Variable Speed Gearboxes?

[carpedav000]

My grandpa has been exploring this concept since the '70s and now he's got me thinking about it, so I was wondering if anyone else has been exploring this idea. So essentially you have two gears; one long, stationary gear and one thinner gear that is free to travel along the length of the other gear. As the thinner gear travels along the length of the stationary gear, the gear ratio changes, giving you a massive amount of different gear ratios to choose from between the starting ratio and the end ratio (example: if the gear ratio at one end is 1:1 and the gear ratio at the other is 18:1, you can have virtually any gear ratio you want that falls between 1:1 and 18:1). I could see this idea being potentially useful in FRC and I was wondering if anyone else had thought about it. I do have a few ideas of my own (including one that will work for low-torque applications), but I haven't seen anyone else working on this concept.