Top Hat Material?

Discussion in 'Piston & Spring' started by NeilM, Sep 9, 2019.

  1. Cooper_dan

    Cooper_dan Well-Known Member

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    Every day is a school day. And quite rightly. I don't need to be adding any more confusion to springers!
    Edited my post ;)
     
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  2. ksts100

    ksts100 Member

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    Before you start shooting again.It s not bad idea to make a barrel cleaning .
    I usually clean the barrel after a new set up or lubing and i repeat an other, after the first 50-60 pellets ,( checking for dieseling) ..
    Many times i had a small dieseling with out to realise that happen..
     
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  3. Malcolm lockhart

    Malcolm lockhart Member

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    Would someone mind putting up the formula to calculate the efficiency.
    Save me searching for it.
     
  4. Nomads HFT

    Nomads HFT Well-Known Member

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    The quick answer is that it's the muzzle energy divided by the kinetic energy available to the piston, multiplied by 100.

    The long answer is that you first have to calculate the spring rate (10000 x (wire diameter raised to the power of four)) divided by (mean coil diameter raised to the power of three x number of active coils), then the energy in the stroke plus preload, minus the energy in preload (0.5 x ((stroke + preload) squared) - (preload squared) x spring rate / 1000 or 12 depending on whether you're using metric (mm and N/mm) or imperial (inch and lbf/inch) measurements).

    I have physically measured spring rate and find that substituting 9800 for the '10000' in the spring rate formula is more accurate for the springs I use.
     
    Last edited: Oct 8, 2019
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  5. Malcolm lockhart

    Malcolm lockhart Member

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    Thank you for that and I'm now sorry for asking for it, I found an article by cloverleaf and realised what I had asked.
    So thank you for taking the time.
     
  6. Cooper_dan

    Cooper_dan Well-Known Member

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    As mentioned, the wire diameter is raised to power 4 in the calculation. This makes wire diameter a very sensitive measurement. 2.95mm will give you a very different output to 3mm.
    Really this wants to be measured with verniers, and averaged over several measurements.

    With my full length TX, if efficiency is above 40% its most likely dieseling a bit. If it drops below 34% i'll change the breech seals.
     
  7. NeilM

    NeilM Well-Known Member

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    It would appear to have a lot to do with pellets.

    After yet another strip down and some bare bones chrono testing and spring changing I have found that with the original set up; one batch of JSB's run at a regular 750fps, a batch of Air Arms runs at 760 to 765 and my original batch of Air Arms, chosen for accuracy now run at 768 to 775, no other changes.

    For the record, the spring I am using appears to be an AA Mk1 and has the following dimensions: 216mm long. 21mm O/D. 14.5 mm I/D. 3.15mm wire dia with around 11mm of preload using a 2mm thick top hat.
     
  8. Nomads HFT

    Nomads HFT Well-Known Member

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    The difference in velocities with pellet is probably due to the weight variation of the pellets in each tin, Neil. I had some rather inconsistent muzzle velocities from one tin of pellets, so weighed the individual pellets before testing them, and the muzzle energy came out very consistent. In fact, after a while of doing this, I could weigh a pellet and pretty accurately predict its velocity based on its weight.

    If it has 23 coils and 3.25mm wire (going from your ID and OD measurements), that's a pretty stiff spring, circa 8.5 N/mm (48.3 lbf/in). Easy to work out the energy efficiency if you could measure the available stroke - if its 85mm, its 37.68% efficient, for instance.
     
  9. NeilM

    NeilM Well-Known Member

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    Nice try, but after something like 20 years of shooting FT / HFT and reviewing rifles, I ain't about to start weighing pellets now, that way leads to extreme obsessiveness, nerdy-ism and ultimately insanity... oh wait, I'm shooting a springer these days... right, where are the powder scales.

    Phil would be able to tell you the exact stroke, he made the latch rod, but it's within a mm or two of the TX200 MK1.
     
  10. hmangphilly

    hmangphilly Can't Re Member

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    Oh that was ages ago .
    I think it gave a couple mm longer stroke than the mk1

    Was i supposed to write it down ? :(


    we found pellet fit could alter fps quite a lot .
    betcha sizing them alters the numbers
     
  11. NeilM

    NeilM Well-Known Member

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    That's what I'm thinking.

    I'm just going to give it a good try with the 760fps pellets and see what I think of it all.
     

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