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Thread: Why flat belts need crowned pulleys.

  1. #11
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    Carnel's Tools
    Hi Tony,

    You are right, I have to specify on which part the forces are working. The forces and the torque I mentioned are working on the belt. And the direction of the forces are are up or down as seen in your drawing fig 3 (so in a horizontal plane). At point 2 the belt is moving faster than the pulley so the pulley brakes the belt at point 2 by applying a force on the belt downward in the drawing. At point 1 the pulley moves faster than the belt and tries to accelerate the belt by applying a force on the belt upward in the drawing. These two forces at point 1 and 2 form a cw torque so at the underside of the drawing he belt is moved to the left of the drawing. Because the movement is upward in the drawing the belt is moving to the higher middle of the pulley.

    "Here is another way to think of it. When you have slip the the tendency of the system will be to eliminate that slip, in this case that means matching the velocities of belt and pulley which is achieved by point 2 trying to move to the top of the crown."

    But in a concave pulley the forces will not come to equilibrium. It constitutes an unstable system.

    Regards,
    Harry

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    TheElderBrother's Avatar
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    This is one of those things that I always thought made sense without knowing why it made sense, possibly because I saw it worked. Nice to understand why.

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    tonyfoale (01-28-2021)

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    old kodger's Tools
    I have a bandsaw with 12" wheels, commercially made (pretty old Australian manufacture) both wheels are perfectly flat, how ever, the top wheel can be 'canted' out of vertical to control the blade position on the wheels would this be your "misalignement" albeit adjustable?

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    Supporting Member tonyfoale's Avatar
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    tonyfoale's Tools
    Quote Originally Posted by old kodger View Post
    I have a bandsaw with 12" wheels, commercially made (pretty old Australian manufacture) both wheels are perfectly flat, how ever, the top wheel can be 'canted' out of vertical to control the blade position on the wheels would this be your "misalignement" albeit adjustable?
    Yes, exactly.
    A misaligned wheel or pulley presents an angle to the belt which is like one side of a crowned pulley. Many bandsaws, mine included ( Homemade vertical bandsaw. ) have a tracking adjustment as well as crowned wheels. This serves two purposes, the crowning keeps the blade on the machine and the adjustment allows you to place the blade where you want on the wheels.

    I only have the top wheel crowned, and I use the adjustment to place the part of the blade just behind the teeth, over the top of the crown. This means that the tightest part of the blade is toward the front and this gives the best directional stability. I explained that with a post on this forum and a video, but I cannot find either right now.

    Why flat belts need crowned pulleys.-dsc_3299.jpg Blade front set to crown.



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