Theoretical Study On The Strength Of Telescopic Booms For Construction Machinery And Structural Optimization

Telescopic booms have the advantages of compact structures and high working efficiency.They are widely used in construction machinery and equipment such as cranes,aerial work platforms,etc.Nowadays the most widely used thin-walled box section telescopic booms are welded high strength steel plate structures.They are key elements for payload lifting.They usually consist of nesting thin-walled hollow segments which can conduct extension and retraction operations controlled by the hydraulic cylinders mounted on the booms.The loads transferred between the neighbouring booms are realized through the sliding contacts between the sliding blocks mounted on one boom and the sliding surfaces of the other boom.The stresses at these contact zones are considerably higher than other parts of the booms.The study of the high stresses in these contact regions are of great importance for the rational design of the telescopic structures to improve the carrying capacity of the booms.Both the analytical method and the finite element method are employed for the study of the high stresses in the vinicity of the contact zones of the telescopic booms in this paper.According to the principle of superposition,an analytical model has been proposed for the analysis of stresses in the contact zones of the telescopic boom structure with hexagonal cross-section.The proposed model calculates the global bending stress according to the beam bending theory,then uses a partial telescopic boom geometry isolated from the contact zones to establish a local analytical model to determine the local stresses according to the plate bending theory,and finally superimposes the two states of stresses to obtain the final stress state which represents the strength of the telescopic booms.For the local stress analysis model,a new assumption has been proposed on the contacts between the adjacent booms.The new model assumes that the contacts only occur on very narrow areas along the edges of the sliding blocks.Numerical result shows that the conventional analysis based on the assumption of uniform stress distribution on the entire surface of the sliding block predicts a far too high stress level.Whilst the results from the new model based on the narrow local contact assumption are in good agreemen t with the carefully conducted experiment results on a real telescopic boom structure of an aerial work platform.Parametric finite element models for the telescopic booms with octagonal cross-section have been established.Static analyses have been carried out under the most critical loading cases.To achieve more accurate results,the contacts between the boom and the contacting sliding blocks have been properly simulated using the surface-to-surface contact elements.The finite element analysis results have been validated by the experimental test conducted on the telescopic boom.In order to improve the computational efficiency of the global model of the telescopic boom and to be able to achieve quite accurate stress analysis result at the local contact areas of the booms in the same time,a balance force system boundary condition sub-modelling method has been proposed in this paper.The newly proposed sub-modelling technique could directly apply forces obtained from the analysis results of the global model onto the sub-model cut-boundaries.When the structural stiffness of the local sub-model region within the global model has a significant departure from the true stiffness as a consequence of the oversimplification of the global geometric model for enhancing the global computational effeciency,if the traditional interpolated displacement type of boundary conditions are used to indirectly apply loading to the sub-model,the amount of loads applied to the sub-model boundaries could have a significant departure from the actual loads derived from the global model.However,the newly proposed method of applying force type of loading boundary condition to the sub-model can overcome this problem.The newly developed sub-model method has been implemented to analysis the structure of an octagonal cross-section booms.The simulation results reveal that the use of optimal sizes of the sliding blocks can significantly reduce the stress level of the contact zones.And then an optimization design has been carried out to minimize the total weight of the telescopic structures with the octagonal cross-section.The geometric shape parameters of the cross-sections and the overlapping lengths between the adjacent booms are the design variables.The constraint conditions include the maximum allowable equivalent stresses,the local buckling of the thin-walled structure and the flexure displacements at the tip of the assembled boom structure in both the vertical direction and the circumferential direction of the rotating plane.The optimized telescopic boom structure satisfies the design requirements for both strength and rigidity.Compared with the conventional original design,the optimization design has achieved a weight reduction of up to 17.8%.The optimized telescopic boom structure has been adopted for a newly designed telescopic crane.