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Variable speed V belts ... was I being particularly clueless?

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  • Variable speed V belts ... was I being particularly clueless?

    The variable speed drive belts on my Rockwell Delta 11x36 lathe started slipping. My first thought was "Oh, they've stretched". I ordered a new bottom belt. When it arrived I was aghast to see that the circumference was the same as the worn belt that it was supposed to replace. So I returned it and asked for "the right belt". When it arrived it was the same dimensions.

    BUGGER... However, checking and rechecking the on-line manual and various replacement charts, it kept coming up as the right belt to replace the old one. So, I decided to try it... what the heck.

    To my surprise it fixed the problem. Then I looked more closely at the belts. The worn one was just under 1" across the top. The new belt was nearly 1.2" across the top. It was WIDER, not shorter. How in heck does that work? Then I sat down a while and had a realization.

    Click image for larger version

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    The variable speed mechanism on my lathe consists of a pair of deep V pulleys made out of a fixed conical plate, a central bi-conical plate which is free to shift left and right, and fixed conical plate on the right. The belt tensions are arranged so that as the pulley shaft is lowered, it creates additional upwards pull on the left side, which shifts the bi-conical plate to the right. That causes the right hand belt to ride higher in the narrowing V... and it changes the pulley ratios. That varies the speed.

    What this means is that the V-belt never rides on the interior flat part of the belt. It's always riding on the beveled faces. Thus, as the faces wear the belt gets narrower and it can sink down deeper into the groove, without forcing a corresponding reciprocal change in the other belt. If they get narrow enough they both lose grip and start to slip.

    Some searching on the intertoobs seems to show me that the inside flat of a V belt is never supposed to ride on the inner cylindrical face of the pulley. Is that correct for all V belts, or have I missed some important aspect of V belts and pulleys?

  • #2
    yep, that's correct for all V belts, including polyV belts - they all ride on the sides of the V, not on the bottom. In fact if they ride on the bottom, it usually means that the pulley (or belt) is worn out.


    • #3
      If the belt were to ride on the bottom there would be a lot of slippage.



      • #4
        If the belt were to ride on the bottom if might as well be a (thin, admittedly) flat belt, yes?
        "A machinist's (WHAP!) best friend (WHAP! WHAP!) is his hammer. (WHAP!)" - Fred Tanner, foreman, Lunenburg Foundry and Engineering machine shop, circa 1979


        • #5
          Make sure the variable sheave is well lubricated. If it doesn't move freely from side to side the belt will wear out quickly and you will have limited variability in the speed.