Weld Constraint Form

A Weld Constraint is actually a set of automatically created Body Constraints used to connect independent but matching meshes. By default, all degrees of freedom at each connected joint participate. However, a subset of the degrees of freedom can be selected to be constrained.

Within the set of joints assigned to the weld constraint, a separate body constraint is internally created for each subset of joints that are near each other, that is, within the Weld Tolerance. Joints not within the Weld Tolerance are not constrained. Each created constraint may contain two or more joints.

Tip: Make sure the Weld Tolerance is not too big, or parts of the model that should be independent may be constrained together. To connect joints that are not very close together, use individually defined body constraints.

Use the following areas on the Weld Constraints form to make the appropriate entries:

Constraint Name edit box. Use the default or enter a unique name for the constraint being defined.
Coordinate System drop-down list. Choose the coordinate system in which the constrained degrees of freedom (below) are being defined. If all degrees of freedom are being used, the choice of coordinate system has no effect on the constraint.
Constrained DOFs options. Choose the degrees of freedom being constrained. By default, all degrees of freedom are constrained, forming a fully rigid body from the constrained joints. If a translation is chosen but not the two rotations that are perpendicular to it, a risk exists that the model will be restrained if the joints are not coincident. For example, if translation X is constrained but not rotation Y, and the joints are separated by a non-zero distance in the Z direction, any moments about Y caused by X direction forces will be restrained.
Weld Tolerance edit box. Enter a small distance (length units) that will be used to determine if joints are close enough to each other to be constrained. It should usually be a small fraction of the typical length of an object in the region where the weld constraint is being defined.

If the Define menu > Joint Constraints command was used to access the Weld Constraints form, the defined constraint must be assigned to the joints.

Access the Weld Constraints form as follows:

Click the Define menu > Joint Constraints command to display the Define Constraints form or select a joint and click the Assign menu > Joint > Constraints command to display the Define/Assign Constraints form. Then do one of the following:

- Select "Weld" from the Choose Constraint to Add drop-down list and click the Add New Constraint button.
- Select a previously defined weld constraint in the Constraints display list and click the Modify/Show Constraint button.