The roller is then removed from the balancing machine and carefully mounted on load cells. The result of mounting the roller on springy load cells is always the same. The roller resonance drops from 120% to 80% of the rated speed. The resonance has been moved to the exact speed the roller will spend most of its life running at.
What can be done to balance the system of load cell and roller to resonate above the running speed?
Wednesday, June 29, 2011 7:42 AM
Load cells are much softer, sometimes two or even three orders of magnitude softer, than the rollers above and the framework beneath. This means, as you've pointed out, that critical speed calculations are meaningless. Worse yet, they are not conservative meaning you think you are safe and you are not.
Running through resonances is nearly inevitable on most machines running more than a few hundred FPM. The trick is to superbalance the rollers. At Beloit, we balanced most rollers to G2.5. However, we spec'ed load cells to G1.25. An audit of the roller shop showed that they balanced even tighter than spec because the machinists had been sensitized by field complaints. To put this in perspective, a roller that weighs as much as a child would be put out of balance with a stick of gum. We then could run through resonances with only a 2x increase in vibration that could only be seen with fast and sensitive strip chart recordings of the raw load cell output or by using a vibration analyzer. In other words, barely detectable and only with instruments. - Dave Roisum, Ph.D.