Imperial College London

Dr Tom Reddyhoff

Faculty of EngineeringDepartment of Mechanical Engineering

Senior Lecturer
 
 
 
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Contact

 

+44 (0)20 7594 3840t.reddyhoff Website

 
 
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Location

 

670City and Guilds BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

88 results found

Reddyhoff T, Choo JH, Spikes HA, Glovnea RPet al., 2010, Lubricant Flow in an Elastohydrodynamic Contact Using Fluorescence, Tribology Letters, Vol: 38, Pages: 207-15, ISSN: 1023-8883

It is well-documented that parameters, such as film thickness and temperature in EHL contacts, can be measured experimentally using a range of techniques include optical interferometry, ultrasonics, capacitance and infrared emission. Considerably less is known, however, about the flow of lubricant through such contacts. Information about lubricant flow would greatly benefit the prediction of friction in machine components. This article describes initial steps to develop fluorescence as a means of observing lubricant flow. An EHL contact was produced between a steel ball and a glass disc and viewed using a fluorescence microscope. The entrained lubricant was dyed using a fluorescent species, so that when illuminated with laser light, a fluorescence intensity map could be viewed. When the contact was fully flooded with dyed lubricant, the fluorescence intensity within the contact correlated well with optical interferometric film thickness measurements under the same conditions. This suggests useful possibilities for mapping film thickness in contacts where conventional optical methods are impractical, such as between rough surfaces and within soft contacts. In order to observe how lubricant flows in an EHL contact, fluorescer-containing lubricant was placed on the out-of-contact track. The boundary between fluorescent and non-fluorescent lubricant was then entrained into the contact and the passage of the boundary through the contact was monitored.

Journal article

Hili J, Reddyhoff T, Olver AV, Pelletier C, Jacobs Let al., 2010, FILM FORMING CHARACTERISTICS OF OIL-IN-WATER EMULSIONS IN ELASTOHYDRODYNAMIC CONTACTS, ASME/STLE International Joint Tribology Conference, Publisher: AMER SOC MECHANICAL ENGINEERS, Pages: 143-145

Conference paper

Hergert R, Ku ISY, Reddyhoff T, Holmes ASet al., 2010, Micro rotary ball bearing with integrated ball cage: Fabrication and characterization, Piscataway, NJ, USA, 23rd IEEE International Conference on Micro Electro Mechanical Systems (MEMS 2010), 24-28 Jan. 2010, Publisher: IEEE, Pages: 687-90

This paper presents a rotary MEMS ball bearing with an integrated silicon ball cage. The device is a deep groove radial ball bearing consisting of steel balls encapsulated between two micromachined silicon wafers. The silicon ball cage is released from the bulk silicon substrate during fabrication. The objective was to show that a simple caged bearing design provides reliable motion at both high and low speeds. The running torque of two identical devices was measured for speeds ranging from 10 to 20,000 rpm. One of the devices was disassembled before failure to provide images of the wear experienced during testing.

Conference paper

Dwyer-Joyce RS, Zhu J, Reddyhoff T, 2010, Ultrasonic measurement for film thickness and solid contact in elastohydrodynamic lubrication, San Francisco, CA, United states, STLE/ASME 2010 International Joint Tribology Conference, IJTC2010, October 17, 2010 - October 20, 2010, Publisher: American Society of Mechanical Engineers, Pages: 111-113

The reflection of ultrasound can be used to determine oil film thickness from the stiffness of the separating film. However, boundary or mixed film lubrication is a common occurrence in elastohydrodynamic lubricated (EHL) contacts, as the nominal thickness of the separating film approaches the surface asperity height. In this paper an ultrasonic investigation was carried out on the interface between a steel ball sliding on a flat disc as the speed was reduced into the boundary regime. The ultrasonic reflection then depends on the stiffness of the interface that now consists of an oil layer and asperity contacts. To distinguish the stiffness contribution from asperity contact and oil layer, a mixed lubrication model for circular contacts was established. This predicted the lubricant film thickness and proportions of solid and liquid mediated contact. The total stiffness predicted by theoretical models showed a good agreement with experimental measurement for kinematic cases. The model can then be used to extract the proportion of real area of contact, and the oil film thickness, from ultrasonic results. Copyright 2010 by ASME.

Conference paper

Myant C, Reddyhoff T, Spikes HA, 2010, Laser-induced fluorescence for film thickness mapping in pure sliding lubricated, compliant, contacts, Tribology International, Vol: 43, Pages: 1960-1969, ISSN: 0301-679X

A laser-induced fluorescence (LIF) technique has been used to measure fluid film thickness in a compliant, sliding contact under low-load/low-pressure conditions. The soft contact between an elastomer hemisphere and a glass disc is lubricated by a liquid containing fluorescent dye. The contact is then illuminated with 532 nm laser light through the glass disc, and viewed with a fluorescence microscope. From the intensity of emitted radiation, film thickness maps of the contact are determined. Previous calibration procedures have used a separate calibration piece and test specimen with possible errors due to differences in reflectivity between the calibration and test specimens. In the work reported in this paper a new calibration process is employed using the actual test sample, thereby avoiding such errors. Results are reported for a sliding contact between PDMS and glass, lubricated with glycerol and water solutions under fully flooded and starved conditions. It was found that, for glycerol, the measured film thickness is somewhat lower than numerical predictions for both lubrication conditions. It is suggested that a combination of thermal effects and the hygroscopic nature of glycerol may cause the lubricant viscosity to drop resulting in thinner films than those predicted for fully flooded contacts. Starvation occurs above a critical entrainment speed and results in considerably thinner films than predicted by fully flooded I-EHL theory. A numerical study has been carried out to determine the effect of the observed starvation on film thickness. Predicted, starved film thickness values agree well with those obtained experimentally. 2010 Elsevier Ltd.

Journal article

Hergert R, Ku ISY, Reddyhoff T, Holmes ASet al., 2010, Micro rotary ball bearing with integrated ball cage: Fabrication and characterization, Hong Kong, China, 23rd IEEE International Conference on Micro Electro Mechanical Systems, MEMS 2010, January 24, 2010 - January 28, 2010, Publisher: Institute of Electrical and Electronics Engineers Inc., Pages: 687-690, ISSN: 1084-6999

This paper presents a rotary MEMS ball bearing with an integrated silicon ball cage. The device is a deep groove radial ball bearing consisting of steel balls encapsulated between two micromachined silicon wafers. The silicon ball cage is released from the bulk silicon substrate during fabrication. The objective was to show that a simple caged bearing design provides reliable motion at both high and low speeds. The running torque of two identical devices was measured for speeds ranging from 10 to 20,000 rpm. One of the devices was disassembled before failure to provide images of the wear experienced during testing. 2010 IEEE.

Conference paper

Andablo-Reyes E, De Vicente J, Hidalgo-Alvarez R, Myant C, Reddyhoff T, Spikes HAet al., 2010, Soft elasto-hydrodynamic lubrication, 233 Springer Street, New York, NY 10013-1578, United States, Publisher: Springer New York, Pages: 109-114, ISSN: 1023-8883

This article examines the use of ferrofluids to control starvation in lubricated contacts. Starvation in a ball-plate contact is experimentally studied under sliding-rolling conditions using a Mini Traction Machine (MTM). Friction is measured and the experimental results are presented in the form of Stribeck curves. The volume of lubricant is controlled in such a way that no free bulk oil is present in the vicinity of the contact. An abrupt change in the slope of the Stribeck curve in the Hydrodynamic Lubrication zone is interpreted as the onset of starvation. It is then shown that the use of ferrofluids in the presence of a magnetic field distribution can change the conditions at which this onset of starvation occurs. Different magnetic field distributions are tested for different values of load and ferrofluid viscosity. It is proposed that ferrofluid lubricants in conjunction with a suitably positioned magnetic field can be used to promote replenishment, and thus control and reduce lubricant starvation. 2010 Springer Science+Business Media, LLC.

Conference paper

Viesca JL, Battez AH, Gonzalez R, Reddyhoff T, Perez AT, Spikes HAet al., 2010, Assessing boundary film formation of lubricant additivised with 1-hexyl-3-methylimidazolium tetrafluoroborate using ECR as qualitative indicator, Wear, Vol: 269, Pages: 112-117, ISSN: 0043-1648

Boundary film formation of ionic liquid (IL) 1-hexyl-3-methylimidazolium tetrafluoroborate, [HMIM][BF4], as an additive of hydrocracked mineral oil is evaluated for a steel-steel contact. Accelerated wear testing was carried out using a high frequency reciprocating rig (HFRR) under these test conditions: maximum contact pressure of 1.04 GPa, two different temperatures (40 and 100 C) and three different times (300, 1800 and 3600 s). Wear volumes were measured using a non-contact 3D profilometer while worn surfaces were characterized using XPS. Furthermore, electrical contact resistance (ECR) was used as qualitative indicator of the formation of electrically insulating films in the sliding contact. Experiments show that the rate of boundary film formation of base oil-ionic liquid blend is faster than neat base oil. Moreover, ECR was in good agreement with film formation and friction behaviour. Ionic liquid as additive not only decreases the time of running-in but also the time of wear-in. Results of neat base oil show that wear-in was not reached during any duration of tests. The improved friction and wear results for the blend are closely related to the boundary film formation on the worn surfaces due to the reactivity of the anion with the steel surfaces. 2010 Elsevier B.V. All rights reserved.

Journal article

Ku ISY, Reddyhoff T, Choo JH, Holmes AS, Spikes HAet al., 2010, A novel tribometer for the measurement of friction in MEMS, Tribology International, Vol: 43, Pages: 1087-1090, ISSN: 0301-679X

A new tribometer has been developed to determine friction under conditions that are representative of MEMS (micro-electro-mechanical-systems). The tribometer consists of a rotating silicon disc, loaded against a stationary silicon disc. Friction and film thickness values are measured using laser displacement techniques. In this study, two different test set-ups were used: a flat on flat specimen geometry, and a moving flat against a structured surface, similar to that of a miniature thrust pad bearing. Using this tribometer, hydrodynamic tests have been carried out with the specimens fully submerged in hydrocarbon lubricants. Results suggest that friction increases with sliding speed and decreases with increasing applied normal load, which is in accordance with the hydrodynamic theory. 2009 Elsevier Ltd. All rights reserved.

Journal article

Reddyhoff T, Choo JH, Spikes HA, Glovnea RPet al., 2010, Lubricant flow in an elastohydrodynamic contact using fluorescence, Tribology Letters, Vol: 38, Pages: 207-215, ISSN: 1023-8883

It is well-documented that parameters, such as film thickness and temperature in EHL contacts, can be measured experimentally using a range of techniques include optical interferometry, ultrasonics, capacitance and infrared emission. Considerably less is known, however, about the flow of lubricant through such contacts. Information about lubricant flow would greatly benefit the prediction of friction in machine components. This article describes initial steps to develop fluorescence as a means of observing lubricant flow. An EHL contact was produced between a steel ball and a glass disc and viewed using a fluorescence microscope. The entrained lubricant was dyed using a fluorescent species, so that when illuminated with laser light, a fluorescence intensity map could be viewed. When the contact was fully flooded with dyed lubricant, the fluorescence intensity within the contact correlated well with optical interferometric film thickness measurements under the same conditions. This suggests useful possibilities for mapping film thickness in contacts where conventional optical methods are impractical, such as between rough surfaces and within soft contacts. In order to observe how lubricant flows in an EHL contact, fluorescer-containing lubricant was placed on the out-of-contact track. The boundary between fluorescent and non-fluorescent lubricant was then entrained into the contact and the passage of the boundary through the contact was monitored. 2010 Springer Science+Business Media, LLC.

Journal article

Reddyhoff T, Ku ISY, Choo JH, Holmes AS, Spikes HAet al., 2009, Lubrication of high sliding MEMS

Effective lubrication for micro-electromechanical systems (MEMS) was studied. A new micro-tribometer was developed to measure friction and film thickness under conditions representative of MEMS. For hydrocarbon lubricants, friction increased with sliding speed and decreased with increasing applied normal load, which is in accord with hydrodynamic theory. Using low viscosity fluids, satisfactory friction coefficient values were obtained with negligible wear. Good agreement was obtained between experimental results and a finite difference model. This is an abstract of a paper presented at the World Tribology Congress (Kyoto, Japan 9/6-11/209).

Conference paper

Reddyhoff T, Underwood RJ, Nikas GK, Sayles RS, Spikes HAet al., 2009, Thermal aspects of debris in EHL contacts

A technique is developed for the measurement of the temperature rise resulting from debris entrainment in an EHL lubricated contact. Under pure rolling, temperature rises are small, because minimal shearing occurs. Under sliding conditions, the temperature rise generally increases from when the particle is entrapped in the inlet zone, to peak near the contact centre where shearing is a maximum. The measured values and the theoretical simulation show a similar trend in temperature rise as the particle passes through the contact. The magnitude of predicted temperatures is significantly higher than those measured. This is an abstract of a paper presented at the World Tribology Congress (Kyoto, Japan 9/6-11/2009).

Conference paper

Nakano K, Reddyhoff T, Cann P, Spikes Het al., 2009, Film formation of liquid crystals in EHD contacts

The film formation of two types of liquid crystals, i.e., 4-pentyl-4′-cyanobiphenyl (5CB) and 4-octyl-4′-cyanobiphenyl (8CB), was studied in EHD contacts. The enhancement of film formation by the addition of palmitic acid affected the traction characteristics of 5CB. The traction coefficient of pure 5CB increased significantly with a decrease in the entrainment speed at entrainment speeds below 0.1 m/sec. The change in the EHD film of pure 8CB was similar to that of 5CB with 0.1 wt % palmitic acid. The traction coefficient of 8CB was lower than that of 5CB, ≈ 0.02, which was maintained at low entrainment speeds. These special properties were attributed to the layer structure of the smectic phase. This is an abstract of a paper presented at the Proceedings of World Tribology Conference (Kyoto, Japan 9/6-11/2009).

Conference paper

Reddyhoff T, Spikes HA, Olver AV, 2009, Improved infrared temperature mapping of elastohydrodynamic contacts, Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology, Vol: 223, Pages: 1165-1177, ISSN: 1350-6501

An effective means of studying lubricant rheology within elastohydrodynamic contacts is by detailed mapping of the temperature of the fluid and the bounding surfaces within the lubricated contact area. In the current work, the experimental approach initially developed by Sanborn and Winer and then by Spikes et al., has been advanced to include a high specification infrared (IR) camera and microscope. Besides the instantaneous capture of full field measurements, this has the advantage of increased sensitivity and higher spatial resolution than previous systems used. The increased sensitivity enables a much larger range of testable operating conditions: namely lower loads, speeds, and reduced sliding. In addition, the range of test lubricants can be extended beyond high shearing traction fluids. These new possibilities have been used to investigate and compare the rheological properties of a range of lubricants: namely a group I and group II mineral oil, a polyalphaolephin (group IV), the traction fluid Santotrac 50, and 5P4E, a five-ring polyphenyl-ether. As expected, contact temperatures increased with lubricant refinement, for the mineral base oils tested. Using moving heat source theory, the measured temperature distributions were converted into maps showing rate of heat input into each surface, from which shear stresses were calculated. The technique could therefore be validated by integrating these shear stress maps, and comparing them with traction values obtained by direct measurement. Generally there was good agreement between the two approaches, with the only significant differences occurring for 5P4E, where the traction that was deduced from the temperature over-predicted the traction by roughly 15 per cent. Of the lubricants tested, Santotrac 50 showed the highest average traction over the contact; however, 5P4E showed the highest maximum traction. This observation is only possible using the IR mapping technique, and is obscured when measuring the traction

Journal article

Ku ISY, Reddyhoff T, Choo JH, Holmes AS, Spikes HAet al., 2009, Lubrication performance of liquids of different viscosities in MEMS devices, Lake Buena Vista, FL, United states, Society of Tribologists and Lubrication Engineers Annual Meeting and Exhibition 2009, May 17, 2009 - May 21, 2009, Publisher: Society of Tribologists and Lubrication Engineers, Pages: 178-180

Conference paper

Reddyhoff T, Spikes HA, Olver AV, 2009, Compression heating and cooling in elastohydrodynamic contacts, Tribology Letters, Vol: 36, Pages: 69-80, ISSN: 1023-8883

In this study, the infrared temperature mapping technique, originally developed by Sanborn and Winer (Trans ASME J Tribol 93:262-271, 1971) and extended by Spikes et al. (Tribol Lett 17(3):593-605, 2004), has been made more sensitive and used to study the temperature rise of elastohydrodynamic contacts in pure rolling. Under such conditions lubricant shear heating within the contact is considered negligible and this allows temperature changes due to lubricant compression to be investigated. Pure rolling surface temperature distributions have been obtained for contacts lubricated with a range of lubricants, included a group I, and group II mineral oil, a polyalphaolefin (group IV), the traction fluid Santotrac 50 and 5P4E, a five-ring polyphenyl-ether. Resulting maps show the temperature rise in the contact increases in the inlet due to compression heating and then decreases and in most cases becomes negative in the exit region due to the effect of decompression. Temperature changes increase with entrainment speed but in the current tests are always very small, and less than 1 C. Contact temperature rises from compression were compared to those from sliding contacts (where a slide-roll ratio of 0.5 was applied). Here the contribution to the contact temperature from compression is shown to decrease dramatically with entrainment speed. The lubricant 5P4E is found to behave differently from other lubricants tested in that it showed a peak in temperature at the outlet. This effect becomes more pronounced with increasing speed, and has tentatively been attributed to a phase change in the exit region. Using moving heat source theory, the measured temperature distributions have been converted to maps showing rate of heat input into each surface and the latter compared with theory. Qualitative agreement between theory and experiment is found, and a more accurate theoretical comparison is the subject of ongoing study. 2009 Springer Science+Business Media, LLC.

Journal article

Reddyhoff T, Spikes HA, Olver AV, 2009, Improved temperature mapping of ehl contacts, Miami, FL, United states, 2008 STLE/ASME International Joint Tribology Conference, IJTC 2008, October 20, 2008 - October 22, 2008, Publisher: ASME, Pages: 265-267

An effective means of studying lubricant film rheology within EHL contacts is by detailed mapping of the temperature of the fluid and the bounding surfaces within the lubricated contact area. This provides a way of directly measuring the rheology of lubricant films under true EHL conditions. Furthermore, temperature measurement itself provides a very effective means of testing and validating computer simulations. In the current work, the experimental approach initially developed by Sanborn and Winer [11 and then by Spikes and co-workers [2], has been advanced to include a high specification infrared (IR) camera and microscope. This is a similar approach to that taken by Yagi and Kyogoku [3]. As well as the instantaneous capture of full field measurements, this has the advantage of increased sensitivity and higher spatial resolution than previous systems used. The increased sensitivity enables a much larger range of testable operating conditions; namely lower loads, speeds and reduced sliding. In addition, the range of test lubricants can be extended beyond high shearing traction fluids. One additional advantage of instantaneous full field measurements is that the weak infrared optical interference caused by the film can be observed and can used to exactly locate the centre of the contact in the resulting temperature maps. These new possibilities have been used to investigate and compare the rheological properties and compression cooling effects exhibited by a PAO, a group II mineral oil, and a traction fluid. Copyright 2008 by ASME.

Conference paper

Reddyhoff T, Dwyer-Joyce RS, Zhang J, Drinkwater BWet al., 2008, Auto-calibration of ultrasonic lubricant-film thickness measurements, Measurement Science & Technology, Vol: 19, ISSN: 0957-0233

The measurement of oil film thickness in a lubricated component is essential information for performance monitoring and design. It is well established that such measurements can be made ultrasonically if the lubricant film is modelled as a collection of small springs. The ultrasonic method requires that component faces are separated and a reference reflection recorded in order to obtain a reflection coefficient value from which film thickness is calculated. The novel and practically useful approach put forward in this paper and validated experimentally allows reflection coefficient measurement without the requirement for a reference. This involves simultaneously measuring the amplitude and phase of an ultrasonic pulse reflected from a layer. Provided that the acoustic properties of the substrate are known, the theoretical relationship between the two can be fitted to the data in order to yield reflection coefficient amplitude and phase for an infinitely thick layer. This is equivalent to measuring a reference signal directly, but importantly does not require the materials to be separated. The further valuable aspect of this approach, which is demonstrated experimentally, is its ability to be used as a self-calibrating routine, inherently compensating for temperature effects. This is due to the relationship between the amplitude and phase being unaffected by changes in temperature which cause unwanted changes to the incident pulse. Finally, error analysis is performed showing how the accuracy of the results can be optimized. A finding of particular significance is the strong dependence of the accuracy of the technique on the amplitude of reflection coefficient input data used. This places some limitations on the applicability of the technique.

Journal article

Reddyhoff T, Dwyer-Joyce RS, Harper P, 2008, A new approach for the measurement of film thickness in liquid face seals, Tribology Transactions, Vol: 51, Pages: 140-149, ISSN: 1040-2004

Face seals operate by allowing a small volume of the sealed fluid to escape and form a thin film between the contacting parts. The thickness of this film must be optimized to ensure that the faces are separated, yet the leakage is minimized. In this work the liquid film is measured using a novel ultrasonic approach with a view to developing a condition monitoring tool. The trials were performed in two stages. Initially tests were based on a lab simulation, where it was possible to compare the ultrasonic film thickness measurements with optical interference methods and capacitance methods. A direct correlation was seen between ultrasonic measurements and capacitance. Where ultrasonic and optical methods overlap, good correlation is observed; however, the optical method will not record film thickness above 0.72 m. A second set of trials was carried out, where the film thickness was monitored inside a seal test apparatus. Film thickness was successfully recorded as speed and load were varied. The results showed that while stationary the film thickness varied noticeably with load. When rotating, however, the oil film remained relatively stable around 2 m. During the normal operation of the seal, both sudden speed and load changes were applied in order to initiate a seal failure. During these events, the measured film thickness was seen to drop dramatically down to 0.2 m. This demonstrated the ability of the technique to predict failure in a face seal and therefore its aptitude for condition monitoring.

Journal article

Reddyhoff T, Dwyer-Joyce RS, Zhang J, Drinkwater BWet al., 2008, Auto-calibration of ultrasonic lubricant-film thickness measurements, Measurement Science and Technology, Vol: 19, ISSN: 0957-0233

The measurement of oil film thickness in a lubricated component is essential information for performance monitoring and design. It is well established that such measurements can be made ultrasonically if the lubricant film is modelled as a collection of small springs. The ultrasonic method requires that component faces are separated and a reference reflection recorded in order to obtain a reflection coefficient value from which film thickness is calculated. The novel and practically useful approach put forward in this paper and validated experimentally allows reflection coefficient measurement without the requirement for a reference. This involves simultaneously measuring the amplitude and phase of an ultrasonic pulse reflected from a layer. Provided that the acoustic properties of the substrate are known, the theoretical relationship between the two can be fitted to the data in order to yield reflection coefficient amplitude and phase for an infinitely thick layer. This is equivalent to measuring a reference signal directly, but importantly does not require the materials to be separated. The further valuable aspect of this approach, which is demonstrated experimentally, is its ability to be used as a self-calibrating routine, inherently compensating for temperature effects. This is due to the relationship between the amplitude and phase being unaffected by changes in temperature which cause unwanted changes to the incident pulse. Finally, error analysis is performed showing how the accuracy of the results can be optimized. A finding of particular significance is the strong dependence of the accuracy of the technique on the amplitude of reflection coefficient input data used. This places some limitations on the applicability of the technique. 2008 IOP Publishing Ltd.

Journal article

Reddyhoff T, Dwyer-Joyce R, Harper P, 2006, Ultrasonic measurement of film thickness in mechanical seals, Sealing Technology, Pages: 7-11, ISSN: 1350-4789

Measurement of the interface film thickness of mechanical seals has been of interest to researchers and industry for many years. This feature describes work to measure the liquid film using a novel ultrasonic approach with a view to developing a condition monitoring tool. Initial tests were based on a laboratory simulation, to compare the method with optical interference and capacitance measurements. Tests were then carried out on a seal test rig. Film thickness was successfully recorded as speed and load was varied. 2006 Elsevier Ltd. All rights reserved.

Journal article

Dwyer-Joyce RS, Reddyhoff T, 2006, Ultrasonic measurement of EHL oil films in a mixed regime contact, San Antonio, TX, United states, STLE/ASME International Joint Tribology Conference, IJTC 2006, October 23, 2006 - October 25, 2006, Publisher: American Society of Mechanical Engineers

In applications, such as gears and rolling bearings, where concentrated contact occurs, extreme loading can result in damaging asperity contact. To study this, an adapted elastohydrodynamic rig was used to measure the film using the reflection of an ultrasonic pulse. Ultrasonic reflection depends on the stiffness of the contact interface. The stiffness of an oil film depends on its thickness. A single contact was operated dry, full film lubricated, and in the mixed regime, by changing the sliding speed. The reflection then depends on the stiffness of both the liquid contact (oil film) and the solid (asperity) contact, acting as two springs in series. When dry, the solid stiffness dominates. As an oil film starts to form, a high stiffness thin liquid film forms. As the speed is increased the stiffness of this layer reduces, as its thickness increases, until it becomes thicker than the roughness and the stiffness then originates from liquid contact alone. By comparing dry, wet and sliding conditions the contributions of solid and liquid stiffness can be separated. Copyright 2006 by ASME.

Conference paper

Reddyhoff T, Kasolang S, Dwyer-Joyce RS, Drinkwater BWet al., 2005, The phase shift of an ultrasonic pulse at an oil layer and determination of film thickness, Proceedings of the Institution of Mechanical Engineers, Part J (Journal of Engineering Tribology), Vol: 219, Pages: 387-400, ISSN: 1350-6501

An ultrasonic pulse incident on a lubricating oil film in a machine element is partially reflected and partially transmitted. The proportion of the wave amplitude reflected, termed the reflection coefficient, depends on the film thickness and the acoustic properties of the oil. When the appropriate ultrasonic frequency is used, the magnitude of the reflection coefficient can be used to determine the oil film thickness. However, the reflected wave has both a real component and an imaginary component, and both the amplitude and the phase are functions of the film thickness. The phase of the reflected wave is shifted from that of the incident wave when it is reflected. In the present study, this phase shift is explored as the film changes and is evaluated as an alternative means to measure oil film thickness. A quasi-static theoretical model of the reflection response from an oil film has been, developed. This model relates the phase shift to the wave frequency and the film properties. Measurements of reflection coefficient from a static model oil film and also from a rotating journal bearing have been recorded. These have been used to determine the oil film thickness using both amplitude and phase shift methods. In both cases, the results agree closely with independent assessments of the oil film thickness. The model of ultrasonic reflection is further extended to incorporate mass and damping terms. Experiments show that both the mass and the internal damping of the oil films tested in this work have a negligible effect on ultrasonic reflection. A potentially very useful application for the simultaneous measurement of reflection coefficient amplitude and phase is that the data can be used to negate the need for a reference. The theoretical relationship between phase and amplitude is fitted to the data. An extrapolation is performed to determine the values of amplitude and phase for an infinitely thick layer. This is equivalent to the reference signal determined by mea

Journal article

Dwyer-Joyce RS, Reddyhoff T, Drinkwater B, 2005, Ultrasonic phase and amplitude and the measurement of oil film thickness, Pages: 515-516

The reflection of ultrasound at an oil film can be used to determine the film thickness. A thin oil film reflects less ultrasound than a thick film. When the film is thin there is a simple relationship between oil film thickness and the proportion of the wave amplitude reflected. The reflection coefficient is in fact a complex quantity with both magnitude and phase. A model for how both the phase and amplitude vary with oil film thickness (and the properties of the bearing materials) has been developed. It has been shown that both can be used to determine film thickness. Tests have been performed to determine the oil film thickness and explore the relationship between reflection amplitude and phase. Experiments are performed both on a static oil film between flat plates, and on an operating journal bearing. Both methods provide a simple accurate method for the measurement of oil film thickness. Copyright © 2005 by ASME.

Conference paper

Dwyer-Joyce RS, Reddyhoff T, Drinkwater B, 2005, Ultrasonic phase and amplitude and the measurement of oil film thickness, Washington, D.C., United states, 2005 World Tribology Congress III, September 12, 2005 - September 16, 2005, Publisher: American Society of Mechanical Engineers, Pages: 515-516

The reflection of ultrasound at an oil film can be used to determine the film thickness. A thin oil film reflects less ultrasound than a thick film. When the film is thin there is a simple relationship between oil film thickness and the proportion of the wave amplitude reflected. The reflection coefficient is in fact a complex quantity with both magnitude and phase. A model for how both the phase and amplitude vary with oil film thickness (and the properties of the bearing materials) has been developed. It has been shown that both can be used to determine film thickness. Tests have been performed to determine the oil film thickness and explore the relationship between reflection amplitude and phase. Experiments are performed both on a static oil film between flat plates, and on an operating journal bearing. Both methods provide a simple accurate method for the measurement of oil film thickness. Copyright 2005 by ASME.

Conference paper

Reddyhoff T, Kasolang S, Dwyer-Joyce RS, Drinkwater BWet al., 2005, The phase shift of an ultrasonic pulse at an oil layer and determination of film thickness, Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology, Vol: 219, Pages: 387-400, ISSN: 1350-6501

An ultrasonic pulse incident on a lubricating oil film in a machine element will be partially reflected and partially transmitted. The proportion of the wave amplitude reflected, termed the reflection coefficient, depends on the film thickness and the acoustic properties of the oil. When the appropriate ultrasonic frequency is used, the magnitude of the reflection coefficient can be used to determine the oil film thickness. However, the reflected wave has both a real component and an imaginary component, and both the amplitude and the phase are functions of the film thickness. The phase of the reflected wave will be shifted from that of the incident wave when it is reflected. In the present study, this phase shift is explored as the film changes and is evaluated as an alternative means to measure oil film thickness. A quas i-static theoretical model of the reflection response from an oil film has been, developed. This model relates the phase shift to the wave frequency and the film properties. Measurements of reflection coefficient from a static model oil film and also from a rotating journal bearing have been recorded. These have been used to determine the oil film thickness using both amplitude and phase shift methods. In both cases, the results agree closely with independent assessments of the oil film thickness. The model of ultrasonic reflection is further extended to incorporate mass and damping terms. Experiments show that both the mass and the internal damping of the oil films tested in this work have a negligible effect on ultrasonic reflection. A potentially v ery useful application for the simultaneous measurement of reflection coefficient amplitude and phase is that the data can be used to negate the need for a reference. The theoretical relationship between phase and amplitude is fitted to the data. An extrapolation is performed to determine the values of amplitude and phase for an infinitely thick layer. This is equivalent to the reference signal deter

Journal article

Dwyer-Joyce RS, Reddyhoff T, Drinkwater BW, 2004, Operating limits for acoustic measurement of rolling bearing oil film thickness, Tribology Transactions, Vol: 47, Pages: 366-375, ISSN: 1040-2004

An ultrasonic pulse striking a thin layer of liquid trapped between solid bodies will be partially reflected. The proportion reflected is a function of the layer stiffness, which in turn depends on the film thickness and its bulk modulus. In this work, measurements of reflection have been used to determine the thickness of oil films in elastohydrodynamic lubricated (EHL) contacts. A very thin liquid layer behaves like a spring when struck by an ultrasonic pulse. A simple quasi-static spring model can be used to determine the proportion of the ultrasonic waves reflected. Experiments have been performed on a model EHL contact between a ball and a flat surface. A transducer is mounted above the contact such that the ultrasonic wave is focused onto the oil film. The reflected signals are captured and passed to a PC for processing. Fourier analysis gives the reflection spectrum that is then used to determine the stiffness of the liquid layer and hence its thickness. In further testing, an ultrasonic transducer has been mounted in the housing of a deep-groove ball bearing to measure the film generated at the outer raceway as each ball passes. Results from both the ball-flat and ball bearing measurements agree well with steady-state theoretical EHL predictions. The limits of the measuring technique, in terms of the measurable rolling bearing size and operating parameters, have been investigated.

Journal article

Dwyer-Joyce R, Reddyhoff T, Drinkwater B, 2004, Operating limits for acoustic measurement of rolling bearing oil film thickness, Tribology Transactions, Vol: 47, Pages: 366-375, ISSN: 1040-2004

An ultrasonic pulse striking a thin layer of liquid trapped between solid bodies will be partially reflected. The proportion reflected is a function of the layer stiffness, which in turn depends on the film thickness and its bulk modulus. In this work, measurements of reflection have been used to determine the thickness of oil films in elastohydrodynamic lubricated (EHL) contacts. A very thin liquid layer behaves like a spring when struck by an ultrasonic pulse. A simple quasi-static spring model can be used to determine the proportion of the ultrasonic waves reflected. Experiments have been performed on a model EHL contact between a ball and a flat surface. A transducer is mounted above the contact such that the ultrasonic wave is focused onto the oil film. The reflected signals are captured and passed to a PC for processing. Fourier analysis gives the reflection spectrum that is then used to determine the stiffness of the liquid layer and hence its thickness. In further testing, an ultrasonic transducer has been mounted in the housing of a deep-groove ball bearing to measure the film generated at the outer raceway as each ball passes. Results from both the ball-flat and ball bearing measurements agree well with steady-state theoretical EHL predictions. The limits of the measuring technique, in terms of the measurable rolling bearing size and operating parameters, have been investigated.

Journal article

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