Adjustable Barrel Tensioner - Harmonics Study

[nervoustrigger]

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

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.

[PakProtector]

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

[hutnicks]

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.

[rsterne]

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"....







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

[MicErs]

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.

[rsterne]

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