What is planetary gear reduction?

Posted on Aug 17, 2020

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What is planetary gear reduction?

The planetary gearbox design is fairly simple, consisting of a central sun gear, an outer ring (also referred to as an internal gear, because its tooth face inward), planetary gears, and a carrier. Input power to the sun gear causes it to spin. The planetary gears mesh with the sun gear, and as the sun gear spins, the planetary gears rotate on their axes. The planetary gears also mesh with the ring gear, which is stationary, causing the planetary gears to revolve around the sun gear. The carrier holds the planetary gears together and sets their spacing. It rotates with the planetary gears and incorporates the output shaft.

In a planetary gearbox, many teeth are engaged at once, which allows high-speed reduction to be achieved with relatively small gears and lower inertia reflected back to the motor. Having multiple teeth share the load also allows planetary gears to transmit high levels of torque. The combination of compact size, large speed reduction and high torque transmission makes planetary gearboxes a popular choice for space-constrained applications.

The combination of compact size, large speed reduction and high torque transmission makes planetary gearboxes a popular choice for space-constrained applications.

But planetary gears have some drawbacks. Their complexity in design and manufacture makes them more expensive than other types of transmissions. Precision production is extremely important for these gearboxes. If one planetary gear is located closer to the sun gear than the others, imbalances in the planetary gears can occur, leading to premature wear and failure. In addition, the compact size of the planetary gears make it difficult to dissipate heat, so applications that run at very high speeds or run continuously may require cooling.

When using a "standard" (ie, in-line) planetary gear, the motor and driven equipment must be in alignment, although manufacturers offer angular designs that incorporate other gear sets (often bevel gears with helical teeth) to provide displacement between entrance and exit.

 

Planetary gears can be made with spur gears or helical gears

Planetary gears can be made with spur gears or helical gears. The spur gears have a zero helix angle and therefore do not create any axial forces. Therefore, the bearings in the planetary spur gear only serve to support the gear shafts. In turn, helical gears have a helix angle of 10 to 30 degrees, which causes them to generate significant axial forces. The bearings used in the helical planetary gear must withstand these axial forces. (Larger helix angles result in higher axial forces, but also provide higher torque capacity, less noise and smoother operation.)

Additionally, in a planetary gear unit - whether it is a spur or a helical design - the bearings play an active role in transmitting torque. But the planetary system allows limited space in the gearbox to accommodate the bearings. Needle roller bearings are a good choice from a size standpoint, but are not designed to withstand significant axial loads. Tapered roller bearings are suitable for high axial loads, but are generally larger than needle bearings.

The inherent limitations of bearing size and type, combined with the dual task of transmitting torque and bearing axial loads, mean that the torque ratings of helical planetary gears may be lower than those of similar helical planetary gears where only the forces resulting from with torque transmission (no axial loads). On the other hand, planetary helical structures have lower noise, smoother operation and higher rigidity than spur planetary gears. These features make helical planetary gears the more common choice in servo applications.

:: Read more : Helical gears or helical rack and pinion?

 

Where are planetary gears used?

Planetary gears are often used when space and weight are a concern, but a large reduction in speed and torque is required. This requirement applies to a variety of industries including tractors and construction equipment where high torque is required to drive the wheels.

Planetary gears are capable of producing high torque as the load is shared across multiple planetary gears. This arrangement also creates more contact surfaces and a larger contact area between the gears than the traditional parallel axis arrangement. As a result, the load is more evenly distributed and therefore the gears are more resistant to damage.

 

Production of planetary gears

The skills required to manufacture planetary gears are the same as for any other type of precision gear manufacturing. Gear Motions is a leading precision gear manufacturer specializing in providing custom cut and ground gears. We have a wide portfolio of gear manufacturing capabilities that includes the ability to manufacture all of the individual gears that make up the planetary gear system. In the case of specific production possibilities, such as minimum and maximum diameter, pitch diameter, and face width.

:: Read more : What is the selection criteria for choosing a gear reducer?

 

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