Among other supportive measures active chatter damping has turned out to be an effective approach to counteract these problems by significantly reducing vibration issues of cold mills. Together with a partner, SMS group currently develops an active chatter damping system using piezo-electric actuators. A first installation of the new technology is scheduled for end of 2016.
In cold rolling mills vibrations of mechanical parts and/or strip can interfere with several quality parameters like thickness, shape, surface quality and others. Since vibrations occur at higher speeds in most of the cases, rolling speed of single- or multi-stand mills for steel and aluminium has to be limited quite often.
3rd octave chatter as a self-excited vibration form within the frequency range 100 Hz to 200 Hz causes the strips to break and impairs thickness quality. On the contrary, the 5th octave chatter produces shape issues and related surface defects (stripes) when resonant modes of the roll stack within the frequency range above 500 Hz are excited by rotating parts like gear wheels or roller bearings. Well known measures against chatter issues and their consequences are the following ones:
- online chatter monitoring with Auto-Slow-Down (ASD) functionality,
- mill design optimization,
- optimized maintenance and service practice,
- passive vibration damping.
As a new, promising approach active vibration damping is currently under development at SMS group. After comprehensive theoretical investigations, SMS group has decided to use piezo-electric elements as actuators which are generally capable of operating in a frequency range of up to 1,000 Hz. An analysis of design situations and efficiencies of piezo-actuators at different locations within the mill stand finally led to an optimum solution: a piezo-actuator box is placed underneath the bottom backup roll chocks (see figures).