Adjustable Barrel Tensioner - Harmonics Study

[rsterne]

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

[CarsonRatSniper]

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

[rsterne]

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

[CarsonRatSniper]

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.



Mike

[rsterne]

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

[CarsonRatSniper]

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

[strever]

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

[PakProtector]

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

[Tomg]

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.

[Rdsail]

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.

[PakProtector]

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

[Tomg]

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.

[Tomg]

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.

[nervoustrigger]

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.

[rsterne]

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

[Tomg]

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.

[rsterne]

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

[Tomg]

Gotcha, in my setup the Bellevilles are not needed for the tq on the breech.

T

[Rescue35]

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.

[rsterne]

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....



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