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Module 1 - Planetary Gear Sets Operation (Page 2 of 5)

Reduction versus overdrive

When we accelerate our vehicles from a stand-still, we start in drive 1 (first gear). As our vehicle speeds up, the automatic transmission shifts into drive 2 (second gear). When we have reached moderate cruising speeds of 45-55kph, the transmission shifts into direct drive (third gear). As we approch highway speeds of 70-80kph, we proceed into overdrive gears.

Each subsequent gear gives gives us less torque but more speed. Torque and speed are always inversely related. Speed increase = torque decrease. Torque increase = speed decrease.

Quite simply, When a smaller gear drives a large gear, the resulting gear is a reduction gear, that is: speed decrease, torque increase. The larger gear spins slowly but with greater torque.

When a large gear drives a small gear, the resulting gear is an overdrive gear. In other words, the smaller driven gear spins faster than the larger drive gear but with less torque.

When two gears of the same size are meshed together and one gear is driving thte other, there is no reduction or overdrive. Each of the gears spins at the same speed. This is called direct drive.

Visit this tutorial for an interactive demonstration of how gear ratio works.

Planetary Gear Set Components

Remember our discussion. In a planetary gear set:

  • The sun gear is considered the smallest gear (pictured below are two sun gears)

  • The internal or ring gear is considered the mid-sized gear. (Pictured below is a ring or internal gear)

  • The planetary pinion carrier is considered the largest gear (pictured below are two planet pinion carriers)

Planetary Gear Set Operation

We can obtain up to seven different gears from one planetary gear set. They are:

  1. Drive 1 - maximum reduction (very high torque, very low speed)
  2. Drive 2 - moderate reduction (high torque, low speed)
  3. Direct drive - no reduction (one : one gear ratio, like 4th gear on manual trans)
  4. Overdrive 1 - maximum overdrive (very low torque, very high speed)
  5. Overdrive 2 - moderate overdrive (low torque, high speed)
  6. Reverse reduction - reduction in reverse (very high torque, very low speed)
  7. Reverse overdrive - overdrive in reverse (very low torque, very high speed)

Not all of these gears are useful. For example, why would we want a reverse overdrive? Have you ever driven on the freeway at 80kph in reverse? Also two overdrives is not really needed either.

Module 1 (Page 2 of 5)