Rigid Link

A rigid link imposes the same velocity on all secondary nodes in one or more directions. The directions are defined to a skew or global frame. Figure 1 displays a rigid link.
The velocity of the group of nodes rigidly linked together is computed using momentum conservation (Section Definition, Equation 1). However, no global moment equilibrium is respected.(1)
V i = i = i N m i v i i = i N m i
Angular velocity for the ith DOF with respect to the global or a skew reference frame is:(2)
ω i = ( j = 1 n I j i ω j i ) ( j = 1 N I j i )

For non-coincident nodes, no rigid body motion is possible.

A rigid link is equivalent to an infinitely stiff spring TYPE8.