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4" piston

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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4" piston

Unread postAuthor: spudtyrrant » Sun Jun 28, 2009 12:58 pm

hey guys i'm making a large 4" diameter piston and i am going to use hot glue i have seen it used successfully in 2" pistons so i'm thinking it should be fine in 4" i'm sure it will work but before buying 4 packs of hot glue i wanted to get some input from you guys it should cost about as much as a bell reducer piston but it will be rubbery and more resilient(hopefully)
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Unread postAuthor: mark.f » Sun Jun 28, 2009 9:37 pm

If you are making a 4" piston, I would use 3" SCH-40 PVC fittings, with space between them to sandwich 1/4" O-ring stock. It's worked for me excellently for smaller pistons (2" diameter made with 1-1/4" fittings and off the shelf o-rings), so with a little tweaking (and gluing your own o-rings), it should work for much larger ones.
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Unread postAuthor: spudtyrrant » Sun Jun 28, 2009 10:21 pm

mark.f wrote:If you are making a 4" piston, I would use 3" SCH-40 PVC fittings, with space between them to sandwich 1/4" O-ring stock. It's worked for me excellently for smaller pistons (2" diameter made with 1-1/4" fittings and off the shelf o-rings), so with a little tweaking (and gluing your own o-rings), it should work for much larger ones.
i am aware the o.d.of 3" = the i.d. of 4" i just think hot glue is more rubbery and less liable to brake or crack plus i can cast it to the length i want and it will be about the same price
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Unread postAuthor: Technician1002 » Mon Jun 29, 2009 1:50 am

mark.f wrote:If you are making a 4" piston, I would use 3" SCH-40 PVC fittings, with space between them to sandwich 1/4" O-ring stock. It's worked for me excellently for smaller pistons (2" diameter made with 1-1/4" fittings and off the shelf o-rings), so with a little tweaking (and gluing your own o-rings), it should work for much larger ones.


Been there done that.. My Dragon 2.5 inch coaxial used that. Short story, it's brittle. It cracks, It shatters, It's only good for a couple shots. That is why my breech photos posted elsewhere don't have a piston. It's long retired itself. I do not recommend PVC pistons. They don't last.

The Apple cannon is my longest lived PVC Piston. It uses a foam football core that is as long as the piston for a bumper and the PVC is double layer thick except for the o rings.

If you used them for smaller pistons, how did you get them to survive? I went through 4 pistons on the Mouse Musket before I retired it. It used a 1 inch pipe cap for a piston.
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Unread postAuthor: mark.f » Mon Jun 29, 2009 2:22 pm

Technician1002 wrote:
mark.f wrote:If you are making a 4" piston, I would use 3" SCH-40 PVC fittings, with space between them to sandwich 1/4" O-ring stock. It's worked for me excellently for smaller pistons (2" diameter made with 1-1/4" fittings and off the shelf o-rings), so with a little tweaking (and gluing your own o-rings), it should work for much larger ones.


Been there done that.. My Dragon 2.5 inch coaxial used that. Short story, it's brittle. It cracks, It shatters, It's only good for a couple shots. That is why my breech photos posted elsewhere don't have a piston. It's long retired itself. I do not recommend PVC pistons. They don't last.

The Apple cannon is my longest lived PVC Piston. It uses a foam football core that is as long as the piston for a bumper and the PVC is double layer thick except for the o rings.

If you used them for smaller pistons, how did you get them to survive? I went through 4 pistons on the Mouse Musket before I retired it. It used a 1 inch pipe cap for a piston.


A sufficient bumper and limited piston travel.

Most of my bumpers consisted of 1/4" of plumbing rubber, and less than an inch of piston travel. I've never had a PVC piston crack or deform in any other way.
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Unread postAuthor: spudtyrrant » Mon Jun 29, 2009 2:30 pm

mark.f wrote:Most of my bumpers consisted of 1/4" of plumbing rubber, and less than an inch of piston travel. I've never had a PVC piston crack or deform in any other way.
well in barrels sealers the 1/2d rule applies and i want 2" of piston travel so i have decided to use hot glue i was just wondering if you guys saw anything wrong with it
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Unread postAuthor: velocity3x » Mon Jun 29, 2009 4:58 pm

spudtyrrant wrote: well in barrels sealers the 1/2d rule applies


I have no idea of what the 1/2d rule is. Will someone please explain it to me?
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Unread postAuthor: spudamine » Mon Jun 29, 2009 5:02 pm

well in barrels sealers the 1/2d rule applies and i want 2" of piston travel so i have decided to use hot glue i was just wondering if you guys saw anything wrong with it


Hot melt glue is quite brittle as well, I made the below projectile from PVC pipe capped with ali and filled with hot glue, then fired it straight into a concrete slab with the hybrid:
Image
As you can see neither the PVC nor the hot glue survived very well (the concrete slab didn't come through to good either) Admittedly this is a little bit 'harsh' as comparisons go but the hot glue doesn't seem to like shocks. You could try reinforcing it with something.
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Unread postAuthor: MountainousDew » Mon Jun 29, 2009 5:38 pm

2 inches of travel is just plain overkill! The results won't be noticeable or even different (of course slightly, but it can't help noticeably).
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Unread postAuthor: mobile chernobyl » Mon Jun 29, 2009 5:57 pm

velocity3x wrote:
spudtyrrant wrote: well in barrels sealers the 1/2d rule applies


I have no idea of what the 1/2d rule is. Will someone please explain it to me?


The 1/2diamter rule is incorrect - it should actually be 1/4diameter.

What this means is that the piston must move back 1/4" the diameter of the port in order to achieve 100% possible flow - The area of the ring between the piston and port sealing face is very close to equal to the porting surface area then.

Of course 100% flow is possible, but there will always be turbulence and constrictions through fittings etc... but the very minimum a piston should open to have a chance at full efficiency is 1/4 the port's diameter.
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Unread postAuthor: spudtyrrant » Mon Jun 29, 2009 6:36 pm

@ mobile general rule for barrel sealers is 1/2d thats what mr.crowley uses and thats i have used. i have never actually seen the math that proves this rule nor have i seen any comparisons between 1/2d piston travel and 1/4d travel would like to know how you guys know whether it is overkill or not and if it is overkill what does it matter this cannon is going to be large enough where if i add another inch to the travel it shouldn't make too noticeable a difference
@spudamine yeah my piston is not likely to come to those kind of forces it also looks like their was a lot of bubbles in the glue i will be casting mine into a mold all at once so it will have much less bubbles also look at how the pvc shattered it looks like the pvc didn't come through much better than the hot glue and that is what most people have suggested for my piston. with the glue i wouldnt have to go to the trouble of cutting peices of plywood to cover each end of a fitting plus it will take a lot less time
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Unread postAuthor: Technician1002 » Mon Jun 29, 2009 8:29 pm

spudtyrrant wrote:@spudamine yeah my piston is not likely to come to those kind of forces it also looks like their was a lot of bubbles in the glue i will be casting mine into a mold all at once so it will have much less bubbles also look at how the pvc shattered it looks like the pvc didn't come through much better than the hot glue and that is what most people have suggested for my piston. with the glue i wouldn't have to go to the trouble of cutting pieces of plywood to cover each end of a fitting plus it will take a lot less time


It's also very unlikely to have that long of an acceleration zone into the pilot. :D

The Mouse Musket did have limited piston travel. It still broke. Several times. Piston travel was limited to 1/3 the piston diameter where it hit the bumper and the bumper was another 1/3 diameter. Space for the piston travel was 2/3 the diameter including the space taken by the bumper. It is possible the pipe plug I was using (dished inside) was swallowing the bumper. It is why the apple cannon (ABS Cellular Core Test Cannon) has a set of washers. 1 behind the piston, 1 in the middle of the bumper, and 1 at the pipe plug. That piston survived.
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Broken Musket Valve.
Valve Assy.jpg
Apple cannon valve assy that survived.
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Unread postAuthor: spudtyrrant » Mon Jun 29, 2009 8:50 pm

@ tech yeah i like the mouse musket but those caps you are using are dwv mine will not be held by threads it will be held by 12 1 and 1/4 inch screws and i'm sure it would pop the whole coupler off the housing before it even came close to failing.

the main question of this was "will hot glue work for a large piston" and i haven't really got a solid reason why it won't yet people are saying its brittle yet its been used in 300psi copper rifles and in my 2" piston gun i just wanted to see if there was any problem with me using it in a large cannon but no one seems to think it will work and i'm just wondering why
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Unread postAuthor: Technician1002 » Mon Jun 29, 2009 9:32 pm

spudtyrrant wrote:@ tech yeah i like the mouse musket but those caps you are using are dwv mine will not be held by threads it will be held by 12 1 and 1/4 inch screws and i'm sure it would pop the whole coupler off the housing before it even came close to failing.

the main question of this was "will hot glue work for a large piston" and i haven't really got a solid reason why it won't yet people are saying its brittle yet its been used in 300psi copper rifles and in my 2" piston gun i just wanted to see if there was any problem with me using it in a large cannon but no one seems to think it will work and i'm just wondering why


The PVC Mouse Musket was not using a DWV plug. The plug did not fail, the piston did. The ABS cannon does as it is entirely DWV as a test. The Mouse Musket operates to 100 PSI and the DWV cannon is tested to 85 PSI and operated up to 65 PSI.

Nobody has tested a large diameter hot melt glue piston, so nobody has any real test data. The above one shattered at high speed after being launched from a long barrel as a projectile. Hardly a fair evaluation of survivabilty from a 2 inch launch into a bumper as a piston.
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Unread postAuthor: mark.f » Tue Jun 30, 2009 3:49 am

spudtyrrant wrote:@ mobile general rule for barrel sealers is 1/2d thats what mr.crowley uses and thats i have used. i have never actually seen the math that proves this rule nor have i seen any comparisons between 1/2d piston travel and 1/4d travel would like to know how you guys know whether it is overkill or not and if it is overkill what does it matter this cannon is going to be large enough where if i add another inch to the travel it shouldn't make too noticeable a difference
@spudamine yeah my piston is not likely to come to those kind of forces it also looks like their was a lot of bubbles in the glue i will be casting mine into a mold all at once so it will have much less bubbles also look at how the pvc shattered it looks like the pvc didn't come through much better than the hot glue and that is what most people have suggested for my piston. with the glue i wouldnt have to go to the trouble of cutting peices of plywood to cover each end of a fitting plus it will take a lot less time


http://www.spudfiles.com/forums/how-far-your-piston-should-travel-t12772.html

I generally use a little longer than 1/4 of the port diameter for my valves.
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