Elastohydrodynamic Lubrication Film Research Articles

Two-dimensional generalized non-Newtonian EHL lubrication: Shear rate-based solution versus shear stress-based solution

Lubricant behaves non-Newtonian at high shear stress and high shear rate. The non-Newtonian shear behavior of oil such as shear-thinning, viscoelasticity, and limiting shear stress could have influences on almost all characteristics of an elastohydrodynamic lubrication (EHL) contact, that is, the central film thickness, the coefficient of friction, and the temperature rise in the lubricating film. For example, for lubricants of large molecular weight or of polymer blended ones, there can be inlet shear-thinning, which would reduce the EHL film thickness. For the EHL traction in a rolling/sliding EHL contact, it cannot be reasonably predicted without the consideration of non-Newtonian rheology. In EHL numerical studies, the non-Newtonian properties and the constitutive equations are expressed by the concept of generalized viscosity [Formula: see text], which can be either a function of shear rate [Formula: see text] or a function of shear stress [Formula: see text]. In this way, a non-Newtonian lubrication problem could be solved as a generalized Newtonian problem based on solvers for a Newtonian EHL problem. According to the function of the generalized viscosity [Formula: see text], numerical solutions can be classified into shear rate-based ones and shear stress-based ones. In this work, these two kinds of numerical solutions are revisited. And their efficiency is compared for a two-dimensional (2D) non-Newtonian point contact EHL problem (here 2D means non-Newtonian flow in both the x and y directions). Results show that the shear rate-based numerical solution has a higher efficiency than the shear stress-based one. The shear rate-based 2D generalized Newtonian method is more suitable to analyze multiple EHL contacts in angular contact ball bearings and gears with complex 2D flow and/or transient EHL lubrication problems.

Professor Duncan Dowson, a source of inspiration

The current paper highlights the contribution of the Dowson and Higginson work to numerical line contact elastohydrodynamic lubrication film thickness prediction and the Hamrock and Dowson contribution to the film thickness prediction in elliptical contacts. This paper shows that, even by today’s standards, both the numerical pressure and film thickness results and the curve-fitted film thickness predictions are very accurate. As for the elliptical results, the authors show that the original predictions remain surprisingly accurate for moderately elliptical contact. For very long elliptical contacts, their prediction does not tend to a line contact asymptote. This paper then concludes that the predicted pressure spikes by Dowson, Higginson, and Hamrock are correct in shape and amplitude, at least near pure rolling conditions.

Starvation and relubrication mechanisms in grease lubricated oscillating bearings

Oscillating rolling element bearings can be found in various industrial applications. A popular example are the rotor blade bearings of wind turbines, which allow the rotor blades to turn around their longitudinal axis. These bearings are predominantly grease lubricated and expected to be running in a state of low velocity starvation. Elastohydrodynamic Lubrication (EHL) films in starved contacts are susceptible to the conditions of inlet lubricant supply. If the bearings are running under moderate starvation no early failure due to wear is expected. If the inlet conditions increasingly lead to a drying out of the contact, resulting in a heavily starved contact, metal-to-metal contact can lead to false brinelling and subsequent bearing failure. Using two different greases bearing experiments are used to gain an understanding of the mechanism of wear initiation. It is found that starvation seems to be a major contribution to wear appearing in the investigated operating conditions (2°-45° osc. angle, 0,2–5 Hz osc. frequency). The degree of starvation in the contact depends on the balance between lubricant displacement by the rolling element and the replenishment of lubricant during operation. The following conclusions could be drawn: Replenishment is highly dependent on the operating conditions such as the oscillation frequency, the oscillation amplitude, and lubricant rheology. For small oscillation angles the ability of the grease to release base oil with high mobility into the contact seams to be essential. For greater oscillation angles, after crossing a limiting angle, a secondary replenishment mechanism seems to become active. This prevents early wear initiation. At critical operating parameters, which mainly include high oscillation frequencies and medium oscillation angles, severe wear is initiated after just a few hundred oscillation cycles. By modifying an existing starvation factor the influences could be visualised and compared to the experimental results.

Achieving macroscale liquid superlubricity using glycerol aqueous solutions

In this work, we explored macroscale liquid superlubricity using glycerol as the base lubricant to form aqueous solutions with different water/glycerol weight ratios. We find that glycerol aqueous solutions with water/glycerol weight ratio of 0.2 achieves superlubricity (coefficient of friction < 0.01) reproducibly within 40 s of testing using steel tribopairs, under a modest range of tribotesting conditions (maximum nominal Hertzian stress up 800 MPa and sliding speed up to 0.15 m/s). Mixed-elastohydrodynamic simulation reveals evidence of apparent mating between asperities on the ball and the disk, along with the formation of microscopic elastohydrodynamic lubrication films that are thick enough to separate the tribopair asperities. The numerical simulation also gives the limiting superlow coefficient of friction. These results lead to a hypothesis by which superlubricity could be achieved with other lubricants.

Traction behaviour of elastohydrodynamic lubrication films with anomalous shapes

This study describes traction behaviours of lubricant films having anomalous shapes under elastohydrodynamic lubrication conditions. The traction generated at a point contact area between a glass or sapphire disc and a steel ball was measured by changing the slide-to-roll ratio. Three alcohols, 1-dodecanol, ethylene glycol and glycerol, and two alkanes of n-tetradecane and n-hexadecane were used as lubricants. Lubricants developing anomalous film shapes exhibited a solid-like behaviour with a sharp traction peak at low slide-to-roll ratios. On the contrary, other lubricants having conventional film shapes indicated a gradual increase in traction coefficient with increasing slide-to-roll ratios. The similarity of the traction behaviour to that of traction fluids supports the solidification of the film, which developed anomalous film shapes.

Macroscale Superlubricity and Polymorphism of Long-Chain n-Alcohols

Simple n-alcohols, such as 1-dodecanol, show anomalous film-forming and friction behaviors under elastohydrodynamic lubrication (EHL) conditions, as found inside bearings and gears. Using tribometer, diamond anvil cell (DAC), and differential scanning calorimetry (DSC) experiments, we show that liquid 1-dodecanol undergoes a pressure-induced solidification when entrained into EHL contacts. Different solid polymorphs are formed inside the contact depending on the temperature and pressure conditions. Surprisingly, at a moderate temperature and pressure, 1-dodecanol forms a polymorph that exhibits robust macroscale superlubricity. The DAC and DSC experiments show that superlubricity is facilitated by the formation of lamellar, hydrogen-bonded structures of hexagonally close-packed molecules, which promote interlayer sliding. This novel superlubricity mechanism is similar to that proposed for the two-dimensional materials commonly employed as solid lubricants, but it also enables the practical advantages of liquid lubricants to be maintained. When the pressure is increased, 1-dodecanol undergoes a polymorphic transformation into a phase that gives a higher friction. The DAC and DSC experiments indicate that the high-friction polymorph is an orthorhombic crystal. The polymorphic transformation pressure coincides with the onset of a dimple formation in the EHL films, revealing that the anomalous film shapes are caused by the formation of rigid orthorhombic crystals inside the contact. This is the first demonstration of a macroscale superlubricity in an EHL contact lubricated by a nonaqueous liquid that arises from bulk effects rather than tribochemical transformations at the surfaces. Since the superlubricity observed here results from phase transformations, it is continuously self-replenishing and is insensitive to surface chemistry and topology. This discovery creates the possibility of implementing superlubricity in a wide range of machine components, which would result in enormous improvements in efficiency and durability.

Recent Trends in Rheological Measuring Techniques of Elastohydrodynamic Lubrication Film Using Fluorescence Method

Recent Trends in Rheological Measuring Techniques of Elastohydrodynamic Lubrication Film Using Fluorescence Method

Impulse Response of the Elasto-Hydrodynamic Lubrication Film of a Rolling Bearing to Dynamic Excitation of a Flat Belt Drive.

The impulse response of a rolling bearing and its principal component, the elasto-hydrodynamic lubrication film (EHDL), are analysed. When measuring the vibrations of bearings, we observed that the impulse response was mostly caused by defects (fatigue damage) on the raceways and/or rolling elements. However, this phenomenon can also occur in new defect-free roller bearings, where it is not commonly expected. This study presents an experiment that identifies the conditions of dynamic excitation for the impulse response of the EHDL, the source of which is not defects, but the EHDL itself. The EHDL responds in the form of impulses in case the velocity of its radial deformation is too fast. This is an unfavourable phenomenon that significantly shortens the service life of bearings. To analyse the dynamic excitation conditions, a testing bench at speeds up to 135,000 rpm with a flat belt drive was used. The testing bench enabled the formation of the so-called beat excitation from two harmonic excitation forces close in rotational frequency. The subject of this study is a defect-free high-speed double-row angular contact ball bearing used in the textile industry. We also present other physical conditions for the occurrence of undesired impulse responses that are caused by the EHDL.

Tribo-dynamic analysis of high-speed roller bearings for electrified vehicle powertrains

Roller bearings are critical components in electrified powertrains. At high speeds, the critical role of the elastohydrodynamic lubrication (EHL) cannot be ignored, necessitating a tribo-dynamic analysis. This paper investigates the behaviour of roller bearings under operating conditions present in high-speed electrified powertrains. The bearing motion is obtained experimentally using a novel experimental rig. The bearing centre's orbital motion is acquired from the experiment and used as the boundary condition for tribological analysis. Results reveal that the EHL film thickness increases from 0.2 to 1.9 μm across a speed range of 0–15 000 rpm. A comparison with conventional dry contact analysis shows that roller load can be underestimated if the lubricant film is neglected in the roller-race contact deformation.

Fault Diagnosis Method for Deep Groove Ball Bearings 6008, 6208 and 6408 Based on 'Hertz's Contact Theory' Using Engine Oil and Gear Box Oil

A bearing is the machine component which allows relative movement between the components of machines and equipment, along with transmitting the load. The most commonly used bearings are the Rolling Element Bearing. Rolling element Bearings generally fail suddenly before giving any indication. Many monitoring methods are in use to monitor the condition of bearings. The existing methods are either very costly or require major changes in the setup. The present work is based on a method, simple and inexpensive and capable for online condition monitoring of rolling element bearings. The methodology includes analytical determination of the Elastohydrodynamic lubricant film thickness based on “Hertz’s Contact theory”. The formula developed includes the Electrical Resistance between balls and races of the bearings and the Electrical Resistivity of the lubricants. The Electrical Resistivity is determined by using a simple Test Rig. Further the Electrical Resistance is also determined by using a simple Electrical Circuit along with measuring instruments on Rolling Element Bearing Test Rig at different loads and speeds. The Electrical Resistance is directly related to the lubricant film thickness present between the balls and races of the bearings, more the thickness, more will be Electrical Resistance. Related formulae of lubricant film thickness were developed for test bearing 6008, 6208, 6408 and test lubricants 20W40 (Engine oil), G 90 (Gearbox oil). Experiments were performed for speed range from 800 rpm to 1200 rpm (in steps of 100 rpm), and load range from 40 Kg to 120 Kg (in steps of 20 Kg), on Rolling Element Bearing Test Rig. Useful conclusions are drawn at the end of the experimental work based on the results.

Study on simultaneous measurement of elastohydrodynamic lubrication film and traction coefficient in low loss automotive lubricants

Study on simultaneous measurement of elastohydrodynamic lubrication film and traction coefficient in low loss automotive lubricants

Interactions of Ethanol with Friction Modifiers in Model Engine Lubricants

When employed as an engine fuel, ethanol can accumulate in the lubricant during use. Previous work has shown that ethanol contamination affects friction and elastohydrodynamic lubrication (EHL) film formation, and also the growth and stability of anti-wear tribofilms. The present work uses spacer-layer ultrathin interferometry and MTM tests to investigate how ethanol (both hydrated and anhydrous) interacts with friction modifiers in model lubricants. Small proportions (5 wt %) of ethanol were added to solutions of friction modifiers (one MoDTC and three organic friction modifiers) in a Group I base oil. For the three organic friction modifiers, the presence of ethanol promoted the formation of thick viscous boundary films so that very low friction coefficients were measured at low entrainment speeds. For the MoDTC additive, the presence of ethanol prevented the formation of a low friction film at low speeds at 70 °C, but this effect disappeared at 100 °C, probably due to ethanol evaporation.

Elastohydrodynamic lubrication line contact of piezoelectric materials

This paper investigates the elastohydrodynamic lubrication (EHL) line contact between a rigid cylindrical punch and a transversely isotropic piezoelectric half-plane. The punch is supposed to be a perfect conductor, and its electric potential is constant within the contact region. The lubricant is assumed as a kind of Newtonian fluid. The lubricant density and viscosity are considered as the function of the fluid pressure. An iterative method is developed to solve the film thickness equation, Reynolds equation, flow rheology equation and load balance equation simultaneously. The effects of the total electric charge, maximum normal pressure, entraining velocity and material properties on the EHL film thickness and fluid pressure are discussed. Furthermore, the EHL contact of the insulating punch is also discussed to analyze the influences of conductivity of punch and piezoelectric effect of materials. The present results could be applied to improve the wear and contact damage of piezoelectric devices.

A Computational Fluid Dynamics Study on Shearing Mechanisms in Thermal Elastohydrodynamic Line Contacts

A computational fluid dynamics (CFD) model of the thermal elastohydrodynamically lubricated (EHL) line contact problem has been developed for the purpose of exploring the physical processes that occur inside a thin EHL film subjected to shearing motion. The Navier–Stokes equations are solved by using the finite volume method (FVM) in a commercial CFD software, ANSYS Fluent. A set of user-defined functions (UDF) are used for computing viscosity, density, heat source, temperature of moving surfaces and elastic deformation of the top roller according to well-established equations commonly used in the EHL theory. The cavitation problem is solved by taking into account multiphase mixture flow. The model combinations of Houpert and Ree–Eyring and of Tait and Carreau were used for modeling the non-Newtonian behavior of Squalane and the results were compared. Both rheological models suggest the existence of shear-band and plug-flow at high fluid pressure. Due to the differences in viscosity at GPa-level pressure, the chosen model has substantial influence on the computed shear stress and temperature distributions in the high-pressure region. This shows the importance of using correct rheology information in the whole range of pressure, temperature, and shear strain rate.

Effect of arbitrary entrainment angle in elastohydrodynamic lubrication elliptical and circular contacts

Conventional elastohydrodynamic lubrication models assume parallel surface velocities such as in cam-follower and ball bearing systems. However, many important machine elements like hypoid gear and spiral bevel, the sliding and entrainment velocities are along different directions that could possibly influence the elastohydrodynamic lubrication performance characteristics significantly. For such complex conditions, the existing film thickness formulae and shear-thinning correction factors available in the literature are not suitable. Therefore, this paper investigates the effect of arbitrarily oriented surface velocity vectors on elastohydrodynamic lubrication characteristics considering realistic rheological models and experimentally established viscosity–pressure and compressibility laws. The Reynolds equation employed herein includes the surface velocity components along both the reference axes in the plane of contact. The elastohydrodynamic lubrication film thickness is found to deviate up to a maximum of 61% with respect to its conventional value. This deviation in film thickness behavior is shown to be a function of ellipticity and shear-thinning parameters.

Boundary and Elastohydrodynamic Lubrication Behaviors of Nano-CuO/Reduced Graphene Oxide Nanocomposite as an Efficient Oil-Based Additive.

Copper oxide/reduced graphene oxide (CuO/rGO) nanocomposite was synthesized by an effective approach combining supercritical hydrolysis and facile thermolysis. The as-synthesized CuO/rGO exhibited a unique microstructure in which ultrafine CuO nanoparticles were encapsulated by several stacked rGO nanosheets. The highly dispersed CuO nanoparticles on the rGO nanosheets were in spherical shape with diameters around 6 nm and had a base-centered monoclinic crystal structure. Tribological performances of the CuO/rGO nanocomposite as a lubricant additive of 10w40 engine oil were evaluated by a ball-on-disc tribotester under boundary lubrication. As compared to the bare 10w40 engine oil, the engine oil with 0.06-0.18 wt % CuO/rGO exhibits the reductions of 46.62 and 77.05% for friction coefficient and wear rate, respectively. In addition, CuO/rGO as a nanoadditive displays superior lubricating abilities than rGO, CuO, and their mechanical mixture. This synergistic lubricating effect of the CuO/rGO nanocomposite was mainly contributed to its special nanostructure rather than only the coexistence of CuO and rGO. The antiwear and friction-reducing mechanism of the CuO/rGO nanoadditive mainly results from its synergistic lubricating effect and the formed tribofilms on the wear surface due to the spontaneous deposition behavior of the nanocomposite. In addition, influences of the nanoadditives including rGO, CuO, the mixture, and CuO/rGO on the rheological behaviors and the elastohydrodynamic lubrication (EHL) oil film of the modified engine oil were also investigated. Results show that these nanoadditives can increase the viscosity of the engine oil to a certain extent throughout the test temperature range of 0-60 °C. The remarkable increase in viscosity by addition of the CuO/rGO nanoadditive corresponds to the thickening of the EHL film and the strengthening of the EHL state for the engine oil with CuO/rGO as compared to the bare one.

Oil film variation and surface damage in the process of reciprocation-oscillation transformation

Abstract In this study, optical interferometry experiments were carried out on a ball-disk test rig to study the change of elastohydrodynamic lubrication (EHL) in reciprocation to boundary lubrication (BL) in oscillation for the contacts of pure rolling and simple sliding motions. Then, using an SRV wear tester, the corresponding tribological performance during the transformation from the reciprocating motion to the oscillating motion under simple sliding condition was evaluated. The experimental results show that the BL area in the contact zone gradually increases with the decrease of the stroke length, so that the area of entrapped oil film by the squeeze effect is gradually decreased. The experiments by the SRV tester revealed that, with the decrease of stroke length, the surface damage become more severe. The article aims to synthesize the variation of the EHL film and the BL film as well as the sliding wear and fretting during the transformation of the reciprocating to oscillating motion.

Parameter design methodologies for vertical skewed line gears taking lubrication performance into account

Based on the elastohydrodynamic lubrication (EHL) theory, design methodologies are proposed aiming to improve the EHL state of the vertical skewed line gear (VSLG) in this article. First of all, the parametric equations of the driving and driven VSLG surfaces are established and then converted into equivalent Hertz contact model. Then, the influences of the design parameters of the VSLG on the ellipticity are analyzed by its equivalent Hertz contact model. Finally, the parameter design method for improving the EHL state of VSLG is deduced. The results of optical elastohydrodynamic experiment and simulation experiment indicate that the parameter design method is effective for the formation of EHL film without changing the kinematic characteristics of VSLG. In short, this paper provides reliable and convenient parameter design methods for VSLG in consideration of EHL.

Mapping of the lubrication regimes in rough surface EHL contacts

Abstract Understanding film formation in rough surface elastohydrodynamically lubricated (EHL) contacts have been an ongoing pursuit in lubrication science for more than half a century. This study furthers that quest by establishing a single combined friction and electrical contact resistance map that forms a clear and comprehensive overview of the lubrication performance. A ball-on-disc machine was operated under a wide variety of heavily loaded rolling/sliding contact conditions. Results show that while sweeping the contact over the SRR- and entrainment speed-domain, the primary sweep direction significantly affects running-in and consequently the transition from EHL to the mixed lubrication regime. Such knowledge sheds new light into the mechanisms that governs EHL film formation and the concurrent interplay with the mixed lubricated friction coefficient.

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

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