Link 1 has ball-shaped head a fitting into two spherical half-collars b of link 3. Link 3 has, at its other end, clevis d in which pin c is secured. Link 2 turns about axis x' -x' of pin c. With respect to each other, links 1 and 2 accomplish four turning motions about three perpendicular axes intersecting at point 0 and about axis x' -x' .
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