A hybrid accessory drive system having two operational modes, a mechanical mode and an electric mode. The accessory is typically a cooling fan. A continuously variable mechanical belt drive mechanism drives the accessory as one mode of operation. An electric motor along with a planetary roller screw mechanism which converts rotary motion to axial motion is used to change the sheave position and thus the drive ratio. Upon disengagement of the mechanical drive mode, the electric motor can be used to drive the accessory in a second mode.