The position of the horizontal beam can be adjusted by turning the machine screw A. Neglec...

The position of the horizontal beam can be adjusted by turning the machine screw A. Neglect the weight of the beam. The pitch of the screw is p = 1 mm, and the mean radius of the thread is r = 4 mm. The coefficients of friction between the thread and the mating groove are μs = 0.20 and μk = 0.18. If the system is initially stationary, determine the couple that must be applied to the screw to cause the beam to start moving (a) upward; (b) downward.