Static Analysis ignoring new position of load

[adminJ3TVX]

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.

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 of rotation.  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 the simulation was still not working properly, even though this body was bonded to the base structure for the purposes of the simulation.  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.

[TheCADWhisperer]

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@adminJ3TVX wrote:

 I expect the tripod to want to tip over...

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Tipping is bodies in motion analysis.

You state you are doing static analysis?

@adminJ3TVX 

Are you a student?  You can do Motion Loads analysis in Autodesk Inventor Professional - Dynamic Simulation.

Students can get Autodesk Inventor Professional for free.

[adminJ3TVX]

I am doing static analysis, but when I fit the rubber feet I can detect when they go into tension as opposed to compression.  Also, even if I do them in metal, with a Rough contact to the floor, I can see when it starts to separate from the floor.  I know this is far from ideal, but it provides an indication which will suffice for my application.

However, the issue that I described in the opening thread makes it more cumbersome to do small adjustments to the model to compare outcomes, which becomes very frustrating and time consuming especially since the solving function now happens in the cloud.

[jesus.rodriguezX55VX]

Could you please share the model?

[adminJ3TVX]

I attach two separate models with the only change between the two being that the upper offset beam was modeled to "join" the main body to form one body (except for mounting pads), whilst in the second model the offset beam was modelled as a "new body" but was bonded to the main body for the purpose of the simulation.

The image shows the difference in the simulation for displacement, clearly showing the fact that the simulation "ignores" the application of the force at the end of the offset beam.

[jesus.rodriguezX55VX · Accepted solution]

The "Tilt Test Basics Mounts Separate Upper Body" case shows an incorrect displacement field.  This is a bug that was fixed and the fix will be available with the March update of Fusion.  There is a workaround in the meantime:  Use "offset bonded" contact:

With this change, the results will be correct:

[adminJ3TVX]

Thank you very much for sharing this. 

I tested my "Tilt Test Basics Mounts Separate Upper Body" with the contact between the separate body as "Bonded" and the result was the same incorrect result as before.

I then changed the contact to "Offset Bonded" and as you indicated the workaround worked and the result is as expected.

However, when I then rotated the separate body into a new position towards the opposite side (as opposed to modeling it in this new position from the start), and did not change the contact from "Bonded" to "Offset Bonded" (I forgot to do it), the simulation also yielded the correct result. 

Although I am very happy with the result, I am not sure how that is possible, as when I then went back and tested with the separate body in the original position just to check if the bug wasn't perhaps fixed during the time between my two simulations, the result was the same incorrect result as before.

I include images of the three scenarios for reference.

[adminJ3TVX]

I am not a student, but thank you for the heads up. 

I am trying to stick to one CAD software program for now, as this in only one component of a much bigger project, and have found Fusion 360 adequate for my purposes thus far.