“Device according to claim 2 characterised in that preferably the lower gripper unit (7) comprises receivers (7a, 7b, 7c) for balancing weights which are located one behind the other transversely to the longitudinal direction of the rotor with which receivers in the gripper closing direction respective counter elements (6a, 6b, 6c) are associated in a corresponding number to the other gripper unit (6).”