Pin b of link 3 fits into the hole in head a of link 1 . Round trunnions d of link 4 fit into holes in prongs c of fork-shaped link 3. Barrel-shaped spherical surface e of link 4 fits into spherical half-collars f of link 2. With respect to each other, links 1 and 2 accomplish four turning motions about the three perpendicular axes intersecting at point 0 and about axis x' -x' .
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