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Thread: Measuring spring stiffness..

  1. #1
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    Measuring spring stiffness..

    I put together a simple way to measure spring stiffness today. Very useful, as seemingly identical springs can actually be quite different. In my case the reverse was true.. I found a 3.2 guage spring that was actually no stiffer than a near identical wind in 3.0mm..

    Figured I'd share...

    Use two spring guies, slightly loose fit. thread into each end of test spring. Get a 9" length of M10 stud, with one end threaded into a small metal plate to support the weight that will be stood on it. Thread rod through the spring / guides. Mount in a vice / clamp, secured vertically by the lower guide flange. Must be very secure. Stick a big weight on top and measure the compression.

    I used a convinent spare car battery (I guess around 18 kg), and got useful differences. e.g.

    225mm long springs, x3

    Spring A 3.2mm wire, 18mm deflection
    Spring B 3.0mm wire, 17mm deflection
    Sping C (std TX mk3 2.9 wire), 33mm deflection

    I found it quite handy, so I will gradually label up my springs so I can compare when looking for the "right" spring.

    Another approach BTW would be to use a sash clamp with some bathroom scales between the end of one guide and the clamp., and measure the deflection at a certain weight, e.g. 30kg. Probably safer than my car battery balancing method But we've no bathroom scales in the house...
    Always looking for any cheap, interesting, knackered "project" guns. Thanks, JB.

  2. #2
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    You might find it easier and perhaps less precarious to apply a tensile load and measure the extension, instead of trying to measure a compressive deflection.

    The number of working turns is also relevant, as well as the wire diameter, but you knew that.

    I did a bit of spring experimentation for Brian Samson when he was developing his knock-over target testing device, and I measured deflection when a load was applied by a weighted lever.
    www.shebbearshooters.co.uk. Ask for Rich and try the coffee

  3. #3
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    Springs are notoriously difficult to make without huge variation. It'd be interesting to see what a batch compared like I think. Airgun springs are a bit of a cottage industry I guess from a volumes point of view.

  4. #4
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    Re my answer to your other excellent spring thread, JB, Jim has done lots of work in this field and has produced some excellent articles on measuring spring rates etc and even on being able to calculate what springs to use through maths, reducing guesswork and trial and error.

    As the very heart of our springers, I'm convinced that this is where our efforts / future developments should be focussed.

    I love springs, me!

    Just wish I had more time to play.
    THE BOINGER BASH AT QUIGLEY HOLLOW. MAKING GREAT MEMORIES SINCE 15th JUNE, 2013.
    NEXT EVENT :- May 4/5, 2024.........BOING!!

  5. #5
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    Or you could compress a spring of known rate, or the one you want to replace, with 'spring x' (both with guides), in tandem on a longer length of threaded rod, and measure the deflections of the two.

    No batteries nor bathroom scales needed

  6. #6
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    Another option is to hang a weight from the spring and measure the frequency of oscillation. The hardest part about this is attaching things to either end without missing any coils.

  7. #7
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    Quote Originally Posted by BTDT View Post
    Or you could compress a spring of known rate, or the one you want to replace, with 'spring x' (both with guides), in tandem on a longer length of threaded rod, and measure the deflections of the two.

    No batteries nor bathroom scales needed
    As above. When I was alive we checked all springs by that method on a jig Sykes Pickavant maybe.
    Same result using a pull gauge as the known spring.
    Now I do this 'cos I'm lazy.

    It can easily employ the stiff known spring in tandem, to replace the scale, and is quicker/easier, for me, than threaded rod.
    That said I quite fancy replicating the tester from the fifties.
    Last edited by deejayuu; 18-04-2016 at 10:12 AM.

  8. #8
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    Some years ago, I welded two brackets, each with a hole in it, at opposite ends of a length of box section steel.

    Pass a long bolt through the spring and the hole one of the brackets, attach a scale to the end (I used a 200 lb dial scale as used by fishermen), attach a threaded rod to the other end of the scale, pass it through the second bracket, attach a nut and compress the spring by around half an inch to get out of the nonlinear response.

    Note the pounds force on the scale, compress the spring by a further inch, and subtract the first reading from the second, and you have the spring rate in pounds force per inch. Divide by 5.71 and you have Newtons per millimetre.

  9. #9
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    Quote Originally Posted by deejayuu View Post
    As above. When I was alive we checked all springs by that method on a jig Sykes Pickavant maybe.
    Same result using a pull gauge as the known spring.
    Now I do this 'cos I'm lazy.

    It can easily employ the stiff known spring in tandem, to replace the scale, and is quicker/easier, for me, than threaded rod.
    That said I quite fancy replicating the tester from the fifties.
    Now that, Sir, is a seriously good method .....

    Cheers, Phil

  10. #10
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    Quote Originally Posted by BTDT View Post
    Or you could compress a spring of known rate, or the one you want to replace, with 'spring x' (both with guides), in tandem on a longer length of threaded rod, and measure the deflections of the two.

    No batteries nor bathroom scales needed
    Now I like that.. just get a long threaded M10 stud and just measure the relative deflection of each....
    Always looking for any cheap, interesting, knackered "project" guns. Thanks, JB.

  11. #11
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    Quote Originally Posted by deejayuu View Post
    As above. When I was alive we checked all springs by that method on a jig Sykes Pickavant maybe.
    Same result using a pull gauge as the known spring.
    Now I do this 'cos I'm lazy.

    It can easily employ the stiff known spring in tandem, to replace the scale, and is quicker/easier, for me, than threaded rod.
    That said I quite fancy replicating the tester from the fifties.
    Now I like that too.... just need some bathroom scales...
    Always looking for any cheap, interesting, knackered "project" guns. Thanks, JB.

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