These two sets of solutions correspond to the two AMs of the four-bar linkage: AM 1 (see Fig. 9b) and AM 2 (see Fig. 9c). Ideally, the mechanism satisfies lAD = l3 and lAB = l2; that is, point C always coincides with point A in AM 1, and thus the mass assigned to C, mC (see Eq. (20.2)), does not induce any moment at point A. According to Eq. (2), the spring stiffness that achieves complete gravity balance can be obtained where mB is the lumped mass at point B, and mp4 is the mass of pulley 4.