FIGURE 10.20 Complex planetary gear train.
of rotation of the sun gear (F) is the same as the direction of rotation of the arm (B). This is similar to what an idler gear does for a fixed axis spur gear train; however, a planetary gear train is more compact, and that is one of its most important advantages.
As before, arm (B) rotates about its own fixed axis so that the planet gear ( A) must roll along the inside of the fixed ring gear (D). As gear (E) moves with gear ( A), it must have the same angular velocity (ωA), given by Eq. (10.44), and have the same direction that is opposite to the direction of the arm (B).
As stated earlier, at the contact point G between compound gear (E) and the sun gear (F), the velocity of point G must be the same velocity whether expressed from gear (E) or gear (F). Therefore, the motion of these two gears must be governed by the expression in Eq. (10.45) as
vG = (r A + rE )ωE = rF ωF (10.45)
where the compound gear (E) appears to be rolling on the inside of the ring gear (D) with a single radius equal to the sum of the radius of gear ( A) plus the radius of gear (E).
Solving for the output angular velocity (ωF ) gives