Square trunnions b of link 3 fit into square holes in prongs a of link 1. Link 3 has box-shaped guide c in which link 4 slides. Lugs f on lug-type head e of link 2 slide along circular grooves d inside link 4. With respect to each other, links 1 and 2 accomplish two turning motions about perpendicular axes y-y and z-z, and two translational motions along perpendicular axes x-x and y-y.
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