During the 2014 off season FRC Team 2363 Triple Helix developed a chain-in-tube WCD drivetrain. A powered drop down omni was also added to help avoid pins.
The CAD files can be found here.
I actually really like this. All the benefits of a 6wd, but it can switch to a powered 2+2 if needed.
The actuation can be really cleaned up if you switch to a pancake cylinder and just push the module down (there is some sliding contact). 3928 did this on their 2013 butterfly, and we ripped it off for our version.
Then just use some spring force to pull it up.
I’d also like to give it a butterfly drive try and put the omnis on both sides. Then you have the ability to drop either side, or both.
Do you have pictures of the method with the pancake cylinders you are talking about?
I can’t take credit for this either, one of our other members cadded this up.
Cylinder just pushes straight down on the top standoff.
More pics on the site if you browse.
Looks pretty cool. What is the approximate weight of the what is pictured?
Also, couldn’t you just replace the set of wheels opposite of the drop down wheels (aka, the back wheels) with omni wheels and have a butterfly drive pretty easily from what you have?
As long as the robot was always sitting on the two sets of colson wheels yes, but in a pushing match you could get rocked back onto the omnis and lose some of your traction.
Here’s a thought - what’s stopping you from having the omni wheel module straddle the drive tube? You could use some delrin blocks to transmit forces applied to the module through the whole frame, which keeps the loads more manageable. As long as you have clearance for the shaft to pivot within your drive tube (doable with chain-in-tube and the right geometry) you might like a setup like that better.