Grease Film Research Articles

Effect of Grease Composition on Impact-Sliding Wear

Impact-sliding experiments were performed by using four self-made lithium-based greases, namely Yangtze Grease 1, Yangtze Grease 2, Yangtze Grease 3, and Yangtze Grease 4. The influence of base oil viscosity, thickener content, and morphology of thickener fiber clusters on the lubricating state were visually explored, combined with field-emission microscopy and two-light interference technology. The grease film distribution at the middle section was measured using Dichromatic Interference Intensity Modulation (DIIM) software. All experiments were executed in a completely flooded environment. The results show that among the components of grease, the base oil’s viscosity has the greatest impact on the anti-wear performance of the grease. As the viscosity of the base oil increases, the grease exhibits better anti-wear performance. The grease film thickness under the condition of high-viscosity base oil is about 10 times higher than that under the condition of low-viscosity base oil. Secondly, the content of thickener in the grease needs to be controlled within a reasonable range. The experiments indicate that the effect of thickener content on the grease’s film-forming properties becomes more pronounced at higher speeds. From the experiment using YG 4, it can be seen that a higher thickener content under high-speed conditions increases the thickness of the lubricating grease film by about 10 times. The dimensions of the thickener fibers and the density of their entanglement structure significantly influence the rheological properties and load-bearing capacity of the grease. Larger fiber sizes and higher entanglement densities result in reduced grease fluidity and recovery but enhance its load-bearing capabilities. In order to obtain the best anti-wear performance during impact-sliding motion, the size of the thickener fiber and the density of the entanglement structure need to be controlled within an appropriate range.

Research on Analysis and Suppression Methods of the Bearing Current for Electric Vehicle Motor Driven by SiC Inverter

The silicon carbide (SiC) inverter brings great advantages to the motor drive systems of new energy vehicles; however, severe challenges to the bearings also happen. The high dc bus voltage and switching frequency of SiC inverter can increase the discharge frequency and energy when the bearing grease film collapses. As a result, the bearing suffers severe electric corrosion, and the service life of the motor drive system can be shortened. In this paper, the characteristics of common-mode voltage and bearing voltage are analyzed, firstly under space vector pulse width modulation (SVPWM). After that, the common-mode equivalent circuit model of the motor drive system is established. The frequency characteristics of bearing voltage are revealed, and the safe working area is determined. Then, the frequency characteristics of bearing voltage and current are verified based on IGBT and SiC inverters in experiments. After that, by designing a common-mode filter, the bearing voltage and current are significantly attenuated. Furthermore, the active zero state PWM (AZSPWM) is adopted to reduce the common-mode voltage from the inverter. At the same time, combined with the common-mode filter, the bearing voltage and current are further reduced. The experimental results show that the switching frequency has a decisive effect on the amplitude of bearing voltage and current. The bearing voltage can be attenuated to around half of the reference bearing voltage by using the common-mode filter and AZSPWM strategy, respectively. The combination of the common-mode filter and AZSPWM strategy can reduce the bearing voltage to around one-fourth of the reference bearing voltage, which can effectively reduce the breakdown time and discharge energy of the grease oil film.

Film Thickness Decay and Wear Behavior of Grease-Lubricated Point Contact under Cyclic Variable Loads

For wind turbine applications, there is a cyclic load-varying process between rolling elements and raceways in pitch bearings. This kind of motion can also lead to radial fretting. However, this is seldom addressed under grease-lubricated conditions in the literature. In this study, grease-lubricated point contact problems have been investigated experimentally under cyclic load-varying conditions. The findings revealed that as the load-varying range diminishes, the variation in grease film distribution becomes more subtle and the rate of discharge of thickener fiber clusters in the stick zone decelerates. This is due to the fact that the rate of change in the Hertz contact radius is reduced and the migration of grease is weakened during the unloading process. Due to the large apparent viscosity of grease with a high soap content, entrapped grease is not easily discharged during loading, and the thickness of the film in the stick zone progressively increases as the soap content of the grease is augmented. This also causes the variable load zone to wear out more easily. As the grease is subjected to repeated loading and unloading, there is a gradual reduction in film thickness, and larger thickener fiber clusters tear, resulting in a flattened form and shear thinning. Grease containing sulphur–phosphorus additives demonstrates a superior effect on reducing fretting wear within the large variable load range but generally proves effective for smaller load-varying ranges. This study may offer insights into the degradation of grease under variable load motion and methods to prevent radial fretting wear.

Energy Dissipation in Tribological Stressed Greases

Lubricating greases that are subject to a continuous friction process are in a non-equilibrium state. In processes far from equilibrium, there is a possibility that dissipative structures will form. In this work, the conjecture is pursued that this is also possible in loaded grease films. On the one hand, the shear process is considered in interaction with structural degradation, and on the other hand, the behavior of energy dissipation mechanisms is investigated. In the two models presented, it is shown that there are conditions under which it is possible to trigger self-organization processes. The next step must be the development of suitable experiments.

Dynamic characteristics of the rack and pinion lift mechanism of the combined command cabin

This paper presents a novel approach to accurately simulate the dynamic characteristics of a rack and pinion lifting mechanism while accounting for the coupling effect of meshing tooth pair stiffness and grease characteristics in real-world environments. A structure-grease coupling meshing stiffness model is established based on the deformation coordination conditions of the tooth pair meshing. The model incorporates the effect of the grease stiffness term on the time-varying meshing stiffness of the rack and pinion. By analyzing the dynamic characteristics of the combined command cabin lifting mechanism using this model, we find that the total meshing stiffness of the gear teeth is lower when the transient thermoelastic flow effect of the grease is considered. Furthermore, the total stiffness value decreases as the normal meshing force at the meshing point decreases. This research reveals that the worst lubrication conditions are typically found on the gear wheel teeth near the base circle, where the grease film temperature rise is highest, the grease film pressure is highest, and the grease film thickness is thinnest.

Influence of Free Water Droplets on Film Formation Under Point Contact Grease Lubrication

Abstract The influence of the grease-water two-phase emulsion on the bearing tracks on the film formation has been studied by carrying out optical interferometry experiments on a ball disk test rig to observe the dynamic film-forming behavior of lubricating grease under steady-state and micro-oscillation conditions. It is found that free water causes a brief rise in the grease film under steady-state conditions, specifically due to the grease-water phase connecting the oil reservoirs on both sides. The experiment determines that the state of the phase before entering the elastohydrodynamic lubrication (EHL) contact is water-in-oil. For the reciprocating motion and oscillation, the effects of the amount of free water and the length of the motion stroke are discussed in detail. It is found that for reciprocating movements with long stroke lengths, the effect of free water is summarized to enhance the fluidity of the fibrous mass of the grease thickener in the first few cycles of the movement. For shorter stroke lengths, the effect of free water is a scouring effect on the contact zone grease after 100 cycles. This paper provides new insights into the effects that cause water pollution to the point of contact lubrication during the transformation from reciprocating motion to fretting.

Dynamic grease lubrication and friction behavior of ball screw mechanism in high-frequency reciprocating motion

Three velocities coexist in ball-groove interface, including spin, rolling and sliding velocity. These velocities change dramatically under high-frequency reciprocating motion, making the analysis of dynamic grease lubrication and friction of the rubbing surfaces extremely complicated. In present study, a transient mixed lubrication model for grease-lubricated ball screw mechanism (BSM) is developed considering the pseudoplastic flow of grease, complicated motion and dynamic load. The dynamic film thickness variation and friction characteristics are revealed in a reciprocating movement. The comparison of lubrication characteristics between ball-screw and ball-nut interface is discussed. The variation tendency of the film thickness is similar to that of the screw speed, while the evolution of the friction torque at ball-screw interface is dependent on the ball spin speed. Compared to the ball-nut interface, the friction coefficient is higher on the ball-screw interface. Besides, when the screw speed frequency and amplitude increase, the grease film thickness rises and friction coefficient decline during the stop time period. This study can provide a reference to evaluate the grease film and friction in ball screw mechanism design and optimization.

Occurrence of Grease Lubricated Impact-Sliding Composite Wear

In industrial chain drives, the sleeve slides on the pin and impact loading occurs due to the polygon effect, while the collision between the ball and cage usually produces an impact-sliding motion in the rolling element bearings. Aiming at addressing the occurrence of surface damage caused by the impact-sliding motion, a ball–disk test rig employing optical interference technology was designed and built to realize load variation. Two kinds of commercial grease types, Klüber Centoplex 3 and Centoplex 2EP, were used in the experiments when the glass disk slides at a constant speed while the steel ball collides into them. The sliding and impact motions were controlled by PLC programming. After the experiments, the mid-section grease film distributions were measured using DIIM software. The results show that surface damage can rapidly occur even in the first working cycle, and that the phenomenon is affected by the sliding speed, maximum load, and grease consistency. When the sliding speed is low, multiple contacts of asperity peaks occur in the interior contact region and develop into adhesive wear. When the sliding speed increases, surface wear starts to occur at the side-lobe position of the elastohydrodynamic lubrication (EHL) horseshoe shape and extends with time accompanied by obvious surface scratches. The wear mechanism investigated provides valuable visible information for the further exploration of impact-sliding composite wear. It is suggested that great attention should be paid to impact-sliding wear occurring with grease lubrication since such working conditions are very common in industrial applications.

Numerical study on mixed thermal-elastohydrodynamic lubrication of grease lubricating lubricated spiral bevel gear for heavy-duty unmanned aerial vehicle

Aiming to solve the problem of the gear lubrication of unmanned aerial vehicles (UVAs), a grease mixed Thermal-elastohydrodynamic lubrication (TEHL) model is established, and the lubrication state of the gearbox of oil-powered heavy-duty UAVs is numerically analyzed. In this study, grease is regarded as a non-Newtonian fluid whose properties change nonlinearly with respect to pressure, temperature, and other conditions. The complex meshing state of a spiral bevel gear tooth surface is analyzed using the loaded tooth contact analysis (LTCA) method, and three points on the tooth surface are selected as samples to describe the lubrication state of the entire tooth surface. The thickness of grease film and the temperature rise of the tooth surface are studied in detail, considering the natural roughness of the tooth surface and influence of different greases. The results show that the established model agrees with results from the literature. The dangerous point of tooth surface lubrication is at the meshing midpoint. Under cruise conditions, the lubricating grease viscosity significantly influences the temperature rise of the tooth surface.

Study on Factors Influencing Film Formation of Grease and Calculation Model for Grease Film Thickness

The grease film thickness was measured in fully flooded elastohydrodynamic lubrication, and the influence of rolling speed, load, consistency, base oil type and thickener type on grease film thickness was analyzed. A new calculation model for grease film thickness was established. The results show that the grease film thickness increases with the increasing rolling speed, and then levels off with the amount of thickener in the contact region reaching an equilibrium. The degree of grease film enhancement comparing to its base oil will depend on thickener type and consistency. The larger the atmospheric viscosity and pressure-viscosity coefficient of the base oil, the higher the film thickness of the greases with the same thickener. The grease film thicknesses with the same base oil and different thickeners are determined by the size of thickener particles at the same consistency or concentration. The larger the consistence of the grease, the larger the effective viscosity of the grease at the contact and the thicker the grease film thickness whose base oil has the same type and viscosity along with the same type of thickener. The calculated values by the new model are in good agreement with the measured values.

Observation of Grease Film Behavior in Sliding-Rolling Concentrated Contacts

Abstract In sliding-rolling concentrated contacts, the grease film is subjected to shear action. In this study, grease film behaviors have been observed under different sliding-rolling ratios (SRRs) by measuring film thickness and friction coefficient simultaneously. The evolution patterns of the grease films with disc revolutions are affected by SRR, entrainment speed, and grease consistency. The severe starvation due to film decay at low shear and the film recovery at high shear has been recognized. The grease with low consistency is in a fully flooded state due to improvement in lubricant replenishment. In addition, the corresponding friction coefficient tends to increase or be constant in terms of contact states. The mechanisms of the grease behavior related to shear actions and grease properties are analyzed.

Plasto‐elastohydrodynamic lubrication performance of greased ellipsoid contact

AbstractThe plasto‐elastohydrodynamic lubrication (PEHL) model of the greased ellipsoid contact is established and solved numerically. In doing so, the increment theory of the plastic surface is adopted to solve the residual deformation, and the fast Fourier transform (FFT) method is employed to accelerate the deformation and stress calculations. Meanwhile, the effeteness of the PEHL model is verified through the residual stress experiment and finite element method. It is found that the plastic behaviour results in a reduction in the maximum grease film pressure. Under heavy load conditions, the grease with large rheology index, large tangential modulus or high entrainment speed of the lubricant is helpful to reducing the maximum residual deformation and maximum residual stress of the greased ellipsoid contact.

A mixed EHL model of grease lubrication considering surface roughness and the study of friction behavior

Grease lubrication is widely used in rolling element bearing because of its special rheological characteristics. Available studies are mainly based on experiments, while the relevant numerical studies of grease are quite limited. The purpose of this article is to establish a mixed elastohydrodynamic lubrication (EHL) model in point contact based on the rheological model of grease, which can calculate film thickness distribution applied to smooth and roughness surface. The obtained result is compared with that of it operated only in base oil. Besides, the impacts of load, surface roughness and rheological parameters on the grease film thickness have been investigated. In addition, the Stribeck curves of four greases covering different base oil viscosity and thickener types were measured on a ball-on-disc device, which can ensure that the contact area was in full film lubrication. From the study, it is concluded that the obtained grease friction behavior agrees well with the variation of grease film thickness calculated by the present mixed EHL model.

Friction Forces in a Linear-Guideway Type Recirculating Ball Bearing Under Grease Lubrication

Abstract This paper deals with friction forces in a linear-guideway type recirculating ball bearing (linear bearing) under grease lubrication. During the experiments, the friction force, temperature, and electric contact voltage of a grease lubricated linear bearing (test bearing) without seals were measured. Experimental results showed that the measured friction forces of the test bearing were fluctuated with the ball passage period. The measured time-average friction force FAVG (measured FAVG) was nearly constant when the grease filling rate x (=grease filling volume/internal space of the bearing) ≥0.13, while the measured FAVG decreased as x decreased when x < 0.13. In addition, the measured temperatures were almost constant, and the measured contact voltages indicated that the contacts of the balls and raceways were electrically insulated by the grease film. Next, the expressions of friction forces due to differential slip (FD), elastic hysteresis loss (FE), and rolling traction (FRT) were shown. The calculated FD + FE + FRT for the test bearing was almost equal to the measured FAVG around the grease filling rate of x = 0, while in cases where x > 0, the measured FAVG was greater than the calculated FD + FE + FRT. This means that when x > 0, an agitating resistance (FA) from the grease might cause the measured FAVG to be greater than the calculated FD + FE + FRT. Finally, an expression for the friction force of a linear bearing, FAVG = FD + FE + FRT + FA (which can estimate the measured ones) is proposed.

Grease film thickness measurement in rolling bearing contacts

Rolling bearings are the second most used machine components. They work in what it is called elastohydrodynamic lubrication regime. The geometry of rolling element bearings makes the direct measurement of the lubricant film thickness a challenging task. Optical interferometry is widely used in laboratory conditions for studying elastohydrodynamic lubrication however it cannot be used directly in rolling element bearings thus the only suitable methods are electrical techniques. Of these, film thickness measurement based on electrical capacitance of the contacts has been used in the past by a number of authors. One of the limitations of the capacitance method, when used in rolling bearings, is that it cannot distinguish between the contacts of every rolling element and raceway on one hand and on the other between the inner and outer ring contacts. In the present study the authors used an original test rig which can measure the film thickness for only one ball and separately for the inner and outer rings of a radial ball bearing. This paper thus shows for the first-time results of the lubricant film thickness, at the inner and outer raceways, in grease lubricated rolling bearings.

Grease film evolution in rolling elastohydrodynamic lubrication contacts

Although most rolling element bearings are grease lubricated, the underlying mechanisms of grease lubrication has not been fully explored. This study investigates grease film evolution with glass disc revolutions in rolling elastohydrodynamic lubrication (EHL) contacts. The evolution patterns of the grease films were highly related to the speed ranges and grease structures. The transference of thickener lumps, film thickness decay induced by starvation, and residual layer were recognized. The formation of an equilibrium film determined by the balance of lubricant loss and replenishment was analyzed. The primary mechanisms that dominate grease film formation in different lubricated contacts were clarified.

Numerical Analysis of Grease Film Thickness and Thickener Concentration in Elastohydrodynamic Lubrication of Point Contacts

This article presents a numerical analysis of elastohydrodynamic lubrication (EHL) with grease in point contacts to predict the grease film thickness at low speeds, which can increase with decreasing speed and exceed the base oil film thickness by orders of magnitude. The grease film thickness is obtained by solving the EHL problem, taking grease flow as a single-phase flow with a non-Newtonian viscosity. The continuity equation for thickener is derived, taking grease flow as a two-phase flow of base oil and thickener and the network structure of the thickener as both a porous medium and a non-Newtonian fluid. The flow rate of the thickener network is obtained as the grease flow rate subtracted by the Darcy flow rate of the base oil. The thickener concentration is obtained by solving the continuity equation for the thickener. Numerical results show that the increase in film thickness at low speeds is due to an increase in the equivalent viscosity in the inlet of the contact, which is due to a decrease in the shear rate and an increase in the thickener concentration. At low speeds, base oil is more easily squeezed out of the inlet of the contact than the thickener network, which leads to an increase in the thickener concentration, because the shear stress due to the Newtonian flow of base oil decreases with decreasing the shear rate, whereas the yield stress of the thickener network is independent of the shear rate. A reasonable agreement between the numerical and experimental results validates the present model.

Numerical Analysis of Grease Film Characteristics in Tapered Roller Bearing Subject to Shaft Deflection

The shaft deflection is one of critical factors that deteriorate the grease lubrication state inside the tapered roller bearings (TRBs). So, in this paper, on the premise of the TRB subjected to the combined loads and the shaft is deflected, the grease lubrication model at the TRB contacts was constructed, in which the interaction loads, linear and angular displacements of the bearing parts were involved. Then the grease film characteristics were numerically analyzed from the perspective of the whole bearing to clarify the negative effects of the shaft deflection on grease film characterizes. The results show that the effect of the shaft deflection on the load distribution in the absence of the radial load is greater than that in the presence of the radial load. The angular misalignment of the roller is mainly affected by the deflection. The deflection results in an irregular film shape and pressure profile at the TRB contacts, and induces a significant pressure spike and necking feature at the roller-end.

Design and development of a wedge shaped magnetorheological clutch

This paper presents a novel drum magnetorheological clutch design having a wedge shaped boundary. The converging film formed due to the direction of motion of the magnetorheological clutch and the inclination of the wedge planes helps to produce the pressure generating mechanism in the magnetorheological grease film. The resistance force developed due to this mechanism in the proposed case exceeds that of the conventional drum clutch. The proposed wedge shaped drum magnetorheological clutch consists of bimetallic discs made up of aluminium and mild steel. Mild steel disc has a certain number of inclined sliders shaped like a wedge at its boundary, which is immersed in the magnetorheological grease. The yield stress of the magnetorheological grease varies as a function of the magnetic field created by electric current passing through the electromagnet. Bingham model has been employed in the present study to analyse theoretically the torque generated by wedge-shaped drum magnetorheological clutch. The proposed magnetorheological drum clutch with wedge shaped boundary and conventional drum clutch have been designed and fabricated with similar material parameters and magnetic circuits. The experiment has been performed for the different current values (0 A, 0.25 A and 0.52 A). Torque results are plotted and compared for both cases. Experimental results suggest that the proposed wedge shaped drum magnetorheological clutch produces more torque than the conventional drum magnetorheological clutch, and thus it has better performance.

Numerical and experimental studies on thin-walled aluminum alloy tube hydroforming using differential lubrication method

In this paper, it was analyzed that the friction forces that affected the material flow were influenced by the friction coefficient and the load path of internal pressure and feeding in T-shaped tube hydroforming process. Therefore, a novel differential lubrication method was proposed to adjust the material flow through changing the friction coefficient in the asymmetric zone in the T-shaped tube besides the loading path design method. The differential lubrication zones in T-shaped tube were divided, and a method called intermediate semiring differential lubrication was designed. The effects of differential lubrication and traditional uniform lubrication methods on the wrinkle, height of branch tube and wall thickness distribution of the T-tube were investigated under the same loading path of internal pressure and axial feeding. Meanwhile, the differential lubrication methods were also simulated under the different loading paths of the internal pressure and axial feeding. The differential lubrication experiments of T-shaped tubes hydroforming were carried out with fluorosilicone grease and PEFT film as lubrication medium. The simulation and experimental results showed that the differential lubrication method more effectively avoided wrinkles in the back zone of the main tube, increased the height of branch tube and weakened the thickening than the traditional uniform lubrication method. And it was a promising way to improve the formability of T-shaped tube hydroforming and reduced the over-reliance on the loading path.