**A bevel-gear implementation of the Yonge phase-shifter, suitable for variable pitch props on planes and boats.**

http://www.yonge-cvt.com/how-it-works-pitch.html

**Implemented on a wind turbine.**

**Using the Yonge phase-shifter to successfully implement a working Dumaresq chain-based CVT.**

http://www.yonge-cvt.com/how-it-works-bike.html

I like this image because it shows how simple this device really is. You can get all clever and "fold" some of the gears back onto the shaft, and make smaller gears that spin within larger gears, but you don't have to do any of that. The concept is really fairly easy.

First thing to know is, we're going to be talking about bevel gears here. Which means that if you pick any two gears that touch each other, they will touch at a right angle. This is different than flat (or "spur") gears, where one gear always lies in the same plane as the gear next to it.

But first let's consider flat gears, since most people have an easier time visualizing them. Visualize that "O" is a large gear, and "o" is a small gear. Now, consider the gear combination "Oo". That's a large gear on the left, that meshes with a small gear on the right. Clear enough? Okay, how about "OoO"? That's a large gear, that meshes with a small gear, which in turn meshes with another large gear. Easy! Now let's go to the ridiculous extreme and imagine "OoOoO". Which is a large gear, then a small gear, then another large gear, then another small gear, then finally one last large gear. All meshing with each other.

Now, try and imagine the "Oo" setup again. But this time, imagine it with bevel gears instead of flat gears. If you have trouble seeing this in your mind, refer to the earlier link again. It's exactly what you see in the picture there. One large gear and one small gear, but they meet at a right angle. Got it? Now try and imagine two large bevel gears, face to face, and one small bevel gear that meshes with them both. (Rather like the red, green and yellow gears here. Or, imagine two large discs both freely spinning on a shaft, touching the opposite sides of a roller-blade wheel.)

Now try and imagine the "OoOoO" setup - but with bevel gears. Three large bevel gears, all free-spinning on a single shaft. And two smaller bevel gears: one small bevel gear that meshes with the first and second large gears, and one small bevel gear that meshes with the second and third large gears.

Got all that?

If you can see all that in your head, then look again at the picture above. In the picture above, there are three large bevel gears (colored green, yellow, and yellow). The outer green and outer yellow gears are attached to shafts, but ignore that for the moment. Just visualize them as gears spinning freely on the shaft. There are also four smaller bevel gears, colored blue or grey. Ignore the top two, and only "see" the bottom blue and bottom grey small gears.

Hey, it looks just like our earlier "OoOoO" bevel gear train! In fact, that's exactly what it is. So now can you see it in your head, how all the gears connect, and can all drive each other? You can? Good. Now this is where it gets interesting...

Imagine the left-hand large green gear is fixed in place, and cannot spin. Also imagine that there is a bar, or handle, which is attached to the central shaft but can spin freely around it. Now imagine that the small blue gear is on the handle, but it can still spin freely. Now, remember that the green gear can't turn. Grab the handle, and spin it around the shaft so that the small blue gear rolls in a circle on the face of the large green gear. Can you see it happening in your mind?

But... the small blue gear touches both the large green gear, AND the middle yellow gear. So when you move that handle and the blue gear spins, it's going to make the middle yellow gear spin as well. (Remember, the green gear is fixed and can't move.) And, of course, the middle yellow gear meshes with the small grey gear. So when the middle yellow gear spins, it makes the small grey gear spin. And the small grey gear also touches the right-hand yellow gear, which of course will also spin.

The net effect of all this is, when you move that handle, you're changing the angle of the large yellow gear with respect to the right-hand green gear. And how much the angle changes is precisely determined by how far you move the handle.

But here's the punchline: This changing of the relative angle thing? It still works EVEN WHEN THE GEARS ARE SPINNING LIKE MAD! The green gear can be going at 1000 RPM, which through the other gears will spin the right-hand yellow gear at 1000 RPM in the same direction. But when you grab that handle with the blue gear on it and move it, the same thing as before still happens! Even though they're both spinning like crazy, the right hand yellow gear will advance a few degrees ahead of the green gear, even as they both continue to spin at the same rate. And it stays a few degrees ahead as long as you keep the handle in that position! I won't try and explain this, as it strains even my mechanical visualization to imagine it in action. But rest assured it does work, and Chris Yonge has built the machines to demonstrate it to the world. And he has pictures of all of them on his website.

Pretty cool. Pretty freaking cool.

Now you'll have to go figure out the fancier implementations, and the chain-based CVT, on your own. ;]

http://en.wikipedia.org/wiki/Continuously_variable_transmission

http://www.yonge-cvt.com/

Not to take anything away from Yonge, but intuition is now whispering in my ear that there's some way to make his chain-based CVT even better...

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