Disclosed is a drive mechanism for driving a spin test rig, comprising a transmission casing (1) which accommodates a transmission gear unit (8) having a vertical output shaft (10) and an input shaft (9) coaxial therewith. The output shaft (10) has an upper end arranged in the transmission gear unit (8) and a lower end adapted to be coupled to a test object. The transmission casing (1) includes a casing lower part (4) detachable therefrom with the gear unit installed, in which casing part the output shaft (10) is axially and radially carried in bearings (28, 29) at a distance from its ends. The transmission gear unit (8) is configured such that the output shaft (10), together with the casing lower part (4) and the bearings arranged therein, is removable from the transmission casing (1).