Down-to-earth gears

Planetary gears cover pic

Just because the automobile is undergoing a transformation from the internal combustion engine to the electric motor, it doesn’t mean gears won’t be needed. After all, we still need to transfer power with uniform rotary motion, right?

electric car gearsWhile it is true that many high-tech electric cars only use one gear ratio since they are able to withstand higher torques, many of todays hybrid cars are keeping the continuously variable transmission (CVT). Enter the planetary gear set. This unique concept combines multiple gear changes into a small package using a combination of gear configurations. The first thing to note, though, is that these planetary gear sets use multiple gears – five at minimum, and they are found in almost all automatic transmissions. Many times, they also work with multiple sets of planetary gear trains. Because of this, inspection of these gears is even more crucial!

Planetary Gear SetA planetary gear, or epicyclic gear, system consists of four parts: a sun gear, a planet gear, a ring gear (or annular gear), and a planet carrier. Their history begins back in the 2nd century, A.D., when Ptolemy used such gear trains to predict the motions of the planets. It was also used in astronomical clocks. If you’d really like to see one, though, simply look inside the old-school (and some current schools!) pencil sharpeners that are affixed to the wall. Remember? Where you had to really crank to get your pencil sharp?!

Old school pencil sharpenersThe idea is that, depending on which of the components is fixed and driven, causing the planet gear, for example, to rotate and orbit the sun, different gear ratios may be obtained. By pairing planetary gear trains together is how car manufacturers can advertise the 9 speed transmissions that you can find in mid-size sedans.

The benefits of a planetary gear train is that they are compact and they have a seamless transition between gear changes. However, they are in constant need of lubrication and they are relatively inaccessible. This is why it is very expensive if your automatic transmission breaks!

Planetary gears movingAgain, in order to make quality gears, these planetary gears should be inspected, but, as in all manufacturing, hopefully in a timely fashion. Since the planet gears in the train are all the same, a simple macro can be written for TGear for simple and faster inspection.

So next time you’ve got your head in the clouds and are wondering if Pluto’s still a planet, spare a thought for the planetary gear train, and make sure to contact me here as well if you have any questions. Visit us at and follow me on Twitter @GearMarks, too. Happy gearing!