Impact of oxidation on grease life in rolling bearings

In this paper the lubrication mechanism of lithium grease lubricated ball bearings in the bleed phase is described. Oxidation plays an important role and therefore grease life “in air” and grease life “in nitrogen” is studied in real bearings. It is shown that grease life is strongly dominated by oxidation but that this is by far not deterministic. Oxidation starts up at a certain time, the induction time, which is the point at which the anti-oxidants have been consumed. This induction time is a function of the bearing operational conditions and the oxygen concentration. Oxidation leads to loss of base oil and loss of lubricity, partly repaired by replenishment and oil release from the grease reservoirs (bleed)

On the film thickness in grease lubricated deep groove ball bearings

Most rolling element bearings use grease as a lubricant. The service life of a bearing is determined by the bearing fatigue life and grease life. Both are influenced by the quality of lubrication, which is strongly determined by the thickness of the lubricating film. The optimal selection of a lubricating grease and/or the prediction of grease and/or bearing life can only be done properly if the film thickness can be determined. However, until now, there is no equation to predict film thickness in grease lubricated bearings. In practice, the equations that were derived for oil lubrication are used. In this thesis, the film thickness immediately after the churning phase is studied under various conditions on different bearings and greases. An improved electrical capacitance method is developed to measure the film thickness by using an electrical model of the bearing, including the effect of starvation. It is shown that the film thickness immediately after churning is determined by the dynamics of the flow of lubricants in and around the contacts and not by oil released by the grease after churning (bleed). It is observed that the film thickness in a grease lubricated bearing is almost constant at higher speeds.It is demonstrated that the level of starvation under pure axial load can be well described using only the product of base oil viscosity, half contact width, and linear speed (ηbu). It is also shown that bearing size/geometry has little to no influence in determining the level of starvation in the ηbu concept. The film thickness study in radially loaded bearings and bearings under combined (axial+radial) loads showed that there is an additional replenishment at the lower loaded zones. This is due to a larger ‘gap’ between the ball and the groove in the low load zone causing a reduction in resistance to viscous flow, thereby increasing the flow of lubricant towards the contacts. Finally, a master curve is created that can be used to calculate the film thickness in grease lubricated ball bearings under radial, axial and combined loads

On the Degradation of Lubricating Grease

A comprehensive literature review on physical and chemical degradation monitoring and life estimation models for lubricating greases is presented in chapter one. Degradation mechanisms for lubricating grease are categorized and described, and an extensive survey of the available empirical and analytical grease life estimation models including degradation monitoring standards and methods are presented. In chapter two, irreversible thermodynamic theory is employed to study the mechanical degradation of lubricating grease. A correlation between the mechanical degradation and entropy generation is established and the results are verified experimentally using a rheometer, a journal bearing test rig, and a modified grease worker machine. It is shown that the degradation rate is linearly related to the entropy generation, and that it can be used for estimation of the mechanically degraded grease life. In chapter three, a model is presented that uses the principles of irreversible thermodynamics to predict the life of a lubricating grease undergoing mechanical shearing action. Here we restrict our attention to operating temperatures far below the initial activation energy needed to initiate chemical degradation or base oil evaporation. Thus, mechanical degradation is the dominant degradation process. The predictions of the model are validated using the experimental results obtained by testing three greases subjected to different shear rates and temperatures. In chapter four, mechanical life of grease in an elastohydrodynamic (EHL) line contact between two steel rollers is studied. Grease traction curves are measured and reported in different conditions. Three successive lubricating phases of “Fully grease covered rollers”, “Slippage and grease separation” and “Formation of liquid lubricant reservoir” are observed and their behaviors are examined. The traction of the grease is monitored during a long term mechanical degradation process. Our mechanical life prediction model is applied to the lubricating grease at the contact. In chapter five, chemical degradation is studied from an energy point of view. A theory is introduced based on acquired experimental results, and is verified using a roller tester rig. The theory is used to estimate the chemical life of a grease at different temperatures. Summary and conclusions are given in Chapter six along with recommendations for future studies

Ball Bearing Mechanics

Load-deflection relationships for different types of elliptical contacts such as those found in a ball bearing are developed. Simplified expressions that allow quick calculations of deformation to be made simply from a knowledge of the applied load, the material properties, and the geometry of the contacting elements are presented. Ball bearings subjected to radial, thrust and combined ball loads are analyzed. A design criterion for fatigue life of ball bearings is developed. The section of a satisfactory lubricant, as well as describing systems that provide a constant flow of lubricant to the contact, is considered

Lubricant effects on bearing life

Lubricant considerations for rolling-element bearings have within the last two decades taken on added importance in the design and operation of mechanical systems. The phenomenon which limits the useful life of bearings is rolling-element or surface pitting fatigue. The elastohydrodynamic (EHD) film thickness which separates the ball or roller surface from those of the raceways of the bearing directly affects bearing life. Chemical additives added to the lubricant can also significantly affect bearings life and reliability. The interaction of these physical and chemical effects is important to the design engineer and user of these systems. Design methods and lubricant selection for rolling-element bearings are presented and discussed

Design of a test rig for grease endurance tests

Background The performance and endurance of lubricating greases is often assessed through model tests (like four-ball tests or SRV tests). This approach has some limitations, especially when greases with special fillers are investigated, because simplified laboratory test conditions can never replicate real operating conditions. Objective For a more reliable assessment on the performance of a lubricating grease, lubricants must be tested in close-to-actual operational conditions, e.g. through component tests. This paper presents both the design and implementation of a dedicated test rig to test lubricating greases through middle-sized thrust ball bearings. Methods The test rig design is optimized to monitor the effectiveness of lubrication and determine the useful life of grease by recording representative parameters, namely bearing temperature at different location in the bearing, vibration level, and electric contact resistance (ECR) through the bearing. Results The results of a couple of assessment tests are presented and confirm that the test rig is working as expected. Conclusions The chart of the parameters being monitored shows that the instant performance of grease lubrication can be tracked during the endurance tests, and grease failure identified. The special mechanical layout of the rig also allows the user to obtain additional information that is usually not available in other commercial greased bearing testers. This work is meant to be the first step of the development a new approach to evaluate the grease lubrication performance and a few experimental results are included as they are intended as a functional validation of the prototyped test rig

Statistical correlation between vibration characteristics, surface temperatures and service life of rolling bearings - artificially contaminated by open pit coal mine debris particles

Nowadays, two most often used methods for rolling bearings condition monitoring are thermographic inspection and vibrodiagno stic. However, analysis of the relevant literature has shown that so far there is no established correlation between rolling bearings surface temperatures and measured radial vibration intensities. These variables also strongly depend on rolling bearing's service life and it's environmental operating conditions, especially in case of open pit coal mine conveyor idler's rolling bearings; where high concentration of debris particles (such as surface dust, dirt and excavated coal) is present. Taking into account previously listed facts; the main goal of the presented research results was to establish statistically significant correlation between listed variables: rolling bearings surface temperatures, radial vibration intensities, service life and concentration level of debris particles in bearing grease. In order to achieve this goal, specific experimental methodology was developed and implemented. Obtained results were then processed using standardized statistical software and appropriate correlation was generated and later verified in praxis. At this moment, developed experimental methodology is applied only to open pit coal mine conveyor idler's rolling bearings, but its principles are universal, so with minor modifications it could be used for prediction of any of listed variables for different kinds of rolling bearings, operating in different environments

Production of high temperature grease from waste lubricant sludge and silicone oil

This research was carried out to study the production of high temperature grease from waste lubricant sludge and silicone oil. The effects of different ratios of waste lubricant sludge to fumed silica and mixing time on grease characteristics were investigated. The ratios of waste lubricant sludge and fumed silica used were 100:0, 90:10, 80:20, 70:30, 60:40, 50:50 and 40:60. In terms of mixing time, it was varied at 1, 2, 3, 4 and 5 hours. Throughout the experiment, the grease was prepared by using the heating and mixing technique. The grease produced was analysed according to the ASTM and NLGI standard via penetration test, dropping point test, copper corrosion test, FTIR and AAS.Fourier transform infra red (FTIR) analysis was carried out to study on the functional groups present in the grease. Atomic absorption spectroscopy analysis was also carried out to determine the concentration of a specific metal element in the grease. The results showed that the grease produced with a minimum ratio of fumed silica to sludge (10:90) was able to produce high temperature grease with a dropping point of 272.5.C.Further analysis verified that the grease is a high temperature grease, has worked penetration of 270-290 which makes the grease fall into the common grease category, has a low tendency to corrode copper and very little metal element was present in the grease. It can be concluded that the ratio of fumed silica to sludge and the mixing time of the grease produced significant effect on worked penetration and dropping point of the grease

Grease

Everybody has made use of grease in their daily lives. The word “grease” originates from the early Latin word “crassus,” meaning fat. For our purposes, in this Special Issue, we will be focusing on lubricating grease, for publication in the eponymous journal Lubricants. According to ASTM, lubricating grease may be defined as “a solid-to-semi-fluid product of dispersed thickening agents in a liquid lubricant”. Other functional ingredients, such anti-wear and extreme pressure additives, may be included, with the overall goal of inducing special properties/functionalities. Grease is a very complex lubricant. We have never had a Special Issue focusing on this key product, and lubricating greases are often underrepresented in the technical literature. In recent years, there has been significant progress in research on greases, ranging from the specific chemical formulation of greases for special applications to how grease interacts with various surfaces, tribological advances in grease properties, new techniques for grease property measurements, etc. Recently, greases have also been evolving, as they and play a key role in the lubrication of electric vehicles. We aim to select the top research avenues and papers worldwide related to lubricating greases to form this compilation. This Special Issue wishes to be the first of its kind, and we plan to make this an annual exercise, where our compendium aims to discuss the latest developments worldwide encompassing all areas related to greases