Simpson Planetary Gearset
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The Simpson planetary gearset is a compound planetary gear train consisting of two planetary gearsets sharing a common sun gear. A Simpson gearset delivers three forward gears and one reverse, plus neutral, and is commonly employed in three and four ratio
automatic transmission An automatic transmission (sometimes abbreviated to auto or AT) is a multi-speed transmission used in internal combustion engine-based motor vehicles that does not require any input from the driver to change forward gears under normal driving c ...
s. It is one of the several designs invented by American engineer Howard Simpson.


Overview

The two planetaries are interdependent via two permanent connections, that commonly but not necessarily have the same gears and
gear ratio A gear train is a mechanical system formed by mounting gears on a frame so the teeth of the gears engage. Gear teeth are designed to ensure the pitch circles of engaging gears roll on each other without slipping, providing a smooth transmission ...
s, both gearsets share a common sun gear. The planet carrier of the first gearset ("first" means closer to the input shaft) is in synchrony with the second gearset's ring. Owing to these linkages, only two bands and two
clutch A clutch is a mechanical device that engages and disengages power transmission, especially from a drive shaft to a driven shaft. In the simplest application, clutches connect and disconnect two rotating shafts (drive shafts or line shafts). ...
es are needed to command the gearsets. 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. In first gear, the input is locked to the first ring. The first carrier is locked to the output and second ring. The second carrier is locked so its ring motion causes the common sun to spin retrograde. The retrograde spin reduces the first carriers reaction to its ring motion thereby doubling the reduction of the first set and output. Command of second gear is simpler, the second carrier is released and the sun drive is braked. Thus input is locked to the first ring, and output is locked to the first carrier. The third gear is always direct drive (1:1), by feeding input to both the sun drive and the first ring, both carriers are effectively locked, so input directly drives the output.


Gear changing

One problem of changing from first to second gear is that two bands must actuate in synchrony: the planet carrier of the second gearset must release at the same time as the sun band is actuated.
Sprag clutch A sprag clutch is a one-way freewheel clutch. It resembles a roller bearing but, instead of cylindrical rollers, non-revolving asymmetric figure-eight shaped sprags, or other elements allowing single direction rotation, are used. When the unit ...
es mitigate roughness when a shift requires a combination of bands to be released or actuated. GM's
Turbo Hydra-Matic Turbo-Hydramatic or Turbo Hydra-Matic is the registered tradename for a family of automatic transmissions developed and produced by General Motors. These transmissions mate a three-element turbine torque converter to a Simpson planetary geartr ...
350 and 400 transmissions utilized
overrunning clutch Freewheel mechanism In mechanical or automotive engineering, a freewheel or overrunning clutch is a device in a transmission that disengages the driveshaft from the driven shaft when the driven shaft rotates faster than the driveshaft. An ...
es 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 The Ravigneaux gearset is a double planetary gear set, invented by Pol Ravigneaux, who filed a patent application on July 28, 1949, in Neuilly-sur-Seine France. This planetary gear set, commonly used in automatic transmissions, is constructed from ...
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.


References

{{Reflist Automotive transmission technologies Gears Epicyclical gearing