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Thread: Even more about flat belts.... improvements

  1. #1
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    Default Even more about flat belts.... improvements

    I finally realized where the issue was.....

    There was a spring as part of the tension mechanism, put in because the pulley sets don't exactly take the same length belt on every step. It took up the difference.

    It is a very short, very stiff coil spring, but it was effectively limiting the belt tension, and hence the max power. More pull would deflect it and tend to hold tension flat. The actual max tension was less than it should be for some mechanical reason. The spring itself didn't limit the max, but limited the maximum on certain pulley steps if it was set correctly for other steps.

    The compensating spring was not at the same deflection for all steps, and hence was not giving the same tension.

    the alternative is to adjust for each step, which is a big hassle. However that is what I have to do now. I need to count turns of my freshly made adjusting nut with thumbwheel, to get the tension up to the approximately 4% stretch which does the job. The result is that I can get a reasonable tension which takes full motor HP for the higher belt speeds.

    I clearly would NOT have been able to do that without polishing the pulleys.....tension made far less difference before polishing.

    I am trying to come up with a provable spring method..... possibly belleville springs would do it, as they don't change force over their entire deflection range. All compensations within their range would have equal tension, unlike the prior situation.

    Since the tight side tension goes up, but the slack side tension reduces, it may be possible to get a tension setting which will be constant for all settings, and also be high enough to allow full HP where needed without slippage.
    Last edited by J Tiers; 07-30-2009 at 11:51 PM.

  2. #2
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    Maybe if you provided tension with a slack side idler the tension would be more constant. Just a thought.

  3. #3
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    Quote Originally Posted by Bruce Griffing
    Maybe if you provided tension with a slack side idler the tension would be more constant. Just a thought.
    Won't work.... there isn't enough length. the C-C distance is 12". And the variation is under 1/8" C-C.

    But its the same basic idea, fairly constant tension over a range of C-C distance.

  4. #4
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    Jerry, have you considered converting the sheaves to Poly-V? They have a lot more traction than a flat belt.

    There have been several articles in HSM magazine and MEW about converting South Bends and Myfords to Poly-V, all with highly enthusiastic results.
    "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

  5. #5
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    Well - I accept your thoughts that the slack side idler will not work, but without a drawing, I cannot see why. If you fix both pulleys (cones) and push a small idler wheel into the slack side - how can there be no room for that? Maybe it is the overall setup that gets in the way, but I can't see how the cones would.

  6. #6
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    There may be room, but not a lot. And it would have to be on top, the underside is the tight side.

    It's mostly a bit of "I don't prticularly want to do that" as much as it is a problem to fit. The belleville springs would be compact, and would not require a major re-do of the belt tension assembly. And I have the belt at this length now, and no more belt material at the moment.

    But I'm also getting lots of power through so no problems now..... Adjusting isn't *that* much trouble. I should be able to stall the motor instead of slipping the belt, so I have little or no incentive to mess with poly-V belts, etc.

    I already have made a poly-V pulley for another piece of gear, so I know about that.....

    it seems to have been all about the tension (that I thought was there, but really wasn't). That, and polishing the pulleys, which did the number.

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