| Hydraulic excavators are always used in the bad working environment,such as mining,building,road surface construction engineering.The dynamic load on the structures is the main reason for the structure failures,which includes the failure of the working device during the excavating operations.Therefore,"improving the structural strength,reliability and durability" is also included in the key development tasks of the "Thirteenth-Five National Strategy Research and Invention Program" in construction machinery industry.The excavating operations are realized by the movement of the working device.The heavy working device leads to the lower productivity and higher energy consumption,whereas the lighter weight will result in the decreasing the structural strength.The precise design of the working device is one of the key technologies to determine the qualities and performances of the excavator.The working device is a multi-cylinder-driven complex mechanical system.Factors,such as the dynamic characteristics of hydraulic cylinders,structural elasticity,and the impact between the bucket and the digging objects,have a significant impact on the dynamic digging loads and dynamic stress.Hence,according to the basic theoretical issues existing in the dynamic design of the hydraulic excavator working device,a flexible-body dynamic model of hydraulic excavator working device,which consists of the hydraulic cylinder dynamic characteristics and the excavator-soil interaction sub-model was established in this thesis.Moreover,the computing methods of the excavating dynamic loads and the structural dynamic stress were proposed systematically.The main contents are as follows:(1)Study on the dynamic model and the stiffness of hydraulic cylindersThe model of the hydraulic cylinder,which is under two different working conditions,was built.the state variables,such as pressures,flow rate,displacements and velocity were systematically considered.The dynamic model of the hydraulic cylinder under the driven condition was built using two-nodes element.Based on comprehensively analyzing the hydraulic cylinder stiffness influenced by factors,such as the compressibility of the hydraulic oil,the gas content of the oil,the volume expansion of the cylinder barrel,the axialdeformation of the piston rod,the volume expansion of the pipes,the sealing deformation,a new stiffness model of hydraulic cylinder was proposed.Compared with the stiffness model presented in current literature,the error rate of the hydraulic cylinder stiffness was reduced from 21% to 3.7%.A more accurate hydraulic cylinder model was integrated into the system dynamic model of the working device to achieve the more accurate results of the hydro-mechanical dynamic simulation.(2)Study on the flexible-body dynamic model of the working device under the digging conditionWith comprehensive considering the factors,such as the dynamic characteristics of hydraulic cylinders,the structural flexibility and the digging objects,a flexible-body dynamic model of excavator working device was established.And the computing method of the dynamic load in all the components was proposed systematically under the digging conditions.To achieve a good balance between accuracy and efficiency,all slender structures like the boom,arm and rods are modeled using the beam finite elements.The complex structures like the bucket are first modeled using the 3D finite elements;a dynamic sub-structuring method is subsequently employed to reduce the degrees of freedom through a well-known condensation scheme.Then it is incorporated into the system dynamic model.The bucket-digging object interactions are modeled using the two-nodes spring-damper elements.With the combination of a hydraulic cylinder model and structural model,a hydro-mechanical-flexible coupling dynamic digging model was built.Finally,in order to realize the target of the highly efficient application,the automatic modeling technology of the structural dynamic model was developed.The dynamic calculation program was proposed based on the NEWMARK numerical algorithms to realize the dynamic digging simulation.(3)Dynamics calculation and experiment under the typical working conditionsIn the case study,the dynamic calculation and experiments of a 50 t excavator were achieved based on the two typical working conditions,to verify the model of the dynamic theory.The working conditions here refers to the arm digging impact condition and the boom lifting impact condition The computing dynamic digging loads are in good agreement with the measurements.The relative error for the peak dynamic load is lower than 8%.Comparingwith the traditional displacement sub-model,the application of the proposed dynamic sub-model can lower the relative error of the bucket dynamic loads from 16% to 3%,while the influence of the structural flexibility on the dynamic digging loads is insignificant.(4)Study on the computed method of the structural dynamic stress based on sub-model.The computed method of structural dynamic stress based on sub-model was proposed,to realize the calculation of the structural dynamic stress of the excavator working device exactly and highly efficiently.A simplified globally dynamic model,using the hydraulic cylinder model,the beam finite elements and the dynamic sub-structuring model by reducing the degrees of freedom,was established to realize to calculate the interface-nodes dynamic loads of the sub-model in high efficiency.Then,the interface-nodes dynamic loads after equivalence were applied on the refined sub-model to calculate the detailed structural stress.In addition,the dynamic sub-model was equivalent to the static sub-model based on the D'Alembert Principle,which aims to realize to calculate the sub-model stress in high efficiency.In order to obtain the time-historical dynamic stress of the sub-model,the stress under the different unit load components and the interface dynamic loads are combined using the linear superposition method to realize the accurate calculation of the dynamic stress.(5)The analysis of the dynamic stress and strength under the typical working conditionThe research results above were used in a 50-ton excavator.As the case study aims to calculate the dynamic stress of the arm detailed structure under the typical working condition of the boom lifting impact.According to the test verification and the comparative analysis,the results showed that the calculation dynamic stress is in good agreement with the measurements.The relative error for the peak dynamic stress between calculation and measurement is lower than 10%.In addition,the analysis results showed that:(a)the impact of the structural flexibility on the excavator dynamic stress is not negligible d.(b)the computed efficiency,using "the dynamic stress calculation method based on sub-model"proposed by this thesis,is much higher than that method of "transient calculation using the whole working device". |
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