pic: CVT drivetrain



i saw this on How Stuff Works and i think its got a real place in FIRST

yeah, Contiunously Variable Transmissions are great. You may want to check this white paper out, from the great team of 217. Do keep in mind its patent pending. I believe they were one of the first teams to try out a CVT, and they were very succesful with it, it’s great to be able to have full horsepower at any speed.

Also, CVT’s are starting to be used in cars too. The 2005 Ford Freestyle is running with a CVT transmission, pretty cool.

all right i came across this in How Stuff Works. the cool thing is that there are no specific ratio it changes how you want it to. its powered by a main motor and then the two servos (A and B) change the position of the two adjustable gears changing from a high to low ratio. note my arrows are slightly off so here’s another pic

cvt-17

heres a link that explains more (http://auto.howstuffworks.com/cvt.htm) this type of transmission is more efficient than your standard gear change transmission. it even was in a couple cars for some time but just never got the publicity needed to make it in most cars. it has a smooth shift and you can have any gear ratio you want.

While the CCT was a very innovative transmission, it was not a CVT (if I remember correctly, the CCT was actually a single speed transmission for blending two motors).

Torroidal CVTs, such as the one in your pictures, were used by 190 in 2002 (and our 2003 bot was designed to use a 2nd Generation CVT, but we ran out of time). I know that plans were made available to the FIRST community at the time, but I don’t know if a formal whitepaper was ever produced. I’ll try to find out.

There are many other types of CVTs out there that would probably be easier to build (the toroidal transmission requires several parts that need to be done by CNC). The Freestyle that you mentioned, like most cars with CVTs, uses a chain that runs in between two sets of two conical pullies. I don’t think anyone has tried this type of transmission on a FIRST robot.

The big problem with CVT’s made for FIRST is that they’re horribly inefficient, and fairly complex.

I’m not sure you’d even have a reason to make one, rather than just making a auto shifting 4 speed like 33 did, other than for the coolness factor.

^It’s all about bragging rights, awards, and pushing the envelope, not necessarily winning or practicality.

What is it about inefficient design and operation that either pushes the envelope or deserves to be awarded?

I was talking with a team from Hawaii (368?) that made an autoshifting team 33 style 4 speed and they said as soon as it hit 2nd it went right to 3rd almost immediately after. So even 4 speeds is excessive for a traditional FIRST game.

actually a CVT is hardly ineficient all it means is nobody has created an efficient one. i once saw a race car with one… VERY COOL the car lost absolutly no RPM when shifting gears and you didnt have to wory about “shocking” your transmission when performing high speed gear shifts. the main benifit with this pertaining to FIRST is you can actually make them VERY simple (although with a few costom done parts) and you have a more complete control over a direct drive torque to speed ratio. i also looked at torque converters but they were too much of a hasle.

So someones read the hows things work on CV T’s . While CV T’s can be and are if well made more efficient then regular transmissions in order to make a efficient one many calculations are needed. Refer to Paul’s white paper. Keep in mind as well that the design you have shown requires three motors or two servos and a motor if you wanted to use servos for the rotation of the inner disc’s a lot to invest in one gearbox at least in my opinion. All I am saying is this is no cakewalk to produce something like this that is efficient.

I was interested so I added my comments after some research,
-Drew
team 1251
Tech tigers

Our team has had good success by following the KISS principle. We would consider a CVT a violation of the KISS principle.

The new Nissna Murano has one and the new Audi A4 have CVT’s too. I <3 CVT’s. Me and my friends were talking about the very low RPM running where as some engines get most of their power out at higher rpms. We were thinking that they could possibly move the cones on command of the driver so that the driver could control the rpm level.

We would love a CVT. We just want one that isn’t entirely friction based, but still they are so awesome. Something that is dependable.

the problem with most current CVTs is that they’re friction based, which causes losses in efficiency, and can limit the amount of power that you can put through the transmission before it slips. thats not saying that a friction based CVT is the only type of CVT; search the internet and you’re bound to turn up some mechanical CVT. in fact, if you do some searching through the chiefdelphi gallery, i’m pretty confident that you’d find one (or more).

basically i would need a company that can make the gear to sponcer us to get these into first (i need four). even if they are given to us and counted as a tax right (i love being a nonprofit org) off it would still need to go into the budget and one of them could take up most of the budget. each gear would have to be cut from a block of aluminum. i have a way I’m sure i can run it off one servo. and what would be the point of giving up i would only be guaranteed failure. i like this design because gears are more reliable than chain. (please no personal stories it my opinion and i in no way claim it as a fact) besides a couple years back if you had said half the things we do now to anyone they would say its impractical or not even possible and yet we manage to get this far on stubbornness (and i say stubbornness because genius can only give man wings not teach him to fly). no smart retorts on that last line!

actually i planned to gear it not run it based on friction

The problem with gears in CVTs is that with a varying diameter comes either one of two things:

A varying pitch, or
A varying number of teeth

And I don’t see how either circumstances could properly mesh with another gear. That’s why I’ve never seen an actual cone shaped gear and that is why most CVTs are friction based.

The only thing I’ve seen that is close to being a mechanical CVT was the ThunderChickens CCT which used a motor to drive the ring gear of the planetary as well as the sun.

How would that work. I am very curious. I would love to know.
edit: in reference to mechanicalbrain’s gearing and not friction

well if you gear it (and i admit the gears would need i bit of slop) correctly the sides will still be able to slide though i couldnt use servos without gearing them. actualy the im positive that if you gear all four of the gears (agian with a little slop) they would rotate perfectly and still slide. to test this im making some oversized wood ones. the concept isnt that impossible and i was believe imployed in some cars or at least that is what im told. to clarify the picture is misleading in that in not tapering that much. admitedly theres some math. it also helps to have fine teeth on the movine gears.

actually now that i think about it both the moving and not moving parts can have the similar change of gears teeth spacing and as you rotate a different spacing is used because a different area touches. its difficult but entirely possible. the trick is to have the least possible area of contact.

or inversely geared teeth would definiteley work but im not sure how it would change the gear ratio.