Dynamic Coupling Through the Rolling Deformation Zone in CSP Mills

Compact Strip Production rolling mills belong to a class of large industrial systems in which mechanical transmission, hydraulic actuation, electrical drive control, and plastic deformation of metal operate as one physically coupled process. The roll system must sustain large deformation resistance from the strip, the drive train must deliver torque under changing load conditions, and the hydraulic screw-down system must maintain the roll gap while responding to force fluctuations generated during rolling. When these actions occur during the production of high-strength thin strip, even small dynamic disturbances can be amplified through the mill structure and through the deformation zone itself. A recurring difficulty in CSP rolling is the appearance of strong vibration during rolling operations, particularly when demanding strip grades and thin specifications are produced. Such vibration is not only a structural response of the mill stand or a torsional response of the drive shaft. It can be accompanied by changes in rolling force, motor torque current, hydraulic pressure, roll displacement, interface friction, and strip surface quality. The scientific problem is therefore not simply to identify a vibrating component, but to understand how different subsystems exchange dynamic influence during rolling.   The deformation zone, where the roll and strip interact under pressure, is the location where torque, vertical force, friction stress, roll gap variation, and material deformation are converted into one another. For modeling clarity, the main drive transmission system and the vertical hydraulic screw-down system are often treated as separate dynamic subsystems. That separation is useful and, in many cases, necessary for establishing solvable mathematical models, but the present rolling condition requires attention to what occurs where both subsystems act on the strip. Torsional fluctuation in the roll can change the circumferential velocity at the interface, thereby modifying friction stress and rolling force. Vertical vibration can change the roll gap and contact arc length, thereby altering rolling load torque and feeding disturbance back into the transmission system. The rolling deformation zone is therefore the site of strip reduction and also a dynamic coupling region.

In a recently published research paper in International Journal of Precision Engineering and Manufacturing Associate Professor Yifang Zhang, Dr. Yiwei Wan, Dr. Huiwei Yan,  Professor Cheng He, Professor Li Cui, Dr. Xu Ding, Dr. Tianyi Chen & Dr. Pingye Wan from Shanghai Polytechnic University developed a vertical-torsional-deformation zone coupling closed-loop dynamics model for a Compact Strip Production rolling mill. The model links the main drive transmission system, hydraulic screw-down system, and rolling deformation zone through parameter pathways involving roll torsion angle, circumferential velocity, interface friction stress, rolling force, contact arc length, and rolling load torque. They also developed and applied a synchronized industrial monitoring approach that allowed the proposed closed-loop mechanism to be checked against field vibration signals and lubrication-adjustment experiments.

The research team began with industrial monitoring on the F3 mill during rolling operations. They developed a measurement system capable of synchronously collecting torsional and vertical vibration information, while also drawing motor current and hydraulic rolling force data from the plant’s online process data acquisition system. Torque and bending moment in the drive system were measured through resistance strain gauges mounted on the rotating shaft, while vertical roll motion was captured using acceleration and displacement sensors installed near the roll bearing seat. This synchronized acquisition was important because the coupling mechanism depends on frequency relationships among signals measured under the same rolling condition.

The field signals revealed a structured frequency relationship between the drive and vertical systems. Torsional vibration in the main drive showed a dominant component near 41 Hz. The vertical vibration contained an 82 Hz dominant component as well as a 41 Hz component, linking the vertical response to the same fundamental frequency present in the torsional response. The hydraulic rolling force and motor torque current signals also contained harmonic components near the same 41 Hz range observed in the torsional and vertical vibration responses and it is this recurrence across drive, hydraulic, and vertical measurements gave the frequency-domain evidence a clear physical coherence, showing harmonically related behavior across subsystems. The team then calculated the inherent torsional characteristics of the F3 rolling mill using a finite element model built from the mill structure. The second-order torsional modal frequency was 42.4 Hz, close to the approximately 41 Hz component observed in the monitored vibration. External disturbances and harmonic combinations generated during rolling could approach the natural torsional frequency of the system, allowing strong vibration to develop.  For the main drive transmission, the researchers used an equivalent two-inertia nonlinear torsional model with excitation from both the electrical drive end and the roll load end. Nonlinear stiffness and damping terms were retained, and the authors used regular perturbation method to obtain the angular response under multi-source harmonic excitation. The solution showed that the torsional response contains frequency components generated by combinations of the excitation frequencies. When one of these combined components approaches the inherent modal frequency, strong torsional vibration can be induced.

That torsional motion was then connected to the rolling deformation zone. Fluctuation in the roller torsion angle changes the circumferential velocity of the roll. Under mixed lubrication conditions, this velocity change affects the tangential friction stress at the rolling interface. Since the total rolling force depends on both deformation resistance and friction stress, torsional vibration can produce rolling force fluctuation, which then excites the vertical roll system. The authors modeled the reverse pathway through the hydraulic screw-down system, represented as an equivalent two-mass system involving the roller and hydraulic cylinder under hydraulic pressure and rolling force fluctuations. Solving the vertical vibration response showed that vertical roll displacement changes the roll gap and contact arc length. Changes in contact arc length alter the rolling load torque, feeding excitation back into the transmission system. In mechanical terms, vertical vibration changes the torque demand placed on the drive; torsional vibration changes the frictional and force conditions imposed on the vertical system.

The authors’ lubrication experiment provided a practical validation of this closed-loop interpretation and by adjusting the lubrication oil supply at the roll gap, the researchers altered a parameter located directly in the deformation-zone coupling path. When lubrication oil content was within an appropriate range, vertical and torsional vibration amplitudes were lower than at either insufficient or excessive lubrication levels. In the comparison before and after lubrication adjustment, the torsional vibration amplitude decreased from 39.68 to 8.35, while the vertical vibration amplitude decreased from 40.01 to 18.52. The dominant vibration shifted away from the strong coupled 41 Hz condition, and the vertical response no longer showed excitation at that critical frequency. A change at the roll-gap interface therefore changed the vibration state of the coupled mill system.

The findings of Associate Professor Yifang Zhang  et al have direct engineering relevance for the diagnosis and suppression of vibration in Compact Strip Production rolling mills, especially during the rolling of high-strength thin strip. The main practical value is the shift from treating torsional vibration, vertical vibration, hydraulic force fluctuation, and motor torque current as separate symptoms toward treating them as linked expressions of one coupled dynamic system. For mill engineers, this means that vibration control should not begin only with the drive train, the hydraulic screw-down system, or the mill stand in isolation.

One important application is in industrial monitoring and the new study shows that meaningful diagnosis requires synchronized measurement of signals from the electrical drive, mechanical transmission, hydraulic screw-down system, and roll system. When related frequency components appear across these signals, especially near the inherent modal frequency of the mill, they can indicate a coupled vibration condition rather than a local disturbance. This provides a practical basis for condition monitoring systems that do more than record vibration amplitude. They can track frequency relationships among motor current, rolling force, torque, roll displacement, and acceleration, allowing operators to identify when the mill is approaching a strongly coupled vibration state.

The results also support more targeted vibration suppression strategies. Since the rolling deformation zone transmits disturbances between torsional and vertical motion, engineering adjustments at the roll gap can influence the behavior of the entire system. Adjusting the lubrication oil supply within an appropriate range reduced the coupling degree of the system and substantially lowered both torsional and vertical vibration amplitudes. This suggests that lubrication control is not only a tribological or surface-quality measure, but also a dynamic control parameter for the rolling mill. The new model can also guide process optimization for high-strength thin strip production and by linking roll torsion angle, circumferential velocity, interface friction stress, rolling force, contact arc length, and rolling load torque, it provides engineers with a structured way to understand how changes in operating conditions may feed back through the mill. Such a model can help engineers define more stable operating windows, avoid excitation near critical modal frequencies, and improve rolling stability through process-level control as well as equipment-level measures.