- [Instructor] This chapter is all about using
the 3-2-1 method because we want to improve the
simulation of stress distribution around the big end.
It's a method I first came across about 15 years ago
and it really changed the way I think
about modeling a lot of components.
So in this video we're going to look
at the background to that method.
We analyzed the conrod previously by
constraining the big end down to the ground.
So how can we avoid doing that?
Constraining in this way, grounding out the big end,
is effectively like superglue-ing the conrod to the crank.
So so far we've had to ignore the big-end results
in terms of both stresses,
and also in terms of displacements.
It really just not a very good way to set up an FEA model.
So our objectives with the new analysis are:
to only apply load at the interfaces,
which in this case means both
the big end and the little end.
We want to avoid constraining any of the interfaces.
In this case specifically,
we're trying to avoid constraining the big end now.
We've got to apply a balanced set of loads
so that load in equals load out.
We want to apply a minimum set of constraints
which won't interfere with any of the interfaces,
but will stop the conrod from having any rigid body motions.
This means in effect that it won't
attempt to fly around in space and cause
major problems in the FE analysis.
The first step in the process is to prepare for the
constrained degrees of freedom by splitting the geometry.
This creates a set of vertices that we want to use.
Any convenient plane will do,
so I've chosen the center of the conrod.
We use three of the vertices that are created
and they will be at right angles to each other.
In other words, they're orthogonal.
We picked the first vertex and apply three
translational constraint in X, Y, and Zed.
This gets rid of the three translational rigid body modes,
but still allows three rotational rigid body modes.
Now we apply constraints in X and Zed.
This eliminates two more rotational rigid body modes.
Finally, we apply a constraint in Y.
This eliminates the last rotational rigid body mode.
This set of exactly six degrees of freedom
provides a minimum set of constraints
required to enable a static analysis to run.
Finally, when we mesh,
the nodes will be created at the vertices
and they will inherit the correct degrees of freedom,
and hence the constraints.
The next step is to apply external loads.
The loads must balance exactly.
Even small out-of-balance errors
just will rip the model apart.
Just to emphasize, all six degrees of freedom must balance.
Here we're using axial direction balance
which is in, say the X direction.
If we had lateral load in Y or Zed, or any moments,
they should all balance to zero.
After the analysis is run,
we need to check reactions of all six degrees of freedom.
Resulting value should be zero or very very close to zero.
Any nonzero values will create
suh-pih-rih-yus local stress concentrations,
so we have to check for these.
Similarly,
there shouldn't be any local displaced shapes.
The reaction system should have
no effect on the overall model.
So in conclusion,
the 3-2-1 method is a really effective way to be
able to handle a free-floating type of model
without having to apply unrealistic
constraints to ground it in some way.
The conrod is a great example.
Neither end is actually grounded.
Other examples include a pressurized scuba tank,
a submarine floating underwater,
and of course a free-flying aircraft
would be a great example.
So in summary,
it's a really useful technique to have in your toolbox.