Trunnions b of ring c of link 3 fit into round holes a of the prongs of link 1. The inner part of link 3, in the form of a bushing, fits over cylindrical guide e of link 4. Square trunnions f at the ends of link 4 slide along guide slots h of link 2. With respect to each other, links 1 and 2 accomplish two turning motions about perpendicular axes x-x and y-y, and two translational motions along perpendicular axes y-y and z-z.
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