A Rod Constraint causes all of its constrained joints to move together as a rigid rod, i.e., it is infinitely stiff in axial direction, but not constrained in any other direction. This constraint can be used to:
Prevent axial deformation in frame objects.
Maintain fixed distances between joints.
The constraint must apply to at least two joints to have any effect on the model. It is intended to be used on a set of joints that lie along a straight line. If the joints do not lie along a straight line, the constraint will effectively restrain the joints against rotation about the two bending axes, which is usually not desirable.
Use the following areas on the Rod 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 constraint axis (below) is being defined. If the constraint axis is automatic, the choice of coordinate system has no effect on the constraint.
Constraint Axis options. The constraint equations will be written for a rod along the selected axis. If the joints lie along a unique straight line, it is usually best to choose Auto and let the program automatically determine the axis. This axis does not necessarily need to be parallel to a coordinate-system direction. If the joints are not colinear, or to force a particular direction, choose the axis in the chosen coordinate system that is parallel to the rod. For example, if the X axis is chosen, this constraint is equivalent to a body constraint in the same coordinate system with only translation X chosen.
If the Define menu > Joint Constraints command was used to access the Rod Constraints form, the defined constraint must be assigned to the joints.
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Access the Rod Constraints form as follows:
Click the Define menu > Joint Constraints command to display the Define Constraints form, or select the joints to be constrained and click Assign menu > Joint > Constraints command to display the Define/Assign Constraints form. Then do one of the following:
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