I have some conceptual doubts about shafts subjected to torsion. When studying Strenght of Materials, to find stress and strain in power transmission shafts, we consider that one of the ends of the shaft is fixed, with all degrees of freedom restricted, and the other one is receiving torque. I'm having trouble to understand this fixed end. If it's a rotating shaft, how can it be fixed? But if it's not fixed, we can't apply equilibrium equations to solve the problem.