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Best homemade alternative to SV ?

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Best homemade alternative to SV ?

Unread postAuthor: Selador » Thu Nov 18, 2010 3:18 pm

Home made 1/2" piston valve.

Works great, when using a blowgun piloted sprinkler valve.

Doesn't work at all with just a blowgun. (No matter how small the pilot chamber volume is. And no matter that the blowgun has been bored out, and modded. Etc.) (I'll post some pics later. I have to go to town today. While there, I'll get batteries for my camera. LOL)

I am trying to work out a cheap homemade SMALL dump valve of some sort, that will clear the pilot chamber fast enough to actuate this piston valve.

I don't want to use the sprinkler valve. I want to use as few and as cheap parts as possible.

Think... K.I.S.S.

So... I want to make a homemade pilot valve behind the home made piston valve...

Any ideas ?
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Unread postAuthor: Technician1002 » Thu Nov 18, 2010 3:22 pm

Small spool valve similar to a QDV, but push button triggered maybe?

Large enough the spool valve works well as a main valve. See some of my cannons for examples.
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Unread postAuthor: saefroch » Thu Nov 18, 2010 4:01 pm

If your blowgun alone can't pilot the valve I'd look into making your equalizer hole smaller first so that it can be piloted by a blowgun.

How tightly is your piston fitting to the rest of the housing it sits in when it's in the forward and sealed position?

BTB's pushbutton pilot valve like a QDV, but it's springloaded to close itself.
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Unread postAuthor: Gippeto » Thu Nov 18, 2010 5:20 pm

Something's WAAAAYY out of whack if you cannot pilot a 1/2" piston valve with just the blowgun. :shock:

Clear pics will be to your advantage.
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Unread postAuthor: Technician1002 » Thu Nov 18, 2010 5:37 pm

So true. I piloted the Mouse Musket with a blowgun part of the time and a 1/4 inch ball valve part of the time. The piston was 1-1/4 inch and used no seals other than the barrel seal. I still had to add an EQ port because grease would seal it too well.

Good construction of a 1/2 inch piston should be no problems to pilot with an unmodified blowgun.
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Unread postAuthor: jackssmirkingrevenge » Thu Nov 18, 2010 11:33 pm

I can pilot a 1" piston valve with a schrader, either your piston is horribly loose/jammed or there isn't much difference between your barrel I/D and piston O/D.
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Unread postAuthor: Selador » Fri Nov 19, 2010 1:32 am

Gentlemen. (And I use that term loosely.) ;)

Long day for me today.

I did get more batteries for the camera today.

Tomorrow I have a couple other jobs to do before I get back to this. But I'll try to get some clear pics for you.

I was asked in a PM, to post some pics and get some help figuring out the problem. I -planned- to start a thread to do just that. But you know how life intervenes when we are making other plans...

I may still start one, just to show all the steps I went through to troubleshoot this. Maybe someone else will be helped.

Depends on what life tosses at me in the next couple of days.

I am treating this as a learning experience. And I have learned a bit, already.

I think I have sussed the problem, and I'll get to that in a bit. First, let me address some of the comments.

I may not be the sharpest knife in the drawer, but I am far from dull. There is not a lot to know about designing a piston.

It may seem daunting at first, but in just one day, I had it figured out, and had a perfectly working 1-1/4" valve built.

What I like about having learned here, is that I have been helped to understand not just the HOW of putting a piston valve together, but the WHY. If you know the why, it is easy to troubleshoot and figure out why something doesn't work.



The piston is perfect. A perfect fit. Not too tight, not too loose. The right length. Not too long, not too short. Not too heavy, not too light. And the seal against the end of the barrel is perfect.

Over the past few days this piston has been by turns, too loose, too tight, too long, too short, too heavy, too light.

But by this point I DO have the piston exactly the way it should be.



Well, and now to the part where I think I have it sussed. And then I'll explain why I am happy it turned out this way...



If you'll look at the picture, you'll see a drawing of the valve as I have built it.

I built it the way I did, because I did not have any pipe smaller than the 1/2" PVC.

But I should have found some piece of pipe somewhere, that would fit inside the 1/2" pipe.

That, or an adaptor that would slip inside the tee, and another, smaller pipe would slip inside the adaptor.

The reason I think this isn't working 'normally', is because the piston has to seal the 1/2" pipe. There is no gap above the seal. Nor on the sides.

The way it is currently built, the only back pressure on the piston, is on the body of the piston itself. I indicated that with the red arrows.

Normally, these valves are built as in the lower part of the picture. Leaving a gap all around the seal face, and providing back pressure not only on the piston body, but around the rim of the face of the seal, as well.


Bottom line, this is why it takes such a fast exhaust of the pilot volume, to get that piston to move backwards.

Had I built the piston as in the bottom of the illustration, a blowgun by itself, would indeed have been enough to trigger the piston.


Since this is already all glued together, the valve is pretty much a fail.

Except....

I can use this valve to experiment, to try to design and build a faster evacuating, homemade pilot valve. :D


In the future, if I find a suitable size pipe, I'll try another 1/2" piston valve. But for now, I think I'll build a 3/4" One, and a 1" one.

I like the size and shape of what I have built up from the 1/2" Valve. So I will try to build a gun this size, but make the chamber more versatile. Able to accept the 3/4" the 1", and the 1-1/4" piston valves.


At this point, I am not sure what I will be working on tomorrow. I have two or three ideas rolling around in my head for homemade pilot valves. But now that I think I have solved the mystery on the 1/2" valve, I may decide to just have fun, and put the 1/2" barrel, and a 2" by 16" chamber... On the 1-1/4" piston valve, and see what kind of damage I can do. LOL


~~~~~~~


Now, to get this thread back on track... Anybody else have any good ideas for homemade pilot valves ?

saefroch, could you give me a link to the thread that you found that valve by BTB, in ? Thank you.
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Unread postAuthor: Gippeto » Fri Nov 19, 2010 2:18 am

Long post...aren't you the loquacious one though. :wink:

"Sussed" it most certainly seems to be. :)

Allow me to congratulate you on NOT being the dullest knife in the drawer. :lol:
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Unread postAuthor: Selador » Fri Nov 19, 2010 2:43 am

Thank you.

:D
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Unread postAuthor: jackssmirkingrevenge » Fri Nov 19, 2010 2:52 am

Oxbreath wrote:Now, to get this thread back on track... Anybody else have any good ideas for homemade pilot valves?


Herewith an example of a "properly" made 1" piston valve for a 3/4" barrel (A perfect fit. Not too tight, not too loose. The right length. Not too long, not too short. Not too heavy, not too light. And the seal against the end of the barrel is perfect ;)) piloted only by a humble schrader valve.

I must insist that if your piston isn't working with a blowgun pilot, then the degree of perfection you claim of it is likely unrealistic.

I digress, this is Brian's pushbutton valve thread, though again before looking for a better pilot I suggest contemplating the true nature of your piston first.
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Unread postAuthor: Selador » Fri Nov 19, 2010 3:21 am

jackssmirkingrevenge wrote:I must insist that if your piston isn't working with a blowgun pilot, then the degree of perfection you claim of it is likely unrealistic.

I digress, this is Brian's pushbutton valve thread, though again before looking for a better pilot I suggest contemplating the true nature of your piston first.


Did you actually read what I posted ? It doesn't seem so.

The piston is a perfect fit. The pilot volume behind the piston is good.

The piston itself "works" perfectly. Moves smoothly. Travels enough but not too much. And seals against the end of the barrel, perfectly.

The problem is at the face of the piston.

There is not enough force at the face of the piston, and the face of the seal. Therefore, it takes a much faster evacuation of the pilot chamber behind the piston, to get the piston to move as it should. (And as it -would- have, had I built the barrel end of things, correctly.)

Thank you very much for the link. :D


EDIT: BTW, my pictures look purtier than yours ! :P :D
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Unread postAuthor: jackssmirkingrevenge » Fri Nov 19, 2010 3:52 am

Oxbreath wrote:Did you actually read what I posted ? It doesn't seem so.


If you read through my previous post, the piston diameter being close to the barrel internal diameter was one of the possible sources of failure ;)

JSR previously wrote:either your piston is horribly loose/jammed or there isn't much difference between your barrel I/D and piston O/D.


In the case of the launcher linked to above, the difference between the piston diameter and barrel internal diameter is less than 1/4", and it still works perfectly with a schrader, hence my doubts as to the veracity of your claims regarding the piston ;)

BTW, my pictures look purtier than yours !


If you're referring to this, those were early days, I got better ;)
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Unread postAuthor: Selador » Fri Nov 19, 2010 4:14 am

Obviously, "perfect" being a very subjective word, is not the right word to have used here. LOL

Let's just say, "pretty good". ;)

I learn best by doing. Try to discuss the math with me, and you'll see my eyes glaze over pretty quickly. But let me get my hands on something, and I can suss out it's intricacies pretty well, and pretty quickly.

The diameter of the piston has been probably 17 different sizes. Some obviously too tight. Some obviously too loose. I just kept trying until I got what seems like the best fit.

Same with the pilot volume. I, like you, prefer to keep the pilot volume at a minimum. But in my project, due to my limited resources, I ran into certain limitations.

The shiney side to that, was that I learned that... if you can't or don't change the -length- of the pilot evacuation pathway... (the path that all the pilot chamber volume is going to have to take, to get out of that chamber)

Making the pathway for the pilot evacuation, smaller, in an effort to reduce the pilot volume... Is not necessarily going to improve performance. And can, in fact, be detrimental.

BTW: I forgot to say earlier... Good job on the cannon you linked me to !

:D

Edit:
jackssmirkingrevenge wrote:
BTW, my pictures look purtier than yours !


If you're referring to this, those were early days, I got better ;)


Yes, I was.

And yes you did. That's great !

:)
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Unread postAuthor: jackssmirkingrevenge » Fri Nov 19, 2010 4:32 am

Oxbreath wrote:Obviously, "perfect" being a very subjective word, is not the right word to have used here. LOL

Let's just say, "pretty good". ;)


Agreed that without measurements it's wholly subjective. I didn't link to my launcher to imply "ha, my piston is better than yours!" but to show that with a "perfect" piston fit (by this I mean that the piston is cast out of epoxy in the exact same tube it will travel in, so it's as airtight as it's going to be without actually fitting o-rings), even a schrader should suffice for piloting.

A practical example:

Let's say air blows out a blowgun through a 1/16" orifice, this means it's passing through a cross section of 0.00307 square inches.

Let's say that the tube in which the piston is travelling is 0.75" diameter exactly, while the piston itself is 0.745" - pretty tight, right? Piston area is 0.43597 square inches, while the internal area of the tube is 0.44184 square inches, so air can flow from the chamber to the pilot past the piston through a gap of 0.00587 square inches - bigger than the flow of the blowgun, so the piston will never fire because the pilot is filling up faster than it can empty.

If you can measure your actual parameters with a vernier and do the math, it might help with your R&D and give a theoretical basis to your experimentation, we're not talking complex math here either, just cross-sectional areas, so no need for your eyes to glaze over ;)

edit: d'oh, I had already fiddled with the math for a similar example :roll:
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Unread postAuthor: Selador » Fri Nov 19, 2010 4:51 am

Ok, dangit !

You -do- realize that now I am not going to get any sleep tonight, right ? LOL

I am going to be keep dinking with this until I have as tight a fit as is possible, without actually getting stuck.

Also, I'll grease the piston and bore with silicone grease.

If I am so tired I can't get anything else done, tomorrow, I am going to blame you.

;)
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