"Shut it down!" the shift supervisor shouts over the "clack-clack-clack" sound coming from the back side of the second dryer's drive. The noise warned of heavy oscillation in the drive, subjecting the machine to potentially damaging duress. The only known cure was a shutdown and restart, costing the mill productivity and dollars.
The mill had allowed this vicious cycle to repeat itself for more than a year. The trouble would not occur for weeks, then rear its ugly head daily for nearly a week. The drive motor would oscillate, driving faster one minute then backing off, regardless of operating speed or paper grade. In addition to causing sheet breaks, the resulting vibration actually loosened several bolts in the gear cases adjoining the drive, posing a potential safety problem. Then, after a few days, the problem would disappear - until next time.
The mill checked bearings and gears on the drive and found no excessive wear. Nor were there indications of dryer evacuation problems.
At the next scheduled shutdown, the drive was re-tuned, and all the electronic control boards were replaced. The control circuit was also replaced to prevent the drive from oscillating when it encountered a zero load condition. After start-up, the dryer performed flawlessly for three months until - "clack-clack-clack" - it started again.
How could a recently tuned drive with no gear or bearing problems and a new control circuit and control boards continue to oscillate?
Upon arrival at the plant, the OEM drive engineer began examining the gearbox and the couplings. A visual inspection of the gearbox showed little dirt build-up, and a vibration reading revealed no excessive movement.
The couplings, however, were another story. The teeth on the gearbox side of the input coupling were severely worn. The output coupling also showed enough wear to merit replacement. The worn couplings were resulting in backlash that was nearly twice the acceptable level. Worn couplings were replaced, reducing the effect of backlash on the drive circuitry.
Still, the drive expert knew the couplings could not be the sole reason for the oscillation. The problem was not purely mechanical and, as was soon discovered, neither was it purely electrical. The situation became clearer after a chart recorder was attached to the drive, which had been rebuilt several years earlier. The drive expert induced the machine to oscillate by speeding and slowing the drive. The data indicated that the start of the drive oscillation was related to a 7 Hz oscillation in the current.
The engineer concluded the 7 Hz oscillation was causing vibrations in the inshaft of the drive. The inshaft was incompatible with a 7 Hz frequency, which had not been present before the drive rebuild. In effect, an electrical problem was triggering a mechanical problem.
Since the drive would be very difficult to tune around a 7 Hz frequency, no attempt was made. Instead, a 7 Hz filter was installed in the drive circuitry to block the 7 Hz frequency and force the drive into other ranges, thus removing the trigger point for the problem.
As a more permanent solution, the drive engineer advised replacing the inshaft with one capable of handling a wider frequency range.
Once in place, the new inshaft, coupled with additional drive tuning, reduced the oscillation by a factor of ten. The new inshaft also provides security in the event of a future drive rebuild or replacement, where a filter would not block all unwanted frequencies.
For assistance with your dryer section mechanical drives and vibration issues, contact your Valmet representative.