Elastohydrodynamic properties of water-based fire-resistant hydraulic fluids

The elastohydrodynamic (ehd) lubricating properties of commercial fire-resistant hydraulic fluids in rolling point contacts are described. Earlier work is reviewed and new work presented on water-in-oil and oil-in-water emulsions. It is shown that in general, elastohydrodynamic film thicknesses of water-in-oil emulsions are close to those of their base oils. For very fine particle size emulsions, however, thicker films are observed. Commercial oil-in-water emulsions in fully flooded conditions have not been found to give measurable elastohydrodynamic films. However, when the emulsions are destabilized, a localized pool of oil collects in the contact zone which enables ehd films to form, though these films do not survive very high rolling speeds. Polyglycol solutions give low ehd film thicknesses, approximately one third those of mineral oils of corresponding viscosity. This can be attributed to the low pressure—viscosity coefficient of these solutions.     

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.     

Numerical analysis of grease thermal elastohydrodynamic lubrication problems using the Herschel-Bulkley model

Grease thermal elastohydrodynamic lubrication (TEHL) problems of line contacts are analysed numerically. The effects of temperature and rheological parameters on grease TEHL are investigated using the Herschel-Bulkley model as a rheological model of greases. The pressure distribution, the shape of grease film, mean film temperature and surface temperature of solid wall in line contacts are obtained. It is found that thermal effects on the minimum film thickness become remarkable at high rolling speeds. The effect of yield stress of the Herschel-Bulkley model on minimum film thickness is negligible, while the flow index and viscosity parameter have significant effects on minimum film thickness.     

Lubricant Replenishment in Rolling Bearing Contacts

It is generally assumed that starved lubricated contacts in rolling element bearings are replenished by side flow. The rolling elements partly push the lubricant to the side of the contacts after which the re-flow will provide fresh lubricant for the next rolling element. This re-flow is driven by surface tension and restricted by the viscosity. Hence, thick oils or greases may yield problems here, which is generally observed in practice. This paper quantifies this re-flow by means of a numerical model using a so-called thin layer/film assumptions. The results here show that this form of replenishment may happen to some extent in single contacts but for sure not in rolling element bearings. There are two reasons for this: the time between successive overrollings is too short and secondly, the centrifugal effects on the inner ring will drive the flow in a vertical direction rather than transversely. This applies to cylindrical roller bearings (CRB)-type of surfaces. This may be different for tapered or spherical bearings, which should be the topic of future research.     

Impact of Water on the Rheology of Lubricating Greases

The operational life of bearings is often determined by the performance of the lubricating grease. The consistency of the grease prevents it from leaking out of the bearing and provides good sealing properties. The possible ingress of water into the bearing will have a considerable impact not only on this consistency but also on the lubricating ability of the grease. There are numerous applications where water ingress may occur, such as in the steel, food, pulp, and paper industries. Some greases are less sensitive to water than others. No specific guidelines are available to select the proper grease for bearings subjected to water ingress. The goal of the article is to contribute to the development of such guidelines for greases subjected to water ingress by studying the impact of water on grease rheology. Fully formulated, commercially available greases with the most common thickeners and base oils are used as model greases. It will be shown that water strongly influences rheological properties such as zero-shear viscosity, yield stress, and storage modulus. Calcium sulfonate greases were found to become stiffer after absorbing a considerable amount of water, leading to an increase in zero-shear viscosity and yield stress. However, lithium, lithium complex, and polyurea greases were found to soften, with appreciable changes in measured rheological properties.     

Conductive and Tribological Properties of Lithium-Based Ionic Liquids as Grease Base Oil

This article reports several conductive greases prepared by ionic liquids (ILs) that are synthesized by mixing lithium tetrafluoroborate (LiBF₄) or lithium bis(trifluoromethane-sulfonyl) imide (LiNTf₂) in diglyme (G2) and tetraglyme (G4) with appropriate weight ratios at room temperature (RT). The ILs have good solution in poly(ethylene glycol-ran-propylene glycol) monobutyl ether (PAG) and thus can be used as a base oil for preparing grease for steel–steel contacts. The electrical conductive properties of the grease prepared with the mixed oil of PAG plus ILs were evaluated using the DDSJ-308A conductivity tester, GEST-121 volume surface resistance tester, and HLY-200A circuit resistance tester. Combining the free volume with viscosity, the conductivity is inversely proportional to viscosity. The tribological properties were investigated using an MFT-R4000 reciprocating friction and wear tester. The results demonstrated that the prepared greases possess better conductive and tribological properties than the commercial grease with Cu powder as an additive.     

Rheological properties of lithium,lithium complex,and sodium greases

The rheological properties of two lithium, two lithium complex, and two sodium greases, all with the most common NLGI grade two, have been investigated. The greases are based on a mineral and a synthetic oil. The apparatus used was a cone-and-plate rheometer and an impacting-ball apparatus. The impacting-ball apparatus used a steel ball, which impacted a lubricated sintered carbide plate, to measure the shear stress-pressure coefficient, γ, of the lubricant. At pressures found in elastohydrodynamic contacts this coefficient determined the limiting shear stress. The γ-value thus affects the coefficient of friction and consequently, lower γ-value means less friction. The results from the impacting-ball apparatus showed that the γ-value was lower for the greases with a synthetic base oil and that the lithium greases gave a lower γ-value than the corresponding base oils. Results from the cone-and-plate rheometer showed the characteristic shear thinning behaviour of the greases and the influence of shear history and temperature. The results from the cone-and-plate rheometer have also been fitted to a four parameter rheological model.     

蓖麻油基含氧化石墨烯润滑脂的制备及其性能研究

以蓖麻油为基础油,以一水氢氧化锂、12-羟基硬脂酸和癸二酸为稠化剂原料,氧化石墨烯(GO)为添加剂,制备3种含GO不同质量分数的混合锂基脂,分别利用锥入度试验器、钢网分油测试仪、滴点试验器、流变仪、热重分析仪、摩擦试验机和三维形貌仪考察其理化性能和润滑性能,探讨GO对蓖麻油基润滑脂的作用机制。结果表明:含GO混合锂基脂的黏度随剪切率和温度的增加均呈非线性减小的变化趋势,但随着剪切速率和温度的升高,GO对蓖麻油基润滑脂的黏度与剪切速率关系和黏温特性影响越来越小;微量的GO能够提高混合脂的热稳定性能;混合锂基脂的平均摩擦因数随GO质量分数的增加呈先减小后增大的变化趋势,适量的GO有助于提高蓖麻油基润滑脂的减摩抗磨性能,这是因为摩擦过程中GO以片层形式进入摩擦副的接触区,能够有效地降低摩擦副表面的直接接触概率,且GO具有自润滑性能,在摩擦副界面上发挥较为有效的润滑作用。     

Producing multigrade lubricating greases from animal and vegetable fat by‐products

This paper reports on some work concerned with the synthesis and characterisation of three lubricating greases based on animal bone fat, cottonseed soapstock, and a mixture of them both. The physico‐chemical properties of these fats were established and showed them to be good thickeners in the preparation of different types of greases. It was shown that the growth of the soap structure and grease consistency depend on the type of fat as well as on the viscosity of the lubricating oil in the grease formulation. The greases so synthesised were assessed in accordance with standard NLGI grades and Egyptian standards. Their rheological and thermal behaviour was studied, which revealed that the synthesised greases have significant yield stress at normal operating temperature and good flow properties. It was concluded that the above mentioned fat by‐products are suitable for producing greases.     

Calculated Performance of Greases in Journal Bearings

A digital computer analysis has been made of the effects of non-Newtonianism on the performance of greases in finite width journal bearings. As anticipated, t he extreme nonlinearity of the shear rate-shear stress relation exhibited by greases makes their deviations from “normal” behavior very marked. Among the effects noted are (1) apparent “anisotropic viscosities,” i.e., viscosities in the side leakage direction as much as 30 times as high as those in the direction of motion, (2) reduced side leakage, (3) flatter pressure profiles, (4) reduced sensitivity of load and friction to speed changes relative to Newtonian lubricants, and (5) the occurrence of “cores” of plug flow. Theoretically interesting is the fact that the apparent axial viscosity approaches the true viscosity while the circumferential viscosity approaches the slope of the flow curve. Many of the conclusions obtained here, especially the advantages of greases at low speeds, are confirmed in practice.     

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.     

Film Formation in EHL Point Contacts under Zero Entraining Velocity Conditions

The contacts of adjacent balls in a retainerless bearing are subjected to the zero entrainment velocity (ZEV). The existence of an effective elastohydrodynamic lubrication (EHL) film between contacts running under ZEV conditions has long been proven experimentally. However, the classical EHL theory predicts a zero film thickness under ZEV conditions. Mechanisms, such as the thermal viscosity wedge effect and immobile film theory, have been proposed to tentatively explain the phenomenon. However, detailed numerical results are needed to provide theoretical evidence for such film formations. This paper aims to simulate, based on the viscosity wedge mechanism, the film formation of EHL point contacts under ZEV conditions. Complete numerical solutions have been successfully obtained. The results show that the thermal viscosity wedge induces a concave film profile, instead of a parallel film (Hertzian) as postulated by some previous researchers. By the simulation solver developed, the variation of film thickness with loads, oil supply conditions and ellipticity parameters have been investigated. Some unique lubrication behaviors under ZEV conditions are demonstrated. Furthermore, preliminary quantitative comparisons with the latest optical EHL experiments are finished. Both results are in good correlation.     

Rheological Properties of Extreme Pressure Greases Measured Using a Process Control Rheometer

A new process control rheometer (PCR) designed for use in industrial process flows has been used to measure the rheological properties of three extreme-pressure greases. The rheometer is a robust yet sensitive instrument designed to operate in an industrial processing environment in either in-line or on-line configurations. The PCR was able to measure rheological properties including the elastic modulus, viscous modulus, and complex viscosity of the greases which in an industrial flow application could be used as variables in a feedback system to control the process and the quality of the product.     

脂润滑轴承静置状态下漏油机理及对策

脂润滑轴承在静置状态下的漏油问题一直是一个难题。枯文旨在地润滑脂的分油及基础油在固体壁面的迁移分析，来探求脂润滑轴承润滑剂泄漏机理；同时介绍了在润滑脂中加入氟表面活性剂、改善轴承壁面结构设计两项措施来降低脂润滑轴承在静置状态下的漏油。     

红外波谱技术在轴承润滑脂分析上的应用

探讨了红外波谱分析技术在轴承润滑脂分析及鉴别中的应用。除了分析脂中各类基础油、添加剂及稠化剂的特征波谱外，还讨论了利用波谱技术的结果来表征轴承润滑脂化学劣变参数，研究不同环境、试验条件及各类污染物对脂润滑机械部件失效的影响。波谱分析技术是一种快速、可靠和极为有效地分析手段，可应用于各类脂润滑的磨损监测及轴承的失效分析。     

Tribological Properties of Polytetrafluoroethylene,Nano-Titanium Dioxide,and Nano-Silicon Dioxide as Additives in Mixed Oil-Based Titanium Complex Grease

Eight titanium complex greases (i.e., benzoic acid/stearic acid and sebacic acid/stearic acid titanium complex greases, and greases containing PTFE, or nano-titanium dioxide, or nano-silicon dioxide) were synthesized using 3-L reaction vessel. Their physical characteristics were characterized and their tribological properties were evaluated by using a four-ball tester. Chemical compositions of the boundary films generated on worn surfaces were analyzed with the use of scanning electron microscope and X-ray photoelectron spectrometer. Results show titanium complex greases containing PTFE, or nano-titanium dioxide, or nano-silicon dioxide exhibited excellent tribological performance. The mechanism for the friction-reduction and antiwear effect of all titanium complex greases was also proposed and discussed from a tribochemistry point of view.     

Comparison of the effects of the lubricant-molecule chain length and the viscosity on the friction and wear of diamond-like-carbon coatings and steel

For steel contacts it is usual for the longer molecular chain lengths of saturated linear hydrocarbons and their acids and alcohols to reduce the coefficient of friction in the boundary-lubrication regime. However, the effect of these lubricant properties on DLC contacts is still unknown. Since the boundary-lubrication mechanisms between DLC coatings and conventional additives do not appear to be as effective as with metals, other potential mechanisms, even though based on weaker interactions or the oil's physical and rheological properties, may thus be very relevant. In this study we focus on the influence of the base oil's chain length and viscosity on the friction and wear in DLC/DLC contacts, and we compare this behaviour with conventional steel/steel contacts, using several simple linear hydrocarbons, i.e., alkanes, and complex branched hydrocarbons, i.e., polyalphaolefins. The results show that in both the steel/steel and DLC/DLC contacts the wear decreases with a longer molecular chain length and a higher viscosity of the oil. However, in DLC/DLC contacts the coefficient of friction increases when oil with a longer molecular chain length or a higher viscosity is used, and decreases with the lower oil viscosity and shorter chain lengths, which is just the opposite to conventional steel/steel behaviour. These results are analysed and discussed in view of lubricant cohesive energy, surface tension, shear strength, viscosity and chain length.     

The grease squeeze film

This paper studies the load capacity and power dissipated by friction for grease squeeze films. The problem is solved both analytically (approximately) and numerically. The results are in agreement with experimental observation regarding the increased damping capacity of oils compared with that of greases having the same viscosity.     

A study of greases based on polyureas

This paper illustrates the thickening effects of diureas and tetraureas formed from monoamines, diamines and diisocyanates on the mineral oils of different viscosity, and on the esters and polysiloxanes of different structures. It also reviews the properties and applications of such greases based on polyureas.     

半流体脂的流变性与润滑机理研究

半流体脂由于其独特的流变性，兼具有润滑油与脂的优点而获得越来越广泛的应用。本文利用毛细管粘度计和ＲＶ２型流变仪研究了半流体脂的弹流润滑机理。