Grease-lubricated Bearings Research Articles

Film Thickness in Grease-Lubricated Deep Groove Ball Bearings—A Master Curve

Film thickness in a grease-lubricated bearing is a critical bearing operation parameter, yet there are no models available to predict it. Our recent articles demonstrated that the relative film thickness ( h g / h ff ) can be predicted using the base oil viscosity ( η ), half contact width (b), and linear speed (u) for any axially loaded deep groove ball bearing. In this article, we extend this study to include radial loads and combined—axial and radial—loads. Radial load application causes an additional replenishment in the unloaded zone, introducing a new length variable z r and surface tension σ . A universal power-law relationship between the relative film thickness in a deep groove ball bearing and the nondimensional number ( η bu ) / ( z r σ ) for all loading and grease lubrication conditions is presented for use in predicting film thickness.

Short-Term Influence of Water Ingress on Wear in Pitch Bearings of Wind Turbines

The pitch bearings of wind turbines are slowly oscillating, grease-lubricated slewing bearings. They facilitate the pitching movements of blades which control aerodynamic loads. These bearings have diameters of several meters, their blade-side sealings can face the environment, bending moment loads can cause radial deformation of the bearing rings, and their highly variable operating temperatures can facilitate condensation of water inside them. All of this makes water ingress into the lubricant possible. There is limited public knowledge with regards to the maximum water content for safe operation in this application. This work presents the results of a series of scaled wind turbine time series tests with both ‘dry’ (no water contamination) and ‘wet’ (10 mass % demineralized water added) greases. A set of four commercially available greases were tested. The time series were scaled from wind turbine operation and represented a 13.7 h worst-case scenario of operation with small oscillation amplitudes and no longer lubrication runs in between. Three of the greases showed reduced friction and no or limited raceway damage in the wet condition, whereas one showed increased friction and raceway damages.

Study on the Effect of Thermal Characteristics of Grease-Lubricated High-Speed Silicon Nitride Full Ceramic Ball Bearings in Motorized Spindles

Grease lubrication is cost-effective and low-maintenance for motorized spindles, but standard steel bearings can fail at high speeds. This study focuses on high-speed full ceramic ball bearings lubricated with grease. The coefficient of friction torque in the empirical formula is corrected by establishing the heat generation model of full ceramic ball bearing and combining it with experiments. A simulation model of grease flow is established to study the influence of grease filling amount on grease distribution. The simulation model of the temperature field of a full ceramic ball bearing is established to analyze the influence of rotating speed on bearing heat generation, and experiments verify the calculation results of the theoretical model. The results show that an optimal grease filling amount of 15~25% ensures even distribution without accumulation. Additionally, when the amount of grease is constant, the outer ring temperature increases with higher rotating speeds. The test results show that when the grease filling is 0.9~1.2 g, it accounts for about 9~12% of the volume of the bearing cavity, and the temperature of the outer ring is the lowest. At a rotation speed of 24,000 rpm, the outer ring temperature of the grease-lubricated bearing is 50.1 °C, indicating a reasonable range for use in motorized spindles. It provides a theoretical basis for the optimization design of macro-structural parameters of full ceramic ball bearings in the future, which can minimize heat generation and maximize bearing capacity.

Visualization techniques of grease fluidity

Energy-saving technology has become increasingly significant as one of the carbon-neutral options for suppressing recent global warming. Grease-lubricated bearings have been used in various automotive and industrial machinery, requiring low torque and long service life for energy-saving performance, which is greatly influenced by grease fluidity. A numerical approach for understanding grease fluidity is very complex since grease is a non-Newtonian fluid with thixotropic properties. Visualization technique is one of the helpful methods to understand the complex grease fluidity and apply it to practical use. This paper describes state-of-the-art visualization techniques, such as fluorescence method, particle imaging velocimetry, infrared spectroscopy, X-rays, and neutron beams.

Vibrations and film thickness in grease-lubricated deep groove ball bearings

Understanding the relevance and significance of the various replenishment mechanisms in grease-lubricated bearings is very important in choosing the appropriate lubrication strategy. This study shows that the lubricant film thickness in the ball-ring contacts of a deep groove ball bearing may increase by 10%–25% due to vibrations. The possible mechanisms behind this film thickness increase are discussed. Of the proposed contact replenishment mechanisms, the micro-lateral motion of the contact edge across the tracks caused by in-operation vibrations appears crucial. Results show that in the starved contacts, vibration plays a vital role in determining the film thickness.

Effect of Grease Viscosity on Channeling Properties of Ball Bearings

Grease-lubricated rolling bearings transition from the churning phase to the channeling phase. This transition property affects grease life and torque properties. Therefore, the relationship between grease yield stress and grease degradation during operation, which affects this transition, has been investigated. However, there have been few studies on grease flow that affects the transition. In this study, the mechanism of grease reduction on the races was investigated for small bearings operated at low speeds, where thermal degradation and softening of the grease are less likely to occur. It was inferred that the grease transfer to the cage affects the channeling transition and that the amount of transfer varies depending on the initial grease viscosity. These findings can be applied to grease composition and cage design and are useful in providing bearings with excellent low-torque characteristics, such as in industrial motor applications.

Measuring Film Thickness in Starved Grease-Lubricated Ball Bearings: An Improved Electrical Capacitance Method

Presented in this article is a new method to measure the film thickness in a rolling bearing running under starved conditions, such as in grease-lubricated bearings, using the electrical capacitance method. The method was used to study the impact of speed on film thickness. Results show that fully flooded conditions prevail at lower speeds where the film thickness increases with increasing speed. At higher speeds, where grease-lubricated bearings run under starved lubrication conditions, this increase is less pronounced. At even higher speeds, the film thickness becomes almost independent of the speed.

Influence of Rheological Properties of Lithium Greases on Operating Behavior in Oscillating Rolling Bearings at a Small Swivel Angle

In this study, the behavior of greases during oscillating bearing operation with a small oscillation angle and high frequency was investigated. This mode of operation entails demands on the lubrication system that differ significantly from those for continuously rotating bearings. In order to determine the variables influencing the suitability of a lubricating grease for small angle oscillating operation, the grease samples were examined with particular regard to their rheological properties. The focus of this investigation was to find a relationship between the rheological parameters and the real behavior in the bearing. Therefore, rheological and physical parameters, which influence the long-term structural changes and lubrication conditions, were identified. For this purpose, the viscosity was measured over a wide shear-rate range. The storage and loss modulus, the work of deformation, and the adhesion force jump are also determined. Afterward, rotational transient flow measurements were performed. These allowed us to analyze the development of the shear stress over time, at a constant shear rate, and to examine the internal friction behavior by evaluating the energy density. Subsequently, grease-lubricated four-point bearings were used in component tests, while the frictional torque was measured. These bearings operated in oscillating motion. Moreover, the yield point of mechanically aged greases was measured and compared with that of fresh greases to examine the influence of the oscillating operation on the lubricant condition. Finally, correlations between grease composition, rheological measurements, and component tests were investigated. Thereby, parameters influencing the frictional behavior of greases in rolling bearings during oscillating operation at small swivel angles were identified.

Greases for electric vehicle motors: thickener effect and energy saving potential

Electric vehicle motors in e-drivetrain are equipped with grease-lubricated bearings operating at both low and high speeds with frequent speed changes. The grease-bearing system must secure a long lifespan and low frictional torque to improve efficiency and sustainability. The present paper focuses on the influence of two types of thickener, lithium complex and polypropylene, on the grease lubrication performance under conditions typical for e-motors. The comparison of both thickeners is performed in terms of friction torque and energy consumption in eight long-duration experiments (337 hr). The results show that the polypropylene thickener provides 21.5% lower energy consumption compared to the lithium complex. Changes in grease rheology and degradation in the tests are analysed and correlated with the grease lubrication performance.

An Evaporation Model for Base Oil from Grease-Lubricated Rolling Bearings including Breathing

The film thickness decay of grease-lubricated bearings is determined by a loss of lubricant from the bearing. One of these loss mechanisms is base oil evaporation. In this article, a model is presented for evaporation where the volatiles leave the bearing by breathing. The model is applied to a typical drive cycle where the temperature is varied in time. The impact of the volatility of base oils and quality of sealing on the evaporation losses can be quantified with this model.

Study on the Thermal Performance and Temperature Distribution of Ball Bearings in the Traction Motor of a High-Speed EMU

The transient thermal performance of rolling bearings affects the mechanical performance and system safety of traction motors. Most of the traditional empirical formulas used in temperature analysis have been simplified and cannot be completely applied to the calculation of heat generation and convection heat transfer coefficients. Based on the comparative analysis of finite element transient temperature and experimental data, this paper proposes a correction method of mathematical model and derives an accurate calculation formula for the heat generation and lubricant convection heat transfer coefficient of ball bearings applicable for the non-driving end in the traction motor of a high-speed EMU (Electric Multiple Unit). The accuracy of the results has been verified by durability experiment data. In addition, with changes in speed, radial load and other factors taken into account, we have analyzed the influence of these time-varying factors on ball bearing temperature, as well as the temperature distribution law of each component in a grease-lubricated bearing, in a bid to lay a foundation for follow-up research on the heat transfer laws of traction motors and rolling bearings.

On the Modeling of Wear in Grease-Lubricated Spherical Roller Bearings

This work presents an approach for modeling grease-lubricated bearing wear by coupling models for boundary-lubricated wear and bearing lubrication state. The lubrication model takes a mass balance approach, using previously developed models for pressure-driven side flow loss and centrifugally driven bleed supply mechanisms. These are combined with a contact model to determine the level of surface interaction and the impact that this interaction has on the lubricated wear and friction coefficients. The developed model was used in numerical simulation of the wear over time during operational conditions representative of 2 Mkm travel distance of a passenger train and the results were compared with experimental investigation of wear particles collected from field test lubricating grease. The results suggest nonlinear behavior with an increasing wear rate over time. Comparison between model results and wear volume estimated from collected wear particles is inconclusive and requires further investigation. A sensitivity analysis of the wear rate found that early in life the model is most sensitive to parameters that directly influence contact properties (load, boundary-lubricated friction coefficient, and wear coefficient). At later stages this shifts as parameters that affect the lubricant loss and supply mechanisms (temperature, base oil viscosity, lubricant fill) become equally or more important.

Replenishment of the EHL contacts in a grease lubricated ball bearing

The film thickness in grease lubricated bearings is usually calculated by assuming fully flooded conditions, using the base oil properties. This paper shows that, for sealed-for-life bearings, starvation already occurs at relatively low speeds, where the film thickness could be described by the product of speed, viscosity and Hertzian semi-contact width. It is shown that replenishment is not significant on the tracks in-between the balls but that it is a local phenomenon. The relation between film thickness and the product of speed, viscosity and contact size is different from what is found in single contacts. The film thickness decreases as a function of speed and viscosity, but the effect is only small leading to an almost constant film thickness with increasing product of viscosity and speed.

Studies of Friction in Grease-Lubricated Rolling Bearings Using Ball-on-Disc and Full Bearing Tests

This article evaluates the frictional performance of different bearing grease formulations in full rolling bearings and a ball-on-disc rig and subsequently assesses whether the ball-on-disc test results can be used to predict the grease performance in actual bearings. A selection of custom-made greases with systematically varied formulations as well as their base oils were tested. Bearing torque was measured in two different cylindrical roller thrust bearings and a thrust ball bearing. The same lubricants were tested with ball-on-disc tribometers, a mini traction machine (MTM) to measure friction and an optical elastohydrodynamic (EHD) rig to measure film thickness. Both lithium complex and diurea greases were observed to produce lower friction than their base oils within the low speed, low nominal lambda ratio region, whereas the greases and oils had the same friction at high nominal lambda ratio values. These relative trends were the same in full bearing and single-contact MTM tests. The reduction in friction was seen to be related to the level of film thickness enhancement provided by greases at lower speeds, which leads to an increase in the effective lambda ratio and hence reduced friction. By extracting the sliding torque component from the overall measured bearing torque, a plot of the friction coefficient against the effective lambda ratio was produced encompassing all bearing and single-contact tests and all lubricants and test conditions. This plot was seen to follow a general shape of a master Stribeck curve, indicating that the numerical values of the friction coefficient from ball-on-disc and full bearing tests overlap and can be related to each other using this approach over the range of conditions employed here. Thus, single-contact ball-on-disc tests can provide a fast and economical way of establishing the frictional performance of bearing greases in full bearings in terms of both relative performance rankings and quantitative values of bearing frictional power losses.

Film thickness in a grease lubricated ball bearing

Abstract The film thickness for grease lubricated bearings is normally calculated using the base oil viscosity, where it is assumed that the bearing is running under fully flooded conditions. It is well known that this is not accurate since grease lubricated bearings are usually running under starved lubrication conditions leading to thinner films. Single contact measurements have shown that, in the case of starvation, the film thickness decreases with increasing speed. It is shown in this paper that this effect seems to be very small though. The film thickness measurements, for three types of thickener material and base oil, show that the film thickness is almost independent of speed. To quantify this starvation effect, the film thickness can be expressed as the ratio between real film thickness and calculated film thickness using the base oil viscosity, hg/hff. The measurements in this paper show that hg/hff > 1 for very low speeds but decreases with speed asymptomatically reaching a constant value. It is shown here that hg/hff depends on speed, load and temperature and on the grease properties. It is not only the base oil viscosity that determines the film thickness. It is also governed by other grease properties that are today not known.

Load distribution in a roller-type rotor blade bearing

Pitch bearings of wind turbines are large, grease-lubricated rolling bearings that connect the rotor blades with the rotor hub. Rolling bearings are the standard bearing type for this application. Most blade bearings are four-point bearings with one or two rows. Three-row roller bearings with two axial rows and one radial row have higher costs, but are an increasingly used alternative. Both rotor blade and rotor hub have a varying stiffness along the circumference of the bearing rings. This results in rotationally non-symmetric load sharing (load distributions) of the bearing rollers. The load distribution depends on the pitch angle, the load magnitude and the load angle. In this paper, we evaluate the load sharing of such a three-row bearing for a reference wind turbine of the 3 MW-class, taking account of the stiffness of the interface parts hub and rotor blade. A set of finite-element simulations with varying loads, load angles and pitch angles has been executed to determine the influence of the named parameters on the loads of the individual rollers. Curve fits of these discrete load points allow the determination of roller loads for any given parameter combination. One application of the results is the determination of the overall bearing load which is a key input for fatigue lifetime calculations.

Wear Prediction of Earth-Moving Machinery Joint Bearing via Correlation between Wear Coefficient and Film Parameter: Experimental Study

ABSTRACTA wear life prediction of a grease-lubricated bearing in earth moving machinery has to be made based on actual wear data for its accuracy, but much effort should be devoted to obtain the wear data from the actual bearing. Currently, manufacturers provide a very conservative maintenance guide to schedule the replacement of the bearings to prevent the mechanical failure due to wear, causing economic losses, and the market requires finding optimal maintenance cycles to reduce maintenance costs. In this study, an economical wear prediction methodology for the grease-lubricated bearing based on typical pin-on-disk (POD) tests is proposed. POD tests were performed under boundary and mixed lubrication regimes considering the actual operating conditions of the bearings. It was found that there is a linear relation between the lubrication film parameter and the wear coefficient in log-log scale. The amount of wear of actual bearings was estimated by the wear coefficient from the linear regression analysis based on the POD test. The wear tests were further performed with the actual bearing under two loading cases and then the lubrication film parameter and wear coefficient were calculated, respectively. The estimated amount of wear shows good agreement with the measurement of wear depth. This leads us to conclude that it is possible to economically predict the wear amount in an actual bearing from POD wear test results by analysis of the correlation between the wear coefficient and the lubrication film parameter.

Mapping of Grease Migration in High-Speed Bearings Using a Technique Based on Fluorescence Spectroscopy

ABSTRACTA novel approach that allowed the identification of (in) active grease reservoirs in high-speed superprecision angular contact ball bearings for spindle applications is described. To improve the design of these grease-lubricated bearings it is necessary to measure grease migration and determine the lubrication role of internal grease reservoirs. Hence, a technique based on fluorescence spectroscopy is successfully applied that allowed the identification of grease migration in such a bearing. Therefore, this technique provides a new way to further improve grease migration in critical bearing locations with the ultimate goal of attaining even higher speeds and longer life.

Study of grease lubricated journal bearing with partial surface texture

Purpose – This paper aims to study the impact of surface texture on grease-lubricated journal bearing, load-carrying capacity, friction and friction coefficient. Design/methodology/approach – The paper opted for a basic application research using the commercial CFD software FLUENT 14.0. Findings – The load-carrying capacity can be enhanced if the surface texture is introduced from a suitable position. The textural shape has a more significant effect on the friction and load-carrying capacity than the friction coefficient. The rheological parameters of grease affect the load-carrying capacity in the same way, i.e. the load increases with the increase in the rheological parameters. With the decrease in the value of cavitation pressure, the cavitation area reduces but the dynamic pressure area gets enlarged. Research limitations/implications – Because of the chosen research method, the research results may lack the analysis of experiments. Practical implications – The research results can be applied to the domain in the high-speed spindle and lubricated multibody system. Originality/value – The investigation on the performances of grease-lubricated journal bearing with surface texture would be significant.

可視化技術を用いたグリース潤滑における転がり軸受トルク発生要因の研究

可視化技術を用いたグリース潤滑における転がり軸受トルク発生要因の研究