Why is the planetary gearbox a true 'mountain' for gear transmission personnel?
What is planetary gear transmission?

The rotational axis of planetary gear transmission is not fixed and is installed on a rotatable support. In addition to rotating around its own rotational axis like fixed axis gears, planetary gears also rotate around the axes of other gears along with the planetary support. The rotation around one's own axis is called "self rotation", while the rotation around other gear axes is called "revolution", just like the planets in the solar system, hence the name "planetary gear", which forms the basis of planetary gear transmission and planetary reduction gearbox.

What is the structural composition of planetary gear reduction transmission?

The simple (single row) planetary gear mechanism is the foundation of the transmission mechanism, and the transmission mechanism of automatic transmissions is usually composed of two or more rows of planetary gear mechanisms. A simple planetary gear mechanism includes a sun gear, several planetary gears, and an inner ring gear, where the planetary gears are supported by a fixed shaft of the planet carrier, allowing the planetary gears to rotate on the support shaft. The planetary gears, adjacent sun gears, and ring gears are always in a constant meshing state.

In a simple planetary gear mechanism, the sun gear is located at the center of the planetary gear mechanism, and the sun gear and the planet gear are constantly meshed. The two external gears mesh and rotate in opposite directions. Just as the sun is located at the center of the solar system, the solar wheel is also named after its position. In addition to rotating around the support shaft of the planet carrier, planetary gears can also rotate around the central axis of the sun gear under the drive of the planet carrier in some working conditions, just like the rotation of the Earth and the revolution around the sun. When this situation occurs, it is called the transmission mode of planetary gear mechanism.

In the entire planetary gear mechanism, if the planetary gear rotates on its own, the planetary carrier remains fixed. This method is similar to parallel axis transmission and is called fixed axis transmission. Ring gear is an internal gear that constantly meshes with planetary gears. It is a combination of internal and external gears that rotate in the same direction. The number of planetary gears depends on the design load of the transmission, usually three or four, and the more they are, the greater the load they bear.

A simple planetary gear mechanism is commonly referred to as a three component mechanism, consisting of the sun gear, planet carrier, and ring gear. To determine the motion relationship between these three components, it is generally necessary to first fix one of them, then determine who is the active component, and determine the speed and rotation direction of the active component. As a result, the speed and rotation direction of the passive component are determined

What are the characteristics of planetary gear transmission?

1. Compact structure, small volume, and light weight

Planetary transmission has the transmission characteristics of planetary motion and power splitting, which is why there is an internal gear pair. It can fully utilize the large load-bearing capacity of the internal gear and the full capacity of the internal gear ring, making the reduction gearbox compact in structure, small in outer dimensions, and light in weight. Generally, when the transmission power and transmission ratio are the same, the volume and weight of the planetary gear reduction motor are about 1/2 to 1/6 of those of ordinary gear transmission

2. Large transmission ratio, capable of synthesizing and decomposing motion

There are many types of planetary transmissions, such as involute planetary transmission, cycloidal pinwheel planetary transmission, harmonic planetary transmission, and movable tooth planetary transmission, which generally have the characteristic of large transmission ratio. When used for transmitting motion, its maximum transmission ratio can reach tens or hundreds of thousands, and as a power transmission, its maximum transmission ratio can reach tens or hundreds. The planetary transmission using differential motors can achieve the synthesis of two movements and the decomposition of one action. In some cases, selecting the appropriate type of planetary gear reduction motor can achieve various complex movements with variable speeds.

3. High efficiency and minimal power loss

Planetary transmission uses several planetary gears evenly distributed between the inner and outer center wheels, which can balance the inertial forces acting on the center wheel and planetary carrier bearings. Adopting this symmetrical structure is beneficial for improving the efficiency of the transmission system. The optimized transmission design can achieve an efficiency of 0.97-0.99 for planetary gear reduction motors.

4. Transmission balance, strong resistance to impact and vibration

By using multiple planetary gears evenly distributed between two central gears, and using a load sharing device to maintain uniform load distribution and power distribution between each planetary gear, not only can the inertial forces of each planetary gear and the rotating arm be balanced, but also the stability and resistance to impact and vibration of the planetary gear reduction motor are significantly improved.