Design And Analysis Of A Deployable Inner Support Fixture For Fuel Tank’s Final Welding Process

Friction stir welding technology is the key technology of improving the manufacture level of launch vehicle.The technical difficulty lies in the design of welding fixtures.The geometrical structure of the tank is closed after the final closed girth welding.The inner support fixture needs to fold from the 3350 mm diameter to the 500 mm diameter and exit through the manhole.The fixture also need to meet the requirement of high rigidity and expansion trajectory.The existing manual internal fixture has many hidden risks,and it can’t be applied to tank’s vertical assembly process.Therefore,there is an urgent need to design a new kind of deployable fixture with automatic unfolding function to solve the existing manual fixture’s problems,and as pre-research for the heavy-lift launch vehicle’s vertical assembly process improvement.In this paper,we proposed a new kind of 3-DOF deployable mechanism,and designed the deployable fixture by using this mechanism.Then,we analyzed the kinematic characteristics of the fixture.A kinematic analysis model is established,the working space,singularity configuration and unfolding trajectory of the fixture is analyzed.A mechanism prototype is manufactured as further validation.Finally,the strength and stiffness characteristics of the fixture are analyzed.The research contents of this paper include the following aspects: 1.The design of the fixture is completed.A new kind of three-DOF deployable mechanism is proposed and used to design the deployable fixture.The designed fixture meets the requirements of the fold ratio and folded trajectory limit.Detailed structural design and component selection of the deployable fixture are completed.The detailed 3D model of equipment prototype is built.2.The kinematic characteristics of deployable fixture are analyzed.The kinematics analysis model of the basic deployable unit is established by using the global exponential product formula(POE).The Jacobian matrix of the basic deployable unit is derived by the screw theory,which can be used to define the index of singularity judgement.The working space of the basic deployable unit under limit of continuity and physical reachability is calculated.In the calculated working space,by the determinant of the Jacobian matrix,we can distinguish the pathological Jacobian region,which is prone to destabilization singularity.According to the defined local and the global transmission performance indexes,a best unfolding trajectory is planned.A mechanism prototype is manufactured to as further validation of the kinematic analysis results.3.The strength and stiffness characteristics of the fixture are analyzed.The maximum sizing force of the tank is calculated and the results of different sizing displacements are analyzed.The strength of the key components and joints in the basic folding unit is validated,and the deformation of each component joint is analyzed.The stiffness analysis model of each kinematic chain is established,and the stiffness characteristic curves are obtained.The elastic deformation error of each kinematics chain is compensated by active input.This paper has finished the design,kinematic characteristic analysis,strength and stiffness characteristic analysis of the inner support deployable fixture,and formed a complete set of design and analysis method.At present,the design and manufacture of mechanism prototype has been completed,and an equipment prototype has been designed.