Not all springs are the same , scragging

Discussion in 'Piston & Spring' started by Nick G, Feb 2, 2019.

  1. Nick G

    Nick G Active Member

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    Some months back , I switched from Titan No 11 to No12 in my 21mm reduced comp tube TX, I noticed that it took a while to settle as the power kept going down and I had to add pre load until it stabilised , so I then took another spring and scragged it for a couple of days , sized it and set the power , and it has remained the same so it looked like the Titan springs aren't scragged in manufacture .
    Fast forward to now and I have a TX on the bench for a 21mm conversion , and I am also experimenting with a new piston design for my own gun , so I buy two No 12's, and scrag them both simultaneously for 24 hrs , they both started off at 295mm, one has ended up at 275mm and the other 257mm, I expected them to lose length, but not different amounts, and 18mm is significant , what it will mean is that the shorter of the springs will have around two extra coils once cut to length so will probably give a softer shot feel .
    so could probably do with sourcing some oe springs that have similar dimensions , and hope for better quality wire .
     
    Last edited: Feb 2, 2019
  2. Malcolm lockhart

    Malcolm lockhart Member

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    Would you explain scragging.
    Do you mean cramping up till coil bound, and leaving a set amount of time.
     
  3. RobF

    RobF Administrator Staff Member

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    Scragging...

    Set Removal | W.B. Jones Spring Co. Blog

    'He' was on about this a while back. Seen similar symptoms as you Nick before with new builds. Compressing springs is scary stuff... when we were measuring the expansion of springs when they were compressed you didn't really want to look at it when you were doing it, polycarb shields to the ready...
     
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  4. Nick G

    Nick G Active Member

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    Yes , i did mine in a lathe with some threaded bar , they do look a little strange until they are fully compressed .
     
  5. Scott Hull

    Scott Hull Member

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    I always scrag my springs to coil bind as the first cycle. I use a piece of all-thread that just fits the ID of the spring. I'm not sure if the time really matters, but I leave them compressed in the jig for 12-24hours, just in case. There are several benefits to scragging a spring BEFORE attempting to shoot with it.
     
  6. hmangphilly

    hmangphilly Floppy Quick Phil

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    is the rate different on the 2 springs nick ?
     
  7. Nomads HFT

    Nomads HFT Well-Known Member

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    In the trade, scragging describes compressing the spring very rapidly to its solid height, and it is a brutal process.

    Mike Wright and I ordered our first batch of springs without specifying they were to be scragged. They started off (from memory) around 220mm, but after a couple of shots with very low muzzle energy, they'd reduced in length to under 200mm. Subsequent batches were scragged three times to the specified length, which they maintained in use.

    Some springs fitted to new air rifles, or sold as aftermarket alternatives, are not properly scragged. I fit a set of guides to keep them straight, thread the lot onto a length of 9mm threaded rod, and compress them. The longer you leave them compressed, the less length they will lose to creep during use.
     
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  8. Malcolm lockhart

    Malcolm lockhart Member

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    Maybe the lgv springs weren't scragged, the ones that gave all the bother.
    No matter, there better without their own Spring.
    Sounds a dangerous process, and easy to knacker the spring.
    Not to be attempted with out proper safety precautions.
    Follow Jim's advice.
     
  9. Nick G

    Nick G Active Member

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    They were both brand new Titan No 12 so should have been the same .
     
  10. Scott Hull

    Scott Hull Member

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    I don't see the need to "scrag" (compress to coil bind) rapidly, or more than one time. If the spring took a set from scragging, that is an indication that some of the material was taken past it's yield point. Every subsequent full range cycle will reduce the fatigue life of the spring.

    After the ONE cycle to coil bind, best to stay away from that point as far as the performance requirements allow.

    The way you personally did it sounds like the proper way (in my opinion) for an airgun spring.
     
  11. Nomads HFT

    Nomads HFT Well-Known Member

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    To add to that, Scott, in describing the way I personally did it, I did state "The longer you leave them compressed, the less length they will lose to creep during use.". Let's put some figures to that.

    I took a spring from a new production rifle, measured its free length, and compressed it to coil bound for an hour, then measured it again. I repeated this for two, four, eight and 24 hour intervals, and this was the result.

    Spring compression.jpg

    You can see how much the free length reduced which, were the spring being used in the rifle, would reduce the preload, reducing the muzzle energy. The spring wasn't weakened by being compressed to solid five times; the loss was creep rather than fatigue, and replacing the creep with preload washers would have pretty well restored the muzzle energy (as it did with a different spring I left compressed for 1,000 hours in an experiment). So, compressing a new spring to solid for 24 hours as you do is good practice... however.

    Subsequent to this little experiment, I refitted the spring in the rifle, and left it uncocked and unused for a week with ~30mm of preload - its free length reduced further to 231.5mm. I then gave the rifle a little use, and the free length settled at 230mm, where it has remained to this day. Had I left the spring compressed solid for about a week rather than a day, I think it would have ended up at or very near to the final 230mm mark. In the test rifle, the loss of 8mm of preload would have reduced muzzle energy by in the order of one ft. lb.

    Spring manufacturers don't have time to leave all their output compressed for long periods, so a quick visit to the scragging machine does the same job in seconds. Scragging puts pennies on the spring unit cost; some manufacturers and resellers pay the price, some skimp.

    Incidentally, when I carried out the 24 hour compression test above, I used a long length of threaded rod, and compressed not only the test spring, but also a new Air Arms TX200 spring in tandem which, unlike the test spring, started the test at 223mm and ended at 223mm.
     
    Last edited: Feb 5, 2019
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  12. Ratinator

    Ratinator 77.74

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    Would scragging be of any benofit to hammer springs ?
     
  13. Scott Hull

    Scott Hull Member

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    My last spring had been compressed to coil bind before use. It got about two years (maybe 10k-12k shots?) before the FPE started to drop, I spaced it up another 1/4" and got another couple of months use but it continued to lose FPE. Spacing it up some more got the FPE back up again but I figured that I was chasing the last cycles before failure. So I decided to replace it. When I removed the spring it was slightly kinked and at about 285mm free length.

    Two new springs that I recently ordered were identical to the first. Both started out at about 308mm. I compressed the first to coil bind for about 14 hours and it ended up about 298mm. The other for 24hours and it ended up at 299mm, so about the same. From your data and other empirical data, I'm guessing that there is some relaxing over time of the outer "fibers" when held near the yield point. Though the relaxation starts to level off over time.

    The preload and FPE of the scragged spring will be less when compared to a "virgin" spring. The FPE can be brought back up by spacing up (restoring preload). A scragged spring that is spaced up should have a longer fatigue life than an unscragged, unspaced spring at the same FPE level. However, I prefer to used a spring with more coils that naturally increases the preload while having the additional active coils reduces the stress slightly. That should give an even longer spring life.
     
    Last edited: Feb 5, 2019
  14. Scott Hull

    Scott Hull Member

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    It might though not really needed. A hammer spring is a low stress application. And easy to design so that you are way below the yield point of the spring material.

    If you keep the stress below a certain level, steel can have an infinite fatigue life, but that is not always easy to do given the constraints of piston gun designs.
     
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  15. hmangphilly

    hmangphilly Floppy Quick Phil

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    exactly that .

    The dimensions of many springers do not lend themselves to springs staying within their yieldy limits .
    Rob put up a calculator ages ago



    SCRAGGING TODAY ...............AYE !
     
  16. Joha

    Joha New Member

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    Is scragging alowed on a spring that has been fit...and used for a only a couple of shot's?

    Why?
    The LGU spring went shorter, so did velocity. Changed it for a HW95 spring. And than this dungeon article came up the STB
     
  17. FPoole

    FPoole Active Member

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    All my TX200 springs are scragged with the first cocking stroke.:D Some last for 10,000+ shots and some only go a few 1,000. Some also break, but most just start messing up the accuracy. They don't come off the guide the same with each shot because they're weakened, that's my theory. A couple of times I've had guns start grouping in a circle pattern around the center of the bull. I pull them down and the spring has a big kink caused by the coils collapsing in one spot. A new spring sorted the oddity both times.
     

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