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  • Pitch circle diameter

    Discussion in 'Calculations' started by Auto Engineer, Nov 29, 2014.

    1. Auto Engineer

      Auto Engineer Active Member

      May 2012
      Likes Received:
      Hi all,

      I have a dilemma trying to keep away from school formulas for calculating the diameter of a circle. The problem as I see it using maths formulas are that they are open to human error in the measurements taken by hand in the first instances.

      Assuming then that I have a crankshaft pulley that operates a serpentine belt (ribbed) what would be the best most accurate way to determine the diameter of the pulley?

      Assuming that I could use the belt passing round the pulley dynamically and also assuming that friction would be uniform throughout the time period, does anyone know a dynamic method to calculate the diameter and then double check that using calculations?

      I have a formula for gear wheels but not for a pulley that drives a belt without teeth?

      Any advice welcome
    3. Lochnagar

      Lochnagar Well-Known Member

      Feb 2011
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      Belts have a myriad of different names - but by the sound of things you are talking about a poly-V belt or micro-V belt - see Gates brochure below - with pictures of the 100 and 1 different belt names/types.

      The very nature of any V belt is that it can slip - so I would advise you to stick to the formulas given in the manufacturers book for the particular belt in question for computing pulley details.


      Hope this helps.
    4. K.I.S.S.

      K.I.S.S. Well-Known Member

      May 2014
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      The name Serpentine is derived from Latin, and means 'snake like' - and this means that it usually performs multiple functions in any mechanism. Because of this multiplicity of functions it is required to simultaneously govern, and because of the increased likelihood of both a tolerance stack up between different components that interact with it and the likelihood of differing torques between these components, serpentine belts almost always make use of at least one sprung or hydraulically tensioned pulley follower, that is strategically positioned in the layout of the belt in a way that will correctly tension the belt to the pulleys that it drives.

      So, if I understand you correctly in that you want to heuristically determine the diameter of a single pulley in the system driven by such a belt, that can become quite complex - you would first need a multiple set of accurate references, including the known diameter of all the other pulleys in the system, the nominal chord length of the belt when new (if it's used, forget it), and the manufacturers specified Nm of torque for the follower (tensioner) at the specified degree of deflection relative to the Youngs modulus of the specific belt material and cross section, to name but a few parameters.

      Are you sure it isn't more effective to simply take multiple measurements of the diameter of the pulley in question with an accurate digital Vernier gauge and then plot the median of those results?
    5. prastaples@yahoo.co.uk

      [email protected] New Member

      Nov 2014
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      I agree with the others.

      rule 1 measure everything in your build. If you don't trust the accuracy of your equipment pay a company to measure them for you using faro arms or cmm. hell you could even use a shadowgraph if your tight on cash.

      You cant measure the diameter of something accurately using another component whose job is to take up tolerances and be in tension by its nature.

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