Automobile steering gear ratio is one of the key factors which effect vehicle steering portability and sensitiveness.In order to obtain good portability and sensitiveness at the same time, it requires the use of variable ratio steering gear. Based on envelope principle, a new digital design method of variable ratio gear was proposed by analyzing the major part characteristics of the mechanical variable ratio steering. This method considers the helical gear of the gear pair with alternating axis as a set of the intersecting planes which are obtained by using a set of some infinite close planes which are perpendicular to its axis to intersect the gear, and each section is called a “gear element ”. The meshing transmission process of the gear pair can be seen as a meshing process of a collection of gear element and variable ratio gear. The height value of every tooth profile point on the variable ratio gear is the minimum value of all intersections of the traverse gear element set during the meshing process and the line which passes through this point and perpendicular to the rack movement direction and the axis of the helical gear with this point meshing transmission. Based on this method, a mathematical model for calculating the variable ratio gear tooth profile point height value of the variable ratio gear pair with alternating axis was established,the solving algorithm was developed and the 3D model of the variable ratio gear was obtained. This method was successfully applied to the development of a certain variable ratio steering gear.Besides that,the feasibility of this method was verified by trial-producing a gear pair sample with alternating axis and testing it.
