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The importance of piston seal fit - an example

Discussion in 'Anything Airgun Related' started by cloverleaf, Aug 22, 2014.

  1. cloverleaf

    cloverleaf Honorary Member

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    As much as I currently dislike spring guns, today I had the "pleasure" of sorting an HW97 that was somewhat lacking in muzzle energy.

    Over the chrono this three-year-old rifle was producing around 10.2ftlb with AA Express. It came to bits, was de-greased, appropriate bits measured, re-lubed and reassembled.

    Despite the replacement of the tacky factory grease with a far nicer moly-based alternative the gun was still producing around 10.2ftlb over the chrono; my magical spreadsheet telling me that it was running at a shade under 32% efficiency. While 32% isn't terrible, a decent underlever ('77, '97, TX200, Superstar and I assume LGU) usually runs in the 35-40% range in my experience.

    So it appeared that the rifle was losing efficiency somewhere; the prime suspects being the breech and piston seals. Adding a drop of oil to the breech seal (not down the transfer port) did nowt to increase velocity; so this seal was assumed to be OK.

    Next, attention was turned to the piston seal; upon stripping the gun again it was found to be pliable but perhaps a little tight in the comp. tube (the scales ran out of capacity but I estimate it took somewhere between 20 and 25N of force to move the piston inside the tube).

    The seal and piston were de-greased and secured together by running a length of sellotape around their circumference; leaving around 4-5mm of the front of the seal exposed. The piston rod was chucked in a pillar drill and the whole lot spun while progressive grades (180, 240 and 400 grit) abrasive paper were used to cut back the OD of the front-most 4-5mm of the seal.

    Overall the "fat" part of the seal directly behind the flexible lip was reduced in OD from around 26.0mm to 25.8mm. After a clean-up the piston was noticeably easier to get moving in the compression chamber - requiring around 5N; around 4-5 times less than previously.

    Back together and fired over the chrono, the '97 was now producing around 11.8ftlb with the Express pellets - for an efficiency of around 36.6%. It's possible that taking a little more off the seal might have yielded more gains, but I thought I'd quit while I was ahead. Anecdotally some state that a correctly-sized seal should fall gently down the cylinder under its own weight - going by the mass of most pistons (typically 200-350g) this would equate to a force of 2-3.5N; so not a million miles away from the 5N measured.

    So there we go - a practical example of how important piston seal fit is; showing how correctly sizing the seal has increased output from 10.2 to 11.8ftlb (an increase of 1.6ftlb or around 16%) and system efficiency from 31.8% to 36.6%.

    :)
     
    Last edited: May 19, 2016
    Desmond.22, Reevo17, SeanE and 12 others like this.
  2. loki_79

    loki_79 Donator

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    Was there any noticeable effect on the recoil, I can imagine that the reduced friction would effect the degree of piston bounce, if present..

    Incidentally, do you know the maximum acceleration produced during a typical springer firing cycle, as I want to make a device to log the output from an accelerometer. You are welcome to have one on permanent loan if I can get a design that works!
     
  3. mattyts

    mattyts Donator

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    Great read up Mike,a question for you;

    Would the piston seal have worn down by 0.02mm through regular use? and if so,how long would that take?
     
  4. terry1001

    terry1001 Major Poster

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    Although I don't want to encourage you to spend money (to be fair it looks as though you don't need much encouragement) you really ought to get a lathe :D
     
  5. cloverleaf

    cloverleaf Honorary Member

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    I'm not sure about the recoil tbh - I'd expect that it would be similar before and after since the lost energy is going into friction rather than recoil etc. Certainly didn't notice any difference.

    As for the accelerations; the highest would no doubt be encountered at the end of the firing cycle as the piston is decelerated at the end of its travel. I did loosely work it out once - I think it was around 50-100g but I'm not 100% sure. How are you planning on data logging - an interface with a PC? This (and selecting an appropriate sensor) were where my planning all went pear-shaped.. hopefully you have a better understanding of electronics than I do.

    I'd certainly be interested to see some results; and of course wouldn't say no to some equipment if it became available :p


    Thanks - and I doubt the seal would have worn down at all - the gun was 3yrs old and the seal showed no witness marks at all.


    I do really need (and would very much like) a good lathe.. sadly and more pressingly, I'd also need somewhere to keep / use said tool!
     
  6. loki_79

    loki_79 Donator

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    I think this is the best candidate:
    http://www.st.com/web/en/resource/technical/document/datasheet/DM00053090.pdf

    I'm planning to make a bluetooth device so that you can stream the data to a mobile or laptop and analyse it post-event. I would prefer an analogue chip, but can't find a triple-axis one with a high enough range, so I2C will have to do. There is an analogue single-axis one that could be a possibility, but just one axis may be a bit limiting:
    http://www.analog.com/static/imported-files/data_sheets/ADXL193.pdf
     
  7. Ballisticboy

    Ballisticboy Posting Addict

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    Models seem to suggest the maximum deceleration on the piston can be as high as 2000g but how accurate they are I don't know. The model was probably not too far off as it gave about the right time to piston stop and the MV was within 1% with gun efficiency more or less spot on.
     
  8. cloverleaf

    cloverleaf Honorary Member

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    Cool - sounds well over my head :p

    Tbh I think three-axis is probably overkill (although worth having if there are not functionality / cost drawbacks) - two axis should be fine while a single axis accelerometer would get the job done for the most part. Did you see you have a PM about this on the other side? ;)


    Sounds reasonably - I've been contacted by Jim Tyler who suggests decelerations (of the rifle) are typically around the 200g mark at the end of the piston stroke - assuming a 10 : 1 rifle : piston mass ratio 2000g for the piston sounds about right. Sounds like you have some very capable software at your disposal!
     
    Last edited: Aug 23, 2014
  9. Ballisticboy

    Ballisticboy Posting Addict

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    Only because I was quoting the set up I used for calibration. So basically I cheated. :cool:
     
    Simonsays and cloverleaf like this.
  10. dave mac

    dave mac Active Member

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    how was the ES before and after??
     
  11. cooper_dan

    cooper_dan Posting Addict

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    There is another less important benefit to having a correctly sized seal which I found in my TX200 with the ABT removed
    When you return the under lever up after cocking, it just glides effortlessly up. With an over sized seal there was more friction and you could almost feel the parts rubbing together. It doesn't make a blind bit of difference to performance, but makes it feel like a higher quality and better put together rifle
     
    Reevo17 and cloverleaf like this.
  12. mikeyhall1

    mikeyhall1 Amazing Poster

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    Nice write up Mike. @Blackmax recently had my 95 in bits only to find that Weihrauch had rammed the piston in chipping the seal on the tube on the way in. This had it running at 10.46 if I remember correctly with FTT’s. Now bedding in a Custom Air Seal it’s heading towards the correct power.

    Makes you wonder as the vast majority of shooters do not own a chrono, just how many are unknowingly shooting a lemon.
     
    Blackmax likes this.
  13. Scubashot

    Scubashot Busy Member

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    Your findings also highlight Jim Tyler's results on the effect of ambient temperature on the output of a springer.
    The difference in the coefficient of expansion of the synthetic seal v the steel cylinder mean that the fit of the piston seal is constantly changing and varying the muzzle energy in the process!
     
    Blackmax likes this.
  14. cloverleaf

    cloverleaf Honorary Member

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    Sorry - really can't remember but I don't think it was terrible in either instance.

    Good point! On another note the seals on the TX are bloody awful IME - always well over-size and difficult to size down on account of their shape and material..

    Thanks and yes, unfortunately that seems to be a fairly common issue with the 95s due to poor QC / sharp edges / careless production. Seems to be worse with the new-type seals too.

    I think if every shooter had & used a chrono there would be a lot more guns going back under warranty!

    Great, another variable! I'd guess this might be another plus for smaller bore guns / those with O-rings as opposed to parachute seals (not that I'm 100% convinced by O-rings yet!).
     
    Blackmax likes this.
  15. Airwolf1

    Airwolf1 Posting Addict

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    I am not familiar with the inner workings of a Tx as I have only every taken HW's apart, how is the TX different as I would assume that once cocked the piston and therefore piston seal stays in place consequently making no difference to under lever friction? Have I missed something? Just wondered what setup is inside the Tx. Thanks
     
  16. cooper_dan

    cooper_dan Posting Addict

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    The compression tube and piston move back together when cocking. The piston latches to trigger. When you return the under lever the compression tube moves back with it and slides over the seal
     
  17. hmangphilly

    hmangphilly Big Poster

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    Mr Leaf .

    Try this .
    Warm the seal up in a cup of hot water .

    Then check your newtons ...................
     
    cloverleaf likes this.
  18. FPoole

    FPoole Big Poster

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    Just a nice smooth glide, of the cocking arm, back to the detent. I've seen seals so tight they almost pull off the piston when the arm is returned. JM of Air Rifle Headquarters, here in the U.S., makes a blue colored seal for the TX that is soft and pliable. Seems to help the shot cycle and it is extremely durable. Just mention you want the old small Apex seal in blue. I sure hope he doesn't stop making them.
     
    Blackmax likes this.
  19. Blackmax

    Blackmax Britseals

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    Hi Freddie,
    I hope you're well mate.

    They're the same as the one you gave me when we caught up in Florida last year.

    I might ping you some dollars and get you to post some to me if you're ok with that?

    Andy
     
  20. kt83

    kt83 Keyboard Hero

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    A wonderful report Mike, so typically full of interesting fact and detail.

    Thanks for sharing that with us.

    But why the sudden dislike for springers?

    Where did that come from mate?
     
    cloverleaf likes this.

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