Input: blue crank that has turning angle α. Output: orange bar that has turning angle β. α, β = 0 when the blue crank and the orange bar are in line with the fixed runway. Distance between revolution joints of the green bar is 16 + 6. Distance between revolution joints (length) of the blue crank is 20. In the α range from 0 to 20 deg., β is nearly double α with max error of 5%. The mechism can be applied for controlling ball retainer in Rzeppa joints.