Study On Transient Dynamics Of Harmonic Gear Drive

Harmonic gear drive transmits power based on the elastic deformation offlexspline made of thin wall and have a great deal of advantages including simplestructure, light weight, high reduction ratio, high driving precision, large number of theteeth in contact regions, large load capacity, little backlash. Up to now, harmonic geardrives are widely used in aviation, robots, general machinery and machine toolinstrument, etc. In the process of transmitting power, flexspline is deformed along thecircle. The mesh load is inhomogeneously distributed along the circumference, and doesnot tangent to the center line of the flexspline. The circumferential torsion and radialdeformation of the flexspline are existed at the same time. As a consequence, the fatiguelife of harmonic gear drive depends on the life span of flexspline. This work is financiallysupported by the National Natural Science Foundation of China (Grant NO.50975295).We develop a transient dynamic analysis model for B1-50-80undermaximum rated conditions. On basis of transient results, fatigue life of flexspline andflexspline made of metal and composite layers are researched.Firstly, the transient dynamic analysis of the harmonic gear transmission system isresearched under the maximum load condition. The results are revealed that the stresschange in sine wave form under work conditions; the change process of stress offlexspline gear ring is from top to bottom; over-stress of the flexspline on tube graduallydecreases along the axial direction. The cracks originate from the dedendum regions offlexspline, then extend along the axial direction, and finally expand along45°slant.Secondly, the high precision testing rig is built and the dynamic strain of flexsplineunder each load conditions is carried out. The experiment results are depicted that thestress of flexspline cup change in sine wave form, and gradually decrease in axialdirection along the bottom of flexspline cup, and the same cross-section radial stress isbigger than axial stress. With the increase of the output load, the strain of flexsplinebottom increases，and less affected by rotational speed. Flexspline bottom radialfrequency change as the superposition of input and output frequency, and the radialstrain is bigger than shear’s.Thirdly, the results between theory results and experiment results are compared.The results are revealed that transient analysis correctly simulates the real stress state offlexspline under the normal condition. The error can be accepted within the allowable range to test contingency of15%of experiment results.Fourthly, the fatigue life of flexspline indicates that the fatigue damage originatesfrom the addendum and dedendum of flexspline tooth and damage expand along theaxis of flexspline form the tooth inside to the tooth tail.Finally, we designed flexspline structure with composite layers. The transientresult are revealed that the maximum Von Mises stress decreases from849.4MPato742.6MPa, and the position of maximum stress changes from the addendum offlexspline to the inner surface of flexspline ring. Fatigue result appears that compositecorrect the phenomenon of a certain slant of flexspline gear ring. As a consequence, thefatigue damage is reduced and effectively improves the fatigue of flexspline.