Understanding Mass Properties

[MadMango]

I have a question about the data shown when you look at Mass Properties in an Assembly. I am trying to get CG of a rotating assembly, along with plotting the axes and moments of inertia, as sub-components of the assembly are incrementally moved away from the central axis and then suppressed.

I have noticed that the Density of my assembly is changing in Mass Props. It had dropped from .300 to .241 pounds per cubic inch. Is this due to the suppression of sub-components, and this displayed Density being only an average?

 

[lumenharold]

I'm using SW09 and I don't see a Density displayed for the assembly. Are you using an older version with "Assigned mass properties" checked? Otherwise the weight of the assembly will go down as the parts are suppressed because they are no longer in the calculation of the total mass.

Harold
SW2009 SP1.0 OPW2009 SP0 Win XP Pro 2002 SP3
Dell 690, Xeon 5160 @3.00GHz, 3.25GB RAM
nVidia Quadro FX4600

http://www.lumenflow.com

 

[MadMango]

SW08 SP5.0, Assigned Mass Properties is unselected. I understand the Mass of the assembly decreasing, but not the Density. You will only see the Density value if you print the Mass Properties from the pop-up window.

 

[lumenharold]

I haven't been able to get the density of an assembly to print either. Perhaps I'm using the wrong menu to print from. Sorry I'm no help. Perhaps you could make a simple assembly of three identical parts and save each part configuration with significantly different mass. That way you could verify what you suspect is happening. I would but I can't see the density displayed.

Harold
SW2009 SP1.0 OPW2009 SP0 Win XP Pro 2002 SP3
Dell 690, Xeon 5160 @3.00GHz, 3.25GB RAM
nVidia Quadro FX4600

http://www.lumenflow.com

 

[CorBlimeyLimey]

Could it be tied to the de-valuing dollar?

I only see Density for a part, not an assy. Are you sure it's not a multi-body part?

Or is this the new style assy which has virtual components?

 

[handleman]

I'm running 2008 SP3.1. I get Density in mass properties, and it seems to be a total average density. That is, total mass divided by total volume. Not particularly useful. although it would tell you if the entire assembly would float/sink in some fluid medium.

-handleman, CSWP (The new, easy test)

 

[MadMango]

CBL, positive it isn't a multi-body part or virtual parts. Your problem is probably between the chair and keyboard.

Harold, no worries.

Handleman, mass/volume is what I suspect the Density value is showing. Fairly useless, I just wanted to double-check I wasn't going crazy.