Astern gear

Astern gear
For astern gear, the ring gear is locked to the casing, and the planet gears are left free to rotate. If the input shaft turns clockwise, this means that the inner planets have to turn anticlockwise, driving the outer planets clockwise. In order to do this, they have to run anticlockwise around the inside of the ring gear, taking the planet holder - and, of course, the output shaft - around with them.
One major snag with this is that although it provides an ahead/astern facility, it can't offer the reduction in shaft speed that is one of the main reasons for wanting a gearbox in the first place. To do that requires a second gearbox in tandem with the first - adding to the cost, weight, bulk and complexity of the whole unit. The second gearbox is usually another epicyclic unit, called epicyclic gearbox, as the planetary gearing but with the important difference that there are no clutches involved, and the output shaft is connected to the ring gear instead of to the planet holder.