Sonic Choke? Maybe not. 2162 FPS

[Buldawg76]

Scott
Have a burger and Mai Tai for me and don't get sunburned.

Mike

[lloyd-ss]

I'll always take a good burger over a mediocre steak.   

------------------------------------

I need some help with o-ring material selection, please.   

This is fitting in a smooth bore  (.245 dia) and I plan on using what might be called a "too small" o-ring. I want to stretch it over the over-sized center post of the projectile so that the O.D. of the o-ring is tight and will resist rolling over.  I might have to experiment with fit, but probably about 7 to 9% compression,  which has worked so far in this experiment. Normal compressions are closer to 20%.

Size 005 o-ring .101 I.D. x .241 O.D.  over a .118 center post will give about .0635 gland depth in a .245 bore.  Compression will be approx 8% after accounting for the thinning due to stretching on the over-sized center post.
But the fit is an experimental thing, and I can putz with that.

Now, for the material selection and I am hoping you folks know more than I do about this.  This o-ring needs to do things that o-rings were never intended to do, but these are one-shot usages, so they can get smoked out the barrel. 

The O-Ring material needs to be:
Low friction for sliding along the smooth wall.
Heat resistant.  The friction will probably burn or melt it.
Stretchy. So that it can be stretched over the head of the pellet.
Abrasion/wear resistant so that it will hold a seal all the way to the end of the barrel.
Must be available  in small qtys (100 or so) without a $$$$ minimum order. It doesn't do any good if I can't buy it.
What do you think? Help! 

Edit--
After some research, am leaning toward Aflas (FEPM) or Viton (FKM, Fluorocarbon).  Lloyd

[rsterne]

          

I got nuthin'....

Bob

[Buldawg76]

With you there Lloyd and if its going to be steak its got to be a Ribeye or better yet Prime Rib.

Mike

[match]

viton?

but you may need to make teflon rings and press them on....

[Buldawg76]

I will second the Viton or polyurethane o rings.

Or buy some delrin round bar from Mcmaster Carr and machine what you need.

Here is a link to 005 urethane o rings on ebay .

http://www.ebay.com/itm/Captain-O-Ring-Polyurethane-Oring-005-90A-Durometer-25-Pack-/171804395797

Mike

[lloyd-ss]

viton?

but you may need to make teflon rings and press them on....

Match, thanks for your thoughts on this.
For the 1745fps, I just used the standard 70 duro Buna-N, but looking at them under magnification, they show wear from the rifling, but they are still intact after a single shot. Definitely not good enough for a second shot, though.

Viton is a Fluorocarbon and is good for heat, but fair for wear.  I don't know about friction.  In a 90 duro it might work.  You have some doubts about the Viton? Please let me know if you think they are not worth trying.

The Aflas looks good for heat, wear, and friction, so it seems like a good choice, but McMaster doesn't have them in size 005.

The teflon would be great for heat, friction, and wear, but I have never had good luck getting Teflon to seal when used as a standard o-ring, and I have my doubts about it sealing at 4500psi.   I think the pellet would have to be a 2-piece configuration to get the teflon ring installed, and I wanted to simplify the construction because of the number of shots I will be taking.

So, do you think the 90 duro Viton are not up to the task?
Thanks,
Lloyd

[I_like_Irons]

I have dealt with larger o-rings that have a PTFE sleeve, but nothing that small. 

What about a normal Buna-N o-ring.  Just lubricate the bore first.  I'd use a barrel cleaning mop with a silicone oil, or grease.

ETA:  The advantage going this route is that you lower the friction (which will be something that all o-rings) will have.  Also, the coating on the o-ring will tend to absorb heat. 

The Buna-N o-rings are inexpensive enough that you can change them for each shot easily.

[lloyd-ss]

The Buna-N are certainly cheap enough and I have plenty of them on hand.  For the 1745 shot, I did lube the bore with Crosman silicone chamber oil. Maybe I should just try that first before making it more complicated than it needs to be.

[rsterne]

K.I.S.S.... and cheap!....

Bob

[rsterne]

Dupont Krytox may be a suitable lubricant for you to run a patch down the barrel.... GPL 205, or possibly another one....

http://www2.dupont.com/Lubricants/en_US/assets/downloads/K-17456-3,%20Automotive%20Product%20Guide.pdf

Non-flammable, good to 200*C....

Bob

[lloyd-ss]

I just happen to have a bit of the Krytox! I'll give it a try.

[rgb1]

Lloyd, for the shots mentioned in reply 123, what was the
calculated residual pressure? How are real gas effects
being taken into account?

Lloyd or Bob, in reply 141, how is efficiency being calculated?

                                                              Ron

[match]

So, do you think the 90 duro Viton are not up to the task?

90 duro viton is exactly what I was thinking - the question mark was because I was not sure if they were available that hard in that size... but found it: https://www.globaloring.com/o-rings/specialty-o-rings/rgd-o-rings/v90rgd005-005-viton-rgd-o-ring.html

75 duro may be good enough in a smooth-bore

Viton is little more expensive than buna-n - you should try the 90 duro buna-n if available and just use them once.

My only concern with buna-n is combustion of the surface in the barrel - leaving residue and messing up the measurements.... this may not be an issue with a lubed smooth-bore.

Be careful of what you use in the bore - I would tend toward moly or graphite but lithium grease may even be an option.... don't know.

[lloyd-ss]

Lloyd, for the shots mentioned in reply 123, what was the
calculated residual pressure? How are real gas effects
being taken into account?

Lloyd or Bob, in reply 141, how is efficiency being calculated?

                                                              Ron

Ron, I am guessing that you mean the shots in reply# 133. 
The residual pressure calculates to 2888 psi for both the 1745and 1714fps shots.

I am not sure what you are asking in the question about the real gas effects, but if you clarify a bit I'll see what answers I can supply.

The efficiency mentioned in reply #141 is the "system efficiency factor" used in the spreadsheet that equates to the rather nebulous system losses for any unique PCP set-up at a particular power and velocity.  That is applied in the force equation in the spreadsheet after the other effects of pellet weight, expanding volume, declining pressure, etc, are all accounted for.  It is the illusive part of the calculation that much of this effort is focused on finding.  The efficiency factor mentioned is NOT the same as the usual air efficiency which is "FPE produced per standard cubic inch of air that is used by the shot".

Lloyd

[lloyd-ss]

So, do you think the 90 duro Viton are not up to the task?

90 duro viton is exactly what I was thinking - the question mark was because I was not sure if they were available that hard in that size... but found it: https://www.globaloring.com/o-rings/specialty-o-rings/rgd-o-rings/v90rgd005-005-viton-rgd-o-ring.html

75 duro may be good enough in a smooth-bore

Viton is little more expensive than buna-n - you should try the 90 duro buna-n if available and just use them once.

My only concern with buna-n is combustion of the surface in the barrel - leaving residue and messing up the measurements.... this may not be an issue with a lubed smooth-bore.

Be careful of what you use in the bore - I would tend toward moly or graphite but lithium grease may even be an option.... don't know.

John,
Thanks for that link to the o-ring site and the other suggestions. 
I have tried the size 005 90 duro Buna-N o-ring and wasn't able to fit it over the head of the pellet without it breaking.  The 90 duro in the small sizes are not very stretchy.  I wonder if the 90 duro Viton in 005 is more stretchy.  I haven't used any Viton o-rings before, except in carburetors, and that was a long time ago.

I remember talking to an application engineer from Seal Science several years ago, who has since retired.  He said that material compatibility is much more important in dynamic o-ring applications than most people think.  I am sure that is true in this application, but given the one-shot nature of the application, getting "close" will hopefully be good enough, but I am sure that the FPS difference between a "close" application, and a "perfect" application would be significant.
Lloyd
Lloyd

[rgb1]

Lloyd , regarding gas expansion what equation of state is being used?
There are at least a half a dozen cubic versions, van der Waals being
the most notable.


                                                                                      Ron

[rsterne]

Ron, the "efficiency factor" is simply a "fudge factor" present in Lloyds PCP Internal Ballistics spreadsheet that allows it to balance with empirical data.... Once you determine that number (by trial and error) for one set of conditions in a given gun, it tends to work for other conditions, within reason.... For the most part, PCPs run about 60-75% efficiency, and the surprising thing is that Lloyd's test gun isn't that far off.... I see Lloyd explained it already....

Bob

[match]

 
I have tried the size 005 90 duro Buna-N o-ring and wasn't able to fit it over the head of the pellet without it breaking.  The 90 duro in the small sizes are not very stretchy.  I wonder if the 90 duro Viton in 005 is more stretchy. 

did you try heat?

drop it in a pot of boiling hot water for a few minutes....

[Scotchmo]

I'm home from the cruise. I'm looking forward to reading about the next test:

.245 cal
6.5 gn
4500 psi
36cc dump shot
47"

My spreadsheet shows about 2060fps using the .22 caliber fluid friction formula and 1 lb. pellet friction. My best guess is that fluid friction will also be proportional to caliber. Doing so yields about 2020fps in the spreadsheet. If the .245 caliber is later tested at different barrel lengths and pressures, we can see if it holds true. Either way, if we see a little over 2000fps, we should be able to predict about what's needed for 2200fps, 2500fps, or 3000fps. And it won't be pretty.

As far as efficiency, there are many ways to calculate it. Instead of using a "fudge factor", I'm trying to account for all losses.
Overall efficiency (pellet energy/air energy) is very low.