Moments of inertia in autocad

[res44]

I am mechanical engineer and I always use autocad 2000 and when I use the mass properties it gives me that:
Mass: 900.0000
Volume: 900.0000
Bounding box: X: 345.7805 -- 375.7805
Y: 800.0214 -- 801.0214
Z: 0.0000 -- 30.0000
Centroid: X: 360.7805
Y: 800.5214
Z: 15.0000
Moments of inertia: X: 577021180.3242
Y: 117483833.5754
Z: 693965013.8996
Products of inertia: XY: 259931293.5648
YZ: 10807039.3183
ZX: 4870537.1931
Radii of gyration: X: 800.7088
Y: 361.2999
Z: 878.1072
Principal moments and X-Y-Z directions about centroid:
I: 67575.0000 along [1.0000 0.0000 0.0000]
J: 135000.0000 along [0.0000 1.0000 0.0000]
K: 67575.0000 along [0.0000 0.0000 1.0000]]
but I know that these numbers are high because I did for a small piece of 30x1x30 mm and the moments of inertia has to be in order of 0.0206 mm4.
Thank you and if there is anybody to help me I ´ll be grateful

 

[IFRs]

Sounds like a units issue?
Drawing is 1:1 ?

 

[cgwilson2]

For mass properties to work corectly the object needs to be located at 0,0,0. Try moving the object to 0,0,0 at the axis that you wish to determine.

 

[IFRs]

Autocad shows Ix and Iy about the centroid as "Principal Moments and X-Y Directions about Centroid" as the last two items in the MassProp results table. I find that moving the object so it's centroid is at 0,0,0 merely make these numbers identical to the "Moments of Inertia" listed several lines above.

I'll wonder if you scale the object down by (/ 1.0 25.4) if you will get the "correct" answers.

Also, does the answer become reasonable if you change it to a 2-D polyline made into a region and then MassProp?

 

[naujoke]

To get rigth answer it is needed use Mass Properties twice:
first find position of centroid
and after the moved origin UCS (Command: UCS O ) to that position you will find products of inertia and moments in that point.
AutoCAD for Mass Properties uses AutoCAD units i.e. milimeters.

 

[cowski]

AutoCAD assumes a density of 1, so you will have to multiply your numbers by the density of the material you are using. You'll notice that Volume is reported as 900 (30x1x30) and mass is also reported as 900 (volume x density).

 

[Sparweb]

I there a variable that allows you to control the density? I've always wondered why AutoCAD would take a guess at the mass if there's no density properties for solids.

STF

 

[cowski]

The last version that I really used it with was ACAD 2000, which did not let you specify a density. Maybe one of the newer versions do, I'm not sure on that.

 

[IFRs]

Release 2004 uses a density of 1.0 also. I have not found a way to change it.