【作者】 张竹林；

【导师】 张勤河； 张志刚；

【作者基本信息】 山东大学 ， 机械设计及理论， 2019， 博士

【摘要】 大型机械压力机是金属板材成形的重要设备,在汽车、轮船、飞机等行业得到了广泛应用。大型机械压力机具有技术含量高、设计和制造难度大、控制系统复杂等特点,其技术水平体现了一个国家的先进制造水平。液压过载保护系统是大型机械压力机中的重要装置之一,通过过载瞬间卸荷来保护压力机和模具,而且卸荷后能快速进行自动补压,实现保护功能的自动恢复。由于该液压过载保护系统是一个由气、液、固三相介质组成的复杂的相互耦合系统,其耦合机理和响应特性还不是十分清楚,在实际应用过程中,常存在卸荷控制精度低的问题,容易造成未达到规定载荷状态提前卸荷和达到规定载荷时不卸荷问题,导致正常生产状态的不连续和设备模具的损坏。本文以大型机械压力机液压过载保护系统为研究对象,针对液压过载保护系统未达到规定载荷状态提前卸荷和达到规定载荷时不卸荷问题,通过力学理论建模、液压系统数学建模与仿真、双向液固耦合分析和试验验证相结合的方法进行了系统研究,主要研究工作如下:在系统分析连接器工作过程中受力情况的基础上,将连接器进行模型简化,构建了连接器和液压垫等效弹簧模型,从理论上分析了连接器等效弹簧刚度、液压垫等效弹簧刚度、液压垫初始预压力分别对连接器传递压力的影响,得到如下结论:(1)卸荷阀卸荷压力越大,连接器传递力越大;(2)卸荷阀卸荷压力一定的情况下,液压垫预压力越小,连接器传递力越大;(3)连接器等效弹簧刚度系数越大,连接器传递力越大;(4)液压垫等效弹簧刚度系数越小,连接器传递力越大。建立连接器三维简化模型,采用有限元的方法,借鉴弹簧刚度公式,通过对液压垫施加不同的预压力,以活塞顶面作为位移参考,推导出连接器等效弹簧刚度拟合公式;在系统分析影响液压垫体积弹性模量因素的基础上,推导出适合工程应用的液压垫等效弹簧刚度计算公式;为了验证所建模型的合理性,运用ADAMS软件对连接器等效弹簧模型进行了建模仿真分析,通过对压力机施加不同的工作力大小来构建不同的工况,分析得出连接器等效弹簧的弹性恢复力占压力机工作力8%左右的结论,即在计算液压过载保护系统静态卸荷压力时,卸荷阀卸荷压力应设置为压力机公称力折算压力的108%,目前采用的经验值一般推荐设置为110%,所建连接器等效弹簧分析模型与工程经验值具有很好的吻合度。验证了所建连接器等效弹簧分析模型和计算方法的正确性,具有一定的工程应用参考价值。研究了大型机械压力机的液压垫预压力、压力机工作力、管道材料体积模量、管道长度、管道直径、管道壁厚等变量对管道中油液压力的影响规律。采用由局部到整体的分析思路,首先建立针对管道分析的管道模型,通过分析获得液压垫预压力、压力机工作力、管道材料体积模量、管道长度、管道直径、管道壁厚等变量各自对充液管道有效体积模量和管道中油液压力的影响规律;在此基础上,构建了包括液压垫充压过程和压力机工作力施加过程的一体化分析模型,采用单变量分析方法,系统分析了液压垫预压力、管道长度、管道直径、管道壁厚、连接器等效弹簧刚度等变量对充液管道中液体压力变化的影响,得出如下结论:(1)液压垫的初始预压力越小,管道中油液压力相对于压力机工作压力超调量越小;(2)管道长度越长,管道中油液压力越低;(3)管道直径越大,管道中油液压力越低;(4)管道壁厚对管道中油液压力变化影响不大;(5)连接器等效弹簧刚度越大,管道中油液压力越低。研究结论为工程应用提供了理论支撑。通过构建中心组合试验设计方案,获得共计60个样本数据点,计算出相应数值,并画出Pareto(帕累托)图,归纳出各参数对充液管道中油液压力的影响规律和灵敏度,获得结论如下:管道长度对管道中油液压力变化影响最大,管道越长,管道中油液压力越低;其次影响较大的是液压垫初始预压力,预压力越大,管道中油液压力越大;再次是管道直径,管道直径越大,管道中油液压力越低;连接器等效弹簧刚度系数影响最小,连接器等效弹簧刚度系数越大,管道中油液压力越低。理论分析与试验结果一致。通过拉丁超立方试验方案获得80个样本点,分别构建了液压过载保护系统多参数和三参数的响应面(Response Surface Methodology,RSM)近似模型。各变量影响管道中油液压力的变化规律与试验结论一致,说明所建RSM近似模型具有很好的工程应用价值。利用双向液固耦合直接求解方法进行了连接器与卸荷阀仿真计算,为了简化模型和提高计算效率,建立了2D双向液固耦合分析模型,研究的方法解决了活塞与缸壁之间小位移滑动下的动网格技术问题,以及先导阀阀芯和阀座之间间隙变化的网格重构技术问题,得出如下结论:活塞微小的移动都会引起液体压力的急剧变化,极易打破先导阀阀芯原有的受力平衡状态,进而引发主阀打开进行压力卸荷。将流体控制方程和结构体控制方程合并为一个控制方程,并用直接法进行求解,通过仿真计算,验证了采用直接法求解双向液固耦合模型方法的可行性,分析结论为连接器和卸荷阀之间采用直连方案的试验数据分析提供了理论支撑。液压过载保护系统具有封闭液体容腔、液体微可压缩、活塞小位移、受冲击载荷作用的特征,为了系统研究充液管道动态响应特性,建立了双向液固耦合的充液管道分析模型,研究的方法解决了封闭液体容腔在类似冲击载荷作用下的双向液固耦合难以收敛的问题,经仿真分析得出:液体和管道的相互耦合作用,会使管道内产生液体压力的波动,导致管道内液体压力大于压力机工作载荷压力,在液体中会产生冲击波,在压力机工作过程中,不同管道位置的液体压力不同,以某个周期来回传递,第一个压力冲击波的数值为最大,然后液体压力逐渐收敛趋于稳定。管道壁在冲击载荷作用下,会产生振动现象,在弯管位置振动幅值最大。研究的双向液固耦合分析方法,为解决具有封闭液体容腔、小位移、微可压缩流体、冲击载荷特征的双向液固稱合问题提供了技术参考。通过试验方法研究了管道长度、液压垫初始预压力、压力机工作力三个变量对管道内液体压力的影响,设计了卸荷阀与连接器采用直连、加长管一和加长管二的三种试验方案,研究了不同液压垫预压力、不同压力机工作力作用下,充液管道内的液体压力响应特性,通过分析对比,归纳出管道内液体压力变化规律,得出如下结论:(1)无论液压垫预压力值取多大,在同样的压力机工作力作用下,较长管道能够降低管道内液体压力,直连方式的管道内液体压力、液体压力超调量、液体压力超调比例都是最大的;(2)在同样的压力机工作力作用下,不同的液压垫预压力、不同的管道长度会对管道内液体压力产生不同的影响;(3)通过试验发现,当液压垫预压力为压力机工作压力的50%(125bar)时,随着工作力的增加,管道内液体压力逐渐减低,有可能受管道结构频响特性的影响,压力机工作力激励频率接近管道共振频率引起管道内液体压力相较其它预压力值下都较大,但随着工作力的增加,导致管道直径变大,管道内液体压力值有所减小。将加长管一的试验数据与所建的三参数RSM近似模型分析数据进行对比,二者具有很好的吻合度,说明所建三参数RSM近似模型具有高置信度。通过将试验数据进行归类,分别构建了三种试验方案的RSM近似模型,该模型能够帮助企业在设计液压过载保护系统过程中,快速得出液压垫预压力、压力机工作力、管道长度和管道内液体压力的耦合响应关系,提高设计效率。更多还原

【Abstract】 Large-scale mechanical presses are high-end equipment and are important equipment for sheet metal forming.They have been widely used in automobiles,ships,and airplanes.Large mechanical presses have the characteristics of high technical content,difficulty in design and manufacture,and complex control systems.Their technical level reflects the advanced manufacturing level of a country.The hydraulic overload protection system is one of the important devices in the large mechanical press.If the pressure is overloaded,the unloading can be performed instantaneously to protect the press and avoids damage to the mold.It also has the function of automatic pressure compensation after unloading and restores automatic protection functions.Because the hydraulic overload protection system is a complex mutual coupling system composed of gas,liquid and solid three-phase medium,the coupling mechanism and response characteristics are not very clear.In the actual application process,the unloading control accuracy is often low.It is easy to cause the unloading problem when the pre-loading is not reached before the specified load state and the specified load is reached,resulting in the discontinuity of the normal production state and the damage of the equipment mold.This paper takes the hydraulic overload protection system of large mechanical press as the research object.Aiming at the problem that the hydraulic overload protection system does not reach the specified load state ahead of unloading and does not reach the specified load unloading problem,the systematic research was carried out by means of mechanical theory modeling.mathematical modeling and simulation of hydraulic system,two-way liquid-solid interaction analysis and experimental verification.The main research work is as follows:On the basis of system analysis of connector working process,the connector model is simplified and the equivalent spring model of the connector and the hydraulic pad is constructed.Theoretically,the effects of equivalent spring stiffness of connector,equivalent spring stiffness of hydraulic pad and initial pre-pressure of hydraulic pad on the connector’s transmission force are studied respectively.The following conclusions are obtained:(1)The greater the unloading pressure of the unloading valve,the greater the transmission force of the connector;(2)When the unloading pressure of the unloading valve is constant,the smaller the pre-pressure of the hydraulic pad,the greater the transmission force of the connector;(3)The greater the equivalent spring rate coefficient of the connector,the greater the transmission force of the connector;(4)The smaller the equivalent spring stiffness coefficient of the hydraulic pad,the greater the transmission force of the connector.The finite element method is used to analyze the three-dimensional model of the connector.By applying different pre-pressure to the hydraulic pad and using the top surface of the piston as the displacement reference,the equivalent spring stiffness fitting formula of the connector is derived.Based on the systematic analysis of the factors affecting the volume elastic modulus of the hydraulic pad,a formula for calculating the equivalent spring stiffness of the hydraulic pad is derived;In order to verify the rationality of the model,ADAMS software is used to simulate the equivalent spring model of connector.By applying different press working forces to construct different working conditions,it is concluded that the elastic restoring force of the equivalent spring of the connector accounts for about 8%of the working force of the press.That is,when calculating the static unloading pressure of the hydraulic overload protection system,the overshoot of liquid pressure should be 108%of the working pressure of the press,and the empirical value currently used is generally recommended to be 110%.Therefore,from the point of view of static load analysis,the analysis model has a good agreement with the engineering experience value.It is verified that the equivalent spring analysis model built for connector and calculation method are correct and have high reference value for engineering applicationThe influence of hydraulic pad pre-pressure,working force,elastic modulus of pipe material,pipe length,pipe diameter,pipe wall thickness and other variables on oil pressure of large mechanical press were studied.Using the analysis idea from partial to whole,firstly establish a mathematical model only for pipeline analysis.Through the analysis,the influences of the hydraulic pad pre-pressure,the working force of the press,the bulk modulus of the pipe material,the length of the pipe,the diameter of the pipe,the wall thickness of the pipe,etc.on the effective bulk modulus of the liquid-filled pipeline and the oil pressure of the pipeline are obtained.On this basis,an integrated analysis model including hydraulic cushion filling process and press working force application process is constructed.Using univariate analysis method,the influence of pre-pressure of hydraulic pad,length of pipe,diameter of pipe,wall thickness of pipe,equivalent spring stiffness of connector and other variables on the pressure change of liquid-filled pipe was analyzed,the conclusions are obtained as follows:(1)The smaller the initial pressure of the hydraulic pad,the smaller the overshoot of the oil pressure in the pipeline relative to the working pressure of the press;(2)The longer the pipe length,the lower the oil pressure in the pipe;(3)The larger the pipe diameter,the lower the oil pressure in the pipe;(4)The wall thickness has little effect on the change of oil pressure:(5)The greater the equivalent spring stiffness of the connector,the lower the oil pressure in the pipeline.The research results provide theoretical support for engineering applicationBy constructing the center combined test design scheme,a total of 60 sample data points was used to calculate the Pareto diagram.The influence law and sensitivity of each parameter on the oil pressure in the filling pipeline are summarized,and the following conclusions are obtained:The length of the pipeline has the greatest influence on the change of oil pressure in the pipeline.The longer the pipeline,the lower the oil pressure in the pipeline;The second major impact is the hydraulic pad initial pre-pressure.The greater the pre-pressure,the greater the oil pressure in the pipeline;It is pipeline diameter again,namely pipeline diameter is bigger,the oil pressure in the pipeline is lower;The effect of the connector’s equivalent spring stiffness coefficient is minimal,that is,the greater the connector’s equivalent spring stiffness coefficient,the lower the oil pressure in the pipeline.The analysis results are consistent with the experimental results.80 sample points were obtained through the Latin hypercube test scheme,and the multi-parameter and three-parameter RSM(Response Surface Methodology)Response Surface approximation model was respectively constructed.The variation law of the oil pressure in the pipeline affected by each variable is consistent with the experimental conclusion,indicating that the established RSM approximation model has good engineering application value.The bidirectional liquid-solid coupling direct solution method is used to simulate the model of the connector and the unloading valve.In order to simplify the model and improve the calculation efficiency,a 2D bidirectional liquid-solid coupling analysis model was established.The research method solved the problem of dynamic mesh technology under the small displacement sliding between the piston and the cylinder wall,and the mesh reconstruction technology problem of the gap between the pilot valve spool and the valve seat.It is concluded that the slight movement of the piston xwill cause a sharp change in the liquid pressure,which easily breaks the stress balance state of the pilot valve spool,and then causes the main valve to open for pressure unloading.The liquid control equation and the structural control equation are combined into a control equation,and the direct method is used to solve the problem.It is proved that it is feasible to solve the bidirectional liquid-solid coupling model Imethod by direct method.The analysis conclusion provides theoretical support for the experimental data analysis of the direct connection scheme.The hydraulic overload protection system has the characteristics of closed liquid chamber,liquid micro-compressibility,small displacement and impact load.In order to systematically study the dynamic response characteristics of the liquid filled pipeline,a two-way liquid-solid coupling liquid-filled pipeline analysis model was established.The research method solved the problem that the bidirectional liquid-solid coupling of a closed liquid cavity is difficult to converge under the action of similar impact load.Through simulation analysis,it is concluded that:The mutual coupling of the liquid and the pipe will cause fluctuations in the liquid pressure in the pipe,causing the liquid pressure in the pipe to be greater than the working load pressure of the press,and a shock wave will be generated in the liquid.During the process of the press,the liquid pressure at different pipe positions is different,passing back and forth in a certain cycle.The value of the first pressure shock wave is the maximum,and then the liquid pressure gradually converges to stabilize.Under the impact load,the pipe wall will produce vibration phenomenon,and the vibration amplitude is the largest at the bend position.The bidirectional liquid-solid coupling analysis method has provided a technical reference for solving the bidirectional liquid-solid coupling problem with closed liquid cavity,small displacement,micro-compressible liquid and impact load characteristics.The effects of three variables of pipe length,initial pressure of hydraulic pad and working force of the press on the liquid pressure in the pipeline were studied by the test method.Three test schemes including unloading valve and connector adopting direct connection,lengthened pipe mode one and lengthened pipe mode two are designed.The liquid pressure response characteristics of the liquid-filled pipeline under different hydraulic pad pre-pressure and different press working forces were studied.Through the analysis and comparison,the variation law of liquid pressure in the pipeline was obtained,and the following conclusions were drawn:(1)No matter what the pre-pressure value of the hydraulic pad is,under the same working force of the press,the longer pipe can reduce the liquid pressure in the pipe.The liquid pressure,overshoot of liquid pressure and overshoot ratio of liquid pressure in the pipe with direct connection mode are the maximum;(2)Under the same working force of the press,different hydraulic pad pre-pressure and different pipe length will have different effects on the liquid pressure in the pipe;(3)Through experiments,it is found that when the hydraulic pad pre-pressure is 50%(125 bar)of the working pressure of the press.as the working force increases,the pipe force is gradually reduced,which may be affected by the frequency response characteristics of the pipe structure,and the working force of the press is excited.The frequency close to the resonance frequency of the pipeline causes the liquid pressure in the pipeline to be larger than other pre-pressure values,but as the working force increases,the diameter of the pipeline becomes larger,and the liquid pressure value in the pipeline decreases.The experimental data of the extended tube one is compared with the three-parameter RSM approximation model,it is verified that the test results are in good agreement with the simulation results,indicating that the three-parameter RSM approximation model has a high degree of confidence.By collating and analyzing the test data,the RSM approximation model is constructed respectively,which can help the enterprise to quickly obtain the coupling response relationship between hydraulic pad pre-pressure,press working force,pipe length and liquid pressure in the pipeline during the design process,improve the design efficiency.更多还原