Elastohydrodynamic Oil Film Research Articles

Exploring Tribological Behaviors of Profile-Shifted Spur Gears: An Experimental Investigation

The tribological behavior of an addendum modified altered tooth sum optimized spur gear set were experimentally compared with that of the standard spur gear under several loading conditions including the wear morphology study and wear debris analysis. The temperature, viscosity, elastohydrodynamic oil film thickness, specific film thickness, and combined surface roughness variation under increasing operating load were analyzed experimentally. Finally, the performance impact of addendum shifted altered tooth sum optimized gear was examined. Results show that the tribological improvement strongly depends on the addendum alteration and modified tooth sum condition. The modification of the addendum in gears with a negatively altered tooth sum facilitates the transition from single-tooth contact to two-tooth contact, broadening the lubrication range while preserving optimal film thickness and improving wear resistance. However, it is reverse to decrease the film thickness and increase wear for standard and profile-modified positively altered tooth sum gear. The gear tribological performance can be significantly improved by profile modification and negatively altered tooth sum approach with a lower rate of degradation of oil and damage on the tooth contacting surface.

A novel self-powered triboelectric sensor for early waring of lubrication failure

In tribology, maintaining good lubrication of friction pairs is an important technical means to reduce wear and failure, which is very important but difficult to monitor the lubrication state of friction pairs. In this paper, when measuring the thickness and shape of the lubricating oil film by the ball-disk elastohydrodynamic oil film measuring instrument, a current amplifier is integrated to measure the electrical signal generated during the friction process. By monitoring the change of lubricating oil film, the change of ball-disk friction force, and the change of triboelectric signal during the friction process in real time, the relationship between the tribological behavior and the triboelectric behavior of the friction pair is explored. The experimental results show that under the conditions of limited oil supply, different rotational speeds and different lubricating oil viscosities, the time of the discharge phenomenon of the steel ball corresponds one-to-one with the time when the friction pair starts to wear under oil-scarce lubrication. The discharge occurs due to the electric charge breaking through the air in the micro-gap of the wear scar. The results demonstrate that the formation of the oil-depleted state has an important influence on the triboelectric process. Based on the relationship between the two, the state of the lubricating oil can be reflected in real time by using the triboelectric signal, which provides a reliable method for the real-time monitoring of the lubrication state.

Monitoring Lubrication and Wear in-situ by triboelectrification Under Vacuum Conditions

Maintaining the condition of the friction pair is an important measure to reduce mechanical failure. This paper integrates a vacuum elastohydrodynamic oil film measurement device and a current amplifier to simultaneously observe the optical interference image, friction coefficient and triboelectric signal during the friction process. In a high vacuum environment, the discharge phenomenon of steel balls corresponds to the time when the friction pair wears. Changes in the friction state in high vacuum environments have a significant impact on triboelectricity. Based on the relationship between the two, a sensor for real-time monitoring of the friction and wear status of the friction pair is designed, which provides a reliable real-time monitoring method for friction pair monitoring in vacuum machinery.

Properties of Elasto-Hydrodynamic Oil Film in Meshing of Harmonic Drive Gears.

Among the essential issues facing designers of strain wave gears, the provision for correct lubrication should be of paramount importance. The present paper presents the results of research on elasto-hydrodynamic oil film in meshing of a harmonic drive with an involute tooth profile. The research was carried out based on theoretical models developed by Dowson and Higginson. For the discussed structural problem, results of the study are presented graphically in the form of static characteristics of the oil film. Correct operation regimes were determined for two different oils. The paper also provides a review of information concerning the design and principle of operation of strain wave transmission.

M1107 O/Wエマルション圧延における導入油膜形成に及ぼす各種油性剤の影響(GS4,11,13 摺動・潤滑特性の向上,修士研究発表セッション)

M1107 O/Wエマルション圧延における導入油膜形成に及ぼす各種油性剤の影響(GS4,11,13 摺動・潤滑特性の向上,修士研究発表セッション)

The Transient Effects of Profile Modification on Elastohydrodynamic Oil Films in Helical Gears

This article presents the results of a transient elastohydrodynamic analysis of the conditions at the contact of two pairs of helical gears operating with different gear ratios. The effects of the variation in contact geometry, kinematics, and tooth loading during the meshing cycle are taken into account together with both tip relief and axial crowning profile modifications. The results are compared with analyses under corresponding steady-state conditions for both point contact and equivalent line contact models. The comparisons show that, away from the ends of the contact lines, steady-state conditions can be used effectively to assess the film-forming capabilities of the gears. Conditions at the end of the contact lines are found to be more adverse with significant transient squeeze film effects. The form of tip relief profile adopted is shown to be highly influential in determining the peak contact pressures and thinnest oil films within the contacts. These conditions are experienced at all axial positions on the tooth flanks at the locations of the start of tip relief profile modifications. If the tip relief profile introduces a slope discontinuity to the gear profile, this can cause a significant stress concentration and very poor film-forming behavior.

Soft tribology of oil-continuous emulsions

Lubrication behaviour of foodstuff is related to mouthfeel perception and consumer appreciation. Soft tribology of food related products has mainly been investigated with semi-solid food, polymer solutions and water continuous emulsions, and this is the first study aimed at investigating soft tribolocigal behaviour of oil continuous emulsions. All the emulsions considered here exhibit the same trends in terms of lubrication behaviour, where little boundary lubrication is observed at the entrainment speed considered. The volume of dispersed aqueous phase affects overall tribology of oil continuous emulsions via an increase in their dynamic viscosity. Increasing the phase volume leads to an increase in friction in the elastohydrodynamic regime whereas the lubrication in the boundary regime is improved. Elastohydrodynamic lubrication is independent of the aqueous phase composition and the type of emulsifier present at the water–oil interface. These parameters affect boundary lubrication of emulsion systems exhibiting droplet size bigger than the elastohydrodynamic oil film thickness. This is expected to have a significant impact on the design of low fat emulsions that match the lubrication properties of their full fat version.

High-Performance Lead-Free Electroplated Composite Bearing Overlay for Heavy Duty Applications

Material selection for engine internal components, e.g., bearings, is becoming increasingly more complex and demanding as the operating environments become more aggressive with the introduction of new technologies for the reduction of CO2 emissions. Historically, engine bearings contained lead, which has excellent fundamental bearing properties such as compatibility (run satisfactorily under conditions of marginal lubrication), conformability (deform and accept small scale geometrical inaccuracies of the crankshaft), and embeddability (tolerance to dirt and other foreign materials) while being readily alloyed to achieve good wear and fatigue resistance. However, facing new challenges, many original equipment manufacturers have started development programs to replace lead-containing with lead-free engine components in order to comply with new end-of-life vehicle directives or anticipated future directives. For more than 15 years, MAHLE has been successfully supplying the light, medium, and heavy duty market, with premium electroplated leaded composite bearings, which are designed to improve wear resistance. Some of this market now demands a switch to lead-free materials, while maintaining or exceeding its aforementioned requirements on bearing material properties. Composites of hard particles in a softer metal matrix are in this context ideally suited bearing materials as they can be tailored to obtain the optimal mix between soft and hard properties for the individual application. Typical hard particles that are commonly used comprise of metal oxides, nitrides or carbides. In addition to higher load carrying capabilities and longer service life, new engine technology trends demand that bearings also must operate under mixed or boundary lubrication conditions without having any adverse effect on the performance and integrity of the engine system. Boundary lubrication is commonly observed upon starting the engine before the elastohydrodynamic oil film is fully established. In this state, load is carried by surface asperities rather than by the lubricant. So far, the incorporation and even distribution of the hard particles into an electroplated lead-free matrix was not achievable using conventional direct current electroplating techniques. MAHLE, therefore, has developed a patented pulse plating technique in order to incorporate hard particles into the overlay metal matrix. The refined and modified crystal structure of the resulting lead-free overlay, with incorporated hard particles, yields a premium electroplated bearing with superior wear and fatigue resistance. Corresponding rig and engine test results have been completed to support the material development.

Bearing Fatigue Life Tests of Two Advanced Base Oils for Space Applications Under Vacuum and Atmospheric Environments

Four series of rolling-element bearing fatigue tests were conducted with 51104 size thrust ball bearings with three balls made from SUJ2 (AISI 52100) steel lubricated with two advanced synthetic base oils used for space applications. The test lubricants were perfluoropolyether (PFPE) and multiply alkyated cyclopentane (MAC). Each oil was tested with bearings under vacuum and atmospheric environments. The bearings were tested at a maximum Hertzian stress of 4 GPa on the inner and outer races. The outer race was rotated at a speed of 250 rpm. A pool lubrication system was used. Fresh lubricant was used for each test bearing. Testing in vacuum conditions was at 5 × 10−2 Pa. The test oils were analyzed to determine whether changes occurred as a result of operating in air and in a vacuum. In a vacuum environment, the PFPE 815Z oil exhibited a longer fatigue life than the MAC 2001A oil. However, in an air environment, the MAC 2001A oil had a longer L10 fatigue life than the PFPE 815Z oil. The fatigue life tests of PFPE 815Z oil in vacuum resulted in a longer L10 life than when tested in an air environment. In an air environment, hydrogen fluoride was generated in the bearing tests with the PFPE 815Z oils. Under vacuum conditions, hydrogen fluoride was not generated with the PFPE 815Z oil, resulting in longer bearing fatigue lives. The fatigue life tests of MAC 2001A oil in a vacuum resulted in shorter L10 fatigue life than in an air environment. The shorter life was attributed to the lower elastohydrodynamic oil film formation with the MAC 2001A oil because of a higher operating temperature and decomposition of the oil in vacuum.

Friction-Enhancing Properties of ZDDP Antiwear Additive: Part I—Friction and Morphology of ZDDP Reaction Films

Many phosphorus-based antiwear films, including those formed by zinc dialkyl dithiphosphates (ZDDP), cause a significant increase in friction in thin film, high-pressure, lubricated contacts. This can have a deleterious effect on engine oil fuel efficiency. Previous work has shown that friction is increased not under boundary, but under mixed lubrication conditions and it has been suggested that this phenomenon results from an effective roughening of the rubbing surfaces by the formation of unevenly-distributed reaction films. In the current paper it is shown that, when other additives commonly used in engine oils are added to ZDDP solutions, quite smooth ZDDP reaction films can result. Despite this, the ZDDP still produces a marked increase in friction in mixed lubrication conditions, which suggests that surface roughening is not the main origin of friction enhancement by ZDDP reaction films. In a companion paper, Part II, it is shown that ZDDP reaction films, whether rough or smooth, enhance friction by inhibiting the entrainment of liquid lubricant into rubbing contacts, thereby reducing the elastohydrodynamic oil film thickness (13). Presented at the 57th Annual Meeting in Houston, Texas May 20–24, 2002

Vibration Modeling of Rotating Spindles Supported by Lubricated Bearings

This paper outlines a five degrees of freedom model of a rotating spindle supported by a pair of lubricated angular contact ball bearings. The ball to raceway contacts are simulated by non-linear contact springs, representing the elastic deformation of the mating rolling members and nonlinear spring/dampers, corresponding to the contact elastohydrodynamic oil film thickness. A regression formula is used to model the latter and includes the damping contributed by the squeeze film effect caused by the mutual convergence of bearing rings. Some results of simulation studies with the model are also presented, in both the time and frequency domains. They include the overall system response, when subjected to varying spindle mass or the number of balls in the support bearings. Furthermore, comparisons are made between the simulated response of the dry and lubricated models. The overall contribution to damping of the elastohydrodynamic oil films between the rolling elements and their raceways is shown to be slight.

Measurements of EHL Films on Gear Teeth Surfaces by Optical Interference Method.

The thickness of the oil film formed between two meshing gear teeth can be calculated on the basis of the elastohydrodynamic lubrication (EHL) theory developed by Dowson. The calculation of the EHL film thickness between meshing gear teeth is based on isothermal and steady-state conditions; in reality, non-isothermal and transient conditions predominate. Thus it is necessary that the film thickness calculated by the EHL theory be compared with the measured value. In this study, the elastohydrodynamic oil films formed between a gear tooth and a sapphire plate were measured by an optical interference method. The agreement between the calculated film thickness and the measured value is recognized under the condition of low velocity.

Model of scuffing based on the vulnerability of an elastohydrodynamic oil film to chemical degradation catalyzed by the contacting surfaces

A model of scuffing is developed based on the premise that metallic surfaces can catalyze degradation of the lubricant film in situ. A failure mechanism for elastohydrodynamic films based on rapid decomposition of mineral and synthetic oils involving chemical reaction between entrapped oil and the containing surfaces is proposed. It is suggested that this destruction of the elastohydrodynamic oil film allows adhesion between nascent metal of opposing surfaces in the contact which in turn causes scuffing. Suppression of scuffing by the application of coatings that do not catalyze the oil decomposition and by the action of some lubricant additives which may block the catalytic effect of metallic surfaces is discussed. Effect of solid lubricant films and contaminant layers on scuffing is also described.

Relationship between EHL Film Thickness Reduction Caused by Sliding Velocity and Lubricant Properties.

The elastohydrodynamic oil films formed between a steel ball and a sapphire disk were observed by an optical interference method. In this study, a paraffin base oil, a synthetic oil and a traction oil were adopted for lubricants. Then the influence of the sliding velocity on the elastohydrodynamic film of each oil was investigated. The experiment reveals that the increase of the slip ratio tends to reduce the film thickness, and fluid properties of the oils influence the rate of film thickness reduction. This experimental result can be explained by the theoretical calculation of the elastohydrodynamic lubrication, which takes account of the effect of viscous heating and the effect of Eyring viscosity.

Application of the ultra-thin elastohydrodynamic oil film thickness technique to the study of automotive engine oils

Recent advances in the measurement of elastohydrodynamic (EHD) oil film thickness have enabled films of only a few molecules width to be measured with good accuracy. As a result, the range of temperature and viscosity that may be permitted in EHD experiments has been significantly extended and the practical value of the information obtained correspondingly increased. This paper explores the potential value of such techniques for investigating the viscometric behaviour of lubricants at high pressure and shear rate. It is shown that the film-forming behaviour of a viscometrically well-characterised Newtonian lubricant continues to obey classical EHD theory at film thicknesses as low as 10 nm. The level of agreement is close enough for pressure-viscosity characteristics to be deduced directly from film thickness information through the application of EHD theory. For polymer-containing, non-Newtonian lubricants, a high shear viscosity can be determined from film thickness data if pressure-viscosity characteristics are first established. A suitably structured sequence of experiments is described which yield estimates of both properties. To help interpret the viscometric information thus obtained, inlet zone viscosities are compared both with conventionally determined high shear viscosities and with the values predicted by a model of polymer solution behaviour.

Influence of a Slip on Elastohydrodynamic Lubrication.

The elastohydrodynamic oil film formed between a steel ball and a sapphire disk was observed by an optical interference method. In this study, the influence of a slip on the elastohydrodynamic oil film was investigated by creating a difference between the steel ball velocity and the sapphire disk velocity. The experiment shows that the film thickness formed under the condition of a constant mean velocity decreases with the increase of the slip. This finding is entirely different from the isothermal elastohydrodynamic theory. There is a possibility that this experimental result is due to the effect of oil temperature increase caused by the shearing heat and the effect of the non-Newtonian response of oils.

Oil Film Behavior in Elastohydrodynamic Lubrication. Effects of Solidification of Oils and Elastic Moduli of Contact Surfaces.

The shapes and thicknesses of the elastohydrodynamic oil films formed between a steel ball and a glass disk are measured using optical interferometry and compared with the established EHL theory. It is found that film profiles and thicknesses are markedly different from those predicted by the EHL theory, and their discrepancies depend upon the non-Newtonian response of oils and the difference in elastic moduli of contacting surfaces. As a result, it is concluded that in order to have a clearer understanding of the cause of surface failure and methods for its prevention, the EHL theory should be reconstructed by considering the solidification phenomenon of lubricating oils and pressure components parallel to the contacting surfaces.

Behaviour of polymer-thickened oils in lubricated concentrated contacts

Measurements of elastohydrodynamic (EHD) oil film thicknesses have been carried out to assess the lubricating performances of polymer-thickened oils using an optical interferometric technique. Five commercially available polymethacrylates (PMAs) with weight average molecular weights of 40 000–900 000 were used as polymer additives. The film thicknesses of all PMA-containing solutions became less than those of corresponding straight oils. Furthermore, provided that the PMA solutions had the same viscosity at a given experimental temperature, the EHD film thicknesses decreased to that of the base oil with increasing average molecular weight of the PMA. In order to elucidate the results and to clarify the influences of permanent viscosity loss, further experiments have been conducted for PMA-thickened oils previously degraded by means of a sonic shear tester of frequency 10 kHz. It is very interesting that an increase in the EHD film thickness with a decrease in the ambient viscosity resulted from a molecular weight decrease. In order to evaluate the above results with respect to antiwear properties, four-ball wear experiments were also conducted with PMA-thickened oils having the highest and the lowest average molecular weights. These results showed that the wear scar diameters for oils containing a PMA of higher molecular weight were larger than those for lower molecular weight oils. It is suggested that it is difficult for the polymer molecules of larger size to pass through the contact region; therefore these molecules may accumulate at the inlet region and/or sweep around the contact. The behaviour of PMA-thickened oils has been investigated by taking photomicrographs in the presence of graphite particles under pure sliding conditions.

Investigation of the temperature distribution in the elastohydrodynamic oil film

Results of investigations of the temperature distribution in elastohydrodynamic (EHD) contacts carried out on a two-disc machine for various contact parameters (rolling and sliding speeds and maximum Hertz pressures) are reported in this paper. The experiments were carried out with the aid of thin layer temperature transducers made of titanium that were evaporated onto the disc's surface in vacuum. Analysis of the errors affecting the temperature measurements in EHD contact was also carried out. It was shown that contact pressure and transducer insulation had a substantial influence on the precision of the temperature measurements made by the thin layer transducer. Error analysis has permitted the determination of the real value of the temperature rise in the contact zone by the application of suitable corrections. It was found that the relative sliding velocity and the contact loading force have substantial and similar influences on the maximum temperature rise in the EHD contact; however, the influence of the rolling velocity is relatively small. A simple empirical formula which permits the calculation of the maximum temperature rise in the contact zone as a function of load and velocity for the investigated oil is presented in this paper. The general formula as a function of non-dimensional parameters is also given. It was found that theoretical formulae which include the exact dependences of the viscosity variation on pressure and temperature ( e.g. the Kannel equation) provide the best fit to the experimental results. Good agreement of the results obtained with other researchers' experimental results was also found.

Temperaturverteilung in elastohydrodynamischen Schmierfilmen

Calculations have been made of the temperature distribution in an elastohydrodynamic oil film after applying more general boundary conditions than those which have previously been applied. In addition, friction coefficients have been determined to obtain experimental verification of the calculations. High compatibility between the friction coefficients and recently published experimental data has been found. The analysis of the temperature distributions obtained from computations permitted the determination of dependences on the non-dimensional maximum increase of temperature in the oil film and on the surfaces of the lubricated elements. Also conclusions concerning heat exchange in elastohydrodynamic contact have been made.