Design of giant magnetostrictive structure for double-nut ball screw pre-tightening 2.1 Present double-nut ball screw pre-tightening structure Currently, there are two kinds of ball screw pre-tightening methods: single nut and double nut. The double-nut method, which can repeatedly adjust and maintain the pretightening force, is used in precision transmission where the return clearance is not allowed to exist. Double-nut pre-tightening methods include the gasket method, the thread method, the tooth difference method, the spring method and the piezoelectric ceramics method. In the gasket method, the two nuts are separated by a gasket and contact with the screw in opposite direction.