Variable Speed Gearboxes?

[IKE]

I have a big passion for CVTs. I did a preliminary (concept design) of one as a Senior Project in college. It was similar to many of the cone style CVTs that are out there. A great reference for these is:

Traction Drives, Selection and Application:

Most run into a scaling and materials issue that causes the user to either need some really impressive material, incredibly high rotational speeds, or power tends to be limited around 10 kW. The belt drives have found a nice way to get to around the 100 Kw levels.

Within the family of CVTs, there is a specific type called IVTs (infinitely variable transmissions). An IVT has its effective gear ratio go to infinity as described as input/output as the output speed can go to "0". In theory, torque would approach infinity, but at the same time, efficiency goes to "0" as well, so the system just becomes a heat generator which can often blow itself up. The most common IVTs I know of are typically using hydraulic (some times called hydrostatic) transmissions.

One of the neatest hydraulic IVTs out there is the Dana Rexroth HVT 3R.
http://danarexroth.com/function.html
It uses planetary sets as a combining function for three different ranges of CVT, with a seamless transition.

The concept you are talking about of a "geared CVT" has trouble because gears work in integers which means you cannot have a continuous tooth changing geometry and count.

Within the world of "Geared" CVTs, there are a couple that come close. Ker-Train a company out of Canada does a lot of cool Transmission research. They have a few really neat transmissions that use actual gears.

The first, they refer to as a "binary" transmission that has fixed gear sets with fixed ratios, and clutches, but the layout doubles the amount of ratios per gearset. Thus 1 set has 2 ratios, 2 sets have 4 output ratios, 3 have 8 output ratios, 4 have 16 ratios, and 5 sets have 32 ratios. While they are 32 unique ratios, you are starting to get really close to a continuously variable transmission.

The second, they often refer to as "variator" transmission:
http://www.google.com/patents/US6849023
This is one of the most interesting gearboxes I have ever seen. It uses "non-circular" gears to create a speed waveform when given a smooth input. That waveform is then combined with a second waveform via a 3 element differential (think planetary set, or Kertrain has a system called Co-planar loop). There is another element which changes the phases between the two waveform generators. By shifting the waves, the net difference varies with a smooth output. This is really the only true "Geared CVT" I have ever seen.

There are also a type of CVTs called Ratchet drives which are pretty neat.

The most likely thought for an FRC application would probably be something akin to the PRIUS transmission.

As others have stated though, It is really overkill for FRC. Electric motors do a pretty good job of acting like a CVT as it is, so there is a lot less advantage.
In theory you could gain more time with the motor operating at or near peak power, but in practice for what we do, the time commitment to controls and fabrication would outweigh any advantages.

CVTs do have a place though. They are sometimes used in variable speed drives for machinist tools (like band saws and lathes).

[Electronica1]

Someone more or less posted something like this last year.