Research On Vertical Assembly Technology Of Large Thin-walled Cylinder

The thin-walled structural tank is the core of the space rocket's body.The assembly process of rocket thin-walled structural tanks faces technical difficulties in controlling the shape and size.In the horizontal assembly process of the thin-walled cylindral section currently common in China,the thin-walled cylinder section is subjected to gravity in its radial direction,the radial deformation of the thin-walled cylinder section is getting greater as rocket size grows,resulting in non-ideal dimensional accuracy.This problem can be solved with the vertical assembly process.Shanghai Jiaotong University developed vertical assembly equipment for rocket storage tanks.First,the two cylindral parts to be welded are placed in the vertical direction,then they are clamped by the inner support and external pressurring fixtures,at last the two cylindral parts are rotated circumferentially and assembled by friction stir welding.Mating and clamping are the key components of vertical assembly of large thin-walled parts.The quality of these processes directly affect the final vertical assembly outcome.Mating and clamping processes face their own engineering challenges.During the clamping process,the thin-walled cylindral section is subjected to over-constraints by the internal support and external pressurring fixtures,resulting in complex deformation conditions.Elastic deformation in fixtures are non neglectable,which further increases the overall deformation of cylindral parts.The deformation of the clamping will cause deviations in the mating features of the cylinders.Even if the two cylinders are placed to ensure the alignment of the docking geometric features,the clamping process still causes deviations in the geometric features.This paper studies the Large Rocket Vertical Aseembly Equipment developed by Shanghai Jiaotong University.For the clamping problem,the internal support and external pressuring fixture stiffness model is established.A model is proposed considering the elasticity of the fixture,from which results of clamping force and clamping deformation are computed.For the mating problem,two data fusing algorithms are studied for laser tracker.A method for extracting the mating features of thin-walled cylindral parts and a geometric mating evaluation index is proposed.A method for determining the position adjustment considering the compensation of the clamping deviation is established.The main research results are as follows:1.A method for modeling the stiffness of the internal support and external pressuriong fixtures is proposed.According to the stiffness of the fixture,a finite element model of the process of thin-walled cylinder loading and clamping is established.For the inner support fixture,the three elastic components of the elastic driving platform,the elastic mechanism and the correctional block are taken into account.Stiffness analysis was carried out according to the serial relationship of the three elastic components.For the elastic drive platforms,a mapping method of joint space stiffness to workspace stiffness is derived.Elastic mechanisms are modelled with beam elements.For the correctional block,a finite element model to determine its end stiffness is proposed.For external pressure clamps,models for its three elastic components including external pressure clamp motor,linear motion platform and external pressure block are established.A finite element model of the thin-walled cylinder clamping process considering the elasticity of the fixture is established.Clamping force and clamping deformation are studied for cylinders with initial geometric deviations.2.A thin-walled cylinder mating method considering the compensation of the clamping deviation is proposed.In the process of point cloud collection,the target ball are usually blocked by the cylinder after moving to a certain position,therefor unable to collect location information.Three-point and multi-point data fusing algorithms are studied.For the problem that the cylinder mating process has few features,a method for extraction of cylinder surface normal and cylinder axis from point cloud is established.A comprehensive geometric mating evaluation index is proposed.A mating evaluation index after clamping deviation is established and an iterating computing algorith is established for solving this optimization problem.By minimizing this index,geometric feature deviation caused by the clamping deformation from the clamping process can be offset.3.A vertical mating experiment was conducted on the thin-walled cylinders.A program for controlling the movement of the experiment is written.This paper discusses the deformation model of over-constrained clamping of large thin-walled cylinders considering the elasticity of the fixture.A complete method of vertical mating of thin-walled parts is formed.The main research object of this paper is Large Rocket Vertical Aseembly Equipment and the internal support fixture have already been setup by SJTU and a trial assembly experiment is conducted.