setting materials for simulation

[safiredesignengineers]

Hi Group

if i am designing an assembly and in the design i have an item which i know the weight of in kg's can i just set the solid model to have the exact known weight so it can be used in the structural simulation?

i.e like a punchbag which is made of a mixture of materials not listed in the materials list.

A

[John_Holtz]

Hi @safiredesignengineers 

If you are including the component in the simulation (such as the punching bag in your example), you cannot enter the known mass. You need to calculate the mass density based on the known mass and volume of the component in the model, and enter the mass density in the material properties.

If you are replacing the component with a concentrated mass, then you can specify the total mass. Replacing a body with a concentrated mass is typically done when you do not know the full details of the body, or the stiffness, and those items are not important in the analysis. For example, if you are designing a structure that holds an electric motor and gearbox, you normally do not model the gearbox and motor in any detail. Instead, you draw a lump, and you replace the lump with a concentrated mass that has the proper CG, mass, and mass moment of inertia.

[safiredesignengineers]

Hi John

thankyou for your reply, can i ask:

- when you say "concentrated mass" is this "point mass" in fusion 360?

- on point mass, i can select a point or a face or a selection of faces and add a weight in Kg's  but what does the "x distance" input box mean

- on a single body should i place the point mass at the COG?  if so how do i show the COG so i can see and select it?

- on a group of components do i group them and then apply a point mass and if so how / where do i place the point mass? or will i be able to see and select the COG for the grouped components?
i.e on a punch bag, i may of drawn the bag, the support chains, the bracket all as individual parts so they will need to be used together for the load on a bracket supporting them.

A

[John_Holtz]

Hi,

1. Yes, I should have written "Point Mass" instead of concentrated mass. (I also should have opened the software to see what terminology was used by Fusion. ☹️)
2. Yes, you generally want the point mass to be placed at the center of gravity (CG). If the body is modeled, use "Loads > Point Mass (Auto)". This will place the point mass at the CG of the selected body (and removed that body from the analysis). If the body is not in the model, then use "Point Mass (Manual)" and specify the X, Y, Z coordinates where you want the point mass to be located. 
3. Same thing.

For both mass types, the last selection is the face (or edge) to which the mass is connected. Rigid connectors are created (but not shown) that attach to the point mass to the selected face (or edge).

For your punching bag example, the chains and all of the other hardware are probably insignificant compared to the bag. No one would question your choice to place the point mass at the CG of the bag. But you can do it either way. (For that matter, it sounds like you are designing the hook, so just model the hook and apply the appropriate forces directly to the hook.)