The steel rod 20 mm in diameter passes centrally through a steel tube of 25 mm internal diameter and 30 mm external diameter. The tube is 800 mm long and is closed by rigid washers of negligible thickness which are fastened by nuts threaded on the rod. The nuts are tightened until the compressive load on the tube is 20kN. Calculate the stresses in the tube and the rod. Find the increase in theses stresses when on nut is tightened by one-quarter of turn relative to the other. There are 4 threads per 10 mm. Take E= 2x10⁵ N/mm² .