Behavior Of Greases Research Articles

Research on Mechanical Aging Behavior of Grease in Rolling Bearing

Grease is extensively used in rolling bearings due to its inherent sealing properties. However, mechanical degradation typically occurs, resulting in a significantly shorter lifespan for the grease compared to the bearing. Investigating aging mechanisms is essential. This study utilized rolling bearings to obtain mechanically aged greases. The aged samples were then subjected to do rheological test, FTIR test and friction test to examine the effects of aging time, radial load, and rotational speed. An innovative disk-ball-disk device was developed to study the friction characteristics of aged grease in rolling bearings. Additionally, a grease lubrication and friction model were proposed to further evaluate the lubrication performance of aged grease, using measured rheological parameters and the kinematic velocities of the disk-ball-disk device as inputs. The calculated friction coefficients matched well with disk-ball-disk experimental results. FTIR analysis revealed that the non-soap thickener's structure remained stable, but additives depleted at high speeds. The structure of lithium soap thickener decomposed severely, leading to oil bleeding and a shortened replenishment duration. Over aging time, both greases had lower friction and viscosity due to gradual thickener breakdown, releasing bled oil and extending replenishment. Radial load minimally affected the properties of aged grease. This study may provide valuable insights into the aging mechanisms of grease and the enhancement of rolling bearing lubrication.

Lubrication mechanism of C@Ag core–shell materials as grease additive

The preparation of a low shearing shell on the surface of carbon spheres (CS) is an attractive strategy to improve the lubrication properties of CS. In this study, core–shell structural C@Ag particles were prepared using CS as the hard core and Ag nanoparticles as the soft shell. Then, the additives were mixed with Polyalphaolefin 20 to prepare the C@Ag core–shell-enhanced lithium complex grease (C@Ag-LCG). The physicochemical properties, corrosion resistance, and tribological behaviors of LCGs were investigated. C@Ag-LCG exhibited the best high-temperature resistance and colloidal stability compared with the CS-enhanced lithium complex grease (LCG) and pure LCG. The dropping point and oil separation rate of 0.05 wt% C@Ag-LCG were 255 °C and 2.36 %, respectively. The tribological behavior of greases was tested using a four-ball friction tester, and the main damage mechanism was abrasive wear. The C@Ag additive reduced the average friction coefficient and wear spot diameter by 27.17 % and 26.12 %, respectively. The improved properties were attributed to the synergistic lubrication of hard CS core and soft Ag shell with the rolling, filling, and self-repairing effects, C@Ag deposit films and the self-lubricating oil film, as well as the thickener film and the tribochemical film at the frictional interfaces.

The effect of thickener structure degradation on tribological properties: Study on the decay mechanism of polyurea grease under mechanical-thermal aging conditions

The aging of lubricants is a primary factor contributing to rolling bearing failures. The rolling stability test simulated the aging behavior of grease under shear and high-temperature circumstances. An investigation was conducted on how mechanical-thermal aging impacts the chemical and physical structures of polyurea grease, as well as how changes in thickener microstructure affect rheological and tribological properties. The results show that under shear and high temperature, structural changes in urea lead to a transformation of the thickener morphologies from the growth of entangled ribbon fiber to a large-diameter rod structure, with a significant decrease in the degree of entanglement. When a certain proportion of fibrous and rod structures coexist, the grease has good oil separation capacity, the structural stabilities of grease become weak, and the deformation resistance decreases. During this phase, the polyurea grease has the best tribological performance.

Effect of Speed and Number of Sliding Cycles on the Film Formation Behavior of Fluorine-Based Grease

Grease, a type of lubricant, finds applications in various mechanical components due to advantages such as shape stability and sealing. However, grease is classified as a non-Newtonian fluid, and its film formation behavior is not fully understood to date due to variations in the mechanism related to the film formation behavior of grease based on factors like the base oil, bleed oil, thickener, and additives. In this study, the film formation behavior of fluorine-based grease was analyzed through the 3D profile of the grease after a friction test. In particular, the film formation behavior of the grease during sliding motion was analyzed. The experimental equipment used was a reciprocating-motion-type experimental equipment. Variations in the 3D profile were observed based on the speed and the cycle, indicating differences in the film formation behavior corresponding to changes in viscosity. In contrast to numerous studies that have observed the film formation behavior of grease under rolling motion, investigating the film formation behavior during the initial cycles of sliding motion will provide a profound understanding of the grease’s film formation process.

Investigation on the tribological properties and electrification performance of grease-lubricated triboelectric nanogenerators

Compared with lubricating oil, grease can firmly adhere to the lubricated surface without loss even on the vertical surface, which is more conducive to improving the durability of triboelectric nanogenerators (TENGs) in practical engineering applications by reducing material wear. However, the correlation between property parameters of grease and triboelectrification is still unclear. In this paper, rotary freestanding TENG with the most severe wear among the working modes of TENGs is taken as the research subject. First, the tribological properties and electrification performance of TENGs lubricated by different greases are studied, and then the effects of the dielectric constant, shear-thinning behavior and cone penetration on the electrical output as well as durability of TENG are explored to reveal the triboelectrification mechanism of the grease-lubricated interface. The results show that, the electrical output of TENG lubricated by the grease with a low dielectric constant can increase by more than 3.5 times compared to dry friction, where the thickener in the grease participates in triboelectrification at the grease-lubricated interface. With increasing sliding distance, the electrical output increases first and then decreases, and the durability is related to the shear-thinning behavior of grease. For greases with similar relative dielectric constant, with increasing cone penetration, the maximum electrical output of TENG decreases, but its durability is improved. This study provides a new approach to improve the electrical output and durability of TENG simultaneously, which further promotes the practical applications of TENGs.

Study on the interaction mechanism between degradation behavior of grease and evolution of friction interface properties under static storage conditions

Long-term static storage of the ball screws result in interactive degradation of the grease and the friction interface, affecting the safety of equipment operation. In this study, the physical structure of the grease under static storage was found to be a sensitive factor for degradation. Subsequently the interaction mechanism between degradation of grease and the evolution of interface property was revealed. The aggregation and adsorption behaviors of thickener-base oil, thickener-metal interface, and base oil-metal interface were investigated by the molecular dynamic simulation. Further, interface characteristics and grease degradation respectively affect the value and duration of the growth phase of friction. This study can provide guidance for the phenomenon of increased starting torque of equipment relying on ballscrew after long-term standby.

Unraveling the lubrication mechanisms of lithium complex (LiX)- and polypropylene (PP)- thickened greases in fretting – Part II: Lubrication model

In a previous investigation, greases with two different thickeners, namely lithium complex (LiX) and polypropylene (PP), and three different base oil viscosities were investigated in fretting with varying displacement amplitude, applied normal load and material combination. The tribological behavior of grease in fretting is extremely complex. Very different lubrication effects were found for the same grease type, depending on test conditions. The grease properties (thickener type and base oil viscosity) together with the applied normal load, displacement amplitude and the material combination all influence the prevailing fretting regime. The aim of the present paper is to unravel the lubrication mechanisms of LiX and PP greases in fretting. Cross sections of selected wear marks from the fretting tests are presented and they are, together with the results from the fretting maps presented in the previous investigation, used as tools to better understand the mechanisms. The exact lubrication mechanisms of the two thickener types are not yet fully understood and confirmed but a lubrication model is suggested, based on the present knowledge and hypotheses.

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.

Diffusion Behavior of Greases in Interfaces in Organic Insulation Materials

The diffusion of greases in organic insulation materials, which is the core parameter of the electrical and mechanical long-term behavior of the interface, is studied. The diffusion of grease depends both on the solid organic insulation material and on the grease. The highest diffusion can be observed by the combination of a silicone paste and silicone rubber. However, for silicone rubber there are low and no diffusion lubricants existing.The diffusion in EPDM, EP and XLPE is commonly much lower. In EP and XLPE, no type-dependency of the grease can be observed. With the knowledge of the diffusion behavior, the desired interface properties can be achieved for the whole lifespan of the apparatus.

Quantitative Observation of Grease Behavior for Rolling Bearings Using Splitting Phenomena of Colorant

Quantitative Observation of Grease Behavior for Rolling Bearings Using Splitting Phenomena of Colorant

Service Life of Lubricating Grease in Ball Bearings (Part 1) Behavior of Grease and Its Base Oil in a Ball Bearing

Service Life of Lubricating Grease in Ball Bearings (Part 1) Behavior of Grease and Its Base Oil in a Ball Bearing

Development of eco-friendly nano-greases based on vegetable oil: An exploration of the character via structure

The grease industry today is saturated with products based on petroleum/mineral oils and harsh (containing harmful chemicals) thickeners and additives. Such greases pose severe threats to the environment during their entire service life and even after being discarded as waste. To acknowledge the issue, researchers have developed a few eco-friendly alternatives (from natural and/or eco-friendly ingredients) so far; however, a potential solution is never reached. The present study puts forward a maiden combination to develop a series of greases using soybean oil as base oil, organo-montmorillonite as a thickener, and CaCO3 nanoparticles (0–3 %w/w) as additives. The developed gels were characterized for fundamental properties (consistency, thixotropy, and rheology) against an equivalent commercial grease to justify the signatures of a typical lubricating grease. Deep exploration of the microstructures was also carried out to understand the mechanism(s) behind the observations. In general, the developed gels exhibited equivalent characteristics to that of the commercial grease. However, the concentration of nanoparticles appeared to influence grease behavior significantly. The distribution of and the level of agglomeration among the nanoparticles were recognized as the two crucial parameters. Among all the formulations, the grease with 2% doping of CaCO3 nanoparticles displayed the optimum properties (thixotropy and rheology) closest to that of the commercial reference grease. Models are proposed to develop an explicit understanding of the grease structure, and TEM (Transmission Electron Microscope) micrographs are presented to support the hypothesis. The structure of the grease and the interaction(s) among various ingredients was observed as the fundamental factor governing the grease character.

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.

On the Fictitious Grease Lubrication Performance in a Four-Ball Tester

The extreme pressure (EP) behavior of grease is related to its additives that can prevent seizure. However, in this study following ASTM D2596 four-ball test method, the EP behavior of greases was modified without any changes to its additive package. A four-ball tester with position encoders and variable frequency drive system was used to control the speed ramp up time or delay in motor speed to demonstrate higher grease weld load and lower grease friction that were fictitious. A tenth of a second delay in speed ramp up time had showed an increase in the weld load from 7848 N to 9810 N for grease X and 6082 N to 9810 N for grease Y. Further increase in the speed ramp up time to 0.95 s showed that the greases passed the maximum load of 9810 N that was possible in the four-ball tester without seizure. The mechanism can be related to the delay in rise of local temperature to reach the melting point of steel required for full seizure or welding, that was theoretically attributed to an increase in heat loss as the speed ramp-up time was increased. Furthermore, the speed ramp up time increased the corrected load for grease X and Y. This resulted in lower friction for grease X and Y. This fictitious low friction can be attributed to decrease in surface roughness at higher extreme pressure or higher corrected load. This study suggests that speed ramp up time is a critical factor that should be further investigated by ASTM and grease manufacturers, to prevent the use of grease with fictitious EP behavior.

X-Ray CT Imaging of Grease Behavior in Ball Bearing and Numerical Validation of Multi-Phase Flows Simulation

X-Ray CT Imaging of Grease Behavior in Ball Bearing and Numerical Validation of Multi-Phase Flows Simulation

The Role of Grease Composition and Rheology in Elastohydrodynamic Lubrication

Grease lubrication is inherently complicated due to its non-Newtonian flow dynamics. The flow behavior of grease is difficult to determine under tribological shear rates as these shear rates are often extremely high, and therefore, are not accessible on a regular rheometer. In this work, we demonstrate the use of shear rate-concentration superposition method to extrapolate the data measured with a rheometer to tribologically relevant shear rates. This method is similar to the other superposition methods routinely employed by rheologists for predicting flow behavior beyond the measurement range of rheometer. In this method, a data master curve, which extends over a broad range of shear rates, is constructed by superposing individual flow curves for greases with different thickener concentrations. The master curve is utilized to determine infinite shear viscosity of grease which is then used for the film thickness estimation. Furthermore, the grease samples are tribologically tested using a four-ball tester to identify the individual and interactive roles of the thickener type, concentration and base oil in lubrication behavior. A change in thickener concentration or type influences frictional torque and wear scar size through modification in infinite shear viscosity and structural stability of grease. The SEM images show deposition of thickener particles on the contact surface, which also apparently affect friction and wear during the test. The lubrication performance is strongly influenced by base oil, which points towards the importance of its role in dictating flow resistance and film-forming behavior of grease inside elastohydrodynamic contact.

Experimental Study on the Physics of Thick EHL Film Formation with Grease at Low Speeds

An unexpected behavior of grease in rolling contact has become known among tribologists that it forms thick film at low speeds, but the nature of the film has still been in dispute. The dominance of EHL effects has been demonstrated in the previous works, in which the increase in the generalized viscosity of grease has been reasoned to cause the increase in the film thickness. This paper describes experiments to measure the film thickness with optical interferometry, to observe the flow in and around the contact with microscopy, and to determine the change in the concentration of the thickener by IR spectroscopy. On the basis of these findings, the physics of the thick film formation with grease at low speeds is discussed.

On the film forming and friction behaviour of greases in rolling/sliding contacts

Abstract The film forming and friction properties of nine greases covering different thickener types, viscosities and NLGI grades have been investigated. Film thickness results are in agreement with previous work that all greases form a thicker film than the base oil counterpart at low speeds, suggesting that lambda ratio and kappa of greases can be much higher than their corresponding oils. The base oil viscosity appears to govern the grease film thickness at low speeds by a direct relationship for polyurea greases whereas by an inverse relationship for lithium and calcium complex greases. Grease friction results indicate that the shape of grease friction curves can differ quite significantly from that of oils. Possible underlying mechanisms that govern friction properties are discussed.

Research on rheological and flow behavior of lubricating grease in extremely cold weather

PurposeThis paper aims to investigate the flow characteristics of lubricating grease in extremely cold weather in which it is difficult to convey the grease due to a huge pressure drop.Design/methodology/approachThe rheological behavior of grease at various temperatures is studied by a rotary rheometer to determine the constitutive equation of lubricating grease. Based on the Herschel–Bulkley (H–B) model, the flow pattern of grease is then simulated by computational fluid dynamics and compared with the test results.FindingsThe yield stress increased dramatically when the shear rate was less than 1s−1 in the rheological experiments of continuous shear mode, and the phenomenon was more significant with the decrease in temperature. The rheological data obtained from the continuous shear mode agrees with the H–B equation after the shear thinned. In extremely cold conditions, there is a large yield stress in the lubricating grease; the numerical results show that the viscosity of lubricating grease increased with an increase in temperature, and the viscosity and velocity of lubricating grease showed uneven distribution leading to difficulty of lubricating grease delivery.Originality/valueThis paper focuses on the flow characteristics of lubricating grease in extremely cold area conditions which is studied rarely. In addition, the continuous shear model and oscillatory model are combined to establish the constitutive equations. Experiment and numerical simulation method are all used by establishing the H–B models.

Experimental Analysis of Grease Friction Properties on Sliding Textured Surfaces

There is comprehensive work on the tribological properties and lubrication mechanisms of oil lubricant used on textured surfaces, however the use of grease lubrication on textured surfaces is rather new. This research article presents an experimental study of the frictional behaviours of grease lubricated sliding contact under mixed lubrication conditions. The influences of surface texture parameters on the frictional properties were investigated using a disc-on-ring tribometer. The results showed that the friction coefficient is largely dependent on texture parameters, with higher and lower texture density resulting in a higher friction coefficient at a fixed texture depth. The sample with texture density of 15% and texture depth of 19 μm exhibited the best friction properties in all experimental conditions because it can store more grease and trap wear debris. The reduction of friction is mainly attributable to the formation of a stable grease lubrication film composed of oil film, transfer film and deposited film, and the hydrodynamic pressure effect of the surface texture, which increases the mating gap and reduces the probability of asperity contact. This result will help in understanding the tribological behaviour of grease on a textured surface and in predicting the lubrication conditions of sliding bearings for better operation in any machinery.