Centre of gravity, balance/tipping of an assembly

[Anonymous]

Hi. I know how to find the centre of mass of a single body/component or an entire project. The centre of mass of an entire project essentially calculates it as if it was in an open space on all 3 axis (if that makes sense, I know what I mean) and finds the point where you could balance your entire assembly on a beam. What I want to do is find the centre of gravity at the point on the floor, if that makes sense.

Let's say I was making a small portable crane or a floor standing support arm which used a big sheet of steel as a base, I want to find the correct place to attach the vertical beam to the base to ensure it's well balanced across the base under load and not going to be vulnerable to tipping in a particular direction. I've tried a few things with modelling the load at the attachment point and using the static force simulation, but haven't yet managed to find the results I'm looking for. Is there a way of using fusion to do this? Or any other way with the data/3D model I already have?

Sorry if that's a bit long winded, I just wanted to make sure I was clear about what I'm looking for. Thanks.

[colin.mcdonald]

Hi Will:

... unless I am misremembering the basics (always a possibility), an easy way to sort this out is to simply project your COG along your gravity vector (say "down along the Z-direction") and see if it falls outside your "footprint" on the "ground" plane.

  (Edit: this assumes that everything is stationary (or static) -- any accelerations are going to change this. In that case, hello dynamics calculations!)

Hope that helps.

Cheers, Colin

[Anonymous]

Yes, I did look at doing it that way but I didn't trust the result. Mainly because the centre of mass of the model was roughly halfway up the vertical beam which means the force at the base will be even stronger with twice the leverage.

It's not really an exercise to make sure it will stand up, I'm certain it will do that, here I am looking at minimising hazard and risk of it tipping if knocked whilst under load etc.

Is it possible to model a set of springs with a particular compression ratio and have them behave during a static stress simulation? I've had an idea that I could model some springs and place them under the four corners and possibly the midpoints along each edge, then simulate reasonable forces pushing it over in Y and X, monitoring the displacement/stress in the springs to give me an idea of how evenly the weight and forces are spread across the base?

I have no idea if that's possible or realistic.

[John_Holtz · Accepted solution]

Hi @Anonymous

You could model a block  or rod at each corner to act like the spring. The spring rate or stiffness of the rod in tension/compression = Area * modulus of elasticity / length.

[Anonymous]

Fantastic, that should certainly do the trick. I haven't had chance to get into Fusion to try anything since making my last post.

Three questions :

Can you make a physical material with your own properties so that you can set things such as mass, tensile strength, elasticity etc to specific values?

Fusion simulation will allow stretch and compression of a material/part? (I haven't tried)

I'm assuming I would make the bottom of each 'spring' a constraint for the simulation?

Thanks.

[John_Holtz]

Hi @Anonymous

The answer to all of your questions (at least the 3 that you asked ) are yes, yes, and yes.

[Anonymous]

Thought I'd update this for anyone else searching for a balance simulation. I ended up doing this quite easily by finding some real springs online with the right spring rate, and copied them (and material, in my case SS 302) using the create>coil function which I hadn't noticed before, and revolved the wire end 90 degrees to point downwards as a 'foot on the floor' to use as a constraint. Then set up a static stress simulation pushing vertically down on the top of the assembly. This worked perfectly and gave me the results I wanted and expected, and for reference using the X and Y centre of mass values is incorrect as I'd thought. My assembly which had a perfect (to within 0.009mm) COM was rear heavy as I'd expected it would be.

[TheCADWhisperer]

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@Anonymous wrote:
Thought I'd update this for anyone else searching for a balance simulation. ... Then set up a static stress simulation pushing vertically down on the top of the assembly.

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You could do this dynamically in Autodesk Inventor Dynamic Simulation Environment.

Use the Input Grapher to change load positions or amount... 

Even apply Friction between components.

[adminJ3TVX]

Hi, this topic is already a number of years old, but I have a problem similar to this, using Fusion 360.

I effectively have a tripod with a horizontal bar fitted at the top, offset to the one side.  When applying a load to the end of this bar, I expect the tripod to want to tip over, but it appears that the simulation ignores the fact that the load is offset.  I have changed the offset distance and load to be "extreme" in context of the design, and still it appears that the load is applied to the center of the design.

I eventually fitted three "rubber" spacers to the end of each leg to try and gauge by how much the different legs move when the load is applied, but even though I can stretch the spacers when the force is applied upwards and compress the pads when the load is applied downwards, the pads movement remains symmetrical, thus ignoring the offset position of the load.

What am I missing?

[adminJ3TVX]

I attach three more images of the tripod with the load downwards.  The deformation has been adjusted to exaggerate the effect.

[TheCADWhisperer]

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@adminJ3TVX wrote:

I attach three more images…

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@adminJ3TVX 

Can you File>Export your *.f3d file to your local drive and then Attach it here to a Reply?

[adminJ3TVX]

Hi, I attach the .f3d file here.

[adminJ3TVX]

A friend just used the STEP file in Solidworks and with the same constraints, the one mount goes into compression while the other goes into tension, exactly as expected.  As there are only three very basic constraints and one load, I am at a loss as to why Fusion 360 is not getting the same result.

[colin.mcdonald]

Hi adminJ3TVX:

... had a look at your F3D: your problem may be with your "fixed constraint". Personally, it makes more sense to allow a prescribed movement of zero-in-the-Z -- which does allow me to poll the reactions at each foot.

Once said though, this does seem like you're still well within reach of hand-calcs. Perhaps get at it with the usual statics equilibrium formulae (?) ...

  And in the end, that load is well outside the footprint of the feet: I'd assume that it would not be hard to tip the assembly.

Cheers, C

[adminJ3TVX]

I managed to get the simulation to function properly, but managed to get it to work for reasons I do not understand.

I originally modelled the beam over the single leg of the tripod with the intention of keeping it as a separate body to be able to rotate it for tests at different angles.  Even though this body is bonded to the “base” after rotating it, the simulation ignores the beam’s new position and the result is calculated as if the beam is in the originally modelled position.

I then tested the simulation by modelling the beam in the position as shown in the test, but still keeping it as a separate body, but the result of the simulation was still not working properly.  The simulation still seemed to “recall” the beam in the original position, even though it was modeled in a different position, as opposed to being rotated into this position.

Only when I modelled the beam in the position as shown in the test and made it part of the same “base” body, does the simulation produce the expected result.

Any insights into this will be greatly appreciated.

[adminJ3TVX]

Hi, I will definitely be using statics equilibrium formulae in the end, but my simulation journey with this mock-up is still of use to me for other reasons, one being to understand the simulation environment better.
As mentioned, the length of the beam was exaggerated in order to try and get to an "obvious" outcome in the simulation.
Please see my comment below.