Please help: Pro Mechanica Newbie

Hi all,

I am working on a design project as a student in a University. We're
designing the frame for a small formula FSAE car. I modeled the frame
as a simple set of datum curves and points and analyzed them as beams.
I was successful checking the stiffness in pro-M by applying a moment
to the front and constraining the rear. My next step is to check the
frame with the engine installed in place of some of the members. The
motor is modeled as a part, so I opened a new assembly with the frame
and motor.

My questions are these: What is the best way to model the connections
between the motor, which is a 3-D feature, and the frame, which is a
set of datum curves? I ran an analyses and it crashed. I assume this
is because the motor and frame are only connected by datum planes in
the assembly. I am hoping pro-M can analyze connections between the
two without remodeling the frame as a 3-D model, or trying to replace
the motor with a series of curves.

Thanks for any suggestions,
Mike

 

<snip>
: My questions are these: What is the best way to model the connections
: between the motor, which is a 3-D feature, and the frame, which is a
: set of datum curves? I ran an analyses and it crashed. I assume this
: is because the motor and frame are only connected by datum planes in
: the assembly.

This is probably a good guess. But, it could also be because you mixed curve and
solid elements in a beam analysis. don't know how well ProM can handle this. You
know, when you do beam analysis, it tries to collapse regular solid and surface
features to mid-planes, loosing contact between assembled components which it also
doesn't handle very well. In any case, get rid of planar constraints. In part
mode, place points on the supporting curve structure in approximately the places
where the motor will be connected. Place corresponding points on the motor block
and use these for assembly. In fact, for beam analysis, instead of solid model,
you should just use a flat surface with some points on it. Or, you might even try
modelling the block as a wireframe outline with datum curves and place the points
on those curves, as well. I suspect that the whole analysis will run better with
similar construction of each part. And it will definitely run the fastest (and
probably the most trouble-free) it can with all curves.

David Janes

 

Dear Mike,

If there is no geometry connection, you could try using 'rigid' connections.
Only put the engine block in your frame model, as not much else contributes
to stiffness. Place datum point on the curves where the engine mounts are (I
assume that there are no rubber mountings otherwise you would use springs)
and use a rigid to connect this point to the mounting surface on the block.
If there isn't a suitable region, create a 'surface region' (Under
Model:Features) beforehand.

By the way, a beam model of a space frame will not give a good estimate of
the true stiffness of the frame, because beam joints are considered
completely stiff, so your frame model will stiffer than the real thing. I
quickly go from a beam model to a shell model of the frame, and I suggest
you have a go at this method if you want good results. However, if time is
short, go with the beam model but remember the real thing will be softer.

Regards,

Rod Giles
Pro/Mechanica Users Group UK

P.S. Please feel free to get in touch, I especially interested in Formula
SAE and helping out universities.

 

Thanks. This helps a lot. I will try some of these methods in the
coming week.

Mike

 

Thanks for the help. What I ended up doing was this:

I determined where the motor mounts would be on the frame. Then I
cross-connected these points with datum curves, and defined them as
massless beams with a modulus of elasticity of about
1,000,000,000,000. This would ensure that no force would be
transferred through them, and they would not bend or twist. They also
would not contribute to the mass of the space-frame.

I don't know if this is correct, but it seems logical. We are
somewhat pressed for time. It didn't add to the stiffness of the
frame, and I didn't figure that it would. The weak point is forward
of the motor, in the driver's compartment. We could make it rigid by
using side pod bracing, but we are trying to get by without them
because it will be lighter and it will look cooler.

Thanks,
Mike