A double-threaded Acme stub screw of 3-in. major diameter is used in a jack having a plain thrust collar of 4-in. mean diameter. Coefficients of running friction are estimated as 0.10 for the collar and 0.15 for the screw. a. Determine the pitch, lead, thread depth, mean pitch diameter, and helix angle of the screw. b. Estimate the starting torque for raising and for lowering a 6000-lb load. c. If the screw is lifting a 6000-lb load at the rate of 4 ft/min, what is the screw rpm