Okay, now that I see that M. intended front and back tensioners, I am ready to pull the rabbit out of the hat...
How about a GENEVA MECHANISM as the shifting mechanism?
Shown above is a 6 lobe version, we would use a 3 lobe variety.
IT'S BRILLIANT!
Hear me out...
The indexed lobe would be the carrier. The shaft with the pin would be driven by a motor, with a switch to stop it at a particular location, specifically, when the drive pin is as far way from the indexed lobe as possible. Looking at the picture above, the pin would be at 3 O'clock.
Here is where the rabbit comes out of the hat: We use the rotation of the pin to disengage and re-engage the chain! Brilliant! Just Brilliant!
Allow me to explain...
We have the "lost motion" of the pin from 3:00 to 11:00 (if the shaft with the pin is rotating CCW) or the motion from 3:00 to 7:00 (if the shaft with the pin is rotating CW) to lift the chain in order to disengage it from the drive sprockets entirely.
Of course this will pull the chain tensioners tight, but that is the reason M. put them there is it not, so that I could do this engineering magic trick? ;-)
After we disengage the chain, we shift (from 7:00 to 11:00 or vice versa), then we lower the chain and engage the chain (from 11:00 to 3:00 CW or from 7:00 to 3:00).
At 3 O'clock we stop the shift motors.
TA DA!
As you can see, I LOVE THIS IDEA. Everything comes together, the shifter is lock when we are not shifting, the chain doesn't bind, we could have one motor drive both sides of the robot (4 Geneva mechanisms total, 2 per transmission), the shifter moves the carriers when there is no load on the sprockets, etc. The list of good things about the design goes on and on...
It is very clever, if I may say so. I think this would be really cool to see in action.
Now, having fallen in love with my idea, I have to tell you all,
I really hope that nobody actually goes down this path for their FIRST robot.
Why? Because I don't feel that a 3 shifter is worth the bother in terms of time and weight and engineering effort.
Part of my reason for writing this message is as a cautionary tale to everyone building FIRST robots in 6 weeks.
Fall in love with your designs. Loving your ideas with all your heart is an important part of the design process. Love your clever designs and be an advocate for them.
But then... ...use your brains to decide what is needed and do what your head tells you, not what your heart wants.
6 weeks goes by SO fast.
Good luck to us all.
Joe J.