Pieter Baart

Oil-Bleeding Model for Lubricating Grease Based on Viscous Flow Through a Porous Microstructure

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 thickener microstructure. The model was verified with experiments at a wide variety of temperatures and rotational speeds.

A new method to visualize grease flow in a double restriction seal using microparticle image velocimetry

A new method to visualize and quantify grease flow in between two sealing lips or, in general, a double restriction seal is presented. Two setups were designed to mimic different types of seals; that is, a radial and an axial shaft seal. The flow of the grease inside and in between the sealing restrictions was measured using microparticle image velocimetry. The results show that grease flow due to a pressure difference mainly takes place close to the rotating shaft surface with an exponentially decaying velocity profile in the radial direction. Consequently, contaminants may be captured in the stationary grease at the outer radius, which explains the sealing function of the grease.

The Influence of Speed, Grease Type, and Temperature on Radial Contaminant Particle Migration in a Double Restriction Seal

Microparticle image velocimetry (μPIV) is used to measure the grease velocity profile in small seal-like geometries and the radial migration of contaminant particles is predicted. In the first part, the influence of shaft speed, grease type, and temperatures on the flow of lubricating greases in a narrow double restriction sealing pocket is evaluated. Such geometries can be found in, for example, labyrinth-type seals. In a wide pocket the velocity profile is one-dimensional and the Herschel-Bulkley model is used. In a narrow pocket, it is shown by the experimental results that the side walls have a significant influence on the grease flow, implying that the grease velocity profile is two-dimensional. In this area, a single empirical grease parameter for the rheology is sufficient to describe the velocity profile. In the second part, the radial migration of contaminant particles through the grease is evaluated. Centrifugal forces acting on a solid spherical particle are calculated from the grease velocity profile. Consequently, particles migrate to a larger radius and finally settle when the grease viscosity becomes large due to the low shear rate. This behavior is important for the sealing function of the grease in the pocket and relubrication.

Review of the lubrication, sealing, and pumping mechanisms in oil- and grease-lubricated radial lip seals

Abstract Radial lip seals are successfully used since the 1940s to seal lubricated systems. Despite extensive experimental and theoretical research in the field, it is still not fully clear how these seals function. Experimental studies, found in the public literature, show that the relatively high surface roughness of the seal lip is very important for good and reliable performance. In addition, the pressure distribution under the lip seems to be a critical factor. Six fundamental hypotheses are presented on the lubrication, pumping, and sealing mechanisms to explain the working principles of these seals. It is generally accepted that lubrication results from micro-elastohydrodynamic film build up between the rough seal surface and the shaft. Non-symmetrical tangential deformations of the lip surface are observed during experiments and assumed to act like spiral groove bearings that generate a pumping action and lubricant film. Another hypothesis suggests that the lubricant will behave non-Newtonian under the very high shear rates experienced in operating conditions. This will reduce friction because of shear-thinning and enhances sealing. Macroscopic aids, like hydrodynamic pumping aids and engineered asperity patterns on the shaft, do improve seal performance. Almost all public literature discusses oil-lubricated radial lip seals while many seals are grease-lubricated, especially in certain technical fields. Due to the non-Newtonian behaviour of grease, the lubrication, sealing, and pumping mechanisms are assumed to differ from the oil-lubricated seals. Lower friction and improved protection against contamination are measured, and it is expected that the interest in grease lubrication will rapidly grow in future.

Non-Newtonian Effects on Film Formation in Grease-Lubricated Radial Lip Seals

In existing models, the only lubricant property used for predicting film thickness in radial lip seals is the (base) oil viscosity. Lubricating greases show non-Newtonian behavior, and additional normal stress components develop that may contribute to the load-carrying capacity. This study investigates the shear rheology of greases and determines whether this “normal stress effect” in grease can significantly contribute to film formation in radial lip seals. First, the rheological behavior of grease is studied in a rotary plate–plate rheometer at small gaps of 25–500 μ m up to shear rates of 5 · 10 4 s −1 . The rheology measurements are used for a rheology model that predicts the first normal stress difference in the grease. Second, a seal lip model was developed to predict the lift force generated by the normal stress effect that separates the seal from the shaft. The model results show that the load-carrying capacity depends very much on the operating conditions: lip geometry, speed, and temperature. The model predicts a lift force that is over 50% of the seal specific lip force for low-contact pressure-bearing seals. The model can easily be used in existing oil seal models and makes it possible to optimize seal design by utilizing the normal stress effect.

Film Thickness Model for Grease-Lubricated Bearing Seals with an Axial Contacting Lip

A theoretical model is presented to predict the oil film thickness in an axial sealing contact based on grease properties and operating conditions. It is assumed that a small amount of grease will form an oil reservoir on the rotating part and slowly supply oil to the sealing contact. The oil bleed model from a previous study is implemented and oil loss due to centrifugal forces and the seal pumping action are taken into account. The results show that, depending on the operating conditions, an oil film is present in the sealing contact for a certain period of time. The oil film thickness decreases in time due to the decreasing oil supply from the grease reservoir and oil loss from the contact due to centrifugal forces. Seal pumping has only a small effect and the seal material and geometry are therefore not important for the predicted time until the mixed lubrication regime is reached. This time depends on the oil viscosity, rotational speed, and seal contact radius and scales with the η /(n2·ds) parameter. The volumetric size of the grease reservoir also has a large impact on maintaining the film thickness in time.

Contaminant Migration in the Vicinity of a Grease Lubricated Bearing Seal Contact

Lubricating grease is commonly used for lubricating sealed and greased for life rolling element bearings. This grease also provides an additional sealing function to protect the bearing against ing ...

Free-Surface Grease Flow: Influence of Surface Roughness and Temperature

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.

On the Normal Stress Effect in Grease-Lubricated Bearing Seals

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.

The Sealing Function of Grease: Contamination Migration in Grease Lubricated Radial Lip Seals

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