Equations (9.1) and (9.2) correspond to two assembly modes (AMs): Eq. (9.1) corresponds to the AM 1 where point B is on the same side of the LBB (see Fig. 5b); Eq. (9.2) corresponds to the AM 2 where point B is on the opposite side of the LBB (see Fig. 5b). Only the balance error of AM 1 will be discussed below. Note that if there is an offset between point A and point C, l2 will no longer be equal to l3. The gravity balance error is caused by their difference Δl2 as