Adjustable Barrel Tensioner

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/o...ps47d42489.jpg

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

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/o...62nsexls_1.jpg

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

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/o...8euque80_1.jpg

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