The invention relates to an electrical stressing means having a housing (2) which can be attached to the drive spindle of a machine tool and in which a threaded rod (3) for adjustment of the clamping jaws of a chuck is mounted such that it can be moved axially, and having an electric actuating motor (4), whose rotor is connected for drive purposes to a drive wheel (6). The drive wheel (6) is connected to the shaft generator (7) of a harmonic drive, and the internal gearwheel (8) of the harmonic drive is connected to the housing (2), with the steel bush (9) (flexspline), which has an external tooth system, of the harmonic drive being provided as a drive for the spindle nut associated with the threaded rod (3).