Machinery Lubrication

Machinery Lubrication March April 2016

Machinery Lubrication magazine published by Noria Corporation

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Page 17 of 67

M achine failures are the bane of every mainte- nance department. Determining their exact cause can also be rather difficult. The equipment failures discussed in this article defied quick solu- tions and were quite costly in manpower, material and downtime. Various resolu- tions were proposed and tried with little success until thoughtful analysis was undertaken. The systematic approach that was employed as well as the results that were achieved in these three examples should provide a better idea of how to analyze similar types of equipment failures in your facility. Failed Back-up Bearings In a steel rolling mill, small rolls approx- imately 2 feet in diameter perform the actual rolling of the steel to reduce its thickness. Since surface quality is so important, these rolls must be re-ground often; the small diameter makes them easier to handle. Due to their small diam- eter, these rolls must be "backed up" by larger 3-foot-diameter rolls to prevent bending and distortion of strip thickness. Normally, the smaller or "work" rolls are driven and have anti-friction bearings, while the back-up rolls (undriven) have Babbitted or plain bearings. The work rolls are lubricated by grease or oil mist. The back-up bearings receive oil from a circu- lating system. The back-up rolls usually stay in the mill for several weeks of rolling before requiring regrinding. Bearing fail- ures on these rolls are rare. Figure 1 illustrates this arrangement. Back-up roll bearing failures suddenly began increasing on the last six finishing stands for no apparent reason. Department management instantly blamed the lubricant in the oil circulating system. Although every load of new oil was checked by a laboratory for quality, samples were taken from the system and checked at an outside lab to be sure there was no contamination. Each checked out perfectly. The oil was purchased on specification, every load was sampled, and years of records were on file to prove consistent incoming quality. The Figure 1. A four-high rolling mill arrangement Back-up Roll and Chock Work Rolls and Chocks Back-up Roll and Chock COVER STORY POSSIBLE CAUSES OF BACK-UP BEARING FAILURES Personnel Related Rebuilding practices, skill-level training, motivation level Systems or Operations Related Incorrect screw-down loading, mill cocked, mill thrusting, excessive water level in oil, low oil pressure (with indicator not on), separator bar loose, water sprays impinging seals Maintenance Related Oil line disconnected, bearing chock dimen- sions out of specification, oil cleanliness, oil temperature, age of bearing, incorrect oil specifications, oil quality not to specification, inadequate pump flow, oil pressure switches not calibrated, improper oil flow to bearing (orifice error), rockers broken, debris in shim pack, cracked sled, incorrect bolt specifications (two-piece chocks), incorrect bolt torque on nuts (two- piece chocks) Material Related Oil viscosity too high or low, improper Bab- bitt quality, incorrect Babbitt thickness, worn housing liners, worn chock liners, worn locator pins (two-piece chocks), bearing too small for mill loads | March - April 2016 | 11

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