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1.25" piston hybrid-- SB15 inspired/clone- UPDATE 8/8

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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1.25" piston hybrid-- SB15 inspired/clone- UPDATE 8/8

Unread postAuthor: JDP12 » Thu Aug 05, 2010 5:17 pm

Before I go any further, let me say this. THIS GUN IS NOT ORIGINAL. I am going to give absolutely all credit to SB15, as it is his piston hybrid that inspired me to get this going, and for the sake of simplicity and time, this will be a very similar copy of his, only slightly smaller.

So with that being said, here are details:

JDP12's Piston Hybrid

1) Piston housed in 1.25" galvanized fittings. Chamber is mostly 2"x8" galvanized, with a 1.25" fitting to get to tee.

2) Meter is SB15 style (again), typical air-through meter. The air comes in through the top, by the pressure gauge and ball valve. Propane is self-explanatory.

3) Piston will be a threaded rod construction.

4) Primary barrel will be 3/4" copper.

5) Pilot area will not be as complex as SB15's (to start) In the interest of time I'm just going to go with a simple style like what he had initially. Better setup will come later.

PROGRESS:

Chamber is all threaded together. Leak testing will be done tomorrow hopefully. All parts are purchased, just have to put it all together.

here are a couple pictures.

Also a note: ALL UPDATES WILL BE PUT INTO THE ORIGINAL POST. As I will be building this primarily on Sunday, I will have lots of updates. i don't want to clog up the thread with constant updates, so always check this post for any gun updates.

Thanks and enjoy!


UPDATE 8/8-- almost complete with piston construction. Just have to JBWeld everything together and cut shaft down. That should be done tomorrow. The flat rubber seals seem like they should work, I have them trimmed so I can push the piston in and out with a pretty fair amount of force, we'll see what some lithium grease can do as well. Hopefully they seal well otherwise I'll have to start a new piston.

The other picture is of the sealing face. It's a little complicated. There's a 1.25"x1" bushing, but I couldnt thread a 1" nipple into the back. So I pushed a 3/4" nipple into a 1" nipple, threaded the 1" into the bushing the normal way, and JBWelded a 1" nipple on the portruding end. Its complex, but the ease of sliding the 3/4" into the 1" allowed for really good straightness.

I'm not sure if this will be done by Saturday or not as I've been having to take care of alot of college things and I'm gone Monday afternoon-Wednesday.

But I'll have more updates tomorrow

UDPATE 8/11-- Unfortunately there's no way this will be done by Saturday. Still too much to do. it's about 95% done. I worked on the piston some more, sanding down the rubber to get it smoother. It slides much easier in the housing now. Also have the rubber plug mounted on shaft for the spool valve. Still have to drill holes in housing endcap, assemble pilot, and then leak test everything. So those things will be done on weekends I am home from college.


Sorry I wasn't able to finish it, my schedule just wouldn't let me.

I do have pictures of what I have done so far.
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Attachments
piston hybrid 1.jpg
piston components.jpg
hybrid piston 1.jpg
1 in porting hybrid port.jpg
hybrid piston pre-jb weld.jpg
another view of piston hybrid.jpg
Last edited by JDP12 on Wed Aug 11, 2010 5:14 pm, edited 3 times in total.
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Unread postAuthor: Gun Freak » Thu Aug 05, 2010 5:20 pm

Looks good, can't wait for another piston hybrid to be completed!
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Unread postAuthor: SpudBlaster15 » Thu Aug 05, 2010 6:52 pm

Looking good so far.

Be sure to seal the washers and nuts when you assemble the piston, as they will leak. Any decent epoxy should work.

How long of a barrel are you planning to use?
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Unread postAuthor: JDP12 » Thu Aug 05, 2010 7:09 pm

6 foot as that is what I have right now, may upgrade later.

Yea I plan on putting some JB Weld on it. Did you just cover the threads in JB Weld like it was pipe dope and put it together that way or just put it on the outside where the nut meets the bolt?

Also, do you think using teflon on the rod would work at first just for adjustment purposes and such so I'm not stuck with a messed up piston?

Thanks
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Unread postAuthor: Gun Freak » Thu Aug 05, 2010 7:14 pm

Teflon on the rod wouldn't work because the air could still leak through where the nut meets the washer on the side. I will seal under the nut bot not on the sides, so you probably have to use some sort of epoxy. Although, you can still adjust it, JB Weld isn't that hard to break :D
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Unread postAuthor: JDP12 » Thu Aug 05, 2010 10:04 pm

SB15--- While I have o-rings, they may prove a little difficult to clamp between two washers. Do you think I could substitute flat rubber sheet for o-rings if necessary?

Also, I was thinking of just using a piece of thick rubber air hose as a bumper-- with such a light piston I don't think I'll need a huge overkill bumper. Thoughts?
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Unread postAuthor: SpudBlaster15 » Fri Aug 06, 2010 12:11 am

JDP12 wrote:Did you just cover the threads in JB Weld like it was pipe dope and put it together that way...?


Yep.

Also, do you think using teflon on the rod would work at first just for adjustment purposes and such so I'm not stuck with a messed up piston?


PTFE tape isn't really designed to seal straight threads, so I don't think it would work effectively. I'd just go straight for the epoxy. Use the slow curing JB weld, and it will give you a few hours to reposition the parts. It's stronger than rapid curing epoxies as well.

SB15--- While I have o-rings, they may prove a little difficult to clamp between two washers. Do you think I could substitute flat rubber sheet for o-rings if necessary?


I'm not really sure how you'd clamp the o-rings between the washers. I sandwiched smaller diameter washers between the large ones to create a groove. The inner surface of the seal actually rides against the outside of the washers.

Flat rubber sheet will likely work as well, assuming you can cut it to shape reasonably well.

Also, I was thinking of just using a piece of thick rubber air hose as a bumper-- with such a light piston I don't think I'll need a huge overkill bumper. Thoughts?


That would probably be fine. If you give me the travel distance of your piston, as well as its mass and the mixes planned for the launcher, I can give you an approximate figure for the energy your bumper would be required to absorb.

Here's a picture of the bumper I used. The overall thickness is somewhere around 1.25", and it uses 2 different types of rubber to achieve a progressive damping effect.

Image

Another terrible picture, this time with my camera. Sorry about that.

Gun Freak wrote:Although, you can still adjust it, JB Weld isn't that hard to break


If the JB weld is properly mixed, and the surfaces are properly cleaned prior to assembly, it will be nearly impossible to break apart the cured joint. After assembling my piston, I strength tested the assembly, and the nuts would not budge. If I had continued to apply torque, the threaded rod would have likely twisted off before the epoxy joint broke.

Once cured, it's a permanent assembly, so your system had better be well engineered from the beginning.
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Unread postAuthor: SpudFarm » Fri Aug 06, 2010 11:58 am

Spudblaster: Looks like you inspired other people with your hybrid to. Good thing!

As to your build, looks good. Since it is mostly the same as SB's launcher it should work out just fine. I am looking forward to this.
The hybrid community is growing like crazy now!
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Unread postAuthor: JDP12 » Fri Aug 06, 2010 2:38 pm

Yup! Off to the store to buy some hosing for a bumper, possibly a spring and a nipple for the back of the bushing.
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Unread postAuthor: niglch » Fri Aug 06, 2010 3:40 pm

Wow. I'm really liking these hybrids that are starting to pop up. I came up with a design last year with a lot of similar ideas but haven't had the time or money to build. This certainly serves as good inspiration to give it a shot.
Are you planning to put some sort of pressure relief valve behind the piston? Or maybe just a large pilot volume with a bumper positioned slightly behind the back of the piston?
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Unread postAuthor: Gun Freak » Fri Aug 06, 2010 3:48 pm

When the pistol moves back from the higher force on the chamber side, the hole in the back cap becomes unsealed and the pressure behind the piston is relieved, making a higher flow valve.
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Unread postAuthor: JDP12 » Fri Aug 06, 2010 4:27 pm

Yup. I picked up some high pressure springs today, with wire thickness of .140 and another smaller one that I'll put inside it with wire diameter of .080" and am thinking of trying that instead of rubber, as I don't have a huge selection of rubber. Thoughts?
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Unread postAuthor: SpudBlaster15 » Fri Aug 06, 2010 6:14 pm

A spring will work, as long as you can absorb all of the piston's energy before the spring fully compresses. Otherwise, the steel coils will permanently bend and/or break.

I would use rubber however, as it is less likely to cause the piston to rebound into the seat with sufficient force to damage the valve's components. It's also much easier to vent the combustion chamber when the piston remains open after firing.
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Unread postAuthor: JDP12 » Fri Aug 06, 2010 6:44 pm

yea. i think I'll pick up some large diameter rubber air tubing and just slide it in there, similar to what you have. Think it should compress alright-- What do you think?
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Unread postAuthor: SpudBlaster15 » Fri Aug 06, 2010 7:00 pm

That should work well.
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