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Pins C and B of links 6 and 4 slide along slot a of slotted link 1 which turns about fixed axis A. Nuts 4 and 5 are connected by screw pairs to links 7 and 3 whose axes are perpendicular to each other. Nuts 4 and 5 are traversed by the rotation of meshing bevel gears 2 and 8. The transmission ratio of gears 2 and 8, and the pitches of screws 3 and 7 are selected so that nuts 4 and 5 travel at the same speed. The nuts are set to comply with the condition: A͞C₀=B͞B₀=x. Then displacement s₆=C͞C₀ of link 6 along the axis of this link is s₆=(1/k)x² where k is a constant dimension of the mechanism.