In this case use a spring because constraint on the weighted link dependent node is not allowed.
The general spring replicates the desired constraint because it has stiffness settings for all 6 d.o.f.
see image.
Some of the ways springs are used:
To constrain a model so it is a static case. (eliminate d.o.f.)
To model some component stiffness as a simplification.
To model bearing/joint stiffness less than rigid. (skunks example is held at a bearing area)
To stabilize the solving for a component held only by contact friction. (use a low stiffness spring)
The option inertia relief also constrains any free d.o.f. And in the case of the unbalanced load mass of weight at the bottom joint vs mass of weight plus mass of lifting rig at the top joint, the inertia relief applies gravity to the model until the load is balanced and all d.o.f are held = static case. That is why I did not have to include gravity in the unbalanced model because inertia relief is a type of gravity load. Inertia relief is why no spring is needed for adding constraint on free d.o.f.
Inertia relief is a powerful tool because it allows the engineer to use a free body diagram with only loads applied and no constraints. Of course the free body diagram must be force and moment balanced as usual but there are small numerical errors that the inertia relief can "fix". In the case of any free body diagram with a gravity load, the gravity load is omitted when doing the FEA with inertia relief because the inertia relief will apply the gravity automatically to re-balance the forces.
