Piet M. Lugt

A Review on Grease Lubrication in Rolling Bearings

Grease lubrication is widely applied to rolling bearings. The consistency of grease prevents it from leaking out of the bearing, makes it easy to use, and will give it good sealing properties. The same consistency prevents an optimal lubrication performance. Most of the grease is pushed out of the bearing during the initial phase of bearing operation and no longer actively participates in the lubrication process, leaving only a limited quantity available, which is stored inside the bearing geometry and on the bearing shoulders (covers or seals). This stored volume strongly determines the remaining lubrication process in the bearing. The distribution of this volume is determined by the grease flow, which is very complex to understand due to the strong nonlinear rheology. There is no consensus on the next phase in the lubrication process. The grease may bleed and provide oil to the raceway; it may be severely sheared in the raceway releasing oil; or small fresh quantities of grease may be sheared off from the volume stored on the shoulder. In addition, the lubrication process may be dynamic. Grease has self-healing properties where fresh grease is supplied in case of film breakdown and self-induced heat development. This article describes the state-of-the-art knowledge on grease lubrication, including grease flow, film formation, film reduction, dynamic behavior, and grease life.

A mixed lubrication model incorporating measured surface topography. Part 1: Theory of flow factors:

Abstract A mixed lubrication model that permits real three-dimensional surface topography as input is developed. The theory of computing flow factors within the model is presented, and with a following paper (Part 2) the method of measuring and adapting the surface roughness, and model validation through flow measurements and application to a bearing is shown. A contact mechanics model is used to calculate the elastoplastic displacement of a periodic topography signal. A method based on homogenization is used to calculate flow factors for all lubrication regimes. The flow factors are compared with the Patir and Cheng method. Results indicate that the two methods compare well for longitudinal roughness lay, but differ significantly for a cross-patterned surface roughness due to the more complete flow description of the current model.

A mixed lubrication model incorporating measured surface topography Part 2: roughness treatment, model validation, and simulation

Abstract A mixed lubrication flow factor model that permits real three-dimensional surface topography as input has been developed. Part 1 gives the theory of computing flow factors within the model. In this article, a method of adapting the measured surface topography signal to suit the numerical models is developed and presented in detail. The mixed lubrication model is validated through flow measurements for three different rough surface test specimens. Simulation of a hydrodynamic bearing was conducted and the results are presented in terms of pressure distributions and Stribeck curves covering all lubrication regimes. The results indicate that the model may be an efficient and accurate engineering design and research tool for tribological devices operating in all lubrication regimes.

Investigation of Grease Flow in a Rectangular Channel Including Wall Slip Effects Using Microparticle Image Velocimetry

The grease flow in a rectangular channel is investigated using microparticle image velocimetry. Of certain interest is to study the behavior close to the boundary where wall slip effects are shown to be present. Three greases with different consistencies (NLGI00, NLGI1, and NLGI2) have been used, together with three wall materials (steel, brass, and polyamide) with different surface roughness. The pressure drop is also varied. It is shown that the velocity profile is strongly dependent on the consistency, having a dominating plug flow structure for a stiff grease. Furthermore, it is shown that wall slip effects occur in a thin shear layer close to the boundary where a very large velocity gradient is present. An analytical solution for the velocity across the channel is described using a Herschel-Bulkley rheology model. The model fits well with the measured velocity profile for all three above-mentioned greases.

Lubrication in cold rolling: Elasto-plasto-hydrodynamic lubrication

A model has been developed with respect to hydrodynamic lubrication in cold rolling. The basic model describes the configuration of a rigid, perfectly plastic sheet rolled by a rigid work roll. The governing equations have been solved throughout the complete contact area, i.e. the inlet, the work zone and the outlet zone. Multi-level techniques have been applied to solve these equations together with boundary conditions, resulting in an algorithm solving the problem in O(n) operations. This means that the distribution of the pressure and the traction force in the lubricant film, and the shape of this film, as well as the plastic deformation of the sheet, can be accurately calculated for a large number of nodal points on a minicomputer. Subsequently elastic deformation, work hardening and dynamic behaviour of the flow stress have been incorporated in the model. It will be shown that the influence of these effects on the film thickness or the pressure distribution is considerable.

On the Chaotic Behavior of Grease Lubrication in Rolling Bearings

Grease-lubricated bearings often show a sharply fluctuating temperature signal. At the start of the bearing operation the temperature rises. This is generally ascribed to grease churning. This phase takes approximately 2–5 h. After that the temperature usually reaches a quasi-steady-state value. Quite often, this situation is not maintained and the temperature signal typically shows “events,” characterized by periods where the temperature rises significantly and falls back to the “steady-state” value after some duration in time. The time intervals between the events and their duration vary significantly throughout a single bearing test. Moreover, this signal varies markedly for individual bearings tested under the same conditions with identical grease. This article gives an analysis of the temperature and relative film breakdown time series signals from various tests using the concepts of nonlinear dynamics and chaos. The analysis demonstrates that the embedding dimension of the system for the group of bearings tested is consistently 5, showing that a five-parameter nonlinear model provides an adequate description. The maximum Lyapunov exponent lies in the narrow range 1.14 < λmax < 1.21, while another exponent lies close to zero. It may thus be concluded that grease lubrication here exhibits “deterministic chaotic” behavior, implying that the initial filling conditions for these bearings will be crucial for the life of the grease in the bearing. The minimum Lyapunov exponent is negative, indicating a dissipative mechanism in the dynamics. The article shows that the film thickness in a grease-lubricated bearing cannot be described using conventional starvation theory only and that it is fluctuating in time in a chaotic manner. This is most important for the development of film thickness and (grease) life models in grease-lubricated bearings.

Free Surface Thin Layer Flow in Bearings Induced by Centrifugal Effects

To be able to predict service life of grease-lubricated bearings, it is of major importance to be able to predict the grease life; i.e., to predict when the lubricant film formation capability has decreased below a certain level. The objective of the present study is to develop theoretical models that can be used in practice to predict different aspects of grease life. As a first step, the influence of centrifugal forces on lubricant migration and the long-term film thickness decay rate due to this effect in a bearing is studied. The authors present a model that can be used to predict the decrease of oil-layer thickness due to centrifugal forces as a function of position across the track for different types of bearings. It is assumed that the oil layers in each of the roller raceway contacts separate equally between the diverging surfaces. To provide some justification for this hypothesis, a roller/plate experiment is carried out. As an example, the model is applied to the geometry of a spherical roller bearing. Two flow types are distinguished, depending on the shape of the lubricated surfaces. For various sizes and for different geometry parameter settings, similar central layer decreases are predicted. It is shown that the centrifugal effects can significantly reduce the layer thickness, within the service life of the bearing.

Friction in Highly Loaded Mixed Lubricated Point Contacts

A model that treats both the EHL and the asperity contact friction in lubricated rough circular contact is presented in this article. In this approach, the well-known Greenwood and Williamson elastic contact model describing dry asperity contact is combined with an isothermal Eyring EHL model of the hydrodynamic film. It is shown that the friction results obtained from this approach are in good agreement with the experimental Stribeck curves measured on a ball-on-disc test rig.

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.