Fresh Grease Research Articles

Experimental Study on In Situ Storage of Grease-Lubricated Ball Screws

Lubricating grease plays an important role in the rolling screw transmission of the actuation system and is particularly concerned with the performance stability of long-term storage in aerospace applications. In this article, a batch of ball screws that were lying flat in the warehouse for about eight years were selected to extract lubricating grease from the screw raceway after being stored in situ. The oxidation performance, friction performance and rheological properties of lubricating grease were tested and compared to those of fresh grease to analyze and summarize the performance changes in lubricating grease. The transmission efficiency, friction torque, and temperature rise of ball screws without removing the original grease and those filled with fresh grease after cleaning were tested and compared. The impact of grease degradation on the transmission performance of ball screws was analyzed to provide a reference for ball screw lubrication schemes and further accelerated storage experimental design.

Effects of Contamination on Selected Rheological and Tribological Properties of Lubricating Greases Working in Underground Mines

This study examines the effect of mining pollutants and wear products on the rheological and tribological properties of a lubricating grease working in the microclimate of the Polkowice-Sieroszowice mine belonging to the KGHM Polska Miedź Group (Polkowice, Poland). The material under investigation is a commercial lubricating grease thickened with complex lithium soap, based on mineral oil with a molybdenum disulfide (MoS2) addition. A sample of the grease was taken from one of the friction junctions of a self-propelled drilling jumbo operated in the mine. Comparative tests of the fresh grease and the spent grease were carried out. For the two greases, rheological tests, i.e., dynamic oscillation tests and tests in steady flow conditions, were carried out at a constant shear rate. The rheological tests were conducted using a rotational rheometer. Moreover, the tribological characteristics of the tested greases under different friction junction loads were carried out using a ball-on-disc tribometer. Besides friction resistance, the lubrication ability of the two greases was also evaluated through an analysis of the wear of the steel disks after the friction process. Contour and topographic maps of the wear traces of the discs together with their wear profiles were compared.

Grease Degradation Alternate Test Method and Storage Interval for Roller Bearings: An Empirical Research

Pandemic caused parts into prolong storage conditions. Empirical experiment was carried out to determine the viscoelastic behaviour and consistency of unworked grease from inside a wheel roller bearing. The grease samples intervals were 9, 10, 11 and 24 months from stored wheel roller bearings. Fresh grease is also tested as a controlled sample. A rheometer amplitude oscillation test was performed. The grease was tested for contaminants such as oxidation, nitration, soot, sulfation, and water using FTIR. Rheology test exhibited changes in viscoelastic behaviour. Grease consistency changed from grade 2 to grade 1. This confirms that the grease tends to soften and degrade over time. FTIR results revealed that there is no significant contaminant for unworked and packed grease in the wheel. Storage and maintenance interval derived from this study.

Effect of high-temperature thermal action on the film-forming properties of a lithium grease

PurposeThis paper aims to investigate the influence of high-temperature thermal action on grease performance from the angle of film-forming performance.Design/methodology/approachA static thermal aging method was used to prepare high-temperature thermal grease samples after high-temperature thermal action. On the basis of optical interference technology, the film-forming characteristics of fresh grease samples and the grease samples after high-temperature thermal action under variable speed and fixed speed conditions were explored.FindingsThe decrease in the structural entanglement performance of the grease after short-term high-temperature thermal action makes its film-forming performance better. The mechanism is that the lubricating grease soap fiber entanglement is reduced. Although the continuous high-temperature thermal action can make the grease film-forming performance better, its mechanism is that the soap fiber structure caused by high-temperature thermal action is damaged and is easy to be cut off under the action of shear.Originality/valueThe effect of structural system change on its film formation performance was discussed in combination with the change in grease structure characteristics, and the mechanism of action was revealed.

The Analysis on Grease Lubrication at Two Tapered Bodies Contact Considering Surface Roughness

Considering surface roughness and non-equal cross-section feature of tapered body and non-Newtonian characters of grease, fresh grease lubrication problem at two tapered bodies contact in the initial lubrication stage was analyzed, and effects of profile crowning, misalignment (tilting and skewing), sliding and loads were included. The multigrid method was applied to grease lubrication simulation by linear interpolation of rough surfaces and the discrete convolution and FFT (DC-FFT) was employed to compute elastic deformation. The results show that grease film thickness decreases with sliding aggravation. Both surface roughness and tilting motion will weaken the effectiveness of crowning and deteriorate the film bearing capacity. The adaptability of the tapered body with the cut-off profile to tilting motion is better than that with the logarithmic profile. However, if the cut-off profile is adopted, the crowning radius at small end of the tapered body should be larger than that at large end. The effect of minor skewing on grease lubrication is relatively slight. In a wide range of speed, the variation of traction coefficient is similar to the typical Stribeck curve, and the contact would experience three states of full-film, mixed and boundary lubrication.

Microstructure Characterization of Degraded Grease in Axle Roller Bearings

Aged mineral-oil-based, lithium-hydroxystearate-thickened grease was collected from used axle spherical roller bearings, which were used for urban commuter trains. Three types of aged grease were categorized based on the total distance that the bearing had traveled, as early life (<0.05 million km [M.km]), medium (0.3–0.6 M.km), and heavily used (1.82 and 2.0 M.km). The microstructure of degraded grease from 20 bearings was characterized in this project using atomic force microscopy (AFM) and scanning electron microscopy (SEM). Base oil of each grease sample was released by a centrifugation method and the oxidation of bled oil was measured by infrared analysis. A notable change in grease microstructure was observed by AFM and SEM. The thickener morphology changed from an entangled helical fibril structure to particle-like nanofragments. The variation in thickener microstructure was observed in the early life grease, which suggested grease can experience an inconsistent aging process in the real-life bearings. Overall, 70.5 ± 1.37 wt% base oil bled from the fresh grease, 68.2 ± 0.73 wt% from the early life grease, 58.1 ± 2.33 wt% from the medium used grease, and 60.0 ± 6.35 wt% from the heavily used grease. The results suggest that the densely packed microstructure observed in the aged grease affects the capability of base oil bleeding from the grease. The retardation of base oil flow due to degraded grease may result in the lubricating ability of grease becoming compromised and hence result in an increased wear rate of the bearings.

Experimental investigation of the chemical degradation of lubricating grease from an energy point of view

Abstract The chemical degradation/oxidation of lubricating grease is investigated experimentally from an energy point of view. A Pressure Differential Scanning Calorimeter (PDSC) is used to measure the activation energies of two lithium-complex greases and to monitor their chemical degradation through induction time. For this purpose, an experimental setup consisting of a heating chamber, a temperature controller and an energy meter is designed, built, and calibrated to heat up grease samples and measure pure energy absorption during the chemical degradation at different test temperatures. A relationship between the energy absorption of the grease and its chemical degradation is established that is independent of the testing time and temperature. It is shown that the energy absorption of the grease approaches to an identical maximum value in a shorter time in high temperatures and a longer time in low temperatures. The maximum energy absorption depends on the grease type and experimental results reveal that it can be used to estimate the grease chemical life. A roller tester rig is used to measure and compare the lubrication ability of two chemically degraded grease samples and to validate the results of the research. To gain further insight, experimental results are presented to examine the flow characteristics of a chemically degraded grease in an elastohydrodynamic (EHL) line contact with a comparison to that of the fresh grease.

New method for lubricating wind turbine pitch gears using embedded micro-nozzles

The increase of power generated by wind turbines has increased the stresses applied in all of its components, thereby causing premature failures. Particularly, pitch and yaw gears suffer from excessive wear mainly caused by inappropriate lubrication. This paper presents a novel method to automatically lubricate the wind turbine pitch gear during operation. A micro-nozzle to inject fresh grease continuously between the teeth in contact was designed, manufactured, and installed in a test bench of a 2 MW wind turbine pitch system. The test bench was used to characterize the fatigue behavior of the gear surface using conventional wind turbine greases under real cyclic loads. Measurements of wear evolution in a pitch gear with and without micro-nozzle show a decrease of 70 % of the wear coefficient after 2×104 cycles.

Formulation, rheology and thermal ageing of polymer greases—Part I: Influence of the thickener content

The aim of this work is to show the correlation between polymer greases’ rheology and its formulation. The tested polypropylene (PP) thickened greases were evaluated regarding their thickener content and its effect on the rheological properties. An artificial ageing procedure was performed by heating fresh grease samples in an oven to study the thermal degradation. The ageing evaluation was performed through rheological measurements, FT-IR spectra, oil loss, bleed-oil viscosity changes and bleed rate. The rheology measurements were performed on a rotational rheometer, emphasizing on the storage and loss moduli values at the Linear Visco-Elastic (LVE) region. The flow curve of each grease was also measured. A modified Herschel–Buckley model was applied and the data was correlated to the thickener content.

쾌속조형기에 의한 기어식 주유장치의 개발

An automatic grease lubricator provides an adequate amount of fresh grease constantly to any type of rotating machine to minimize friction heat and reduce friction loss. This study seeks to develop an automatic grease lubricator by means of rapid prototyping with a gear-driven mechanism and a controlled operation time. The ultimate design is to lubricate an adequate amount of grease by a simple dip-switch clicking mechanism according to an advanced set cycle. The backlash of the gear was minimized to increase the power, and to increase the power of the mechanism, the binding frequency and the thickness of the coil were changed. To control the rotation cycles of the main shaft according to certain set numbers, different resistances and chips were used in the design of the circuit which controls the electrical signals via a pulse. A digital mock-up was analyzed and the rapid prototyping (RP) trial products were tested with a PCB circuit and grease. An evaluation of the outlet capacity of RP trial products was conducted, as the friction caused by the outlet on the wall surface was an important factor in the operation of the equipment. Finally, a finishing process was applied to decrease the roughness of a surface to a comparable level to test the performance of the product.

Detection of Elemental Composition of Lubricating Grease Using Laser Induced Breakdown Spectroscopy

The elemental composition of lubricating soft grease used in rail engines are studied using laser induced breakdown spectroscopy (LIBS) technique. LIBS spectra of fresh, partially used and fully used grease samples are recorded using time-gated ICCD spectrometer for verification of compositional degradation of the used grease. LIBS spectra of grease samples are analyzed by comparing with emission spectra of elements published by NIST standard database. Many spectral lines of impurity elements like Fe, Cu, Ba, Mg, Mn, Ni, S, Zn, Si, Pb, Ti, Ca and Al present in the grease in ppm or ppb level in trace level concentrations are observed in excess in the used grease mainly due to wear and tear. On the other hand in fresh grease, spectral lines of Ca, Al and Na are observed predominantly.

Ageing Evaluation of No.2 Lithium Lubricating Grease

Contamination, acid number, moisture, thickness and dropping point are summarized as ageing indexes of lubricating grease. Size distribution, composition and weight of contamination which influence lubricating performance most and are used to evaluate performance of lubricating grease broadly in No.2 lithium lubricating grease which is used in main clutchs release ring of a certain type vehicle were analyzed after a vehicle which had run for about 20000 kilometers. Results showed that, the number of large size particle (larger than 25 μm) in aged grease was much larger than that in fresh grease, concentration of element Fe, Cr, Ni, Si, P, Ti, Mo, V, Na, Ca in aged grease was much higher than that in fresh grease, weight percentage contamination occupied in aged grease was very higher.

Fractal Analysis for Vibrational Signals Created in a Ball-Screw Machine Operating in Short- and Long-Range Tribological Tests

In the present study, the vibrational and frictional torque signals acquired from the forward-backward movements of a commercial ball-screw system were considered via mono fractal analysis. The short-range tests were carried out in order to investigate the effects of operating conditions, a nut's inner surface roughness and the applied pretension (preload) on the fractal dimension (Ds) and topothesy (G). The long-range test was conducted to observe the variations of vibrational and frictional torque signals and thus the fractal parameters acquired from the ball-screw operations under the condition of no fresh grease supply during the testing process. The effects of the ball-screw rotational speed and pretension on the G parameter of vibrations were greater than the Ds parameter. In the backward movement, the highest G value always occurred at the highest rotational speed (3000 rpm in this study). The Ds parameter generated in the forward movement by the nut's inner surface before polishing produced a value greater than that by the nut with a polished surface. The G parameter related to vibrational amplitudes showed a value before polishing greater than that after polishing. The unusual vibrational signals are assumed to be related to ball passing behavior. Their experimental frequency was verified to be consistent with the frequency predicted by the ball pass theory. An increase in the rotational speed can bring a significant increase in the number of ball-pass signals. The G parameter and its skewness data, defined for the number distribution function of the G peaks, showed values that in general increased with the test time if the fresh grease was not supplied during the long-range test.

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.

Grease Degradation in R0F Bearing Tests

This paper is the second in a series that examines grease lubrication mechanisms and failure in rolling element bearings. The aim of the work was to understand the grease condition changes during use and the relationship to lubrication performance and failure. R0F bearing tests were carried out with two lithium hydroxystearate greases and the effects of the temperature, the speed, and the additive package on lubrication life was studied. Post-test, one pair of bearings (fail and non-fail) was dismantled and grease distribution and condition assessed. IR spectroscopy was then used to determine the lubricant composition and the oxidation level of the grease remaining on the shields, the inner raceway, and the cage pockets. The additive package increased the grease life by 100-700% depending on the test condition. Most of the grease remaining in the bearing was found on the shields, with only trace amounts in the cage pockets or close to the rolling track. The IR analysis showed that the composition of the shield sample was similar to the fresh grease although the base oil oxidation was evident and this increased with the running time. The cage pocket and inner raceway films contained a number of chemical species; these included the base oil and the thickener and their oxidation products. The study concludes that after an initial running-in period the “active” lubricant is heavily degraded grease, which contains oxidized species from the base oil and the thickener. Different failure mechanisms are identified depending on the test condition. High-speed tests that fail relatively quickly are due to poor boundary lubrication performance or cage failure rather than the lubricant reaching its “oxidation” limit. Long-term tests at slower speeds suffer considerable base oil oxidation. Under these conditions, failure is due to a reduction in the amount and/or mobility of the raceway lubricant.

An Investigation into Grease Behavior in Thermal EHL Circular Contacts

The behaviors of two lithium lubricating greases were investigated under EHL circular contact through measurements of traction coefficients on a self-made rig in which the contact was continuously fed with fresh grease. The average values of Erying shear stress and shear modulus of the two lithium greases were obtained from traction experimental data using this rig. Based on the Evans-Johnson model and thermal analysis, we calculated the values of shear stress and traction coefficients of the two greases. The results show that the calculated traction coefficients agree fairly well with the measured data.

Elastohydrodynamic Grease Lubrication Theory and Numerical Solution in Line Contacts

In this paper, film thickness and pressure distribution of EHL in line contacts for four kinds of greases and one type of oil are calculated under a series of rolling velocities by using an inverse solution method. The Herschel-Bulkley model is used for the rheological characteristics of the grease. Based on the theoretical analysis and numerical calculation, the following influences on film thickness and pressure distribution in EHL of grease are analyzed: the effects of starvation, yield stress, presheared grease and fresh grease, and approach and separation between two cylinders. The effect of different rheological characteristics of grease on film thickness is also investigated. Presented as a Society of Tribologists and Lubrication Engineers paper at the ASME/STLE Tribology Conference in San Diego, California, October 19-21, 1992

Flow of Greases Under the Action of Complex Shear

Experiments were performed in a coaxial cylinder viscometer in which there were realized separately and simultaneously flows of grease and bright stock (Newtonian fluid) in the axial and peripheral direction. The latter was at uniform shear stress field. The effect of peripheral on axial-flows was a great decrease in resistance toward deformation in the axial direction. This is a very important peculiarity of greases, which are non-Newtonian media, when considering their flow in labyrinth boxes and some other devices. The effect of axial on peripheral flow showed somewhat increased resistance toward the latter. Such behavior was attributed to continuous inflow of fresh grease into the annular space where it did not remain long enough for structural destruction to reach a limiting value.