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Module 1 - Planetary Gear Sets Operation (Page 4 of 5)

Overdrive 1 - Maximum overdrive (very low torque, very high speed) - useless gear

Maximum overdrive occurs when the planet pinion carrier is connected to the engine and driving the sun gear. The planet pinion carrier is considered the largest gear. The sun gear is considered the smallest gear. When the largest gear drives the smallest gear, we have a resulting gear with very low torque and very high speed. This is not a very useful gear because the sacrifice in torque is too severe.

Sun gear – OUTPUToutput to rear wheels
Ring gear – HELD by the transmission
Carrier – INPUT front the engine

RULE: Because the planet carrier is considered the largest gear, the resulting gear is ALWAYS overdrive when the planet carrier is the input gear (driven by engine)

Overdrive 2- Moderate overdrive (low torque, high speed)

When we leave the roads and enter the freeway, we increase our speed. We can approach speeds of up to 100kph. In order to promote good fuel economy, most manufacturers provide some sort of overdrive gear. In overdrive, torque is further reduced and speed is increased. The engine turns at a lower RPM thereby promoting good fuel economy.

The only difference between overdrive 1 and overdrive 2 is the ring gear is now the output gear to the wheels and the sun gear is held by a device in the transmission.

By doing this, the largest gear (planet carrier), is now driving the second largest gear (the ring gear) instead of the smallest gear (sun gear).

Sun gear – HELD by the transmission
Ring gear – OUTPUT to the rear wheels through the output shaft
Carrier – INPUT from the engine

RULE: Because the planet carrier is considered the largest gear, the resulting gear is ALWAYS overdrive when the planet carrier is the input gear (driven by engine)

Module 1 (Page 4 of 5)