"行星齿轮变速箱" can be translated into English as "planetary gear transmission" or "planetary gearbox".
"Planetary gear transmission" emphasizes the functional aspect of the mechanism, indicating it's a type of transmission system using planetary gears.
"Planetary gearbox" focuses more on the physical component or the device itself that incorporates planetary gears for speed and torque conversion.
Both terms are commonly used in technical and engineering contexts, and the choice between them may depend on the specific context or the emphasis desired.