Re: Maximum bore/stroke length for cylinders?
 

You could use smaller bore cylinders if you added powerful constant force springs (free from Vulcan Spring).

For example, if you pre-loaded your lift with 40 or 50 lbs of force then you have to push the springs up when you're extending, but only have to pull 70-80 lbs when retracting.

Of course you want to design in a fudge factor for safety.

Re: Maximum bore/stroke length for cylinders?
 

Our Team tested this cylinder with ~120 in^3 of storage capacity lowering and lifting 135lbs. Here's a video link of the test:

https://youtu.be/M5YBYYYhEY0

We used the pneumatic system from last years robot to get an idea of the air flow and the load on the compressor.

Re: Maximum bore/stroke length for cylinders?
 

1518's video seems to confirm this

Re: Maximum bore/stroke length for cylinders?
 

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

Re: Maximum bore/stroke length for cylinders?
 

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.

Re: Maximum bore/stroke length for cylinders?
 

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.

Re: Maximum bore/stroke length for cylinders?
 

If you use separate solenoid valves for each side, or use a 3-state solenoid valve with blocked ports in the middle state, and had some limit switches, you could actuate the "return" stroke pneumatically using much less air (probably 75-80% less) than the power stroke.

The time for the power stroke is the biggest problem. You'd probably be better off with two 18" stroke cylinders mounted to each other, pointing in opposite directions and pressurizing them through different solenoid valves. The returns could be through one solenoid valve.

Not quite, you need to add atmospheric pressure to both sides. The high pressure tank only holds about 80% more air than the low.

Re: Maximum bore/stroke length for cylinders?
 

I expect you could accomplish the same thing by putting a regulator set to a lower pressure in the line between the solenoid valve and the cylinder extension (we lift with the 'return'). Legal? Don't know.

i don't understand why you think two cylinders would be better than one? The lift area (piston in2) will still be determined by the weight of the robot and the total volume will just be the lift area times the lift distance.

Unless you mean you extend with a small diameter cylinder while the 2" is open to free atm, then open the little one to atm and lift with the big one.

And all of those people that are worried that the valves and the fittings won't be big enough to be fast --pretty unlikely that they'll even be a factor. You'll run out of air and have to wait for the compressor.

Re: Maximum bore/stroke length for cylinders?
 

Yes, the force will be the same.
I'm concerned that they might not be able to get enough air through one of those tiny 1/8" solenoid fittings. Since you cannot legally plumb the output of two solenoid valves together, you need two cylinders to each do half the work.

Actually, thinking it through a bit farther, two 1.5" cylinders with a 36 inch stroke would be even better. You could locate these to the sides of the robot and it would stabilize the rotation of the lift as well.

Re: Maximum bore/stroke length for cylinders?
 

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.