A rotary device with a speed governor as a part of a centrifugally radially expandable rotor and having radially adjustable prestress means for preselecting the maximum speed at which the governor operates to limit the speed to this maximum. The rotor which contains both torque imparting means and governing means as integral parts is a hollow flexible rotor that expands radially under centrifugal forces to move the sides of the rotor toward each other during increasing centrifugal expansion with increasing speeds of rotation. The rotor also has a fluid valve that is moved toward closed position by the centrifugal expansion to govern the speed of the rotor to a preselected maximum.