Research on Ultra-high Rotary Bending Fatigue Strength of Flexible Bearing Steels for Robot Harmonic Speed Reducer

Abstract

The flexible bearing steel with long fatigue life using for robot harmonic speed reducer has been successfully de⁃ veloped by applying EAF process technology. The ratio of scrap to hot metal was controlled more than 80%. This paper compared the metallurgical quality of flexible bearings and traditional rolling bearings in terms of non-metallic inclusions， austenite grain size and carbide banding structure， the fatigue strength of steel used for flexible bearing and traditional roll⁃ ing bearing under 107  cycles was tested by the method of rotary bending fatigue test. The O content of steel used for flexible bearings of robot harmonic reducer was less than 0. 000 4%， the Ti content was much less than 0. 001%， non-metal inclu⁃ sions of class A sulfide≤1. 0 grade， non-metal inclusions of class B and D oxide ≤0. 5 grade. The size of the maximum spherical inclusion was less than 30 μm. The test results show that the flexible bearings have ultra-high purity. By increas⁃ ing Al and N chemical composition in the smelting process， the austenite grain size was 10 grade in the flexible bearing steel， much higher than 8. 5 grade in the traditional rolling bearing steel. The proportion of carbide banding structure of 7. 1 and 7. 2 grade was higher than that of traditional bearing steel in the means of extending the high temperature diffusion time. The ultra-high austenite grain size and the decrease of the width of carbide banding structure showed that the flexible bearing steel had an ultra-high structure uniformity. In addition， the rotary bending fatigue strength of the flexible bearing steels was 1 016 MPa， its fatigue life was slightly higher than that of the traditional rolling bearing steels.

Keywords

• harmonic speed reducer； flexible bearing steel； grain size； non-metallic inclusions； rotary bending fa tigue； ultra-high cycle fatigue