- [Instructor] In this video,
we're going to look and see
how an example of rigid body modes can be created
in a normal modes analysis.
We're also going to identify
the characteristics of the rigid body modes.
So we go to File Open,
and if you're using SolidWorks version 2018 or later,
then you can open the file tray3_begin.
Alternatively, if you're using a SolidWorks version
prior to 2018, you need to open the Parasolid file.
That Parasolid file is version-independent.
So I'm going to open the SolidWorks file.
Now, go to the Simulation tab,
click on New Study, and open up New Study,
and choose, under the General Simulation,
the Frequency solution.
Let's name this as tray fixed, check OK.
Now, let's right-mouse-click on the part name here
and Apply Material to All Bodies.
And I'm going to choose the Alloy Steel again,
click Apply, and then click Close.
So this time, we're having the same material property
for the payload, as for the tray.
Now, as before, we're going to set up constraints
to simulate the bolts in the support regions.
So right-mouse-click on Fixtures,
choose Roller/Slider, and then go down
to the Advanced Option,
and we choose On Cylindrical Faces.
Again, for the bolts, we're constricting them radially.
We're also preventing them from moving actually.
And we need to find the surfaces,
so I'm gonna go ahead and select these surfaces.
Again, I'm going to fast-forward the video
to when we're ready, and that's completed.
And that's the task done
with all six sets of bolts now constrained.
Now right-mouse-click on Mesh
and select Create Mesh.
Go in to set the size of the element
slightly bigger than usual to deal, again,
with the payload, which is just a block.
Click OK.
Now, right-mouse-click on the study name,
select Properties, and we're going to set
10 natural frequencies to be calculated.
Click OK, and then we can click Run This Study.
Now, let's copy this study,
so we'll right-mouse-click Copy Study,
and we're going to name this as tray free.
Check OK.
I'm coming down to Fixtures,
and I'm deliberately going to
suppress the constrain, too.
So now, the model is effectively
a free-free model, as we call it.
There's no constraints to ground.
So let's run the study.
Now, let's run through the first six modes,
animating as we go.
So here's mode one, Animate.
You can see it's a rigid body motion,
moving in the x direction, zero hertz.
Go to the next mode, Animate.
This is a rigid body mode,
moving in the y direction, zero hertz.
Go to mode three, and Animate.
This is a rigid body mode in the vertical direction,
zero hertz.
Mode four, Animate,
this is a rigid body mode rotating about the x-axis,
zero hertz.
Mode five, Animate.
Rigid body mode rotating about the y-axis, zero hertz.
Mode six, Animate.
And this is a rigid body mode
that's spinning about the zed-axis, zero hertz.
Now, these are the six characteristic rigid body modes.
They've been calculated
because the model is completely unconstrained.
Now, if we go to mode seven,
this is now the first elastic mode.
It's at 266 hertz, and we can animate that.
Now, 266 hertz for the free-free model,
that is not the same as the mode one in the previous study,
and we're gonna compare that shortly.
Now, we've only got four elastic modes.
We asked for 10 modes.
Six have been taken up with rigid body modes,
so that leaves us the four elastic.
Let's just finish off and look at
those four elastic modes.
Here's mode eight, mode nine, and mode 10.
Now, to compare modes
between tray free and tray fixed.
Let's split the screen.
So we go to Window, Viewport,
Two-View Vertical, let's choose mode seven,
our first elastic mode.
Select a nice symmetric view,
and let's tidy up the display.
So go to Chart Options, switch off the legend,
and set this to Discrete.
Check OK.
Now click in the right-hand window
and choose the tray fixed study,
and we want to go and set up a nice symmetric view.
This time, we want mode one,
and let's tidy up that output plot.
Go to Chart Options, switch off the legend,
change the Settings to Discrete.
And now, we can just move the models into a position
so they're side-by-side.
So that's comparing the first elastic mode,
just 266 hertz,
with the fixed first elastic mode,
which is 136 hertz.
Let's move each one up one.
So let's go to the second fixed mode,
and with the tray fixed mode,
we've gone to mode two, the second elastic mode.
Click on the left-hand window,
and let's up that to mode eight,
second elastic mode.
So we got 405 hertz,
playing 226 hertz.
Let's move this up one.
Now I have mode nine at 489 hertz.
And the corresponding mode on this side,
we want mode three, here's at 440 hertz.
And we only have one more elastic mode in our free-free.
So let's compare mode four of the fixed,
which is 538 hertz.
Let's come to the left-hand window here,
and up that to our final elastic mode we recovered,
which is mode ten, which is at 619 hertz.
Now that exercise reproduces the images
in the earlier background video.
And we can see that, although the frequencies are similar,
the modes are very different.
The different boundary conditions, fixed and free,
have changed this difference in distribution.
And what I suggest you do
is to experiment further with this model.
You could unsuppress the constraints
but delete some of the constraints
that stay on the left-hand side.
That would result in just one rigid body motion,
and that will be rotation about the x-axis.
So let's save the file now,
and we'll save it as tray3_end.
We do want to copy the results files, as usual.
And that means we'll be looking for
that SolidWorks part file tray3_end.
And also, the complementary result sets,
tray3_end/tray fixed,
and tray3_end/tray free.
So, in this video, we've seen how to set up
rigid body modes by just removing the constraints,
and we've the characteristics of the rigid body modes.
We've also identified the frequencies.
All of the frequencies are very close to zero hertz.