Simpson planetary gearset
Encyclopedia
The Simpson planetary gearset consists of two planetary gearsets
Epicyclic gearing
Epicyclic gearing or planetary gearing is a gear system consisting of one or more outer gears, or planet gears, revolving about a central, or sun gear. Typically, the planet gears are mounted on a movable arm or carrier which itself may rotate relative to the sun gear...

, both having the same gears and gear ratio
Gear ratio
The gear ratio of a gear train is the ratio of the angular velocity of the input gear to the angular velocity of the output gear, also known as the speed ratio of the gear train. The gear ratio can be computed directly from the numbers of teeth of the various gears that engage to form the gear...

s. It was employed in early automatic transmission
Automatic transmission
An automatic transmission is one type of motor vehicle transmission that can automatically change gear ratios as the vehicle moves, freeing the driver from having to shift gears manually...

s. It was one of the several designs invented by engineer Howard Simpson
Howard Simpson
Howard Woodworth Simpson was an automotive engineer whose designs were extensively used in automatic transmissions.-Biography:...

. 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 identical planetaries are interdependent. Their suns are bonded - in practice, both gearsets share a longer sun gear. The planet support of the first gearset ("first" meaning closer to engine input shaft) is in synchronism
Synchronism
Synchronism is deliberately achieved coincidence in a point of time.In telecommunication the term synchronism has the following meanings:1. The state of being synchronous....

 with the second gearset's ring, and both are in synchronism with the output shaft.

Due to these links, two bands
Band brake
A band brake is a primary or secondary brake, consisting of a band of friction material that tightens concentrically around a cylindrical piece of equipment to either prevent it from rotating , or to slow it . This application is common on winch drums and chain saws and is also used for some...

 and two clutch
Clutch
A clutch is a mechanical device which provides for the transmission of power from one component to another...

es are all that is needed to command the gearsets and extract the different gears. A four-speed box can easily be developed from a Simpson box, just by adding a third gearset which is operated independently, to create the fourth, long gear (the Simpson part being in 1:1 gear).

First gear

In Simpson's gearbox, the first gear is the most difficult to understand, and probably the one that causes most stress in the components. It happens because there is a feedback: 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. The mechanical energy has actually to "loop" in the gearset system until settling the ratio.

Gear changing

Another problem of changing from first to second gear is that two bands must actuate in synchronism: the planet support 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.

Due to both factors abovementioned, gear changing patterns in Simpson gearboxes, including modern 4-speed units, avoid first gear as much as they can, going up to 2nd as soon as possible, and especially avoiding reducing to 1st unless absolutely necessary (very slow speed plus floored throttle
Wide open throttle
Wide open throttle refers to an internal combustion engine's maximum intake of air and fuel that occurs when the throttle plates inside the carburettor or throttle body are "wide open", providing the least resistance to the incoming air...

).

Modern gearsets

The Simpson gearset is now more or less obsolete. Other gearsets can make 4 forward gears with just two planetaries. Most modern gearboxes employ the Ravigneaux planetary gearset
Ravigneaux planetary gearset
The Ravigneaux gearset is a double planetary gear set commonly used in automatic transmissions. This planetary gear set is constructed from two gear pairs, ring-planet and planet-planet. The Ravigneaux set has two sun gear wheels, a large sun and a small sun, and a single carrier gear with two...

design, which produces 4 forward gears from a single set (and 6+ gears by adding another standard planetary).

Nevertheless, many new cars are still sold with automatic transmissions based on Simpson gearset. Such transmissions are typically four-speed, designed at late 1990's, they are electronically controlled versions of older projects, and are kept in production because they are cheap, reliable, well-known and good enough for the target application.
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