Oil Analysis Confirms Supercallender Motor Bearing Failure
Dave Burgess, a PdM (Predictive Maintenance) engineer with GE EMP at the Boise Cascade Paper Mill in Rumford, Maine, suspected a wear problem was occurring with the supercallender main drive motor on a specialty paper processing line. The motor sleeve bearings with tin base babbitt (5.5" i.d.), were lubricated with an ISO 68 anti-wear mineral oil. The oil supplier’s analysis report found no abnormalities, but Dave, with the support of Boise Cascade’s Kim Osgood opted for a second opinion. Two samples were sent in July 1993 to the BTS lab and the standard analysis program was performed.
Paper mills have a strong focus on equipment reliability, with many predictive technologies in place. Of particular concern are bearing assemblies with critical nonferrous components, which are ideal candidates for comprehensive tribology analysis. Many mills have large forced lubrication systems with central reservoirs, serving many bearings on rolls and drives. One such piece of equipment is the supercallender roll set ( Fig 1) a vital part of the finishing process for high quality speciality papers used throughout the world today.
The lab routinely applied four screening tests, and on an exception basis, performed four analytical tests. Four of the tests were used for evaluating the lubricant while the other four were specific to Wear Particle Analysis (WPA). The four tests for metals were:
- Emission Spectroscopy (SPECTRO)
Screening test for fine metal particles, dissolved particles, ferrous and nonferrous, additive and contaminant elements, results in parts per million (ppm).
- Rotrode Filter Spectroscopy (RFS)
Screening test for coarse metals, ferrous and nonferrous as well as silicon, results in parts per million (ppm)
- Analytical Ferrography (AF)
Analytical test done when SPECTRO and RFS suggested severe wear; involves preparing a display of wear particles for optical microscopy (principally used for ferrous particles although nonferrous particles often deposit).
- MilliporeÔ Patch Test (MPT)
Analytical test done when SPECTRO & RFS suggested severe nonferrous wear, and when AF did not confirm the nonferrous wear as indicated by SPECTRO & RFS. It involves filtering an oil sample through a filter membrane, and displaying wear particles for microscopic examination.
Results & Corrective Action
The results (Fig 2) show severe bearing wear taking place, and remedial action was suggested as soon as possible. The oil was immediately changed and monitored after two weeks. The BTS results were again showing severe wear and coupled with vibration data, convinced that the engineers that their suspicions were correct, i.e. the motor was not coupled properly, and it appeared to be thrusting away from the gearbox and into both bearing thrust faces. The motor was immediately taken out of service and overhauled.
Figure 2. The SPECTRO and RFS results for both samples. The 7/7/93 sample showed extremely high values of nonferrous metals. The sample was sent to the oil supplier, and was reported satisfactory. The operator changed the oil based on the BTS report. the 7/21/93 sample was also sent to both labs; BTS reported an Alert! condition, while the other lab gave a satisfactory. While coarse and fine iron were higher than desired in the BTS results, the levels were not excessive. It was the nonferrous coarse reading which could not be ignored, especially coarse tin.
One bearing assembly (Fig 3) was replaced. The motor was reinstalled and aligned, eliminating a soft foot. Had the motor run without the overhaul, it would have failed, requiring a rewind of the armature, which together with the other maintenance costs would have cost about $32,000. No production loss estimates were available but they were considered to be substantial.
Figure 3. A badly worn sleeve bearing taken from the supercallender drive motor Edges of the sleeve are worn due to misalignment