Got torque?
A simple planetary gear unit viewed from its delivering end. Dual roller bearings at the output help isolate the gearing (spur gears) from the effects of external transverse loads. The heavy disc is the planet carrier. (Courtesy of GAM Gear.)

Planetary gearing, with its inherent in-line shafting and cylindrical casing, is often recognized as the compact alternative to standard pinion-and-gear reducers. Being suited for a wide range of applications – from electric screwdrivers to bulldozer power trains – these units are strong contenders when space and weight versus reduction and torque are chief concerns. To fully understand their operation, you need details. Examining the construction and mechanics of planetary systems reveals some of the less-obvious factors that come into play.

The arrangement

The most basic form of planetary gearing involves three sets of gears with different degrees of freedom. Planet gears rotate around axes that revolve around a sun gear, which spins in place. A ring gear binds the planets on the outside and is completely fixed. The concentricity of the planet grouping with the sun and ring gears means that the torque carries through a straight line. Many power trains are "comfortable" lined up straight, and the absence of offset shafts not only decreases space, it eliminates the need to redirect the power or relocate other components.

In a simple planetary setup, input power turns the sun gear at high speed. The planets, spaced around the central axis of rotation, mesh with the sun as well as the fixed ring gear, so they are forced to orbit as they roll. All the planets are mounted to a single rotating member, called a cage, arm, or carrier. As the planet carrier turns, it delivers low-speed, high-torque output.

A fixed component isn't always essential, though. In differential systems every member rotates. Planetary arrangements like this accommodate a single output driven by two inputs, or a single input driving two outputs. For example, the differential that drives the axle in an automobile is planetary bevel gearing – the wheel speeds represent two outputs, which must differ to handle corners. Bevel gear planetary systems operate along the same principle as parallel-shaft systems.

Even a simple planetary gear train has two inputs; an anchored ring gear represents a constant input of zero angular velocity.

Designers can go deeper with this "planetary" theme. Compound (as opposed to simple) planetary trains have at least two planet gears attached in line to the same shaft, rotating and orbiting at the same speed while meshing with different gears. Compounded planets can have different tooth numbers, as can the gears they mesh with. Having such options greatly expands the mechanical possibilities, and allows more reduction per stage. Compound planetary trains can easily be configured so the planet carrier shaft drives at high speed, while the reduction issues from the sun shaft, if the designer prefers this. Another thing about compound planetary systems: the planets can mesh with (and revolve around) both fixed and rotating external gears simultaneously, hence a ring gear is not essential.

Increasing reduction

Planet gears, for their size, engage a lot of teeth as they circle the sun gear – therefore they can easily accommodate numerous turns of the driver for each output shaft revolution. To perform a comparable reduction between a standard pinion and gear, a sizable gear will need to mesh with a rather small pinion.

Planetary gearing (sun gear driving)

Simple planetary gears generally offer reductions as high as 10:1. Compound planetary systems, which are far more elaborate than the simple versions, can provide reductions many times higher. There are obvious ways to further reduce (or as the case may be, increase) speed, such as connecting planetary stages in series. The rotational output of the first stage is linked to the input of the next, and the multiple of the individual ratios represents the final reduction.

Another option is to introduce standard gear reducers into a planetary train. For instance, the high-speed power might pass through an ordinary fixedaxis pinion-and-gear set before the planetary reducer. Such a configuration, called a hybrid, is sometimes preferred as a simplistic alternative to additional planetary stages, or to lower input speeds that are too high for some planetary units to handle. It also provides an offset between the input and output. If a right angle is needed, bevel or hypoid gears are sometimes attached to an inline planetary system. Worm and planetary combinations are rare because the worm reducer by itself delivers such high changes in speed.

Taking the torque

Since planetary gears mesh with the sun gear and ring gear at several locations, more teeth are engaged to drive the load, compared to a conventional gearand- pinion mesh. Therefore, for the same load, planetary gearing requires smaller gears (although in greater number) than a standard pinion-to-gear reduction. Likewise, the radial arms of the planet carrier transfer substantial moment to the output shaft – another illustration of the efficiency of a concentric arrangement.

Fixed-axis gearing (pinion driving)

Perhaps a less obvious but no less significant consideration is that with multiple, equally spaced planets (as is usually the case) the input and output shaft bearings are spared the radial loads resulting from separating and tangential gear reaction forces, because these reactions cancel out. Plus, since no such forces act on these bearings, there is less potential for distortion of the outer casing.

With more planets comes an increase in load capacity and torsional rigidity; the more divided the load, the less deflection and wear of gear teeth. It follows that quite a large load can be driven in a comparatively small and streamlined planetary gear unit. Three is typical for the number of planets, but there are often more, and sometimes less. Again, it is common for multiple planets to be equally spaced.

Helical gears can be used for load capacity beyond spur gears, given comparable gear sizes and numbers of planets – because helicals are angled, not straighttoothed, even more teeth mesh at once. But with helical planetary gearing there are axial reactions, and these don't cancel with multiple planets like the tangential and separating gear reactions do, so bearings have to account for the thrust load.

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