On Sept. 24, 2003, a chatter reduction team was formed in order to determine why USS-POSCO Industries' (UPI) tandem cold mill (TCM) was experiencing an abnormally high incidence of delays (roll changes) due to what had been termed "bearing chatter." Figure 1 indicates that the number of roll changes due to bearing chatter could be as high as approximately three roll changes a week plus or minus two roll changes. The average number of roll changes was unacceptable, and the variability was too great, denoting a process out of control. Chatter is the result of a resonant response of the cold mill stack. This causes the stack to vibrate uncontrollably when excited by a periodic force. This force can be produced by a variety of things. While discussing the problem in further detail, operations personnel noted that the typical response to bearing-chatter was a roll change. It was believed that once the bad bearings were out of the mill, then the resonance condition would go away. The only other fact made known to the team was that the physical effect of this type of cold mill chatter was a short cycle pattern across the width of the strip, visibly seen at the exit of the TCM. Figure 2 is an example of the pattern seen on the strip.
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