How to Preserve Small Gaps in a Model

Whenever a model is checked or analyzed, the geometry and finite element data are automatically verified. Part of this process involves converting the lines of the model drawing into nodes and elements. However, some lines might not be connected correctly due to user error, mathematical round-off or difference in precision from an imported CAD model. To ensure element connectivity, an automatic tolerance is used to decide whether closely spaced endpoints should be merged together or not. For the most common cases, the model validation process merges endpoints appropriately to create a finite element model that is suitable for analysis. 

One exception may be when a model includes a very small gap (such as between parts involved in surface-to-surface contact). Sometimes, parts can be merged rather than preserving the gap between them. This is due to the following:

• The automatic tolerance that is used for merging endpoints is based on several model parameters. The most common parameter is the "merge tolerance", which is 1/20 of the shortest line in the model.
• If the shortest line length in the model is much larger than the gap, then the automatic tolerance will be set too large and the endpoints on either side of the gap might be merged together.

So, how can a very small gap be preserved? The most common method is to add a small line to the model. If the line is the same length as the smallest gap between the parts, then the automatic tolerance will be set smaller than the gap.  The parts will not be merged and the gap will be preserved. The following figures illustrate this method.

Figure 1: This model of gear teeth includes a very small gap between parts.	Figure 2: After the model geometry and finite element data are checked, the Superview IV Results environment shows that the parts were merged at node 40.

Figure 3: To preserve the gap, the model is modified by adding a line between the closest endpoints.	Figure 4: With the added line, the gap is preserved as shown in the Superview IV Results environment.

When using this method, there are some additional considerations depending on the type of elements that you are using in your model:

• If you are using truss elements, then draw a line the same length as the smallest gap anywhere in space, not connected to the model. Restrain both ends of the added line.
• If you are using beam elements, then draw the line connected anywhere in the model to one of the parts.
• For all other element types, you can use the method shown in Figures 1-4. That is, draw a line between the two closest endpoints of the gap using one of the existing element groups. The added line will not form a complete element, so it will be effectively ignored except for establishing an automatic tolerance that will preserve the gap.