Plant Services - January 2008 - (Page 43) Power Transmission EFFICIENCY ment. Here, it’s sometimes necessary to fi x the rotor-side bearing so that thermal expansion and contraction of the shaft doesn’t cause the rotor to rub. Recently, at a steam-generation plant, roller bearings in an overhung, direct-drive fan failed within two months of installation. Production came to a halt while a replacement fan was brought on-line. e cost of the shutdown was significant in diminished productivity and output. e initial inspection revealed that the fi xed and floating bearing positions had been reversed during installation. e more heavily loaded bearing, which should have been axially held, was allowed to float; the other bearing, which should Basic fan types Impeller attached directly to motor Held bearing Direct-driven shaft with cantilevered impeller Free bearing Avoid using tools made of softer metals such as aluminum or brass when installing fan bearings. have been free, was fi xed within the housing. As a result, roller skidding occurred during operation, leading to high bearing operating temperatures, oxidation of the bearing’s lubricant, and ultimate bearing failure. Identifying the fi xed and floating positions and correctly installing the replacement bearings solved the problem and improved bearing life expectancy in the application. Held bearing Direct-driven shaft with straddle-mounted impeller Free bearing Held bearing Belt-driven shaft with cantilevered impeller Free bearing Held bearing 2. Use the right tooling You must use the right tools to avoid damaging the bearings. Recommended installation tools include a set of feeler blades to determine bearing clearance measurements, a steel hammer and a hook spanner wrench. A fan bearing often is installed inside a pillow block housing, on an adapter sleeve. e pillow block housing limits access to the bearing locknuts and precludes using hydraulic mounting devices. Specialized hook spanners, available from bearing manufacturers, provide better access to the bearing within the pillow block base. ese tools allow installers to grip locknut notches securely and rotate the locknut within the housing. Avoid using tools made of softer metals such as aluminum or brass when installing fan bearings. Pieces of the softer metal can flake off during mounting and lodge in the bearing, causing premature failure. Belt-driven shaft with straddle-mounted impeller Free bearing Figure 1. In these common bearing con guration for centrifugal fans, the more heavily radially loaded bearing is held or xed in the housing while the other bearing is allowed to oat to accommodate shaft expansion. 3. Measure consistently When measuring a bearing’s radial clearance, designate one technician to perform all measurements. To obtain the correct hoop stress on the inner ring, maintenance technicians should take a series of radial clearance measurements when mounting a tapered bore roller bearing. Failing to do so could result in the bearing spinning on the shaft during operation. No two workers have an identical touch with feeler blades. To ensure consistency, the same person should perform the J clearance measurements on a bearing. A bench clearance can be taken when the bearing is removed from its packaging. is verifies that the bearing was manufactured according to the correct specifications. e starting radial clearance is measured with the bearing loosely positioned on the shaft and under shaft load. Subsequent clearance measurements also are taken while the bearing is under load. Using a moderate amount of force, the technician inserts a feeler blade through the gap at the 12-o’clock position between the topmost bearing roller and the bearing’s outer ring. e objective is to insert the thickest blade possible through the gap. e clearance should be taken on one roller set at a time. is method establishes the initial radial internal clearance. is clearance reading serves as the baseline to measure subsequent reductions in clearance during mounting. For 43 .PLANTSERVICES. http://www.PLANTSERVICES.com
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