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Practicing Oil AnalysisMachinery Lubrication

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Lean Manufacturing

 

Stephen Shakeshaft, Shakeshaft Consulting  

Roller Bearing Lubrication Failures Solved

 

Machinery DetailsThe machine was a double-entry centrifugal fan,horizontally mounted with an inboard and an outboardbearing. The shaft diameter was approximately 150mm (6 inches) and due to the configuration, theoriginal equipment manufacturer specified split rollerbearings at both the inboard and outboard bearinglocations. The drive was directly coupled from anelectric motor via a flexible gear-type coupling. The fanwas driven at 1,500 revolutions per minute.

Initial I nvestigations

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A global

chemicalmanufacturingcompany isoperating aplant with alarge rotarykiln. The kilnis part of asingle-streamprocess, soanydowntime atthe kiln planthasimmediateand severeeffects on several downstream manufacturing units. Theresulting lost production has serious cash loss implications as theplant is operating on a sold-out basis; therefore, reliability is of paramount importance. The strongly acidic and toxic productgases are drawn from the kiln at high temperature(approximately 400°C) by an induced draft fan. This fan isknown as "the hot gas fan" for obvious reasons. Historically, thefan and the kiln had been remarkably reliable, requiring onlyroutine maintenance between annual maintenance shutdowns.

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The plant did not have comprehensive maintenancerecords for this piece of machinery. However,conversations with both the operations andmaintenance departments did support the opinion thatthe fan historically had not been a source of majorunreliability on the plant.

Problems with the fan had a major effect at the site because if the fan broke down, the kiln stopped and the feed to fivedownstream plants was instantly interrupted.

The fan operated on flue gases with a temperature that couldexceed 400°C (800°F), so there was a significant amount of heatconducted into the bearings from the shaft via the inner bearingrace.

Commissioning records showed that when originally installed,the bearing housings were water-cooled. Early in the startup of the plant, there were problems with the bearings overheatingand failing. Attempts to increase the cooling water flow to thebearing housings failed to solve the problem. Eventually, thecooling water was removed from the bearing housings and thebearings ran successfully, which initially seems counter intuitive.

Apparently, the cooling water kept the outer bearing race cool,but the heat input to the bearing came from the inner race fromthe shaft. The combination of the outer race and the hot innerrace was causing the internal clearance of the bearing to be lost.Losing the internal clearance reduced the oil film thickness andcaused metal-to-metal contact which resulted in internal heatgeneration in the bearing. Eventually, the bearing overheatedand failed.

Turning off the cooling water may not have been the firstresponse when faced with an overheating failure, but it allowedthe bearing to reach an equilibrium temperature where it wouldrun satisfactorily between yearly overhauls. What was notdocumented at that time was whether the clearance of thebearings was altered from the original specifications.

Further I nvestigationsRecent inspection of actual failed bearings showed thatthe races were badly spalled by fatigue. There hadclearly been a reduction in clearance caused by theheat generated in the bearing.

Of special interest was the accumulation of material between therollers, adhering to what remained of the bearing cage. Some of this was scraped off and sent to a lab for analysis. The

laboratory reported that the substance was clay. This tied inexactly with what was happening on the fan.

The grease supplier had recommended changing to a Bentonite-thickened grease (clay) due to the high service temperature. Themaintenance department subsequently made the change. Theheat generated within the bearings caused the oil from theBentonite grease to evaporate, leaving the clay compoundbehind. The bearing, then effectively running without lubricant,continued to overheat and failure rapidly followed. Air hosesdirected at to the bearing housings were turned on in an attemptto keep the bearings running longer when overheating.

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Issue Number: 200707Machinery Lubrication

Case Studies 

bearing problems have occurred on the hot gas fan for severalyears. As a bonus, the maintenance team now has a betterunderstanding of the application of bearings and is better trainedto handle problems on other machines.

Please reference this article as: Stephen Shakeshaft, Shakeshaft Consulting, "RollerBearing Lubrication Failures Solved". MachineryLubrication Magazine. July 2007