Tribological Performance of Some Pennzane®-Based Greases for Vacuum Applications

Commercial greases for space applications usually fulfill the requirements imposed by the severe conditions of use (load, ultrahigh vacuum, speed, etc.). The main requirement is their ability to create an elastohydrodynamic lubrication film, boundary film, or both, for the speed, load, and temperature conditions under which the mechanisms will operate. Three greases, all based on a multiply alkylated cyclopentane (Pennzane®) base oil, were studied. The thickeners were an n-octadecylterephthalamate soap, a lithium soap, and a urea derivative. A four-ball tribometer and a spiral-orbit tribometer were employed to evaluate the greases under ultrahigh vacuum. The results indicated that all three greases yielded very low wear rates and extended lifetimes. In addition, routine physical property data are reported for each grease.     

Friction torque in grease lubricated thrust ball bearings

Thrust ball bearings lubricated with several different greases were tested on a modified Four-Ball Machine, where the Four-Ball arrangement was replaced by a bearing assembly. The friction torque and operating temperatures in a thrust ball bearing were measured during the tests. At the end of each test a grease sample was analyzed through ferrographic techniques in order to quantify and evaluate bearing wear.A rolling bearing friction torque model was used and the coefficient of friction in full film lubrication was determined for each grease, depending on the operating conditions.The experimental results obtained showed that grease formulation had a very significant influence on friction torque and operating temperature. The friction torque depends on the viscosity of the grease base oil, on its nature (mineral, ester, PAO, etc.), on the coefficient of friction in full film conditions, but also on the interaction between grease thickener and base oil, which affected contact replenishment and contact starvation, and thus influenced the friction torque.     

Self-diffusion of oil in lubricating greases by NMR

Self-diffusion of base oil in six model lubricating greases and in the base oils themselves was measured using pulsed-field gradient nucleic magnetic resonance (NMR) at 23, 40, 70, and 90°C. Three of the greases were based on naphthenic mineral oils, and three were based on synthetic polyalphaolefin oils. Soaps of 12-hydroxy stearic acid were used as thickener agents in all six greases. The purpose of the investigation using NMR was to see if structure varies with temperature, and if NMR is a viable method for structure determination. The ratio between the diffusion coefficient of the base oil in the grease and the neat base oil in itself was evaluated as a function of temperature. This ratio was taken as a direct indicator of the obstruction effect that the thickener has on the oil. The thickener showed a stronger obstruction effect on the base oil in the synthetic grease than in the mineral-based grease. This was related to the soap content, which, for these greases, is roughly twice as high in the synthetic greases as in the mineral ones. The obstruction effect was constant in the temperature range 40–90°C for mineral- as well as synthetic-based greases. NMR was judged to be a promising method for investigating how the gellant affects the diffusion coefficient of oil in a lubricating grease.     

The Slow-Speed Wear Behavior of Case-Carburized Gears Lubricated with NLGI 00 Grease under Boundary Lubrication Conditions

NLGI 00 greases are often used to lubricate gears running at low pitch line velocities, such as, for example, in large open gear drives. At low pitch line velocities, sliding wear, which under these operating conditions is referred to as slow speed wear, is often the limiting factor to gear lifetime. A thorough knowledge of the effect of different grease components on the wear behavior is therefore important when selecting a grease to effectively reduce gear wear in a given gear drive. In order to systematically investigate and analyze the influence of different grease components on the slow-speed wear behavior of case-carburized gears, systematic gear tests using the Gear Research Center's (FZG) back-to-back gear test rig were conducted. Primarily, the focus of the experimental investigations is on the influence of the base oil viscosity and type, the additive type, and also the type of soap thickener on the gear wear behavior at low pitch line velocities. To experimentally determine the influence of these different grease components on the wear behavior of case-carburized gears, a modified, more stringent wear test, based on the standard DGMK slow-speed wear test for gear oils, was developed. Different NLGI 00 greases with base oil viscosities between ν₄₀ = 70 and 1,200 mm²/s were investigated.

Base oil type and base oil viscosity were shown to have only a minor effect on the wear behavior under boundary lubrication conditions. On the other hand, the thickener type and especially the additive type play an important role in determining the wear behavior.     

Influence of grease rheology on thrust ball bearings friction torque

The friction torque and the operating temperatures in a thrust ball bearing were measured for seven different types of greases, including three biodegradable greases having low toxicity. These friction torque tests were performed using a modified Four-Ball machine.Rheological evaluations of the lubricating greases were made using a rheometer. Bleed oils were extracted from the greases and the dynamic viscosities were measured.In order to compare the performance of the lubricant greases in terms of friction, the grease characteristics were related to experimental results, showing that the interaction between thickener and base oil have strong influences in the bearing friction torque.     

Grease lubrication of rolling element bearings — role of the grease thickener

The contribution of grease thickener to lubricant film formation was examined in this paper. Lubricant film thickness and friction were measured for different grease thickener types in a bearing simulation device. The results showed that the greases formed thick (20–80nm), low friction surface layers at low speeds, which were much greater than the corresponding base oil film. These films appeared to be formed by the physical deposition of thickener in the track during overrolling of the grease. This was confirmed by infrared reflection analysis, which showed the deposited films to have increased thickener content. The ability of grease to form renewable physically deposited solid films has significant implications for optimising lubricant formulation for certain applications, e.g. bearings operating at high temperatures and low speeds where a conventional elastohydrodynamic lubricating film would be inadequate. Copyright © 2007 John Wiley & Sons, Ltd.     

Synthetic aluminium complex grease: A tribological evaluation using a four‐ball test

This paper describes the preparation and performance evaluation of a synthetic aluminium complex grease. The aluminium complex grease was prepared in situ by reacting stearic acid and benzoic acid in the presence of a synthetic base oil, polyalphaolefin. The preparation of the soap (aluminium benzoate stearate hydroxide) was monitored using Fourier transform infrared. Two different extreme‐pressure additives blended with the prepared aluminium complex grease were evaluated for their extreme‐pressure activity in four‐ball and Timken tests. These greases exhibited higher Timken OK loads (60 lb), lower values of wear‐scar diameter at higher loads, and higher values of weld load in the four‐ball test than the aluminium complex grease without additive. The blended greases were also found to pass rust and corrosion, oxidation‐stability, and shear‐stability tests. The topography of the specimens after four‐ball testing was studied with scanning electron microscopy.     

Film Forming Behavior of Greases Under Starved and Fully Flooded EHL Conditions

Improving knowledge on the film forming behavior of greases is essential to be able to develop efficient greases. This article examines how operating conditions (e.g., temperature, lubrication condition [fully flooded/starved]) and base oil viscosity influence the film forming properties of greases by comparing the behavior of two lithium-based greases and their respective base oils in rolling point contact. It is found that the onset and degree of starvation is controlled by speed (u) × viscosity (ν)/load (W) factor (uν/W) and temperature and that low uν/W values promote entrainment of thickener into contact. Thus, grease with low base oil viscosity shows significant thickener entrainment in the low speed region compared to the one with high base oil viscosity, which leads to the formation of thickener-rich viscous material during extended running with the low base oil viscosity grease. The results suggest that the shape of the film thickness versus speed curve is viscosity and uν/W range dependent. Furthermore, for the test conditions used in this study, grease-lubricated contacts appear to shift from the initial fully flooded condition to starved condition over a prolonged running of 2 h. The results from this study concur with those reported in the literature that fully flooded oil elastohydrodynamic lubrication (EHL) theory or film thickness cannot be directly applied or taken as a guideline in grease-lubricated contacts.     

An assessment of beeswax as a thickener for environmentally friendly lubricating grease production

Environmentally friendly lubricants are preferred in many applications where groundwater or soil pollution is possible. The main purpose of the current study was to evaluate the possibility of beeswax use as a thickener for lubricating greases. European beeswax was used as a thickening material for the preparation of lubricating greases. Rapeseed oil and two mineral oils were used as base oils. The consistency, dropping point temperature, and tribological properties of the prepared lubricating greases were investigated and compared with conventional soap‐thickened greases. The prepared greases exhibited good tribological properties, especially the rapeseed‐oil‐based grease. However, tribological properties of conventional lubricating grease were superior to prepared ones. The weakness of the prepared lubricating greases is their narrow working temperature range. However, the beeswax together with rapeseed oil has great potential in the production of environmentally friendly, completely renewable lubricating greases. Copyright © 2014 John Wiley & Sons, Ltd.     

A study on tribological behaviours of ZDDP in polymer thickened lubricating greases

Zinc Dialkyl Dithiophosphate (ZDDP) is a well‐known multifunctional additive for soap based lubricating greases. Polymers are being studied for their performance as lubricating grease thickeners and rheology modifiers. In this work the tribological study of ZDDP was carried out in polypropylene (PP), maleated polypropylene (mPP) and linear low density polyethylene (PE) thickened lubricating greases. Performance of lithium grease was taken as a reference. The tribological properties were evaluated using a four ball tester for EP properties and Optimol SRV‐III machine for coefficient of friction. The mechanism of action of ZDDP in polymer greases was established through analysis of the worn surface of steel balls with scanning electron microscope (SEM) and energy dispersive x‐ray spectroscopy (EDAX). The results show that the performance of ZDDP depends on the thickener system. The antiwear and EP property exhibited by ZDDP varies from polymer to polymer. Poor performance was observed with PP type thickener whereas performance in PE and lithium grease was comparable. Copyright © 2015 John Wiley & Sons, Ltd.     

往复冲击对脂润滑成膜性能影响的试验研究

冲击现象广泛存在于工业链、滚动轴承等机械零部件中,严重的情况下会引起冲击磨损。为探究冲击载荷对脂润滑条件下成膜性能的影响,在点接触光干涉弹流试验台上对锂基脂润滑条件下的膜厚演化进行冲击试验研究。试验时钢球和玻璃盘的初始间隙设置为0,冲击载荷按三角波往复变化。结果发现：在第一个冲击周期内,接触区存在大块的增稠剂纤维团,该纤维团造成接触区内的脂膜凹陷;随着冲击周期的增加,接触区内的大块增稠剂纤维团消失,脂膜厚度逐渐降低,润滑状态进入到薄膜润滑状态,最后发生了表面损伤;在任何一个冲击周期内,中心膜厚和最小膜厚大部分的时间都呈现固定值;中心膜厚随着冲击周期数的增加而减小,最小膜厚在最初的100个周期内变化很小,此后逐渐降低,最后为0。     

Influence of oil polarity and material combination on the tribological response of greases formulated with biodegradable oils and bentonite and highly dispersed silica acid

Different biodegradable lubricating greases formulated with esters of fatty acids, as base oils, and bentonite and highly dispersed silica acid, as thickener agents, were tribologically investigated in a nanotribometer and compared with polyalpha olefin greases with equal thickeners. Material combinations of steel ball on steel disc and sapphire ball on steel disc were used with different normal loads. Several friction and wear effects were found depending on the thickener and the base oil. The influence of grease components is also different in both material combinations evaluated. On the one hand, the base oil exerts a much higher impact on friction and wear in grease systems thickened with highly dispersed silica acid than in those thickened with bentonite. On the other hand, the latter reacts more sensitively to a change in material combination. Results were discussed and explained on the basis of polarity influences of the base oils and solid surfaces. Copyright © 2012 John Wiley & Sons, Ltd.     

New fluorinated greases: A tribological evaluation

New fluorinated greases based on perfluoropolyether (PFPE) fluids and perfluorinated polymer thickeners were prepared. Both the type of fluid (branched and linear PFPE) and the thickener (polytetrafluoroethylene, PTFE) were varied, and different oil/thickener ratios were also evaluated. The greases were characterised in terms of penetration, oil separation, and wear. Some relationships regarding the influence of both the amount and the type of thickener used were found. Torque values of ball bearings lubricated with these greases were investigated at low temperature and at high bearing speed. Specifically, the use of a new PTFE grade obtained through micro‐emulsion polymerisation technology allowed the formulation of greases with a substantially reduced amount of thickener. These greases show improved antiwear properties as well as lower friction coefficients under high‐speed conditions, thus paving the way to their use in applications not generally fulfilled by fluorinated greases.     

Film Thickness of Mechanically Worked Lubricating Grease at Very Low Speeds

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.     

A New Form of Rolling Contact Damage in Grease-Lubricated,Deep-Groove Ball Bearings

A new form of rolling contact damage was discovered in the fatigue tests of series 6206 deep groove ball bearings under pure radial load with grease lubrication. The appearance of the damage reveals a few cracks at right angles to the rolling direction of the ball, along with the formation of a dent about 20 μ m deep in the raceway surface. This is found to occur only on the stationary outer ring raceway and the ball surface and is distributed widely within the load zone. Furthermore, under the raceway surface several cracks propagating into the substrate at an angle of about 60-80 degrees relative to the raceway surface are observed not only under the damage site but also in other nearby locations. Only the cracks at the damage position open up to the surface. The grease used for the test contained a lithium complex thickener with mineral oil as the base oil with a kinematic viscosity of 141 mm²/s at 40°C. On the test bearings two pure radial load levels of 9.14 and 12.13 kN were applied. In order to prevent the occurrence of seizure at each load, the speed of the inner ring of the test bearing was maintained at 1800-2500 min^{? 1} and 600-800 min^{? 1}, respectively, to keep the outer ring circumference temperature below 65°C. It is suggested that the damage is caused by metal-to-metal contacts due to lubricant starvation under grease lubrication and to a decrease in oil film thickness due to local increases in temperature.     

Impact of polyurea structure on grease properties

The thickener structure of polyurea grease has a crucial effect on its properties. Therefore, it is very important to study the relationship between a thickener structure and physical and performance properties. In this study, polyurea greases were synthesised, having a different number of urea groups in per polyurea thickener molecule. Properties such as dropping point, penetration, oil separation, shear stability and four‐ball wear and extreme pressure (EP) were evaluated. The result shows that properties of polyurea grease vary with the number of urea group per polyurea molecule. Based on molecular theory, the mechanisms explaining these effects are briefly discussed. Copyright © 2010 John Wiley & Sons, Ltd.     

Differences in Mechanisms for Fretting Wear Reduction between Oil and Grease Lubrication

The different mechanisms of fretting wear in oil and grease lubrication and methods to reduce fretting wear were examined by means of thrust ball bearings in this study. Tests of fretting wear under oil lubrication were conducted. It was confirmed that high-viscosity oil can reduce fretting wear at high velocity (i.e., high frequency) through oil film formation. In the case of grease lubrication, the influence of velocity on fretting wear was significantly different for low- and high-viscosity greases. Grease with low-viscosity base oils could reduce fretting wear at high velocity. In contrast, grease with high-viscosity base oils could reduce fretting wear at low velocity. Grease thickeners were found to be effective in forming a layer that could prevent fretting wear. These results highlight the large differences in effective fretting wear reduction mechanisms between oil and grease lubrication.     

Bearing Fatigue Life Tests of Two Advanced Base Oils for Space Applications Under Vacuum and Atmospheric Environments

Four series of rolling-element bearing fatigue tests were conducted with 51104 size thrust ball bearings with three balls made from SUJ2 (AISI 52100) steel lubricated with two advanced synthetic base oils used for space applications. The test lubricants were perfluoropolyether (PFPE) and multiply alkyated cyclopentane (MAC). Each oil was tested with bearings under vacuum and atmospheric environments. The bearings were tested at a maximum Hertzian stress of 4 GPa on the inner and outer races. The outer race was rotated at a speed of 250 rpm. A pool lubrication system was used. Fresh lubricant was used for each test bearing. Testing in vacuum conditions was at 5 × 10^?2 Pa. The test oils were analyzed to determine whether changes occurred as a result of operating in air and in a vacuum. In a vacuum environment, the PFPE 815Z oil exhibited a longer fatigue life than the MAC 2001A oil. However, in an air environment, the MAC 2001A oil had a longer L₁₀ fatigue life than the PFPE 815Z oil. The fatigue life tests of PFPE 815Z oil in vacuum resulted in a longer L₁₀ life than when tested in an air environment. In an air environment, hydrogen fluoride was generated in the bearing tests with the PFPE 815Z oils. Under vacuum conditions, hydrogen fluoride was not generated with the PFPE 815Z oil, resulting in longer bearing fatigue lives. The fatigue life tests of MAC 2001A oil in a vacuum resulted in shorter L₁₀ fatigue life than in an air environment. The shorter life was attributed to the lower elastohydrodynamic oil film formation with the MAC 2001A oil because of a higher operating temperature and decomposition of the oil in vacuum.     

聚脲润滑脂稠化剂结构与其性能的关系

以脂肪胺、芳香胺、脂环胺的不同组合和异氰酸酯为稠化剂,在不同的工艺条件下制备一系列的聚脲润滑脂;利用扫描电子显微镜(SEM)对润滑脂皂纤维微观结构进行表征;探讨不同的稠化剂组成、不同制备工艺对聚脲润滑脂的微观结构和性能的影响。结果表明:单纯均匀颗粒状稠化剂比纤维与颗粒状复合的稠化剂所制备的聚脲脂具有更优的性能;纤维结构以单根形式存在为主的聚脲润滑脂性能优于纤维结构为多根纤维聚集为一股,各股之间交织的聚脲润滑脂。     

Study of the Conductivity and Tribological Performance of Ionic Liquid and Lithium Greases

Ionic liquid (IL) lubricating greases were prepared using 1-hexyl-3-methylimidazolium tetrafluoroborate and 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl) amide as base oil and polytetrafluoroethylene (PTFE) as thickener, respectively. Three kinds of lithium greases were also prepared using lithium ILs ([Li(PAG)]X) as base oil and PTFE as thickener. 1-Ethyl-3-methyl imidazolium hexafluorophosphate as an additive was added to the PAG grease, which was prepared using polyalkylene glycol monobutyl ether (PAG) as base oil and PTFE as thickener. The conductivities and tribological properties of the prepared lubricating greases were investigated in detail. Scanning electron microscopy and X-ray photoelectron spectroscopy were employed to explore the friction and wear mechanism. The results showed that the IL and lithium lubricating greases have conductivities and excellent tribological properties. Especially, IL greases have the highest conductivity. The excellent tribological properties are attributed to the formation of boundary films consisting of both tribo-chemical reaction films and physical absorption films, while high conductivities are attributed to the intrinsic electric fields of the ILs.