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RCorley2-Explorer
2-Explorer
October 17, 2014
Solved

Simple moment applied to shaft, all rotational calcs = 0.00000

  • October 17, 2014
  • 3 replies
  • 7976 views

I'm out of options here. I'm running a simple static analysis on a crankshaft (just imagine a solid round bar). I've created a cylindrical coordinate system. I constrain one end in R, T, and Z as fixed. The other end I apply a 1Nm moment. I've created two measures at points on the end where the moment is applied. One is at 12 o'clock and the other is at 6. The measures are set as rotatonal (radians).

The results are fine and reasonable for everything except the measures and the rotational calculations. See below:

Measures:

max_beam_bending: 0.000000e+00

max_beam_tensile: 0.000000e+00

max_beam_torsion: 0.000000e+00

max_beam_total: 0.000000e+00

max_disp_mag: 5.520417e-04

max_disp_x: -1.152828e-05

max_disp_y: -5.520416e-04

max_disp_z: -3.781698e-04

max_prin_mag: 1.840552e+00

max_rot_mag: 0.000000e+00

max_rot_x: 0.000000e+00

max_rot_y: 0.000000e+00

max_rot_z: 0.000000e+00

max_stress_prin: 1.840552e+00

max_stress_vm: 2.288784e+00

max_stress_xx: 4.405939e-01

max_stress_xy: -3.826951e-01

max_stress_xz: 4.070771e-01

max_stress_yy: -7.137716e-01

max_stress_yz: -1.207598e+00

max_stress_zz: 1.763673e+00

min_stress_prin: -1.327123e+00

strain_energy: 6.989292e-03

Measure2: 5.031145e-04

Measure3: 2.162827e-04

Measure4: 5.520417e-04

Rotational1: 0.000000e+00

Rotational2: 0.000000e+00

Ultimately I'm trying to compare the rotational stifness between two different shaft designs. Thanks

      Best answer by sdensberger

      Shouldn't that be in radians?

      No, because we've defined a displacement measure; for this, the displacement is the magnitude of a vector, and the direction is a unit vector that is in the theta direction with respect to the initial configuration of the system.

      EDIT: I thought about this a little more and there is a way you could directly measure the rotation (provided you know the axis about which the rotation is occuring).

      1. Create a datum point along the axis of rotation and offset it such that the point does not fall within the geometry.
      2. Create a weighted-link between the cross-section of your shaft (i.e. a surface) and the datum point you just created.
      3. Define a measure for rotation on the datum point.

      This should give to the rotation value you're looking for.

      3 replies

      S
      sdensberger2-Explorer
      2-Explorer
      October 19, 2014

      I assume you're model uses solid elements; if so, then the reason you're getting 0s for your measures are because solid elements don't have rotational DOF. If you want to measure the rotation of this shaft you'll need to do the following:

      1. Create a cylindrical coordinate system oriented such that the twist of your shaft is in the theta direction.
      2. Create a measure to calculate the displacement a point on your geometry with respect to the theta direction of your cylindrical coordinate system.
      3. Create a custom measure that calculates the inverse tangent of the value of your previously defined displacement measure divided by the distance from the point of rotation to the point that the distant measure is define at.

      You could also link a dummy beam element to the geometry and measure the rotation of the beam element itself, but there are a bunch of caveats to this method.

        R
        RCorley2-ExplorerAuthor
        2-Explorer
        October 20, 2014

        Thanks Shaun. I'll look into this. What you're saying makes sense

        But if I create a measure relative to theta it still is in a length unit (mm, in my case). Shouldn't that be in radians?

        S
        sdensberger2-ExplorerAnswer
        2-Explorer
        October 20, 2014

        Shouldn't that be in radians?

        No, because we've defined a displacement measure; for this, the displacement is the magnitude of a vector, and the direction is a unit vector that is in the theta direction with respect to the initial configuration of the system.

        EDIT: I thought about this a little more and there is a way you could directly measure the rotation (provided you know the axis about which the rotation is occuring).

        1. Create a datum point along the axis of rotation and offset it such that the point does not fall within the geometry.
        2. Create a weighted-link between the cross-section of your shaft (i.e. a surface) and the datum point you just created.
        3. Define a measure for rotation on the datum point.

        This should give to the rotation value you're looking for.

          R
          RCorley2-ExplorerAuthor
          2-Explorer
          October 20, 2014

          Yes. Angular deflection.

          R
          RCorley2-ExplorerAuthor
          2-Explorer
          October 20, 2014

          My hand calculations for moment of inertia do not match what Creo is giving me either. Any thoughts on why? I was trying to correlate hand calcs back to the machine.

          S
          sdensberger2-Explorer
          2-Explorer
          October 20, 2014

          Are you sure both calculations (hand and Creo) are with respect to the same origin?

          R
          RCorley2-ExplorerAuthor
          2-Explorer
          October 20, 2014

          I got them to work out when measuring the cross section. So we may have had the origins off. I don't that is holding me up though.

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