The most confusing systems on any car is the automatic transmission. The main stumbling block for most technicians is not the electronic control systems, but the humble planetary gear. The planetary gear sets in an automatic transmission provide forward underdrive and overdrive gear ratios, plus reverse. The machinery handbook describes planetary gearing as a compact means of driving inline shafts with speed reduction (underdrive) and induction (overdrive).
Advantages of planetary gears over parallel axis gears include high power density, large reduction in a small volume, pure torsional reactions and coaxial shafting. Disadvantages include high bearing loads, inaccessibility and design complexity. The planetary gearbox arrangement is an engineering design that offers many advantages over traditional gearbox arrangements. One advantage is its unique combination of both compactness and outstanding power transmission efficiencies.