Boundary Conditions for Elastohydrodynamics of Circular Point Contacts

The paper presents the solution of an elastohydrodynamic point contact condition using inlet and outlet lubricant entrainment with partial counter-flow. The inlet and outlet boundaries are determined using potential flow analysis for the pure rolling of contiguous surfaces. This shows that Swift–Stieber boundary conditions best conform to the observed partial counter-flow at the inlet conjunction, satisfying the compatibility condition. For the outlet region, the same is true when Prandtl–Hopkins boundary conditions are employed. Using these boundary conditions, the predictions conform closely to the measured pressure distribution using a deposited pressure-sensitive micro-transducer in a ball-to-flat race contact. Furthermore, the predicted conjunctional shape closely conforms to the often observed characteristic keyhole conjunction through optical interferometry. The combined numerical–experimental analysis with realistic boundary conditions described here has not hitherto been reported in the literature

Study of film formation in EHD contacts using a novel method based on electrical capacitance

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.The elastohydrodynamic lubrication regime (EHD) is found in many machine elements, such as rolling element bearings, gears, cam/tappet, where a combination of hydrodynamic effect, elastic deformation of the surfaces and an increase of the lubricant’s viscosity with pressure create a continuous lubricant film which is capable of supporting pressures of the order of tens of thousands of atmospheres. One of the most important features of these films is their thickness, as this determines whether the bounding surfaces are completely separated, thus avoiding premature wear and failure of the contact. Consequently for many years scientists were interested in finding methods for measuring the lubricant film thickness in elastohydrodynamic conditions. One of the most versatile and widely used techniques for measuring lubricant film thickness in EHD contacts is the optical interferometry method. Apart from numerous advantages, this method has the limitation in the fact that one of the contacting surfaces must be transparent, usually glass or sapphire, thus it does not replicate real conditions found in machine elements contacts. On the other hand, the other group of methods used for studying the behaviour of elastohydrodynamic films includes a variety of electrical methods. Historically, these appeared before the optical methods, but gradually lost importance with the success of the later. Most capacitive, resistive, inductance methods developed so far use specially designed sensors for monitoring the lubricant film thickness. In the case of electrical techniques, both elements of the contact are metallic, which means that these can be used for measuring film thickness in real machine elements. One of the main disadvantages of electrical methods though, is the difficulty with which the calibration of various electrical quantities, against lubricant film thickness is obtained. This thesis describes the work carried out by the author on the application of a capacitive method for studying lubrication of elastohydrodynamic contacts. The novelty of the method used consists in the calibration of the capacitance of the contact with optical interferometry. This project started from the premises that a thicker Chromium layer will supply the phase change needed to precisely measure the lubricant film thickness by eliminating the fragile silica layer, and it has been shown that an increase in Cr thickness results in a increase in reflection of the glass–Cr interface making the resulting images hard to process. Modifications to the existing experimental rig were carried out in order to apply/collect an electrical signal from both the disc and the ball. Signal collection from the disc was quite straightforward and a graphite brush paired with a copper nut was used, as this is the oldest method of collecting/applying and electrical signal from a rotating element. Collecting an electrical signal from the ball presented quite a challenge as the ball is submerged in oil. A number of brushes was designed, made and tested and the one that provided the most stable results chosen. For calibration purposes a base oil and two additives were chosen, the additives were chosen in such a way that the improvement made to the lubrication process to be very different from one additive to the other. The chosen additives were a Viscosity Index Improver [VII] and an Organic Friction Modifier [OFM]. The VII is used by many researchers in order to obtain multigrade lubricants using the same base oil by varying its percentage in the mix. The OFM is used to provide protection between the two contacting bodies when EHD film fails and EHD lubrication is replaced by mixed lubrication by forming a boundary layer on the contacting surfaces. Optical measurements were carried out on the base oil and the two resulting lubricants from the additive mixes using the Ultra Thin Film Interferometry [UTFI] method. The measurements were used as a benchmark against which the capacitive measurements were calibrated. Tests were conducted in a number of controlled conditions for speed, temperature, load and sliding conditions. Results showed that the highest influence on the lubrication process was given by the speed, an increase in speed results in an increase in optically measured film thickness and a decrease in electrically measured film thickness. Phenomenon explained by a large amount of lubricant pushed into the contact. Another parameter that influenced the results quite significantly was temperature, a rise in temperature supplies a decrease in optically measured film thickness and an increase in capacitive measured film thickness which was explained by lubricant viscosity dropping with a rise in temperature. Three different sliding conditions were employed and a small drop in optically measured film thickness followed by a small rise in electrically measured film thickness was recorded due to a local increase in contact temperature when sliding was employed. The capacitive method developed in this project is precise enough to accurately measure lubricant film thickness down to 100nm; a model for thicknesses lower that 100nm was proposed Results from the optical and capacitive methods were compared and a good correlation was found, indicating that the developed capacitive method can be used as a tool for measuring metal on metal contacts without further calibration.EPSR

Elastohydrodynamic film thickness and traction in rolling, spinning and sliding point contact

Imperial Users onl

Study of film formation in EHD contacts using a novel method based on electrical capacitance

The elastohydrodynamic lubrication regime (EHD) is found in many machine elements, such as rolling element bearings, gears, cam/tappet, where a combination of hydrodynamic effect, elastic deformation of the surfaces and an increase of the lubricant’s viscosity with pressure create a continuous lubricant film which is capable of supporting pressures of the order of tens of thousands of atmospheres. One of the most important features of these films is their thickness, as this determines whether the bounding surfaces are completely separated, thus avoiding premature wear and failure of the contact. Consequently for many years scientists were interested in finding methods for measuring the lubricant film thickness in elastohydrodynamic conditions. One of the most versatile and widely used techniques for measuring lubricant film thickness in EHD contacts is the optical interferometry method. Apart from numerous advantages, this method has the limitation in the fact that one of the contacting surfaces must be transparent, usually glass or sapphire, thus it does not replicate real conditions found in machine elements contacts. On the other hand, the other group of methods used for studying the behaviour of elastohydrodynamic films includes a variety of electrical methods. Historically, these appeared before the optical methods, but gradually lost importance with the success of the later. Most capacitive, resistive, inductance methods developed so far use specially designed sensors for monitoring the lubricant film thickness. In the case of electrical techniques, both elements of the contact are metallic, which means that these can be used for measuring film thickness in real machine elements. One of the main disadvantages of electrical methods though, is the difficulty with which the calibration of various electrical quantities, against lubricant film thickness is obtained. This thesis describes the work carried out by the author on the application of a capacitive method for studying lubrication of elastohydrodynamic contacts. The novelty of the method used consists in the calibration of the capacitance of the contact with optical interferometry. This project started from the premises that a thicker Chromium layer will supply the phase change needed to precisely measure the lubricant film thickness by eliminating the fragile silica layer, and it has been shown that an increase in Cr thickness results in a increase in reflection of the glass–Cr interface making the resulting images hard to process. Modifications to the existing experimental rig were carried out in order to apply/collect an electrical signal from both the disc and the ball. Signal collection from the disc was quite straightforward and a graphite brush paired with a copper nut was used, as this is the oldest method of collecting/applying and electrical signal from a rotating element. Collecting an electrical signal from the ball presented quite a challenge as the ball is submerged in oil. A number of brushes was designed, made and tested and the one that provided the most stable results chosen. For calibration purposes a base oil and two additives were chosen, the additives were chosen in such a way that the improvement made to the lubrication process to be very different from one additive to the other. The chosen additives were a Viscosity Index Improver [VII] and an Organic Friction Modifier [OFM]. The VII is used by many researchers in order to obtain multigrade lubricants using the same base oil by varying its percentage in the mix. The OFM is used to provide protection between the two contacting bodies when EHD film fails and EHD lubrication is replaced by mixed lubrication by forming a boundary layer on the contacting surfaces. Optical measurements were carried out on the base oil and the two resulting lubricants from the additive mixes using the Ultra Thin Film Interferometry [UTFI] method. The measurements were used as a benchmark against which the capacitive measurements were calibrated. Tests were conducted in a number of controlled conditions for speed, temperature, load and sliding conditions. Results showed that the highest influence on the lubrication process was given by the speed, an increase in speed results in an increase in optically measured film thickness and a decrease in electrically measured film thickness. Phenomenon explained by a large amount of lubricant pushed into the contact. Another parameter that influenced the results quite significantly was temperature, a rise in temperature supplies a decrease in optically measured film thickness and an increase in capacitive measured film thickness which was explained by lubricant viscosity dropping with a rise in temperature. Three different sliding conditions were employed and a small drop in optically measured film thickness followed by a small rise in electrically measured film thickness was recorded due to a local increase in contact temperature when sliding was employed. The capacitive method developed in this project is precise enough to accurately measure lubricant film thickness down to 100nm; a model for thicknesses lower that 100nm was proposed Results from the optical and capacitive methods were compared and a good correlation was found, indicating that the developed capacitive method can be used as a tool for measuring metal on metal contacts without further calibration.EThOS - Electronic Theses Online ServiceEPSRCGBUnited Kingdo

Fundamentals of fluid lubrication

The aim is to coordinate the topics of design, engineering dynamics, and fluid dynamics in order to aid researchers in the area of fluid film lubrication. The lubrication principles that are covered can serve as a basis for the engineering design of machine elements. The fundamentals of fluid film lubrication are presented clearly so that students that use the book will have confidence in their ability to apply these principles to a wide range of lubrication situations. Some guidance on applying these fundamentals to the solution of engineering problems is also provided

Master of Science

thesisMore than 285,000 total hip replacement (THR) surgeries are performed in the United States each year. Most prosthetic hip joints consist of a cobalt-chromium (CoCr) femoral head that articulates with a polyethylene acetabular component lubricated with synovial fluid. The statistical survivorship of these metal-on-polyethylene prosthetic hip joints declines significantly after 10 to 15 years of use, primarily as a result of polyethylene wear and wear debris incited disease. The current engineering paradigm to increase the longevity of prosthetic hip joints is to manufacture ultra-smooth articulating surfaces. In contrast, this work shows that adding a patterned microtexture to the ultra-smooth CoCr femoral head reduces friction when articulating with the polyethylene acetabular liner. The microtexture increases the load-carrying capacity and the thickness of the joint lubricant film, which reduces contact between the articulating surfaces. As a result, friction and wear is reduced. A lubrication model is used to design the geometry of the patterned microtexture, and we experimentally demonstrate reduced friction for the microtextured compared to conventional smooth surrogate prosthetic hip joints

Measurement of film thickness in lubricated components using ultrasonic reflection.

Many everyday objects are used without consideration of the fact that they rely on a tiny layer of lubricant as thin as 1/1000th of the thickness of a human hair in order to operate correctly. However, without the lubricant, problems are quickly noticed: door hinges squeaking, wear in engines or failure of hip implants. This thesis documents the application of ultrasound to the measurement of this layer of lubricant

Laser interference metallurgy of metallic surfaces for tribological applications

Tribological phenomena play a decisive role in diverse systems. For many years, researchers have sought to alleviate these problems and to understand their origin. There are many potential solutions to manipulate friction. In particular, the rapidly growing field of laser surface texturing has attracted a lot of attention in the last decades and shown to be an effective means of improving tribological properties. A possible approach of laser surface texturing to scrutinize the effects of various pattern geometries and lateral feature sizes in one single laser shot is the so called Laser Interference Metallurgy (LIMET) which will be applied within this thesis. The aim is to study the microstructural and topographic possibilities of LIMET concerning the tribological performance of laser-patterned thin film systems (Au and TiAl multilayer) and bulk aluminium as well as steel surfaces. It will be shown that depending on the laser fluence for example, distinct grain size arrangements and intermetallic phase composites can be created with superior tribological properties compared to the unpatterned reference situation. Moreover, a successful process combination of micro-coining and LIMET will be presented with an enhanced oil retainment capability under lubrication. Finally, the results of laser-textured steel surfaces and their ability to geometrically interlock will be shown. Depending on the relative alignment between the textured sliding surfaces and the selected pattern line-spacing, the frictional response can be significantly influenced. Most of the experimental results will be directly correlated to simulations in order to reveal the underlying phenomena.Reibung spielt eine zentrale Rolle in vielen Bereichen. Insbesondere die Steuerung von Reibung ist dabei von enormer Bedeutung. Zur Minimierung von Reibung sind in den vergangenen Jahrzehnten bereits unzählige Methoden für trockene und geschmierte Bedingungen entwickelt worden. Besonders laserstrukturierte Oberflächen scheinen hierbei vielversprechend für tribologische Anwendungen zu sein. Ein Ansatz, mikrostrukturell und topographisch ma geschneiderte Werkstoffe zu erzeugen, ist die Laserinterferenzmetallurgie (LIMET). Das Ziel der vorliegenden Arbeit ist die Untersuchung der erzielbaren mikrostrukturellen und topographischen Effekte durch LIMET und deren Auswirkungen auf die tribologischen Eigenschaften von metallischen Schichtsystemen (Au und TiAl-Multilagen) sowie massiven Aluminium- und Stahloberflächen. Hierbei wird gezeigt, dass es z.B. abhängig von der gewählten Fluenz möglich ist, Korngrößenarchitekturen oder intermetallische Phasenkomposite definiert zu erzeugen, deren Reibeigenschaften denen der unbehandelten Ausgangssituation überlegen sind. Des Weiteren wird die erfolgreiche Kombination des Mikroprägens mit der LIMETMethode vorgestellt. Die Ergebnisse zeigen, dass insbesondere die Ölspeicherfähigkeit in den hierarchischen Kavitäten unter geschmierten Bedingungen deutlich gegenüber einer unstrukturierten Oberfläche gesteigert ist. Schlie lich werden die tribologischen Auswirkungen von beidseitig strukturierten Stahloberflächen in Abhängigkeit von deren gegenseitiger Orientierung und den gewählten Strukturabständen näher untersucht. Je nach Ausrichtung und den genannten Strukturperiodizitäten lässt sich das Reibverhalten reproduzierbar in gewissen Grenzen manipulieren. Die experimentellen Ergebnisse werden mit Simulationen verknüpft, um dadurch die Wirkmechanismen näher zu beleuchten

Improved tribology and materials for a new generation of hip prostheses

Not availabl

The strength and mechanical behaviour of quartz slip interfaces: an experimental investigation

An experimental study has been undertaken to explore the strength, mechanical behaviour and microstructural evolution of bare interfaces in quartz sandstone during slip. These experiments were designed to simulate fault processes with increasing depth in the continental crust. Two main aspects have been explored: (1) the effect of temperature and confining pressure on the behaviour and stability of favourably-oriented faults, and (2) the influence of reactivation angle on the mechanical behaviour and associated microstructural evolution of a fault zone. Experiments were conducted on Fontainebleau sandstone using a triaxial deformation apparatus, at normal stresses comparable to that in the continental seismogenic regime and over small slip displacements. The first suite of experiments was conducted at temperatures of 400-927°C and confining pressures of 50-200MPa. Experiments reveal complex transitions in fault behaviour between stick-slip and stable sliding regimes. Mechanical results are coupled with microstructural analysis using multiple techniques (including high resolution FE-SEM, and FIB-TEM) that provide insights into fault surface processes down to the nano-scale. Significant findings include the identification of a partially amorphous layer formed during aseismic creep and the generation of pure-silica frictional melt (pseudotachylyte) during high temperature seismic slip events. The pseudotachylyte is recognisable by the formation drawn-out glass filaments and fractured glass patches on the fault surfaces, forming a discontinuous layer up to 2µm thick and covering 10-60% of the fault surface. At normal stresses > 200MPa, frictional melt develops within the first 50µm of rapid slip, correlating with changes in slip acceleration and velocity. High temperature hydrothermal treatment of melt-covered fault surfaces indicates that the pseudotachylyte has a short lifespan (<1 hour) in the presence of high temperature, reactive fluids. The second suite of experiments explores reactivation of fault surfaces inclined between 25º and 70º to the maximum shortening direction, representing faults that vary from optimally-oriented to severely-misoriented for failure. These faults have been reactivated in both dry and fluid-saturated conditions, using two different loading mechanisms. ‘Stress-driven failure’ involves increasing the axial load at constant rate until failure, whereas ‘fluid-driven failure’ is achieved by maintaining a constant axial load and increasing pore fluid pressure until slip occurs. While the initial reactivation of faults obeys frictional theory, continued reactivation is strongly influenced by the microstructural evolution of the fault surface, most notably through the development of frictional melt. Rapid-slip events form a locally-continuous layer of frictional melt in both the dry and water-saturated samples. The presence of pseudotachylyte increases fault cohesive strength through a process termed ‘melt-welding’. Melt-welded regions serve as a nucleation point for the development of off-fault damage and on the most unfavourably-oriented faults, cause lock-up and the failure of a new, more favourably-oriented fault. This work provides new insights into the behaviour and microstructural development of fault surfaces during the early stages of seismic instability. These results have implications for the interpretation of slip processes in natural fault zones, and also more generally for understanding slip mechanics, weakening distances and coseismic fault strength within the continental seismogenic regime