An interesting feature about planetary gearbox working principle is that, you can achieve many speed ratios from a compact and robust structure.

First, I want you to understand that, the working principle of planetary gearbox involves three main components:

• Sun gear
• Planet gear
• Ring gear

By changing the state of any of these gears, you can achieve many gear ratios.

In these gearboxes, any of the three components can be held stationary and they may act as either input or output.

Depending on the type of planetary gearbox component that is an input, output or held stationary, you can achieve a range of gear ratios.

The good thing about these gears is that, you can achieve any gear ratio without necessarily having to engage or disengage other gears.

Let’s look at this example of planetary gear explanation (Source: How Stuff Works):

In this example, the ring gear has 72 teeth while the sun gear has 30 teeth.

Clearly, you can get a wide range of gear ratios depending on the specific working requirements.

Take for instance:

• 4:1 is basically a reduction process; the output speed is lower than the input speed
• 71:1 is an overdrive; the output speed is faster than the input speed

Clearly, you can see how convenient a planetary gearbox is in most mechanical systems.

Analysis of the Planetary Gearbox Working Principle

Basically, a normal gearbox has three sets of gears as I had mentioned earlier.

All the gears have different degrees of freedom.

Under normal circumstances, we have:

• Planet gears revolving around a sun gear
• Planet gears rotating freely on a movable arm. At times, they may rotate relativelyto the sun gear.
• During the operation, the entire process incorporates ring gear or annulus

Depending on the gear arrangement, we may have either simple or complex planetary gearbox.

That is, a simple planetary gearbox has planet gears, carrier, ring gear and sun gear.

On the other hand, compound planetary gears have mashed-planet, stepped-planet and multi-stage structures.

Basically the differences between these two types of gears depend on the desired gear ratio, torque to weight ratio and flexibility in configurations.

So, what do all these imply?

In a simple planetary gearbox, as the input power turns the sun gear, the planets around the central axis begin to rotate.

That is, the planets mesh with the sun thereby, forcing them to rotate as a whole.

By the end of the process, as the planet carrier rotates, it delivers the desired output torque.

Again, by keeping any of the parts at a fixed point, while using the other sections as either output or input results in different degrees of power or speed.

You can see this as shown in the table below:

Well, a comprehensive analysis of planetary gearboxes requires a lot of calculations and that may be beyond the scope of this article.

This is why I can recommend Gear Trains (An analysis of gear trains).

In short, the planetary gearbox working principle is based on fixing one point while using others as either output or input.

With this, the gears should mesh seamlessly thereby either increasing or reducing speed.