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Module 1 - Planetary Gear Sets Operation (Page 1 of 5)

Introduction to Planetary Gear Sets

Planetary gear sets are used in many applications. Some of the more common applications include gear reduction starter motors (ie: for large diesel engines), electric winches and automatic transmissions.

There are three parts to a planetary gear set: the yellow arrow is pointing to the sun gear. The green arrow is pointing to the ring or internal gear. The red arrow is pointing to the planetary gears or planet pinions.


Planetary gear sets have several important advantages over standard transmission gear sets. First, planetary gear sets are in constant mesh. They do not use synchronizers and blocker rings to engage and disengage gears. Automatic transmission gears do not "slip out of gear" or "grind when shifting gears."

Secondly, planetary gear sets are small in size yet offer a wide variety of gears. One planetary gear set can offer first gear (maximum reduction), second gear (moderate reduction), direct drive (1:1 gear ratio), reverse reduction gear, reverse overdrive (not very useful) and two overdrive gears.

Thirdly, the spreading forces of the planetary gear set is contained by the ring or internal gear. Gears that are meshed together under load will always tend to spread apart from each other. If they are allowed to spread apart in operation, it will cause severe damage to the transmission and gears.

Manual transmissions relies on the strength of the transmission case to contain the spreading forces. Thats why manual transmission cases are often made of cast iron. Automatic transmission cases do not need to contain these spreading forces, they rely on the strength of the ring or internal gear to contain these spreading forces. Therefore even a heavy-duty automatic transmission cases can be made of lightweight aluminum.

Fourthly, planetary gear sets are strong and reliable because of the large area of contact between gear components. The force of the load that the gears handle is distributed among a relatively large surface area due to the design of a planetary gear set. They are also quite in operation for the same reason.

Module 1 - (Page 1 of 5)