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This study investigates and characterizes, via thermodynamic laws, a widely employed standard for measuring oxidation-thermal stability of lubricating greases—ASTM 5483—based on first-order chemical kinetics, using pressure differential scanning calorimetry (PDSC). Steps and active mechanisms in the controlled test are analyzed and modeled using energy and entropy transformations. Applying the degradation-entropy generation (DEG) theorem, oxidation induction time is related to accumulated oxidation entropy and entropy transfer by mass flow to obtain characteristic degradation coefficients. These DEG coefficients are calibrated using available measured data from the literature and subsequently used to predict induction times at various temperatures. DEG elements—trajectories, planes, and domain—presented here consistently characterized all greases studied. Results show that the high-pressure entropy transfer by oxygen mass flow in the final step of the ASTM 5483 procedure is the primary contributor to oxidation induction time, verifying the significance of this process in accelerating antioxidant depletion, which would otherwise take months to years under atmospheric conditions. DEG-predicted induction times at various temperatures are compared to induction times predicted by the first-order chemical kinetics model, revealing a close match. The new DEG model is also herein used to predict induction times at atmospheric pressure, whereas the first-order kinetics model is independent of pressure and limited to the specific conditions in ASTM 5483. The DEG approach has useful implications for the accelerated and unaccelerated evaluation of grease oxidation stability and grease useful life.

Grease aging in a rolling bearing happens primarily during the churning phase due to high shear and temperature caused by macroscopic flow. This study utilizes a temperature profile to quantify the impact of thermo-mechanical aging on grease. Two different methods are explored to evaluate the energy dissipated in the grease during churning: one estimates the imposed energy density and the other calculates the heat transfer entropy density. Both models show a good correlation with the measured grease life. Grease life test results show that a high imposed energy density or heat transfer entropy density during churning results in shorter grease life. Therefore, a low energy density or heat transfer entropy density is preferred during churning.

Five common ball bearing greases were studied using a grease-on-paper test configuration, where the bleeding of greases is described by the dynamics of a driving capillary pressure provided by the paper and its opposing counterpart in the grease matrix. The Washburn-like model for extracting the retaining capillary pressure, determined by the affinity of the oil for the grease matrix, has proven to be applicable for greases made of various type of thickeners and base oils, and can potentially contribute to modelling the bleeding of stationary grease as well as to monitoring the change in bleeding properties of greases in a bearing.

Applications operating at high temperatures may suffer from oxidative lubricant degradation. The current test methods to evaluate thermoxidative stability and performance are resource intensive and they often provide results that are difficult to extrapolate to application conditions where the temperatures are lower than the test temperature. In this paper, we present a method to evaluate the thermoxidative performance of lubricants via non-isothermal thermogravimetric experiments under conditions closer to application conditions. We can provide effective kinetic parameters for the oxidation process. Based on these parameters, we can simulate mass-loss in experiments at a constant temperature, and estimates of induction times of oils with different antioxidant concentrations agree well with experimental values. Additionally, the kinetic parameters may be used in grease life models.

The evolution of the film thickness in a grease lubricated rolling bearings is given by a continuous process of loss and replenishment of oil in the rolling track. It is believed that in a long run, the oil film is maintained by the oil that is bleeding from grease reservoirs. The bleeding capabilities of a grease thus have an impact on the grease and/or bearing life. The ability of a grease to bleed the oil changes during the operation of the bearing due to mechanical ageing, which affects the fiber structure of the grease. In this study the lubricating grease is considered as a porous medium formed by the grease thickener and the base oil. The oil bleed behavior of a polyurea grease was explored as a function of mechanical degradation of the grease in numerous full bearing tests. The measured degradation in bleed was interpreted as an evolution of the grease matrix permeability based on Darcy's law. A grease ageing master curve was built to describe the change of permeability of the grease due to mechanical ageing.

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.

Grease life in hybrid bearings is longer than that in all-steel bearings. This will be shown in this article by means of grease life tests with a large number of greases for both deep groove ball bearings (DGBBs) and cylindrical roller bearings (CRBs). The results show that grease life for hybrid bearings is always larger than that for equivalent all-steel bearings and that this varies between a factor of 2 and 9 depending on the grease type used. For cylindrical roller bearings, grease life does not increase with decreasing speed below a minimum value. However, at these relatively low speeds, hybrid bearings give a longer grease life than all-steel bearings.

The bleed rate of a grease may change over time due to degradation of the thickener microstructure and a decrease in oil content, as oil is released from the grease. In this work, the bleed properties—quantified by the permeability—of five types of lubricating greases are studied. Controlled shear ageing is performed using a grease worker. It is shown that the bleed rate does not change significantly in a grease worker at room temperature, as opposed to changes in rheology and microstructure. This is explained using a permeability model. It is shown, via a comparison with rolling bearings results, that ageing grease in a grease worker is too mild to yield significant changes in the permeability of lubricating greases.

Calcium sulfonate complex greases have excellent extreme pressure, antiwear, and anticorrosion properties and are widely applied in rolling bearings, particularly in humid environments. In this article, the shear stability of dry and water-contaminated lubricating calcium sulfonate complex greases is described using a novel aging method. Unlike lithium and polyurea greases, no shear softening is observed for the dry greases due to the good mechanical stability of the particle-like thickener structure. For water-contaminated greases, no water separation was found during the prolonged aging. Instead, a homogeneous water–calcium sulfonate thickener micellar structure is generated. These micelles function as apparent thickeners and effectively increase the thickener concentration, which thickens the grease. This may explain why calcium sulfonate complex grease has excellent water absorption properties.

The lubricating film formation of greases has been widely studied using single contact ball-on-disc configurations. However, other than for oil, the film formation with grease is governed by starvation, which in a full bearing is clearly different from that of the single contact. In this paper, film thickness measurements for various greases in a shielded deep groove ball bearing will be presented. The results were obtained using an in-house-made film thickness measurement rig based on the ‘electric capacitance method’. It will be shown that the film thickness may deviate from the theoretical elastohydrodynamic film thickness calculated from the base oil and that the film thickness is not constant in time. It will also be shown that the film thickness for longer times is not only determined by the classical properties such as oil viscosity and grease bleed but also by shear degradation of its microstructure.

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.

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.

The churning phase, i.e., the early phase in bearing operation, ends when the grease has been migrated from in-between the rolling elements to the unswept area. This process is often called “channeling or clearing”. In the present study, early stage temperature profiles of running bearings are used to identify the length of this phase using different greases. It is shown that grease undergoes a microstructural change during the channeling/churning phase. Good (“peak type”) channeling greases maintain their structural integrity and undergo only a very short period of churning, whereas for poor (“plateau type”) channeling greases a significant microstructure degradation occurs. This is ascribed to the longer churning phase. The channeling properties may be attributed to the microstructural flexibility of greases.

Understanding the mechanical degradation of grease is crucial for the development of grease life models and to predict grease leakage from bearings. It was found recently that this process can be described using a Master-Curve where the yield stress is given as a function of the imposed energy corrected for temperature using an Arrhenius equation. This paper shows that this concept can be applied to the mechanical degradation of grease in rolling bearings provided that the oil content in the grease has not changed. So it does not apply to those areas in the bearing where the centrifugal forces are high such as under the cage/inside the running track. However, it does apply to the grease that is aging in the unswept area of the bearing, i.e., the area in the bearing where the grease undergoes moderate shear. This Master-Curve makes it possible to develop a remaining grease life model for bearings operating under moderate temperatures.

Water contaminated grease samples are investigated with attenuation spectra in the visible and near-infrared (NIR) region in this paper. The purpose of this investigation was to identify a model wi ...

The impact of start-stops on the thickness of lubricating films in the Elasto-Hydrodynamic Lubrication (EHL) contacts in grease lubricated rolling bearings is different from that in oil lubricated bearings. This paper shows that the film thickness drops to zero quickly due to the absence of hydrodynamic effects during a stop. It quickly increases to an unexpected large value when the bearing is stopped for a long time and started up again, although this large value only lasts for a very short time. This is due to additional track replenishment during the stop leading to an initially thicker film than expected. This increase is found to be determined by the rotational speed of the bearing, stop-time, base oil viscosity and width of the contacts. The start-stop effect is similar for unidirectional and oscillating speeds, provided that the stopping time is long enough.

A new model was developed for the simulation of wear particles formation in mixed lubricated sliding contacts. The simulations based on a previously developed half-space algorithm coupled with a numerical elasto-hydrodynamic lubrication solver utilizing the load-sharing concept. A particle removal criterion based on a critical Von Mises stress and a geometrical boundary condition was successfully implemented. The resulting model allows for the determination of the size of the particles generated in different conditions. The model was applied to the simulation of running-in and validated using experimental measurements of the initial and run-in friction coefficient in a mixed lubricated contact. Additionally, dynamic light scattering (DLS) measurements of the wear particles size were performed and compared to the simulation data.

In this article, the mechanical aging behavior of lubricating greases with a fibrous structure is studied by aging fresh samples at controlled shear rates and temperatures in an in-house-made Couette aging machine. The rheological properties of fresh and aged samples were evaluated in a plate–plate rheometer. In the absence of oxygen, no chemical reactions occurred. The results showed that shear degradation is accelerated by increasing the temperature. This thermal effect can be described by an Arrhenius law. A grease aging master curve was created to describe the influence of shear and temperature on the mechanical aging of fibrous structured grease. Next, the model was validated using a conventional grease worker test. Finally, the model was applied to full ball bearings using an R0F + bearing test rig. The master curve forms an important contribution to the development of grease life models.

This article discusses the functions of lubricants to prevent premature failure of rolling element bearings and the advantages of fluid lubrication. It describes the composition of refined mineral oil for rolling bearing applications. The article reviews the types and properties of nonpetroleum oils, such as polyglycols, phosphate esters, silicone fluids, dibasic acid esters, and fluorinated polyethers. It discusses the properties of greases, including grease speed limits, grease composition, relubrication intervals, corrosion prevention behavior, and grease compatibility. The article concludes with a discussion on polymeric lubricants and solid lubricants.

Grease lubrication is traditionally used in a great variety of mechanical systems such as rolling bearings, seals, and gears where it has been shown more advantageous than oil, mainly due to its consistency allowing the grease to stay inside the system and not leak out. Knowledge of the flow dynamics of grease is important for the understanding and prediction of grease distribution for optimum lubrication and for the migration of wear and contaminant particles. Free-surface effects play an important role in rolling bearings and open gears as the configuration normally is filled with about 30 % grease to avoid heavy churning. In this study, an analytical model of the stationary uniform flow on a rotating disc is developed and validated with experiments. The model results in the velocity profile for the flow in the thin fully yielded viscous layer in connection to the surface as well as an expression for the plug flow region on top of the viscous layer. Furthermore, the depth-averaged velocity is derived as is the shear stress value on the plate. From the latter, follows a condition for the grease to start moving and in turn yielding an expression for the viscous layer thickness as a function of the grease yield stress value, grease density, angular velocity, and radial position. In addition, an expression of the layer thickness containing the ratio between the flow rate and the layer width which in turn can account for effects not included in the model such as wall slip and surface adhesion and thus add another degree of freedom into the model. Experiments with two different greases having NLGI grade 1 and 2, respectively, shows it is possible to obtain a good fit with the analytically obtained thickness using the rheological parameters for actual greases

The film formation in lip seals, due to the non-Newtonian rheology of the lubricant, has been a topic of speculation. Earlier work suggests that normal stresses in grease would be favorable for the film buildup between the seal lip and shaft or bearing ring. In the current article, we evaluate this earlier work and our earlier theoretical seal lip model with a series of experiments. We use a modified concentric cylinder geometry and a model fluid to study the fluid pressure distribution in the seal-type geometry. The results are then related to grease-lubricated seals and our earlier theoretical predictions. The present analysis shows that this earlier work and our earlier predictions are not correct and indicate that normal stresses in the grease pull the seal lip toward the shaft, increasing the contact pressure. However, normal stresses also ensure the presence of grease on the shaft or bearing inner ring, which enhances replenishment of the sealing contact.

One of the criteria in selecting lubricating grease for rolling-element bearing applications is its ability to bleed oil, sometimes called “grease bleeding.” Oil bleeding is assumed to be the dominating mechanism supplying new oil to the rolling track for lubrication. In this study, a physical model has been developed to understand the relation between parameters that control oil bleeding. In the model, lubricating grease is described as a porous network, formed by the thickener fibers, that contains the base oil. This type of structure is confirmed by SEM and AFM images of a lithium complex grease showing a matrix of rigid fibers with random orientation. A relatively simple flow model based on Darcy’s law for viscous flow in porous media and an anisotropic microstructure deformation model was developed. The model relates the pressure gradient, oil viscosity, thickener structure deformations, and permeability to the volumetric oil flow out of the thickener network. The permeability depends strongly on the...

It is generally assumed that starved lubricated contacts in rolling element bearings are replenished by side flow. The rolling elements partly push the lubricant to the side of the contacts after which the re-flow will provide fresh lubricant for the next rolling element. This re-flow is driven by surface tension and restricted by the viscosity. Hence, thick oils or greases may yield problems here, which is generally observed in practice. This paper quantifies this re-flow by means of a numerical model using a so-called thin layer/film assumptions. The results here show that this form of replenishment may happen to some extent in single contacts but for sure not in rolling element bearings. There are two reasons for this: the time between successive overrollings is too short and secondly, the centrifugal effects on the inner ring will drive the flow in a vertical direction rather than transversely. This applies to cylindrical roller bearings (CRB)-type of surfaces. This may be different for tapered or sph...

Grease flow in grease-lubricated systems can often be qualified as free-surface flow. It occurs, for example, in rolling bearings after the churning phase or on open gears. Here only a fraction of the bearing or gearbox volume is filled with grease. Part of the grease is flowing in relatively thin layers induced by centrifugal forces caused by rotation of the various components. In this paper, a model problem is investigated in the form of a free-surface flow of grease on a rotating disc. Experiments have been performed where the onset of flow and remaining grease have been studied varying the surface roughness, temperature and the centrifugal forces. The experiments have been coupled to analytical models describing the flow and temperature distribution in the grease. It was found that the impact of surface roughness could be neglected. The flow is determined by the centrifugal forces and rheology of the grease. Temperature effects the rheology but also the oil separation creating low shear strength/low viscosity layers at the surface.

International audience; Several different types of commercially available greases, aged in a roll stability tester, have been tested in a ball-on-disc machine where the film thickness was measured using the interferometry method. The grease film thickness at very low speed is thicker than the base oil film thickness due to the contribution of the thickener in this regime, which practically vanishes at higher speeds. The (mechanical) aging process reduces the thickener material, decreasing the grease film thickness at very low speeds (entrainment speed less than 0.01 m/s) with aging time, whereas no significant difference was found at higher speeds where the grease film thickness still approached the calculated base oil film thickness.

International audience; The contribution of the thickener to the thickness of the lubricating film in grease-lubricated contacts is investigated. Four different types of greases were tested in a ball/spherical roller-on-disc machine, where the film thickness was measured using the interferometry method, varying the temperature, load, and slip. The test results show that there are two lubrication regimes. Below a transition speed the grease thickener plays an important role where the film thickness increases with decreasing speed. At higher speeds the film thickness is primarily governed by the base oil. This transition speed is a function of the temperature and not of the load and slip. At higher speeds the grease film thickness can be calculated using the base oil viscosity. The electrical capacitance method was applied to measure the film thickness at ultralow speed in a real bearing, showing that the single contact results are indeed applicable to a full rolling bearing.

Grease is extensively used to lubricate various machine elements such as rolling bearings, seals, and gears. Understanding the flow dynamics of grease is relevant for the prediction of grease distribution for optimum lubrication and for the migration of wear and contaminant particles. In this study, grease flow is visualized using microparticle image velocimetry (μPIV). The experimental setup includes a concentric cylinder configuration with a rotating shaft to simulate the grease flow in a double restriction seal geometry with two different grease pocket sizes. It is shown that the grease is partially yielded in the large grease pocket geometry and fully yielded in the small grease pocket. For the small grease pocket, it is shown that three distinct grease flow layers are present: a high shear rate region close to the stationary wall, a bulk flow layer, and a high shear rate boundary region near the rotating shaft. The grease shear thinning behavior and its wall slip effects have been identified. The μPIV experimental results have been compared with a numerical model for both the large and small gap size. It is shown that the flow is close to one-dimensional in the center of the small pocket. A one-dimensional analytical model based on the Herschel-Bulkley rheology model has been developed, showing good agreement with the measured velocity profiles in the small grease pocket. Furthermore, wall slip effects and shear banding are observed, where the latter imply that using the assumption of uniform shear in conventional concentric cylinder rheometers may result in erroneous rheological results

Rolling bearings contain seals to keep lubricant inside and contaminants outside the bearing system. These systems are often lubricated with grease; the grease acts as a lubricant for the bearing a ...

Lubricating grease is commonly used for lubricating ‘sealed and greased for life’ bearings. This grease lubricates the rolling contacts. It also provides an additional sealing function to protect the bearing against ingress of contamination. The sealing function of lubricating grease in the vicinity of the seal lip contact has been studied experimentally. The effects of the lubricant rheology on the migration of ingress particles has been examined. In grease, experimental results reveal that contaminant particles consistently migrate towards the sealing contact where the shear rate reaches its highest value. In contrast, for a Newtonian base oil and a non shear thinning elastic fluid, it has been observed that the migration effect takes place in the opposite direction, and brings particles away from the sealing contact. It is concluded that the sealing function of grease in the vicinity of the sealing contact is due to the fluid rheology and more specifically to the shear thinning behaviour of the lubricant.Copyright © 2010 by ASME