Terminology
turbofan engine gearbox
1. gear box

source: MTU

In conventional engines, the low-pressure turbine and the fan sit on one shaft, but here the gearbox decouples the two components. This lets the components run at their optimum speed: the large-diameter fan slower, and the smaller-proportioned low-pressure compressor and low pressure turbine considerably faster. The gearbox is a planetary gear train with a reduction ratio of 3:1 or higher.

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