mass property and COG

Can someone advise on how to apply a mass and a center of gravity to a
imported assembly (STEP) consisting of a few hundred parts. I don't
want to individually apply a material to each component.

Thanks in advance.
HB

 

: Can someone advise on how to apply a mass and a center of gravity to a
: imported assembly (STEP) consisting of a few hundred parts. I don't
: want to individually apply a material to each component.
:

If all of the parts just happened to be the same material or at least their
densities were close, the only problem would be getting the hundreds of parts to
act as a single part. You could do this by creating a shrink wrap model or
possibly, by importing the STEP file as a part instead of an assembly. In both
case I think you can make a merged solid to which you can apply a material or at
least a density and do a model mass properties analysis. It would be nice though
to have a check, like knowing the actual weight. If the mass calculation is within
5% of measured weight, the cog should be very close, as well. If you need to get
closer to measured weight, adjust the density number.

This might also work where some of the component densities vary greatly from an
average. Before making the shrink wrap, suppress the components with extreme
densities and just apply density to then separately. You'd cut down the work
compared to doing them all individually.

David Janes

 

Thanks for your reply. The imported parts are of multiple materials,
but I know the mass of the imported assembly and I know where the
center of gravity (COG) is located. I want to apply these properties
to the imported assembly so that I can determine the mass and COG of
the complete assembly consisting of all my components and the imported
assembly, but I just don't know how to do it. Any information would
be greatly apprecieted.
HB

 

: Thanks for your reply. The imported parts are of multiple materials,
: but I know the mass of the imported assembly and I know where the
: center of gravity (COG) is located. I want to apply these properties
: to the imported assembly so that I can determine the mass and COG of
: the complete assembly consisting of all my components and the imported
: assembly, but I just don't know how to do it. Any information would
: be greatly apprecieted.
: HB
:
You can record mass property values in a part/assembly with 'Edit>Setup>Mass
props'. If there is any material assigned, you should remove it first. Also, as
much as possible, make sure the assembly components use consistent units; it just
makes life easier when units are consistent through all levels. The Setup Mass
Props input box hass four sections; about half are greyed out. If you change the
default Source from Geometry to Geometry and Parameters, two of the input sections
are activated, the two you actually need, Density and Actions. Density, in the
case of an assembly, will be average density or a simple calculation of weight
divided by volume. The volume is obtained by a model analysis; the weight I think
you said you know. To give the assembly some numbers for mass without assigning it
a material and to input cog, go to the Actions section and pick Edit. This gives
you three additional tabs to input the rest of the known mass properties. I'm not
sure, but since it allows you to input mass and volume, it may actually calculate
the density for you. The second tabbed page gives you input lines for x, y and z
values for cog that reference some assembly coordinate system.

Now you can do an analysis on the top level assembly and use the values from the
subassembly. It will interrupt the calculation to ask how you want to treat the
subassembly with the input mass props. You have a choice between Assign or
Calculate. Leave it on the default Assign and hit Close. It will generate the
usual Mass Props analysis, including average density, mass, volume and surface
area plus assign the whole thing a cog. Record the cog and try it again, this time
selecting Calculate for the imported assembly. The cog should shift as the one you
input in Mass Props will be ignored.

David Janes


: > : Can someone advise on how to apply a mass and a center of gravity to a
: > : imported assembly (STEP) consisting of a few hundred parts. I don't
: > : want to individually apply a material to each component.
: > :
: >
: > If all of the parts just happened to be the same material or at least their
: > densities were close, the only problem would be getting the hundreds of parts
to
: > act as a single part. You could do this by creating a shrink wrap model or
: > possibly, by importing the STEP file as a part instead of an assembly. In both
: > case I think you can make a merged solid to which you can apply a material or
at
: > least a density and do a model mass properties analysis. It would be nice
though
: > to have a check, like knowing the actual weight. If the mass calculation is
within
: > 5% of measured weight, the cog should be very close, as well. If you need to
get
: > closer to measured weight, adjust the density number.
: >
: > This might also work where some of the component densities vary greatly from
an
: > average. Before making the shrink wrap, suppress the components with extreme
: > densities and just apply density to then separately. You'd cut down the work
: > compared to doing them all individually.
: >
: > David Janes

 

Thanks for your response, David, you've been very helpfull ! Looks
like it's working now. Is there a way to display the COG on the model
?

Thanks again,
HB

 

: Is there a way to display the COG on the model?

Yes, the analysis you did can be captured in a feature which stays with the model.
When you are at the point of doing the top level analysis, go to 'Insert>Datum
feature>Datum Analysis'. Pick Model Analysis/Next and Compute/Done. You can now
concretize any or all of the values of the analysis as parameters by simply
picking the analysis from the list and clicking the radio button for Yes. To get
the COG, click Next again and specify either a CS or Point to identify the COG.

Because this is added to the model as a feature, it remains in the model and
continues to be visible, unless blanked with a layer.

David Janes