Abstract: A tensioning device has an inner end plate mounted on a frame, a sleeve fixed to the end plate and extending along an axis toward an idler wheel over which is spanned a flexible endless element (e.g. a drive chain or conveyor belt), and an axially compressible stack of inner and outer spring rings. The stack is closely surrounded by the sleeve, forms an annular friction spring, and has one end bearing axially on the plate and an opposite end. An outer end plate bears axially toward the frame end plate on the opposite end of the annular friction spring, and a hydraulic unit is braced axially between the outer end plate and the idler wheel. The inner spring rings have substantially frustoconical outer surfaces and the outer spring rings have substantially frustoconical inner surfaces engaging the outer surfaces of the inner rings.