GTA
All Springer/NP/PCP Air Gun Discussion General => "Bob and Lloyds Workshop" => Topic started by: rsterne on December 22, 2014, 06:56:29 PM
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After reading the idea of using Belleville Spring Washers to tension a barrel on the GTA (thanks, Cal), I decided to draw up something to show how it could be done.... The idea is to thread the end of the barrel (I chose 1/2"-20 NF because that is a common size for barrel attachments) and use 1/2" ID x 1" OD Belleville washers to provide an adjustable tension in the barrel to allow tuning it.... The compression load is taken by a shroud, transmitted to the breech, so the shroud has to be strong enough in compression and the barrel mounted solidly enough in the breech to stand the tension load, which can be considerable.... If, for example, you use 0.050" thick Bellevilles, they have a load when compressed flat of 600 lbs.... A stack of three of them, as shown, has a total travel of 0.075", and since 20 TPI is 0.050" per turn, if you start about hand tight (1/4 turn) and use 1 turn of adjustment, you can adjust the tension from about 100 to 500 lbs.... Here is the general idea....
(http://i378.photobucket.com/albums/oo221/rsterne/Parts%20for%20Sale/BarrelTensioner_zps47d42489.jpg) (http://s378.photobucket.com/user/rsterne/media/Parts%20for%20Sale/BarrelTensioner_zps47d42489.jpg.html)
The red is a bushing in the end of the shroud (blue) to take the load from the Bellevilles, it slides on the barrel (black).... The adjuster (purple) threads on to the barrel, and the Bellevilles are captive inside a chamber for appearance and to keep them clean.... You could have an index mark of the top of the shroud, and a vernier scale around the adjustment collar so that you can return to accurate settings while tuning.... There is a pair of flats machined on the adjuster so that you can use a 3/4" wrench to tighten it.... If you want more tension, just double up on the Belleville stack, (( )) (( to go from 200-1000 lbs. in that same 1 turn of adjustment.... just make sure everything is strong enough.... Different thickness Bellevilles can be used to achieve different adjustment ranges of course.... I think this is worth a try....
Bob
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What exactly is the purpose of the bellevilles as opposed to simply using the half in thread on fitting and a torque wrench to set the desired tension?
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Pretty neat. It poses a certain issue if you want to use the shroud volume for anything. I wonder how much more repeatable the tensioning would be v. the torque wrench.
cheers,
Douglas
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Using a torque wrench would depend on thread lubrication to be consistent or the tension would vary for a given torque setting.... Also, the torque would be under about 100 inch-lbs. for a 1/2" OD dry thread (less if lubricated), that represents about 1000 lbs. tension.... so a torque wrench in ft.lb. would not be accurate enough....
Yes I realize it doesn't allow using the shroud volume, but you can't really do that if you are using the shroud to apply barrel tension anyway, right?.... The choice between accuracy and quiet is easy north of the border anyway, all guns have to be noisy....
ADDITION: From QVTom, he thinks about 800 lbs. tension worked on his .30 cal Rainstorm, so we probably need the double Belleville stack, giving 200-1000 lbs. adjustment over 1 turn....
(( )) ((
Bob
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I'm just thinking that shot cycle vibrations and bellevilles are going to lead to oscillation in the tension value during the shot.
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The goal is to find a tension that improves accuracy.... If you can't find it, what are you out?.... a few dollars and a bit of work.... It's sure a lot cheaper than another barrel, IMO....
Bob
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Nice design and cad work Bob. I will save this one to my best of bob folder. The best thing about winter is that you have more time for r&d.
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Nice. I might be doing something with a new mrod barrel. I might conserder something like it
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What exactly is the purpose of the bellevilles as opposed to simply using the half in thread on fitting and a torque wrench to set the desired tension?
To my thinking, Temperature and "conditions" compensation is the big plus to using a known compliance element in the tension. (cone washers)
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So Bob,
When pressure is applied, is it just pulling on the barrel, kind of like its trying to stretch it? Which I would guess keep a constant pressure on it and keep down certain vibrations and such!
What I see is, the shroud is secured to the other end of the barrel or the breech and the Belleville Spring Washers are pushing or compressing against the shroud and in turn the adjuster on the muzzle is pulling on the barrel!
I think I see the concept now, and looks like a rather good one at that!
Took me a minute, but I got it...it's a barrel stretcher ::)
William
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YUP!.... stretch the barrel by pushing on the shroud.... barrel in tension, shroud in compression, loads at the other end taken at the breech....
I agree, Cal, using the Bellevilles will provide a much more uniform tension than just torqueing, over a wider range of temperatures.... without them, differences in the rate of expansion could nearly eliminate all the tension on the barrel if the barrel expanded more than the shroud.... That's another good reason not to just use a single Belleville, a small change in length would change the tension 3 times as much as having 3 of them.... That is also a good reason to use the "softest" Bellevilles you can for a given amount of tension required.... If you need 400 lbs. of tension and you use 3 Bellevilles that can provide 600 lbs. in 0.075" of travel you will get a much smaller change in tension than you would if you used 3 stiffer Bellevilles that can provide 1335 lbs. of tension in 0.054" of travel (the next ones stiffer).... If you get a relative length change of 0.005", the tension on the 600 lb. Bellevilles will change about 10% (40 lbs.).... but with the 1335 lb. Bellevilles it would change over 30% (125 lbs.).... With no Bellevilles, it would change even more, of course....
Bob
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Hi Bob,
I have found that removing even a small amount of material (threading) from the OD of most barrels will open up the ID. Don't know if that is an issue with your barrel or not.......just mentioning it, as the muzzle can be a bad place to loosen things up.
Mike
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I'll watch that, thanks....
Bob
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Turning the OD of a barrel could cause the bore to open up on a barrel that was previously shooting accurately.
Now, are you going to 'single point' the barrel threads or use a 1/2 die?
Depending on the wall thickness, using a die can actually add choke to a barrel.
Just throwing that out there ... And watching the developments closely.
Cheers,
Smoketown
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Just an update.... I have ordered some Bellevilles from McMaster Carr, and also some tubing to try out this idea.... I will be using my Disco Double as the test bed, using a 1/2"OD TJ's barrel in .25 cal and 1" Bellevilles.... and a .22 cal Crosman barrel (the usual 7/16" OD) with 7/8" Bellevilles, and a piece of 7/8" OD x 0.058" wall 6061-T6 aluminum tubing for the shroud/compression member.... I am sleeving the 7/16" Crosman barrel up to 1/2" to fit the breech and I will be using at least three 10-32 setscrews in drilled pockets to hold the barrels in place.... The barrels were already threaded with 1/2"-20NF and 7/16"-20NF threads for air strippers.... I'll report back when I have some results one way or the other....
Bob
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I am doing this, with a slightly larger shroud, the pusher nut (threaded piece on the muzzle) have been drilled with 8 holes through to the shroud cavity between the breech and muzzle. But essentially do the same thing as you've outlined in your first post Bob, and it works.
So with some careful planning you can utilize the shroud as intended, while still using it as a ram against the breech to put tension on the barrel.
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Curious as to how you get the air past the Belleville Washers?.... They are typically at least twice as large OD as ID, so for a 1/2" barrel at least 1" OD.... That works great for pushing on a 1" shroud, not so good for allowing air into it....
I have the .22 cal Crosman barrel mounted on my Disco Double, and tethered it to a regulator at 1800 psi, fed from a SCUBA tank, so the velocity is stable (1% ES).... I had to back off the hammer strike a LONG ways before the velocity dropped from the plateau at 1070 fps with 18 gr. JSBs, but now have it shooting at 1000 fps, and I can dial it down to 600ish with the port restrictor.... I've noticed some interesting things going on with the POI as I change the velocity with just that 24" barrel free floated.... I'll report on that when I do my baseline testing....
Bob
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Hi Bob,
I have found that removing even a small amount of material (threading) from the OD of most barrels will open up the ID. Don't know if that is an issue with your barrel or not.......just mentioning it, as the muzzle can be a bad place to loosen things up.
Mike
I've threaded quite a few TJ liners in 22 30 and 35 caliber, all are 1/2" OD and have not observed and change in bore diameter (measured with gauge pins). I use a 1/2-28 thread with a full profile insert in the lathe.
Tom
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Bob, I dont use washers, so your point is well taken.
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I like the washer idea too. I'm glad Bob is taking the lead on this idea....... By using a steel shroud I've minimized thermal expansion effects but a nice light carbon or aluminum shroud would sure shed a few ounces.
Tom
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Tom, I checked the thermal expansion coefficients, and interestingly Aluminum is almost identical to Chrome Moly.... and both are about twice what steel is.... CF is almost 10 times more stable....
Aluminum - 13.0 PPM / *F
4140 - 13.7 PPM / *F
Steel - 6.7 PPM / *F
CF - 1-2 PPM / *F
It would seem to me that an aluminum shroud on a Cr.Moly barrel would work just fine....
Bob
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Good info. I assumed that the CM was similar to steel not aluminum. It pays to do your homework! I'll probably stick with the CM shroud for now for its additional stiffness and its much quieter than an aluminum shroud.
Tom
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I did some in interesting testing today on my fully floated .22 cal Crosman barrel that I just fitted to my Disco Double, in preparation for trying out the Belleville washer barrel tensioner system proposed above.... I tethered the gun to a regulator at 1800 psi, and reduced the hammer strike until the velocity with the 18 gr. JSB Heavies dropped off the plateau at 1070 fps.... I had to back off the preload a LONG ways before it lost any velocity, but eventually, with the hammer spring rattlilng around loose, I got the velocity down to 1000 fps, with less than a 1% ES, usually about 5-6 fps over a 5 shot group.... The gun has an adjusting screw on the back of the breech that controls how far the bolt retracts across the barrel port (it's a probeless, retractable design), and there is 7 turns on the adjuster to go from closed to fully open.... The last 2 turns made no difference, so at 1 turn increments I ended up with 6 settings, at 0 through 5 turns out from closed, yielding the following FPE levels, with the heavier pellets having a slighty higher FPE, as expected.... I tested 7 pellets, from 15.9 to 25.3 gr....
5T: 38.6 - 40.6 FPE
4T: 36.5 - 38.3 FPE
3T: 32.6 - 34.8 FPE
2T: 26.8 - 29.0 FPE
1T: 19.7 - 21.6 FPE
0T: 14.4 - 15.9 FPE
I have a target with 42 small scope targets on it, which worked out perfect, 7 columns (pellets) and 6 rows (velocities), and I shot 5 shots per group, through the Chrony, at my normal indoor range of 20 feet.... Even at that distance, the groups moved around as the velocity changed, which I believe is due to barrel harmonics.... The smallest groups were 1/4" OD, and all but four were under 3/8" OD, and those four were still one ragged hole, well under 1/2".... The target has a grid on it of 0.10" squares, so I recorded the estimated center position of each group both vertically and horizontally, using visual inspection, to what I feel is an accuracy of plus or minus 0.02".... I then calculated the average group center for each pellet, over the six different velocities, and then plotted how far the group center was from that average position vs. the velocity, with the following results.... On the horizontal graph, positive values are to the right, negative indicate the center of the group was shifted left from the average....
(http://i378.photobucket.com/albums/oo221/rsterne/PCP%20Internal%20Ballistics/BarrelHarmonicsCrosmanFloated_zps1e9524a2.jpg) (http://s378.photobucket.com/user/rsterne/media/PCP%20Internal%20Ballistics/BarrelHarmonicsCrosmanFloated_zps1e9524a2.jpg.html)
At first glance it looks pretty confusing, but I see a pattern that indicates that the barrel is waving around a fair amount, particularly side to side.... Look at the lower graph first, and you will notice that at high and low velocity, the pellets are hitting to the right, and at around 800 fps, they are hitting to the left.... The groups are moving side to side about 1/8", which works out to about 2 MOA.... This has nothing to do with how tight the groups are, just where the POI is shifting as the velocity changes.... The upper graph, showing the vertical movement of the groups, has only half the movement, and it's possible that I'm seeing things, but I think there is a pattern where most of the groups are high at 600 fps, low at 700, high again at 800, low again at 900, and then moving up again at 1000 fps.... I'm wondering if the primary movement side to side is the fundamental harmonic and in the vertical plane it is the second harmonic....
At the moment, this data is just a curiousity.... The important thing is whether or not the group sizes shrink when the barrel is placed in tension, and also whether or not the POI becomes more stable.... While not really important as we don't shoot over a wide range of velocities, I think it may be useful as proof about whether we tamed the movement of the barrel or not.... I'm looking forward to the rest of the testing....
Bob
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Excellent work Bob,
I am very curious as to see if those graphs level out with tension on the barrel, as well as just how much tension it takes to level them out!
Can you imagine the difference in POI at longer distance!
I will be waiting on the results thats for sure!
Once you read the explanation and look at the graphs I can actually envision the pellet groups on paper!
Thank you,
William
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I'm always looking for new sources for carbon Fiber tubes. I found this site that has very good prices and a great selection.
http://www.acpsales.com/Carbon-Tubes.html (http://www.acpsales.com/Carbon-Tubes.html)
I was reading the spec sheets on the 3 different tubes and had a question.
I think one of the important number to look at is the Tensile Modulus of the tube. Please correct me if I'm wrong. The higher the number the stiffer the material which will be beneficial for our purpose.
I noticed that the UNI wrap and the Twill wrap tube had a T.M of 17 msi but the pultruded tube has a T.M of 19.5. This will make it stiffer and better for our applications. Both Warp tubes are A higher tensile strength but that is not the post important part for us.
I just find this interesting specially because the cost of the pultruded tube is less significantly less that the other options. Also notice that they have twill tubes that would slide that would slide perfectly over the pultruded tube to make a tensioner.
Daniel
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On quick inspection it looks like the periods vertical are comparable to horizontal...can you do Horiz^2 + Vertical^2=Distance^2 and plot Distance v. velocity?
cheers,
Douglas
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Rsterne:
Shooting at different speeds and plotting the poi shifts shows a few things, or dynamic relationships between how much barrel movement there is, and how long the pellet is in the barrel, i.e. how effected the pellet is by the barrel movement. Obviously at the lower FPS it stays longer in the barrel (relative) to the high FPS.
I am sure there is a relationship between the FPS/Barrel Length/Amount of POI shift in any given barrel.
Looking at the graph it seems that the lower speeds throws the POI further of centerline than the faster FPS.
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Douglas, if the movement of the barrel is circular (as one might expect?) doing what you suggest would leave the deflection constant in quantity, but varying in direction.... Unfortunately, squaring the quantities makes them all positive, losing that polar plot possibility.... I think that the methodology is simply not accurate enough given the range (small measurements) and the difficulty in finding the group center.... In some ways, the results seem clearer if you just look at one pellet instead of all seven.... I noticed this phenomenon when setting up the velocities with the 18 gr. JSB Heavy, as it consistently shot left at 834 fps and right at 613 fps.... I thought that perhaps by testing a bunch of different pellets I might see a pattern throughout, but it's hard to discern (especially the vertical movement) from the noise....
Tomg, every barrel will have a different resonant frequency.... At it's simplest (the fundamental frequency), the muzzle is just whipping back and forth (or perhaps in a circle or ellipse).... and when the pellet arrives at the end (primarily a function of velocity, which is the time in the barrel) it gets spit out in whatever direction the muzzle is pointing at that time.... I'm pretty sure that is what we are seeing in the horizontal graph, the muzzle is pointing right at ~ 650 fps and ~ 950 fps and left at about ~ 830 fps.... It is going left again below 500 fps, and seems to be going left again at around 1000 fps.... The vertical is harder to make out, except that the barrel seems to be pointing upwards at ~ 830 fps, low at ~ 950 fps, and moving up again at around 1000 fps.... So combining those, it looks like the barrel is moving mostly from upper left to lower right when it whips back and forth....
Bob
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I'm a slow leak at best, but is it possible that the projectiles mass "spinning" causes an action against the barrel? Maybe the mass of the projectile is "attempting" to straighten out the twists in the barrel it's causing an opposite reaction within the barrel? Of course we all know it will never be able to do that, but the projectiles mass beginning to spin could be causing the barrel to vibrate is one direction over another at different speeds? Could it be possible that the mass of the projectile is attempting to "torque" the barrel in some way? :-)
Or has my cheese completely slipped off my cracker :-) hehehe
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Alright...looks like 5 of the pellets show a similar behavior( the green yellow and three shades of blue). These are the mid-weight pellets. In the vertical plot, is the low point happening at the same delta from sear release? I figure Lloyd's spreadsheet should be able to spit out that number...LOL
Also the heaviest is showing the most radical d(POI)/dVel at the upper end of achievable energy.
I did not make the assumption the barrel tip motion is circular. the rifle mass distribution gives me no inclination to believe the excitation is circular, or that the reaction would result in circular motion.
cheers,
Douglas
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rsterne:
Interesting correlation with the 880 fps = best accuracy fable.
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Actually, looking at the groups, although in the mid 800s they were among the best with each pellet, some of the best groups occurred at 1000 fps and over, and the rest were mostly in the mid 900s.... However a few good groups occurred at other velocities as well.... The differences between pellets were generally larger than the difference between velocities with the same pellet.... All this data looks at is the position of the center of the group, not the size of it.... That will have to wait until the spring when I can get outside to test at longer ranges....
Bob
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That will have to wait until the spring when I can get outside to test at longer ranges....
such a tease..............
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I use to shoot .22 benchrest and just about everyone had a barrel tuner of some sort on their rifles. One of the most popular is the Harrels style tuner. Very simple to use and additional weights could be added to the end of the tuner if needed. The barrel does not have to be threaded. The part of the tuner that attaches to the barrel would probably need to be machined to fit the barrel. After the tuner is installed, most people I know used the Hopewell method for tuning at that point. It can be very frustrating when tuning and you have to walk away for awhile.
http://www.killoughshootingsports.com/index.php/tuners/harrel-style-muzzle-tuners-sil.html (http://www.killoughshootingsports.com/index.php/tuners/harrel-style-muzzle-tuners-sil.html)
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Actually, looking at the groups, although in the mid 800s they were among the best with each pellet, some of the best groups occurred at 1000 fps and over, and the rest were mostly in the mid 900s.... However a few good groups occurred at other velocities as well.... The differences between pellets were generally larger than the difference between velocities with the same pellet.... All this data looks at is the position of the center of the group, not the size of it.... That will have to wait until the spring when I can get outside to test at longer ranges....
Bob
Or visit us here in Phoenix....we can do those things year round....lol
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That "tuner" sounds very much like the "Browning Boss" system.
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That "tuner" sounds very much like the "Browning Boss" system.
The Browning Boss is indeed similar to a barrel tuner. Just a different name. Some people even use a devise that attaches to the barrel in different places. Two or three can be used. These are moved up and down the barrel until the desire group is found. The big difference is when using ammo and you change brands, weight, etc it will have to be retuned because the harmonics of the barrel change again with the new load. When shooting benchrest we would find an Eley round that worked well and then buy as much as the company would sell us. That way it was all the same lot number and we didn't have to retune as much. The same holds true with pellets. Some brands shoot outstanding and others you can't give away, but you may come across some that have potential. With the tuner, you should be able to dial it in as needed.
Here is some more information on the Browning Boss; http://www.browning.com/customerservice/qna/detail.asp?id=108 (http://www.browning.com/customerservice/qna/detail.asp?id=108)
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Good stuff...
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To still have a fully functioning shroud, I'd just make the bushing/Belleville stack at the rear of the tube by the breech so when you thread the shroud on to the muzzle the bushing will push the Belleville stack against the breech or barrel band.
Same concept and operation but it leaves the business end of the shroud open for baffles and venting.
;)
Mike
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Except that the shroud, sitting against the Bellevilles (which are curved) can now wiggle around against the breech instead of bearing on it with its full diameter.... Yes, the barrel will still be in tension and the shroud in compression, but the joint between the assembly and the breech is only the barrel diameter (or slightly larger), the way I look at it.... Kind of like sleeving the barrel with a huge OD but leaving the sleeve with a gap between it and the breech.... You have a club mounted on a hinge, which would suppress all the harmonics but leave the fundamental (muzzle waving) frequency....
Bob
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Regarding the two barrel tuners mentioned, I believe they (both) work by moving a weight back and forth to change the fundamental frequency of the barrel.... This is a valuable tuning tool, and as mentioned, any change of bullet or velocity will require a change in setting.... I used a series of seven simple muzzle weights on a Crosman barrel, in 5 gram increments, and for any given pellet, changing the weight would change both the POI and group size.... I would select the weight that produced the tightest group with the pellet I was using at the velocity I was using, and it could make the difference between a 1/4" and 1/2" group at just 20 feet.... Unfortunately, changing either the pellet or velocity required going through the whole procedure again, usually with different results....
The idea behind the adjustable barrel tensioner (yet unproven by me, but used successfully by others) is to increase the vibration frequency while decreasing the amplitude.... This should minimize the difference between pellets and velocities (in theory) in the same manner as a bull barrel does (larger OD, therefore stiffer).... Further improvements could likely be found with a tuner as above.... Why not combine the two, and make the fixed part of the tuner the "nut" that places the Bellevilles in compression?.... You increase the tension until the barrel vibrations are minimized, and then tune out what is left.... Seems logical to me....
Bob
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Take my shrouded Millennium Pumpers for example...what do you think if I made a stepped barrel band for the breech shoulder - it could clamp down on the breech shoulder then the end plug of the CF tube that holds the Belleville stack could have another shoulder (maybe .800" OD) at the same time.
That would get rid of the pivot point between the barrel & breech.
What do you think Bob?
I looked for Belleville's with an OD of 1 1/8" and ID of .750" but didn't see any...would it affect the washers performance if we machined the ID to .755"?
I *really* like this idea...it adds some measurable adjustability to the shroud tension.
There are 3 rifles being built with 20" barrels (mine + 2 customers) and a 1.25" OD CF shroud over the already CF sleeved barrels @ .750" OD.
The shroud floats freely inside the caps, yes? It simply held in place under tension between the breech and muzzle?
TIA
Mike
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If the ID and OD of the Bellevilles are too close, they can fail (flatten) as I understand it.... 2:1 seems common, or larger....
I'm not sure how you could contain the shroud and yet allow the Bellevilles to work.... Since they have to sit convex to convex, they can rock sideways.... If that happens, then the barrel diameter where it exits the breech is the governing factor in stiffness for the fundamental frequency (barrel whipping sideways)....
Bob
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I was thinking an end cap (not a plug) for the breech side of the shroud.
The cap would have an ID that matched the OD of the CF shroud...the cap would have a reduced diameter shoulder (.750" ID x .850" OD) that has an o ring gland inside (like an Mrod barrel for the bolt o ring) and would be a slip fit over the barrel.
The breech/barrel/tube band would clamp onto the breech, shroud end and tube and be 'fixed' in place by the band.
The washers would slip over the barrel, into the cup followed by the CF shroud...the CF shroud would slip into the cup and compress the washers in place when the LDC end of the shroud is screwed onto the muzzle.
With this design I don't see how it would be any different than a muzzle install version as everything is just as rigid as a CF sleeve that slides onto a breech shoulder...I've just moved the washers to the other side of the barrel so we have a ventable area for the LDC/Shroud to be effective.
Am I missing something or looking at this the wrong way?
I will try it on my personal gun for R&D...I don't see how it would negatively affect the barrel/shroud/tension relationship...I would think the barrel band being *clamped* down by a 8-32 screw like BKL mounts would solve the pivot junction issues at the barrel & breech.
:P
Mike
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You lost me, Mike.... How can the shroud be free to move forward to apply tension to the barrel and yet be fixed in place at the breech end?....
Bob
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There would be roughly 1/4" of muzzle threads that the LDC end has to engage on the muzzle before the CF shroud hits the washers in the cup at the other end - leaving 1/4" movement to the rear before the muzzle threads bottom out.
The LDC end of the CF shroud is permanently attached to the insert that threads onto the barrel.
The breech side into the cap with washers would be able to slide in and out thereby adjusting the washer tension on the shroud/barrel.
I wish I had a way to draw a picture...
I will e mail you some pics or post them here when I have one assembled...
Mike
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Great idea Bob
wonder if we can add an air stripper ?
might be a new super add on for all serious shooters
Why not combine the two, and make the fixed part of the tuner the "nut" that places the Bellevilles in compression?.... You increase the tension until the barrel vibrations are minimized, and then tune out what is left.... Seems logical to me....
Bob
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Except that the shroud, sitting against the Bellevilles (which are curved) can now wiggle around against the breech instead of bearing on it with its full diameter.... Yes, the barrel will still be in tension and the shroud in compression, but the joint between the assembly and the breech is only the barrel diameter (or slightly larger), the way I look at it.... Kind of like sleeving the barrel with a huge OD but leaving the sleeve with a gap between it and the breech.... You have a club mounted on a hinge, which would suppress all the harmonics but leave the fundamental (muzzle waving) frequency....
Bob
Bob, I think that this scenario you present will be the case no matter what end the springs go on. The 'hinge' will still function because the springs allow the shroud to compress them as the barrel attempts to wave back and forth.
cheers,
Douglas
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Bob,
Here is what I thought up this morning.
Instead of using the shroud as the ram, you insert a free floating "tight" sleeve to act as the ram.
(http://i54.photobucket.com/albums/g97/renderbrandt/BarrelTension.png)
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Bob,
Here is what I thought up this morning.
Instead of using the shroud as the ram, you insert a free floating "tight" sleeve to act as the ram.
This is how I would approach it myself and What I will be using on my mrod build if need it. I have a sleeve with OD .625 and if I need a tensioner I will use a sencond sleeve with ID of .625. should be straight forward.
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The reason as I see it to use the shroud to push against is to lock the barrel to a tube with considerably more stiffness than something barrel OD. So...the main benefit to tensioning the barrel is to stiffen it by locking it to something much stiffer than the barrel itself; the tension the barrel is under is a trivial effect IMO.
Putting a few tight fitting bushings on the barrel, say two spaced every 6" of the exposed barrel( or what ever spacing the barrel length dictates), will lock un-supported barrel to the shroud. The sleeving in uni-directional CF tubing will be aided by this addition.
cheers,
Douglas
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The reason I (pun) suggest this version is cause I've already CF sleeved the 0.5" OD barrel for stiffness. And want to force the rest of the vibrations in line with the barrel direction, rather than side to side motion. My research, albeit not hugely conclusive, tells me that this is the most effective and easiest way to do that.
I have tried the tensioning on various barrels, including sleeved ones, and one of the main benefits is less POI shift at different FPS and Pellet types.
In one test I did, shooting 8 different pellets (5 shot groups or more) at 800, 850 and 900+ the POI stayed exactly the same in the vertical plane. Obviously there where POI shift up and down depending on the weight/speed. This was at 35yds.
I took off the tension system, and the Poi center started to wander left and right.
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I also wanted to know, if anyone would know this.
And have ZERO to do with the tensioning of a barrel. But rather stiffness. What if you went the opposite route, instead of epoxy sleeve a "thick" barrel, you get a barrel as thin as you possible/safely can get. and double/tripple sleeve sleeve it. At least in Airgunning there is no heat to worry about as opposed to powder burners.
I cant but wonder if the lack of harmonic base material (steel) will allow for an incredibly stable barrel. It would almost be like an inner steel lined thick CF tube.
Just food for thought.
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I had been wondering the same thing. Intuitively, both mass and stiffness should mitigate high-amplitude harmonics.
CF has high stiffness but low mass.
I can't help but think that if CF had superior properties to steel, the top benchrest powderburner shooters would wrap their meaty bull barrels in CF for an incremental advantage. Well, maybe they do and I'm just not familiar with it but usually what I see reference to are just big heavy steel barrels.
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Wow, there are a lot of posts to answer, I'll do my best....
Douglas, you first.... I agree that the "hinge" effect will occur between the muzzle and the front of the shroud (where I propose the Bellevilles should be), but that can only effect the second and higher harmonics, the very thing that the barrel tension is designed to minimize.... If that hinging occurs at the breech, then the FUNDAMENTAL vibration (the barrel waving side to side) is not damped.... Further, your comment about the large OD shroud pushing in compression against a large OD seat on the breech is right on, it all adds to the stiffness to resist that fundamental vibration (which let's face it, is the worst one).... I have also used intermediate bushings to dampen second and higher order harmonics, and they may aid even the tensioned barrel.... but I am hoping enough tension will eliminate the need for them....
Tomg, I think introducing yet another element (a small OD sleeve on the barrel inside a larger shroud) will only give the benefit of the smaller sleeve, as the entire unit will now vibrate within the shroud.... Two things to consider, one is the buckling strength of the small tube under compression, you would have to do the math for that.... If it can't buckle (but wants to) because there is no space between it and the barrel, then you may as well glue the two together, because they will act as one, IMO.... Secondly, the blue "cup" you show would wobble around on the Bellevilles, allowing the barrel/sleeve/shroud combination to vibrate at the fundamental (whipping) frequency.... Only the thin barrel to breech joint is there to stabilze that.... The cup cannot touch the breech or the Bellevilles are not free to move and provide the tension....
I have tried the tensioning on various barrels, including sleeved ones, and one of the main benefits is less POI shift at different FPS and Pellet types.
In one test I did, shooting 8 different pellets (5 shot groups or more) at 800, 850 and 900+ the POI stayed exactly the same in the vertical plane. Obviously there where POI shift up and down depending on the weight/speed. This was at 35yds.
I took off the tension system, and the Poi center started to wander left and right.
I agree completely, and that is consistent with the graphs I posted above.... I EXPECT the pellet and velocity sensitivity to greatly reduce or disappear, exactly as it should with a bull barrel....
There is a difference between propagating sound waves (steel does it better than CF, no argument) and stiffness to resist bending due to the energy of that wave, IMO.... CF is very stiff for the weight, although not much stiffer than (the same diameter of) steel.... If weight is not a consideration (or indeed a benefit), then just use a steel bull barrel, long enough to get the velocity you want, and big enough in diameter that it doesn't vibrate enough to worry about....
Mike....
The LDC end of the CF shroud is permanently attached to the insert that threads onto the barrel.
Not sure how you are going to do that to resist 1000 lbs. of shearing force, but you will have to overcome that once the LDC end sticks forward of the threads.... Look at the drawing that Tomg submitted, particularly the blue cup (inner or outer, it won't matter) pushing on the Bellevilles which can still ROCK on the breech, allowing the fundamental (whipping) frequency.... I hope you can solve that....
strever, it may well be possible to have a "nut" that provides the barrel tension and acts as a mount for a tuner/air stripper, but that added flex point may not mount the air stripper solidly enough (a tuner doesn't need to fit close to the pellet path).... I have already thought of just using an air stripper as the nut, I will be trying that....
Did I miss anyone?.... *LOL*
Bob
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Bob: "...The cup cannot touch the breech or the Bellevilles are not free to move and provide the tension..."
How so, once you set the tension by the nut in the muzzle end, tighten it down, the Bellevilles will be in constant tension against the breech, pressing against the front nut.
Whether or not the cup is touching the breech is "irrelevant" as I would think this is not a dynamic relationship that have to change during the shot cycle, but rather a fixed force tension along the barrel direction.
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I repeat, the blue cup cannot touch the breech to stabilize the barrel/sleeve/shroud at that point, or the cup/sleeve will be taking the compression, not the Bellevilles, so just throw them away and torque the setup against the breech to adjust it....
If the Bellevilles are "active", ie free to move, apply force, and adjust for changes in temperature, then they will rock against each other or the breech, and the whole assembly can be pushed to the side just as easy as if the barrel was there by itself (almost)....
Put the Bellevilles at the muzzle, and let the full diameter of the shroud sit against the breech shoulder, and you have a completely different situation.... The OD of the shroud prevents the assembly from moving sideways with applied force....
Also note, you didn't draw the Bellevilles properly, the convex part that touches is adjacent to the barrel, not half way between the barrel and shroud/cup, so they can easily rock on each other.... refer to my drawing, post #1....
Bob
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Gotcha, in my setup the Bellevilles are not needed for the tq on the breech.
T
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I also wanted to know, if anyone would know this.
And have ZERO to do with the tensioning of a barrel. But rather stiffness. What if you went the opposite route, instead of epoxy sleeve a "thick" barrel, you get a barrel as thin as you possible/safely can get. and double/tripple sleeve sleeve it. At least in Airgunning there is no heat to worry about as opposed to powder burners.
I cant but wonder if the lack of harmonic base material (steel) will allow for an incredibly stable barrel. It would almost be like an inner steel lined thick CF tube.
Just food for thought.
Increasing mass lowers the natural frequency. Increasing stiffness raises the natural frequency.
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My Belleville washers arrived today, and I already had the shroud ready.... Here is a photo of what it looks like on my Disco Double....
(http://i378.photobucket.com/albums/oo221/rsterne/Parts%20for%20Sale/Belleville%20Tensioner_zps62nsexls_1.jpg) (http://s378.photobucket.com/user/rsterne/media/Parts%20for%20Sale/Belleville%20Tensioner_zps62nsexls_1.jpg.html)
The barrel is a 7/16" OD Crosman, threaded 7/16-20 NF, and the shroud is 7/8" x 0.035" 6061-T6 Aluminum with the appropriate plug at the muzzle to hold it concentric with the barrel but slide on it.... The Bellevilles I used are 0.045" thick, I am using three in series, and it takes 495 lbs. to collapse them flat.... I marked the point of the nut that was at the top when the nut just touches them, and I can only tighten the nut one flat with my fingers.... With a wrench it's not a problem, of course, and I have at least a turn and a half of useable travel available.... The nut is tightened 1/2 turn in the photo, so the tension is about 150 lbs....
I plan to tighten it a flat at a time and see what happens....
Bob
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Nice proof of concept. Can't wait to see some results.
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Here are some initial test results with two pellets, JSB 15.9 gr. Exacts and JSB 18.1gr. Heavies.... With my Disco Double I have the ability to change the velocity over a wide range very easily by varying the amount the retractable bolt pulls back to clear the barrel port.... My testing is limited to 20 ft. indoors until the snow clears in a couple of months, so I tested for shift in the POI at various velocities.... I did the tests with the 7/16" OD Crosman barrel free floated, and again with it in tension, with the shroud free floated, and then with the shroud supported about 60% of the length out from the breech by a barrel band with an O-ring.... Group were 5 shots, and sizes were overall tighter with the barrel in tension, and slighty tighter again when supported by the O-ring barrel band, but to be truthful they are all just small ragged holes, so much further distances are required to evaluate the group sizes....
However, it was amazing to watch the group wander horizontally and vertically across the paper as I changed the velocity.... With the Exacts, the group moved 0.26" to the left from 638 fps to 871 fps, and then 0.11" back to the right at 1048 fps.....Once tensioned, the total movement left to right shrank to just 0.04".... The Heavies had 0.18" of horizontal movement with velocity, but it also was cut to just 0.04" when tensioned.... The vertical movements of the groups was about half the horizontal movement with both pellets, but it was reduced to about a third of the movement when tensioned, less than 0.04" with both pellets.... Here are the results....
(http://i378.photobucket.com/albums/oo221/rsterne/PCP%20Internal%20Ballistics/Barrel%20Harmonics%20Crosman%20Tensioned_zps8euque80_1.jpg) (http://s378.photobucket.com/user/rsterne/media/PCP%20Internal%20Ballistics/Barrel%20Harmonics%20Crosman%20Tensioned_zps8euque80_1.jpg.html)
The vertical axis is the distance the group moved from it's average position in inches, and the horizontal axis is the velocity.... The upper graph is the vertical movement of the group, and the lower graph is the horizontal movement.... The solid lines are with the barrel tensioned, and the dashed lines are without the shroud, just the 7/16" barrel floated on its own.... The red lines are the Exacts, and the blue lines are the Heavies.... Overall, the barrel has only about one quarter of the deflection as velocity changes once tensioned, so just on that basis the experiment is a screaming success as long as the groups were not worse, and that is certainly the case, they are definitely tighter as well.... There is no doubt in my mind that tensioning the barrel in this manner drastically tames the whippy Crosman barrel....
I did some testing with the Heavies at 980 fps, with various tensions on the Bellevilles, both with and without the shroud support by the O-ring barrel band, and there is significant side to side, and some vertical, movement of the groups as I change the tension.... and as the group size changes as well, but not by a lot.... It appears at it's worst to be as good as before, and at best basically a tiny 1/4" hole.... Both with and without the O-ring 1 turn of tension on the Bellevilles (about 300 lbs.) seemed to be a sweet spot, which is why I used it for the POI testing....
Bob
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Ho-Lee-Smokes!
And that's with a 7/16" barrel!
That gives me hope for how I'm doing the fully shrouded Millenniums.
:)
Mike
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Mike, I hope I have convinced you NOT to put the Bellevilles at the breech end.... Consider this drawing....
(http://i378.photobucket.com/albums/oo221/rsterne/Parts%20for%20Sale/BreechRocker_zpsa039bd54.jpg) (http://s378.photobucket.com/user/rsterne/media/Parts%20for%20Sale/BreechRocker_zpsa039bd54.jpg.html)
The shroud CANNOT touch the breech or the Bellevilles won't hold the tension.... Therefore, since the Bellevilles are convex, they can rock.... Therefore, the barrel can bend back and forth at the breech.... It is TOTALLY different than putting the Bellevilles at the muzzle and having the shroud sit on a shoulder at the breech, forced against it in compression....
Bob
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I got that part...I'm simply having the the shroud tension against the breech/barrel band via being tightened against the muzzle threads...no Belleville's.
It should still help tension the barrel, although maybe not as well as with the Belleville's - but the tensioning is secondary to its primary function as a full integral LDC.
I will post pics in my Millennium thread when we get there...sometime during the week of Feb 8.
Mike
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Here are some initial test results with two pellets, JSB 15.9 gr. Exacts and JSB 18.1gr. Heavies.... With my Disco Double I have the ability to change the velocity over a wide range very easily by varying the amount the retractable bolt pulls back to clear the barrel port.... My testing is limited to 20 ft. indoors until the snow clears in a couple of months, so I tested for shift in the POI at various velocities.... I did the tests with the 7/16" OD Crosman barrel free floated, and again with it in tension, with the shroud free floated, and then with the shroud supported about 60% of the length out from the breech by a barrel band with an O-ring.... Group were 5 shots, and sizes were overall tighter with the barrel in tension, and slighty tighter again when supported by the O-ring barrel band, but to be truthful they are all just small ragged holes, so much further distances are required to evaluate the group sizes....
However, it was amazing to watch the group wander horizontally and vertically across the paper as I changed the velocity.... With the Exacts, the group moved 0.26" to the left from 638 fps to 871 fps, and then 0.11" back to the right at 1048 fps.... Once tensioned, the total movement left to right shrank to just 0.04".... The Heavies had 0.18" of horizontal movement with velocity, but it also was cut to just 0.04" when tensioned.... The vertical movements of the groups was about half the horizontal movement with both pellets, but it was reduced to about a third of the movement when tensioned, less than 0.04" with both pellets.... Here are the results....
(http://i378.photobucket.com/albums/oo221/rsterne/PCP%20Internal%20Ballistics/Barrel%20Harmonics%20Crosman%20Tensioned_zps8euque80_1.jpg) (http://s378.photobucket.com/user/rsterne/media/PCP%20Internal%20Ballistics/Barrel%20Harmonics%20Crosman%20Tensioned_zps8euque80_1.jpg.html)
The vertical axis is the distance the group moved from it's average position in inches, and the horizontal axis is the velocity.... The upper graph is the vertical movement of the group, and the lower graph is the horizontal movement.... The solid lines are with the barrel tensioned, and the dashed lines are without the shroud, just the 7/16" barrel floated on its own.... The red lines are the Exacts, and the blue lines are the Heavies.... Overall, the barrel has only about one quarter of the deflection as velocity changes once tensioned, so just on that basis the experiment is a screaming success as long as the groups were not worse, and that is certainly the case, they are definitely tighter as well.... There is no doubt in my mind that tensioning the barrel in this manner drastically tames the whippy Crosman barrel....
I did some testing with the Heavies at 980 fps, with various tensions on the Bellevilles, both with and without the shroud support by the O-ring barrel band, and there is significant side to side, and some vertical, movement of the groups as I change the tension.... and as the group size changes as well, but not by a lot.... It appears at it's worst to be as good as before, and at best basically a tiny 1/4" hole.... Both with and without the O-ring 1 turn of tension on the Bellevilles (about 300 lbs.) seemed to be a sweet spot, which is why I used it for the POI testing....
Bob
Amazing. The effect on the horizontal is a nearly magnitude larger than the vertical. I'd expect the opposite. Are you shooting from some type of machine rest? Initial results are promising. Very nice work.
Tom
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Hi Bob,
I'm in agreement with you on the effectiveness of the bellevilles being drastically reduced when located at the breech end.
You may not be aware.....but I have been using a similar tension loading technique on my LV class guns with effective results.
My version has the tension tube threaded onto the breech block, and uses a mating taper on the nut and the muzzle end of the tube. I used a brass sleeve with a shrink fit on the end of the barrel, rather than threading the barrel, though. This setup is very rigid, and as long as the nut is tight....it does the job well without POI wander.
If the nut comes loose.....trouble ensues. I have not detected any difference between tight to really tight. Snug, however, is not good enough. I use an aluminum tube that exhibits thermal expansion properties very near to the barrel......it just has a carbon fiber looking film on it. Carbon fiber itself was way different, of course.
I have been interested in the belleville idea, but as of yet, I cannot come up with a way to completely get rid of any possible movement like the setup I have. For benchrest at 25m...... .001" of movement at the muzzle is about .035" at the target......and staying exactly the same from card to card is very important.
Mike
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Tom, I prefer to look at it like the total movement is reduced to about 0.03-0.04" at the target in both (all) planes.... For some reason, the barrel oscillated a lot more horizontally than vertically when simply floated.... Crosman barrels are notoriously flexible, so I'm not surprised (but pleased) to see such a huge difference.... I'm just shooting off a front bag and holding the stock against my shoulder with my left hand lifting the butt to align vertically with the target.... These results are a pretty good indication that the stock barrel is waving all over the place (0.26" at 20 feet is nearly 4 MOA) when you change the velocity drastically.... It had similar changes in POI between pellets, I don't know yet if that is better or not....
Mike.... that's a really nice setup, and the proof it works is the great scores your gun is turning in.... You can't argue with success.... If the thermal coefficients are similar for the barrel and shroud (like CrMoly and aluminum) then you can get away without the Bellevilles, I think, but I'd like to be able to torque the tension for consistency.... Your results RE no difference between "tight" and "really tight" are consistent with Tom's, who also found a big difference to just "snug".... I can't reach the tension levels (~800 lbs) that Tom used with a single stack of these Bellevilles, but it may not be needed with the skinny Crosman barrel.... If it is, I can always double them up.... I REALLY like the repeatability of using the Bellevilles, it's easy to just use a flat at a time, and of course 1/2 flat could be used for fine tuning if necessary.... I have two sets (different thicknesses) of 1" Belevilles and a 1" shroud for my 1/2" TJ's .25 cal barrel, and the thick set will max. out at 1300 lbs..... so no problem getting lots of tension if I need it....
Bob
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Just thinking out loud...regarding the discrepancy in the horizontal versus vertical shift. If the barrel is secured in the conventional manner, a set screw (or two) driving down through the breech in the vertical axis, it seems logical that the barrel would want to "wag" more left to right than up and down.
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Ding....Ding....Ding.... I think we have a winner, Jason.... That could be the reason for the greater horizontal dispersion with the bare barrel floated in my breech.... It has three screws, but only in the top.... They are 10-32, in pockets to stand the tension forces, and TIGHT.... but I have no other explanation for the sideways wandering.... Yours is as good as any....
Bob
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Just thinking out loud...regarding the discrepancy in the horizontal versus vertical shift. If the barrel is secured in the conventional manner, a set screw (or two) driving down through the breech in the vertical axis, it seems logical that the barrel would want to "wag" more left to right than up and down.
Excellent thought!
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Ding....Ding....Ding.... I think we have a winner, Jason.... That could be the reason for the greater horizontal dispersion with the bare barrel floated in my breech.... It has three screws, but only in the top.... They are 10-32, in pockets to stand the tension forces, and TIGHT.... but I have no other explanation for the sideways wandering.... Yours is as good as any....
Bob
Couldn't we then use locking screws on sides (or one side) as well to reduce the horizontal size of groups?
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very nice work indeed. Results are great. It is funy but I was thinking on putting set screws on the sides of the breech once my new barrel comes in. The reason relates to this. I found a little wandering of the barrel sided to side. I'm guessing that if the barrels attached differently it would be a different matter. Ex: Screws into the breech.
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As pr. Usual Bob, you present solid data. It looks similar to my findings, although I have never plotted out all the data as much as you have, to actually see the effect of this over a wide FPS range. And you did it on a stock Crosman barrel to exemplify the results.
Now, I have question. You are using belleville washers, I assume you doing that to be able to tell how much tension there is.
Cause even if you took them off and just tighten the nut(with a spacer on the shroud) you would still have the same tension.
I believe as others do, that the reason for sideways flex is due to at least 3 points of contact in the vertical axis (2 set screws and the tport, at least on the discovery) which would act as nodes.
I was toying with the idea to thread the front of my BNM, lathe the front inside into a wedge, and add a flanged spacer(kinda like a crush washer) that will center the barrel when threaded onto it.
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Putting in setscrews on the side, might be a little tricky, not doing it exactly, but it might push the barrel out of line in relationship to the sight system. Any thoughts?
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I agree about the concern of skewing the barrel laterally with grub screws installed in the side of the receiver. If the machining tolerances result in a slip fit between the barrel and receiver, not so much a concern, but that probably doesn't match reality most of the time.
It's been a while since I fitted a Disco-type barrel into a Crosman steel breech but as I recall, the barrel does not have O-rings. So to help prevent blow-by and to better register the barrel in the receiver, all I did was wrap a few turns of tape on the barrel until it was a slip fit.
This was on a 2289 pumper, and the barrel was typical Crosman fare with a scratch running through the rifling. I gave it a little TLC (crown, leade polish, port debur, and J-B bore paste treatment), and the first two groups at 25 yards were right at 3/8". Abysmal by your standards, Tom and Michael, but remarkably good by my standards, at least for a cheap MSP.
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I received a PM suggesting a more likely candidate for the horizontal dispersion.... a bent barrel with the hook running sideways.... The thought was that the pellet travelling down the bore tries to straighten it, setting up the horizontal vibration.... As you change the velocity, the pellet exits at a different muzzle angle, impacting in a different horizontal location.... I have heard Tim @ Mac1 suggest exactly the same thing, and both suggest rotating the barrel until it is hooked downwards as the straightening force has to overcome gravity, minimizing its effect.... While this is not very practical on a predrilled Crosman barrel, I will in future spin all my barrels in the lathe (by hand) when held at the breech end, and mark the greatest deflection at the muzzle and place that at the bottom when I cut my port.... It isn't as good as making a thimble and rotating it, but at least it should help minimize the effect mentioned....
I think this is a much more likely reason for the horizontal deflection than the setscrews only on the top.... but if you are concerned, it would certainly be easy to put some on the side of the breech.... I have done this on QB's, right at the back of the loading tray (one on each side) as they tend to move a lot due to the short (and sloppy) engagement.... Threading the barrel into the breech is a great idea, particularly if you get into the hundreds of pounds of tension on the barrel.... My current setup (three 10-32s in drilled pockets in the barrel) seems solid enough for the ~500 lbs. maximum I can exert with the current Belleville setup.... After all the testing, I backed off the nut, and it's finger tight position had not changed at all, my index mark was still straight up.... Any movement was therefore less than 0.001".... and the setscrews were still tight....
Tomg, you can just torque the nut to determine the tension, and make it repeatable.... but if the barrel and shroud have different coefficients of expansion you can get a huge change of tension.... The Bellevilles reduce change that to a great extent.... http://www.gatewaytoairguns.org/GTA/index.php?topic=81226.0 (http://www.gatewaytoairguns.org/GTA/index.php?topic=81226.0)
Bob
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Set screw barrel retention is just plain cheese ball. What surprises me is that FX, Datstate, AA and other pricey manufactures think they are just fine.
My arrangement seems to work well and is completely axial when applying tension.
(http://i1147.photobucket.com/albums/o553/QVTOM/RainStorm/013_zpse14c900a.jpg)
(http://i1147.photobucket.com/albums/o553/QVTOM/RainStorm/012_zps3a06037b.jpg)
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Awesome cheese!!!! QV just what I see in my head!!!
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>Bob, yes that make sense about different expansions.
I just converted my own BNM disco into a thimble system, so I can index the barrel. I'll have it together tonight actually.
And after seeing what QVT did for the retention of his barrel, my head is filled with too many ideas, and trying to justify finding time to do them...lol
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I've been watching this thread with interest in what really is going on during the shot cycle to cause the POI shifts. Is it really the muzzle effectively pointing in a different direction as the wave propagates down the barrel or some other effect? The propagation speed of the impulse down the barrel and reflection of the wave back from the muzzle will change with tension in the barrel so the fundamental frequency will go up. Other effects like mass loading on the muzzle will also effect the experiment. One thing that I didn't think of is that the shock wave actually changes the bore diameter as it goes down the barrel. I found this study (http://www.the-long-family.com/shock%20wave%20theory%20summary%20explanation.pdf (http://www.the-long-family.com/shock%20wave%20theory%20summary%20explanation.pdf)) that was done by a fellow electrical engineer and explores all of this in powder burners to some depth with some surprising results (to me at least). I would be interested to hear how this relates to your results, Bob and if it brings more understanding to why the tensioning works. It seems that simply changing the vibration frequency of the barrel would not accomplish much w.r.t. reducing the amplitude of the wave at the end of the barrel. There are so many factors involved but this study filled in a few of the blanks for me
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QVTom, how is the ferrule attached to the barrel, silver solder?.... not that I have the ability to machine anything that nice anyway.... *eyeroll*
I'll have some whine with those cheese-balls, please.... Sometimes you just gotta do what you can....
Bob
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Eric, the magnitude of the pressure pulse / shockwave is much greater with a PB, but on the other hand, we tend to play with skinnier barrels.... I will read that study to ponder what might apply to us, thanks for the link.... There is a barrel harmonics calculator online, and one of the inputs is the chamber pressure, and we play in hundreds of psi, not thousands.... so the initial disturbance is an order of magnitude less for airguns....
Part of the success of tensioning the barrel is that the effective OD is increased by the shroud, which basically stiffens the barrel.... Since the resonant frequency increases with stiffness, the result is the same, IMO, regardless of the mechanism to get there....While a complete understanding of the Physics involved would be nice, just the practical result is at least as important....
I have used untensioned shrouds, supported by an O-ringed barrel band, in the past, and they CAN help the accuracy, by supporting the muzzle.... Often they are improved by an intermediate support of the barrel inside the shroud, just as a barrel is sometimes aided by a barrel band at the right location.... Tensioning the barrel inside the shroud seems to stabilize the assembly even more.... but a lot more work is going to be required to really prove anything.... QVTom has already proven that the system can work with a rigid shroud, rigidly mounted barrel, and a lot of tension (~800 lbs.)....
Bob
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QVTom, how is the ferrule attached to the barrel, silver solder?.... not that I have the ability to machine anything that nice anyway.... *eyeroll*
I'll have some whine with those cheese-balls, please.... Sometimes you just gotta do what you can....
Bob
Yes silver bearing solder. The soldering is a lot of work so next time around I will tread and install with permanent loctite.
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Eric, I found one thing that is hugely different on an airgun RE the article in that link before I got very far into it.... Using the speed of sound in steel given (0.227" per microsecond), and 24" barrel, we get ~211 microseconds for one round trip of the shockwave.... The typical time a pellet spends in the barrel is about 2.8 milliseconds (2,800 microseconds) at 950 fps MV, allowing time for about ~13 round trips during the time the pellet travels up the bore, compared to "4-5 trips" for a typical PB according to that article....
In addition, the stress applied to the barrel is proportional to the pressure, and we are dealing with, as I said earlier, pressures about 1/20th that of a PB.... Therefore, we have 1/20th of the disturbance, varying over a timeframe 3 times as fast, relative to the pellet leaving the muzzle.... I had a quick look at the graphs explaining the timing, and the magnitude of the event is obviously related to the chamber pressure, as is the change in muzzle diameter.... At our pressures, instead of a total bore diameter change of ~ 0.00025", we would be dealing with ~ 0.00001".... Imagine if you will, graph #4 with the peaks 1/20th of the magnitude.... and then graph # 5 adjusted for that and the much slower relative bullet exit timing.... At that point (rightly or wrongly) I decided that while the model and method may be valid, it is not likely a major concern for us....
Bob
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The way we're doing the Millennium top ends is to try and make the entire assembly as rigid as possible...the barrels are threaded into the breech for approx 1.5" then the last 2" or so is turned down to .500" with .0005" clearance inside the breech. The barrel step down at the breech shoulder sets against the breech giving .010" clearance from the magazine port.
The breech shoulder is .750" OD x .400" long...to stiffen the junction of the breech and barrel we are using a 1" wide barrel band that locks this joint together and also acts as the forward breech securement, eliminating the tiny 4-48 breech screw (our mag ball detent is located there anyway).
I am designing the 1.25" OD CF shroud to thread onto the muzzle threads and the o ring floated rear tube cap over the barrel will tension against the breech band.
Both caps will be permanently attached to the CF tube.
The muzzle will be an insert with an air stripper built in that threads onto the muzzle and will have aluminum cups that slide into the sleeve, followed by the muzzle cap.
I'm not sure *how much* tension will be applied - we may make holes for a spanner wrench to really tighten into tension...
I won't know how well it works for a couple of weeks when I can test a completed rifle but I will post my findings.
Thanks for this thread Bob...it shows an improvement in using a tensioned barrel for sure.
Mike
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Carson.........PIC's!
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Eric, I found one thing that is hugely different on an airgun RE the article in that link before I got very far into it.... Using the speed of sound in steel given (0.227" per microsecond), and 24" barrel, we get ~211 microseconds for one round trip of the shockwave.... The typical time a pellet spends in the barrel is about 2.8 milliseconds (2,800 microseconds) at 950 fps MV, allowing time for about ~13 round trips during the time the pellet travels up the bore, compared to "4-5 trips" for a typical PB according to that article....
In addition, the stress applied to the barrel is proportional to the pressure, and we are dealing with, as I said earlier, pressures about 1/20th that of a PB.... Therefore, we have 1/20th of the disturbance, varying over a timeframe 3 times as fast, relative to the pellet leaving the muzzle.... I had a quick look at the graphs explaining the timing, and the magnitude of the event is obviously related to the chamber pressure, as is the change in muzzle diameter.... At our pressures, instead of a total bore diameter change of ~ 0.00025", we would be dealing with ~ 0.00001".... Imagine if you will, graph #4 with the peaks 1/20th of the magnitude.... and then graph # 5 adjusted for that and the much slower relative bullet exit timing.... At that point (rightly or wrongly) I decided that while the model and method may be valid, it is not likely a major concern for us....
Bob
Bob your numbers all make sense but the point that the impulse timing and pressure effects are minimal compared to a PB for airguns raises the question ...Then why does adding tension which should only serve to raise the velocity of the impulse down the barrel (slightly) v~ sqrt (T/M) have any appreciable effect in airguns? It sounded like the pressure wave down the barrel was what the tensioning was supposed to fix, right? I would suspect that without adding some kind of lossy mechanism (ala dashpot in mechanical models), all that is being done is changing timing so the pellet doesn't reach the end of the barrel at the wrong time when an impulse hits the same place. If adding tension will do that I would maintain that it would only work for one particular gun setup (pellet, barrel length, pressure). Otherwise, it seems there is something else going on. It seems that adding some material around the barrel that will dampen the wave impulse would give the best result over all. Other things that come to mind are adding vibrational "nodes" to the barrel by hanging weights or by placing barrel supports to the shroud or stock at critical points. This again doesnt dump the vibration energy ... it just moves the standing wave pattern around so the end of the barrel doesnt move as much for that particular setup.
My linear system analysis training is rusty so maybe I'm missing something here. Thanks for all your experimental work. Obviously there is something very cool going on there.
Eric
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I will take more pics as we progress the week of Feb 8.
(http://i94.photobucket.com/albums/l83/mhossack/IMG_20141114_130710943_HDR.jpg) (http://s94.photobucket.com/user/mhossack/media/IMG_20141114_130710943_HDR.jpg.html)
(http://i94.photobucket.com/albums/l83/mhossack/_20141114_140345.jpg) (http://s94.photobucket.com/user/mhossack/media/_20141114_140345.jpg.html)
(http://i94.photobucket.com/albums/l83/mhossack/IMG_20141114_130737600.jpg) (http://s94.photobucket.com/user/mhossack/media/IMG_20141114_130737600.jpg.html)
These are all the pics I have so far but you can see what's going on.
It will be much clearer when we have the barrels CF sleeved to .750" OD.
The barrel TP ports are married to each breech to avoid clocking issues - the barrels will be tightened down then spot marked for the TP location.
We *could* vice tighten the barrels to the breech AND tension the 1.25" CF shroud in the fashion I mentioned but it will requite a vice to remove the barrel for caliber/barrel changes.
They will be screwed in tightly but will not require any special tools to unscrew from the breech.
There will still be a grub screw in the top of the breech to ensure it cannot loosen at all.
Mike
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Hi Bob,
I have found that removing even a small amount of material (threading) from the OD of most barrels will open up the ID. Don't know if that is an issue with your barrel or not.......just mentioning it, as the muzzle can be a bad place to loosen things up.
Mike
Except a Hammer forged barrel, which will constrict the bore when removing material of off of the O.D.
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Eric, I think the primary thing that is happening is getting rid of the fundamental frequency (the muzzle waving from side to side like clamping a ruler to the table and bending it and letting go) because the shroud, which is twice as large as the barrel stiffens the assembly by like a factor of eight (think of what happens if you double the thickness of the ruler).... Then in addition to that, the first and all other harmonics increase in frequency and reduce in amplitude as well, just like tightening a guitar string.... At least that is my understanding of what is going on.... Any effect that the tensioning has on fore-and-aft pulses, while theoretically helpful, is IMO insignificant....
Bob
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If a dial indicator was placed at the muzzle end. Would the dial movement show the harmonic movement?
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The issue with using a contact method is measuring without influencing the outcome. Maybe with an interferometer.
Bob's shop is decently equipped but I don't think he's been holding out on us ;D
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Bob, That may be what is happening but at the same time, depending on the spectrum content of the exciting source (shot impulse), the actual amplitude of a tensioned string or barrel may actually go up. You can sort of see that when listening to how your guitar sounds when you strike the string with a pick versus a soft thumb. Think of it as a filter on the input excitation source ... higher tension filters out the lower frequency content. So, it really depends on what that pulse spectrum looks like. If it were a true impulse, the power would be equally spread across all frequencies and that would make it an equal amplitude response for the higher or lower tension cases. If we knew what the excitation pulse looked like we could tune things to have a minimum response with some assurance. I guess without pushing the experiment too far we can make some assumptions based on results.
Reading your post on the "hooked barrel" being moved by the pellet may be the most credible explanation for what is going on. Good info.
Eric
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After reading the "hooked barrel/shot wants to straighten it" theorem, it certainly explains the effect you documented, and also the effect of tensioning the barrel combating it. Which brings back the practice of indexing a barrel to where any POI change is in the vertical plane.
Some really good information by all participants, thank you all for the free flow of info!!!
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I'm still having a hard time wrapping my mind around the pellet inducing the sideways excitation forces.
What magnitude forces are at play? Presumably the barrel (an/or the bore) isn't shaped like a banana. I'm assuming more like 0.030" arc swept gently over the length of a 20" barrel. A pellet striking a blade of grass would have its trajectory changed more than that, right?
I would have thought the impulse of the blast of high pressure air would impart more energy into the rifle by several orders of magnitude, setting up vibrations in the barrel and causing it to buzz like a guitar string.
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It doesnt take much to produce a slight horizontal shift in the flight path.
I think this is how it works: If the barrel is bent, even slightly, the harmonic wave propagation will be most notable in the perpendicular plane to the bend.
But smarter heads than mine might explain this better.
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A dial indicator would need some kind of telltales to give a minimum/maximum reading.... however, I can tell you that for the group to move side to side 0.26" at 20 ft., the end of the 2 ft. barrel has to be moving 0.026".... That isn't necessarily on one shot, that is the maximum difference between the right deflection of the group center at 638 fps to the left deflection of the group center at 871 fps.... The barrel may in fact be swinging further than that on any given shot (or less).... Using any measuring device that touched the barrel would likely affect the result, as mentioned....
I think what may be getting lost here is that we don't really care WHAT is causing the vibration (although that is interesting by itself).... We are more interested in the dance the barrel does once disturbed, and how we can minimize that.... Thicker barrels, or shorter barrels, both are stiffer, bend less, and at a higher frequency.... This tensioning, using a shroud with twice the diameter, makes the barrel act like it is thicker, bending less, and at a higher frequency.... but without the weight penalty.... If weight is an advantage, as in a bench-rest gun, then forget all this nonsense, and use a barrel that is 7/8" OD instead of 7/16".... It will weigh over 4 times as much, but be even stiffer (and more accurate) than the tensioned one....
Bob
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It doesnt take much to produce a slight horizontal shift in the flight path.
Well, that is precisely what I was saying. If it doesn't take much force to deflect a pellet's path a few 10 thousandths of an inch, the other way of saying it is that the pellet does not exert much lateral force on the barrel as its course is altered. Thus not inducing much in the way of vibration.
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You could get a feel for the force applied to the barrel using Newton.... for every action there is an equal and opposite reaction.... If the barrel is bent 0.10" it has to change the path of the pellet by that much, compared to the direction it started out in at the breech.... If the flight down the barrel takes 3 mSec, you can calculate the lateral acceleration on the pellet, in the direction of the bend, and hence the force.... The barrel will see that same force away from the bend.... Using the mass of the barrel, you can calculate how much the barrel will "recoil" away from the bend....
and NO, I'm not going to do the math, 'cause I'm not that interested, suffice it to say that it happens....
Bob
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Where F = ma or F = (ML)/T, in the scenario of a barrel bent 0.1", an 18.1gr pellet taking 3ms to accelerate down the barrel would exert a force of 0.331 Newtons or 0.074 pound*force. That's a little over 1 ounce of lateral force on the barrel.
A 1/2" diameter steel barrel bored out to .22 cal would weigh about 0.9lbs. Plugging the preceding force into the equation says there would be 0.00028" of deflection to the barrel, so we'll call it 3 ten-thousandths of an inch. Pretty tough to measure with a dial indicator ;D
If I calculated it right, that is ??? Big if.
But whether or not those are the correct numbers, your point is well taken...the sources of the excitation forces aren't essential for us to understand if tensioning a barrel mitigates their deleterious effects on precision. What I was thinking was if we knew more about where the source and nature of those forces, there might be some opportunity to reduce them with other design features.
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In the expected configuration these air rifles have, it seems to me there are a few excitation sources. One being the hammer acceleration on sear drop, and at the other end, the hammer impact on the valve. These are all far enough from center of mass to give a solid shove to the rifle( even on an AF topology, though it is likely to be slightly closer to CoM ).
cheers,
Douglas
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A dial indicator would need some kind of telltales to give a minimum/maximum reading.... however, I can tell you that for the group to move side to side 0.26" at 20 ft., the end of the 2 ft. barrel has to be moving 0.026".... That isn't necessarily on one shot, that is the maximum difference between the right deflection of the group center at 638 fps to the left deflection of the group center at 871 fps.... The barrel may in fact be swinging further than that on any given shot (or less).... Using any measuring device that touched the barrel would likely affect the result, as mentioned....
I think what may be getting lost here is that we don't really care WHAT is causing the vibration (although that is interesting by itself).... We are more interested in the dance the barrel does once disturbed, and how we can minimize that.... Thicker barrels, or shorter barrels, both are stiffer, bend less, and at a higher frequency.... This tensioning, using a shroud with twice the diameter, makes the barrel act like it is thicker, bending less, and at a higher frequency.... but without the weight penalty.... If weight is an advantage, as in a bench-rest gun, then forget all this nonsense, and use a barrel that is 7/8" OD instead of 7/16".... It will weigh over 4 times as much, but be even stiffer (and more accurate) than the tensioned one....
Bob
If it's one axis movement you could rig up an optical lever and video the movement as you are shooting indoors.
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I finally had a chance to do some testing of the tensioned Crosman barrel today.... The results, unfortunately, are not conclusive.... The gun was set up shooting 18 gr. JSB Heavies at 950 fps.... I sighted the gun in at 50 yards with 1 turn (6 flats) of tension in the Bellevilles, and then reduced the tension 1 flat (1/6th turn) at a time until I got to no tension, and then removed the shroud and tried the barrel free floating.... I then set the tension back to 1 turn, and increased the tension 1 flat at a time until I got to 9 flats (1.5 turns), which put the Bellevilles nearly fully compressed, so the tension would be about 450 lb.f.... I shot 10 shots at each setting, and here are the targets.... The small squares are 0.15" and the large black squares are 0.60"....
(http://i378.photobucket.com/albums/oo221/rsterne/PCP%20Internal%20Ballistics/Crosman%20Tensioned001_zpsurkz8kif.jpg) (http://s378.photobucket.com/user/rsterne/media/PCP%20Internal%20Ballistics/Crosman%20Tensioned001_zpsurkz8kif.jpg.html)
(http://i378.photobucket.com/albums/oo221/rsterne/PCP%20Internal%20Ballistics/Crosman%20Tensioned002_zpshdbv6lbl.jpg) (http://s378.photobucket.com/user/rsterne/media/PCP%20Internal%20Ballistics/Crosman%20Tensioned002_zpshdbv6lbl.jpg.html)
(http://i378.photobucket.com/albums/oo221/rsterne/PCP%20Internal%20Ballistics/Crosman%20Tensioned003_zpsnf3zziey.jpg) (http://s378.photobucket.com/user/rsterne/media/PCP%20Internal%20Ballistics/Crosman%20Tensioned003_zpsnf3zziey.jpg.html)
The groups all showed horizontal stringing, and there wasn't much wind, most of the time I couldn't feel it on my face and the leaves at the target weren't moving, or at least I didn't notice the wind, with the exception of the target at 7 flats.... If I could see any trend, it would be that the groups from 5 through 9 flats were the tightest, followed by the group with no tension.... and the group with 4 flats of tension (2/3 turn) was the worst.... Unfortunately, there was no real marked trend, other than a lot of tension seemed better than light to moderate tension.... Each time I changed the tension the POI moved on the target, with a large shift to the right occurring at 7 and 8 flats of tension.... The single hole to the left on the 8 flat target was not part of the group.... The group at 9 flats (1.5 turns) of tension had 9 shots within 1"....
It is possible that the small amount of wind spoiled the test and hid any significant results.... With that pellet at that speed, a 2 mph crosswind will move the pellet 0.6", or the width of the large black squares.... The group with the smallest vertical dispersion by a large margin was the group at 9 flats, with the most barrel tension.... I left the gun set up like that....
Bob
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It is harmonic, Bob, much less than a sixth of a turn ... =) Mind you, I am an idiot but I do have a gift for geometry.
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The basic idea is to increase the frequency (and reduce the amplitude) of the harmonics by tensioning the barrel.... The best group was obtained with ~ 450 lb. of tension in the barrel and compression in the shroud.... QVTom used a similar setup but without the Bellevilles (torquing the nut), and found that the groups improved after a certain tension was reached and then had little change after that.... He estimates the tension at about 800 lb., but that was on a 1/2" barrel, this one is only 7/16".... My barrel seemed to settle down once the tension went over about 1 turn (~300 lb.), but I suspect the wind was affecting the results as all groups had a much larger horizontal spread than vertical.... It only takes a 3 mph crosswind to change the last group from one hole to what you see, and anything less than 4 mph you cannot feel on your face and it doesn't make leaves move, so very hard to even know if that is the problem.... other than by looking at the groups....
Bob