Simpson planetary gearset
The Simpson planetary gearset consists of two planetary gearsets, that commonly but not necessarily have the same gears and gear ratios. It is employed in automatic transmissions. It is one of the several designs invented by American engineer Howard Simpson. A Simpson gearset delivers three forward gears and one reverse, plus neutral. Actually there are four forward gears, but two of them have exactly the same ratio, so there are three distinct gears. Third gear is always direct drive (1:1).
Overview
The two planetaries are interdependent via two permanent connections. Typically one bonds their suns gears - in practice, both gearsets share a common sun gear. The planet carrier of the first gearset ("first" means closer to the input shaft) is in synchronism with the second gearset's ring, and both are in synchronism with the output shaft.
Owing to these links, two bands and two clutches are all that is needed to command the gearsets and extract the different gears.
Gear changing
In the first gear, the first gearset actuates on the second gearset and output shaft; the second gearset reacts and makes the sun turn in reverse, causing the first gearset to increase the reduction ratio.
One problem of changing from first to second gear is that two bands must actuate in synchronism: the planet carrier of the second gearset must release at the same time as the sun band is actuated. As a result, this is a gear change that tends to be rougher than the others, where just one band is released or actuated. This also means more wear in auto box parts.
This is solved by the use of one-way overrun clutches which allow one or more components to "free wheel" when the transmission shifts into a higher gear. GM's Turbo Hydra-Matic 350 and 400 transmissions utilized overrun clutches in both the low and intermediate gears, allowing for fully progressive shifting with no "overlap".
Gear changing patterns in some Simpson gearboxes, including modern 4-speed units, avoid first gear as much as they can, going up to 2nd as soon as possible, and avoiding reducing to 1st unless absolutely necessary (very slow speed plus floored throttle).
Modern gearsets
In the 1990s and 2000s, the Simpson gearset went out of fashion and was replaced by more complex transmissions. A Ravigneaux planetary gearset was combined with a simple planetary gearset to form a Lepelletier gearset, that offers six useful forward gears.
Even the latest 4-speed transmissions are not Simpson; they have only two planetary sets, while a Simpson transmission needs three planetary sets to get four speeds, the third planetary being an integrated overdrive.
Nevertheless, it is still possible to find new automatic transmissions based on the Simpson gearset. Such transmissions are three- or four-speed, and are often electronically controlled versions of older projects. They are kept in production because they are cheap, reliable, well-known and good enough for the target application.