RCA Solves Persistent Bearing Problem

RCA Solves Persistent Bearing Problem Using their expertise in Root Cause Analysis (RCA), Stockport based AV Technology (AVT) have identified the reasons behind a recurring bearing problem on a large electric motor operating at the Solvay Interox Warrington site. AV Technology have a broad experience in providing structured and sustainable RCA solutions in industrial situations involving some form of persistent equipment failure. As independent consultants they have no 'axe to grind' with any of the parties involved, thereby allowing them to look at the problem in an unbiased way. The AVT engineers have developed a blend of excellent observational skills, analytical capabilities and mechanical engineering skills, together with the wherewithal to utilise different external resources as necessary to arrive at a conclusion. Coupled with this is a pragmatic understanding of tribological and metallurgical principles.The 350 kW, variable speed, inverter-driven motor drives a screw compressor providing process air for the Hydrogen Peroxide plant.Failure of this motor would have resulted in a significant reduction in throughput. AVT senior engineer Andy Mellor explains how his company got to the 'root' of the problem: 'We were awarded a condition monitoring contract for the site and soon identified a developing bearing problem on the motor. A bearing change was scheduled for a production outage and we asked for the damaged bearing to be returned to us so we could conduct an RCA.Interviewing a range of staff, we found out that based on noise, bearings had been changed approximately every two years and the unreliability of the motor was well known. Inspection of the bearing revealed fluting on the outer race and darkening of the balls - a sure sign that an electric current was passing through the bearing during operation. A call to the motor manufacturer revealed that the bearing should have been insulated to prevent shaft voltages causing circulating currents to pass through the bearing and frame. The motor and plant were both mature - we believe that at some time in the past the insulated bearing fitted by the manufacturer was replaced by a motor repairer for a non-insulated type. Each time a replacement bearing was damaged through the passage of current another un-insulated bearing was fitted and the mistake had perpetuated'.