keeping double action pistons in the middle

We have a bit of a dilemma. We have a double acting piston that in order to be in size can’t be all the way in or all the way out. We would have to have something that blocks the motion of the piston and then when it move away from what is blocking the piston the block is released (in the match we have to be able to go all the way in and all the way out). Our mentor says that this might damage the piston by breaking a seal, is the a danger? If it is a danger we are going to look into using a spring return piston but we don’t know how powerful those are and don’t want to buy one just to find out. So is the spring reasonably powerful?

Could you post a picture of this mechanism?

Based on what little you’ve said, I don’t see why any seals would necessarily be broken, but maybe there is something else going on. It seems to me that so long as you don’t pressurize one side or the other of the double piston and so long as there are no elastics involved or weights or other forces, the piston will remain wherever it is until it gets pressure. Of course, you would need to be sure the piston can’t fire until you’re ready to un-stow it.

Is the “something that blocks” going to drag across the end of the piston and possibly bend the piston rod as something else moves? If that’s the case, then perhaps a seal could be damaged. We would need more description or photos to know.

So how do I avoid pressurizing one side or the other without using the main cut off valve or not pumping the pneumatic up? I though the solenoids could either send air to one side or the other. If there is a way to solenoid to allow no air through then how?

By the way our mentor is ignorant of pneumatics, don’t worry he’s my dad and he told me to say that.

here are pictures.

Piston all the way in

Piston all the way out

Piston at the starting position

We know that our robot is a mess, we don’t compete or even practice with it in this state.

Would it be possible to use two single acting solenoids with the dual acting cylinder. If so, you could power in, out, passive - no action, and pushing from both sides to hopefully put it in the middle. This is more expensive and would take a lot of programming and testing to make it work well, but it might work.

Interesting design though, I look forward to a reveal, or seeing this at worlds!

I like that idea. We have plans to use the other piston that came with the kit for worlds so if we do that we would have to buy a solenoid but it would be pretty easy to program. The piston only starts in the middle, so the solenoid that allows the the piston to start in the middle would only have to activate once at the beginning of autonomous.

One possible solution might be to have something like a bar that hooks onto the frame of your turntable mechanism. The bar is held in place by the tension of the piston trying to push the turntable all the way around. When you are ready to deploy, you command the piston to retract. When the turntable turns around away from the hook, the tension on the hook is released so the hook falls away. Next, when you command the piston to extend all the way, the hook is no longer there, so the turntable can turn to its full extent. From then on, the turntable is free to turn back and forth to its full extent in either direction.

Just a thought.

We used to have a mechanism that needed the piston to start in the middle, so we used 2 single acting solenoids, as Inspector Gadget suggested. You can also place a single acting solenoid before the double acting solenoid, to stop air from getting to it.

This is pretty much what we had inn mind, we were just wondering what danger there is to break a seal in the piston.

Ok, I had slightly misunderstood what he said. I though what he said was that we should have a solenoid before the double acting piston to cut off air flow. Either way we would have to buy another solenoid if we want to use the 2nd piston that came with the kit since the kit only comes with 2 solenoids. Not to mention another solenoid driver cable. The cost to buy the 2 would be $58.49.

I’m no pneumatic expert, but the only way I can think that a piston might break a seal is if it is over pressurized or you put way too much lateral (sideways) load that deflects the piston rod. So long as the forces are kept along the same line as the piston rod, I can’t imagine how the seal could be damaged.

you can just put spacers on the shaft of metal that is pushed out at the end of the piston so that the contracted position is not all the way in, which would reduce the length of the stroke.

You are misunderstanding the question. He needs both the normal settings for during the match but would like to start the match in the middle of the piston stroke for spec.

To the OP. Each solenoid has 2 options so if you want 3 options you will need another solenoid. There are a handful of ways to do this as state above but inherently you need more than 1 solenoid.

An extra idea to throw around might be this type of thing

We are probably going to go with FullMetalMentor’s idea because it is simple, and we don’t need another solenoid. We only ever need to be in the middle at the start of the match to stay within size.

PS: I subscribed to your youtube channel :wink:

I see, my bad.