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Old 07-04-2012, 07:11 PM   #15 (permalink)
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Originally Posted by niky View Post
Aren't planetary gears simply... one gear?
Hello niky,
I saw MPGranger's post, and I didn't think it really addressed your question.
I'm assuming you know what a planetary gearset is, and you've seen or studied them for aircraft or toys or something. So yes, a single planetary gearset presents one gear ratio.

But for something like an automatic transmission, in general using planetary gears you need one planetary system for 2 ratios, plus you need an additonal planetary set as a reverser. So for a 6 speed transmission you need 4 gearsets.
Matthieu Rihn developed the "ravigneaux" gearset using 2 sun gears, 2 ring gears and 1 planet carrier. By using clutches and brake bands, you can have 2 planetary gearsets produce 3 forward ratios and reverse.

However, the planetary CVTs that vskid3 was referring to is the electrically controlled "power-split device" used in the Prius and other hybrids. It is based on the 1970s U.S patents 3,566,717 and 3,732,751.

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