The planet bearings are critical components of the planetary gear set, and work under revolution-rotation coupled (RRC) conditions. With the development of higher speeds and heavier loads of transmission devices, higher failure risks of planet bearings are more likely observed in contrast to the traditional non-revolution conditions, including roller-race contact failure and cage fracture. This paper presents the findings of an investigation into the strength of planet bearings based on a comprehensive model of PGS system. Results suggest that the higher risk of roller-race contact failure can be attributed to the larger roller-race contact stresses and sliding ratios. With regard to cage fracture, the primary causes are identified as excessive impact stress and cage instability under RRC conditions. This study illuminates the increased failure risks of planet bearings under RRC conditions, and offers insights that can inform the design and application of planet bearings.