Full Film Lubrication Research Articles

Mixed Elastohydrodynamic Lubrication With Three-Dimensional Machined Roughness in Spiral Bevel and Hypoid Gears

Spiral bevel and hypoid gears are key components widely used for transmitting significant power in various types of vehicles and engineering machineries. In reality, these gear surfaces are quite rough with three-dimensional (3D) topography that may significantly influence the lubrication formation and breakdown as well as components failures. Previous spiral bevel and hypoid gears lubrication studies, however, were limited mostly to cases under the full-film lubrication condition with smooth surfaces. In the present study, a comprehensive analysis for gearing geometry, kinematics, mixed lubrication performance, and friction and interfacial flash temperature in spiral bevel and hypoid gears is developed based on a recently developed mixed elastohydrodynamic lubrication (EHL) model that is capable of handling practical cases with 3D machined roughness under severe operating conditions and considering the effect of arbitrary entrainment angle. Obtained results from sample cases show that the simulation model developed can be used as an engineering tool for spiral bevel and hypoid gears design optimization and strength prediction.

Elastohydrodynamic lubrication properties and friction behaviors of several ester base stocks

Abstract This paper reports a series of studies on the lubricant properties, elastohydrodynamic film thickness, and coefficients of friction of several commercially available ester base stocks, i.e., diisooctyl phthalate (DIOP), diisodecyl phthalate (DIDP), diisotridecyl phthalate (DITDP), diisooctyl sebacate (DOS), diisotridecyl sebacate (DTDS), trihydroxymethylpropyl trioleate (TMPTO), and pentaerythritol tetraoleate (PETO). The results include densities and viscosities from 303 to 398 K, and elastohydrodynamic lubricant film thicknesses and friction in the boundary, mixed and full-film lubrication regimes measured at several temperatures, loads, and speeds. These ester base stocks have different lubrication abilities owing to their chain lengths, geometric configurations, and molecular rigidity. This study provides quantitative insight into the use of ester-based lubricants for low friction through the entire lubrication regime (boundary to full film) by utilization of suitable type and size of the ester base stocks.

From Full-Film Lubrication to Boundary Regime in Transient Kinematics

Friction and lubrication regimes are usually characterized using generalized Stribeck curves. Lubricated friction experiments were carried out using the IRIS tribometer, which allows simultaneous measurement of film thickness distribution in the high-pressure zone and friction force by controlling, accurately and independently, the motions of solids. Transient kinematics combining pure rolling, acceleration, pure sliding and deceleration phases was chosen in order to get access to friction and lubrication mechanisms. It is shown that these transient operating conditions cover all the lubrication regimes that are traditionally obtained through steady-state Stribeck curves. The analytical modelling proposed here emphasizes the role of the local shear rate distribution in full-film regime and the influence of surface in the mixed and boundary regimes.

Experimental Analysis of Damping and Tribological Characteristics of Nano-CuO Particle Mixed Lubricant in Ball Bearings

Experimental analysis of damping capacity and tribological characteristics of nano CuO added Servosystem 68 lubricant is attempted. CuO nano particles were synthesized by aqueous precipitation method and characterized. Prior to dispersion into lubricant, CuO nano particles were coated with 0.2 wt.% surfactant (Span-80) to stabilize the nano fluid. Tribological characteristics of particle added lubricant were tested in ASTM D 4172 four ball wear tester. Scanning electron microscopy test results of worn surfaces of nano CuO particle added lubricant were smoother than base lubricant. The particle added lubricant was applied in a new ball bearing and three defected ball bearings. When particle added lubricant was used, the ball defected bearing's vibration amplitude was reduced by 21.94% whereas it was 16.46% for new bearing and was ≤ 11% for other defected bearings. The formation of protection film of CuO over ball surface and regime of full film lubrication near the ball zone were observed to be reason for improved damping of vibrations.

Spontaneous Blinking from a Tribological Viewpoint

The mechanical forces between the lid wiper and the ocular surface, and between a contact lens and the lid wiper, are reported to be related to dry eye symptoms. Furthermore, the mechanical forces between these sliding partners are assumed to be related to the ocular signs of lid-wiper epitheliopathy (LWE) and lid-parallel conjunctival folds (LIPCOF). Recent literature provides some evidence that a contact lens with a low coefficient of friction (CoF) improves wearing comfort by reducing the mechanical forces between the contact lens surface and the lid wiper. This review discusses the mechanical forces during spontaneous blinks from a tribological perspective, at both low and high sliding velocities, in a healthy subject. It concludes that the coefficient of friction of the ocular surfaces appears to be strongly comparable to that of hydrophilic polymer brushes at low sliding velocity, and that, with increased sliding velocity, there is no wear at the sliding partners' surfaces thanks to the presence of a fluid film between the two sliding partners. In contrast, in the case of dry eye, the failure to maintain a full fluid film lubrication regime at high blinking speeds may lead to increased shear rates, resulting in deformation and wear of the sliding pairs. These shear rates are most likely related to tear film viscosity.

Grease Aging Effects on Film Formation under Fully-Flooded and Starved Lubrication

Several film thickness measurements were performed with three fresh and aged lubricating greases, their base and bled oils under a wide range of operating conditions using a ball-on-disc test rig with optical interferometry. The analysis of the film thickness measurements is in agreement with the observations of several authors and adds some important aspects regarding separate film properties in EHL contacts. At full film lubrication and moderate to high speeds, the bled oil showed a similar behavior of its lubricating greases. At fully-flooded condition, low speeds and thin films, it was observed that the thickener lumps play a major role on film formation, overcoming the bled oil effects. A relationship between thickener type and film formation was evidenced. The same trends were observed under starved lubrication, where the thickener type that contributes the most to locally increase the film thickness follows the order of PP > Ca > Li. The aging process of the greases was shown to change their rheological response in different manners—softening or hardening—depending on the grease formulation. Grease aging increased the film thickness under fully-flooded and starved lubrication, regardless of the level of degradation.

LUBRICATION PREDICTIONS FOR TOTAL KNEE PROSTHESIS MADE OF HARD MATERIALS

The lubricated contact zone in a total knee prosthesis is modeled, where femoral and tibial components are supposed to be made of hard materials (high stiffness and hardness) and lubricated by a non Newtonian fluid (synovial liquid). To determine optimal relations among geometric parameters, load and film thickness, a generalized Reynolds equation was solved by means of finite element method. Although mix lubrication regime is predominant, it is shown that increasing conformity, it could be assure a full-film lubrication regime in most of the gait cycle. These results might be a reference to make more durable total knee prostheses.

Analytical and experimental study on the sliding contact of the sealing ring in the wet clutch

Sealing rings in the wet clutch of heavy vehicles are vulnerable and weak. A key issue in designing and applying sealing rings is predicting the state of the seal contact quickly and accurately. A mixed-lubrication model of the sealing ring is developed on the basis of the load-sharing concept. A theoretical model is proposed to estimate the hydrodynamic and asperity contact behaviour of the sealing ring. The friction coefficients are calculated and compared under different operating conditions. The results cover the entire lubrication regime, including full-film lubrication, mixed lubrication and boundary lubrication. Experiments are conducted on a special test rig for sealing rings. General agreement is observed between the experiments and the simulations. The proposed method is simple but realistic and significantly improves the simulation efficiency. Designing the sealing ring quickly and accurately benefits the transmission system of heavy vehicles.

Lubrication Enhancement for UHMWPE Sliding Contacts through Surface Texturing

Considered a self-lubricating, abrasion-resistant biomaterial, ultra-high-molecular-weight polyethylene (UHMWPE) has become the most common acetabular cup material used in artificial hip joints. Artificial hip joints exist primarily in the boundary and mixed regimes, which can lead to premature failure due to surface wear, debris generation, and subsequent osteolysis. The tendency of failure due to the generation of wear debris has turned the hip problem into a complex tribological challenge with a focus on abrasion-resistant surfaces. Surface texturing in the form of microdimples has been shown in other applications to provide enhanced wear resistance through the trapping of wear debris and through the enhancement of fluid pressures to levels that promote mixed- to full-film lubrication even at low speeds. The objective of this investigation is to explore the potential for surface texturing of UHMWPE sliding contacts in enhancing the lubrication regimes present in total hip replacements. Surface textures were created using a mechanical micromachining process, and experiments were conducted on a pin-on-disk tribometer under lubricated conditions present in the hip for textured and untextured samples. Results show that the surface textures were able to reduce friction by up to 50% over that observed with untextured UHMWPE samples, with evidence that suggests a regime shift from boundary lubrication to mixed and full-film lubrication as the primary mechanism for friction reduction.

Shear stress characteristics of microtextured surfaces in gap-controlled hydrodynamic lubrication

Microtextured surfaces can reduce friction in lubricated dynamic contact. However, no prior experimental study has explored the dimensionless parameter space of Reynolds number and dimensionless gap, since normal force is easier to control than the fluid film thickness. Here, we develop a custom precision-aligned setup for gap-controlled tribo-rheometry based on a rotational rheometer. The novel experimental setup allows for measurement of full film lubrication of parallel disks down to 20μm gaps with gap precision ±3μm, over a range of Reynolds numbers. We show for the first time in gap-controlled conditions that microtextured surfaces reduce friction. The reduction in dimensionless shear stress is nearly independent of velocity for Re<10 which matches computational results in this work.

Influence of the Geometry on the Rolling Friction Torque in Lubricated Ball-Race Contacts

Based on a theoretical model and an experimental methodology for defining the friction torque in a modified thrust ball bearing having only 3 balls and presented in [2], the authors experimentally investigated the influence of the ball diameter on friction torque when operating in mixed and full film lubrication conditions and maintaining the normal load, the race curvature and oil viscosity constant. The experiments were realized using ball diameters between 7.938 mm and 3 mm corresponding to maximum Hertzian pressure between 0.264 GPa and 1 GPa and a rotational speed between 60 rpm to 240 rpm. The experiments confirmed that the measured friction torque can be explained using hydrodynamic rolling force relationships respecting the transition from IsoViscous Rigid (IVR) to ElastoHydrodynamic Lubrication (EHL) regime presented in [2].

Tribological behavior of ionic liquid-based magnetorheological fluids in steel and polymeric point contacts

A range of ionic liquids (ILs)-based magnetorheological (MR) fluids are formulated and their tribological properties investigated in steel–steel, polyoxymethylene–polyoxymethylene POM–POM and polydimethylsiloxane–polydimethylsiloxane PDMS–PDMS point contacts. The effect of particles in dispersion is investigated by measuring neat ILs in the same tribopairs. The boundary lubrication performance of neat ILs is dominated by the Lambda ratio. A correlation is found between the contact angle, wear-scar diameters and friction coefficient. In POM–POM contacts, NTf2− containing ILs do exhibit the largest friction meanwhile full film lubrication occurs in PDMS–PDMS contacts. MR fluid lubricated contacts operate within the boundary regime. However, friction levels are significantly lower than polyalphaolefin PAO based conventional MR fluids. SCN−-based ILs do provide the largest friction coefficients.

Studying The Effect of The Surface Roughness on the Maximum Eccentricity Ratio and The Load Carrying Capacity in The Journal Bearing

The focus of this investigation is to study the effect of the two dimensional isotropic surface roughness of both , journal and bearing on the maximum eccentricity ratio and the load carrying capacity of the finite length plain journal bearing , on the basis of full film lubrication regime to reduce the wear and heat produced by friction to extend the machine element life , in the steady state operation conditions. The appropriate equation to predict the suitable minimum film thickness which include the surface roughness is used . The Conventional Reynolds equation in two dimensional forms are solved numerically . The change of viscosity due to temperature and pressure , using adiabatic solution ,were taken into account. To investigate the effect of surface roughness on the maximum eccentricity ratio value and load capacity , it was assumed that there are two surfaces smooth and rough and are compared with each other . The results of this work show that the increase of the surface roughness case the maximum eccentricity ratio decreases at L / D ratios of 0.5 , 1 at range of roughness 0.3 – 1.2 Mm , which means that the limit of bearing to work with a steady state , full film lubrication . The load ratio at L / D = 0.5 is greater than at L / D = 1 for clearance ratio 0.001 , 0.002 . The surface roughness have no influence on the bearings that have 90 mm diameter or larger at clearance ratio 0.002 .

Dynamic Analysis of Spiral-Groove Rotary Seal Ring for Wet Clutches

A tribo-dynamic model of a spiral-groove rotary seal ring is developed through coupling lubrication and dynamic equations. Effects of centrifugation, hydrodynamics, cavitation, and asperity contact are considered. To represent real rough surfaces, asperity contact is described by a statistics-based model. A global time marching scheme is developed to obtain the motion of seal ring and key parameters such as bearing force, friction torque, and leakage rate. Dynamic behaviors and seal characteristics of spiral-groove rotary seal ring under real and step change oil filling conditions are analyzed. The result shows that the rotary seal ring operates steadily under real conditions and has fast and stable step response. It is also indicated that the seal ring can achieve full film lubrication under high speed conditions through the oil filling and dispersing stage. The steady lubrication performance is experimentally validated.

Thermo-elasto-hydrodynamic Mixed Lubrication of Main Bearings of Marine Diesel Engines, Based on Coupling between Flexible Engine Block and Crankshaft

Flexible whole engine block and flexible crankshaft are built by CMS method. Under coupling between them, based on the generalized Reynolds considering mass-conserving boundary condition and Greenwood/Tripp asperity contact theory, Thermo-elastic-hydrodynamic mixed lubrication model of main bearings of marine diesel engines is developed and validated indirectly. Simultaneously, calculation shows that mixed lubrication is normalcy, while full hydrodynamic film lubrication is transient. Cavitation erosion becomes a very real possibility only when cavitations, centripetal or centrifugal movement of crankshaft and suddent change of film pressure gradient occur simultaneously. As oil leakage dissipated power is more than 80 percent of the whole dissipated power, outlet film temperature can be a key parameter for monitoring journal bearing lubrication. 8 pieces of bearings meet the requirement of lubrication basically, but work condition of two of the bearings is worse and they need be optimized furthermore.

The influence of the lubricant viscosity on the rolling friction torque

Authors propose a theoretical model and an experimental methodology for defining the friction torque in a modified thrust ball bearing, operating in mixed and full film lubrication conditions.The friction torque was measured at low loads and large Λ parameter range using a spin-down method.A comprehensive analytical bearing torque model is described using elastic rolling resistance, curvature effects, inertia forces, disc-air resistance and ball-races hydrodynamic rolling forces, the latter explaining 98% of the final bearing torque. Several sets of hydrodynamic rolling force relationships respecting the transition from IVR to EHL lubrication regime were tested. Final numerical results are shown to be very close to the experimental ones in both full film and mixed lubrication conditions.

Multiscale Analysis of the Roughness Effect on Lubricated Rough Contact

Determining friction is as equally essential as determining the film thickness in the lubricated contact, and is an important research subject. Indeed, reduction of friction in the automotive industry is important for both the minimization of fuel consumption as well as the decrease in the emissions of greenhouse gases. However, the progress in friction reduction has been limited by the difficulty in understanding the mechanism of roughness effects on friction. It was observed that micro-surface geometry or roughness was one of the major factors that affected the friction coefficient. In the present study, a new methodology coupling the multiscale decomposition of the surface and the prediction of the friction coefficient by numerical simulation was developed to understand the influence of the scale of roughness in the friction coefficient. In particular, the real surface decomposed in different roughness scale by multiscale decomposition, based on ridgelets transform, was used as input into the model. This model predicts the effect of scale on mixed elastohydroynamic point contact. The results indicate a good influence of the fine scale of surface roughness on the friction coefficient for full-film lubrication as well as a beginning of improvement for mixed lubrication.

Application of Smoothed Particle Hydrodynamics to Full-Film Lubrication

An in-house solver was created in order to simulate hydrodynamic lubrication utilizing smoothed particle hydrodynamics (SPH). SPH is a meshfree, Lagrangian, particle-based method that can be used to solve continuum problems. In this study, transient hydrodynamic lubrication in a pad bearing geometry was modeled utilizing the SPH method. The results were validated by comparison to computational fluid dynamics (CFD) and an analytical solution provided by lubrication theory. Results for the pressure distribution between SPH and CFD were agreeable while lubrication theory failed to capture any inertial effects of the fluid. Velocity profile comparisons differed slightly between all three methods. However, since smoothed particle methods have been shown to have the advantage of being able to model large deformations, as well as allowing easy definitions of fluid-solid interfaces, they can be useful tools for complex problems in tribology.

A Parametric Study on Fatigue Life for Mixed Elastohydrodynamic Lubrication Point Contacts

The lubrication characteristics and fatigue life are numerically analyzed under full film and mixed lubrication regimes, in which the three-dimensional sinusoidal surfaces with changeable wavelengths in x and y directions are used, the geometry changes of the contact areas are described by the various ellipticity, and the non-Newtonian flow of lubricant is described by the sinh-law rheology model. The results show that the influences of characteristic shear stress, wavelength ratio, and ellipticity on lubrication characteristics and fatigue life are remarkable. The effect of surface topography on lubrication characteristics has a close relationship with speed. Increasing the ellipticity and decreasing wavelength ratio and characteristic shear stress can prolong the fatigue life.

The Effect of Wavy Groove Liner on Pressure Distribution of Journal Bearing

The ability of bearing liner to maintain the fluid film lubrication is crucial to its performance. This study is to investigate the pressure distribution for full film lubrication of wavy bearing liner. The results were compared to that from smooth bearing liner. These bearings were used with the palm oil based lubricant. CFD analysis was developed to determine the numerical data. A 60 mm bearing with ratio 0.5 of its diameter to length is simulated. This bearing has 250 µm in clearance and 200 µm in amplitude with semi rectangular circumferential surface waviness liner in shape. Pressure distribution of the bearing was influenced by the increment of the eccentricity ratio and the speed of shaft. The surface waviness liner bearing produces higher in pressure by comparing it with the smooth bearing. The maximum pressure was noted in the vicinity of minimum fluid film thickness where it was distributed at the area around the peak of wave.