Cold Rolling Mill System Vibration Control

Author: Zou Jiaxiang / et al.
Editor-in-Chief: Ge Zhiqi
Publisher:
Publish Date: 1998-11-01
Features:
Introduction This book is a specialized monograph on the vibration problems of strip cold tandem rolling mills and a summary of significant research achievements. The book consists of eight chapters: Chapter 1 introduces research findings on vibration in rolling mill systems both domestically and internationally; Chapter 2 presents a comprehensive testing and analysis of cold tandem rolling mill vibrations; Chapter 3 discusses the mechanism of self-excited vibrations in cold tandem rolling mill systems; Chapters 4 and 5 cover the dynamic characteristics and system simulation of torsional vibrations in the main drive of the rolling mill and the vertical vibrations of the machine base; Chapter 6 explores the influence of lubrication conditions on rolling mill chatter vibrations; Chapters 7 and 8 detail the dynamic characteristic analysis of the main drive automatic control system and the downstand system, as well as the vibration warning device for rolling mills. Through the study of specific mechanical equipment vibration problems, the book elaborates on the principles and research methods of rolling mill vibrations, providing reference value for analyzing and studying vibrations in various types of machinery. This book is suitable for use by mechanical manufacturing and design professionals, rolling mill machinery designers, manufacturers, and operators, as well as for reference by faculty and graduate students in relevant fields at universities.
Excerpt:
1.1 Advances in Research on Rolling Mill System Vibrations
1.1.1 Types of Rolling Mill System Vibrations
From a dynamic perspective, any mechanical vibration can be categorized into three types based on its cause:
- Free Vibration: This occurs when a mechanical system, after being disturbed and losing its equilibrium, is maintained by the system's elastic restoring force. When damping is present, the vibration gradually attenuates due to energy dissipation without external input. The frequency of free vibration is the system's natural frequency.
- Forced Vibration: This is vibration induced and sustained by a continuous external excitation force. The vibration frequency is the excitation frequency.
- Self-Excited Vibration: This is a periodic vibration generated by an alternating force produced by the system itself under certain conditions, without external excitation. The vibration frequency is close to the system's natural frequency.
Based on the loading characteristics of rolling mills, components can be analyzed for vibrations under two different load transfer systems:
1. The loaded system is the main drive system of the rolling mill, including rolls, coupling shafts, gearboxes, reducers, and the armature of the main motor. The primary external loads on this system are rolling torque and the torque acting on the armature due to the main motor's magnetic field.
2. The loaded system is the working machine base of the rolling mill, including rolls, bearing seats, downstand screws, downstand nuts (or hydraulic downstand systems), bending units, and frame girders. The primary external loads on this system are rolling force, bending force, and balancing force.
The vibration form of the main drive system of a rolling mill is primarily torsional vibration, while the vibration form of the working machine base is primarily vertical vibration. Below are examples of vibrations in rolling mills both domestically and internationally, along with related research, to illustrate the characteristics of these two types of vibrations.
1.2 Examples and Research on Torsional Vibrations in Rolling Mill Drives
Torsional vibrations in rolling mills were gradually recognized through frequent failures of transmission components in the production process. Such failures could not be explained by mechanical static strength theory. It was found that under external loads, the transmission system of a rolling mill undergoes torsional vibration. When the amplitude of torque waves in transmission components exceeds a certain value and acts for a certain duration, fatigue damage occurs, reducing the service life of the parts. Severe vibrations can also cause sudden fractures or significant noise. The response of a mechanical transmission system to a specific external load is typically evaluated using the "torque amplification factor." The torque amplification factor is expressed as:
Torque Amplification Factor (TAF) = Maximum Torque of the Elastic System / External Torque
Where TAF is the abbreviation for Torque Amplitude Factor.
1.2.1 Testing and Research on Rolling Mill Vibrations by Bethlehem Steel Company in the U.S. [1]
In the 1960s, Bethlehem Steel Company conducted on-site testing and theoretical analysis on three of its rolling mills:
The first was the 1125 mm × 2250 mm universal slab rolling mill at Sparrows Point plant, which went into operation in 1958. The horizontal rolls were driven by two × 4410 kW dual-armature DC motors (as shown in Figure 1.1); the vertical rolls were driven by one × 2940 kW dual-armature DC motor.
The second was the slab rolling mill at Lackwanna plant, which went into operation in 1961 (the same as the first mill except for program control).
The third was the 1350 mm roughing rolling mill at Sparrows Point plant, which went into operation in 1927. This mill was driven by a single × 5145 kW dual-armature motor through a gearbox.
Figure 1.2 shows an oscilloscope trace of the impact on the 1125 mm × 2250 mm universal slab rolling mill's transmission system with backlash. Torsional vibration exhibited nonlinear vibration characteristics.
By carefully examining the torque curve of transmission shaft 3-4 near the rolls in the oscilloscope trace, it can be observed that it bears rolling torque 15 ms earlier in time compared to transmission shaft 2-3 near the motor. This indicates that the torque is generated by the workpiece dragging the rolls. It can be inferred that the workpiece speed is faster than the roll speed, resulting in slippage. The peak torque of shaft 3-4 is approximately ±1.8 units. Interestingly, when this curve transitions from positive to negative and passes through zero, it shows a small horizontal segment, proving the presence of backlash and its repeated closure and opening. The amplitude of torque T1-2 between the two motors is approximately 5 units and -3.8 units. At this moment, the torque of shaft 3-4 is only 1.5 units, and the torque of shaft 2-3 is similar to that of shaft 3-4. Calculations on the nonlinear torque of this transmission system show that when the second natural frequency is 17 Hz, the torque amplification factor reaches its maximum value of 5.12. The frequency of the torque curve of shaft 3-4 on the oscilloscope is also approximately 17 Hz, while the frequency of the torque curve of shaft 1-2 is 70 Hz, which is exactly the natural frequency of this shaft section.
Figure 1.2 shows the measured oscilloscope trace when the workpiece does not enter the rolls and remains in a slipping state.