All automatic transmissions rely on planetary gearsets to transfer power and multiply engine torque to the drive axle. Compound gearsets combine two simple planetary gearsets so load can be spread over a greater number of teeth for strength and also to obtain the largest number of gear ratios possible in a compact area.

A simple planetary gearset consists of three parts: a sun gear, a carrier with planetary pinions mounted to it, and an internally toothed ring gear or annulus. The sun gear is located in the center of the assembly. It can be either a spur or helical gear design. It meshes with the teeth of the planetary pinion gears. Planetary pinion gears are small gears fitted into a framework called the planetary carrier. The planetary carrier can be made of cast iron, aluminum, or steel plate and is designed with a shaft for each of the planetary pinion gears. (For simplicaity, planetary pinion gears are called planetary pinions.)

Planetary pinions rotate on needle bearings positioned between the planetary carrier shaft and the planetary pinions. The carrier and pinions are considered one unit - the mid-size gear number.

They planetary pinions surround the sun gear's center axis and they themselves are surrounded by the annulus or ring gear, which is the largest part of the simple gearset. The ring gear acts like a band to hold the entire gearset together and provide great strength to the unit. To help remember the design of a simple planetary gearset, use the solar system as an example. The sun is the center of the solar system with the planets rotating around it; hence, the name planetary gearset.

How Planetary Gears Work

Each member of a planetary gearset - the sun gear, pinion gear carrier, and ring gear - can spin (revolve) or be held at rest. Power transfer through a planetary gearset is only possible when one of the members is held at rest, or if two of the members are locked together.

Any one of the three members can be used as the driving or input member. At the same time, another member might be kept from rotating and thus becomes the held or stationary member. The third member then becomes the driven or output member. Depending on which number is the driver, which is held, and which is driven, either a torque increase or a speed increase is produced by the planetary gearset. Output direction can also be reserved through various combinations.

7. When any two members are held together, speed and direction are the same as input. Direct 1:1 drive occurs.

8. When no member is held or locked together, output cannot occur. The result is a neutral condition.

The above table summarizes the basic laws of simple planetary gears. It indicates the resultant speed, torque and direction of the various combinations available. Also, remember that when an external-to-external gear tooth set is in mesh, there is a change in the direction of rotation at the output. When an external gear tooth is in mesh with an internal gear, the output rotation for both gears is the same.

MAXIMUM FORWARD REDUCTION With the ring gear stationary and the sun gear turning clockwise, the sun gear rotates the planetary pinions counterclockwise on their shafts. The small sun gear (driving) rotates several times, driving the mid-size planetary carrier one complete revolution, resulting in the most gear reduction or the maximum torque multiplication that can be achieved in one planetary gearset. Input speed is high, but output speed is low.

MAXIMUM FORWARD REDUCTION In this combination the sun gear is stationary and the ring gear rotates clockwise. The ring gear drives the planetary pinions clockwise and walks around the stationary sun gear. The planetary pinions drive the planetary carrier in the same direction as the ring gear - forward. This results in more than one turn of the input as compared to one complete revolution of the output. The result is torque multiplication. The planetary gearset is operating in a forward reduction with the large ring gear driving the small planetary carrier. Therefore, the combination produces minimum forward reduction.

MAXIMUM OVERDRIVE With the ring gear stationary and the planetary carrier rotating clockwise, the three planetary pinion shafts push against the inside diameter of the planetary pinions. The pinions are forced to walk around the inside of the ring gear, driving the sun gear clockwise. In this combination, the mid-size planetary carrier is rotating less than one turn and driving the smaller sun gear at a speed greater than the input speed. The result is overdrive with maximum speed increase.

SLOW OVERDRIVE In this combination, the sun gear is stationary and the carrier rotates clockwise. As the carrier rotates, the pinion shafts push against the inside diameter of the pinions and they are forced to walk around the held sun gear. This drives the ring gear faster and the speed increases. The carrier turning less than one turn causes the pinions to drive the ring gear one complete revolution in the same direction as the planetary carrier and a slow overdrive occurs.

SLOW REVERSE Here the sun gear is driving the ring gear with the planetary carrier held stationary. The planetary pinions, driven by the sun gear, rotate counterclockwise on their shafts. While the sun gear is driving , the planetary pinions are used as idler gears to drive the ring gear counterclockwise. This means the input and output shafts are operating in the opposite or reverse direction to provide a reverse power flow. Since the driving sun gear is small and the driven ring gear is large, the result is slow reverse.

FAST REVERSE For fast reverse, the carrier is held, but the sun gear and ring gear reverse roles, with the ring gear now being the driving member and the sun gear driven. As the ring gear rotates counterclockwise, the pinions rotate counterclockwise as well, while the sun gear turns clockwise. In this combination, the input ring gear uses the planetary pinions to drive the output sun gear. The sun gear rotates in reverse to the input ring gear, providing fast reverse.

DIRECT DRIVE In the direct drive combination, both the ring gear and the sun gear are input members. They turn clockwise at the same speed. The internal teeth of the clockwise turning ring gear try to rotate the planetary pinions clockwise as well. But the sun gear, which rotates clockwise, tries to drive the planetary pinions counterclockwise. The opposing forces lock the planetary pinions against rotation so the entire planetary gearset rotates as one complete unit. This ties together the input and output members and provides a direct drive.

NEUTRAL OPERATION When no member is held stationary or locked, there is input into the gearset, but no output. The result is a neutral condition.