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Calculation of moment of inertia

rlewin
3-Newcomer

Calculation of moment of inertia

Hello

 

I am having issues with the mass analysis tool in CREO parametric 4.0 giving me rubbish in the moment of inertia calculation. To test the software I drew a simple rod with a diameter of 50mm, a length of 100mm and a density of 7.8270820e-06 kg / mm^3 (steel).

A simple (and exact) hand calculation for the moment of inertia is:

I=(1/12)*mass*length^2

around the centre of gravity and:

I=(1/3)*mass*length^2 around one end.

 

My hand calculation gives:

Mass = 1.54kg

I around CoG=1.28e3 kg/mm^2

I around one end = 5.12e3 kg/mm^2

 

The figures produced by the mass analysis tool are given as:

mass=1.54kg

I around CoG=1.52e3 kg/mm^2

I around one end = 5.36e3 kg/mm^2

 

As the inertia around one end should be exactly 4x the inertia around the CoG this is clearly wrong as it is not even self consistent.

 

Where am I going wrong as I need to trust this tool to correctly calculate the inertia of large, complex assemblies.

 

All the best

3 REPLIES 3
TomU
23-Emerald IV
(To:rlewin)

Does this information help at all?

https://www.ptc.com/en/support/article?n=CS180789

TomU
23-Emerald IV
(To:rlewin)

I think Creo is using the moment of inertia of a cylinder, not a rod.  Compare these online calculators:

 

http://www.amesweb.info/SectionalPropertiesTabs/Mass-Moment-Inertia-Rod.aspx

Inertia_04.PNG

 

 

http://www.amesweb.info/SectionalPropertiesTabs/Mass-Moment-of-Inertia-Cylinder.aspx

Inertia_05.PNG

rlewin
3-Newcomer
(To:TomU)

Hi TomU

 

You are right: thanks for that.

 

All the best

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