Hello!
I have not previously used pneumatics, and if you could help me out that would be great. We are considering using pneumatics directly on our 6-bar lift as a locking mechanism for hanging. If we mount it directly to the arm, will the cylinder slide in and out freely until we activate the pneumatics?
Thanks!
Daniel
No, unless you bring the pressure waaaay down (using the regulator). Even then, it may not be possible. You’ll be losing lots of air pressure by doing that because you’re fighting a non-perfect system trying to compensate. Now that I think about it, you might be able to get it to work with single acting pistons - the only thing keeping them from moving around when not activated is a weak vacuum and springs, but almost certainly not with dual acting.
I have some ideas for a locking mechanism that doesn’t attach to the lift tower (or the lift itself). I haven’t done any investigation, so I won’t share just yet. I’m saying this simply to encourage you to think outside of the box… or rather, think outside the lift.
So I see you guys finally are beginning to see the potential value of hanging? 
Its better to use a piston that is attached to the chassis and protrudes outwards onto the intake tray so that the arm cannot go up(and the robot stays up).
When no air is supplied to a piston it may freely move, but only if the solenoid is not feeding air.
How do we have no air going to the cylinder during the match, but supply air when we want to hang at the end?
Use the solenoid
Can you please expound on this? As I said before, I have never used pneumatics before.
The solenoid is what controls air flow. You put that in your tubing system, plug it into your cortex, and you can use a digital out port to set the solenoid to 0 (closed) or 1 (open).
Check out this page: http://botsnstuff.com/wiki/Pneumatic
You can also Google the official VEX pneumatics guides.
So if the solenoid is closed, will the piston move freely?
If a solenoid is set to 0, the pistons retract. If it is set to 1, the pistons extend, simple as that.
This might help you: http://content.vexrobotics.com/docs/inventors-guide/VEX_PneumaticsKit-INST-083111.pdf
When the robot is disabled, will the piston stay retracted or expanded?
I do believe that pistons should stay in the position they last were.
I completely disagree, I would have thought a double acting piston would work better than a single acting one. Just use only one of the connectors on the piston. Just because it’s a ‘dual acting piston’ doesn’t mean it has to use pressurised air in both directions.
For a latch on an arm, I would have thought you would only need pressurised air flowing through one connector, maybe some weak elastic to help the piston move in the other direction if you wanted to ‘un-latch’, i.e. in autonomous. What you are after is reservoir → solenoid, one output → piston, the other output on the solenoid blocked off (if using a double acting solenoid). That will let the piston slide freely when it is not being activated. 
Make sure the un-used connector on the piston is left open, probably take the connector off to help air flow if you want.
Or, a little more complicated now, if you wanted to control air in both directions and be able to let the piston slide freely, you could go reservoir → solenoid (with one end blocked off) → another solenoid → piston. However, I would predict that the arm wouldn’t move as easily then as the air inside the piston has to flow all the way back to the first solenoid to escape, so if you were for some reason wanting to be able to control a double acting piston and be able to let the piston slide freely, I would recommend this instead: reservoir → T junction, splitting to 2 solenoids with one end blocked off and the other going to a connector on the piston (one solenoid per connector on the piston).
But based off the original post, I think I have gone a little beyond what was needed, I think you are just controlling the piston through one connector so yes that will work absolutely fine 
Hopefully the extra stuff I mentioned will be of help to someone, and if someone has a better suggestion please tell me, as I would be interested
And yes, at the end of a match, the pistons hold their position.
~George
Thanks for the tips! We will try it out and let you know how it goes.
Another idea is to use one of the strokes as counter tension until you want the lift to stay in the lowered position.