Blow Out "Plug" for jammed gun 4

[kmount]

I'm trying to design a blow-out plug for a gun to relieve chamber pressure if the projectile jams the gun barrel. I have thoroughly read the Machinery Handbook as well as other various materials and can't find a set of consistent equations for thread shear. This problem seems relatively simple, and I feel like I've confused myself by reading too much into it. I need a bolt whose threads shear (assume bolt much weaker material than chamber, ie brass vs hardened 4340 steel) @ 100,000psi chamber pressure. Most of the equations in the literature are for "safe design", not for purposeful destruction. I'm also concerned with load sharing in the threads; its been my experience that the 1st thread bears the most load and failure subsequently engages the next thread. Additionally, how would initial torquing of the bolt affect its ultimate failure? Any assistance with this problem would be greatly appreciated. The plug diameter can be as large as 1" and the length of thread engagement as long as 1.25".

 

[ornerynorsk]

Would a blowout disk be more suitable? I'm familiar with many bolt action rifles that have a pressure relief hole drilled through the action behind the locking lugs in event of such a blow out. Scary stuff here, you're talking some high pressures, in excess of 50k.

 

[kmount]

To ease your mind, the gun is an overdesigned, multi-barreled R&D gun that operates around 80ksi. Testing on a range. The problem arises because a switch was made from steel to combustible cartridges, and there is fear that the chamber would basically become a pressure vessel if the projectiles didn't find their barrels. We have thought of using a disc, but I don't see how a disc could withstand the high pressures.

 

[strokersix]

kmount,

How about a spring loaded pop-off valve? This might be easier to design and probably more accurate than shearing threads.

As stated by ornerynorsk; be careful.

 

[EdStainless]

This will take trial and error calibration. I suppose that you want to reach 80ksi but not exceed 100ksi, or something like that.

Pressure reactors that I used to use had small diameter blow out discs in them. They were only about 3/8" diameter. They were domed and seated against a large step. They were replaced every time we had an 'event'.
In your case that might mean every shot.

A spring loaded relief sounds good, but I don't think that it will work. If the pressure loaded area is 0.250" diameter then you only need a spring with about 5,000 lb of load. That is a tall order.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[kmount]

Thank you guys for the advice. We are certainly sensitive to "events" in my field and therefore the gun is encased in an inch thick steel housing and fired remotely from a distance behind steel walls.

I will put some thought into the idea of a blow out disc and bounce it off the other guys I work with. It obviously has a mass advantage over a threaded plug in the sense of ballistic ejection. I think I'll throw it into an FEA run and see what pops out the other side.

 

[kmount]

Did some research on rupture discs with the FIKE Co. and Continental Disc. Seems that the major application for these types of pressure relief devices are in the low pressure range. I'll give them a call tomorrow and see if they have anything that can handle 100ksi.

 

[unclesyd]

I believe you can get a valve to operate at your pressures. The valve would be opened (remotely) only if an excursion occurred. I have seen small vent valves that work at 55,000 psi. That look at Ultra High Pressure Water systems. I know they are operational at 77,000 psi.

This system must have a very quick acting valve.

http://www.tempresstech.com/watercannon.htm

 

[Screwman]

The calculations that you want are in Bickford. It will give you the strength level and/or length of engagement for different material strengths and thread fits.

I don't have a copy with me right now, I think that this was covered in a previous thread. Try searching for thread shear calculation.

Dick

 

[unclesyd]

You might want to talk to these people about a vent valve for the system. Their system helps with the thread situation both with the stresses and the reusability of the connection.

http://www.highpressure.com

 

[unclesyd]

Here is thread form you might want to look at since it distributes the load on the threads a lot better than standard form. The good part is that they accommodate reuse.

http://www.spiralock.com/

Also you may want to look at the thread arrangement on my previous post.


Here is some good information on thread stripping.

http://www.hexagon.de/dose/dose1e.ht

Look at lesson 28

http://www.utm.edu/departments/engin/lemaster/machine_design.htm

 

[unclesyd]

Do you want bursting, relieving or maybe venting.

http://users.rcn.com/harwood.ma.ultranet/safety.html

http://users.rcn.com/harwood.ma.ultranet/releavlv.html

 

[kmount]

Thanks unclesyd for your suggestions...I think Harwood may have a feasible solution for us, I'll look into them further.

 

[arto]

Helicoils also distribute the load out & give a hardened [& replace-able] thread to minimize wear - not pressure-tight though.