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| Planetary Gear |
I have a planetary gear set on my desk from an old TH400 transmission; as I spun the assembly, and listened to the gear chatter, I thought that someone over the last 50 years must have developed an improvement on torque transmission and reduction. It didn’t take too long for me to find something called a Cycloidal Gearbox. Always taken in by a good high definition 3D rendering, I decided this would be a good subject for a blog. For the uninitiated, a planetary gearbox is a very simple system comprised of three components: a sun gear, a varying number of satellite or “planet” gears, and an internal ring gear. An input shaft transfers rotational motion to the sun gear, which transfers motion to planet gears, which turn the ring gear, part of the gearbox housing. The planet gears rotate on shafts rigidly mounted to plates. This part of the assembly is called the carrier. The rotation of the carrier transfers motion to the output shaft, which gives the output shaft a lower rotational speed and higher torque than the input shaft. Planetary gearboxes typically provide single or double reduction, from 3:1 up to 100:1; further reduction can also be achieved with other modifications. Not a bad set up, but hey, what year is this anyway? The planetary, aka “Epicyclic Gearing,” has been around since 87 BC--that’s older than my toothbrush. So what do you do if you need a reduction of more than 100:1? Enter the cycloidal gearbox/reducer.
I could watch these all-day
Cycloidal Gearbox Animation  |
| Cycloidal Gear |
Once the realm of watchmakers, cycloidal gearboxes are now commonly used in applications such as industrial-automation in conjunction with servomotors to control heavy loads at high cycle rates. They are also ideal for a number of other applications, including Hybrid automobiles, indexers, machine tools, tool changers, robotic positioners, and much more.
Ratios aside, when selecting a type of gearbox, there are a number of things to consider. If positioning accuracy is important, then cycloidal gearboxes offer the highest precision; their design delivers very precisely positioning/backlash. This is especially important with servo motors that perform high cycle, high-frequency moves. This feature is relatively constant throughout its life cycle.
As I mentioned earlier, for ratios of 3:1 to 100:1, planetary gearboxes offer the most advantages, such as torque density, weight, precision, and low cost. There are a number of manufacturers that offer planetary gearboxes designed to mate with servomotors; however, if space is an issue, planetary gearboxes increase in length as stages are added to accommodate increased reduction.
Cycloidal gearboxes can provide ratios below 30:1. In fact, they can provide excellent performance in ratios as low as 10:1, but their strength lies in reduction ratios greater than 100:1. Their design inherently eliminates the space issue because stacking is not required. Though cycloidal gearboxes don’t require the added length, they are larger in diameter for the same torque. Another big advantage is that a cycloidal gearbox can handle all reduction ratios with the same footprint, so the higher the ratio, the shorter the gearbox.
Both cycloidal and planetary reducers can be applied to virtually any servo or stepper motor application. However, if high ratios, low wear, compact design, and precision backlash are major factors, then the cycloidal design really stands out.
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