Simulation study settings

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Simulation study settings.

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In this video, we'll take a look at removing rigid body modes.

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In Fusion 360, we want to start with a supply dataset rigid body mode.F3D.

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This will automatically open up in the simulation workspace and this design has its load cases already defined.

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What we want to do is we want to take a look at an option called remove rigid body modes.

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One important aspect of setting up a linear static stress study is to make sure that it is static.

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We're looking at small displacements over the linear range of a material that is isotropic.

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So when we talk about these options, when we set up our simulation study, we can have completely opposing loads that we know are equal and opposite,

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which in reality would mean that we are statically stable.

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However, that doesn't necessarily mean that the software understands that, it needs to be explicitly told.

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So if we expand our loads and constraints, we can see that this design has a fixed constraint on this end face,

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preventing it from moving in the Y direction and we can see that there is a bearing load on both sides that are equal and opposite.

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The problem with this is the degrees of freedom are partially fixed.

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It knows that this phase cannot move in the Y direction, but it's still free to move in X and Z and it's free to rotate.

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So when we look at solving this simulation study, we need to understand what the rigid body mode is going to be.

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To follow along you can solve the simulation study either on the cloud or locally,

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remembering that five cloud credits will be used to solve in the cloud.

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Now, I know because this design is not statically stable in the case of Fusion 360 simulation that it is going to fail to solve.

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So if you decide to solve this and you're using cloud credits and solve does fail, those cloud credits will be refunded to your account.

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So once again feel free to follow along and try to solve the simulation study,

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but do note that it will intentionally fail because Fusion 360 does not consider this statically stable.

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You can see here now that the simulation study did fail, it failed to solve,

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and if we take a look at the details, it gives us a little bit of information as to what happened.

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If we take a look at the cause, you can see here that it's telling us that the stiffness matrix has detected a problem,

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during the preconditioning phase, so it didn't even get in to solve.

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And then if we take a look at the actions, it's telling us to investigate the model for a lack of constraints.

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So noting that it's thinking that there are lack of constraints and that's what's causing the problem,

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even though we know that it's statically stable based on the loads, there's another option that we can do.

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We go into manage and into our settings for a simulation study.

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We have an option to remove rigid body modes.

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If you want to learn more this will direct you to the Fusion 360 website and tell you more about what remove rigid body modes does.

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Note that turning the remove rigid body modes on has not removed any warnings that we had in this design.

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It will still allow us to solve but removing rigid body modes is going to allow us to solve,

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and it's going to add its own inertial dampeners and springs preventing the entire design from moving,

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but allowing us to solve knowing that the intention is statically stable.

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The reason that we would want to do this is because the simulation setup itself by adding more constraints will give us non realistic results.

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By adding additional fixed constraints on our design we are essentially going to negate some of the loads that we would naturally seek.

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So let's take a look at the stress.

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If we're looking at the distribution of stress and we take a look at the deformation and if we use inspect and take a look at surface probes,

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we can see that these max values are going to be the same if not identical on both sides.

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When we're looking at a design like this that we know is statically stable and we know we can evaluate,

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remove rigid body modes helps us get to that point without adding additional constraints,

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simply for the fact of holding it in place so that we can analyze it.

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So this is a great option to use especially if your design is statically stable based on the loads that are opposing,

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and you want to evaluate what happens in that design.

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In our case, everything looks good.

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Once we've removed those rigid body modes, we were still able to solve even though we have a warning and there was no issue with the solve failing.

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So once we've solved, we don't have to save because it automatically saves a new version with those results.

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But it's important for you to explore removing rigid body modes more,

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and if you need to use that option again, going to remove rigid body modes and the help file is going to give you additional information,

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on what the tool does and when to use it.

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If any changes are made, of course, make sure that you do save before moving on.