In a conventional manual transmission (gearbox) application there are two (2) gears involved (driven and Drive) for any given gear ratio. Each gear rotates on its own shaft. These gears can only offer two (2) variations between the input shaft and output shaft, forward or reverse but the ratio remains the same.

With a planetary gear set, because the gears all revolve about the same axis and are in constant mesh with each other, the gear set is stronger, quieter and more compact. Eight (8) different outputs are possible. In a planetary gear set all the gears are helical cut gears.

The term planetary refers to the fact that the pinion or planet gears revolve around the central or sun gear, the same way the planets revolve around the sun.

Sun Gear

The sun (central) gear is at the center of the gear set. This gear may or may not be splined to a main shaft running through the center of the assembly. If the gear is not splined to the shaft then the gear is supported by a bushing or needle bearings. Usually the sun gear has a metal shell (cup shaped) attached to it in order to allow the gear to be utilized.

Planet Gears and Planet Carrier

Planet (pinion) gears revolve around the sun gear. They are held together by a carrier assembly. Their can be either three (3) or four (4) planet gears in the planet carrier. The planet carrier keeps the gears equally spaced apart allowing them to evenly support the ring gear from the sun gear. The planet carrier is usually splined to the main shaft (either input or output) or another member of the gearbox assembly in order to allow it to be utilized.

Ring Gear

The ring (internal) gear surrounds the planet gears. Unlike the other two (2) gears, it is an internal cut helical gear. It is usually splined to another part of the gearbox assembly in order to allow it to be utilized.

Gear Combinations/Ratios

In order for any gear combination to be obtained three (3) things must be done. One gear must be designated as the input gear, another must be designated as the output gear and the third gear must be held stationary.

The following chart shows how the various gear combination/ratios are achieved. In order to obtain direct drive (1:1) there are two (2) inputs needed and for neutral, one of the gears is allowed to spin freely.

Gear Combinations/Ratios
Slow Forward ReductionHeldInputOutput
Fast Forward ReductionInputHeldOutput
Slow ReverseOutputInputHeld
Fast ReverseInputOutputHeld
Slow OverdriveOutputHeldInput
Fast OverdriveHeldOutputInput
Direct DriveInputInputOutput

Bands and Clutches

In order for the automatic transmission to change gears various parts of the planetary gear set must be connected to the input shaft (from the engine) while others must be held stationary. This is done through the use of bands and clutches.

Bands are steel straps with friction material on the inside that are wrapped around a particular component. The band is tightened by the use of a hydraulic piston called a servo. One end of the band is attached to the transmission casing and the other to the servo. When fluid is directed from the pump to the servo, the fluid forces the piston forward tightening the band.

Clutches use multiple discs which are squeezed together by a large ring type piston. The clutch casing is usually driven by the input shaft. When fluid is directed from the pump to the clutch, the fluid forces the piston forward clamping the discs together so that they all rotate as one along with the component that is connected to the clutch.

The high pressure fluid from the pump is circulated and directed by a series of valves referred to as the valve body in order to change gears as well as lubricate the transmission.

Simpson Gear Train

A common use of a planetary gear set is in an automatic transmission where two (2) planetary gear sets are combined. One of the popular arrangements is called the Simpson gear train. This arrangement uses a common sun gear shared by both planetary gear sets. The gear sets are referred to as the front and rear gear set. By using a series of clutches, bands and a one-way (over-running) clutch, three (3) forward gears and a reverse gear can be achieved.

Simpson Gear Train Chart
GearFront BandFront ClutchRear ClutchRear BandO. R. Clutch
Note: In some automatic transmissions the rear band has been replaced by a holding clutch.

Key Terms and Definitions

Band: A device used to hold a part of the gear train assembly from turning in order to achieve a particular gear ratio.

Clutch: A device used to lock two (2) components together in order to transmit motion. Primarily used for engaging the input shaft to the gear assembly.

Mesh: To engage the teeth of one gear with those of another.

Overrunning Clutch: A clutch mechanism that will drive in one direction only. If driving torque is removed or reversed the clutch slips. Also referred to as a one way clutch.

Planet Gears: The gears in a planetary gear set that are in mesh with both the ring gear and the sun gear. Referred to as planet gears in that they orbit or move around the central or sun gear.

Planetary Gear Set: A gear unit consisting of a ring gear with internal teeth, a sun or central gear with external teeth, and a series of planet gears that are meshed with both the ring gear and the sun gear.

Ring Gear: A gear with internal teeth. The outer gear in a planetary gear set.

Simpson Gear Train: A compound planetary gear set where the sun gear is common between both sets of planetary gears.

Splined: A joint between two parts in which each part has a series of cuts or grooves along the contact area. When connected the splines from one part fit into the grooves of the other and vise versa.

Sun Gear: The central gear around which the planet gears revolve.

Rules of Planetary Gears

To determine the gear condition in a simple planetary gear set always look at what the carrier is doing.

  • If ever the carrier is an output in a simple planetary gear set, the condition will be forward reduction.
  • If ever the carrier is held in a simple planetary gear set, the condition will be reverse.
  • If ever the carrier is an input in a simple planetary gear set, the condition will be overdrive.


When working on a transmission, there are many hazards to be aware of in order to avoid personal injury. The following safety rules should be observed when working on any transmission.

  • Sharp edges and unfinished casting pieces could cut you severally. Never run your hands freely across any surfaces without some form of protection (work gloves or a rag).
  • Oil is a carcinogenic and should not be left on your skin. Be sure to wipe off any excess oil from your hands when you have finished your work for the day.
  • If a transmission has any burned oil on its surfaces and is giving off excessive odour, be sure to keep the work area well ventilated.
  • Some transmission parts are extremely heavy and could cause harm if they are dropped. Always handle all transmissions parts with care. Never attempt to lift any transmission parts that exceed your own capabilities. Ask for assistance.
  • Most internal transmission parts have very low tolerances and should not be forced into place on the transmission. Follow all recommended procedures when every dismantling or assembling any transmission.
  • Be sure to have another person support the transmission, when applying excessive force (tightening or loosening a fastener) to prevent the transmission from toppling over and causing any bodily harm or damage.

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What are the 5 laws of a simple planetary gear set?