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Chuck Schultz is a licensed engineer, Gear Technology Technical Editor, and Chief Engineer for Beyta Gear Service. He has written the "Gear Talk with Chuck" blog for Gear Technology since 2014.

Planetary Gearboxes: A Wheel Inside a Wheel

June 26, 2018

Charles D. Schultz

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We will never know who invented the internal gear — much less the multitude of “epicyclic” or “planetary” gear systems — but we know from archeological evidence that these versatile arrangements have been in use for a very long time. As with many gear topics there is a lot of information about them floating around the internet and — no surprise — some of it is inaccurate. Unless you work with epicyclics on a regular basis, a “wiki” level of knowledge about them is probably sufficient. For those who aspire to master all types of gear design, there are many books and technical papers available for study. My goal in the next few postings is to take some of the mystery away and prepare the curious for more detailed investigations.

Long Service Life and a Systematic Approach

I first crossed paths with planetary gearboxes in 1979. My new employer had a legacy product line that certain customers would not let them abandon. I do not know if it is true that the design had its origin in German equipment obtained as reparations after World War I; many of the drawings were from that time period, so an element of truth might be there. The reason the customers loved the product was the extremely long service life they enjoyed. We rebuilt some of the lowest serial numbers and only had to change the bronze planet bushings after 40 years of around-the-clock service with through hardened teeth. One of the units achieved this despite leaving the factory without a “pilot shaft;” this demonstrated the “self-centering load sharing” feature of a well-engineered planetary far better than any textbook description. We looked at the product through different eyes; it required lots of gears and more inventory than management wanted to deal with going forward. Then there was the “math” problem; you had to understand the various tooth counts that would and would not assemble. Once I did my homework I appreciated the beauty of the systematic approach the original designers employed, and was able to develop some “special ratios” upon request. Next, we will consider why planetary gearboxes enjoyed such popularity in the early 20th century — and why they remain popular today.