Research On Digital Design Theory And Method For Variable Ratio Tooth Profile Of Automotive Variable Ratio Steering Gear

“Variable transmission ratio”is the common demand of automobile in different driving conditions.At present,there are mainly three types of steering systems that can realize the variable ratio steering function:variable ratio steering-by-wire system,electronic power variable ratio steering system,electric control system and mechanical variable ratio steering system.Mechanical variable ratio steering gear has wide application prospects because of its low production cost and high reliability.The variable ratio tooth profile is the key to realize“variable transmission ration”for mechanical variable ratio steering gear,and it is a kind of envelope surface formed by the envelope process of involute tooth profile in accordance with the variable ratio transmission relationship.But it is this special envelope process that leads to difficulties in modeling and CNC?Computer Numerical Control?machining of the variable ratio tooth profile.Currently,there are mainly two modeling methods for the variable ratio tooth profile:the meshing principles and the Boolean subtraction operation.Establishing the analytical model and point cloud model of variable ratio tooth profile by the meshing theory has some disadvantages^[44],and the variable ratio tooth profile designed by the Boolean subtraction operation is non-continuous surface with poor accuracy.This thesis focuses on the digital design method,CNC machining method and design verification method for variable ratio tooth profile of mechanical variable ratio steering gear,which lays theoretical foundation for the popularization and application of the variable ratio tooth profile.The main research works are as follows:?1?In aspect of digital design theory of the variable ratio tooth profile:Starting with the meshing characteristics of variable ratio tooth profile,this thesis studied on a digital design theory of the variable ratio tooth profile based on the meshing principles,established the mathematical model for calculating variable ratio tooth profile points and completed its theoretical deduction;It lays theoretical foundation for solving the accurate variable ratio tooth profile points.?2?In aspect of digital design method of the variable ratio tooth profile:Aiming at the mathematical model for calculating variable ratio tooth profile points,this thesis studied on its solving algorithm and the generation method of variable ratio tooth profile point cloud;To achieve better engineering application and fast modeling of variable ratio tooth profile,a modeling method based on the generating machining principle was studied,and the variable ratio tooth profile entity model was established by the Boolean subtraction operation and tooth profile modification technology in this thesis.?3?In aspect of CNC machining method of the variable ratio tooth profile:Using the tooth profile point cloud solved by the digital design method in this thesis to planning the tool path,size selection of the finishing tool and calculation of the cutter bar vector at cutter-contact point were completed,and CNC end milling of the variable ratio tooth profile was achieved in CAM system;The machining precision was analyzed by comparing the tooth profile points get by machining with the corresponding points of the variable ratio tooth profile model.?4?In aspect of verification of the design method for the variable ratio tooth profile:Due to the fact that the validity of the design method cannot be verified by comparing the designed variable ratio tooth profile and the theoretical model with the lack of a theoretical variable ratio tooth profile model,this thesis started with the key design parameters-"variable ratio curve"and"variable ratio drive characteristics"to study the verification method and to complete the simulation and test verification of its validity.