Fig. 2
Realization conditions for the case that l12 does not cross line l34 between J3 and J4: (a) A force w12 applied to the mechanism along line l12 must result in a twist t12 located on line l34 between joints J3 and J4 and (b) a force w34 applied to the mechanism along line l34 must result in a twist t34 located on line l12 between joints J1 and J2
