1 . A capping head ( 1 , 1 A) for applying pre-threaded caps to containers, including: a shaft ( 2 ) movable according to a rotary motion about its axis and a translational motion along said axis; a first electric motor ( 5 ) for driving the rotary motion of the shaft ( 2 ), the first electric motor ( 5 ) having a stator ( 50 ) and a rotor ( 51 , 51 a ) coaxially arranged around the shaft ( 2 ) and mounted in a stationary housing ( 6 ) so as to be decoupled from the shaft ( 2 ) as far as the translational motion is concerned; a first bushing ( 12 ), which is mounted at one end of the stationary housing ( 6 ) so as to seal the internal cavity thereof at said end, is passed through by the shaft ( 2 ) and is arranged to transfer the rotary motion of the rotor ( 51 , 51 a ) to the shaft ( 2 ) thanks to the cooperation between ribs ( 15 ) provided on an inner surface thereof and grooves ( 14 ) provided on an outer surface of the shaft ( 2 ); Wherein: the head ( 1 ; 1 A) further includes a second bushing ( 11 ) that is mounted at a second end of the stationary housing ( 6 ), opposed to the first one, so as to seal the internal cavity thereof also at that second end, and is passed through by the shaft ( 2 ) that is mounted in the same bushing ( 11 ) in a freely rotatable manner; the first electric motor ( 5 ) forms, together with the stationary housing ( 6 ) and the first and second bushings ( 12 , 11 ), a self-standing module for driving rotation of the shaft ( 2 ); and said module ( 5 , 6 , 11 , 12 ) can be associated, for driving the translational motion, either with a member ( 41 ) carrying cam follower rollers ( 42 ) arranged to cooperate with a mechanical cam ( 104 , 104 a ) along the whole of the path run by the head ( 1 ) during a capping operation, or with a second electric motor ( 200 ) arranged to impart the translational motion to the shaft ( 2 ).
2 . The capping head ( 1 ; 1 A) according to claim 1 , wherein the shaft ( 2 ) is translatable jointly with a decoupling joint ( 40 ) that is mounted at the end of the shaft ( 2 ) proximal to the second bushing ( 11 ), to allow rotation of the same shaft ( 2 ) about its axis and can be fastened, in a detachable manner and so as to be jointly translatable, either to the member ( 41 ) carrying the cam follower rollers ( 42 ), or to a piston ( 201 ) that is moved by the second electric motor ( 200 ) according to a linear motion.
3 . The capping head ( 1 ) according to claim 2 , wherein the member ( 41 ) carrying the cam follower rollers ( 42 ) is associated with means ( 7 , 43 ) for preventing, while the head ( 1 ) is advancing along the cam ( 104 , 104 a ), unwanted rotary movements of the same head ( 1 ) occurring during the translational motion.