Dynamic stiffness and damping coefficients of aerodynamic tilting-pad journal bearings

The dynamic gas–film forces of aerodynamic bearing often can be characterized by eight linear stiffness and damping coefficients. How to theoretically predict these coefficients is a very difficult issue for tilting-pad gas bearing design because of its structural complexity. The current study presents a novel and universal theoretical analysis method for calculating the dynamic stiffness and damping coefficients of aerodynamic tilting-pad bearing. The gas–film pressure within the bearing is expressed in the form of dimensionless compressible gas-lubricated Reynolds equation, which is solved by means of the finite element method. With the assumption that the journal and the pads are disturbed with the same frequency, the dynamic coefficients of tilting-pad gas bearing are computed by using the partial derivative method and the equivalent coefficient method. Finally, the investigations are conducted about the effects of bearing number, perturbation frequency of the journal and the pads, eccentricity ratios, preload and length-to-diameter ratio of the bearing on the dynamic coefficients of aerodynamic tilting-pad journal bearing. The numerical results indicate that the dynamic stiffness and damping coefficients of tilting-pad gas bearing are closely related with these factors. The proposed analytical method provides a valuable means of predicting dynamic performances of tilting-pad gas bearing. The solution can be used for the purpose of prediction of dynamic behavior of the rotor systems supported by aerodynamic tilting-pad bearings.     

A study on calibration of coefficients in end milling forces model

This paper presents an improved approach to calibrate the cutting coefficients in an end-milling model. In order to predict end-milling forces, lots of simulative models are established. In order to use them, coefficients in the models, for example, cutting pressure constants etc., must firstly be calibrated experimentally, and simulative precision and applicability of the models are influenced by them. For simplicity, using average forces to calibrate cutting parameters are widely adopted by lots of researchers. However, the existence of an instruments zero-drift, noise, etc., will have effect on the precision of experimental data, so, it is difficult to directly obtain exact average-cutting forces through experimental data. Aiming at the above problem, the paper investigates milling forces in the frequency domain, discusses the impact of experimental data at different frequencies on cutting force coefficients and the influence of sensitivity of error on experimental data at different frequencies on coefficients is studied. Based on the research, an improved method to calibrate the cutting coefficients is provided. Based on a series of experiments and numerical simulations, the validity of the method is confirmed. At the end of the paper, some useful conclusions are drawn.     

角接触球轴承动特性的数值与试验研究

角接触球轴承的动特性对机床主轴的回转精度、可靠性及寿命等有重要影响。以弹性力学理论、滚动轴承动力学和沟道控制理论为基础,研究角接触球轴承的动特性。推导了计及过盈配合量和预紧力影响的高速角接触球轴承动刚度计算方法,并对轴承动刚度的影响因素进行了理论分析和试验验证。结果表明:随着过盈量的增大,轴向刚度逐渐减小,径向刚度则逐渐增大,外圈过盈量影响更显著;预紧力过小时,刚度会出现明显的波动现象,适当的预紧力对角接触球轴承刚度的稳定至关重要。     

Experimental determination of the stiffness and damping coefficients of fluid film bearings by means of step forces

A method of identifying bearing dynamic coefficients by means of step forces is described. Obtaining an accurate fast Fourier transform of the rapidly varying step function is difficult since the limitations of real time analysis by minicomputer make truncation necessary. This paper therefore proposes a feasible approach to direct analysis of the spectrum of the step function and introduces its application to measure the stiffness and damping coefficients of journal bearings. The approach followed is to use a step exciting force on the test bearings. By using the proposed direct spectrum analysis method, the transfer functions of the bearing systems and eventually the dynamic coefficients of the journal bearings can be derived.     

An experimental investigation of oblique cutting mechanics

In this study, an experimental investigation of oblique cutting process is presented for titanium alloy Ti-6Al-4V, AISI 4340, and Al 7075. Important process parameters such as shear angle, friction angle, shear stress, and chip flow angle are analyzed. Transformation of the data from the orthogonal cutting test results to oblique cutting process is applied, and the results are compared with actual oblique cutting tests. Effects of hone radius on cutting forces and flank contact length are also investigated. It is observed that the shear angle, friction angle, and shear stress in oblique cutting have the same trend with the ones obtained from the orthogonal cutting tests. The transformed oblique force coefficients from orthogonal tests have about 10% discrepancy in the feed and tangential directions. For the chip flow angle, the predictions based on kinematic and force balance results yield better results than Stabler's chip flow law. Finally, it is shown that the method of oblique transformation applied on the orthogonal cutting data yields more accurate results using the predicted chip flow angles compared to the ones obtained by the Stabler's rule.     

非稳态油膜力的转子-定子-轴承系统碰摩故障研究

利用拉格朗日（Lagrange）方程建立了非稳态油膜力的转子-定子-轴承系统碰摩故障的力学模型,应用数值分析对其进行研究,得出转子系统在激励频率作为唯一控制参数时系统的轴心轨迹图和分岔图;对该系统响应的非线性行为和故障机理进行分析,从而为该类转子系统故障诊断和系统的安全运行提供理论依据.研究结果表明,当激励频率作为唯一控制参数时,系统存在周期运动、拟周期运动和混沌运动等丰富的非线性现象.     

An experimental study on the adhesion at a nano-contact

Nano-adhesion characteristics between scanning probe microscope (SPM) tips of various radius of curvature and flats of different materials were experimentally studied. Adhesion and friction forces between Si-wafer (1 0 0) and Si₃N₄ tips were measured under various applied normal loads, and the results were compared to those of diamond-like carbon (DLC), tungsten incorporated diamond-like carbon (W-DLC) and octadecyltrichlorosilane (OTS) self-assembled monolayer (SAM) formed on Si-wafer surfaces. Also in order to study the effect of capillary force, tests were performed in various relative humidity. Results showed that the adhesion increased with the tip radius. When the applied normal load increased from 0 to 40 nN, the adhesion did not change, but the friction increased linearly. Results generally showed that surfaces of the more hydrophobic property revealed the lower adhesion. The adhesion forces increased with the relative humidity. The nano-adhesion phenomenon was discussed on the basis of JKR model and capillary force exerted by meniscus.     

An experimental study on ejection forces of injection molding

In injection molding, the ejection forces decrease when the surface roughness of mold decreases. However, the ejection forces increase contrary in the area of surface roughness is less than 0.2 μmRa. Several kinds of PVD films are coated on the mold cores to prevent the increasing of ejection forces. Some of these PVD films showed effective prevention of increasing of ejection forces.     

轮胎刚度和阻尼非线性模型的解析研究

根据轮胎的静态和动态试验结果,建立了轮胎刚度和阻尼的非线性解析模型.该模型分析了轮胎刚度与轮胎变形量、充气压力和振动频率之间的数值解析关系;同时还分析了轮胎阻尼与振动速度、轮胎变形速度和充气压力之间的关系.     

气体动压轴承非线性动力学行为和稳定性预测

以球面螺旋槽气体动压轴承为研究对象,建立三维微气膜瞬态流场数学模型,采用流体动力学软件,对轴承三维气膜压力场进行仿真分析,得到使轴承承载能力最大的结构参数和运行参数。研究在最大承载力下,不同转速和偏心率对气体轴承瞬态刚度系数和阻尼的影响规律,探索气浮轴承瞬态非线性动力学行为。模拟出转子受转速影响下的轴心轨迹图,研究轴承-转子系统的稳定性。研究结果表明:槽宽比、槽深比、偏心率对轴承承载特性的影响明显;提高转速和偏心率有助于轴承的稳定,但随着转速的升高,轴承转子系统的稳定性接近临界状态,导致轴承转子系统不稳定,而较大的偏心率,易导致轴承产生碰摩现象,也破坏了轴承的稳定性,因此,在轴承设计过程中应合理选择轴承的设计参数,提高轴承的综合性能。     

GH4169高温合金铣削力试验研究

针对高温合金材料GH4169的切削加工性差的特点,设计了铣削力测量试验,以研究切削用量、冷却措施等因素对高温合金数控铣削过程产生的铣削力的影响。通过铣削试验分析,得到了切削参数与铣削力的变化曲线,提供了合理的切削参数;利用线性回归方法建立了铣削力经验公式,并通过了试验验证;进行了干切削与乳化液冷却的铣削力对比试验。试验表明选择合理的切削参数和冷却措施能有效抑制铣削力的产生。     

角度伺服阀芯阻力矩的测试与研究

就新型液压伺服关节中内置伺服阀芯阻力矩问题进行了探讨.在理论分析的基础上,建立了伺服阀芯阻力矩测试的实验系统,应用支反力法测得了实验数据,并对测试结果进行了比较分析,得出了摩擦阻力矩与系统压力的关系曲线,为选择合适的步进电机,进而进行结构设计打下了基础.     

螺旋槽气体轴承动态特性及失稳分析

针对高速动静压气体轴承气膜的复杂非线性动力学行为,以球面螺旋槽动静压气体轴承为研究对象,建立润滑分析数学模型;采用有限差分法与导数积分法进行求解,得到动态扰动压力分布及动态特性系数,并研究切向供气条件下螺旋槽参数、径向偏心率、供气压力、转速对气膜刚度阻尼系数的影响规律;建立线性稳定性计算模型,预测气膜涡动失稳转速,分析运行参数对失稳转速的影响。结果表明:气膜阻尼是一种抑制涡动的因素,气膜的稳定性取决于气膜刚度与阻尼的协同作用;气膜刚度阻尼随着槽宽比、槽深比、螺旋角的增大,整体上呈先增大后减小的趋势;刚度随转速的升高而增大,阻尼则随转速的升高而减小;径向偏心率和供气压力越大,气膜刚度和阻尼越大;在一定范围内,提高供气压力、增大径向偏心率能够提高系统失稳转速;合理地选取轴承结构参数和运行参数,能够优化轴承动态特性,保证气体轴承较高的运行稳定性。     

玻璃钢复合材料夹层结构钻削试验研究

玻璃钢复合材料夹层结构凭借其优越的性能在航空航天、国防等领域得到了广泛应用,但在钻孔的过程中经常因钻削轴向力过大而出现分层、撕裂等缺陷.为此,利用正交试验方法,选用不同材质的钻头、不同切削参数对玻璃钢复合材料夹层结构进行钻削试验,分析这些因素与钻削轴向力之间的关系,得出最适合的切削参数.然后通过方差分析,得出各因素对钻削轴向力的影响程度.最后从钻削工艺角度提出改进措施,进一步提高钻孔质量.     

An Improved Method for the Determination of 3D Cutting Force Coefficients and Runout Parameters in End Milling

A simple improved method is suggested for determining constant cutting force coefficients, irrespective of the cutting condition and cutter rotation angle. This can be achieved through the combination of experimentally deternimed cutting forces with those from simulation, performed by a mechanistic cutting force model and a geometric uncut chip thickness model. Additionally, this study presents an approach that estimates runout-related parameters, and the runout offset and its location angle, using only one measurement of cutting force. This method of estimating 3D end milling force coefficients was experimentally verified for a wide range of cutting conditions, and gave significantly better predictions of cutting forces than any other methods. The estimated value of the runout offset also agreed well with the measured value.     

可调式线性油压减振器的许用磨损量研究

为减振器的寿命、材料、密封设计以及维修工作提供依据和检测标准,以可调式线性油压减振器为研究对象,对其许用磨损量进行了研究,建立了减振器阻尼性能受其关键磨损量影响的数学关系模型,对关键磨损量影响阻尼性能的数值分析结果、机理进行了分析,并以额定典型工作点最大可调阻尼力允许下降的极限值为目标,对油压减振器的许用磨损量者了求解。给出了试验验证的方法和结果。     

基于非线性理论的齿轮机构动力学模型的建立及实验

在研究分析了轮齿的变啮合刚度和轮齿间的油膜动力学性能的基础上，提出了把此油膜看成是一个质量-弹簧-阻尼系统的假设，并运用弹流理论，分析了油膜厚度和油膜等效刚度，建立了相应的动力学模型和运动微分方程。采用GEAR方法求解非线性动力学微分方程组，并通过实验验证齿侧间隙对齿轮机构动态特性的影响。     

油气悬架在全液压底盘上的应用与建模

为克服传统的弹簧悬架系统线性特性的缺点,在全液压底盘上采用先进的油气悬架技术。油气悬架系统具有非线性的变刚度特性和变阻尼特性,有助于提高车辆的平顺性和稳定性。根据车辆工作要求,对悬架系统进行部分参数计算,为油气悬架的设计提供理论依据。根据双缸耦连物理模型,建立刚度特性和阻尼特性的非线性数学模型,为油气悬架的参数选择和优化奠定基础。合理地选择油气悬架系统的工作参数,将会进一步提高车辆的平顺性和稳定性。     

Improved dynamic cutting force model in ball-end milling. Part I: theoretical modelling and experimental calibration

An accurate cutting force model of ball-end milling is essential for precision prediction and compensation of tool deflection that dominantly determines the dimensional accuracy of the machined surface. This paper presents an improved theoretical dynamic cutting force model for ball-end milling. The three-dimensional instantaneous cutting forces acting on a single flute of a helical ball-end mill are integrated from the differential cutting force components on sliced elements of the flute along the cutter-axis direction. The size effect of undeformed chip thickness and the influence of the effective rake angle are considered in the formulation of the differential cutting forces based on the theory of oblique cutting. A set of half immersion slot milling tests is performed with a one-tooth solid carbide helical ball-end mill for the calibration of the cutting force coefficients. The recorded dynamic cutting forces are averaged to fit the theoretical model and yield the cutting force coefficients. The measured and simulated dynamic cutting forces are compared using the experimental calibrated cutting force coefficients, and there is a reasonable agreement. A further experimental verification of the dynamic cutting force model will be presented in a follow-up paper.