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Crankshafts, which class of lever

  1. Feb 21, 2016 #1
    For those that are familiar with the 3 classes of lever (and the fact that all simple machines are fundamentally either a lever or an inclined plane) I have a question regarding rotational levers such as in a crank shaft.

    It seems to me that a crankshaft could simultaneously be considered more than one class of lever. Lets say for example that a piston exerts its force on the crank. It does so with a lever arm (or crank throw) of x. Now lets say the crankshaft directly drives a wheel with a radius of 2x. The effort arm would be x, the resistance arm 2x and the fulcrum the very center of the shaft.

    Seeing as this is a round system, could you not look at the system as being effort arm ->fulcrum->load (thereby being considered a first class lever) and yet also look at it as fulcrum->effort arm->load (thereby being considered a third class lever)?

    I realize this is all just definition and no matter how you think about it in terms of lever classes, the reality does not change. Just curious if the reasoning for it being both simultaneously is sound.
  2. jcsd
  3. Feb 21, 2016 #2


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    Even a simple seesaw can be seen as different lever classes, depending on the reference frame. I never saw much use in those categorizations.
  4. Feb 21, 2016 #3


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    You have written it for me. :smile: Just stop worrying about it and treat each case according to the situation. Nowhere, except in school, will anyone want you to quote the class of lever. It's one of those curriculum things which waste everyone's time and can only cause worry. See AT's comment.
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