Planetary Gears 101 – UnderhoodService
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Planetary Gears 101

The most confusing systems on any car is the automatic transmission. The main stumbling block for most technicians is not the electronic control systems, but the humble planetary gear. The planetary gear sets in an automatic transmission provide forward underdrive and overdrive gear ratios, plus reverse. The machinery handbook describes planetary gearing as a compact means of driving inline shafts with speed reduction (underdrive) and induction (overdrive).
Advantages of planetary gears over parallel axis gears include high power density, large reduction in a small volume, pure torsional reactions and coaxial shafting. Disadvantages include high bearing loads, inaccessibility and design complexity. The planetary gearbox arrangement is an engineering design that offers many advantages over traditional gearbox arrangements. One advantage is its unique combination of both compactness and outstanding power transmission efficiencies.

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FIGURE 1: A planetary gear set is made up of a sun gear, planetary gears and a ring gear.  The ring is used as the final drive to the vehicles front or rear drive wheels.  The gears have three modes of operation: Driver, Fixed and Follower. 
 

 
FIGURE 2: Overdrive uses the sun gear in a fixed position, planetary gear in a driver position and the ring gear as the follower.  
 

 
FIGURE 3: Underdrive uses the sun gear in a driver position, the planetary gear set as the follower and the ring gear is in the fixed position.  To drive the ring gear in the proper direction, the planetary gear set uses a second gear to turn the ring gear. 

 
FIGURE 4: Reverse uses the sun gear in a drive position, the planetary gear set in a fixed position with the ring gear in a follower position. One of two types of clutches or bands are used to hold a particular member of the planetary gearset motionless, while allowing another member to rotate, thereby transmitting torque and producing gear reductions or overdrive ratios. These clutches are actuated by the valve body and their sequence controlled by the transmission’s internal programming. Principally, a type of device known as a sprag or roller clutch is used for routine upshifts/downshifts.
 

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