1 2 1 3 4 MDPI02 11 2019· processes Article Research on the Vertical Vibration Characteristics of Hydraulic Screw Down System of Rolling Mill under Nonlinear Friction Yongshun Zhang 1 2 Wanlu Jiang 1 3 Yong Zhu 4 and Zhenbao Li 1 1 Hebei Provincial Key Laboratory of Heavy Machinery Fluid Power Transmission and Control Yanshan University Qinhuangdao 066004 China