Planetary Gears – a masterclass for mechanical engineers

Planetary gear sets contain a central sun gear, surrounded by several planet gears, held by a planet carrier, and enclosed within a ring gear
Sunlight gear, ring gear, and planetary carrier form three possible insight/outputs from a planetary equipment set
Typically, one portion of a planetary set is held stationary, yielding an individual input and an individual output, with the overall gear ratio depending on which part is held stationary, which is the input, and that your output
Instead of holding any kind of part stationary, two parts can be utilized as inputs, with the single output being truly a function of the two inputs
This is often accomplished in a two-stage gearbox, with the first stage generating two portions of the second stage. A very high gear ratio could be realized in a concise package. This type of arrangement is sometimes called a ‘differential planetary’ set
I don’t think there exists a mechanical engineer out there who doesn’t have a soft place for gears. There’s just something about spinning items of steel (or some other materials) meshing together that is mesmerizing to watch, while opening up so many possibilities functionally. Particularly mesmerizing are planetary gears, where the gears not only spin, but orbit around a central axis as well. In this article we’re likely to consider the particulars of planetary gears with an vision towards investigating a particular family of planetary equipment setups sometimes known as a ‘differential planetary’ set.

The different parts of planetary gears
Fig.1 The different parts of a planetary gear

Planetary Gears
Planetary gears normally contain three parts; A single sun gear at the center, an interior (ring) equipment around the exterior, and some number of planets that go in between. Generally the planets are the same size, at a common center length from the center of the planetary gear, and kept by a planetary carrier.

In your basic setup, your ring gear will have teeth equal to the number of the teeth in the sun gear, plus two planets (though there might be advantages to modifying this somewhat), due to the fact a line straight across the center from one end of the ring gear to the other will span the sun gear at the center, and space for a world on either end. The planets will typically be spaced at regular intervals around the sun. To do this, the total amount of tooth in the ring gear and sun gear mixed divided by the amount of planets has to equal a complete number. Of program, the planets need to be spaced far enough from each other so that they do not interfere.

Fig.2: Equal and reverse forces around the sun equal no aspect push on the shaft and bearing in the center, The same could be shown to apply to the planets, ring gear and planet carrier.

This arrangement affords several advantages over other possible arrangements, including compactness, the possibility for sunlight, ring gear, and planetary carrier to employ a common central shaft, high ‘torque density’ because of the load being shared by multiple planets, and tangential forces between the gears being cancelled out at the center of the gears because of equal and opposite forces distributed among the meshes between your planets and other gears.

Gear ratios of regular planetary gear sets
Sunlight gear, ring gear, and planetary carrier are normally used as input/outputs from the apparatus set up. In your standard planetary gearbox, one of the parts is definitely kept stationary, simplifying points, and giving you a single input and an individual output. The ratio for any pair can be worked out individually.

Fig.3: If the ring gear can be held stationary, the velocity of the planet will be seeing that shown. Where it meshes with the ring gear it will have 0 velocity. The velocity boosts linerarly over the planet equipment from 0 to that of the mesh with sunlight gear. Consequently at the centre it’ll be shifting at fifty percent the acceleration at the mesh.

For example, if the carrier is held stationary, the gears essentially form a standard, non-planetary, equipment arrangement. The planets will spin in the contrary direction from the sun at a relative speed inversely proportional to the ratio of diameters (e.g. if the sun has twice the diameter of the planets, sunlight will spin at fifty percent the acceleration that the planets do). Because an external equipment meshed with an interior gear spin in the same path, the ring gear will spin in the same direction of the planets, and again, with a speed inversely proportional to the ratio of diameters. The velocity ratio of the sun gear in accordance with the ring thus equals -(Dsun/DPlanet)*(DPlanet/DRing), or simply -(Dsun/DRing). This is typically expressed as the inverse, called the gear ratio, which, in cases like this, is -(DRing/DSun).

One more example; if the band is kept stationary, the side of the planet on the ring part can’t move either, and the earth will roll along the within of the ring gear. The tangential swiftness at the mesh with sunlight equipment will be equal for both sun and planet, and the center of the planet will be shifting at half of this, getting halfway between a spot moving at full rate, and one not shifting at all. Sunlight will end up being rotating at a rotational speed in accordance with the swiftness at the mesh, divided by the size of the sun. The carrier will become rotating at a quickness in accordance with the speed at

the center of the planets (half of the mesh rate) divided by the diameter of the carrier. The gear ratio would hence become DCarrier/(DSun/0.5) or just 2*DCarrier/DSun.

The superposition approach to deriving gear ratios
There is, nevertheless, a generalized method for determining the ratio of any kind of planetary set without needing to figure out how to interpret the physical reality of every case. It is known as ‘superposition’ and works on the principle that if you break a motion into different parts, and piece them back again together, the result will be the same as your original movement. It’s the same principle that vector addition works on, and it’s not really a stretch to argue that what we are performing here is in fact vector addition when you get because of it.

In this case, we’re going to break the motion of a planetary set into two parts. The foremost is if you freeze the rotation of most gears relative to one another and rotate the planetary carrier. Because all gears are locked jointly, everything will rotate at the speed of the carrier. The second motion is to lock the carrier, and rotate the gears. As observed above, this forms a more typical equipment set, and equipment ratios can be derived as features of the various gear diameters. Because we are merging the motions of a) nothing except the cartridge carrier, and b) of everything except the cartridge carrier, we are covering all motion taking place in the machine.

The information is collected in a table, giving a speed value for every part, and the apparatus ratio when you use any part as the input, and any other part as the output can be derived by dividing the speed of the input by the output.


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As one of leading manufacturers, suppliers and exporters of mechanical products in China, We offer reducers, sprockets, industrial and conveyor chain, belts, pulleys, gears, racks, gearboxes, motors, PTO Shafts, taper lock Bushing, vacuum Pumps, screw air compressors and many other products. Please contact us for details.

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