Lester Roundtree from Moventas , the manufacturer of our Yankee Dryer Shaft Mounted Gearbox was in the mill today during the washup to perform a boroscope inspection of the gearbox as we sought to determine the increased audible and vibration noise coming from the high speed input shaft area of the gearbox.
Our condition monitoring consultant, Machine Diagnostics Inc. had identified a bearing defect frequency during the survey they performed on the gearbox on Monday. Our inspection today was intended to verify the bearing defect.
The photo below shows the presence of 'fluting' on the bearing race and rollers. These are the lighter colored paralell lines on the surface of the roller. Left un-addressed, this condition would surely result in a catastrophic bearing failure and possible damage to the precision gearing components in the gearbox.
Changing the Yankee Dryer Gearbox would require 2 -3 days of downtime. Since we have no spare on the property we would have been down an extended length of time awaiting the arrival of a replacement gearbox. (A spare gearbox was ordered last September and is due to arrive in early March).
Fluting is caused by induced electric current in the shaft of a motor passing through the bearing via the rollers. http://technicallyspecializedsolutions.com/Electrical_Fluting.html This phenomenon normally occurs in motor bearings but can occur in the connected machinery if the path of least resistance of the induced current is along the shaft.
We have taken several measures to eliminate this particular root cause of failure (fluting) in various locations in the mill by specifying ceramic coated bearings in our motors, (diverts the path of the induced current in the shaft so it does not go through the bearing to get to ground ), installing shielded cabling between the AC drive and the motor (to reduce induced currents) and installation of grounding brushes on the motor shafts (to provide a low resistance path to ground).
Replacement of the Yankee Dryer Gearbox input shaft bearings will require about 12-16 hours and removal of the gearbox cover during our upcoming outage to grind the Yankee Dryer shell surface. This will extend the grind outage by at least 12 hours – we can do this work during the time we are inspecting the dryer internals. Moventas will provide a field technician and mill maintenance technicians will perform the work.
We should all be pleased we identified this defect before and not after the grind and especially before a catastrophic failure occurred.
This failure was initially identified by operators paying close attention to changes in the noise level of the machinery as we operate day to day. By noting changes in noise level, temperature and/or vibrations we can augment and assist predictive maintenance/condition monitoring efforts to verify if a failure is/is not in progress. Identifying failures in the early stages allows us to plan repairs, reducing maintenance expense and the downtime associated with repairs which helps to make our mill more productive and profitable.
Well done AST Team !!
Think Safety, Work Safely
Mike Sherbak
VP - Operations
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