Center of Gravity - am I missing something?

[GeneArch]

Hello,

I am creating an assembly of a tiltbed truck trailer, and I need to find its center of gravity. I know I can go to the Physical tab of the iProperties and update its COG there, but when I did that, I got some unexpected results.

Actually, the COG is basically where we expect it to be (about the center of the bed), but we want to make sure that it's reliable. To test it, I put a huge steel i-beam in the assembly, and constrained it on the back of the bed. I expected this to move the COG waaay back on the trailer, but it only moved about an inch. This was a 14-inch i-beam, it should have moved more than that. Is there something I'm missing? Most of the parts I'm using for this are the standard Inventor content center structural steel parts (c-channel), and for the parts I created myself, I made sure they all had the correct material designations (all steel). Shouldn't I get an accurate reading of my assembly's COG, or am I forgetting something?

Any help is greatly appreciated.

[Anonymous]

Make sure you define the material for each part in the assembly.
Then the CoG should work properly.
Chris
wrote in message news:5219141@discussion.autodesk.com...
Hello,

I am creating an assembly of a tiltbed truck trailer, and I need to find its
center of gravity. I know I can go to the Physical tab of the iProperties
and update its COG there, but when I did that, I got some unexpected
results.

Actually, the COG is basically where we expect it to be (about the center of
the bed), but we want to make sure that it's reliable. To test it, I put a
huge steel i-beam in the assembly, and constrained it on the back of the
bed. I expected this to move the COG waaay back on the trailer, but it only
moved about an inch. This was a 14-inch i-beam, it should have moved more
than that. Is there something I'm missing? Most of the parts I'm using for
this are the standard Inventor content center structural steel parts
(c-channel), and for the parts I created myself, I made sure they all had
the correct material designations (all steel). Shouldn't I get an accurate
reading of my assembly's COG, or am I forgetting something?

Any help is greatly appreciated.

[tyler.henderson]

The default material may not have the correct density. Your I-beam may only weight a fraction of a pound which obviously won't change the COG much.

Tyler Henderson (Autodesk)

[Anonymous]

Also, remember to update the propertys.
wrote in message news:5219141@discussion.autodesk.com...
Hello,

I am creating an assembly of a tiltbed truck trailer, and I need to find its
center of gravity. I know I can go to the Physical tab of the iProperties
and update its COG there, but when I did that, I got some unexpected
results.

Actually, the COG is basically where we expect it to be (about the center of
the bed), but we want to make sure that it's reliable. To test it, I put a
huge steel i-beam in the assembly, and constrained it on the back of the
bed. I expected this to move the COG waaay back on the trailer, but it only
moved about an inch. This was a 14-inch i-beam, it should have moved more
than that. Is there something I'm missing? Most of the parts I'm using for
this are the standard Inventor content center structural steel parts
(c-channel), and for the parts I created myself, I made sure they all had
the correct material designations (all steel). Shouldn't I get an accurate
reading of my assembly's COG, or am I forgetting something?

Any help is greatly appreciated.

[GeneArch]

Ok, I took a look, and realized my steel i-beams and c-channels (which are the Autodesk Standard Content Center Parts) only have a density of about .284! How would I change this? I can't edit the density, and I would think that it would calculate the mass and volume correctly based on the dimensions of the part (but apparently not). The density is saved in the part as a user parameter, called "G_RHO" - and it has the correct value. But I can't use that in the COG calculation (or can I?). Any other suggestions?

[kpanik]

.284 what? The density of steel is approximately 0.284 Lbs/in^3.

kp

[GeneArch]

Ah, then I was getting confused by units of measure. So apparently the density is correct. Here, I'll paste the properties for one of my c-channels, and see if anyone can make heads or tails of it (I know I can't - I'm not an engineer).

Physical Properties for 171.060_76.5
General Properties:
Material: {Steel, Mild}
Density: 0.284 ( lbmass/( in^3 ) )
Volume: 184.067 in^3
Mass: 52.268 lbmass
Area: 1.422E+003 in^2
Center of Gravity:
X: 38.250 in
Y: 3.000 in
Z: 0.513 in
Mass Moments of Inertia
Ixx 300.677 lbmass in^2
Iyx Iyy 1.273E-012 lbmass in^2 2.551E+004 lbmass in^2
Izx Izy Izz 1.591E-013 lbmass in^2 -3.978E-014 lbmass in^2 2.578E+004 lbmass in^2
Principal Moments of Inertia
I1: 300.677 lbmass in^2
I2: 2.551E+004 lbmass in^2
I3: 2.578E+004 lbmass in^2
Rotation from XYZ to Principal
Rx: 0.00E+000 deg
Ry: 0.00E+000 deg
Rz: 0.00E+000 deg



This is a 76 1/2-inch long piece.



Otherwise, I don't know what is wrong, if my materials are indeed set up correctly. The COG of the entire assembly SHOULD be closer to the rear. Right now, it's about dead center. Message was edited by: GeneArch

[GeneArch]

Oh, here's a screenshot of the assembly. I can't attach it, because it's too large. Maybe a view of it will give you a better idea of what I'm talking about.

Thanks.

[tyler.henderson]

From what I can see in the image the COG looks correct. Can you send one with your I-beam in the assembly? Remember to do an update after you insert the I-beam. Also, check the mass of the assembly before and after inserting the I-beam, that should verify that the I-beam is adding the expected mass.

Tyler Henderson (Autodesk)

[kpanik]

without knowing what channel you're using I can only assume the numbers are correct. It would be somewhere around a 8 lb/ft channel. I doubt 52 lbs will make that much difference in your assembly.

kp