Abstract: A power unit having an increased capacity for utilizing the potential energy of a stressed spring over periods of extended duration. The power unit is characterized by a housing, a drive shaft projected from the housing and supported for rotation about its longitudinal axis, a radially expansive spring unit of a substantially circular configuration disposed within the housing and connected to both the shaft and the housing, and an expansion control unit including a fluid-actuated ram radially oriented with respect to the spring unit having a pressure pad of an arcuate configuration forcibly engaging the periphery of the spring unit for selectively accommodating radial expansion of the spring whereby energy is released at a selectively controlled rate.