During a routine vibration analysis, the maintenance team at an integrated North American mill discovered unusually high vibration frequencies emanating from the back internal bearing on the second press suction roll of PM5. Subsequent weekly follow-up measurements revealed the vibration amplitude was increasing, and after just 90 days, had surpassed acceptable tolerance levels. The crew replaced the roll, but the problem soon returned.
Two rolls were used interchangeably at this position, and were switched every nine to fourteen months, corresponding with routine maintenance procedures. No unusual wear or deterioration had been detected on the back internal bearing, but suddenly it was failing every three to five months.
When the problem was initially discovered, the mill increased lubrication frequency on the bearing from once a month to biweekly. When the problem persisted, it was further increased, but still the failures continued.
When the mill disassembled the roll, it found no sign of over-greasing, despite the dramatic increase in lubrication frequency. The condition of the grease appeared normal, as well.
The problem did not appear to be related to the roll, either. The same mill mechanics rebuilt every suction press roll, but only the rolls at this position experienced the problem. In short, there were very few substantive clues to point the mill toward a resolution.
Although the cause could not be pinpointed, the effect of these failures was clear - downtime and lost production were costing the mill several hundred thousand dollars. The mill formed a team, comprised of mill maintenance personnel and representatives from both the bearing manufacturer and the machine OEM field service personnel. The team reviewed every detail of the situation, including the operating specs from the bearing manual. It was there that they found the solution.
At running speeds below 210 rpm, the rollers in the bearing rode against the inner race of the bearing assembly. Above 210 rpm, centrifugal force pulled the rollers against the outer race. The running speed of PM5 had recently been increased to 3,000 fpm, which corresponded almost exactly to 210 rpm at the bearing.
At this speed, the rollers virtually "floated" in a "free" space between the inner and outer races. In the free space, the lubricant impeded the rollers' movement, so they were slowed down as they approached the load zone, or the lowest point of the revolution. The difference in speed caused them to actually skid against the outer race. It wasn't long before the bearing ran through enough cycles for the skidding to cause deterioration and ultimately failure.
After this discovery, immediate recommendations from the team included using a lower viscosity lubricant that wouldn't interfere with bearing speed, as well as installation of a temperature probe / vibration transducer for more accurate data collection and tracking.
The short-term modifications helped alleviate the problem. To guard against future bearing failures, the team began investigating alternative bearings where the rollers are not "free" in the mill's operating speed range.
For assistance with your bearing maintenance and vibration analysis programs, contact your Valmet representative.