Limiting the Cylinder stroke

[Darth Pbal]

Okay so the deal is that I've heard a lot about increasing the pushing power of a pneumatic cylinder by limiting how far it can pull back. I want to try that but I don't know how I can do it. Can someone tell me how I can get that done?

[jamie_1930]

Originally Posted by Darth Pbal

Okay so the deal is that I've heard a lot about increasing the pushing power of a pneumatic cylinder by limiting how far it can pull back. I want to try that but I don't know how I can do it. Can someone tell me how I can get that done?

I'm pretty sure you should be able to add in some physical stops for it. Or am I on the wrong track? What exactly are you trying to do?

[Darth Pbal]

I'm trying to take my Cylinders and increase their force that they can give. And I know you have to put something on the shaft or something so that when it retracts it can't come back all of the way and it then acts like another part where some air pressure can be stored. This shortens the stroke, but increases it's power. I know you have to limit it from retracting all the way but I can't think of anything off the top of my head that would be able to stay on for a long time while it's being powered.

[RyanB]

PVC pipe on the end has worked in the past for us when we need to limit the stroke.

[Matt H.]

From your posts I believe you are talking about preloading the cylinder.

To do this you need to hold the cylinder partially extended and then fill the cylinder.
In an optimal configuration the piston will be extended ~.3-.4* the length of cylinder.

Most teams hold the piston in place using some kind of latch. Team 842 uses a standard gate latch and U bolt as part of their configuration while other teams use quick releases designed for water sports etc.

On the physics:
This does not increase the maximum force provided by the cylinder. That will always be given by system pressure*area.

Preloading the cylinder increases the system's energy output by maintaining that force over a longer distance. (Work is force integrated over distance).

Here are some useful links:
Regarding safe kickers:
http://www.chiefdelphi.com/forums/sh...ad.php?t=80679

Regarding the math/physics:
http://www.chiefdelphi.com/forums/sh...ad.php?t=81145

An Example (with photos!):
http://www.chiefdelphi.com/forums/sh...ght=842+kicker

I hope this helps and would be more than happy to answer any additional questions you might have.

[Ether]

Originally Posted by Matt H.

Preloading the cylinder increases the system's energy output by maintaining that force over a longer distance.

For a given kicker design, preloading gives a higher force over the same distance, yielding more energy.

Consider two identical robots which both have mechanical latches to hold the kicker in the "armed" position, and which both have the piston in the 35% extended position when in the armed position.

The only difference between the two robots is that Robot A pre-charges the cylinder with 60psi, whereas Robot B does not.

The piston in Robot A and the piston in Robot B both travel through the same stroke when kicking.

Because of the pre-charge, the pressure in Robot A's cylinder will be greater than the pressure in Robot B's cylinder at the start of the kick (and perhaps even throughout the entire kick).

The greater pressure results in greater force, and the greater force acting over the same distance results in more energy in Robot A's kick.


Now remove the constraint that the two Robots are identical. Instead, Robot B's kicker design is modified to provide a longer stroke, by retracting the piston further.

Now it isn't so obvious any more which system will delivery more kicking energy. For a very interesting discussion of this situation, see vamfun's post#12 in the following thread:

http://www.chiefdelphi.com/forums/sh...ad.php?t=81145

His conclusion, in a nutshell, is that it is better to use the full stroke (and not pre-charge) the small-bore cylinders, whereas it is better to pre-charge (and not use the full stroke) for large-bore cylinders. The reason for this ultimately traces back to the 1/8" .32 valve restriction. Note that the conclusions in that post are based on using greater stroke for the non-charged scenarios (as in the "non-identical" kicker design scenario above). Note also that the conclusions in vamfun's post are based on a set of operating assumptions (especially load) that may be different in your kicker design.


~

[kramarczyk]

You must spread some Reputation around before giving it to Matt H. again.

<edit> Ether does a nice job of explaining the pressure effects over the length of the stroke. Matt's links pertain to the pre-charging required to get around the flow limits of the system that enable those higher pressures. Both pieces are required to maximize power out of the cylinder. Work is equal to force over distance (W=F*d), which is what Ether is maximizing. Power is work divided by time (P=W/t); Matt is trying to reduce the time the work is done in.