Finite element analysis and experimental study on roll-forming method in iso-wall thickness stator bushing of screw drilling

Abstract

In this paper, roll-forming experiments are analyzed with the method of single-roller rotary feed (S-RRF). By exploring insufficiencies of the S-RRF, an improved method named multi-roller rotary feed (M-RRF) is proposed. The mechanical model of the roll-forming is set up. On the basis of the unit pressure differential equation by Karman, the rolling-force formula is obtained. Taking the stator bushing with the type of 5LZ73×7.0 as the research object, operation processes and rolling parameters of the M-RRF are investigated in reference to the finite element method (FEM). The influences of rolling speed, roller size, rolling temperature, and friction coefficient upon the rolling-force and the axial-force are studied. It is concluded that the rolling-force is the most sensitive to the temperature, and the axial-force is the most affected by the roller size. The effects of rolling temperature on residual stress are further investigated, and the evaluation index of the residual stress is established by exploring the distribution of stress. It is found that the fatigue strength of the stator bushing is improved as the radial working stress is counteracted by the radial compressive residual stress. It is also found that the forming efficiency of the M-RRF is m times than the S-RRF, and the forming quality is improved when billet pipe is compressed uniformly at the same time. Two-pass rolling can also improve the forming quality, but the forming stress in the secondary rolling is larger than that in the preforming as work-hardening phenomenon appears after one-pass rolling, and this phenomenon is more prominent in hot rolling.