The pitch of a \(50 \mathrm{~mm}\) mean diameter threaded screw of a jack is \(12.5 \mathrm{~mm}\). The coefficient of friction between the screw and the nut is 0.15 . Determine the torque required on the screw to raise a load of \(20 \mathrm{kN}\). Assume that the load does not rotate with the screw. Calculate the ratio of torque required for raising and lowering the load. Also calculate the efficiency of the screw in both the cases.