Maximum bore/stroke length for cylinders?

[Sperkowsky]

Originally Posted by cmrnpizzo14

Have you found anyone who sells these? I'm sure they have them but remember that it might be the heaviest single part of your robot and will definitely take away from your weight/pneumatic budgets.

We have one we accidentally bought 4 years ago. it's actually only around 11 pounds.

[Kyle Butzerin]

Here's one

McMaster-Carr, http://www.mcmaster.com/#6498k485/=10n6o8z

That's one way to go, we are thinking of almost the same concept, and are preparing to arm many tanks on our bot this year.

Expensive, but it could work, honestly we are looking at putting 2 on the robot stored at an angle. Therefore, roughly half the power per cylinder is needed, but twice the air... the bore wouldn't need to be a mile wide though, which is a plus.

[cmrnpizzo14]

Originally Posted by Sperkowsky

We have one we accidentally bought 4 years ago. it's actually only around 11 pounds.

Sweet Jesus...

[Colin935]

Originally Posted by Sperkowsky

We have one we accidentally bought 4 years ago. it's actually only around 11 pounds.

Those batteries should be wrapped in electrical tape or heat shrink... those look dangerous

[hrench]

We prototyped this and learned a couple things that I'm happy to pass along.

1. Bimba sells double-wall cylinders that go much longer than the standard 24" that the singles do. They weigh slightly more, but since it's lifting the robot, I would've considered putting the cylinder in a tube to protect it anyway. 6.7 lbs for a 36" we bought on ebay.
Here's a 28" for sale on ebay today.
http://www.ebay.com/itm/BIMBA-DWC-6028-2-DOUBLE-WALL-PNEUMATIC-CYLINDER-2-1-2-BORE-X-28-STROKE-/151469157440?hash=item234443ec40:g:X30AAOSwrx5UXm0 g

2. a 2-inch cylinder on pull (subtract out the shaft area) can lift about 170lbs at 60 psi. Don't forget that your 120 lb robot has a battery and bumpers. Also, the friction of the bumpers dragging against the tower will be significant, based on your bumper design and the placement of the lift-point on the robot. We lifted a very heavy older robot (150lbs?), but we had to go to 65 psi to actually climb the wall with a 2-inch cylinder. Not doable there.

3. Yes, we had two black tanks and they emptied in the first six inches. But the overall lift powered by a Thomas compressor still finished in the 20 seconds. More like 15 seconds.

[JesseK]

The relevant equation here is the ideal gas law, or P1*V1 = P2*V2, supposing that temperature is the same and no gas leaks.

60 PSI * cylinder volume = 120 PSI * stored volume

If you presume storage pressure to be around 100 PSI near the end of the match, you will want to calculate and test using 100 PSI for storage rather than 120 PSI. This is very likely if you use more than a couple of pneumatic-driven mechanisms.

Separate from storage, there is hysteresis when determining needed cylinder force. It is something I know exists but I don't know how to calculate. Effectively, it takes more force to get the cylinders moving and to continue moving than it does to keep the cylinders held in place. In my experience, it takes about 20% extra force to get the cylinder moving (e.g. 180lbs of force for a 150 lb robot) and 5% extra to keep it moving (e.g. 158 lbs of force for a 150-lb robot). You can play with hysteresis by adjusting your working pressure after the cylinder stops/starts moving.

[Mike Schreiber]

Originally Posted by JesseK

The relevant equation here is the ideal gas law, or P1*V1 = P2*V2, supposing that temperature is the same and no gas leaks.

60 PSI * cylinder volume = 120 PSI * stored volume

This is assuming the system expands isothermally since PV = nRT which is likely not the case. The expansion will lower the temperature of the gas. I would likely model this process as polytropic, but this is probably over analyzing it. Add an extra air tank or two as a buffer.