A spherical joint is a joint with 3 rotational degrees of freedom. An instance of this class is used as cut joint to break a kinematic loop of a multibody system. The 3 constraint equations on position level state, that the distance between the origins of frame_a and frame_b is zero. As constraint forces fc (= Lagrange multipliers) of the joint fc = fa is used, i.e., the cut-forces at frame_a.