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Homework Help: Calculating the force on a Crankshaft in a Steam Engine

  1. Jun 5, 2012 #1
    Given that I have a steam engine, and now its size, the size of the piston, the length of its stroke and the force acting on the piston at certain points, I need to calculate the force on the crankshaft.

    Now, I now that Fx should be the same where the piston meets the crank, and we can ignore Fy at this point I think, since there is no friction. But how do I calculate the force at the other end of my crank? Where the crank pushes the flywheel?

    Since i don't really want people doing all the work for me I figured I needn't include variables, just some theory work from you guys would be much appreciated.

    Finally, am I right in thinking i need to know the velocity of my piston, and thus my crank(Or at least the velocity of one point) I also now the RPM of the flywheel when the piston is a t top dead centre (I.e fully closed)

    Hope that covers everything thanks in advance!
  2. jcsd
  3. Jun 6, 2012 #2


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    I guess you have a pistom, connecting rod, and crankshft ( attached flywheel).
    Perhaps you should do a fbd of the connecting rod to determine your forces at your pins, knowing that with a pin connection the force will be along the centrerline of the member ( or from centre of one pin to the other ), Is there really no y-force?
  4. Jun 7, 2012 #3
    Done a free body.
    I cant think of what the y force would be? there should be none on the piston (other than gravity and some force N pushing up on the piston but since there is no friction i ignore these right?)
    Though there will be an x and y component of the forces on the pin at the other end of the crankshaft where it meets the fly wheel.
    Finding these forces is proving to be a problem.

    I know the force on the piston at a point B as the steam expands. (Pressure*pistonArea - ambientPressure*piston diameter) so that should also be my Fx where the piston meets the rod, but how then do I go about finding the force at the other end of the rod? Does finding the velocity help?

    I think a friend mentioned finding an equation X in theta (where theta is the angle of the flywheel out of the 360 degrees in a revolution) but im unsure how to do that, Trigonometry using the flywheel radius and the rod maybe?

    Thanks for your time
  5. Jun 7, 2012 #4

    rude man

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    I would think the germane question is what is the torque, not the force, on the crankshaft.

    The force is just the force of the piston. The torque is that force times the lever-arm imparting torque to the crankshaft. That torque is of course time-varying with the rotation angle of the crankshaft.
  6. Jun 7, 2012 #5
    The torque you reckon? Which is the force translated to the fly wheel?
    By lever arm do you mean the rod length? or the distance from the pin on the fly wheel to the fly wheel centre?
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