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PostPosted: Mon Dec 22, 2014 5:52 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....

Image

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 1/2" 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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PostPosted: Mon Dec 22, 2014 6:22 pm 
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Just like the "Truss Rods in a Guitar Neck", this allows the neck of the guitar to be straighten when the rod is tightened/compressed/reduced vibrating and if pressure is released, increases vibration or in a guitar, induces a bow in the neck for increasing string to fret clearance.
This is the same idea as the Crosman Mark 1.

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PostPosted: Mon Dec 22, 2014 6:25 pm 
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I wasn't aware they used Belleville washers, the one I had apart didn't.... it was either "tight" or "not"....
Bob

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PostPosted: Mon Dec 22, 2014 8:59 pm 
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No Bob the MrkI or II never had a washer they just butted up to the shroud. Believe the principle of butting up is what he was referring too. I have used orings upon the Cr Mrk series for holding tension angainst the bbl and the nut from loosening off.

It is an interesting way of putting a bbl into tension. But it then leads to the question of what is the bbl dia? I can understand thin dia bbks but what the purpose upon bbls that are larger than 1/2"

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PostPosted: Mon Dec 22, 2014 9:50 pm 
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We use a lot of TJ's liners these days, and they run 7/16"-9/16" so it would be good on any of those I would think.... of less use on 5/8" and up, you might have to go to more tension to find out if there is any advantage.... One of the guys on the GTA that runs a barrel in tension (but no Bellevilles) say he figures about 80 in.lb. of torque on 1/2" OD threads, which works out to ~ 800 lbs. of tension.... and that made a BIG difference in group size....

To convert torque to tension here is a handy calculator.... http://www.engineersedge.com/calculator ... e_calc.htm

He suggested to double up on the Bellevilles.... )) (( )) ..... That would give a range of 200-1000 lbs. with 1 turn on the 1/2"-20 NF threads, and not difficult to do with a wrench.... You would sure have to look at how the barrel is mounted in the breech, however (his was threaded I think) and make sure the shroud would stand the compression load....

Bob

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PostPosted: Tue Dec 23, 2014 12:18 am 
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Tensioning the barrel would IIRC my physics, reduce the amplitude of the harmonic vibration, t'would also increase the frequency.

Kinda like tightening a guitar string. :)

Not sure how well the bellevilles will work in this scenario...a vibration would induce a variable load on them, and since they are a spring and will move with load....the tension during the shot cycle would vary as well.

Have to wonder what the net effect would be when the bellevilles release their tension back into the barrel.....

Try it and find out I guess. :)

Al


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PostPosted: Tue Dec 23, 2014 12:52 am 
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I don't know for sure, of course, but I think the Bellevilles will simply allow the barrel to resonate at whatever frequency it wants to under the tension applied to it.... My thoughts are exactly the same as yours, Al, that the frequency will increase and the amplitude will decrease.... That is the whole purpose of the experiment.... Incidently, the same thing happens when you increase the OD of the barrel, higher frequency and lower amplitude.... which we look at as the barrel being "stiffer".... I think it's worth a try....

Instead of using six of the 0.050" thick x 0.075" high Bellevilles and doubling them (1200 lbs. flat load) I may just use three of the 0.073" thick x 0.091" high which are 1335 lbs. flat load.... If I get both types, I can use the thinner ones in a single stack for 100-500 lbs. in 1 turn, and the thicker ones for 50-1300 lbs. in 1 turn, at 20 TPI.... I realize I could use just 1 or 2 and get the same force, but I wouldn't get the full turn of adjustment on the vernier, losing resolution.... so I think a stack of 3 makes sense because of that....

Bo

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PostPosted: Tue Dec 23, 2014 8:46 am 
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Sounds like a great idea, especially for those whippy Crosman barrels. I wonder what it would take to provide adequate anchoring for the barrel in a steel breech? A couple of extra set screws with indents in the barrel, or will it need something more drastic?


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PostPosted: Tue Dec 23, 2014 10:53 am 
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IMHO for setting a bbl into that much tension. The bbl being threaded into the breech block would be the most secure mount. As for frictional and pressure forces from the projectile traveling down the bbl. That's another set of calculations, if one is trying to find the sweet spot in the harmonics equation.
It is however an interesting way of improvising, using the washers to set tension there fore duplicating a thicker heavier bbl. Curious to see where this will go with the thinner TJ liners and the commonly used Cr bbls. Barrel harmonics are like a whole different science, as like the rifling of a barrel. JMO

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PostPosted: Tue Dec 23, 2014 11:45 am 
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The best method I have seen so far for stiffening thin barrels is carbon fiber tubes glued on.... but to get a 1/2" barrel up to 3/4" takes two layers and they aren't cheap (but they are STIFF).... The Bellevilles with a 1/2" bore are 1" OD, so it would make sense to use that size shroud.... Once in tension/compression the unit should work like a 1" barrel, or close to it, at least that is the hope.... All I can tell you is that the guy who is using about 800 lbs. of tension in his barrel said that when he reached that the group sizes shrunk dramatically and no further increase in tension seemed to help further.... I'm just hoping the Bellevilles will give a very repeatable method of finding a tune, as seldom do we put a gun together just once, and torqueing can be affected by the surfaces and lubrication and not be very consistent, particularly at the relatively low torques involved here....

I'll be ordering some Bellevilles to try this on my new 7mm barrel on the Hayabusa, that should be a good candidate for it.... The barrel is 9/16" diameter and 28" long....

Bob

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PostPosted: Wed Dec 24, 2014 8:33 pm 
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Bob I have have been working on similar for some time, have you the means to substantially check the results?
Reading between the lines data you have printed (thread length to pitch) I have come up with similar and know it works when methodically setup by trial and error about 100 shots.

10M rifles are sleeved up to make rigid at 20mm diameter, they have barrel flex but not as much as unsleeved thinner.

Grain structure throughout the barrels if even I wouldn't dismiss this working on to a certain extent but having turned down many over the years have seen soft and hard patches over several areas down the lengths. So if tensioning the barrel it will cause it to deflect at weakest point to one side. Upon firing that bend becomes greater, still not a bad thing.
If the tensioning pulls in a straight line it will still have the symptoms I will lead on to nothing to do with spring loading.
Flex isn't a bad thing but harder to control. Testament to that was first rifle made in 92 barrel turned down to 12mm dia with 1" rad recess in to it 1mm deep, it twanged like a tuning fork. Groups most would be amazed to show off shot at 8 yards that this would leave at 55 and 60 repetitively.

I'll let you answer first


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PostPosted: Thu Dec 25, 2014 12:07 am 
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The only question I saw was can I check the results.... When I eventually get the parts and the time, and can get to the range, I plan on systematic testing with what I know to be the best pellet for the barrel to start with.... I will have marks on the vernier nut, and record setting and group size, and hope to see a trend, within my limited shooting ability.... If it works, I will test other pellets as well, as time permits....

Bob

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PostPosted: Thu Dec 25, 2014 7:20 pm 
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There is a cheap source for Carbon fibre tube on eBay from China I use for shrouds. http://www.ebay.com/itm/13MM-OD-x-11MM-ID-X-1M-Carbon-fiber-wing-tube-100-carbon-fiber-Quadcopter-/321612729649?pt=Radio_Control_Parts_Accessories&hash=item4ae19ccd31 I have only ever sleeved Remington 700 Actions in aluminum when I was gunsmithing for the Benchrest game and 1000yd guns.

A carbon sleeved 7/16" Crosman Barrel might really help with the longer range hunting guns built up?

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PostPosted: Thu Dec 25, 2014 7:33 pm 
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Bob try it at least 50 yards the results will be more pronounced and easy to learn.

As i mentioned earlier I couldn't dismiss tensioning not working but confident its not that that will create the effect you will see.
Do exactly the same without the disc springs! Just rotating the graduated purple weight.

How heavy is the purple?
Made loads from 7gr to 33.2gr. Heavy easy to learn the symptoms and effects but very hard to control spot on alignment. I usually aim for 12 to 16gr either on 1mm or 0.75mm pitch thread.
On 1mm pitch with 33gr, every 60 degree rotation will see a distinct pattern appear. Every subsequent 60 degree a further pattern and so on 4 times. Start to finish 2/3 of a rev that's 0.66mm your purple would move back and forth for max effect. Its also why those air strippers cannot work, at best pot luck no way can anyone make something that critical and assemble to that tolerance being 1/4 thickness of your hair.
One pattern will be -----, next Vertical, Next big spread, then single hole either low or above aim point.

I fit them to most of the P800 I have converted, to the untrained they look nothing special, great to keep an edge.
Done this June 2012 but made several different interchangeable weights.
http://i1140.photobucket.com/albums/n56 ... 0001_8.jpg

Jan 2012 weight screws along barrel in to back venting quietener with 1/2"UNF at front with cap. Graduations every 30 degrees. http://i1140.photobucket.com/albums/n56 ... CN4198.jpg
http://i1140.photobucket.com/albums/n56 ... 001_14.jpg
http://i1140.photobucket.com/albums/n56 ... 001_11.jpg

This ones in the grey bulbous thing at muzzle, most of the others directly in to barrel sleeve 15mm ish bore.

Customer quickly setting tuner up at 55 yards off the knee with unweighed, unsorted pellets. Looks like around 18 shot groups
http://i1140.photobucket.com/albums/n56 ... rsetup.jpg
He ended up with this though personally I would be far from happy having put a full charge down range 55 yards on my own with unweighed, unsorted pellets not leaving 3/8" edge to edge in a drift. Decent 5 shot group looking .22 pellet size hole. http://i1140.photobucket.com/albums/n56 ... ttuner.jpg

some months back 0.75mm pitch, weights 12gr/pair. Should be looking within 1 1/2 revs setup due to lighter weight and reduced thread pitch.
http://i1140.photobucket.com/albums/n56 ... 001_15.jpg
http://i1140.photobucket.com/albums/n56 ... 001_11.jpg


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PostPosted: Thu Dec 25, 2014 7:38 pm 
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I'd be interested in trying the carbon fiber approach. What inner diameter do you use and how do you glue it? I can see 11 and 12mm ID on ebay at fairly reasonable prices. Not sure if the 11 mm could be fitted over a 7/16 barrel, but 12 sounds quite loose?


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