Machinery Lubrication magazine published by Noria Corporation
Issue link: http://www.e-digitaleditions.com/i/655822
14 | March - April 2016 | www.machinerylubrication.com guarantee that they would do it properly. If the pump does not pick up a prime, air entering the suction line will cavitate and destroy the pump. While plant personnel were deciding what to do about these pump losses, anti- wear hydraulic fluids were just coming on the market. Oil companies claimed longer pump life would be possible with these fluids. This strategy was added to the list of possible actions. Overheating oil was also persistent on all the systems and generally started with a problem in the unloading system. Assuring the coolers worked properly helped, but quick diagnosis and repair of the unloading system only brought minimal improvement in pump losses. Action Since it appeared that the pump losses had multiple causes, the decision was made to correct the easiest possible cause first — the fluid. Switching to anti-wear fluids made a small improvement. Next, better filters were installed on each system. Changing these filters on a monthly basis became the routine because the bypass indicators were not trusted. In addition, it was easier to schedule the changes on the same shift. This also resulted in some improvement. The practice of "filter-fill" was then begun. Previously, in order to fill a system, millwrights would bring a drum of fluid to the site and insert an air pump into the large bung hole. They would pump oil directly from the drum into the system through an opening in the top of the tank. When the drum was empty, they often would place the air pump on the ground, take the empty drum away and return with a full drum. To counter this, all openings in the top of the tank were plugged except for the breather and a spin-on-type filter attached to an opening in the tank. The mechanic had no choice but to connect the hose to the filter when filling the tank. By this method, all dirt on the pump or in the oil was stopped by the filter. This led to improved pump life. Subsequently, the vane pumps were changed to gear pumps in order to prevent the pump gang from attempting repairs. When a gear pump fails, it cannot be repaired properly except in a specialty shop. The strategy was to keep low-skill personnel from trying to make repairs. Pump life improved markedly with this action. Still not satisfied, the plant converted its hydraulic tanks to vertical tanks with the pumps mounted beside them. This gave the pumps a "positive head" and lessened the chances of them being starved for fluid. This move was the most productive of all. Pump life increased so much that the pump gang thought someone else was doing their work. This problem provides an excellent example of what to do when there's a multitude of possible causes and all are believed to be contributory. If none can be eliminated as a possible cause, then correcting the easiest one first is a good strategy. Plants frequently live with a problem that should have been corrected years ago. These "lived-with" problems take up a lot of maintenance time and become routine or part of the "how we do things around here" syndrome. Constantly be on the lookout for these problems and eliminate them, but only after proper analysis. Work Roll Bearing Failures In the mill described previously, the work roll bearings were the anti-friction type. A total of 40 bearings were in the mill at one time, and all were lubricated with grease. The grease's performance characteristics were specified by the mill, and every load was tested when received. Typically, 15 to 20 bearings were lost each year, primarily on the faster finishing stands. The losses, which usually were attributed to age or misdirected water sprays and only occasionally to grease, were considered normal and difficult to reduce. Figure 2 illustrates the typical bearing. When the work roll bearings suddenly began failing on the finishing stands at the rate of one per day, a fishbone diagram was prepared to list the possible ways the bearings could fail (see the table below). Operations management immediately blamed the grease. Even though years of records were available to confirm the grease's quality, samples were taken. All the results were perfect. Each item on the list below was then checked or discarded due to recent verifi- cation. Only one item stood out as suspicious — the location of the losses. An investigation proved that all the losses had occurred on one particular stand and only on the operator's side of the mill. This pointed directly to that stand housing as contributing to the failures. The wear plates on the inside of the stand housing, which the bearing chocks rubbed against, were checked and found to be badly worn. They were changed immediately. Failures soon returned to normal levels. These three examples illustrate the effectiveness of using a fishbone diagram to ensure that all possible causes of a failure are considered. The Kepner-Tregoe method can also help to establish what, where, when and the extent of the problem. The first and last examples arose suddenly with too many people panicking and jumping to conclusions. In all three COVER STORY POSSIBLE CAUSES OF WORK ROLL BEARING FAILURES Personnel Related Rebuilding practices, skill-level training, inad- equate greasing, personnel changes Systems or Operations Related Duration of rolling schedule, water sprays impinging seals, location of the losses Maintenance Related Grease change, grease quality, worn bearing chocks, wear plates on mill, wear plates on bearing chocks Material Related Bearing manufacturer, spacer change, age of bearings