科创中国

Corner contact in gear pair causes vibration and noise,which has attracted many attentions.However,teeth errors and deformation make it difficulty to determine the point situated at corner contact and study the mechanism of teeth impact friction in the current researches.Based on the mechanism of corner contact,the process of corner contact is divided into two stages of impact and scratch,and the calculation model including gear equivalent error-combined deformation is established along the line of action.According to the distributive law,gear equivalent error is synthesized by base pitch error,normal backlash and tooth profile modification on the line of action.The combined tooth compliance of the first point lying in corner contact before the normal path is inversed along the line of action,on basis of the theory of engagement and the curve of tooth synthetic compliance&load-history.Combined secondarily the equivalent error with the combined deflection,the position standard of the point situated at corner contact is probed.Then the impact positions and forces,from the beginning to the end during corner contact before the normal path,are calculated accurately.Due to the above results,the lash model during corner contact is founded,and the impact force and frictional coefficient are quantified.A numerical example is performed and the averaged impact friction coefficient based on the presented calculation method is validated.This research obtains the results which could be referenced to understand the complex mechanism of teeth impact friction and quantitative calculation of the friction force and coefficient,and to gear exact design for tribology.

Wear is a major factor of disc cutters' failure. No current theory offers a standard for the prediction of disc cutter wear yet. In the field the wear prediction method commonly used is based on the excavation length of tunnel boring machine(TBM) to predict the disc cutter wear and its wear law, considering the location number of each disc cutter on the cutterhead(radius for installation); in theory, there is a prediction method of using arc wear coefficient. However, the preceding two methods have their own errors, with their accuracy being 40% or so and largely relying on the technicians' experience. Therefore, radial wear coefficient, axial wear coefficient and trajectory wear coefficient are defined on the basis of the operating characteristics of TBM. With reference to the installation and characteristics of disc cutters, those coefficients are modified according to penetration, which gives rise to the presentation of comprehensive axial wear coefficient, comprehensive radial wear coefficient and comprehensive trajectory wear coefficient. Calculation and determination of wear coefficients are made with consideration of data from a segment of TBM project(excavation length 173 m). The resulting wear coefficient values, after modification, are adopted to predict the disc cutter wear in the follow-up segment of the TBM project(excavation length of 5621 m). The prediction results show that the disc cutter wear predicted with comprehensive radial wear coefficient and comprehensive trajectory wear coefficient are not only accurate(accuracy 16.12%) but also highly congruous, whereas there is a larger deviation in the prediction with comprehensive axial wear coefficient(accuracy 41%, which is in agreement with the prediction of disc cutters' life in the field). This paper puts forth a new method concerning prediction of life span and wear of TBM disc cutters as well as timing for replacing disc cutters.

The accurate measurement on the compressibility and thermal expansion coefficients of density standard liquid at 2329kg/m3(DSL-2329) plays an important role in the quality control for silicon single crystal manufacturing. A new method is developed based on hydrostatic suspension principle in order to determine the two coefficients with high measurement accuracy. Two silicon single crystal samples with known density are immersed into a sealed vessel full of DSL-2329. The density of liquid is adjusted with varying liquid temperature and static pressure, so that the hydrostatic suspension of two silicon single crystal samples is achieved. The compression and thermal expansion coefficients are then calculated by using the data of temperature and static pressure at the suspension state. One silicon single crystal sample can be suspended at different state, as long as the liquid temperature and static pressure function linearly according to a certain mathematical relationship. A hydrostatic suspension experimental system is devised with the maximal temperature control error ±50 μK; Silicon single crystal samples can be suspended by adapting the pressure following the PID method. By using the method based on hydrostatic suspension principle, the two key coefficients can be measured at the same time, and measurement precision can be improved due to avoiding the influence of liquid surface tension. This method was further validated experimentally, where the mixture of 1, 2, 3-tribromopropane and 1,2-dibromoethane is used as DSL-2329. The compressibility and thermal expansion coefficients were measured, as 8.5′10–4 K–1 and 5.4′10–10 Pa–1, respectively.

Tribological properties of impregnated graphite are greatly influenced by preparation technology and working conditions and it's highly susceptible to corrosion environmental impacts,but the experimental research about it are few.In this paper,three kinds of impregnated graphite samples are prepared with different degree of graphitization,the tribological properties of these samples in the dry friction environment and in a corrosive environment are analyzed and contrasted.The tribo-test results show that the friction coefficient of samples is reduced and the amount of wear of samples increase when the graphitization degree of samples increases in dry friction condition.While in a corrosive environment(samples are soaked N2O4),the friction coefficient and amount of wear are changed little if the graphitization degree of samples are low.If the degree of graphitization increase,the friction coefficient and amount of wear of samples increase too,the amount of wear is 2 to 3 times as the samples tested in the non-corrosive environment under pv value of 30MPa?m/s.The impregnated graphite,which friction coefficient is stable and graphitization degree is in mid level,such#2,is more appropriate to have a work in the corrosion conditions.In this paper,preparation and tribological properties especially in corrosive environment of the impregnated graphite is studied,the research conclusion can provide an experimental and theoretical basis for the selection and process improvement of graphite materials,and also provide some important design parameters for contact seal works in a corrosive environment.

The T-junction model of engine exhaust manifolds significantly influences the simulation precision of the pressure wave and mass flow rate in the intake and exhaust manifolds of diesel engines. Current studies have focused on constant pressure models, constant static pressure models and pressure loss models. However, low model precision is a common disadvantage when simulating engine exhaust manifolds, particularly for turbocharged systems. To study the performance of junction flow, a cold wind tunnel experiment with high velocities at the junction of a diesel exhaust manifold is performed, and the variation in the pressure loss in the T-junction under different flow conditions is obtained. Despite the trend of the calculated total pressure loss coefficient, which is obtained by using the original pressure loss model and is the same as that obtained from the experimental results, large differences exist between the calculated and experimental values. Furthermore, the deviation becomes larger as the flow velocity increases. By improving the Vazsonyi formula considering the flow velocity and introducing the distribution function, a modified pressure loss model is established, which is suitable for a higher velocity range. Then, the new model is adopted to solve one-dimensional, unsteady flow in a D6114 turbocharged diesel engine. The calculated values are compared with the measured data, and the result shows that the simulation accuracy of the pressure wave before the turbine is improved by 4.3% with the modified pressure loss model because gas compressibility is considered when the flow velocities are high. The research results provide valuable information for further junction flow research, particularly the correction of the boundary condition in one-dimensional simulation models.

The identification of maximum road friction coefficient and optimal slip ratio is crucial to vehicle dynamics and control.However,it is always not easy to identify the maximum road friction coefficient with high robustness and good adaptability to various vehicle operating conditions.The existing investigations on robust identification of maximum road friction coefficient are unsatisfactory.In this paper,an identification approach based on road type recognition is proposed for the robust identification of maximum road friction coefficient and optimal slip ratio.The instantaneous road friction coefficient is estimated through the recursive least square with a forgetting factor method based on the single wheel model,and the estimated road friction coefficient and slip ratio are grouped in a set of samples in a small time interval before the current time,which are updated with time progressing.The current road type is recognized by comparing the samples of the estimated road friction coefficient with the standard road friction coefficient of each typical road,and the minimum statistical error is used as the recognition principle to improve identification robustness.Once the road type is recognized,the maximum road friction coefficient and optimal slip ratio are determined.The numerical simulation tests are conducted on two typical road friction conditions(single-friction and joint-friction)by using CarSim software.The test results show that there is little identification error between the identified maximum road friction coefficient and the pre-set value in CarSim.The proposed identification method has good robustness performance to external disturbances and good adaptability to various vehicle operating conditions and road variations,and the identification results can be used for the adjustment of vehicle active safety control strategies.

Thermal damage caused by frictional heat of rolling-sliding contact is one of the most important failure forms of wheel and rail.Many studies of wheel-rail frictional heating have been devoted to the temperature field,but few literatures focus on wheel-rail thermal stress caused by frictional heating.However,the wheel-rail creepage is one of important influencing factors of the thermal stress.In this paper,a thermo-mechanical coupling model of wheel-rail rolling-sliding contact is developed using thermo-elasto-plastic finite element method.The effect of the wheel-rail elastic creepage on the distribution of heat flux is investigated using the numerical model in which the temperature-dependent material properties are taken into consideration.The moving wheel-rail contact force and the frictional heating are used to simulate the wheel rolling on the rail.The effect of the creepage on the temperature rise,thermal strain,residual stress and residual strain under wheel-rail sliding-rolling contact are investigated.The investigation results show that the thermally affected zone exists mainly in a very thin layer of material near the rail contact surface during the rolling-sliding contact.Both the temperature and thermal strain of rail increase with increasing creepage.The residual stresses induced by the frictional heat in the surface layer of rail appear to be tensile.When the creepage is large,the frictional heat has a significant influence on the residual stresses and residual strains of rail.This paper develops a thermo-mechanical coupling model of wheel-rail rolling-sliding contact,and the obtained results can help to understand the mechanism of wheel/rail frictional thermal fatigue.

冷凝系数为冷凝流率与碰撞流率之比,是气体冷凝过程的重要性质.本文采用分子动力学方法,探讨了不同温度(95.5、104-3、113-3、123.2)K下,氩流体气液两相平衡体系中气相的冷凝过程.模拟得到了氩流体气液相主体范围、Gibbs界面位置及界面厚度;并分别以气相主体与界面区的分界面|z8|及Gibbs界面作为碰撞界面,统计得到了氩流体的冷凝系数,并与文献值进行了比较.模拟结果表明,在相同温度条件下,以Gibbs界面为碰撞界面得到的碰撞粒子数目明显高于以|z8|为碰撞界面得到的碰撞粒子数目.当采用|z8|界面作为碰撞界面时,冷凝系数出随着温度的变化规律与文献值一致,均随着温度的升高而降低,变化范围在0.822与0.596之间;但以Gibbs界面作为碰撞界面时,所得冷凝系数口,基本上与温度无关,其值在0.335左右,且a2明显小于a1.

以探讨我国公共图书馆资源配置公平性为目的,应用基尼系数和和洛伦茨曲线,对我国31个地区的公共图书馆数、从业人数、高级职称人数、馆藏总量、本年新购藏量、财政拨款、图书购置费、计算机数、年流通人次、书刊文献外借册数、累计发放有效借阅证数、读者活动参加人次12个指标的人口公平性进行分析.结果显示,公共图书馆数、从业人数、计算机数3个指标的公平性较好,其他9个指标的公平性较差,其中图书购置费的不公平性很大.研究结论可为我国公共图书馆资源公平配置提供实证依据.

声学多普勒水流剖面仪是目前较先进的河道测流设备.按《规范》要求,利用该仪器前,需要对该仪器与流速仪测流的实测流量进行对比实验,计算其换算系数.在分析ADCP测流原理的基础上,利用引黄干渠张二庄水文站实测流量资料,对两种测流方法测量结果进行对比分析.根据最小二乘法原理,建立回归直线,并对其进行检验.对回归直线进行拟合优度检验,拟合优度系数为0.982;对回归直线用F检验法进行截距检验,回归效果显著.在该断面用ADCP测流时,流量换算系数为0.9911.