Shell Modeling and Analysis
By D Cheshire Page 1 of 7
You have probably already realised that the initial model is very important
and can affect both the result accuracy and the time taken to perform the
analysis. For example analysis is often undertaken on models where the
majority of radii and other small features which have no significance on the
results have been removed of suppressed – this can reduce analysis time
tremendously. Of course it is down to skill of the operator to decide which
features can be suppressed without affecting the results.
A particular area where correct modelling can improve analysis speed is in
parts which have lots of thin walls of constant thickness. Examples of
these include sheet metal parts (simple brackets or complex car bodies)
and even moulded parts (since good moulding practice requires constant
wall thicknesses wherever possible). The modelling technique used for
these parts is called shell modelling. Here the designer will model the
centreline of a feature then assign a thickness to the feature. Pro Engineer
combines the information to generate a solid model which looks identical
to one made from normal modelling techniques. W hen analysing the
model the shell information can be used to reduce the analysis time –
experience has shown that this can be by as much as 100 times in
extreme cases.
Here is an example of the techniques involved. The tutorial uses a realistic
part so the process is quite complex. Pay careful attention as you read –
especially if you have not completed all of the modelling exercises in this
series.
Even if you don’t intend to use shell modelling the tutorial is worth
completing as it introduces other techniques related to analysis. If you find
the modelling instructions difficult to follow then have you completed the
modelling tutorials? If you haven’t you might find it helpful to do so.
Shell Modelling
Figure 1 : The Chair Base
The part we are going to analyses is the injection moulded base to a
swivel chair as shown in Figure 1. The first thing you should notice about
such a part is that it has 5 identical legs. This should immediately show
you that you can save both modelling and analysis time by only looking at
one of the five legs. Even more time can be saved if you recognise that
each leg has a plane of symmetry along its length (see Figure 2) so even
more modelling and analysis time can be saved.
Figure 2 : The Leg Half Model
Here is how to model the leg. Create a new part using FILE > NEW with a
name of chair_leg. Choose the mmns_part_solid template.
