Abstract: A 1/8 mill screw pair experiment model of heavy and medium plate mill is made to analyse the rapping mechanism and the forming reason of plastic bulge quantitatively. Based on the experiment model, a series of experiments are done like rapping distance testing, vibration experiment and FEM nu-merical simulation of the copper nut tooth plastic bulge. The mill screw pair is oriented on medium radius, which is force constraint and is unreliable. There is time difference between the mill screw and balance mechanism when bite and toss of steel, which is the main reason for the forming of rapping and plastic bulge. Compound nut is provided, which can take place of force constraint with geometric constraint which depends on the rear of nut tooth. In the compound nut, the back side of slave nut tooth is the working face. The results of 1/8 model experiment show that it can control screw-pair dynamic gap, realize self-balance of screw, reduce the effect of mill screw-pairs rapping and prevent the emergence of a plastic bulge of copper nut tooth with compound nut. At the same time, adopting compound nut can reduce the abrasion of copper nut effectively and elongate the nut lifetime.

Key words: Compound nut, Heavy and medium plate mill, Lateral gap, Mill screw-pairs rapping, Plastic bulge