Machinery Lubrication

Machinery Lubrication March April 2016

Machinery Lubrication magazine published by Noria Corporation

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12 | March - April 2016 | www.machinerylubrication.com oil flow to the bearings was also checked and determined to be on target. At this point, the mill was losing a bearing per week. Previously, one lost bearing per year was normal. Costs were skyrocketing, and a mill shutdown was a real possibility. A meeting with supervisors and repairmen was held to consider all the ways a back-up bearing could fail. The compiled list is shown in the table on page 11. This list was meant to be all-inclusive, so some items might not apply to a particular mill. When trying to determine the cause of any bearing failure, you should concentrate on what has changed recently if the problem is of recent origin. Something has changed, and that possibly includes current practices. Analysis Several items on the list were deemed very unlikely or had been recently checked, so they were not considered. The remaining items were divided up among all personnel. After all other items on the list checked out, the bearing chock dimensions were investi- gated. A bearing chock is a housing into which the bearing is fitted before being placed on the roll. The bore into which the bearing was inserted was 3 feet in diameter. The internal dimensions of the chock are critical. The difference between the vertical and horizontal measurements cannot exceed 0.05 inches or the bearing will not seat properly. The location of the bearing failures was random, so no pattern could be discerned. The losses occurred on both the drive side and the operator's side of the mill on six finishing stands. The bearing shop was fortunate to have a meticulous supervisor who recorded everything on a computer. Each bearing chock was numbered and a record kept of which stands each was in during any rolling schedule. The computer records proved that four chocks were involved in all the bearing failures. This was surprising, but the cause still had to be proven. The four chocks were set aside and unused for a time to see what would happen. All bearing failures ceased. Checking the internal dimensions of these four chocks showed wear well beyond toler- ances for good bearing seating. These chocks were immediately sent out for rebuilding, and normal bearing life resumed. The Kepner-Tregoe method was also helpful in the analysis of the failed bearings. Rather than listing all the possible causes of a problem, the Kepner-Tregoe method seeks to describe what the problem is or is not, where it occurs or does not, when it occurs or does not, and its extent. Basically, you are building a fence around the problem to keep good information inside and under consideration and bad information out. You are determining what has changed from the previous "problem-free" condi- tion. The true cause will satisfy all the conditions unearthed by using this method. If one condition cannot be satisfied by the suspected cause, it must be discarded and another considered. With this bearing problem, the worn chocks satisfied all the conditions of loca- tion, timing and capability of causing the problem. None of the other possible causes could do that. Also, setting the suspected chocks aside amounted to changing only one parameter at a time, which prevented confusion of the issue. Hydraulic Pump Failures In this particular plant, most of the hydraulic systems used vane pumps. Losses were very high, and it didn't take much analysis to determine that 80 percent of the pump gang's time was being occupied changing pumps. It was also necessary to increase the crew size to keep up with the work. A fishbone diagram was prepared that listed all the possible causes of short pump life (see the table below). Analysis Because vane pumps are very sensitive to dirt, and steel mills are inherently dirty, it was suspected that the vane pumps might not be the correct type for this envi- ronment. An investigation determined that the pump gang was rebuilding failed pumps with parts from other failed pumps. The pump manufacturer advised strongly against this practice and insisted that only new, matched sets of vanes, rotors and wear plates be used when rebuilding units. This clashed with long- standing practice in the plant. The strategy was to change pump types. Gear pumps are less expensive than vane pumps and much more resis- tant to dirt. They also fail gradually, giving a warning by moving all cylinders more slowly. Vane pumps fail suddenly without warning. It was also noticed that all systems in the plant were designed with the pump and motor sitting atop the tank. When the pump is started on this type of system, the mechanics are cautioned to "jog" the pumps, meaning to start and stop the motors at least three times before walking away to ensure the pump has picked up a prime. However, the operators often would start the pumps, and there was no POSSIBLE CAUSES OF EXCESSIVE PUMP FAILURES Personnel Related Rebuilding practices, skill-level training, moti- vation level, inadequate time (rush to complete) Systems or Operations Related Improper pump starting by operators, low oil level warnings ignored Maintenance Related Dirty oil (filters need to be changed), undersized suction lines, systems receiving dirt during maintenance, loose suction lines (air entering), suction lines are too long, inattention to overheating oil Material Related Hydraulic oil quality level, incorrect parts used, inferior parts purchased, incorrect pump for the job, incorrect system design for the job, incorrect fluid used COVER STORY

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