Abstract:On the basis of the modification theory,relative modification concept of cycloidal gear pair and the appraisal index of the initial backlash are built to solve the problem that cycloidal gear works unsmoothly in pin wheel of the full-hydrostatic steering control units.The major goal of modification is a uniformity of initial backlash,so equidistance modification that has good effect on improving quality of fluid cycloidal pair is chosen.As an example of rp=34 mmrrp=11.1 mm a=3.8 mm,the BZZ full-hydrostatic steering control units cycloidal pair parameter,the initial backlash of equidistance is compared with that of equidistance and the moved distance.The obtained result shows that the initial backlash of modification of equidistance is small and symmetrical in no tie bar cycloidal pair,cycloidal gear is smoothly running in pin wheel,and the sealing performance improves.