Would it be a good idea to use Euler's eigenaxis ##\vec{e} = \begin{bmatrix} e_x & e_y & e_z \end{bmatrix}^T## as the rotational axis for the spring? And then compute the spring moment as the product of the spring constant and the rotation around the Euler axis?
##
\vec{M} =...
What I want to do is model the rotational behaviour of two bodies (1 and 2). They are connected by three virtual rotational springs (representing a link between them). For a normal (translational) spring the forces on body 1 in X, Y, and Z would be easily calculated as:
$$
\mathbf{F}_1 =...