The combined planetary-gear unit is designed to allow the same number of transmission steps using fewer gear-wheels in a housing more compact than when using two single planetary-gear units and in addition, to economise on multi-disc clutches or brakes.
The basic form shows two differently sized sun-wheels and the swivel bearer (3), normally with six planetary gears, all mounted on the swivel bearer and at an angle of 120 ° to each other. Three of these (4) are arranged closer to the center and mesh with the larger sun-wheel (2). They are wider than the other three planetary gears (5) thus connecting both parts of the combined planetary-gear unit.
The three short planetary gears (5) are, at the same time, positively linked with the small sun-wheel (1), the hollow wheel (6) and with the other planetary gear unit (4). In contrast to the single planetary-gear unit, there are four instead of three entrances and exits. Variations of this basic form, either with additional planetary gears or in the case of each having only two sets, are possible. Theoretically, only one narrow and one wide planetary-gear would be sufficient, however, this would be mechanically very unfavourable.
A certain indicator of a Simpson unit is the common sun-wheel which connects two complete planetary units with each other. If both planetary units are the same, the outcome is a 3-speed automatic gearbox as it was primarily used at the beginning of the second half of the last century in American vehicles. Nowadays it has become very rare. Mostly a combination of Ravigneaux and simple units is used. 07/08