CAKE III Preview

[mlz3000]

The glue is drying, and a paint job is in the planning, but here's what I have so far for my first piston valve. It's a barrel sealer.

Much thanks to Hubb, I used his piston 2" design.

The piston housing is a 2" threaded tee joint. The chamber in portion of the tee was already a 2" female threaded port, and to each of the other two ports I added a 2" female threaded adapter, allowing for interchangeable barrels and a fully serviceable piston.

The seat is a 1.5" section of PVC. I filed down a 2" to 1.5" bushing so that the 1.5" pipe could go all the way through. One end of this 1.5" pipe is what the piston seals against, and the other end goes into a second 2" to 1.5" bushing which is inside the 2" female adapter which is where the barrels are attached.

The bumper is two pieces of a tennis ball, with a very weak spring between them.

The pilot valve is the modified 1" sprinkler as seen on CAKE I and CAKE II





Any comments/tips are welcome, but I was particularly wondering if anyone could help me figure out how far back the piston should move? I've seen this equation, but I didn't quite understand what values to use. Currently the spring (barely) pushes the piston into the sealed position, though I might cut the spring so that the piston does not touch the bumper until it is nearly to the minimum opening distance.

Also, I'm not sure how to find the Flow Coef for ggdt, if anyone could tell me, that would be much appreciated.

[Gun Freak]

Looks like my first 2" piston valve. I also used Hubb's piston. How did you cut grooves on the piston for the o-ring? It is difficult to make airtight grooves... Anyways you will be very proud of this, it has a lot of power. Good job.

The sealing face is a 1.5" section of PVC.

That would be the seat. The sealing face is on the piston. That is where you put the rubber washer.

[mlz3000]

Ah, thank you. Edited.

I cut the groove VERY SLOWLY with a handheld bow saw intended for cutting metal. The cutting edge can best be described as high grit sandpaper made of metal. It took a good bit of tweaking before the o-ring fit properly, and it's not a perfect seal, but it's the best I can get with the tools at hand.

A family friend has a woodworking lathe, so I might make a wooden piston at some point in the future.

[Selador]

I used pretty much the same design, for my 1-1/4" piston valve.

The piston itself was an end cap, and a bit of pipe and a connector. All of which fit tightly inside the piston chamber.

I described to you, the way mine operates, in the other thread. But I'll explain a couple other things, here.

First, the end cap is rounded. So the "nose" of my piston is not flat. The piston itself, is bullet shaped.

The seal itself is supported by the washers, just as I explained in the other thread.

This setup gives the chamber pressure more of an advantage, once the pilot valve is opened.. Than just a flat-faced piston.

As I said before, the barrel seal instantly becomes a piston chamber seal, when the piston is blow back. Eliminating any loss of chamber pressure, through the piston bore. And eliminating bounceback of the piston.

Next, the bumper I built was a piece of kneepad type foam, sandwiched between two more pieces of the same red rubber that the barrel seal is made of.

Your bumper may work. But I am betting it slows down the evacuation of the piston bore. Maybe cut a 3/4" hole in the center of the two pieces of tennis ball ?

I'll try to attach a .gif file of the piston operating. (Manually, of course. I don't have a camera I can stick into the gun to watch the valve as it operates pnuematically. LOL)

Your piston travel seems to be fine.

More than many will tell you is needed. But I don't see the waste that some do, in letting it travel farther. Mine has nearly the same amount of travel and works beautifully.



Well, here's hoping the image works...

[mlz3000]

Your bumper may work. But I am betting it slows down the evacuation of the piston bore. Maybe cut a 3/4" hole in the center of the two pieces of tennis ball ?

As you might be able to see in the picture, I did cut a bit of a hole in each piece of tennis ball, but I think I'll make the hole larger.

The spring in the picture is very weak, so I don't think it will reduce performance enough for me to not use it (with a bike pump, it's nice to have the piston already against the seat when I start pumping). I do have another spring of the same size which is considerably more stiff, and I'm thinking I'll cut it and twist it together with the shown spring so that it doesn't slow down the piston until it's open, at which point it does slow down the piston, and acts as a bit more of a firm bumper than I have right now.

First, the end cap is rounded. So the "nose" of my piston is not flat. The piston itself, is bullet shaped.

That looks like it works quite well, I'll probably end up building one. (Another advantage to serviceable pistons: you can try new piston designs without having to build a new housing )

[Selador]

I do have another spring of the same size which is considerably more stiff, and I'm thinking I'll cut it and twist it together with the shown spring so that it doesn't slow down the piston until it's open, at which point it does slow down the piston, and acts as a bit more of a firm bumper than I have right now.

That sounds like a pretty good idea.

If you are going to stick with the tennis ball bumper, maybe you should use marine GOOP to glue the springs to them.

First, the end cap is rounded. So the "nose" of my piston is not flat. The piston itself, is bullet shaped.

That looks like it works quite well, I'll probably end up building one. (Another advantage to serviceable pistons: you can try new piston designs without having to build a new housing )

Note that what would work even better would be a flat faced piston, with a shaft, (bolt), coming out of it, on which the seal is mounted.

I beleive several others here have done that.

[Ragnarok]

This setup gives the chamber pressure more of an advantage, once the pilot valve is opened.. Than just a flat-faced piston.

You'll have to explain that one to me, but it sounds very like the typical "curved/multiple surface fallacy" we get around here regularly.

When pressure is concerned, only the cross sectional area is relevant, not the total surface area.
The simplest proof of this is that if the inverse were true, there would be a big business of increasing the surface area of the back of bullets (be it with grooves, curved surfaces, etc) in order to get MOAR POWAH.

[Selador]

You'll have to explain that one to me, but it sounds very like the typical "curved/multiple surface fallacy" we get around here regularly.

Not at all.

Here is a pic to explain what I meant.

[mlz3000]

If you are going to stick with the tennis ball bumper, maybe you should use marine GOOP to glue the springs to them.

I currently have the spring through a pinhole in each tennis ball, and twisted in a little. If they end up falling out of that, I might use some GOOP on them.

This setup gives the chamber pressure more of an advantage, once the pilot valve is opened.. Than just a flat-faced piston.

Maybe he's referring to the fact that there is surface for the chamber pressure to act on before the piston unseats?

That seems to me like it would generate negligible performance increase at best...

[Gippeto]

You'll have to explain that one to me, but it sounds very like the typical "curved/multiple surface fallacy" we get around here regularly.

When pressure is concerned, only the cross sectional area is relevant, not the total surface area.

@ OX

First one won't work.

Second...What Rag said.

Third...is correct enough

[Selador]

First one won't work.

You're preaching to the choir.

Second...What Rag said.

Look again at the .gif I posted above. The second one IS my valve. And it works very well.

Third...is correct enough

Third was what I intended to make, in the beginning. Then I took a shortcut and used what I had easily at hand. LOL

It works very well with a sprinkler valve as pilot, so I didn't bother changing it.

[Technician1002]

OK, my 2 cents worth.

1 The seal on the piston not going to work. The little bolt holding on the seal simply will not prevent it from being blown out the barrel. You will need a washer in front to lift the seal from the barrel. This washer should cover most of the rubber face of the piston except the area touching the valve seat.

2 You will want to polish the valve seat. Some sandpaper on the piston face rotated a few times to wear the end of the 1.5 inch pipe nice and flat will go a long way to getting the piston to seal so you can add pressure with a bike pump.

3 The rate the piston flies open will mangle the spring on the first high pressure shot. I was unable to keep a spring in the Mouse Musket for this very reason. The speed launches one end of the spring through the other end resulting in a tangled mess of a spring. I dropped the spring.

4 If your o ring leaks, use a larger flow pilot valve such as a ball valve instead of a blow gun. Otherwise the chamber pressure will vanish out the pilot as you try to fire it.

5 When it works you will be impressed with the performance. Congratulations. Welcome to the larger piston club.

[mlz3000]

@ Tech

1. Ok, I changed out the bolt for a longer one so that I could fit a washer. (The largest washer I had on hand was a quarter that I drilled a hole through ) I'll pick up a bigger washer next time I'm at the hardware store.

2. Beat you to that one! I flattened it out with a file and then took some sandpaper to it before I glued the housing together. Looks like I'm learning.

3. Alrighty... I was a bit worried about that. I'll test it out, but I suspect that I'll have the same problem. What do you use as a bumper?

4. Did you read the first post? I'm using a modified 1" spinkler valve as the pilot valve.

5. I'm looking foreward to it.

[Technician1002]

Nice job. My bad I missed your pilot was a sprinkler valve.

I've had good luck using the dense foam knee pads used for gardening for bumpers. They do eventually break down and need to be replaced as they take a beating.

I hope your sanding got the end of the pipe perfectly flat. A slight wrinkle in the flat surface can create a huge leak that makes it hard to seal so it can be pumped with a hand pump. The new bolt may need the threads sealed where it goes through the rubber. This is often missed as air leaks out the threads and out the barrel.

Enjoy.

[mlz3000]

The new bolt may need the threads sealed where it goes through the rubber. This is often missed as air leaks out the threads and out the barrel.

I considered that, and I think I'm going to open up the piston and pour in a layer of craft glue to create the seal. It's hardly pressure rated stuff, but I figured a half inch to an inch in there would be enough to seal that little hole.

I'm I totally wrong on that?

EDIT: Oh duh, before I try that, I'm just going to try sealing it with a bunch of teflon tape...