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Built a hybrid cannon? Then post it here! This section is for completed, finished cannons that you have built. Please include pictures and information.
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Unread postAuthor: SpudFarm » Fri Aug 13, 2010 4:36 pm

SpudBlaster15 wrote:Very nice. I've seen many videos demonstrating hydraulic shock, but certainly nothing of that scale. Impressive stuff.

The velocity was lower then expected but I think it is because the burst disk slips out of the union on one side and resticts flow.


That is quite likely. I had a similar issue with unmodified hardware store unions on my smaller hybrid designs. If the disk managed to seal effectively enough to allow the launcher to be fired, it would often partially slip out of the union, lodging in the barrel port and heavily constricting flow.

I achieved a solution by grinding down the tapered surface on the female component of the assembly. As your McMaster union uses flat mating surfaces, it should solve this issue as well.


Thank you, I hope I can get my union and a new fuel gauge soon. I have some interesting shots in mind.

What do you mean by the female part? Didn't you grind the male lip off?
I would like to find a way to fix the unions over here for use in hybrids since I would hate paying 50USD for shippin on every union I need..
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Unread postAuthor: MrCrowley » Fri Aug 13, 2010 5:25 pm

I'm not sure if this is what SpudBlaster did, but when I built my hybrid I had the same problem and read about his modification so I also tried it and it worked well:
http://i75.photobucket.com/albums/i290/ ... G_4461.jpg
http://i75.photobucket.com/albums/i290/ ... G_4476.jpg

Yeah, the union is on different sides in the photos, must've been when I was testing it to see whether the male side should be on the chamber or barrel side. Think I settled with the female modded side (flat, no threads) was to be attached to the barrel since it had a greater surface area for the disks to be held against.
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Unread postAuthor: SpudBlaster15 » Fri Aug 13, 2010 5:36 pm

Yes, the picture Crowley posted is exactly what I was referring to.

Works great on smaller hybrid launchers with thin sheet metal burst disks, but I haven't tried it with large unions. I believe it will work well if you are able to tighten the connection sufficiently.
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Unread postAuthor: SpudFarm » Sat Aug 14, 2010 2:44 am

This seems promising since I have allways used the union the other way around..
Do I have to use a lathe to cut the lip off or is it good enough to grind it off with a angle grinder?
Also, does it need a gasket to seal properly with aluminum foil?

I think I should be able to tighten it a fair bit, the thing is that my wrench came in at a mind boggeling 527USD so needless to say I didn't get two.

Thanks for the help guys, but if it works and I have used 70 dollars or so on a new one I will be kind of pissed :D
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Unread postAuthor: MrCrowley » Sat Aug 14, 2010 2:47 am

Do I have to use a lathe to cut the lip off or is it good enough to grind it off with a angle grinder?
Also, does it need a gasket to seal properly with aluminum foil?

I used a grinder and I use two gaskets glued to each other (adds thickness so you can tighten the union more).

Not sure how well gaskets stand up to $527USD worth of wrench-tightening though :shock:
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Unread postAuthor: SpudFarm » Sat Aug 14, 2010 2:55 am

MrCrowley wrote:
Do I have to use a lathe to cut the lip off or is it good enough to grind it off with a angle grinder?
Also, does it need a gasket to seal properly with aluminum foil?

I used a grinder and I use two gaskets glued to each other (adds thickness so you can tighten the union more).

Not sure how well gaskets stand up to $527USD worth of wrench-tightening though :shock:


Since I get a new one later I just go out and try to remove the lip.

I have some kevlar reinforced rubber that I can use. If it kind of mush out I can probably get away with using a O-ring or having a layer of tape on the disk.

Thanks!
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Unread postAuthor: SpudBlaster15 » Sat Aug 14, 2010 3:46 am

I used an angle grinder for my union way back in '07. If you're careful, you can get the surface very smooth and uniform, though a lathe would obviously be preferable if you have access to one.

I also used a rubber gasket on the modified surface, and a machined aluminum washer on the opposite side of the disk to create additional clamping area. With this setup, there was not one instance of a disk slipping free or failing to burst uniformly.
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Unread postAuthor: SpudFarm » Sat Aug 14, 2010 4:21 am

I grinded it down, I still have the part that was male on the chamber side. It also has a gasket on the same side. The other side is not changed for now.
But I might take your tip on the washer if this won't work.

I am currently waiting for the thread lock to cure as I found a small leak at the chamber side of the 2" nipple. I will have to enter hell and build a snowman to get it off again once I get the new union :D

Also, I took it to 30psi in the shop and the burst disk sealed perfectly. It was the leak in the nipples threads that stopped me from going higher as I wouldn't have heared a eventual leak in the burst disk.
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Unread postAuthor: SpudBlaster15 » Sat Aug 14, 2010 4:32 am

Any particular reason why you used threadlocker on the threads?

Not that it won't seal effectively, but removing it will likely require a couple of very large pipe wrenches and the application of some heat to the bonded area. I would have just used PTFE tape or thread sealant.

I think you'll be pleased with the results of removing the tapered surface from the union. With a proper gasket, there's no reason it shouldn't form a nearly perfect seal.
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Unread postAuthor: SpudFarm » Sat Aug 14, 2010 4:49 am

The thread locker was the only thing I had. It also secures the joint so it won't unscrew while I am tightening the union.

I used PTFE tape when I noticed the leak. I have tried different amounts but that joint have allways been leaking. Could be Lardas thread job or mabe different standards in Sweden since it is not leaking with just PTFE tape on the union side.

When I am going to take it off I will clamp the chamber in a vice and heat the nipple from the inside while using my wrench on it.

(I used some teflon tape with thread lock between every layer to ensure I can get it off)

EDIT: I tested the setup. I have the modified part on the chamber side but I will try it the other way today if I can get the union off.

The way it is now the union seals good enough (Perfect I think) but there are another leak somewhere that causes the pressure to drop slowly.

The first time I tried it I used one wrench to tighten the union. The disk slipped out at about 6bar. I got my hands on another one that I can hold the barrel with so I can tighten it properly and it didn't slip out after the chamber was pressurized to 7bar five times.

I still don't completely trust it though. I hope to see improvements once I turn the union around. I might end up using a metal washer as Spudblaster said also.
I will update you once I have some fresh results.
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