Author Topic: Expansion of air tanks  (Read 220 times)

d_boom

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Expansion of air tanks
« on: December 06, 2018, 05:24:20 AM »
Benjamin Marauder has a free floating barrel with a barrel band attached at the end of the pressure tube.
The "O" ring inside this barrel band is large and does not make contact the barrel.  I made a bushing that
is a slip fit onto the barrel but fits tightly inside of the barrel band.  I though this bushing would stabilize
the barrel, wrong!  The tank was degassed but instead of proceeding with some planned work, I decided to
shoot some more and recharged the tank.  The point of impact at 10 meters moved 4" left!  An engineering
friend pointed out that an air tank even of steel expands/contracts with pressure changes which moves the
barrel band which changes the point of impact as the barrel moves.  Now I understand why as I shot I was
chasing the impact point of the pellets as the pressure changed!  Needless to say, the barrel band bushing
has now been removed.


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bnowlin

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Re: Expansion of air tanks
« Reply #1 on: December 06, 2018, 05:48:15 AM »
I have one question.  Did you remove the O-ring from the band as I have all my Mrods non floating with a bushing and have not noticed any change when pressure drops or on refills?  Just curious.
Bobn

d_boom

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Re: Expansion of air tanks
« Reply #2 on: December 06, 2018, 06:29:32 AM »
No, I left the "O" ring on inside the barrel band.  The bushing was a slip fit onto the barrel shroud. 
If the air tank only swelled in length then the bushing should slip back and forth.  Evidently the air
tanks swells in diameter as well as in length, and does not swell/contract evenly.  I did not measure
the wall thickness of the air tank, but that might shed some light to which way the tank swells with
air pressure changes.
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bnowlin

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Re: Expansion of air tanks
« Reply #3 on: December 06, 2018, 07:02:00 AM »
I would think that the barrel band, even with a bushing would not have anything to do with any swelling of the air tube because being attached to the fill adapter should not cause that.  Now I don't know about length maybe some one else could chime, however it doesn't seem to affect mine.  All mine are solid a do not move enough to affect POI.  No play at all in the shroud area.
Bobn

Alan

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Re: Expansion of air tanks
« Reply #4 on: December 06, 2018, 07:25:34 AM »
I have owned 5 Marauders, and still have two. I've tried what you tried, but the truth is, all of them shot better after removing the barrel band.
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Alan

I have an Omega compressor. If you're a fellow Guild member, and you pass through Roswell, NM, I'll fill your portable tank as a courtesy.

rsterne

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Re: Expansion of air tanks
« Reply #5 on: December 06, 2018, 08:45:31 AM »
The sideways shift in the POI could be because the barrel band is twisting (or was installed with a twist), and moving the muzzle to the left.... The DAQs use a barrel band at the muzzle, and the instructions tell you to twist it until your shots are centered left-to-right and then tighten the setscrews....

Bob
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bnowlin

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Re: Expansion of air tanks
« Reply #6 on: December 06, 2018, 10:10:09 AM »
Bob I have had one of my Mrods do that when I first unfloated the barrel shroud because I didn't have the the fill adapter screwed all the way in even though it was sealed.  I finally loosened the band and turned the fill adapter almost another full turn until it butted up fully against the air tube.  I lined up the band, tightened the set screws and Bingo didn't happen again. So his could be doing that ever so slightly.

Bobn

d_boom

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Re: Expansion of air tanks
« Reply #7 on: December 06, 2018, 11:20:43 AM »
Barrel band was not installed with a twist, the bushing was a slip fit over the barrel shroud but
a tight fit into the barrel band.  Installation was made by slipping the bushed barrel band into
the barrel shroud then onto the fitting on the end of the tank, then after all was in place and
free from stress the screws were tightened.  When I started shooting accuracy seamed improved,
but as pressure was depleted shots started going in all directions.  After completely degassing the
air tank and refilling is when I have the 4" shift in my shots.  If the tank had expended evenly in
all direction, there should not have been a shift in shots.     
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mobilemail

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Re: Expansion of air tanks
« Reply #8 on: December 07, 2018, 04:04:11 AM »
4" at 10m is an awful lot of POI change to think the change of a tube swelling - something normally measured in microns - would cause it.  Especially since the barrel band doesn't fasten on bottom to the air tube itself, but the the front air port assembly, which is a pretty good chunk of metal.   I'm not trying to discredit your report, it just seems to defy the laws of physics...
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bnowlin

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Re: Expansion of air tanks
« Reply #9 on: December 07, 2018, 06:39:33 AM »
d_boom
Did u check for pellet clipping?  Do u have a LDC and u keep calling it tanks do u have a bottle on the gun or regular air tube?  I'm just throwing things out. Also do you have the gun tuned too high or too low.  All are possible.  Just trying to help.
Bobn

ShawninIL

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Re: Expansion of air tanks
« Reply #10 on: December 07, 2018, 07:40:31 AM »
I used large o rings to unfloat the barrel on my .25 and accuracy improved considerably. If I take off the shroud, I have to adjust the band to make sure the POI matches the POA, then all is well. The only shift I see with pressure drop is vertical, which is to be expected.
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Motorhead

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Re: Expansion of air tanks
« Reply #11 on: December 07, 2018, 08:08:53 AM »
Have been VERY successful in "Semi-Floating" ... Or in reality UNFLOATING marauder barrels for years.
Placing a thin strip of 1/16" hobby foam between band & shroud ( o-ring removed ) and making sure the stripper o-ring is well greased.
* Also making sure that the end of shroud in contact with receiver is PERFECTLY square.

The foam damps vibration of the straw barrel within, allows minor motion so barrel stays in position if air tube shifts with pressure etc ...

If you did a search on other forums ( Marauder / GTA for instance ) that were around 6-7 years ago when M-rods first were getting popular you would find MANY posts on the subject.
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d_boom

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Re: Expansion of air tanks
« Reply #12 on: December 07, 2018, 10:57:07 AM »
Thanks for all the advise, now I have additional stuff to try.   I should be headed in the right direction.
I check the barrel shroud and check if it is square.   I though I had my Marauder shooting well until I
degassed to dry lube the hammer after removal of the sticky grease.  I decided to put off the hammer
cleaning and recharge the Marauder to 2800 psi, that is when the 4" shift with pellets impact to one side
happened.  Since I was told the air tank would only expend microns (.000034") I decided to check it out. 
My background is in a gauge lab work.  Pressure was at 1,400 PSI on the Marauder gauge, air tank dia.
averaged 1.2529" (18 measurements taken).  Slowly degassed the tank and allowed it stab.  The next
18 measurements taken at the same locations checked 1.2516" a diameter change of .0013".  I was not
setup to check how much air tank grew in length or if the air tank warped in any direction or not.
Hope you found my measurements of interest.  Again thanks for the helpful advise, I will get my Marauder
shooting accurately soon.
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Frank in Fairfield

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Re: Expansion of air tanks
« Reply #13 on: December 07, 2018, 02:02:09 PM »
.0013  of an inch may not mean much to most of you guys, but I can use it! ;D
"Good judgement comes from experience, and a lot of that comes from bad judgement."

bnowlin

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Re: Expansion of air tanks
« Reply #14 on: December 07, 2018, 02:25:41 PM »
.0013  of an inch may not mean much to most of you guys, but I can use it! ;D

OK now