Journal
A vibration strip from a mill motor, read without theatre
After a bearing is changed, someone often wants a figure that proves the motor is “quiet now.” A pen record can show whether the stored amplitude sat lower than it did in the fortnight before the job. It cannot show the inside of the new bearing, and it cannot show a knock that fell between stored points.
In a recent mill commission the interval was five minutes, which is kinder than a quarter-hour for this question. We drew the same axis before and after the date of the job, with the hours of the change hatched. The line after the job sat lower on most shifts and not on the Friday the mill ran a harder product. The written reading said that. It did not say the bearing was healthy.
The shift lead had hoped for a single number. We gave him two bands and the Friday exception. He was not pleased, then used the Friday figure in the next planning meeting anyway. That is about as much as a strip should be asked to do.
If you send a vibration series, name the axis the instrument uses (mm/s, g, or a pen’s own scale) and whether the sensor was moved during the job. A probe on a new bracket is a new story, even if the motor is the same.