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Homework Help: Determine bolt size

  1. Nov 27, 2013 #1
    1. The problem statement, all variables and given/known data

    We have a project on third semester where we should calculate different things on a 4 stroke engine.
    My assignment is to prove that the bolts between the cylinder and the cylinder head has the right size.

    The following information is given:

    Maximum pressure in cylinder: 59.5 bar
    Tightening torque: 25 Nm
    Bolt: M8x125 DIN EN ISO 4014
    and all the dimensions on the cylinder and the cylinder head

    So far, I've calculated the stiffness of the bolts and the cylinder (assuming that the cylinder is a pipe)
    I want to calculate the deflection and then draw a deflection diagram but I'm stuck

    2. Relevant equations

    k_b = F_b / δ_b , where k_b = stiffness, F_b = the load and δ_b = displacement

    3. The attempt at a solution

    I know that the maximum force in the cylinder can be defined by F_max = p_max * A_max
    but should I use that force to find the deflection?
    or maybe the force ratio * F_max ??

  2. jcsd
  3. Nov 27, 2013 #2


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    Science Advisor
    Homework Helper
    Gold Member

    Not really my field so I may have the wrong approach but..

    I was looking at it from the point of view of the net force on the head. There are two forces...

    1) the preload tension on the bolts (downwards)
    2) the force due to the pressure in the cylinder (upwards)

    If the net force on the head is upwards the head will accelerate upwards away from the block breaking the seal. So in short the preload force must be greater than the force due to the pressure.

    The force due to the pressure in the cylinder is easy enough to calculate (pressure * area of cylinder bore) but I don't know how to calculate the tension in the bolts from the applied tightening torque.
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