The dynamic stability of a rotating pre-twisted asymmetric cross-section Timoshenko beam subjected to lateral parametric excitation

A finite element model is presented to study the dynamic stability of a pre-twisted Timoshenko beam having asymmetric aerofoil cross-section subjected to lateral parametric excitation. Solutions referred to as combination resonance are investigated. The effects of shear deformation and rotary inertia are included in the analysis. The effects of coupling due to centre of flexure distance from the centroid and the shear coefficient on the stability are considered. The change in these effects due to pre-twist angle is investigated. It is concluded from the results that the pre-twist angle has an influence on the effects of coupling and the shear coefficient.     

Dynamic stability of a rotating asymmetric cross-section blade subjected to an axial periodic force

The finite element method is applied to study the static and dynamic stability of an aerofoil cross-section rotating blade subjected to an axial periodic force. The effects of coupling due to the center of flexure (shear center) distance from the centroid, rotational speed, stagger angle and disk radius on the stability are considered. Critical load parameters are calculated and dynamic instability regions of the blade with different reference values are illustrated graphically. The numerical results indicate that the coupling effect is very important in the third and fourth modes depending on d_Y distance. The increase in stagger angle makes the blade less stable. However, the increase in rotational speed and disk radius make the blade more stable.     

Dynamic stability analysis of pretwisted aerofoil cross-section blade packets under rotating conditions

This paper presents a finite element model for the static and dynamic stability of a rotating pretwisted aerofoil cross-section two-bladed packet subjected to uniform radial periodic force. The effects of various parameters such as shroud dimensions, pretwist angle, stagger angle, rotational speed and distance of shear center from the centroid on the stability of the blade packets are presented. The numerical results show that pretwist angle and the coupling effect are important in high-frequency modes depending on shear center distance from the centroid on dynamic stability regions. The increase in stagger angle makes the pretwisted two-bladed packet less stable. However, the increase in rotational speed makes the pretwisted two-bladed packet more stable.     

Dynamic stability analysis of rotating asymmetric cross-section blade packets

In this paper static and dynamic stability of rotating asymmetric cross-section two-bladed packets subjected to uniform radial periodic force are studied using the finite element method. The effects of various parameters such as shroud dimensions, stagger angle, rotational speed and distance of shear center from the centroid on the stability of the blade packets are presented. The numerical results indicate that the coupling effect is important in frequency modes depending on shear center distance from the centroid. The increase in stagger angle makes the two-bladed packet less stable. However, the increase in rotational speed makes the two-bladed packet more stable.     

Dynamic response of coupled shaft torsion and blade bending in rotor blade system

This study develops a computational model for the dynamic characteristics of a rotor-blade system. The rotor-blade coupled model with pre-twisted blade attached to a rigid disk driven by a shaft is developed using the Lagrange equation in conjunction with the assumed mode method to discretize the blade deformation. The effects of axial shortening due to blade lagging deformation, centripetal force caused by the rotating blade, and gravity are included in the model. The coupled equation of motion is formulated based on the small deformation theory for the blade and shaft torsional deformation to obtain the dynamic characteristics of the system for various system parameters. Numerical simulations show that the pre-twist angle of the blade and the shaft torsional flexibility strongly influence the dynamic behavior of the rotor-blade system.     

回转梁动力学方程求解的数值方法研究

考虑驱动力矩、重力、哥氏力、运动阻力和惯性耦合力，根据考虑横向弯曲变形的Euler-Bemoulli细梁模型，并考虑离心力作用下的拉伸变形，建立回转运动梁的动力学模型。将梁根部的回转角度分解为整体刚性运动回转角度和刚体运动和柔性运动激发刚柔耦合回转角。将梁的柔性振动位移也分解为由整体刚性回转角所激发的动力激励振动和刚柔耦合回转角运动及刚柔耦合项共同激发的刚柔耦合振动。整体刚性回转角和动力激励振动，通过数值方法(如龙格-库塔法)求解，而将刚柔耦合回转角和刚柔耦合振动，通过奇异摄动法求解。通过数值方法和奇异摄动法相结合的方法，能够正确计算刚柔耦合回转角及其对梁振动位移的影响，更精确、深入地分析回转梁的动力学特性。     

基于带反馈Hopf振荡器的六足机器人斜坡步态发生器设计

为了提高六足机器人斜坡运动的稳定性,基于三支撑足步态,分析六足机器人的斜坡运动,得到斜坡运动静态稳定裕度与躯体俯仰角的定性关系;研究带反馈Hopf振荡器的输出特性与收敛系数、反馈量之间的关系,并设计基于带反馈Hopf振荡器的单腿三关节信号和斜坡步态发生器模型;确定收敛系数的组合,并引入躯体俯仰角构造反馈信号,实现在只改变膝关节摆角而不影响步态其他特性的情况下提高六足机器人斜坡运动的稳定性;搭建Matlab-ADAMS联合仿真平台与实物样机进行验证。仿真表明：与Hopf模型相比,基于带反馈Hopf模型六足机器人上12°斜坡稳定裕度提高6.3%,下12°斜坡稳定裕度提高7.2%;试验表明：在12°斜坡上前进1 m时,基于Hopf模型的六足机器人向左偏移0.3 m,基于带反馈Hopf模型的六足机器人向左偏移0.05 m,稳定裕度显著提高。     

钛酸酯偶联剂含量对磁流变液性能的影响

分别以不同含量的钛酸酯偶联剂为稳定剂,以羰基铁粉为磁性颗粒制备了硅油基磁流变液,比较了不同体积分数钛酸酯偶联剂对磁流变液沉降稳定性和剪切性能的影响.结果表明:磁流变液的沉降稳定性随钛酸酯偶联剂含量增多而下降,添加2%时具有最好的沉降稳定性;添加4%钛酸酯偶联剂时,在400 mT磁场下剪切屈服强度可达20 kPa;并得到...     

基于斜角切削理论的钛合金螺旋铣孔切削力建模

通过分析螺旋铣孔的加工原理和计算加工过程中的运动向量,结合侧刃和底刃对切削力的影响,建立了螺旋铣孔过程的切削力解析模型。提出了基于斜角切削的切削力系数辨识方法,并根据斜角切削过程几何关系推导出摩擦角、剪切角、剪切应力的约束方程。开展切削力系数辨识试验和钛合金螺旋铣孔试验对仿真值进行验证,结果表明,切削力的仿真值与试验值误差较小,平均误差为9.55%,从而验证了斜角切削系数辨识方法的有效性和切削力模型的正确性。     

Attenuation modelling of bulk waves generated by a point source in an isotropic medium

Attenuation of a bulk wave, generated by a point source, propagating in an isotropic medium, is due to the geometry and nature of the material involved. In numerical simulations, if the complete domain of propagation is modeled, then it captures the attenuation of a wave caused due to its geometry. To model the attenuation of the wave caused due to the nature of the material, it is required to know the material’s attenuation coefficient. Since experimental measurement on attenuation of a wave involves both the effects of geometry and material, a method based on curve fitting to estimate the material’s attenuation coefficient from effective attenuation coefficient, is proposed. Using the material’s attenuation coefficient in the framework of Rayleigh damping model, numerical modeling on attenuation of both the bulk waves - longitudinal and shear excited by a point source was carried out. It was shown that the proposed method captures the attenuation of bulk waves caused on account of geometry as well as nature of the material.     

某发射装置串联式扭杆平衡机的设计

根据平衡机需要在满载和空载两种工况下工作,设计空间限制大等特点,提出了扭杆平衡机由四连杆机构与串联式扭杆连接而成,左右两侧完全对称布置的设计方案.给出了双向串联式扭杆平衡机设计主要参数的计算方法及其加工工艺.引入扭杆单位长度刚度系数,使设计具有一定的新颖性.经预扭试验测定,其扭转角和相应的扭矩均达到了设计要求.     

低附着系数路面车道保持模型预测控制及汽车稳定性控制

提出了车道保持模型预测控制算法。在低附着系数路面上分析了双移线工况下转向角和侧向加速度二次低通滤波之差的峰值与汽车失稳程度之间的关系,基于可测信号(转向角、侧向加速度、横摆角速度和车速)提出了模糊控制算法,设计了汽车稳定性控制器。为了阻止低速和不严重失稳工况下汽车稳定性控制器的误启动,提出了不稳定指标模糊控制算法;为了避免汽车稳定性系数急速减小,设计了稳定性系数保持器。CarSim/Simulink仿真结果表明,在低附着系数路面上所设计的转向控制器和汽车稳定性控制器具有更好的车道保持能力和稳定性。     

Buckling and dynamic stability of spinning pre-twisted beams under compressive axial loads

The equations of motion of a spinning pre-twisted beam under compressive axial loads are formulated using Euler beam theory and the assumed mode method. The equations of motion are first transformed to the standard form of an eigenvalue problem for determining the critical buckling loads for various combinations of prescribed spinning speeds, aspect ratio of the cross-section and pre-twist angle of the beam. The equations of motion are then transformed to the form of another eigenvalue problem for determining the critical spinning speeds. Both the critical spinning speeds and critical buckling loads are found to exhibit similar trends of variation and curve veering phenomenon with respect to changes in the aspect ratio of the cross-section. For a spinning pre-twisted beam under axial compressive loads, the critical spinning speeds corresponding to divergent behaviours are found to be no longer the dividing points for separating the speed zones into stable and unstable regions, contrary to the stability behaviour of a nonpre-twisted beam which have distinct regions of stable and unstable spinning speed zones separated by critical spinning speeds.     

微极流体对错位轴承的润滑性能研究

错位轴承比普通径向轴承表现出了更好的性能,因此对错位轴承的静态性能和动态性能的研究具有重要意义。推导了微极性润滑时错位圆和错位椭圆轴承的动静特性方程,采用有限差分法计算微极错位圆和错位椭圆的静特性,同时采用偏导数法计算其动特性,研究耦合数和特征长度对轴承性能的影响。结果表明：随耦合数增大,错位圆和错位椭圆轴承承载力和摩擦力增大、摩擦因数减小、刚度系数和阻尼系数绝对值增大,轴承稳定性提高;随特征长度增大,错位圆和错位椭圆轴承承载能力和摩擦力减小、摩擦因数先减小再增大、刚度系数和阻尼系数绝对值减小,轴承稳定性降低;相比于牛顿流体,微极流体的承载力更大,摩擦因数更小;微极流体会加大轴承的阻尼系数和刚度系数的绝对值,并且会提高轴承的稳定性;与错位圆轴承相比,错位椭圆轴承承载力大、摩擦力大但是摩擦因数小、稳定性更好。     

十字轴式万向联轴器传动性能分析

以大型养路机械的十字轴式万向联轴器为研究对象,应用运动学理论分析了当十字轴式万向联轴器的输入轴做匀速转动时,其输出轴做圆周性不均匀转动的性能。分别从角速比、不均匀系数和角加速度等方面对十字轴式万向联轴器定量表达了其不均匀性。结果表明,随着十字轴式万向联轴器输入、输出轴夹角、转速的增大,其输出轴最大角加速度增大,使其最大惯性力矩增大。十字轴式万向联轴器的输入、输出轴夹角是影响其不均匀性的关键参数,当十字轴式万向联轴器输入、输出轴夹角增大时,其输出轴转角波动性增大。减小输入、输出轴夹角是提高十字轴式万向联轴器传动效率的唯一途径。     

The Merchant's model of orthogonal cutting revisited: A new insight into the modeling of chip formation

The Merchant's model, as the most famous approach of orthogonal cutting, is widely used in introductive courses on machining. However, the shear angle predicted by the Merchant's model from the criterion of minimization of the cutting energy, does not generally agree with experimental data and numerical simulations. The aim of this paper is to elucidate the theoretical reason for which the Merchant's model fails to predict the correct orientation of the primary shear zone. It is shown that the principle of minimum of the cutting energy must be supplemented by a stability criterion of the chip morphology. A modified Merchant's formula is then obtained for the value of the shear angle.     

A NEW MECHANISTIC MODEL FOR PREDICTING WORN TOOL CUTTING FORCES

An enhanced model for predicting worn tool cutting forces in metal cutting without the need for any worn tool calibration tests is presented in this paper. The new model utilizes a previously developed slip-line field approach in conjunction with a mechanistic force model to predict the shear flow stress and shear angle for a range of cutting conditions with only a minimal number of sharp tool calibration tests. The shear flow stress and shear angle values are then used as inputs into a worn tool force model to predict the cutting forces due to tool flank wear. Predictions of worn tool cutting forces from the new model have been compared to experimental data from both a steel and a ductile iron workpiece. Ductile iron tests are significant because previous shear flow stress and shear angle models require chip measurements which cannot be made with the chips produced by iron workpieces. Model predictions are also compared to literature data obtained using an aluminum workpiece. An excellent comparison between the model predictions and the experimental data is found for all of the materials considered.     

钻井液与钻柱的耦合纵向振动分析

考虑钻井液与钻柱的泊松耦合,基于管内外钻井液与钻柱间的耦合纵向波动方程组,应用特征矢量法,研究地层切变模量和钻柱泊松比边界对钻柱内纵向应力波和钻井液纵向压力波传输的影响。在此基础上推导钻柱纵向振动的频率方程,引入等效质量分析管内外液动压力下的钻柱固有频率。理论求解表明,钻柱的泊松效应造成应力波与压力波耦合,且管内外不同介质间的波速耦合对压力波的影响明显,而对应力波的影响较弱,故耦合对泥浆脉冲传输的影响远大于钻柱声遥测。钻井液的液动压力增加纵振的惯性质量,导致固有频率与不考虑钻井液相比有较大下降。综合等效粘惯性质量、粘性阻尼系数和钻井液惯性力,建立多因素影响下的钻柱纵向耦合振动模型。     

光折晶体Cu:KNSBN的双光束耦合——能量转移研究

二波耦合是研究光折晶体特性和品质的一种重要方法。本文着重分析基于光析变理论的二波耦合能显转移原理,研究所得的Cu:KNSBN晶体的增益系数γ与两光束入射角20的关系,以及ρ与光束比β的关系曲线。测量结果显示:Cu:KNSBN晶体具有大的τ值。最后,本文还报告了该晶体的高频时间调制传输特性,以及测得的衍射效率η与光束夹角20及调制频率f的关系曲线。     

Dynamic instability characteristics of laminated composite plates subjected to partial follower edge load with damping

The study of vibration and dynamic instability behaviour of laminated composite plates subjected to partially distributed non-conservative follower forces is presented by using the finite element technique. The first-order shear deformation theory is used to model the plate, considering the effects of shear deformation and rotary inertia. The modal transformation technique is employed to the resulting equilibrium equation for subsequent analysis. Structural damping is introduced into the system in terms of equivalent viscous damping to study the significance of damping on stability characteristics. The effects of load width, boundary condition, aspect ratio, ply orientation, direction control of the load and damping parameters are considered for the stability behaviour of the plates. The results show that under follower loading, the system is susceptible to instability due to flutter alone or due to both flutter and divergence, depending on system parameters.