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Thread: Bandsaw tension

  1. #1
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    Default Bandsaw tension

    This might sound like a dumb question, but how is the tension in the tension spring transferred to the blade? When I first apply tension on mine, the upper wheel rises and the blade takes up slack, but on further tightening of the tension lever the wheel doesn't keep rising further, while the spring gets more compressed. I imagined that the wheel has to move to keep increasing blade tension, but apparently not.
    I haven't seen this explained.

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  3. #2
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    The wheel is attached to the spring, and the tension lever arm. When you move the tensioning lever, you move the whole wheel assembly, and this puts the spring under compression tension; this is exerting more force on the wheel than when it's not under compression, when the spring wants to return to it's uncompressed form.

  4. #3
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    so therefore, should you release the tension when not using ?
    I would love to grow my own food, but I can not find bacon seeds

  5. #4
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    Yes, correct

  6. #5
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    Quote Originally Posted by Tonto View Post
    so therefore, should you release the tension when not using ?
    yep, as the spring would (eventually) lose it's spring capability... obviously depending on the quality of the spring steel is how critical this is, but essentially occasionally forgetting isn't a big deal, but don't make it a habit

  7. #6
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    Thanks for that Poundy, but I'm still not getting the full picture. The spring wants to return the wheel downwards, but how is that tensioning the blade?

  8. #7
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    Default Bandsaw tension

    When you tighten the blade, you are actually stretching it. Because the blade is not elastic, the tension goes from zero to 10,000+ psi very quickly. The spring acts as a tension buffer and a shock absorber. Without the spring you could easily over tension the blade.

    There is no evidence to support any benefit to releasing tension on the blade or vice versa. Do you release the load on the coil springs on your car? Not all compression springs are created equally so make sure you have a good spring. Along the way you will replace guides, tires, thrust bearings. Just add spring to the list.

    Don

  9. #8
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    Don,
    that sounds like the spring is actually a tension resister, rather than a tensioner, which is what it feels like. But when people replace their spring with a stronger one I thought they did so because they couldn't get enough tension?
    Chris

  10. #9
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    When you screw down the adjustment handle the base of it is on a solid part of the saw frame and the thread goes through a collar under the spring. As you tighten the adjuster it pushes down on the frame and screws the collar upward pushing against the spring. The top of the spring then pushes against the top of the housing that the wheel is attached to trying to lift that housing higher. Because the blade will not allow the wheel to go any higher the collar starts to exert pressure on the spring so the more you turn the adjuster the tighter the blade becomes.

    Bandsaw spring.jpg
    Dallas

  11. #10
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    I was with you until this bit:
    "Because the blade will not allow the wheel to go any higher the collar starts to exert pressure on the spring so the more you turn the adjuster the tighter the blade becomes."
    I agree the blade will not allow the wheel to go any higher, and that the collar exerts pressure on the spring (until the spring is fully compressed). But then you say "the tighter the blade becomes". If the spring is resisting any more tensioning, how does that make the blade tighter?
    I've taken a good look at my machine, but unfortunately the key bits connecting the wheel are hidden from view.
    I'll just have to accept that it works.

  12. #11
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    When the spring just starts compressing, let's say it's 5% compressed, it's exerting 5% of it's max. possible tension on the blade.
    Halfway compressed? It's exerting 50% of it's max. possible tension on the blade.
    Fully compressed? It's exerting all the tension it's going to.

    Newton's third law states: For every action, there is an equal and opposite reaction.

    In the case of the BS, the blade exerts equal and opposite pressure to that of the spring... ie. it increases in tension.


    Another way of looking at it: It can be likened to resting your thumb lightly on a counter-top, pressing your thumb down firmly or pressing it down with all your might. The counter-top won't move but your thumb is exerting different degrees of pressure (or tension) on it... and, of course, Newton's 3rd Law still applies.
    I may be weird, but I'm saving up to become eccentric.

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  13. #12
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    The big saws at mills have to be released when not in use as they work on time; you get 'x' hours under tension before it needs to be sent to a saw doctor, whether it's running or just sitting there. There's no need to worry with 'normal' bandsaws.

  14. #13
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    The advice that I've seen says to release the tension because tension springs kept under constant tension will lose some of their ability to extend. Plus it will put a flat spot on the tyre.

  15. #14
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    Quote Originally Posted by Treecycle View Post
    When you screw down the adjustment handle the base of it is on a solid part of the saw frame and the thread goes through a collar under the spring. As you tighten the adjuster it pushes down on the frame and screws the collar upward pushing against the spring. The top of the spring then pushes against the top of the housing that the wheel is attached to trying to lift that housing higher. Because the blade will not allow the wheel to go any higher the collar starts to exert pressure on the spring so the more you turn the adjuster the tighter the blade becomes.

    Bandsaw spring.jpg
    Ok, now I think I have it. Mine looks a bit different to that pic and I couldn't see that the tensioning screw passes through the housing, but now I can see that the spring is free to try to force the housing up, thus trying to lift the wheel - which as you say is resisting because of the blade.
    Thanks to all participants - I didn't mean to turn it into a saga.

  16. #15
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    Quote Originally Posted by Chrism3 View Post
    ... release the tension because ... it will put a flat spot on the tyre.
    This!!!!!

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