CVT, which teams are planning on using them for next year?

[team222badbrad]

Originally Posted by Kevin Sevcik

Wouldn't that aluminum wheel + rubber wheel have a fairly large amount of friction losses? I mean, unless the rubber wheel has a very small contact point, then it's going to slip along pretty much all of its surface cause the aluminum disk will be travelling faster at the outer edge vs the inner edge. As I understand it, that's why other roller designs use hard rubber designed to contact at a point.

And in answer to the thread, 57 doesn't look to be using a CVT in the foreseeable future.

The gum rubber drive wheels as about 3'' in diameter and was about 1/2'' thick.

We probably could have Knurled the aluminum disk, like Sanddrag said, but that would probably eat up the rubber quickly.

Like you said, I think the design would have probably worked better with a drive wheel that had a curved contact point rather than one with a flat contact point 1/2'' wide on the aluminum disk.

We basically scrapped the prototype because we knew that it would be very difficult to get enough power the the wheels, without the drive wheels slipping.

I am sure we could still make it work.

Thinking back, we could probably have added a lot pressure to the drive wheels, thus we would have needed to add thrust bearings on the wheels.

We also probably could have made a better method of moving the drive wheels in and out on the disk.

[santosh]

Originally Posted by Veselin Kolev

Using belts is definately a good idea. You can put tons of tension through a v-belt and get brilliant traction on pulleys. So.. just figure out how to get a pulley that changes size. Not so hard anymore, eh? I'm sure the brilliant young minds of FIRST can figure out how to do it.

Well there are these cone pulleys (STOP WHAT YOU ARE THINKING NOW). These pulelys arent being used as most cone pulley desigins put them to use.
These cone pulelys ride next to eachother as in the flattned tip of the cones almost touch eachother. There is also a V Belt riding in between these cones. The cones have grooves in them that do expand, but so does the V-belt.
There is another pulley at the other end that the belt also wraps around. The single pulley is the one that has the drive motor running it.

When the cones are pushed together, the belt has to ride up the cones becasue it has nowhere to go. as the cones are pushed together, the overall diameter of the touching point of the 2 cones grows and your gear ratio changes.

And that Nissan Murano with the powerplant under its hood, the 3.5 liter engine, with a CVT will be awesome.

1002 the CircuitRunners would love a CVT on next years bot, but that is if we can find one that can handle a lot of torque an one that will be reliable. We however will not build one if the game next year doens't call for one. Purpose over pimpness is a policy that we are trying to adhere to.

[Jeff K.]

Santosh, are you talking about something like this?

Taken from HowStuffWorks.


If it is, how could you change the diameter of the pulleys and still have a reliable drive train?

[Holtzman]

Originally Posted by Stonefan5271138

Santosh, are you talking about something like this?

Taken from HowStuffWorks.


If it is, how could you change the diameter of the pulleys and still have a reliable drive train?

These are called variable diameter pulleys. They have been used for many years for variable speed drives on things like drill presses and mills. Team 1114 successfully adapted this concept for use on their robot cart in the fall of 2003. You can buy variable diameter pulleys from a company called hi-lo . The website is www.hi-lo.com

You might also want to look into a design by Jay Tenbrink from 494 a couple of years ago. They successfully designed and build a set of variable diameter pulleys that used a fly weight system to adjust the pulley diameter automatically.

[Jeff K.]

I have spent some time looking over the pictures of their CVT on their website repeatedly, and I have some difficulty in understanding just how the diameter of the pulleys is changed by the weights. From looking at the pictures, I see that the weights fold out when the shaft is spinning faster, but I see that the pulleys remain at the same diameter. Is this a friction based CVT, because I see that the man's hand in the background is holding onto the shaft tighter in one of the pictures than in the other.

Can someone on team 494 possibly give more information?

[sanddrag]

I too have always wonered exactly how that one worked, or if there was a video of it running.

Anyway, I wonder if it would be better do to a CVT on a smaller scale like right off the motor so it is in the first stage of reduction. That way, it wouldn't have to carry as much torque. But, it might be too fast.

Also, the key is not just to having a CVT, but having a CVT that can react to present conditions. It has to be fully automated either mechanically or electronically.

[mechanicalbrain]

i like the idea of a small one right off the motor since the size really doesnt matter and you can make it as small as possible. im curious though why it has to be automated? why not use it like you would a normal transmission and just hace more control over torque and speed. however if you want to have autonomous control i would recomend some type of backgroung program that uses info from the accelerometers. for example when it marks that you are incountering esistance (moving slower than you shoud based on speed controllers) it ups the torque. you would need something to govern how much it ups the torque based on what and when to lower the torque (maybe when your speed suddenly increases after overcoming an obstacle).

[sanddrag]

I think you'd want current draw to abe a big factor in the auto "shifting" program. Also, it would have to be automated because I don't think a driver is skilled enough to control an infinitely variable transmission to the best of its potential. There is too much other stuff going on to think about exactly what torque/speed setting you need. Some drivers don't even make good use of a two speed.

[Jeff K.]

Well...maybe have it automated by using shaft encoders, but then if you got into a pushing match, and there was a problem and the CVT was still in high gear, maybe using the buttons on the controller, the driver could manually change the pulley diameters to low gear by pressing the buttons to slowly change the diameters. Or instead...maybe preset diameters that when the driver presses the left joystick button, it automatically goes to lowest gear for more torque, and then the right joystick button, middle gear for getting across the field more quickly.

For automating, you could probably have it that when it starts from 0 fps, the pulleys could automatically be at the smallest pulley to biggest pulley for most torque, and then gradually increase. Or something like that..

[mechanicalbrain]

i think it would be easiest to have the driver control it. instead of specific ratios a driver could push a button for more torque or one for more speed. i think my biggest fear would be the gears just slipping and a sliding.

[santosh]

^agreed. You get most of your power at a certain RPM. And yes if you do have a pushing match, you want to keep a very high RPM on a high torque setting.

[mechanicalbrain]

i wonder if you use a simpler design if you could make a mini disk CVT. as long as it stands up to the stress i think you can make them miniscule.

[John Wanninger]

Originally Posted by mechanicalbrain

i wonder if you use a simpler design if you could make a mini disk CVT. as long as it stands up to the stress i think you can make them miniscule.

Remember that torque x speed=transmitted power. On a cvt disk,( or even with gears or sprockets) the smaller the diameter, the greater the force required for a given torque. In other words if you make it small, it needs to spin really fast, or have really high contact force (or both). Even if the materials could withstand the contact forces, heat buildup due to inefficiency of non-point contact could be an issue and would be more troublesome as you go smaller as there is less surface area to dissipate the heat.

[mechanicalbrain]

ooh good point. darn physics.
actually my second fear is the heat build up. any suggestions on how to minimize it?

[Veselin Kolev]

The V-belt variable pulley size drive was exactly what I was hinting at. How it works.. simply, is that you take a standard v-belt pulley, cut it in half, and ride a belt over it. If you bring the half pulleys together, the belt rides higher on the pulleys. If you bring them apart, the belt rides lower. The problem is not how to get traction from the pulleys, thats the easy part. The hard part is how to reliably controll the distance between the pulleys. If that was figured out, CVT would be a joke for me. But untill then, I'm cadding new ideas.