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Spot Weld Tips & Tricks in Pro/MECHANICA® Structure
Rod Giles BSc.,MSc.,NRA
CEP Functional Simulation
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Spot welds are a great way to connect two or more parts together in
Pro/Mechanica.They combine beams and links to give the correct type of
connection that simulates any discrete connection that is used in
engineering. So they can be used to model not only spot-welds but also
rivets, pins screws and bolts.
Spot-welds work by creating a beam that is perpendicular to the surfaces
that you want to join. For this reason the surfaces have to be
reasonably parallel where you want to join them (within 5 degrees). It's
perfectly possible to spot-weld two cylindrical surfaces together. The
beam is connected to the two surfaces by a special link, or multi-point
constraint, that rigidises the surface at the diameter of the spot-weld.
This means that moments are correctly transferred to the surfaces.
Spot-welds can be created on parts that are solid or shells or both.
Also bear in mind that the point that you use to specify the spot-weld
location does not have to be on either of the two surfaces that you
pick, merely that Mechanica must be able to project this point onto the
surface. If the part has a shell definition then at run time the beam
ends are projected to the mid-surface, together with the links.
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There are times when Mechanica won't create a spot weld or creates one
in the wrong place. Be sure to check that you picked the correct surfaces,
always use query-select. If Mechanica still doesn't create the spot-weld
correctly, try selecting a different pair of surfaces if the parts are
shell-paired. Mechanica will not create spot-weld if there is zero distance
between the surfaces. In this case make sure to select the two opposing surfaces
to make sure that the is a gap between them.
Although it's pretty straightforward to join two parts together, when you want
to join three or more things can get a bit tricky. Mechanica will not create
co-axial spot welds if the one of the surfaces that is selected is the same as
was used for a previous spot-weld. So if you have three parts to join, A,B &
C, then follow this procedure:
 | Join outer surface A to nearest surface of B. |
| Join Outer surface of C to nearest surface of B. |
| Join inner surface of A to inner surface of C. |
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This works for solid and shell models. If you have more parts to join then this
technique can be used if the number of parts is odd. If the number of parts are
even then you run out of surfaces to pick. The only option is to create a
slightly offset spot weld, as shown below:
In Mechanica 2000i2 there's also a slight problem with co-axial
spot-welds: the analysis will run properly, but it finishes with a fatal error!
This is OK if you are interested in just looking at results, but if you want to
use the analysis for something else, like using the results of a modal analysis
for a dynamic analysis, then it won't work! The only recourse is to offset the
spot-welds.
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