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.     

Analysis of the elastic punch-substrate contact under fretting: Monotonic and cyclic loading of the punch

The object of this paper is to analyze the elastic behaviour of a 2-D contact problem between a right-angled flat punch and a semi-infinite substrate, subjected to a constant normal compressive load and a cyclic shear load using a finite element code. The knowledge of the stress and strain fields produced close to the corners of the punch under different loading conditions as a function of the friction coefficient will allow insight to be gained into the fretting fatigue problem associated to this cyclic loading. In order to better understand the behaviour and analyze the possibility of using fracture mechanics approaches to study the stress field close to the punch corners, two different models have been compared to each other: one without continuity solution between the punch and the substrate and the other with a couple of contact surfaces between them. Using the continuous model, a particularization of the general analytic solutions of Williams has been proposed. The complete stress field around the corners of the punch for this model has been obtained for any values of the punch size, normal and shear loads. Some general guides for understanding and systematizing the punch-substrate behaviour have been extracted from the above solution and that of a sliding wedge, provided by the literature, which enable a systematic numerical analysis of the problem. Further on, a more detailed study of the slip between punch and substrate, as well as of the stress field, has been accomplished using finite element analysis guided by the previous semi-analytical results. The study has been completed for the whole load process: compressive normal load, monotonic shear load, and cyclic shear load.     

The use of rolling-fatigue test methods over a range of loading conditions to assess the performance of fire-resistant fluids

A means has been proposed whereby the results of rolling-contact fatigue tests under different loads and on different machines can be brought together to provide a coherent picture of lubricant performance over a wide range of loading conditions. From results obtained on the rolling four-ball machine, the Unisteel machine and a ball-bearing test rig, relationships have been determined between load and bearing life for several fire-resistant fluids. These relationships explain why derating factors obtained at one load level may not be applicable at other load levels. In addition it has been proposed that fluids possessing certain characteristics may perform worse in multi-bearing systems than might be expected from their performance in single bearing tests.     

Critical stress and load conditions for pitting calculations of involute spur and helical gear teeth

Calculation methods of spur and helical gear drives for preliminary designs or standardization purposes available in technical literature, use the Hertz equation to evaluate the contact stress, assuming the load to be uniformly distributed along the line of contact. However, this model presents some discrepancies with experimental results because the changing rigidity of the pair of teeth along the path of contact produces a non-uniform load distribution, which implies that some load distribution factors are required to compute the contact stress. In this paper, a non-uniform model of load distribution along the line of contact, recently developed, obtained from the minimum elastic potential criterion, has been used. This model combined with the Hertz equation yields more accurate values of the contact stress. As the load per unit of length at any point of the line of contact and any position of the meshing cycle has been described by a very simple analytic equation, a complete study of the location and value of the critical contact stress has been carried out. From this study, a recommendation for the calculation of the pitting load capacity of spur and helical gears is proposed.     

Load capacity of components with initial point contact

Formulas have been derived for the critical loads corresponding to the onset of plastic deformation in different loading stages. Likewise, a formula has been obtained for the limiting load corresponding to sharp deviation of the size of the surface impression from the values predicted by the Hertz equations, which describes elastic behavior. A means of increasing the load capacity with initial localized contact is considered.     

Contact stress calculation of undercut spur and helical gear teeth

The presence of undercut at the pinion root may affect the load sharing among couples of spur gear teeth in simultaneous contact, as well as the load distribution along the line of contact of helical gears. This occurs if the outside points of the wheel profile do not find, due to undercut, mating profile to mesh with, which is called vacuum gearing. Under these conditions, the effective contact ratio is reduced, and the critical contact points may be shifted from their locations at non-undercut profiles. In this paper, a non-uniform model of load distribution along the line of contact, obtained from the minimum elastic potential criterion, has been used combined with the Hertz equation for the calculation of the contact stress. A complete study on the critical load conditions and the value of the critical contact stress has been carried out, considering a wide range of the values of the geometrical parameters. As a result, a recommendation for pitting load capacity calculations of vacuum-gearing spur and helical gears is proposed.     

A six axis load cell for the analysis of the dynamic impact response of a hybrid III dummy

An innovative six axis load cell is presented and employed for frontal impact tests on a Hybrid III 50th percentile dummy. The new six axis load cell, designed at Politecnico di Milano (Technical University of Milan), is a quasi-statically determined structure consisting of a central three spoke element fixed to an external frame by means of special joints appositely designed to minimize friction. A specifically designed electronic board is embedded in the load cell and allows real-time signal processing. The developed load cell has been employed during frontal impact tests on a Hybrid III dummy. An impact force is applied to the dummy’s head by means of an impact pendulum. The accelerations in two different points of the dummy’s head as well the impact force have been measured. A simple 1 DOF analytical model of the head and neck has been developed. Neck stiffness and damping have been identified directly from experimental data.     

海上浮动核电站管道系统摇摆载荷分析

巨大的海上风暴会使海上浮动核电站产生剧烈的摇摆,在对海上浮动核电站核反应堆管道系统进行力学分析时,必须考虑摇摆载荷。采用载荷系数法,在专用管道计算分析软件PIPESTRESS上,取虚拟节点的载荷系数,计算出了在摇摆作用下管道所承受的载荷;同时,采用ANSYS软件的谐响应分析功能,计算出了在摇摆作用下管道系统的稳态响应,并对载荷系数法计算出来的弯矩和谐响应法计算出来的弯矩进行了对比分析。分析结果表明,载荷系数法计算出来的结果具有较大的保守性。对于管道应力超出限值的节点,通过用该节点处实际坐标的载荷系数,重新计算该节点的摇摆载荷,可以降低不少保守性。     

Measurement and analysis of the structural performance of cellular metal sandwich construction

The bending performance of sandwich construction with thin cellular metal cores has been measured and simulated. A mechanism map has been generated to characterize the predominant failure phenomena based upon collapse load criteria for face yielding, core shear and indentation. A previously developed constitutive law for the core material has been incorporated into numerical simulations. Comparisons have been made with the measured response. Initial discrepancies attributed to a core thinness effect were rectified by inputting core shear properties measured on materials having the same thickness. Analytical estimates for the stiffness, yield load and limit load are compared with the numerical simulations and experimental results.     

Limit analysis of circular plates subjected to arbitrary rotational symmetric loadings

The lower bound technique of limit analysis has been implemented in order to calculate the critical load factor for loadings on circular plates. The obtained results are applicable to any rotational symmetric loading on clamped or simply supported plates. Both the square and Tresca yield criteria have been considered in the analysis. It has been shown that the calculated lower bound for the critical load factor is the exact answer for the considered problem. In order to perform the calculations, the loading function is expanded in the form of a power series and the critical load factor calculated by use of a numerical technique. Finally, variation of this factor with the loading function has been illustrated.     

车辆齿轮油承载能力的估算

从纳米摩擦学角度考察了车辆齿轮的四球机测定结果,发现最大无卡咬负荷ＰＢ和烧结负荷ＰＤ值不能很好地代表承载能力。阐明了由这两个指标难于预测车辆齿轮油承载能力的原因是：（１）钢球材质与实际摩擦副的不同;（２）钢球的接触方式是点接触;而齿轮是线接触;首次提出一套利用四球机测试估算车辆齿轮油承载能力的方法,指出实际选择齿轮油时可根据经的啮合压力来确定油品在四球机试验中应达到的比压力。     

Influence of in-plane displacements at the boundaries of rigid-plastic beams and plates

An approximate theoretical rigid-plastic study has been undertaken into the influence of small in-plane displacements at the boundaries of various laterally loaded beams. The most interesting feature of these theoretical predictions is the remarkably small in-plane displacements at the supports which can change the response from that of an axially restrained beam, with considerable reserve strengthening beyond the limit load for moderate lateral deflections, to that of a freely supported beam with no concomitant increase in strength beyond the limit load. A comparison has also been made between some theoretical predictions for a laterally loaded square plate and the corresponding experimental results obtained by Clarkson on rotationally fixed plates with edges free to slide inwards.     

Closed-form solution of axisymmetric contact problem for a viscoelastic base within cycle of increasing and decreasing of load on the indenter

The axisymmetric contact problem for a spherical indenter and viscoelastic base has been considered within a cycle of increasing and decreasing of load. The loading-unloading cycle has been performed at a constant speed of indenter movment. The quasi-static approximation has been used. The study has been carried out according to linear viscoelasticity theory, while the material model allows one to take into account an arbitrary number of relaxation times. The dependence of load on the indenter on time has been defined. Expressions for determining the unknown boundary of the contact area in the loading–unloading process has been given. The hysteretic loop area at different values of the material relaxation time has been analyzed. The applicability of the offered analytical model of elastomer indentation experiments data processing has been discussed.     

A new approach for polymer composite gear design

A new design method for polymer composite gear has been proposed in the current paper. This design method is based on the link between polymer gear wear rate and its surface temperature. It has been found from the tests that the polymer (acetal) gear wear rate will be increased dramatically when the load reaches a critical value for a specific geometry. The gear surface will wear slowly with a low specific wear rate if the gear is loaded below the critical one. The possible reason of the sudden increase in wear rate is due to the gear operating temperature reaching the material melting point under the critical load condition. Gear surface temperature has been then investigated in detail through three components: ambient, bulk and flash temperatures. Through extensive experimental investigations and modelling on gear surface temperature variations, a general relation has been built up between gear surface temperature and gear load capacity. The method has been related to test results under different operating ambient temperature and gear geometries. Good agreement has been achieved between the proposed method predictions and experimental test results. Experimental investigation on polymer composite (glass fibre reinforced nylon with PTFE as internal lubricant) gears have also been carried out and two forms of failure have been found, root and pitch fractures.     

对数母线圆柱滚子凸度量的有限元分析

以42724QT型铁路客车圆柱滚子轴承为分析对象,对其滚子进行对数母线凸度的分析,借助有限元软件ANSYS完成了在不同凸度量下的接触应力分析及比较,得到了一定载荷下的较优凸度量区间,同时进行了相同凸度不同载荷下的接触应力比较。结果显示,为避免"边缘效应",一定的载荷对应一个较优的凸度区间,并且理论凸度近似值与分析后得到的最优凸度有一定的误差,即使是对数母线的滚子,当载荷超出正常加载范围时,仍会出现"边缘效应"。     

Polymer gear surface thermal wear and its performance prediction

This paper will concentrate on acetal gear wear behaviour and its performance prediction based on the extensive investigations on the gear thermal mechanical contact both experimentally and numerically. It has been found from the tests that acetal gear wear rate will be increased dramatically when the load reaches a critical value for a specific geometry and running speed. The gear surface will wear slowly with a low specific wear rate if the gear is loaded below the critical one. The possible reason of the sudden increase in wear rate is due to the gear operating temperature reaching the material melting point under the critical load condition. Gear surface temperature has been then investigated in details through three components: ambient, bulk and flash temperatures. Through extensive experimental investigations and modelling on gear surface temperature variations, a general relation has been built up between gear surface temperature and gear load capacity. An approach for acetal gear transition torque prediction has been proposed and this method is based on the link between polymer gear wear rate and its surface temperature. The method has been related to test results under different operating speeds and gear geometries. Good agreements have been achieved between the proposed method predictions and experimental test results.     

Investigation of vibrotactile sensation of human fingerpads by observation of contact zones

An experimental apparatus has been developed to observe the contact zones between human fingerpads and a smooth flat surface of a total-reflection prism. Using this apparatus, the following experiments have been carried out: (1) measurement of the apparent and real contact areas of fingerpads under a controlled contact load, (2) measurement of the vibrotactile thresholds of fingerpads under a controlled contact load, and (3) investigation of the discrimination of tones by tactile sensation under a controlled contact load. From the third experiment, it is found that human fingerpads can discriminate between tones from the differences in their harmonic components.     

核电用石墨密封垫片的试验方法研究

核电用石墨密封垫片是一种新型结构的密封垫片,目前还没有相关测试方法的标准。本文通过对石墨密封垫片测试方法的研究,重点对垫片压缩率、回弹率、应力松弛率及密封泄露率的试验载荷进行了试验分析,提出了试验载荷的要求。对于制订该产品测试方法标准有重要的参考价值。     

Effects of partial-grooving on the performance of spiral groove bearings: analysis using a perturbation method

The perturbation method has been extended to investigate the effects of partial-grooving on the performance of spiral-grooved spherical and conical bearings. A ‘partial-grooving ratio’ (PGR), which has a value of 0 for a fully-grooved bearing and a value of 1 for a plain bearing, has been introduced to represent the degree of partial-grooving. It has been found that the load capacity and stiffness of a bearing are not significantly affected by partial-grooving as long as the value of PGR is below 0.2. When PGR increases above 0.2, the axial load capacity and axial stiffness will decrease sharply whereas improvements in radial load capacity and radial stiffness may be achieved. The study has also revealed that radial stability of a bearing is improved by partial-grooving, provided PGR is not too large. However, a bearing becomes inherently radially unstable if the bearing gap is not completely filled with lubricant, regardless of it being fully or partially grooved.     

基于离散元的模拟月岩切削负载特性数值模拟及试验验证

钻取采样是人类获取月球岩石类样品的重要手段,钻进负载的稳定性是确保月岩采样任务成功的关键因素。由于岩石属性直接决定其可钻性等级,并影响钻进负载,有必要在地面环境中,分析岩石本构模型与钻进负载之间的关系,构建不同可钻性等级的岩石样本模型,开展模拟钻进试验,获知不同钻进参数下岩石采样的钻进载荷变化情况。据此,将地面条件下大理岩样本选作模拟月岩切削特性的研究对象,基于二维颗粒流程序(Particle flow code 2 dimensions,PFC2D),建立小切削深度下仿真分析模型。利用Plackett-Burman试验(PB试验)和中心组合设计(Central composite design,CCD)方法,确定影响岩石切削特性的微观参数,开展不同切削深度下的模拟月岩切削负载试验验证。经仿真分析与试验验证能得到与岩石切削特性相对应的离散元模型,试验结果表明该仿真模型在较小切削深度条件下的切削负载变化规律与实际情况一致。通过研究获得了一种建立模拟月岩离散元切削模型的建模方法,为后续分析钻头/切削刃构型参数对钻进/切削负载的影响提供了研究条件。