A dynamic analysis tool for calculating cage stress in needle roller bearings under planetary motions was developed to examine the mechanism of rising cage stress. This analysis considers three degrees of freedom of a cage and rollers and two degrees of freedom of a planetary gear in two-dimensional model. Moreover, the elastic deformation of the cage is implemented to determine the cage stress by using a Component-Mode-Synthesis method. In order to validate this dynamic analysis, two needle roller bearings with different structural cage strengths were tested in a planetary gear system. In cases where the weaker cages were damaged, the analyzed stress of the cages nearly reached or exceeded the material fatigue strength. This high stress was observed when a roller passed the load zone collided with the pocket bar due to the centrifugal force of planetary motion. The maximum cage stress increased with the carrier rotation speed and the stress of the damaged weaker cage only exceeded the fatigue strength in the experimental range of carrier speed. These results indicate that the dynamic model is effective and valid for the current application.

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