In automatic transaxle vehicles, the planetary gear unit handles deceleration, reverse, direct connection, and acceleration. The planetary gear unit consists of the planetary gears, clutches, and brakes. The front planetary gear set and the rear planetary gear set are connected to the clutches and brakes that connect and disconnect the power. They switch the input section and fixing elements and produce various gear ratios and neutral.

**HINT:**

3- speed planetary gear unit (A130 series). Basically, this model will be used to explain operations of the planetary gear unit.

**Construction**** **

The planetary gears have three types of gears – the ring gear, pinion gears, and sun gear – and the planetary carrier. The planetary carrier is connected to the center shaft of each pinion gear and makes the pinion gears revolve. With this set of mutually connected gears, the pinion gears resemble planets revolving around the sun, so they are called the planetary gears. Normally, plural planetary gears are combined in the planetary gear unit.

**Principle of Operation**

By switching the input, output, and fixing elements, it is possible to decelerate, reverse, directly connect, and accelerate. The outline of these operations is explained below.

**1. Deceleration**

**Power input: Ring gear**

**Power output: Planetary carrier**

**Fixed: Sun gear**

When the sun gear is fixed, only the pinion gear rotates and revolves. Therefore, the output shaft decelerates in proportion to the input shaft only by the rotation of the pinion gear. The length of the arrow shows the rotational speed and the width of the arrow shows the torque. The longer the arrow, the greater the rotational speed and the wider the arrow, the greater the torque.

**2. Reverse**

**Power input: Sun gear**

**Power output: Ring gear**

**Fixed: Planetary carrier**

When the planetary carrier is fixed in position and the sun gear turns, the ring gear turn on its axis and the rotational direction is reversed. The length of the arrow shows the rotational speed and the width of the arrow shows the torque. The longer the arrow, the greater the rotational speed and the wider the arrow, the greater the torque.

**3. Direct connection**

**Power input: Sun gear, ring gear**

**Power output: Planetary carrier**

As the ring gear and sun gear rotate together at the same speed, the planetary carrier also rotates at the same speed. The length of the arrow shows the rotational speed and the width of the arrow shows the torque. The longer the arrow, the greater the rotational speed and the wider the arrow, the greater the torque.

**4. Acceleration**

**Power input: Planetary carrier**

**Power output: Ring gear**

**Fixed: Sun gear**

When the planetary carrier turns clockwise, the pinion gear revolves around the sun gear while turning clockwise. Therefore, the ring gear accelerates based on the number of teeth on the ring gear and sun gear. The length of the arrow shows the rotational speed and the width of the arrow shows the torque. The longer the arrow, the greater the rotational speed and the wider the arrow, the greater the torque.