Bob's Resource Website (2007)

(Transmission Info)

Planetary gears provide for the different gear ratios needed to move a vehicle in the desired direction at the correct speed. A planetary gear set consists of a sun gear, planet gears, and a ring gear. These gears are typically helically cut gears, which offer quiet operation.

At the center of the planetary gear set is the sun gear. The placement of this gear in the gear set is the reason for its name; the earth's sun is at the center of our solar system. Planet gears surround the sun gear, just like the earth and other planets in our solar system. These gears are mounted and supported by the planet carrier and each gear spins on its own separate shaft. The planet gears are in constant mesh with the sun and ring gears. The ring gear is the outer gear of the gear set. It has internal teeth and surrounds the rest of the gear set. Its gear teeth are in constant mesh with the planet gears. The number of planet gears used in a planetary gear set varies according to the loads the transmission is designed to face. For heavy loads, the number of planet gears is increased to spread the work load over more gear teeth.

The planetary gear set can provide a gear reduction or overdrive, direct drive or reverse, or a neutral position. Because the gears are in constant mesh, gear changes are made without engaging or disengaging gears, as is required in a manual transmission. Rather, clutches and bands are used to either hold or release different members of the gear set to get the proper direction of rotation and/or gear ratio.

Compound Planetary Gear Sets

A limited number of gear ratios are available from a single planetary gear set. To increase the number of available gear ratios, gear sets can be added. The typical automatic transmission with three or four forward speeds has at least two planetary gear sets.

In automatic transmissions, two or more planetary gear sets are connected together to provide the various gear ratios needed to efficiently move a vehicle. There are two common designs of compound gear sets: the Simpson gear set in which two planetary gear sets share a common sun gear; and the Ravingeaux gear set, which has two sun gears, two sets of planet gears, and a common ring gear. Some transmissions are fitted with an additional single planetary gear set, which is used to provide an "add-on" overdrive gear.

Reaction Members

Reaction members are those parts of an automatic transmission that hold members of the planetary gear set in order to change gears. One-way overrunning clutches, bands, and multiple-disc clutch packs are examples of reaction members. One-way overrunning clutches are purely mechanical devices, whereas clutches and bands are hydraulically controlled mechanical devices. Most automatic transmissions use more than one type of these reaction members; some use all three.

These pages and programming contained therin are © 1996 - , Bob Johnstone and may not be reproduced without the permission of rfjohnstone at cox.net (Bob Johnstone) All Rights Reserved.
Some technical opinions are my own from experience, other informational data is from online sources with credits when available and while care has been taken to be as accurate as possible, it is offered only as a guide and caution should be exercised in the application of it.