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Romax Dynamic Analysis Model Check

Premjeet Singh

A common issue people run into when using Dynamic Analysis in Romax:

1. What is the most important check to do in Romax before running Dynamic Analysis to ensure a successful run?

2. Why am I getting 0Hz modes?

[This question and answer has been posted by Hexagon support for the benefit of our community members]

  • 0

    Lets look at a simple shaft arrangement in Romax as shown below (exaggerated view scale).

    A shorter shaft (Shaft 2) is connected by a Radial Clearance Bearing to the longer shaft (Shaft 1).

    Shaft 1 is then supported by a taper roller bearing on either side and a main Power Load in between the left taper roller bearings and the edge of shaft 2 with a zero power load on the other end.

    The Taper Roller Bearings are connected to ground for simplicity.

    Assume we wanted to run a Dynamic Analysis on the setup.

    The result would be the appearance of 0Hz modes in the dynamic analysis window (scale shown is normalised).

    When playing the animation (with increased scale of deflection) we can see the effect in the video below.

    nexus.hexagon.com/.../0511.RomaxDT_5F00_5QL1b6J1Bk.mp4

    This occurs as shaft 2 has no Axial or Torsional constraints causing unrealistic rigid body modes at 0Hz.

    So the most important check when running a dynamic analysis is to ensure all components are constrained in all degrees of freedom.

    You can do this by adding 1D bearing connections between shaft 1 and the edges of shaft 2) and a rigid connection with only torsional stiffness.

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