# Planetary Gear

Planetary gears can rotate around their own axis of rotation (B-B) like a fixed-axis gear.

Their axes of rotation also rotate with the blue bracket (called the planet carrier) about the axis of the other gears (A-A).

The rotation around its own axis is called “rotation”, and the rotation around the other gear axis is called “revolution”.

Just like the planets in the solar system, hense it gets its name.

## The feature

The main features of the planetary gear transmission are small size, large carrying capacity and stable operation.

However, the high-power high-speed planetary gear transmission structure is complicated and requires high manufacturing precision.

Some types of planetary gears are highly efficient, but the gear ratio is not large.

Other types have a large transmission ratio but are less efficient.

When they are used as reducers, their efficiency decreases as the gear ratio increases;

When the speed increaser is used, it is possible to generate a self-locking.

## Application

1. Reduced transmission with large transmission ratio

In the planetary gear system, if the number of teeth of each wheel is z1 = 100, z2 = 101, z2′ = 100, and z3 = 99, the transmission ratio of the input member H to the output member 1 is 100.

It can be seen that a large gear ratio can be obtained according to the needs of the planetary gear system.

1. Realize compact power transmission

Planetary gear system can use several evenly distributed planet wheels to simultaneously transmit motion and power.

The centrifugal inertial force generated by these planetary wheels due to the revolution and the radial component force of the reaction force between the tooth profiles can be balanced with each other, so that the main shaft has a small force and a large transmission power.

In addition, since it uses internal gears, the space of the transmission is fully utilized, and the input and output shafts are in a straight line, so the space size of the entire system is much smaller than that of the conventional fixed-axis gear system under the same conditions.

This type of system is particularly suitable for aircraft.

1. Achieve the synthesis of motion

The synthesis of motion combines two input motions into one output motion.

The degree of freedom of the differential gear system is equal to two, and the motion of the other member can be determined given the determined motion of any two members.

The use of this feature of the differential gear system enables the synthesis of motion.

The rotational speed of the carrier H is a combination of the rotational speed of the wheel 1 and the wheel 3.

Therefore, this type of system can be used as an adding mechanism.

When the carrier H, the sun gear 1 or 3 is the original, the wheel train can be used as a subtraction mechanism.

This characteristic of differential gear trains for motion synthesis is widely used in machine tools, computing mechanisms, and compensation adjustment devices.

1. Achieve the decomposition of motion

The differential gear train can also decompose the rotation of one of the primary moving members into different rotations of the other two driven basic members.

The picture shows a schematic diagram of the rear axle differential of the car. In the figure, the members 5 and 4 form a fixed axle train, the wheel 4 is fixed to the carrier H, and the planet wheels 2 and 2′ are mounted on the H.

The gears 1, 2, 2′, 3 and the planet carrier H form a differential gear train that decomposes the motion of the engine to the gear 5 into different movements of the sun gears 1, 3.

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