Compound Epicyclic Gear Train:
A compound epicyclic gear train consists of two co-axial shafts S1 and S2. An annulus gear A which is fixed, the compound gear B-C, the sun gear D and the arm H. The annulus gear has internal teeth and the compound gear is carried by the arm and revolves freely on a pin of the arm H. The sun gear is co-axial with the annulus gear and the arm but independent of them.
The annulus gear A meshes with gear B and the sun gear D meshes with the gear C. It may be noted that when the annulus gear is fixed, the sun gear provides the drive and when the sun gear fixed, the annulus gear provides the drive. In both cases, the arm acts as a follower.