Quote:
Originally Posted by MechEng83
Side note: I think it's beautiful that the ratios offered are all clean integer ratios*, not messy repeating decimals. Planetary gear boxes of this variety have single stage ratios of Ring Gear Teeth/Sun Gear Teeth +1. The ring gears on Versaplanetaries are 72 teeth, which has factors 2x2x2x3x3. Using those factors in different combinations results in sun gears of 36, 24, 18, 12, 9, and 8 resulting in 3:1, 4:1, 5:1, 7:1, 9:1, and 10:1 respectively. 6:1 and 8:1 ratios would have required use of prime factors 5 and 7 respectively, which would increase the ring gear tooth count by that many, which is impractical.
*I understand by definition gear ratios are integer ratios, but when reduced to the mechanical advantage factor, only evenly divisible ratios give integer results.
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The one ratio which always bothers me is the 9:1. The sun gear has 9 teeth. Since the ring gear pitch diameter is the sun diameter plus twice the planetary diameter, working backwards from 72 teeth on the annular gear leaves 31 1/2 teeth on each of the planetary gears. I know it's possible to play some tricks with pitch diameter vs tooth counts, but I hope to be able to work around needing a 9:1 VP stage.
Gear ratios are always rational, that is ratios of integers, but not necessarily integers. It is possible to get close to the 6:1 and 8:1 integer ratios with a single stage 72t annular gear: with 10 teeth on the sun and 31 on the planets, a VP-compatible stage could be made with a gear ratio of 8.2:1; 14 on the sun and 29 on the planets would result in 6.14:1.
If you wanted to make a single "exact" 6:1 or 8:1 ratio stage compatible with VP, the first step would be to make a (most likely) 70t annular gear. Then your sun gears would be 10 and 14 teeth. If you were to do this, your "nearly identical to the standard" annular gears would probably be quite unpopular unless you found some clever way (e.g. color or a milled exterior similar to a U.S. quarter) to make them look quite unlike the 72t annular gear. I concur that this is not likely to be commercially viable, as the existing VP ratios provide a maximum gap of 40% (5:1 to 7:1) for a single stage. Above 9:1 (allowing multiple stages), the maximum gap is 25% (12:1 to 15:1 and 16:1 to 20:1) below 20:1 and 20% above that (50:1 to 60:1). There are few applications that cannot be designed around a 20% gap in capabilities.