Planetary Gear Basics

The planetary gear is one of the epicyclic gear, which is a gear system consisting of one or more outer gears, or planet gears, revolving about a central, or sun gear. And it can be wildly used in industry, such as printing lathes, automation assembly, semiconductor equipment, and automation system. Since its commercial value, our project is aimed to build a 3D model of multiple layers of planetary gear by using Solidworks software. And with the help of COS- MOSMotion, visual movement simulation can be realized more efficiently. Moreover, using an FDM machine, a solid model of a sophisticated planetary gear can be built successfully. The planetary gear is a type of gear, used in AC /DC geared motors and planetary gear reducers.


Planetary gear trains are one of the main subdivisions of the simple epicyclic gear train family. The epicyclic gear train family in general has a central “sun” gear that meshes with and is surrounded by planet gears. The outermost gear, the ring gear, meshes with each of the planet gears. The planet gears are held to a cage or carrier that fixes the planets in orbit relative to each other. The planetary gear is a widely used industrial product in the mid-level precision industry, such as printing lathe, automation assembly, semiconductor equipment, and automation system. Planetary gearing could increase torque and reduce load inertia while slowing down the speed. Compared with traditional gearbox, planetary gear has several advantages. One advantage is its unique combination of both compactness and outstanding power transmission efficiencies. A typical efficiency loss in a planetary gearbox arrangement is only 3% per stage. This type of efficiency ensures that a high proportion of the energy being input is transmitted through the gearbox, rather than being wasted on mechanical losses inside the gearbox. Another advantage of the planetary gearbox arrangement is load distribution. Because the load being transmitted is shared between multiple planets, torque capability is greatly increased. Greater load ability, as well as higher torque density, is obtained with more planets in the system. The planetary gearbox arrangement also creates greater stability due to the even distribution of mass and increased rotational stiffness.

Based on so many advantages of planetary gear above, we did our 3D model of multiple layers of planetary gear to get the speed reduction. And by using COS- MOSMotion, we achieve the visual movement simulation of it. Also, we build the solid model using an FDM machine. Finally, we hope our product can be used in industry in the future.

By using SolidWorks software to build the 3D model of planetary gear, there are some things to do first, such as keeping a constant velocity ratio between two adjacent gear teeth, generating an involute curve, and setting parameters of spur gear.

Constant Velocity Ratio

To get a constant velocity ratio, the common normal to the tooth profiles at the point of contact must always pass through a fixed point (the pitch point) on the line of centers. In figure 1, although the two profiles have different velocities at the contact point K, their velocities along N1N2 that is passing through the pitch point P are equal in both magnitude and direction. Otherwise, the two tooth profiles would separate from each other. Therefore, we get the velocity ratio, which is equal to the inverse ratio of the diameters of these two tooth profiles. This is called the fundamental law of gear-tooth action. For each two mating gearing tooth profiles, they should satisfy the fundamental law to get a constant velocity ratio.

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