According to Nooks catalog, a 1/2-10 acme screw with a plastic nut needs .624 oz-in of torque to lift one pound. 240/.624 = 384 pounds. That's how much weight you can lift, but very slowly. As speed increases, torque drops off, possibly very rapidly. Also, if the screw is binding, it can drastically reduce the force. Disconnect the motor, and you should be able to fairly easily lift the Z axis by turning the screw with your fingers. If you can't, something is binding somewhere.