Numerical and experimental analysis of the dynamic effects in compact cluster mills for cold rolling

Prevention of dynamic instability caused by chatter phenomenon may be difficult in case of the so-called “cluster mill” because of the number of back-up rolls used to avoid bending of the work roll. A numerical simulation of the dynamics of the cluster mill may help to prevent the strip defects and to predict the life of bearings and rolls. Analytical models proposed in the literature for the two- and four-high mills are unfortunately inapplicable. This paper investigates whether the multi-body dynamics approach can be effective in analysing the so-called “Z-Mill”. Service monitoring detected in a real case some clear and dark bands on the strip surface, which disappeared after a calibration of rolls position. A numerical model was built by assuming that the mill stand and the strip could be analysed separately. Validation allowed concluding that modelling is effective, if relevant parameters are tuned on the experimental evidences. In particular damping and friction coefficients look the most critical to predict the actual value of rolling force. Mechanical nonlinearities introduced by the contact among rolls and by bearings increase the computational effort, but their role is somewhere overestimated in the literature. In practice, the multi-body dynamics approach still shows some limits, but they should be overcome, if a deeper experimental validation will be performed on a dedicated test rig, suitably designed to be largely instrumented.