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What Is a Planetary Reducer?

Posted on Sep 17, 2020

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What millennia-old technique is the basis of many of the most innovative technical breakthroughs at the moment? Robotics, 3D printing and new modes of transport have one thing in common: they are often driven by a planetary reducer. What if this is the first time you encounter this technique? We decided to make it clear to everyone - in this article we cover the basics of the planetary reducer.

What is a planetary gear?

A planetary reducer is a gearbox with an inline input and output shaft. Planetary reducer is used to transmit the greatest torque in its most compact form (known as torque density).

A bicycle's acceleration hub is a great example of a planet gear mechanism: have you ever wondered how you can get so much power and capability in such a small hub? In the case of a hub with three speeds, a single-speed planetary gear is used, in the case of a five-speed hub - a two-speed. Each planetary gear system has a reduction state, direct engagement, and acceleration mode.

In mathematical terms, the smallest reduction ratio is 3: 1, the largest is 10: 1. With a gear ratio of less than 3, the sun gear becomes too large for the planet gears. With a gear ratio greater than 10, the sun gear becomes too small and the torque drops. The ratios are usually absolute, ie integer.

It is not known who invented the planetary reducer, but it was described functionally by Leonardo da Vinci in 1490 and has been used for centuries.

Why is a planetary gearbox?

The planetary reducer got its name because of how the different gears move together. In the planetary reducer, we see a sun (sun) gear, a satellite (ring) gear, and two or more planetary gears. Typically the sun gear is driven and thus moves the planet gears locked in the planetary holder and forms the output shaft. Satellite gears have a fixed position in relation to the outside world. It looks similar to our planetary solar system and hence the name. What helped was that ancient gear constructions were used extensively in astrology for mapping and following our celestial bodies. So it was not such a big step to make.

In practice, we often speak from the perspective of using planetary reducers for industrial automation. Therefore, we call the sun gear the input shaft, planetary gears and the output shaft bracket, and the satellite gear (or ring gear) the housing.

Where is planetary gear standard (in gear)?

In the robot to increase the torque

In a printing press to reduce the speed of the rolls

For precise positioning

In a packaging machine for repeat products

Buying a planetary gear : what you need to look for

What are the purchase criteria for a planetary reducer? It is difficult to answer this question as it greatly depends on where exactly the gearbox is used. First, the primary specs (e.g., torque, backlash, gear ratio) have to be correct, but then the secondary specifications (eg.: corrosion resistance, noise level, construction) and the tertiary (eg.: delivery time, price, global availability, service) are important.

Grease or oil as lubricant in planetary gearbox

Even with the precise manufacture and assembly of the planetary gear unit, there are always rolling or sliding surfaces inside. Therefore, every gearbox contains a lubricant - whether it is oil, grease or synthetic gel, to ensure the proper operation of the gear and prevent wear. In addition, the lubricant often also provides cooling and reduces noise or vibration.

The design of the planetary reducer is quite simple, it consists of a central sun gear, an outer ring (also called an inner gear because its teeth are facing inwards), planet gears, and a bracket. The power input to the sun wheel causes it to rotate. The planetary gears mesh with the sun gear, and as the sun gear rotates, the planet gears rotate on their axes. The planetary gears also mesh with the ring gear, which is stationary, causing the planetary gears to rotate around the sun gear. The carrier holds the planetary gears together and adjusts their spacing. It rotates with planetary gears and includes an output shaft.

:: Read more : What is planetary gear reduction?

Planetary gears are also referred to as epicyclic gears

In a planetary gear, many teeth are engaged simultaneously, which allows to achieve high speed reduction with relatively small gears and lower inertia reflected back to the motor. Having multiple load sharing teeth also allows the planetary gears to transmit high torque. The combination of compact size, large speed reduction and high torque transmission make planetary gears a popular choice in applications with limited space.

But planetary gears have some drawbacks. Their complexity in design and manufacture makes them more expensive than other types of gears. With these gearboxes, precise production is extremely important. If one planetary gear is located closer to the sun gear than the others, an imbalance in the planetary gear 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 run in very high speed or experience continuous operation 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. 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, the 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 roller bearings.

The inherent limitations of bearing size and type, combined with the dual task of transmitting torque and supporting axial loads, mean that the torque ratings of helical planetary reducers may be lower than those of similar helical planetary gear units whose bearings are only exposed to forces resulting from the transmission of torque (no axial loads). On the other hand, helical planetary designs have lower noise, smoother operation and higher rigidity than helical planetary gears. These attributes make helical planetary reducers the more common choice in servo applications.

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