(1) The main reasons for fan vibration are: inaccurate balance and center alignment during maintenance; balance weights welded on the impeller falling off during operation; severe blade wear or ash accumulation causing imbalance, which can lead to serious accidents such as fan blades flying out; severe bearing wear resulting in excessive clearance between the shaft and bearing bushing; roller (or ball) bearing damage. When air enters from both sides, inconsistent adjustment of the dampers on both sides or blockage of the flue on one side may cause an excessive difference in airflow on both sides. Loosening of the bearing nut or foot bolt nut; unstable bearing bracket; blade deformation, etc.
(2) Reasons for excessively high bearing temperature include poor quality of bearing installation and maintenance, such as misalignment of the shaft and bearing, too small a gap in the bearing shell, defects in the shaft and bearing shell, etc.; inferior oil quality or lack of oil, such as poor quality of lubricating oil, excessively high or low oil level, or lack of oil, etc.; interruption or blockage of cooling water; excessive fan vibration; for induced draft fans, it could also be due to excessively high exhaust gas temperature.
Source: Beijing Ferroli Wall-Hung Boiler Repair Phone http://blog.tianya.cn/blogger/blog_main.asp?BlogID=3778889. Ferroli Wall-Hung Boiler Repair Q&A; Causes of Fan Failures.