Slide-roll Ratio Research Articles

Transient mixed thermal elastohydrodynamic lubrication analysis of aero ball bearing under non-steady state conditions

A transient mixed thermal elastohydrodynamic lubrication model of bearing considering the thermal effect and the changes in velocity, load, and contact geometry is proposed. Based on the bearing dynamic and lubrication analysis, the effects of bearing rotational speed and start-stop conditions on the dynamic behavior and lubrication performance are studied. Results show that when the bearing rotational speed exceeds a certain critical value, the increasing speed will reduce the film thickness, especially the outer raceway. The increasing bearing acceleration can cause a higher slide-roll ratio and film temperature at the initial start-up process. Compared with the start-up process, the lubrication change in the shut-down process has a similar opposite trend, but the overall lubrication performance is better.

An Experimental Investigation Replicating the Surface Behavior of Ground Steel Gears in Mixed Lubrication Using Twin-Disk Testing Part 1: Running-in

ABSTRACT The surface of ground gear teeth is changed during the initial period of operation through a process termed running-in. During this process asperity peaks are plastically deformed and removed to better distribute the load across the surface, resulting in modification of the surface finish. In this work the influence of pressure, slide-roll ratio, and entrainment velocity on two-dimensional surface roughness parameters is evaluated through the running-in process using a full-factorial experimental programme. Hardened steel disks are used to simulate gear tooth contacts via the use of a twin-disk rig. Results showed that all three variables strongly influence the change in surface geometry, both individually and through both two- and three-factor interactions.

An Experimental Investigation Replicating the Surface Behavior of Ground Steel Gears in Mixed Lubrication Using Twin-Disk Testing Part 2: Micropitting

Tooth surfaces in heavily loaded power-transmission gearing can often suffer from surface fatigue on the surface roughness scale, known as micropitting. In this paper, the influence of three variables (pressure, slide-roll ratio, and entrainment velocity) on micropitting is investigated using a full-factorial experimental program using a twin-disk test rig. The results showed that pressure was most influential on micropitting growth during the latter phases of the test, with slide-roll ratio being a more important factor during micropit initiation. The full-factorial nature of these tests allowed two- and three-factor effects to be investigated, demonstrating that micropitting is a complex phenomenon influenced by a network of interconnected effects.

Elastohydrodynamic Traction and Film Thickness at High Speeds

A renewed interest in elastohydrodynamic lubrication (EHL) phenomena at high speeds, for which thermal effects strongly influence both traction and film thickness, has grown out of the challenges presented by high-speed geared transmissions in electric vehicles. This study uses a new ball-on-disc set-up employing the well-known ultra-thin-film interferometry technique to simultaneously measure EHL film thickness and traction at entrainment speeds up to 20 m/s and slide-roll ratios up to 100%. The effect of fluid composition is examined for Group I, II and III mineral oils, for two polyalphaolefins in Group IV, and for the traction fluid Santotrac 50. The effect of viscosity in the range 4–180 mPa.s is investigated by varying bulk fluid temperature. At high speeds, both film thickness and traction are considerably lower than predicted by conventional EHL theory. The contact is seen to be fully-flooded for all conditions tested. The widely-used thermal EHL correction of Gupta is shown to overcorrect for the film thickness reduction even at modest SRRs. Finally, the influence of the sliding direction on traction and film thickness is discussed for this set-up, and a thermal model is proposed to explain the observed behaviour.Graphical abstract

Thermal lubrication characteristics of hypoid Gear with non-Newtonian fluids considering roughness

Based on Hertz contact theory and EHL, the model of elliptic contact elastohydrodynamic lubrication considering load, speed, roughness, thermal effect, and shear-thinning effect of non-Newtonian fluids is established. The variation trend of oil film pressure, thickness, and temperature with load, entrainment velocity, and the slide-roll ratio is analyzed. Then, several lubrication states considering roughness, the thermal effect, and the characteristics of non-Newtonian fluid are compared. Compared with the smooth surface, the roughness when RMS is 0.15 μm increases the maximum film pressure (P max) by 8.34%, increases the maximum film temperature (T max) by 7.18%, and reduces the minimum film thickness (H min)by 9.13%. The pressure and temperature are more sensitive to light load. The results are approximate to that of the smooth surface when the entrainment velocity is big enough. The pressure and thickness decrease with the increasing slide-roll ratio. The decreasing speed of the oil film thickness becomes faster as a result of the thermal effect. The roughness and the characteristics of non-Newtonian fluid have a significant impact on temperature.

Skidding Analysis of Exhaust Cam-Roller Unit in the Steady/Startup Operation of Internal Combustion Engine

This paper develops a coupling model of cam-roller contact and roller-pin contact considering thermal effects based on the exhaust cam-roller unit of an internal combustion engine. For the steady/startup running processes, the skidding and the lubrication performance are obtained in a complete cam rotation cycle, and the effects of oil viscosity or modified load are also discussed. The results show that the most significant effect of skidding on the cam-roller unit is the increase in friction between the cam and the roller. In a cam rotation cycle, the slide-roll ratio is not constant, and its value may even be negative. Compared with the steady running process, skidding is more pronounced during the startup running process, especially at the beginning of the acceleration stage (0–18°). Reducing oil viscosity or increasing modified load can effectively reduce the skidding situation, and its effect is more obvious in the steady running process than in the startup running process.

Energy-Conserved Hydrodynamic Lubricated Components with Wall Slippage

The hydrodynamic thrust slider and journal bearings as well as hydrodynamic lubricated gears with the merit of energy conservation by the wall slippage are reviewed. The principle for designing these hydrodynamic contacts is to artificially set the wall slippage on the stationary surface in the hydrodynamic inlet zone. To design the wall slippage on the moving surface in the hydrodynamic outlet zone can also give additional benefits. The technical merits of these mechanical components are the improved load-carrying capacity and the lowed friction coefficient, i.e., the energy conservation due to the wall slippage. Owing to the designed wall slippage, the carried load of the hydrodynamic step bearing can be increased by 200%~400% while its friction coefficient can be reduced by 50%~85%, and the load-carrying capacity of the hydrodynamic journal bearing can be increased by nearly 100% while at the same time, its friction coefficient can be reduced by more than 60%. For hydrodynamic lubricated gear contacts, by covering ultrahydrophobic or oilphobic coatings on the slower moving surface, the friction coefficient can be approaching to vanishing and the contact load-carrying capacity can be increased very significantly for large slide-roll ratios under medium or heavy loads.

Numerical Solution for Thermal Elastohydrodynamic Lubrication of Line Contact with Couple Stress Fluid as Lubricant

In this paper, the detail analysis of the influence of thermal and non-Newtonian aspects of lubricant (couple stress fluid) on EHL line contact as a function of slide-roll ratio is presented. The novel low complexity FAS(Full approximation scheme), of Multigrid scheme, with Jacobi dipole and Gauss Seidel relaxationis used for the solution of coupled equations viz.modified Reynolds equation, film thickness equation and energy equation satisfying appropriate boundary conditions. The analysis reveals the combined influence of non-Newtonian, thermal and slide-roll ratio (of bearing movingwith different speeds) on pressure, film thickness and pressure spike covering wide range of physical parameters of interest. Results show that pressure spike is strongly influenced by thermal, slide-roll ratio and non-Newtonian character of lubricant with negligible effect on overall pressure distribution. Also, minimum film thickness is slightly altered and it increases with increase in couple stress parameter. These findings confirm the importance of non-Newtonian and thermal effects in the study of EHL.

Wear induced changes in surface topography during running-in of rolling-sliding contacts

Running-in occurs in rolling-sliding contacts under mixed-friction conditions and typically involves asperity smoothing through mild wear and plastic deformation. To improve the prediction of service life or friction of rolling-element bearings under mixed-friction conditions, knowledge of surface topography changes during running-in and their dependence on the operating condition is an important prerequisite. Therefore, this study aims to describe the surface mechanisms due to wear, during running-in and their dependence on one of the variables, slip.AISI 52100 steel specimens were tested in a mini traction machine (MTM) instrumented with friction and contact potential measurement in the presence of a PAO base oil, operating in the mixed lubrication regime. A novel method of pre and post-test surface relocation with 3-D optical profilometry and scanning electron microscopy (SEM) was implemented. Rolling-sliding tests were performed to study the effect of slide-roll ratio on the surface topography changes during running-in. Additional tests were conducted to track the changes to surface topography during this period.The results exhibit the rapid nature of running-in and how most of the surface topography changes occur in the first few load cycles. Surface topography transitions such as asperity removal due to wear, increase in the load-bearing area of the asperities and in tests with slip, plastic flow of material from peaks into adjacent valleys are shown at high magnification. Surface profilometry measurements show that the reduction in depth of the valleys due to plastic flow adds to the reduction in roughness, thus accurately identifying the surface mechanism, and redressing the consensus that the reduction in roughness during running-in is mainly due to wear and plastic deformation diminishing the height of the peaks, which is true for ideal running-in alone.

Development of lubrication film and influence on friction in a total knee replacement during a gait cycle

The presented study dealt with the experimental analysis of friction and lubrication mechanism observed with fluorescent microscopy in total knee replacement represented by CoCrMo wheel and PMMA plate. Model synovial fluid with marked constituents was used as test lubricant. The experiments investigated three main parameters. Slide-roll-ratio (SRR) showed the greatest influence on lubrication film formation. Friction coefficient was mostly influenced by axial load; its increase led to a decrease in friction. Relative speed was manifested for SRR 1.25 and higher where its increase caused a decrease in friction. The complex experiment showed a development of friction coefficient and lubrication film along the whole gait cycle. The results of this experiment correspond with the initial results concerning the influences of parameters.

Discussion on Ball Screw Slide–Roll Ratio and Entrainment Velocity Calculation

The slide–roll ratio and entrainment velocity are critical parameters in ball screw mechanism tribology investigations and are often determined by Chin-Chung Wei’s approach. Their findings indicated that substantial sliding always occurs between the ball and the raceways, which appears to violate the ball drive principle. We validated Wei’s approach using the Harris method, which is widely used in rolling bearing research. When the helix angle is set to zero, significant differences occur: when the Harris method is utilized, the entrainment velocity at the inner contact points is essentially equal to that at the outer contact points, and the slide–roll ratio is zero for both; however, when Wei’s method is utilized, the entrainment velocity at the inner side is nearly three-times that of the outer side, and the slide–roll ratio at the outer side approaches two—the level of pure sliding—which is clearly incorrect. To overcome this issue, we present an accurate approach for obtaining the slide–roll ratio and entrainment velocity for ball screws by regarding the Frenet frame as a virtual cage, which is particularly applicable to those with a long lead and operating at high speeds. Moreover, we investigated the effect of structural factors on the slide–roll ratio and entrainment velocity utilizing this model.

Modeling and analysis of sliding wear based on fractal surface reconstruction in line contact mixed lubrication

In mixed lubrication, the direct rough contact between the two contact surfaces causes material wear and lead to the variation of the surface micro-topography. In turn, the variation of the micro-topography affects the lubrication state between the two contacting surfaces. For the better comprehension of the interaction of lubrication and wear on rough surfaces in mixed lubrication, in this paper, a coupled model of fluid flow, rough contact, and surface wear was established, with fractal theory being applied to predict the changes in surface microtopography. Furthermore, the effects of fractal dimension, roughness direction, contact load and slide-roll ratio on the surface contact area ratio, accumulated wear, film thickness and friction coefficient are analyzed. The results demonstrated that surfaces with large fractal dimension have denser roughness peaks, resulting in larger contact areas and more severe wear. Transverse roughness surfaces exhibit better lubrication and wear performance compared to longitudinal and isotropic surfaces. In addition, increasing the contact load and slip-roll ratio increases the contact area ratio, resulting in lower lubricating performance and increased wear.

Modeling the effects of solid thermal expansion and thermal stress on elastohydrodynamic lubrication

Temperature has a significant effect on the performance of elastohydrodynamic lubrication (EHL), and this topic has been extensively covered in many studies. These studies are focused on how temperature affects lubrication performance, without considering the effect of temperature on the deformation and the subsurface stress of the contact bodies. However, there will be a significant rise in temperature in the contact area under such conditions as high speeds, heavy loads, and high slide-roll ratios. This will generate significant thermal stress inside the solid, thus inhibiting the elastic deformation of the contact surface and impairing the lubrication performance in the contact area. In such cases, it is essential to consider the effects of solid thermal stress and thermal expansion on thermal elastohydrodynamic lubrication (TEHL). In this paper, a TEHL model that consider solid thermal expansion and thermal stress to solve point contact problems is developed. The effects of solid thermal expansion and thermal stress on pressure, film thickness, temperature, and subsurface stress are investigated. The results show that solid thermal expansion partially inhibits the elastic deformation of the contact surface, resulting in a decrease in film thickness and an increase in pressure. It is also found that solid thermal stress causes the subsurface von Mises stress of the upper contact body to increase and that of the lower contact body to decrease.

Boundary Slip-Induced Temperature Rise and Film Thickness Reduction Under Sliding/Rolling Contact in Thermal Elastohydrodynamic Lubrication

Abstract Temperature rise and film thickness reduction are the most important factors in elastohydrodynamic lubrication (EHL). In the EHL contact area, interfacial resistances (velocity/thermal slips) induced by the molecular interaction between lubricant and solid become significant due to the large surface/volume ratio. Although the velocity slip has been investigated extensively, less attention has been paid on the thermal slip in the EHL regime. In this study, numerical simulations were conducted by applying three cases of boundary slips to surfaces under sliding/rolling contacts moving in the same direction for the Newtonian thermal EHL. We found that the coupled velocity/thermal slips lead the most significant temperature rise and film thickness reduction among the three cases. The velocity slip results in a lower temperature in the lubricant and solids, whereas the thermal slip causes a temperature rise in the entire contact area in the lubricant as the film thickness decreases simultaneously. Furthermore, the effect of thermal slip on lubrication is more dominant than that of velocity slip, which increases the entrainment velocity or slide–roll ratio.

Effects of Thermal Properties of Contact Materials and Slide-Roll Ratio in Elastohydrodynamic Lubrication

Abstract The effects of thermal conductivity, heat capacity, thermal inertia, and slide-roll ratio (SRR) on point elastohydrodynamic lubrication (EHL) are discussed with engineering ceramics and steel by a non-Newtonian thermal EHL analysis. When the thermal conductivities of contacting materials are significantly different, the film thickness is greatly affected by which material has the higher velocity. However, the film thickness is dominated by the heat capacity when the difference in thermal conductivity is not large. In contact of materials with the same mechanical and thermal properties, the film thickness and friction coefficient are influenced by the thermal inertia.

Tribo-dynamic analysis for aero ball bearing with 3D measured surface roughness

Tribological failures, like wear, scuffing and pitting, etc, often occur in aero ball bearings due to large dry contact area, high friction-flash temperature and stress concentration at the interface between the contacting ball and race. In this paper, a numerical tribo-dynamic analysis procedure is developed for the predictions of dynamic performance, lubrication state, friction-temperature, and subsurface stress of the aero ball bearing with 3D measured roughness. The numerical model is verified by comparing the friction coefficients with the testing results, which show well agreements and proves the presented model is available. The dynamic performance of bearing at different azimuth angle are studied, which indicate the increasing radial load and speed result in more asymmetric distributions of the contact load, stress and slide-roll ratio, the maximum values and the unloaded-region become larger. With the increase of the inner race groove coefficient and the contact angle, the pv value and the radial stiffness decrease clearly. The tribology behaviors of the ball bearing are also investigated, the obtained results indicate that the heavy-load and low-speed lead to large dry contact areas, high friction-temperature, and high local stress peaks, the lubrication state become worse, which may further cause the failures of mass wear, scuffing and micro-pitting.

The Causes of Asymmetric Deformation of Surface Roughness Asperities in Elastohydrodynamic Lubrication Contacts

Abstract This paper provides the main causes of asymmetric or directional deformation of surface roughness based on a transient non-Newtonian thermal elastohydrodynamic lubrication (EHL) model, where the contact materials have different thermal conductivities and elastic moduli. In order to obtain the actual shape of the surface asperity, the surface shapes of contact bodies are evaluated separately. It is clarified that the asymmetric deformation of the asperities appears due to two causes. One depends on the slide-roll ratio (SRR) and the difference in thermal conductivity between contact materials, and the other is caused by the contact pressure between the asperities through the oil film.

Boundary lubrication performance of polymeric and organic friction modifiers in the presence of an anti-wear additive

The effect of oleic acid, oleyl alcohol and oleyl amine based organic friction modifiers (OFM), ester and ethoxylated fatty ester based di-block polymeric friction modifiers (PFM) and Zinc dialkyldithiophosphate (ZDDP) on the boundary lubrication of steel surfaces was studied using a ball-on-disc tribometer equipped with optical interferometry. Frictional performance of oil blends containing the OFM/PFM, with and without ZDDP, was investigated at a slide roll ratio (SRR) of 100% and 130 °C for a rubbing duration of 24 h. Surface topography measurements using 3D surface profilometer and scanning electron microscopy (SEM) were used to quantify the anti-wear performance. Comparison of optical interferograms and friction data showed that formation of a thicker tribofilm does not always translate to effective friction reduction. Our experiments showed that PFMs generally provide better friction reduction than OFMs with or without ZDDP by effectively eliminating the boundary regime. This was attributed to the ability of PFMs to form a tenacious low shear strength film on the metal surface due to its polymeric nature. In contrast, both the friction modifiers had a negative effect on the anti-wear performance of ZDDP. The observed tribological response is attributed to either preferential adsorption of the friction modifier over ZDDP or ZDDP decomposition products. This notion was further corroborated by results obtained from energy-dispersive X-ray (EDX) spectroscopy and X-ray photoelectron spectroscopy (XPS) with depth profile analysis, which showed lower concentration of ZDDP derived products for the formulation containing both the friction modifier and ZDDP.

Lubricant properties of trimethylolpropane trioleate biodegradable oil: High pressure density and viscosity, film thickness, Stribeck curves and influence of nanoadditives

Lubricant properties of trimethylolpropane trioleate synthetic base oil (TMPTO) were experimentally determined under different temperatures, pressures and rolling-sliding conditions. With the aim to obtain the viscosity-pressure coefficient, density and viscosity measurements were performed up to 150 MPa with a falling-body viscometer and a vibrating tube densimeter, respectively. Film thickness and friction properties were determined with a ball-on-disc apparatus from temperatures of 303.15 to 353.15 K, from slide-roll ratios from 5 to 50% at 50 N (applied load). Finally, it has also been evaluated if the use of nanoparticles as additives could involve changes on film thickness and Stribeck curves of TMPTO base oil. For this aim, hexagonal-boron nitride nanoparticles (h-BN) and graphene nanoplatelets (GnPs) were used at mass concentrations of 0.25, 0.5 and 1.0 wt%. The viscosity of TMPTO increases from 15 mPa s (at 10 MPa and 353.15 K) to 525 mPa s (at 150 MPa and 303.15 K). The Stribeck curves for TMPTO are placed between elastohydrodynamic and mixed lubrication. All nanolubricants show very similar Stribeck curves, being the lowest friction coefficient obtained for 0.25 wt% of GnP. It has been found that for most of the experimental conditions the addition of the GnP promotes an increase of the film thickness.

Tribological properties of hexagonal boron nitride nanoparticles or graphene nanoplatelets blended with an ionic liquid as additives of an ester base oil

AbstractAntifriction synergies between an ionic liquid (IL) and nanopowders as additives of an ester base oil were analysed at rolling conditions. For this aim, two nanodispersions based on hybrid combinations of hexagonal boron nitride nanoparticles (h‐BN) or graphene nanoplatelets (GnP) with tri(butyl) ethylphosphonium diethylphosphate ionic liquid as additives in triisotridecyltrimellitate (TTM) base oil and the mixture of TTM and IL were analysed using two tribometer techniques. Film thickness characterisation and tribological tests were performed at rolling conditions [5% slide‐roll‐ratio (SRR)], under 50N and at operating temperatures of 30, 50 and 80°C. Stribeck curves showed that friction coefficients for the prepared lubricants are lower than those found for the TTM, being the best friction behaviour for the hybrid GnP nanolubricant. Furthermore, friction torque loss tests were performed in rolling ball bearings, observing that for the two hybrid nanolubricants lower friction torque values were obtained.