It was pretty hard for anyone in the machine room not to notice the vibration coming from No. 5. It had become so that the floor actually shook. The vibration seemed to be coming from the gearboxes at the pull stack and reel.
The odd thing about the vibration was that it wasn't constant, but came in waves measured at a frequency of 30 cycles per minute. It intensified, held and then dissipated. To lower the intensity of the vibration, the operators had been forced to back off the machine’s top-end speed of 2,000 fpm. The machine was losing approximately 10% daily production.
Maintenance checked for other machinery in, above and below the room for other possible sources of the vibration. None of the other machinery exhibited an odd, peaking frequency or operated at a level near the lineshaft rpm. Gearbox inspections showed no problems with the gears. And even with the clutches disengaged, the vibrations persisted. Maintenance concluded the vibration had to be coming from No. 5, but why? What was causing it?
The mill staff called in a service engineer from the machine's manufacturer. His first order of business was to conduct vibration scans on a full scale of 500 Hz to 20 Hz on the dryer section – an area unaffected by the vibration. This data would act as a reference point to compare with similar scans performed on the calender and reel.
The 100 Hz scans from the calender and reel showed multiples of a frequency with a wide spike, which indicated more than one source of the vibration. The 50 Hz scans revealed a frequency of approximately 16.57 Hz and another frequency of 17.25 Hz.
The difference between the two frequencies pointed to a phenomenon called a "beat frequency." Characteristically marked by a pulsating frequency, a beat frequency normally indicates two vibration sources close in frequency to each other. The magnitudes of the two frequencies add and subtract, causing pulsations.
The service engineer suspected that in this case, the readings indicated a mass imbalance with the gearbox pulleys, which in turn may have been affecting the operation of the pinions to cause the vibration. To confirm his suspicions, he examined the belt positions of the reel and pull stack units to calculate the rpm of the pinions. Sure enough, there was a mismatch in the operating speeds of the gearbox pinion shafts.
But what was causing the imbalance between the pulleys? Was it bearing wear or system looseness?
For the answer, the service engineer recommended an examination of both gearbox pulleys. Mill maintenance complied and removed from inside each paper dust, dirt, oil and grease, which had accumulated in one spot in the pulley. The unbalanced point must have created the extreme vibration at high speeds. To further reduce the vibration, mill maintenance dynamically balanced each pulley with counterbalance weights and adjusted the bearing assemblies.
In this case, simple housekeeping would have reduced the vibration. Of course, belt-driven machines are not as common as they used to be. But the idea of accumulated crud somewhere on the machine causing unwanted vibration is always a valid issue.
For assistance with vibration or other runnability issues, contact your Valmet representative.