Analysis of Viscosity Interaction and Heat Transfer on the Dual Conical-Cylindrical Bearing©

Viscosity is an essential property in hydrodynamic lubrication. In general, the lubricant is not considered to have uniform viscosity within a given bearing. The viscosity of the lubricant is affected by both pressure and temperature. The viscosity of the lubricant increases with pressure and, for most lubricants, this effect is much larger than that of temperature or shear when the pressure is significantly above atmospheric pressure. This study analyzes the thermal effect of dual conical-cylindrical bearing performance parameters via the viscosity-pressure-temperature relationships of lubricants. The results reveal that pressure increases both the film viscosity and temperature.     

水润滑动静压滑动轴承的临界温度研究

建立水润滑动静压滑动轴承的弹流润滑几何模型,采用考虑热效应的Reynolds方程,对水润滑动静压滑动轴承进行弹流理论分析,计算一定供水压力下当润滑剂局部最高温度达到临界值时不同的速度、载荷极限值;分析润滑介质、轴瓦材料以及供水压力变化对速度、载荷达到极限值时的临界温度曲线的影响,得到润滑剂局部最高温度达到临界温度时的轴承速度和载荷极限值的临界温度曲线,并通过拟合得到曲线的函数表达式。研究结果显示:海水润滑下的临界温度曲线高于纯水润滑,塑料轴瓦的临界温度曲线高于陶瓷轴瓦;供水压力对不同轴瓦材料的临界温度曲线变化趋势影响不同,对不同润滑介质的临界温度曲线的变化范围影响不同。     

Properties and Performance of Gas-Expanded Lubricants in Tilting Pad Journal Bearings

Lubricants enable proper function and reduce friction in rotating machinery, but they can also contribute to power loss and heat buildup. Gas-expanded lubricants (GELs) have been proposed as tunable mixtures of lubricant and CO₂ under pressure with properties such as viscosity that can be controlled directly in response to changing environmental or rotordynamic conditions. In this work, experimental results of GEL viscosity, gas diffusivity, and thermal conductivity were combined with high-pressure phase equilibrium data to understand how these mixtures will behave in tilting pad journal bearings under a range of industry-relevant high-speed conditions. Simulations were carried out using the experimental data as inputs to a thermoelastohydrodynamic model of tilting pad journal bearing performance. Viscosity could be easily tuned by controlling the composition of the GEL and the effect on bearing efficiency was appreciable, with 14–46% improvements in power loss. This trend held for a range of lubricant chemistries with polyalkylene glycols, polyalpha olefins, and a polyol ester tested in this work. Diffusivity, which drives how readily CO₂ and lubricants form homogenous mixtures, was found to be a function of the viscosity of the synthetic lubricant, with more viscous lubricants having a lower diffusivity than less viscous formulations. Model results for a bearing in a pressurized housing suggested that cavitation would be minimal for a range of speed conditions. Other bearing parameters, such as eccentricity, temperature, and minimum film thickness were relatively unchanged between conventionally lubricated and GEL-lubricated bearings, suggesting that the efficiency improvements could be achieved with few performance tradeoffs.     

Performance of flexible shell journal bearings with variable viscosity lubricants

In heavily loaded rotating machines, both the deformation of the elastic bearing shell and the dependence of lubricant viscosity on pressure become significant and may result in an appreciable change in the performance of the journal bearing system. In this paper, stable solutions for bearing deformation and the lubricant flow field are obtained which combine the effects of the elastic deformation of the bearing shell with the pressure-viscosity dependence of the lubricant. Two elastic models were tried for deformation calculations in the bearing. One which was computationally economical and consistent in accuracy was adopted for the detailed computation. The effects of bearing deformation on the performance characteristics of the journal bearing system are reported for both isoviscous and variable viscosity lubricants.     

动载荷下径向轴承的非牛顿介质润滑

为分析动载荷条件下非牛顿介质的流变特性对径向轴承润滑效果的影响，推导了相应的雷诺方程。在方程中用差分粘度和第一正应力差函数表征非牛顿介质的流变特性，用挤压项表示动载荷作用。轴承润滑的数值计算结果表明，差分粘度的变化是影响承载力的主要因素，它取决于非牛顿介质的动态参数和剪切频率范围，使得非牛顿介质润滑的承载力并不总是高于或低于牛顿介质。在动载荷条件下，第一正应力差效应明显增强了油膜压力和承载力，并对轴心轨迹产生影响。     

Pressure viscosity coefficients and traction properties of synthetic lubricants for wind turbine gear systems

Synthetic lubricants are increasingly used to provide equipment reliability for wind turbine gear boxes. The majority of synthetic lubricants used today are based on polyalphaolefins. In gear systems where contact pressures are high, the pressure viscosity coefficient and traction values of the lubricant are important fundamental properties. A comparison of these properties for a wind turbine lubricant based on a polyalphaolefin and two lubricants based on polyalkylene glycols has been undertaken. Pressure viscosity coefficients were calculated from viscosity measurements made using an ultra‐high pressure falling needle viscometer at pressures up to 50 000 psi. Significant differences in properties were observed with both polyalkylene glycol lubricants showing lower pressure viscosity coefficients and much lower traction values. A calculation of the film thickness values in the Hertzian contact zone suggests that polyalkylene glycol lubricants may provide elastohydrodynamic films that are approximately 25% thicker than polyalphaolefin lubricants. Copyright © 2012 John Wiley & Sons, Ltd.     

Effects of ultra-thin liquid lubricant films on contact slider dynamics in hard-disk drives

This paper describes the effects of ultra-thin liquid lubricant films on contact slider dynamics in hard-disk drives. In the experiments, the contact slider dynamics as well as ultra-thin liquid lubricants behavior are investigated using three types of lubricants, which have different end-groups and molecular weight as a function of lubricant film thickness. The dynamics of a contact slider is mainly monitored using acoustic emission (AE). The disks are also examined with a scanning micro-ellipsometer before and after contact slider experiments. It is found that the lubricant film thickness instability occurs as a result of slider–disk contacts, when the lubricant film thickness is thicker than one monolayer. Their unstable lubricant behavior depends on the chemical structure of functional end-groups and molecular weight. In addition, it is also found that the AE RMS values, which indicate the contact slider dynamics, are almost same, independent of the end-groups and molecular weight for the lubricants, when the lubricant film thickness is approximately one monolayer. The molecular weight, however, affects the contact slider dynamics, when the lubricant film thickness is less than one monolayer. In other words, the AE RMS values increase remarkably as the molecular weight for the lubricant increases. When the lubricant film thickness is more than one monolayer, the AE RMS values decrease because of the effect of mobile lubricant layer, while the lubricant instability affects the contact slider dynamics. Therefore, it may be concluded that the lubricant film thickness should be designed to be approximately one monolayer thickness region in order to achieve contact recording for future head–disk interface.     

Influence of lubricants on the performance of journal bearings – a review

ABSTRACT

The lubricant properties have a significant influence on the static and dynamic performance characteristics of journal bearing such as load-carrying capability, minimum fluid film thickness, maximum pressure, lubricant flow rate, damping coefficients, stiffness coefficients, etc. The present document reviews the behaviour of various lubricants such as power-law lubricants, couple stress lubricants, micropolar lubricants, ionic liquid lubricants and space lubricants. The influence of these lubricants on the performance of hydrostatic, hydrodynamic and hybrid journal bearings is discussed. An effort is made to develop the understanding to choose the suitable lubricant for journal bearings for different journal bearing configurations. Journal bearings operated with non-Newtonian lubricants have shown better performance compared to Newtonian lubricants. Ionic liquid lubricants have shown high potential in vacuum applications and extreme temperature environment such as in bearings of spacecraft moving mechanical assemblies.     

Lubricant Flow and Evaporation Model for Heat-Assisted Magnetic Recording Including Functional End-Group Effects and Thin Film Viscosity

The lubricant covering a hard disk in a heat-assisted magnetic recording drive must be able to withstand the writing process in which the disk is locally heated several hundred degrees Celsius within a few nanoseconds to reduce the coercivity of the media and allow writing of data. As a first step in modeling a robust lubricant, we have developed a simulation tool based on continuum theory that incorporates previously proposed variations of viscosity and an additional component of disjoining pressure due to functional end-groups with film thickness. Here we apply this simulation tool to a conventional perfluoropolyether lubricant, Zdol 2000, for which there exists experimental data. The simulation tool can be used equally well for other lubricants once their properties become known. Simulations at small length and time scales that are unobservable with current experimental capabilities are performed. We investigate the effect of the total disjoining pressure and thin film viscosity on evaporation and lubricant flow for different initial thickness. For films thicker than 1 nm, the inclusion of polar disjoining pressure suppresses the lubricant thickness change due to evaporation and thermocapillary shear stress compared with cases without this component. Thin film viscosity is an important property to consider for thinner lubricants. We also consider how lubricant depletion depends on laser spot size and thermal spot maximum temperature. The smaller spot profiles exhibit side ridges due to thermocapillary shear stress while the larger spot profiles show no side ridges, only a trough due to evaporation. The lubricant depletion zone width and depth increase with increasing thermal spot maximum temperature.     

A study of optimum load capacity of slider bearings lubricated with power law fluids

A general theoretical study of an infinitely wide lubricated slider bearing is presented, considering the lubricant to be an incompressible, isothermal, power law fluid. A set of algebraic equations is developed to obtain the pressure gradient for any value of the power law index. To illustrate the mathematical development, the set of equations is used to calculate the pressure gradient for two special forms of slider bearings, viz., inclined and parabolic slider bearings. In these two cases, the variation of pressure, load capacity, coefficient of friction, etc. for a range of fluid- and bearing-parameters is presented. In order to obtain optimum load capacity for an inclined and a parabolic slider bearing, the variations of load capacity, coefficient of friction, etc., with respect to the simultaneous changes of the inlet–oulet film height ratio and of the power law index of lubricants are also analysed. The results are presented both numerically and graphically. The results reveal that in the cases of an inclined and a parabolic slider bearings the inlet–outlet film height ratio for the optimum load capacity depends on the power law index of lubricants. Further, for each value of power law index, there exists a value of inlet–outlet film height ratio for which the frictional coefficient is minimum.     

Viscosity–pressure–temperature behaviour of mineral and synthetic oils

A new high‐pressure viscometer that can measure viscosity at pressures up to 0.8 GPa has been developed in the authors' laboratory. The ‘modulus equation’ has been used to compare the behaviour of mineral and synthetic lubricants. Among the oils investigated there was one ester that biodegraded rapidly both before and after ageing in a long‐term test‐rig operation. To facilitate a comparison or application of the results to other oils, an analysis of the correlation between the viscosity—pressure coefficient and the kinematic viscosity measured at atmospheric pressure has been provided. A prediction of lubricant film thickness based on high‐pressure viscosity data is compared with film thickness measurements in a roller bearing.     

固体颗粒对低黏度介质润滑特性的影响

基于水-固两相单流体模型,研究润滑过程中的固体颗粒对低黏度润滑介质的影响。以无限宽楔形滑块为计算模型,推导出特定假设条件下考虑惯性力和界面滑移的低黏度介质润滑的雷诺方程,并通过不同条件下雷诺方程的求解,探讨固体颗粒含量对低黏度介质润滑特性的影响。结果表明:在对低黏度介质润滑特性进行研究时,需考虑其惯性力和界面滑移影响;固体颗粒的存在一定程度上增大润滑膜的压力和承载能力,颗粒含量越大,承载能力越大,但颗粒含量对润滑膜整体的压力分布状态几乎不会产生影响;惯性力增大固体颗粒对润滑介质的影响程度,而界面滑移减小固体颗粒对润滑介质承载能力的影响,发生滑移时,颗粒基本不影响润滑膜的承载能力。     

The Determination of Vapor Pressures of Low Viscosity Lubricants from Distillation Data

The vapor pressure of low viscosity aliphatic hydrocarbon lubricants is shown to be determined with good accuracy from data obtained from a modification of the ASTM Distillation Test D86-67. This test gives the initial boiling point of a lubricant from which the vapor pressure at any other temperature can be calculated by use of an empirical equation relating the boiling point to the vapor pressure.

Two other equations were derived, one relating the viscosity of an aliphatic hydrocarbon lubricant to the vapor pressure at any temperature, and the other extending the use of this distillation method to calculation of the vapor pressures of other types of lubricants as, for example, esters, alcohols, and acids.

The data presented indicate that this method is superior to the isoteniscope method in accuracy and ease of operation.     

Synthetic Lubricants in Hydrodynamic Journal Bearings: Experimental Results

Synthetic lubricants and additives have seen many major improvements in recent years. However, very little is known about the performance peculiarities of these new lubricants in actual machines. To fill this gap, a new full-scale hydro-dynamic journal bearing test rig has been constructed to evaluate the behavior of conventional and new bearing designs, synthetic lubricants and variations in operating parameters. This test rig’s bearing has diameter 180 mm with measuring capabilities including continuous film thickness and film pressure as well as temperature. The new machine was used to compare a number of synthetic lubricants to mineral based lubricants, finding that performance of the synthetic lubricants was superior to their mineral based counterparts of much higher viscosity grade. These tests showed that high viscosity index (VI) synthetic lubricants had higher viscosity in the region of maximum pressure and lower viscosity elsewhere in the bearing than similar mineral based lubricants. This reduction in viscosity in low pressure zones was found to produce a measurable reduction in friction and power loss in the bearing system. This paper provides comparative performance results of several formulations of current and future turbine oils from measurements of losses, oil film thickness, and temperature under a range of operating parameters. Lubricants tested include ISO VG68 and VG32 mineral based turbine oils (VG68 and VG32), ISO VG32 synthetic ester based oil (SE32), two ISO VG22 synthetic ester based oils (SE22 and SV22), and ISO VG15 synthetic ester based oil (SE15). It was found that SE32 and VG68 provided similar performance at lower speeds while SV22 provided similar performance to VG68 at the highest speed. Likewise, SE22 and SV22 provided similar performance to VG32 at low speeds while SE15 provided similar performance to VG32 at medium to higher speeds. Generally, the results demonstrate the potential for replacing mineral based lubricants with high performance synthetic lubricants of significantly lower viscosity grade without sacrificing bearing safety.     

Calculated journal bearing lubrication of non-Newtonian medium with surface roughness effects

To analyze the effects of non-Newtonian lubricants and surface roughness in journal bearing lubrication, a modified Reynolds equation is derived. In the equation, differential viscosity and the first normal stress function are defined to specify the rheological properties of non-Newtonian medium. Flow factors are used to specify the effect of surface roughness. The modified Reynolds equation is numerically calculated using super over relaxation method. Numerical results of the lubrication show that the differential viscosity is the principal non-Newtonian property affecting the lubrication, it is determined by the material parameters of the lubricant and is affected by the shearing rate. Under its effect, the load capacity of non-Newtonian lubricants is not always higher or lower than that of Newtonian lubricants’. The effect of the first normal stress difference increases under the conditions of dynamic loading. Surface roughness shows an obvious effect on load capacity when it is greater than one-tenth of the film thickness, and the surface with longitudinal pattern affects the load capacity most. However, surface roughness has less effect on lubrication results than lubricants’ non-Newtonian property. __________ Translated from Tribology, 2005, 25(6) (in Chinese)     

Effect of Lubricant Degradation on Contact Fatigue

It is well known that contact fatigue is affected by contact pressure, frictional stress, residual stress, initial distribution of material flaws, and so on. The behavior of contact pressure and, primarily, the frictional stress is determined by the viscous properties of the lubricant used. It is also recognized that lubricants degrade while passing through lubricated contacts. Degradation of lubricants causes viscosity loss that, in turn, reduces the frictional stress and raises contact fatigue life. The objective of this study was to find out the extent to which lubricant degradation may change contact fatigue life of elastic surfaces completely separated by lubricant. The analysis was performed numerically based on the models of contact fatigue and lubricant degradation recently developed by the author. The results showed that contact fatigue life of solids completely separated by lubricants with the same ambient viscosity may vary significantly due to the specific way lubricants are formulated. In particular, contact fatigue life is strongly affected by the initial molecular weight distribution of the polymeric additive (viscosity improver) in the lubricant and contact operating conditions, which in some cases promote fast lubricant degradation caused by high lubricant shearing stresses.     

Test of a Fluid-Film Wave Bearing at 350°C with Liquid Lubricants

A novel fluid-film wave bearing has been run at a higher temperature (350°C) than ever before with a perfluoropolyether (PFPE)-K liquid lubricant. Additionally, the wave journal bearing (45 mm diameter and 24 mm long) completed an 8-h endurance test at the NASA Glenn Research Center. The lubricant was PFPE-K XHT 500. After being maintained at 350° C for 8 h, the bearing temperature was raised to 356°C for the last 30 min of the run. The speed was 29,000 rpm and the load ranged from 2670 to 3560 N. The bearing was perfectly stable both dynamically and thermally. The observed temperature was more than 150°C above that run with current turbine engine lubricants. The use of high-temperature bearings as tested here would allow efficiency increases of more than 5% in aero or terrestrial turbine engines.     

Optimal profiles for one dimensional slider bearings under technological constraints

The optimal slider bearing profile for maximum bearing load is studied by using direct constrained optimal control techniques. The constraints include the Reynolds and the energy equations. The energy equation takes into account the shear strain rate in the lubricant. The dependence of lubricant viscosity on temperature is considered. Technological constraints such as the maximum lubricant pressure and temperature and the minimum lubricant film thickness are included into the model. The realistic problem considered here yields optimal bearing profiles which are much more complex than the classical Rayleigh step bearing profile. The optimal bearing profile consists of an alternation of regions of constant height and more or less abrupt height variations. The number of constant height regions depends on the type of the constraint and in many cases is larger than three. The minimum value of the bearing height is one of the most important constraints. Four levels of model approximations have been tested. The most important model improvement is to take into account the temperature dependence of the lubricant viscosity. Several bearing design and operation parameters, such as bearing length, inlet height, sliding velocity and lubricant inlet pressure and temperature, have been considered. They all have complex influence on the optimal bearing profile.     

Effects of lubricant viscosity on the mixed lubrication of a piston ring pack in an internal combustion engine

A theoretical analysis is presented of the mixed lubrication of a piston ring pack. The analysis comprises Patir and Cheng's average flow model and Greenwood and Tripp's asperity interaction model, and is developed to consider the shear thinning effect of a non‐Newtonian fluid, multigrade lubricant. The friction characteristics of the piston ring pack for both monograde and multigrade lubricants are investigated. It is found that a decrease in the lubricant viscosity is effective in reducing the friction loss, although this increases the boundary friction at the beginning of the expansion stroke. The friction characteristics are markedly affected by the shear thinning effect when multigrade lubricants are used.     

The influence of the lubricant film on the stiffness and damping characteristics of a deep groove ball bearing

This paper experimentally investigates the formation of a lubricant film in a deep groove ball bearing and its effect on the bearing dynamics. A novel test rig is introduced, which allows testing different types and sizes of bearings in real-life conditions. The test rig dynamics are optimised such that the dynamic properties of the bearing are measured in a frequency range below the resonances of the flexible modes. Two properties of the bearing, both its stiffness and damping value in the direction of the static bearing load, are identified. The behaviour of the lubricant film between the rolling elements and raceways is measured based on the electrical resistance through the bearing. For this purpose, the bearing housing is electrically isolated from the surrounding structure. The electrical resistance, stiffness and damping of the test bearing are identified during a speed run-up. The influence of the bearing temperature is analysed as well. During a run-up at constant bearing temperature, the measurement of the electrical resistance describes the formation of the lubricant film. Due to the formation of the lubricant film, the bearing stiffness increases by 3.2% while the damping increases by 24%. During a warm-up of the bearing, the viscosity of the lubricant film decreases strongly. A resulting decrease in electrical resistance, stiffness and damping is measured. Finally, the electrical resistance, stiffness and damping are identified at different speeds, after the bearing has reached a stable temperature at each speed. A combined effect of both rotation and temperature is observed and discussed.