Abstract: An energy storage and conversion apparatus (1) comprising a containment (5) defining a vacuum chamber (7), a support shaft (9) within the vacuum chamber (7), a stator (11) on the shaft (9) and a cylindrical rotor (13) which, in use, is driven by the stator (11) to store energy as kinetic energy of the rotor (13) and acts with the stator (11) as a generator to release energy, wherein the rotor (13) is supported by the shaft (9) via an end cap (29) which, as the rotor (13) speed increases, deflects from a conical shape towards a planar shape. As a result, failure of the apparatus (1) does not occur when the rotor (13) expands due to centrifugal forces. Further, the position of the rotor (13) relative to the stator (11) and magnetic bearing on the shaft (9) is kept essentially constant, even if the length of the rotor (13) decreases due to Poisson effects.