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how heavy is too heavy? Edit New problem

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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how heavy is too heavy? Edit New problem

Unread postAuthor: Jeeperforlife » Mon Nov 02, 2009 2:20 pm

I am building my first piston cannon. It is a coaxil with 4" SCH40 chamber 5' long and 2" SCH40 6' long barrel. I will be piloting it with a modded 1" rain bird sprinkler valve. My biggest question is on the piston, it is 2 3" end caps sanded down to fit as tight as I could with very low friction. it has a oring in the middle for a good seal, it seals and moves good enough that I can push it out the back by blowing down the barrel.

My conceren is the piston weight is right at 1.5 pounds; heaver than I wanted.
What do you think?

Marcus
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Last edited by Jeeperforlife on Mon Nov 02, 2009 7:48 pm, edited 1 time in total.
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Unread postAuthor: Jack_Hogg » Mon Nov 02, 2009 2:36 pm

I have made several pistons. Still figuring out how I will make a better piston though. I have acces to a lathe, but still need a better piston. Same problem. Too heavy and stuff. I suggest you test it. If not good, just make another one or modify your piston, just like I did.
Linky: http://www.spudfiles.com/forums/work-in ... 19458.html
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Unread postAuthor: c11man » Mon Nov 02, 2009 3:30 pm

get rid of the second endcap and that will get rid of alot of weight. if it is a very good fitting piston then you will not need the oring that i am guessing was being held in by the second end cap. so by getting rid of a cap and the pipe between you should have a lightweight piston
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Unread postAuthor: ramses » Mon Nov 02, 2009 3:43 pm

keep in mind that by removing the second end cap, you massively increase pilot volume.
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Unread postAuthor: JDP12 » Mon Nov 02, 2009 4:31 pm

^^

ditto to what ramses said
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Unread postAuthor: c11man » Mon Nov 02, 2009 5:07 pm

yes i would massivly increase pilot volume but he could cut off the thread adaptor on the back, and glue a new on one closer to the barrel.

also if he hasnt glued the barrel on yet he could just glue in it farther down.

and if i were you i would turn the endcap so the open end faces the barrel so that it lowers the space needed for the piston to ride in, reduce more volume, and make it hit the bumper yet, this would require the seal to be on the inside of the cap instead of outside
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Unread postAuthor: spudtyrrant » Mon Nov 02, 2009 6:36 pm

c11man wrote:yes i would massivly increase pilot volume but he could cut off the thread adaptor on the back, and glue a new on one closer to the barrel.

also if he hasnt glued the barrel on yet he could just glue in it farther down.

and if i were you i would turn the endcap so the open end faces the barrel so that it lowers the space needed for the piston to ride in, reduce more volume, and make it hit the bumper yet, this would require the seal to be on the inside of the cap instead of outside
ooooorrrrr.... you could make a very long bumper :wink:
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Unread postAuthor: c11man » Mon Nov 02, 2009 6:38 pm

that could work too
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Unread postAuthor: Jeeperforlife » Mon Nov 02, 2009 7:16 pm

Thanks for the quick replies. But now this is the last of my problems. My cat knocked the gun off the couch and cracked it were the barrel is glued to the bushing. When I pressurized it next the barrel broke free and shot out of the chamber. :?
Now the bushing is fine But I am going to replace the barrel so I have a clean surface to glue. I wire brushed the inside of the bushing and removed all of the old glue. I don't think it was glued well enough the first time.
So what great advise can I get from you? How is the best way to glue a joint when over a foot sticks past the joint? I feel very confident in all the other Joints in the gun.

Marcus
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Unread postAuthor: Moonbogg » Mon Nov 02, 2009 9:06 pm

Damn, it cracked from falling over? I'm sorry to hear that. Goes to show how careful you have to be with PVC cannons. Luckily you knew it cracked.
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Unread postAuthor: c11man » Mon Nov 02, 2009 9:59 pm

so when you replace the barrel remember to mod your piston and glue it in farther so there is not too much deadspace
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Unread postAuthor: Technician1002 » Mon Nov 02, 2009 10:32 pm

I have a quick note on dead space as this is often very much in the forefront of cannon performance suggestions. I have played with dead space in my cannons and found the debate over rated.

I found another parameter which is more important.

In barrel sealer piston designs, chamber air on one side of the piston is balanced by the pressure in the pilot area. The pilot area has more surface area exposed to pressure than the chamber side so the pressure is able to hold the piston closed. When the force on the rear of the piston (Area X Pressure) drops to equal or is less than the force on the front of the piston, the piston can begin to open.

Depending on how low the pilot pressure is when this happens has a large impact on the piston performance. If the pilot pressure is still relativly high, the piston as it moves back compresses the pilot area. A small volume compresses rapidly limiting the piston speed. As the chamber pressure drops this may re close the piston before it fully opened. A really large pilot valve is required to overcome this, or a slightly larger pilot volume.

With too large a pilot volume, the time it takes to drop the pilot area pressure is increased. This increases the length of time chamber pressure can blow off into the pilot area. This IS the issue with too large a pilot area. It is also an issue with too large an EQ port. A smaller pilot and smaller EQ port or fixing piston blow by will fix this. Too small a pilot area with a small pilot valve is bad.
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Unread postAuthor: Jeeperforlife » Mon Nov 02, 2009 11:12 pm

Thanks,
I have decided to go with just 1 3" end cap turned around with 2 O ring grooves to seal it. this will probaly move my barrel in about 5" witch is a big plus for me and take 2/3rds of the weight off. I played with GGDT and there was little to no effect with the reduced weight but I think it may be better so wile I have it apart why not?

My new problem is still a issue I do not know how to properly glue the bushing to the new pipe. Should I use PVC glue and no primer on the bushing or would Epoxy work better? I may run a few screws in the barrel through the bushing wile it is drying to help hold it in place and offer some added safety. This is a father son project so safety is a big concern.
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Unread postAuthor: c11man » Mon Nov 02, 2009 11:34 pm

use primer AND glue and you will be good. just prime the bushing a bunch before sliding the barrel into it then glue it in.
screws would not be nessesary if you glue it right.

remember primer is supposed to still be wet when glue is applyied
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Unread postAuthor: Jeeperforlife » Mon Nov 02, 2009 11:40 pm

c11man wrote:use primer AND glue and you will be good. just prime the bushing a bunch before sliding the barrel into it then glue it in.
screws would not be nessesary if you glue it right.

remember primer is supposed to still be wet when glue is applyied


This is were I have an issue by the time i slide the barrel on the primer will have come off and there is no way to get glue in there other than apply it to the barrel and hope it makes it way to the bushing.

If the general consensus is that it will work I will try it, but this time I want to be sure.
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