A Method for the Measurement of Hydrodynamic Oil Films Using Ultrasonic Reflection

The measurement of the thickness of an oil film in a lubricated component is essential information for performance monitoring and control. In this work, a new method for oil film thickness measurement, based on the reflection of ultrasound, is evaluated for use in fluid film journal bearing applications. An ultrasonic wave will be partially reflected when it strikes a thin layer between two solid media. The proportion of the wave reflected depends on the thickness of the layer and its acoustic properties. A simple quasi-static spring model shows how the reflection depends on the stiffness of the layer alone. This method has been first evaluated using flat plates separated by a film of oil, and then used in the measurement of oil films in a hydrodynamic journal bearing. A transducer is mounted on the outside of the journal and a pulse propagated through the shell. The pulse is reflected back at the oil film and received by the same transducer. The amplitude of the reflected wave is processed in the frequency domain. The spring model is then used to determine the oil film stiffness that can be readily converted to film thickness. Whilst the reflected amplitude of the wave is dependent on the frequency component, the measured film thickness is not; this indicates that the quasi-static assumption holds. Measurements of the lubricant film generated in a simple journal bearing have been taken over a range of loads and speeds. The results are compared with predictions from classical hydrodynamic lubrication theory. The technique has also been used to measure oil film thickness during transient loading events. The response time is rapid and film thickness variation due to step changes in load and oil feed pressure can be clearly observed.     

Measurement of circumferential viscosity profile in stationary journal bearing by shear ultrasonic reflection

Viscosity is the most important property of a lubricant that can affect bearing performance. It controls the film thickness that is established during an operation. In this study, the ultrasonic method was used to measure the static viscosity profile around a journal bearing by using shear reflection coefficients at several locations around the journal bearing. This enables the viscosity profile to be established. The technique introduced was found to be successful and acceptable results were obtained from certain regions of the journal bearing flow. This study serves as a preliminary work for developing viscosity measurement in a rotating journal bearing.     

The Effects of Viscosity and Clearance on the Performance of Hydrodynamic Journal Bearings

In the present work, the effects of viscosity and clearance ratios have been analyzed on the reliable performance and design of a steady-loaded, pressure-fed, hydrodynamic cylindrical bearing. The technique for bearing performance evaluation has been developed based on the maximum oil temperature in the region of load carrying oil film, and variation of oil viscosity with temperature.

In this paper viscosity coefficients have been determined using iterative procedures for different oils. And also, a viscosity integral has been evaluated by method of splines as a function of inlet oil temperature to the load carrying oil film and exit oil temperature from the oil film. The viscosity integral has also been evaluated for a bearing operating under different conditions. Thus, by comparison of the viscosity integrals, exit oil temperature from the load carrying oil film and safe load carrying capacity of a bearing with different clearance ratio and using different oils have been examined. Nomographs have also been plotted for easy assessment of bearing performance.

The developed technique gives a more realistic approach for design and performance evaluation of a bearing as compared to conventional procedures. This may have potential as an effective tool to assess performance of a hydrodynamic journal bearing.     

考虑润滑油黏温效应的动压滑动轴承性能分析

滑动轴承的相关研究很多都基于等黏度的情况下,这与轴承的实际工作情况有较大的出入。使用计算流体力学FLUENT通过编写的黏温方程UDF程序进行动压滑动轴承润滑油黏度的计算,并考虑黏温效应对动压滑动轴承性能的影响,比较等黏度与变黏度情况下动压滑动轴承的油膜压力与承载力、油膜的轴向与周向温度分布。结果表明:在考虑黏温效应条件下,轴承的承载力、油膜压力、摩擦力均小于定黏度条件下,这是由于温度升高导致黏度降低,从而减小了油膜静压力和承载力;在轴承轴向方向上,从油膜中心位置向两端部,油膜温度逐渐升高;在轴承圆周方向上,从收敛区到发散区,油膜温度先升高后降低,油膜温度峰值出现在轴承发散区的端部位置。     

Operating Limits for Acoustic Measurement of Rolling Bearing Oil Film Thickness

An ultrasonic pulse striking a thin layer of liquid trapped between solid bodies will be partially reflected. The proportion reflected is a function of the layer stiffness, which in turn depends on the film thickness and its bulk modulus. In this work, measurements of reflection have been used to determine the thickness of oil films in elastohydrodynamic lubricated (EHL) contacts. A very thin liquid layer behaves like a spring when struck by an ultrasonic pulse. A simple quasi-static spring model can be used to determine the proportion of the ultrasonic waves reflected.

Experiments have been performed on a model EHL contact between a ball and a flat surface. A transducer is mounted above the contact such that the ultrasonic wave is focused onto the oil film. The reflected signals are captured and passed to a PC for processing. Fourier analysis gives the reflection spectrum that is then used to determine the stiffness of the liquid layer and hence its thickness. In further testing, an ultrasonic transducer has been mounted in the housing of a deep-groove ball bearing to measure the film generated at the outer raceway as each ball passes. Results from both the ball-flat and ball bearing measurements agree well with steady-state theoretical EHL predictions. The limits of the measuring technique, in terms of the measurable rolling bearing size and operating parameters, have been investigated.     

Journal bearing performance under starved lubrication

The hydrodynamic performance of a journal bearing under starved lubrication conditions is studied theoretically and experimentally. When lubricant is supplied to a journal bearing from an oil groove at an insufficient oil flow rate, the hydrodynamic oil film cannot cover the full width of the bearing at the inlet of the convergent bearing clearance, and the covered width increases gradually with the decrease in film thickness in the circumferential direction. This phenomenon affects the static and dynamic performance of the journal bearing. The oil film boundary in the bearing clearance is calculated numerically for a constant load and a constant speed under the assumption of laminar, iso-viscous lubrication. Measurements are in good agreement with predictions. Some more calculations of journal centre loci are given.     

汽轮机组椭圆轴承瞬态响应的研究

联立求解广义雷诺方程、油膜瞬态三维能量方程、轴瓦瞬态三维固体热传导方程及轴颈的运动方程,并考虑粘度和密度随温度及压力的变化,在油膜与轴瓦界面使用热流连续性边界条件,得到了在载荷突然变化时汽轮机组椭圆轴承的瞬态响应。介绍了一种有效的用于求解轴承瞬态性能的改进Newton-Raphson算法。结果表明,载荷突然变化后,各性能参数经振荡衰减后均能稳定在一新的恒定值,只是最小油膜厚度和最高油膜压力的变化量很大。     

弹性金属塑料瓦径向滑动轴承启动过程热弹流分析

研究了采用弹性金属塑料轴瓦时径向滑动轴承的瞬态润滑特性。建立了该轴承的三维热弹流模型，通过数值求解对比了启动过程中采用弹性金属塑料轴瓦和金属材料轴瓦时油膜和轴瓦的瞬态温度场分布、轴瓦的热弹变形量，以及转子轴心轨迹。结果表明，在启动初期弹性金属塑料瓦径向滑动轴承的瓦体温度要高于金属瓦，转子偏心率也要大于采用金属瓦时的偏心率；因润滑油在弹性金属塑料瓦径向滑动轴承轴瓦表面存在一定的滑移速度，随着油膜边界滑移作用的出现，采用弹性金属塑料瓦径向滑动轴承时的油膜温度最终低于采用金属瓦时的温度，且转子的偏心率也最终小于采用余属瓦的偏心率。     

均压槽与静压气体轴颈轴承承载特性的关系研究

主要研究通过开设不同结构形式的均压槽来提高静压气体轴颈轴承的承载能力和刚度,利用加权余量法和有限元离散化方法求解雷诺方程进行数值计算和仿真,针对单排孔和双排孔轴颈轴承,分析不同长度的周向均压槽,以及不同数量、不同位置和不同长度的轴向均压槽对轴承承载能力和刚度的影响规律。结果发现:开设周向均压槽和轴向均压槽都可以提高轴承的承载能力,相比开设周向均压槽,开设轴向均压槽对提高轴承的承载能力更为有效,而且只开设一条或两条轴向均压槽就能显著提高轴承的承载能力,通常将轴向均压槽置于气膜间隙较小的位置时能使轴承的承载能力和刚度最大。数值仿真结果通过试验进行验证,研究结果可用以指导高承载和高刚性静压气体轴颈轴承的设计。     

A study of thermohydrodynamic lubrication in a circular journal bearing

An analytical method to determine the film temperature of circular journal bearings was developed, which considers the cavitation of the oil film and also recirculation and mixing of the lubricant. The results were verified experimentally. The theory is in good agreement with experiment over a wide range of operating conditions. The effects of journal speed, clearance ratio, lubricant viscosity and specific load on the bearing temperature were examined, and the following conclusions derived. (1) T_b,max, the maximum temperature on the bearing metal surface, increases considerably with the increase of speed and lubricant viscosity and with the decrease of clearance ratio. (2) With the increase of speed, the angular position of T_b,max varies considerably towards the direction of journal rotation from the upper stream side of the location of minimum film thickness, h_min, to the lower stream side of it. The change of angular position of T_b,max is greater than that of h_min. The contrary happens with decrease of the clearance ratio. (3) These characteristics of T_b,max correspond to those of the maximum temperature, T_f,max, in the oil film.     

An Experimental Investigation into the Temperature Profile of a Compliant Foil Air Bearing

A series of tests was performed to determine the internal temperature profile in a compliant bump-type foil journal air bearing operating at room temperature under various speed and load conditions. The temperature profile was collected by instrumenting a foil bearing with nine type-K thermocouples arranged in the center and along the bearing's edges in order to measure local temperatures and estimate thermal gradients in the axial and circumferential directions. To facilitate the measurement of maximum temperatures from viscous shearing in the air film, the thermocouples were tack-welded to the backside of the bumps that were in direct contact with the top foil. The mating journal was coated with a high-temperature solid lubricant that, together with the bearing, underwent high-temperature start-stop cycles to produce a smooth, steady-state run-in surface. Tests were conducted at speeds from 20 to 50 krpm and loads ranged from 9 to 222 N.

The results indicate that, over the conditions tested, both journal rotational speed and radial load are responsible for heat generation with speed playing a more significant role in the magnitude of the temperatures. The temperature distribution was nearly symmetric about the bearing center at 20 and 30 krpm but became slightly skewed toward one side at 40 and 50 krpm. Surprisingly, the maximum temperatures did not occur at the bearing edge, where the minimum film thickness is expected, but rather in the middle of the bearing, where analytical investigations have predicted the air film to be much thicker. Thermal gradients were common during testing and were strongest in the axial direction from the middle of the bearing to its edges, reaching 3.78° C/mm. The temperature profile indicated the circumferential thermal gradients were negligible.     

Energy loss due to misalignment of journal bearings

An analysis of a journal bearing describing a maximum allowable value of misalignment at a length to diameter ratio of unity is presented. The journal misalignment is allowed to vary in direction up to a direction normal to the axial plane containing the load vector. The results demonstrate that journal misalignment influences bearing behaviour. For the same load carrying capacity, a misaligned bearing consumes more power due to friction than an aligned one. An optimum position of misalignment has been found to be almost along the axial plane containing the load vector since it produces lower frictional losses and less lubricant leakage than any other.     

Calculated Performance of Non-Newtonian Lubricants in Finite Width Journal Bearings

A numerical procedure has been developed for the calculation of the performance of non-Newtonian, polymer-thickened lubricants in finite width journal bearings. Such oils were found to act as if they had averaged “anisotropic viscosities,” i.e., different viscosities in the circumferential and side leakage directions, even though the viscosity was taken to have one definite value, a function of the resultant shear stress, at each point in the oil film. Overall, polymer oils carried less load at a given eccentricity, gave less friction and a flatter pressure distribution than mineral oils of the same low shear viscosities. By analogy with the previously calculated infinite width case, which gave similar results, it is expected that the flatter viscosity temperature slope of the polymer oils will compensate for their apparent viscosity decrease. The program has also been adapted to “natural” boundary conditions, which improve upon the delineation of the cavitation region on the inlet side of the bearing.     

A parametric study on bubbly lubrication of high-speed journal bearings

The influence of aerated oil on high-speed journal bearings is examined by classical thermohydrodynamic lubrication theory coupled with analytical models for viscosity and density of the air–oil mixture in the fluid-film bearing. Convection to the walls, mixing with supply oil and recirculating oil, and some degree of journal misalignment are considered. The parameters considered for the study of bubbly lubrication are oil aeration level, air bubble size, shaft misalignment and shaft speed. The results show that air bubbles can more clearly bring on increasing load capacity under high-speed operation of a plain journal bearing than under previous normal speed operation. Moreover, the load capacity may be increased more by oil aeration under the conditions of shaft misalignment and increasing speed.     

Three-dimensional hydrodynamic analysis of a journal bearing with a two-component surface layer

The article presents a three-dimensional hydrodynamic analysis of a bearing with a two-component surface layer journal. This kind of bearing is characterized by an experimentally proved susceptibility to oil contaminants which is five times lower than that of a typical bearing. The oil flow was described with the Navier–Stokes and energy equations. The equation system was solved by the finite element method. The distributions of pressure, temperature and oil flow velocity were determined. The computation results made it clear that a journal with a two-component surface layer causes a reduction in the bearing load capacity for high eccentricity ratio values (0.7–0.9). Besides, contrary to a typical bearing, considerable velocity changes in oil flow in all directions (circumferential, radial and axial) were observed in the bearing under consideration. Particularly great velocity changes occurred in the radial direction. However, no essential changes were found in the temperature of the oil film and in the attitude angle.     

TEHD Analysis for Tilting-Pad Journal Bearings Using Upwind Finite Element Method

A general approach for incorporating heat transfer and elastic deformation effects into a tilting-pad journal bearing simulation model is presented. A global analysis method is used, which includes variable viscosity and heat transfer effects in the fluid film, elastic deformation and heat conduction effects in the pads, and elastic deformation effect in the pivots. The two-dimensional variable viscosity. Reynolds equation produces pressure distributions in the axial and circumferential directions. The energy equation is two-dimensional, assuming that the temperature variation in the axial direction is negligible. The elasticity and heat conduction models are also two-dimensional, being in the midline cross-section of the bearing, including the circumferential and cross-film directions. An upwind technique is used in the finite element formulation of the energy equation to remove numerical instability due to the convective term. Simulation results are compared with the test and predicted values of previous researchers.     

Thermal effects in cylindrical journal bearings of high‐speed gearboxes

The high rotational speeds and loads of gears operating in acceleration gearboxes causes problems related to the correct choice and design of journal or rolling bearings. In the case of journal bearings, these problems are connected with thermoelastohydrodynamic lubrication theory and the dynamics of the bearing system. However, of major importance is the problem of thermal effects in journal bearings. This has been considered for bearings used in a double‐helical gearbox with the pinion and output shaft operating in cylindrical journal bearings. The oil film pressure, temperature, viscosity distributions, and maximum and mean oil film temperatures have been determined. In calculations, laminar adiabatic and turbulent adiabatic models of oil films have been applied. The results of the calculations can be used in the design of cylindrical or other types of journal bearing in rotating machinery, including acceleration or reduction gearboxes.     

卧式水电机组局部多油楔径向滑动轴承研究

针对卧式水电机组用径向滑动轴承载荷日益提高的现状,设计一种局部多油楔瓦面结构的径向滑动轴承。通过联立求解膜厚方程、雷诺方程、密度方程、黏度方程、能量方程和固体热传导方程等,获得轴承的热流润滑特性,并与椭圆径向滑动轴承的热流特性进行对比。结果表明,局部多油楔径向滑动轴承具有较大的动压承载区域和较小的油膜压力梯度以及较低的油膜温升,可以大幅度提高轴承的承载能力。     

The High-Pressure Rheology of a Soap-Thickened Grease

The high-pressure rheology of a soap-thickened grease and the base oil has been characterized using a falling-body viscometer, high-pressure Couette viscometer, and a high-pressure penetrometer. At shear stress from 0.1 to 1 MPa, the response of the grease was approximately linear with rate of shear with a viscosity about twice that of the base oil. Apparent yield stress behavior is observed at lower shear stress. A new modified Bingham model is offered to describe the experimental observations. These results are relevant for simulations of elastohydrodynamic lubrication and journal bearings operating at elevated ambient pressure.     

基于动网格方法的油膜轴承性能的研究

考虑气穴的影响,建立了油膜轴承所支撑转子系统的动力学模型,并利用新的动网格更新方法,编制了求解油膜轴承压力分布、转子静平衡位置以及轴心轨迹的程序,验证了其正确性。利用该程序考察了气穴压力和转速对油膜轴承压力分布和所支撑转子的轴心静平衡位置的影响。计算结果表明,在相同的速度和载荷下,随着气化压力升高,轴承偏心率和最大油膜压力增大,偏位角减小,并且最大油膜压力的周向位置受气化压力的影响较小;而在相同的载荷下,转速对转子静平衡位置影响较大,并随着转速增加,轴承偏心率减小,偏位角增加。