Plant Services - January 2008 - (Page 42) NT HOW TO RUIN FAN BEARINGS Maintaining fan bearings to achieve long-term reliability ndustrial fans play a critical role in a host of manufacturing activities, including the production of steel, paper, cement and pharmaceuticals. Fan problems, however, can reduce productivity and even bring production to a halt. Bearing failure analysis demonstrates that most fan problems are preventable. Often, they can be traced to mistakes associated with fan bearing installation or lubrication, such as tightening locknuts while bearings are loaded or fi lling bearing pillow blocks with excessive amounts of grease. e following recommendations can help you avert common bearing-related mistakes and keep the fans in your facility running efficiently. I 1. Get oriented One cause of fan failure is confusing the fi xed and floating bearing positions. A typical fan has two bearings supporting the shaft. In most cases, the bearing exposed to the highest radial load should be fi xed, or axially held, within the housing. Radial bearings generally perform better when the axial-to-radial load ratio is low. Also, the axial force needed 42 to float a bearing is lower when the radial load is lower (axial force to move bearing = coefficient of sliding friction x radial load, or Fa = µ x Fr). e fan’s other bearing should be allowed to float internally or within the housing bore, thus accommodating shaft expansion and contraction. In most fan applications, the bearing closest to the motor or belt drive has the highest radial load and should be the fi xed bearing. is is the case in most of the common centrifugal fan arrangements – overhung belt-driven, center-hung belt-driven, and center-hung direct-driven fans. Figure 1 shows the motor, bearing and rotor locations in common centrifugal fan arrangements. Overhung direct-driven fans are an exception to the rule. Here, the bearing close to the motor is nearly unloaded and should be allowed to float. e bearing farther away from the drive handles the heavier radial load and should be fi xed in the housing. Another case in which it might not be appropriate to fi x the most heavily loaded bearing is when the fan rotor has a very tight clearance with surrounding structure (typically the inlet cone) in an overhung fan arrangeJ .PLANTSERVICES. BY JOHN CLARK http://www.PLANTSERVICES.com
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