# Planetary Gear CVT

Would it possible to create a CVT with planetary gearing? I know that turning both motors in the same direction gives one gear ratio, and turning them in opposite directions gives another, but what about turning one motor full speed forward, and the other half speed backward? Does that give a gear ratio somewhere in between? If so, what is the math for determining the gear ratios?

No. It might output a variable speed but the torque wouldn’t magically appear.

So that use of a planetary gearbox would result in a significant loss of power, since you would be decreasing speed without increasing torque. Are you sure that is correct, that it wouldn’t become a CVT? Maybe if I can find the time and get the parts, I can build one and test that with rubber bands to “measure” torque.

Wouldn’t that be a CVT? An infinite number of gear ratios, albeit none would be higher (more torque) than 1:1.

To get around that, could you gear up the inputs?

I think he means that the output speed will change, though never higher than when the two motors are working together/against each other (whichever one is faster, I’m not totally familiar with the mechanics of a planetary gearbox), but the torque will remain constant at the amount of the faster gear ratio.
On a side note, is the gear ratio input:output or output:input? I only see documents online for input:output, and my mechanical engineering major dad said that it’s input:output, but I see both on this forum, so we basically have to use common sense to decide whether it increases speed or torque.

Edit: The last paragraph is worded terribly. Basically, I’m asking if the gear ratio is written as the mechanical advantage, or its inverse.

Speaking of cvts, I wonder if it would be possible to build a cvt like this one with vex parts for a drive train or flywheel (it would be especially nice for a flywheel).

I think, for the purposes of VEX, it’s whatever is convenient for the current game. I used output:input for skyrise, because lifts need torque, but now I use input:output because flywheels are fast.

So it would be a CVT with inconvenient endpoints? I don’t know the math for a planetary gearbox (that’s why I’m asking), but could you gear the inputs appropriately to change this?

It’d take awhile and a lot of planning, but you could use a 2.75in omni for the rollers and a bent plate or a bunch of sanded down gears for the discs.

I’m not sure if the gears are thick enough to be practical for that use, but it’s probably ~possible~ somehow, if not feasible.

Do you guys think that would work with helical gears, or would they need to be custom, high-friction discs with omni wheels as rollers? I’ve been looking for more ideas for the Autodesk Make It Real Challenge, and I want to do something involving transmissions.

You could use a differential transmission, if you go to the IQ forum someone posted recently about this.

http://www.vexiqforum.com/forum/main-category/vex-iq-robot-showcase/11329-apalrd-s-cvt-evt

I dont think that gears would work since there would have to be more teeth at the top of the cone shaped disk and less teeth at the bottom.

Problem is creating the planetary ring gear.

2R a while ago made a planetary gear set using chain as the annular gear, but I’m not sure how they got the sun and planetary gears to mesh. Go to YouTube and search for Vex planetary transmission and you will find it.

That’s a really good point; discs and omni wheels it is. Wish me CAD luck, right after I finish with Blender (hopefully today) and Candide by Voltaire (thanks, school, for homework over CHRISTMAS BREAK. Scrooges).

Precisely.

Think about your objective and effort-reward ratio. Given a few types of transmission or non-transmission drives. Given the game of the year. What’s your strategic requirement on your base? What option gives the highest reward?

It’s okay to be like, “this mechanism is cool so that I want to use it in game”. But that might not be the best option.

If you want a transmission, probably a 2 speed gear shifting transmission will be much more reliable.

If you are only changing speed, why even build a gearbox?

That’s why we have basically abandoned the concept of a planetary gearbox behaving as a CVT, since using it as such reduces efficiency, rather than increasing torque. The point of using a gearbox to reduce speed is to increase torque, and apparently planetary “CVTs” don’t do that.

I’m not asking for my robot. I’m just curious about the math and all.

Puzzler7,
What you are suggesting (if I understand it correctly) is exactly how hybrid automotive transmissions combine gasoline engine torque with electric motor torque. Increasing the electric motor speed as the car accelerates allows the gasoline engine speed to stay at constant speed that provides the best engine efficiency.

Keep those grey cells thinking!