clide's take on semi-auto (now with animation)

Post questions and info about pneumatic (compressed gas) powered launchers here. This includes discussion about valves, ratios, compressors, and anything else relevant to launchers powered by compressed gas.

Postby clide » Tue Oct 05, 2004 12:42 am

Thanks tim. Are you talking about the check valve components of the piston? Well the hole near the back is just for a metal rod. I use it to grab onto and pull the piston out. I used thinwall 1" for the pipe that the piston slides in, so I had to lathe the threads on the female adaptor down some. The o-rings still sometimes get caught on the threads when I pull it out so I can use that metal rod to rotate the piston and back it out through the threads. But yes I do use a set screw that you can't see in the pic to hold that inner pvc piece in. I wanted to be able to acess the check valve components incase the spring was too weak/strong or if the disk didn't work properly, so I didn't glue it in. This also came in handy when that piston shown broke. I was able to transplant the check valve components to the new piston easily.
As for retaining the spring, the barrel end of the piston is plugged, so the spring is resting against the plug. Hope that answered your question, if not just tell me what to clarify.

I sent you an email Freefall. Looking forward to hearing your idea.
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Postby tim jones » Tue Oct 05, 2004 9:35 am

Clide:

Thanks for your reply; it cleared a few things up for me. You are correct that I was asking about the piston and it's check-valve components. This is the photo I referred to: http://members.cox.net/ecyrb/slider/piston%20parts.jpg

If I understand correctly, the piston has a 1" Schedule 40 plug (does the bulkhead end face forward, or to the rear?) at its forward end , and a section of 1" thin-wall (SDR 21 or 26?) as the cylinder. The plug is grooved for an o-ring near its forward end, and its rear end is glued into the piston body. The 1" thin-wall section is glued into the rear end of the piston body. The forward end of the check-valve spring rests inside, and is retained by the forward end of the 1" plug. The rear end of the spring extends rearward into the 1" thin-wall pipe section, and holds the rubber valve plunger against the valve seat. The valve seat (made from 3/4" Schedule 80?) fits snugly inside the rear end of the 1" thin-wall section, and is held in place by a single set screw. This set screw is threaded radially through the piston body (between the o-rings?) and thin-wall pipe. The valve seat has a small, radially drilled hole, which accepts a piston removal tool. Four vent holes (about 1/4" diameter?), equally spaced around the piston's circumference, are drilled radially through the piston body and thin-wall pipe section near the mid-point of the piston's axis. Is this correct?

EDIT: added parenthetic questions.
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Postby clide » Tue Oct 05, 2004 1:47 pm

um. Kinda but not really. I'll explain the parts of the piston in the order I made it. First the outside of the piston is a 3/4" conduit bell end. It is lathed down to a slide fit in thin-wall 1"(sdr21). A piece of 3/4" conduit is glued into the bell end to give support. Running through the entire piston and sticking out the front is a lathed down, glued in peice of 1/2" sch40 pvc. At the barrel end (the part sticking out)there is a piece of pvc rod glued into the 1/2". O-ring grooves are cut in the 3/4" bell to make a seal with the inside of the 1" thin-wall. On the piece of 1/2" sticking out there is an o-ring groove to seal inside 3/4" sch40(the outlet going to the barrel). At this point it is just a solid piston no holes or anything, if you pressurized it from the rear all the air would stay in the pilot chamber. The rear insert(the light gray thing below the rubber disk in the picture)is made to be a friction fit. 4 vent holes (yes about .25") are drilled around the piston's circumference about right where the insert stops. A single set screw between the 2 rear o-rings goes through the bell end, 3/4" , and 1/2" pvc to contact and indentation on the insert to hold it in place. A hole is drilled trough the rear insert with a small section of rod the length of the OD of the insert placed in the holes. That is what I grab onto to remove the piston. So the components of the check valve are all inside the 1/2" pvc in this order: PVC rod>spring>rubber disk>insert
And I believe what I am referring to an insert is what you were referring to as the valve seat, it was made out of pvc rod.
And it is hard to see in the picture, but right about where the 4 holes are, the piston reduces in diameter slightly, that is the end of the bell. And where it turns white is the 1/2" pvc

OK, hope all that made sense. If not, let me know.
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Postby tim jones » Tue Oct 05, 2004 2:52 pm

Wow, I really was off scale! Looking back at your earlier reply, I see that you described the piston as fitting INSIDE 1" thin-wall pipe. I thought the spring, rubber disk and rear insert (what I called the valve seat) were a tight fit inside the 1" pipe, not inside a 1/2" pipe. Thanks for correcting my misconception.

You indicated that the piston in the picture broke. Was it dammaged by normal operating stresses? More importantly, is the replacement piston built differently from the one pictured?

I'm attempting to scale your piston/check-valve system up for use with a golf ball launcher (this is probably why I was thinking on a larger scale), so I appreciate your insightes.
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Postby BillyBobJoe » Tue Oct 05, 2004 3:49 pm

<blockquote id="quote"><font size="1" face="tahoma,verdana,arial" id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Freefall
[br]I've held off viewing this thread for a while, but now, seeing the improved diagrams and video, I must say:

<font size +2><b>HOLY [EXCREMENT]!</b></font>

Nice work!<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">

Same here, thats friggin' awesome clide, excellent job, and design:D:D[:p][:0]
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Postby clide » Tue Oct 05, 2004 6:40 pm

I'm not to sure what broke it exactly. I found that the pvc had cracked where the 1/2" pvc enters the 3/4" which also happens to be where the plug ended. I made some slight changes of lengthening the piston slightly in the back and also lengthening the plug to give it more strength in that area. And the previous o-ring groove was too close to the back end and had broken, so as far as the spacing goes, most of the extra length went behind the rear o-ring. I am also upscaling this design for a golfball cannon. I can send you some pics and info on what I have done so far if you are interested.
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Postby tim jones » Tue Oct 05, 2004 8:17 pm

Clide:

I'm very interested to hear your progress on the golfball version, but I'm disappointed that I did not come up with the idea first. Perhaps I'll have to build the first semi-auto tennis ball cannon, or soda bottle cannon, or pumpkin cannon, etc.

On the first piston failure: Could the 1/2" forward extension of the piston have been mis-alligned with its recieving port, causing the extension to impact the side each time it was driven into battery? That would fit with the dammage you described.

By the way, what does the 1/2" forward extension fit into? I would guesse that you lathe-turned a 3/4" x 1/2" bushing down to fit inside a 1" Sch 40 pipe at the front end of the tee fitting, but most of my guesses have been wrong lately.
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Postby clide » Wed Oct 06, 2004 4:01 pm

If mis-allignment of the recieving port was the problem the in was likely that the problem comes from the 3/4" not being cut square, since the 1/2" runs all the way through the piston I doubt it was mis-alligned. The breaking was probably a combination of many things. Luckily I notice it when I took out the piston for maintenence. It could have been bad if the plug was shot out of the barrel unexpectedly.
It fits into 3/4" sch40. On the side of the tee where the barrel connects, there is a 1"x3/4" bushing in both the female adaptor and the tee. Connecting them is a piece of 3/4" which is what the front of the piston seals inside.
I'll try to get you an email sent later today. Heh many people have come up with the semi-auto golfball gun idea. AFAIK nobody has carried it through. Feel free to be the first. I have 2 projects infront of the golfball gun ATM.
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Postby tim jones » Thu Oct 07, 2004 1:17 pm

OK, I'll continue work on a golfball version. Would you mind describing the components of the auto-loading mechanism? Some of the component parts are obvious, but some are a bit strange.
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