of different viscosities, including low viscosity silicone oil, hexadecane, squalane, and water. Testing was carried out using a newly developed MEMS tribometer in which a rotating silicon disk is loaded against a stationary silicon disk. Two different test setups were used: one where both disks are flat, and the other where the stationary disk is structured as in a thrust pad bearing. In all tests the disks were fully submerged in the lubricant. With the flat-on-patterned disk combination, the variation of friction with rotation speed was found to follow classical Stribeck curves for all the lubricants tested. The friction at high speeds also decreased with increasing normal load, in accordance with hydrodynamic lubrication theory. For the least viscous lubricants, it was found that the hydrodynamic friction coefficients remained relatively low even at higher speeds. In particular, for water the friction coefficient for water was around 0.1 at 10,000 rpm. However, boundary friction was found to be unacceptably high at low speeds where there was insufficient lubricant entrainment. The experimental results have been compared with a finite difference solution of Reynolds equation and reasonable agreement is seen between theory and experiment. The results indicate that liquid lubrication is potentially an effective means of lubricating MEMS components with high levels of sliding.

Oil film thickness measurement: a contribution to the understanding and control of lubrication in the piston-ring packs of IC engines

Oil film thickness measurement is a vital experimental technique which has been widely applied to investigate lubrication in a range of tribological components. This article focuses on its development and use as a tool to study the hydrodynamic lubrication of piston-rings in internal combustion engines. This includes the development of experimental sensors to measure lubricant film thickness and a review of phenomena related to the hydrodynamic lubrication of piston-ring packs. Future applications of oil film thickness measurement as part of a feedback-based system to control the lubrication of ring packs are also discussed.

Measurements of lubricant film thickness in the iso-viscous elastohydrodynamic regime

This paper describes the application of ultrasonic reflection techniques to measure the lubricant film thickness in iso-viscous elastic lubrication regime. The reflection of ultrasonic pulses from the interface between the sphere and disk was recorded for a range of lubricated, dry, static, and dynamic contact conditions. In this way profiles of oil film thickness were created for various loads and sliding speeds. It was also possible to observe cavitation effects on the signal in the exit region. The measured central film thickness results are compared with published models of the lubrication mechanism and experimental data obtained from optical methods.

Lubricating film thickness measurements with bovine serum

Lubricant film thickness measurements were made for bovine serum solutions under steady state rolling and sliding. The effect of low (30MPa) and high contact pressures (200MPa) was examined. In the high pressure rolling tests BS initially formed films 5–50nm thick over the speed range. However, in subsequent speed sweeps, a relatively speed independent film of 40–50nm developed. In some cases thick (up to 100nm) films were formed at low speeds; this behaviour was considered representative of high-viscosity surface layers rather than of solid films. At the end of each test residual boundary films of 9–19nm were measured under static loading. These are attributed to the multilayer adsorption of protein molecules and will provide surface protection during stance or on initiation of gait. The results at low pressure showed that much thicker films (∼60–80nm) were formed over the same speed range. Again thicker films were formed at the lower speeds. There was significant scatter in the film thickness results, possibly due to the inherent nature of the fluid, which is an inhomogeneous biological solution. The film thickness/speed behaviour was not representative of a simple Newtonian fluid and this has considerable implications for the development of predictive film thickness models and new designs of artificial hip joints.

Development of a multipurpose, efficient and inexpensive bearing test rig

The basic objective of this research is to develop an inexpensive, multipurpose and efficient test rig for condition monitoring of rolling element bearings. The fundamental philosophy of measurement of lubricant film thickness for monitoring the condition of rolling element bearings has been exploited and transformed in the form of a test rig. The developed test rig is potent enough for performance evaluation and condition monitoring of rolling element bearings which is vital for its prolonged life and enhanced performance. A significantly correct prediction of life of bearing can also be made which in turn gives an idea about possible mode and time of failure. Certain machineries like aircraft, battle tanks, rocket launchers, ships, etc. need impeccable working of the system. Failure of such systems is not acceptable at any cost. In the heart of such system, bearings play a central and key role. Still there is no such standard mechanism available for ascertainment of performance and testing of rolling element bearings with high degree of reliability. In the continuing process of improvement and modification in the existing test rig, a simple, versatile, and inexpensive test rig has been developed and presented through this paper. Key words: Condition monitoring, low cost, lubricant film thickness, performance evaluation, rolling element bearing, test rig.

Discussion on the Technique of Relative Optical Interference Intensity for the Measurement of Lubricant Film Thickness

The technique of relative optical interference intensity (ROII) has been developed for measuring lubricant film thickness. Here, we describe in detail the principle and the characteristics of the film thickness measurement system based on the existing ROII method. Some researchers have reported a large calculation error in film thickness with the use of ROII method. The description and analysis of the resolution, accuracy and important factors influencing the measurement results of this measurement technique presented here illustrate that the ROII method should be used in a specific optical system with negligible multiple beam reflections, but not in an ordinary one. The reported calculation error in film thickness using the ROII method comes from using an inappropriate optical system and inappropriate coating parameters. In fact, the ROII method, which is a very accurate method for measuring the thickness of a thin lubricant film, with a high resolution in film thickness of 0.5 nm and a horizontal resolution of 1.4 μm, can only be used in a two-beam interference system.

Friction and lubricant film thickness measurements on simulated synovial fluids

This article is part of a research programme to study fundamental aspects of synovial fluid lubrication of artificial hip joints and focuses on the role of proteins (albumin and γ-globulin) in film formation. Friction and film thickness measurements have been carried out on a series of simple protein solutions (simulating synovial fluid SF) and bovine serum (BS). Protein solutions were prepared with different buffer solutions to simulate healthy and periprosthetic SFs. Simple bench-top tests using a ball-on-disc geometry were used to simulate hip joint articulation. All measurements were made under steady-state load and speed (5—50 mm/s) conditions. The friction results showed that BS and simple protein solutions demonstrate boundary properties by reducing friction in the slow-speed regime. The film thickness results with BS provide evidence of the formation of an adsorbed solid surface layer (<20 nm), which is augmented by a thicker hydrodynamic film. Film thickness was fairly constant over the speed range, but tended to decrease with increasing speed. The choice of buffer solution affected the lubrication properties; much thicker films were recorded for the phosphate-buffered saline compared to 2-amino- 2-(hydroxymethyl)-1,3-propanediol protein solutions. The friction and film thickness results were time and/or rubbing distance dependent, suggesting that the formation of the deposited film was pressure/friction related.

Interaction of Asperities on Opposing Surfaces in Thin Film, Mixed Elastohydrodynamic Lubrication

A novel experimental method has been developed to investigate how model asperities, on opposing surfaces in an elastohydrodynamic (EHD) contact, interact to influence the lubricant film distribution. This technique allows direct measurements of lubricant film thickness during asperity-asperity collision. A surface having a single transverse ridge asperity was rubbed against a second surface having three different roughness features, a transverse ridge, multiple transverse ridges, and an array of hemispherical bumps to study the resultant micro-EHD films. This work reveals how the film thickness is greatly reduced when the peaks of opposing asperities coincide, and how asperities can combine to cause a larger volume of lubricant to be entrapped at their leading edges. The new technique described shows considerable promise for the study of mixed lubrication.

Measurements of Lubricant Film Thickness in Reciprocating Piston Cylinder Assemblies and Engines

The thickness of the lubricant film between the piston and the liner has a very important role in controlling friction and wear, and also influences exhaust particulate emissions. Measurements obtained with LIF and a capacitance probe, complemented by direct imaging through optical windows, have allowed the dependence between film thickness, speed of reciprocation and load to be quantified and the onset and development of cavitation to be revealed. A simplified piston-cylinder assembly was mainly used in this investigation but also a limited number of tests in a single-cylinder diesel engine were obtained to confirm the presence of cavitation in the lubricant film.

Lubricating Properties of Water in Oil Emulsions

In this study the effect of water as a contaminant in lubricated contacts was analytically and experimentally investigated. A steel ball on glass disc apparatus was used to measure lubricant film thickness of pure oil and water in oil emulsions under various operating conditions. A steel ball on steel disc rig was used to measure friction as a function of various loads, slide to roll ratios and water in oil emulsions. A finite difference numerical model was developed using the continuum theory of mixtures and results were corroborated with the experimental measurements. Numerical results are in excellent agreement with the experimental results and indicate that water will flow around the contact. The experimental and analytical results suggest that for heavily loaded contacts water-in-oil emulsions perform essentially the same as pure oils.

ESCA-thickness metrology and head-medium spacing impact of disk lubricant

Electron spectroscopy for chemical analysis (ESCA) has been commonly used as a metrology tool for lubricant film-thickness measurement on magnetic hard disks. The accuracy of the ESCA-thickness measurement rests solely with the calibration accuracy of the characteristic photoelectron attenuation length in the lubricant film. Several past studies on this subject yielded widely divergent results, due to the difficulty in obtaining an accurate, absolute lubricant film-thickness measurement. In this paper, we revisited the calibration issue and, instead of following the same paths pursued in the past, used a derivative method to yield an accurate calibration of the photoelectron attenuation length. We also compared the various methods for lubricant film-thickness calculation based on ESCA measurements and determined that the most accurate method is to use only the photoemission signal from the lubricant film. In addition, by studying the lubricant film-thickness effect on the electrical readback signal, we found that the lubricant film leads to an increase in the head-medium spacing by an amount greater than one times, but less than two times, its physical thickness.

The on-line measurement of lubricant film thickness for condition monitoring

The ultrasonic reflectivity of a lubricant layer between two solid bodies depends on the ultrasonic frequency, the acoustic properties of the liquid and solid, and the layer thickness. In this paper, ultrasonic reflectivity measurements are used as a method for determining the thickness of lubricating films in bearing systems. An ultrasonic transducer is positioned on the outside of a bearing shell such that the wave is focused on the lubricant film layer. The transducer is used to both emit and receive wide-band ultrasonic pulses. For a particular lubricant film the reflected pulse is processed to give a reflection coefficient spectrum. The lubricant film thickness is then obtained from either the layer stiffness or the resonant frequency. The method has been validated using fluid wedges at ambient pressure between flat and curved surfaces. Experiments on the elastohydrodynamic film formed between a ball sliding on a flat were performed. Film thickness values in the range 50-500 nm were recorded which agreed well with theoretical film formation predictions. Similar measurements have been made on the oil film between the balls and outer raceway of a deep groove ball bearing.

Fourier analysis of a single transverse ridge passing through an elastohydrodynamically lubricated rolling contact: A comparison with experiment

An image analysis technique is employed to measure lubricant film thickness fluctuations introduced by a single flat-top transverse ridge passing under pure rolling conditions through an elastohydrodynamically lubricated contact. The experimental observations are compared with the film thickness profiles predicted by the discrete Fourier transform method for real surface roughness given by Morales-Espejel et al. in 2000. The results show that the theoretical approach adequately predicts the overall film thickness, thus giving credence to its potential practical application. On a local scale, however, important differences are seen regarding the deformed shape of the feature.

A multi-beam intensity-based approach for lubricant film measurements in non-conformal contacts

Studies of elastohydrodynamic lubrication (EHL) frequently utilize the phenomenon of optical interference which is exhibited when an illuminated steel ball is in contact with a partially Cr-coated glass disc in conjunction with a lubricant layer of known refractive index. The use of the steel and chromium surfaces enables lubricants having similar refractive indices to glass to be examined. The EHL contact is composed of two microscopically thin layers: the Cr coating and the lubricant film, bounded by the glass disc and the steel ball. That the two-beam interference method frequently used in optical EHL research may induce significant errors in the measurement of film thickness is reported in this paper. A theoretical analysis which includes consideration of the effects of both multi-beam interference and optical absorption within the Cr film and the steel surface is presented. The analysis is used to devise a multi-beam intensity-based test technique which has been validated for a wide range of lubricant film thickness measurements at a resolution of 1 nm and offers a practical minimum detectable film thickness as thin as 1 nm.