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Atmospheric Pressure on Shaft of piston cylinder assembly

  1. Sep 1, 2016 #1
    1. The problem statement, all variables and given/known data
    In a vertical piston cylinder assembly, a vertical shaft whose cross sectional area is 0.8 cm^2 is attached to the top of the piston. Determine the magnitude F, of the force acting on the shaft, in N, required if the gas pressure inside the cylinder is 3 bar. The masses of the piston and attached shaft are 24.5 and 0.5 kg respectively. The piston diameter is 10 cm. The local atmospheric pressure is 1 bar. The piston moves smoothly in the cylinder and g=9.8 m/s^2

    2. Relevant equations

    3. The attempt at a solution
    I converted the atmospheric pressure and gas pressure to forces. I summed all the forces acting on the cylinder. F= -Wshaft - Wpiston + Fgas -Fatm -F=0 Now my problem is, when I find the force caused by atmospheric pressure, do I subtract the area of the shaft from the area of the piston >> Fatm = Pressure (Area(piston)-Area(shaft)) ? How does atmospheric pressure not act on the top of the shaft, shouldn't Fatm = Pressure(Area(piston))?
  2. jcsd
  3. Sep 2, 2016 #2


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    If there is no seal or other pressure controlling feature at the top of the shaft then atmospheric pressure acts on the shaft end in the normal way .

    Area to be used for calculating atmospheric pressure force is effectively just the total area of the piston - the hatched area shown in diagram below .
    Last edited: Sep 2, 2016
  4. Sep 2, 2016 #3


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    Pressure area.jpg
  5. Sep 2, 2016 #4
    You should be considering the equilibrium force balance on the combined piston and shaft, not the cylinder.
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