What’s up Doug? Fan issue – how hot alignment solved the problem
In a recent investigation in frequency failures of an electric motor that was directly coupled to a fan, the fan assembly utilized a two-bearing housing (such as an SKF PD unit). This type of arrangement is uncommon in North American industry and creates a very stiff fan arrangement.
When vibration analysis was used on the fan, the diagnostics had pointed to issues with the electric motor bearings. But on inspecting the bearings, the condition of the bearing suggested that the outer ring of the bearing was being driven around its fit in the housing (that is, the outer ring was “creeping” in the housing). What this suggested was that there was an external influence causing the bearing to creep in its seat.
The difficulty was that the user could not accept the suggestion that the problem was not the motor, but rather what the motor was coupled to. This was especially hard to demonstrate since the “damaged” components were part of the motor and not the two-bearing housing that the fan support bearings were assembled in.
In addition, when the motor and bearing assembly was “cold” aligned with the fan support bearing assembly, there were no flags that suggested that there was a problem. However, the motor bearings pointed to an alignment problem, but assembly records suggested that the initial alignment was done properly.
A technical specialist from the motor rewind company was called in to assist with the alignment and he supervised the “hot” alignment of the motor/fan bearing assembly. What this means is that after a period of running, the fan/motor assembly was shut down and realigned at temperature to ensure proper alignment of the motor/fan bearing assembly at the operating speed. Since this “hot” alignment was undertaken the chronic vibration problem has not reappeared.
Douglas Martin is a heavy industry engineer based in Vancouver. He specializes in the design of rotating equipment, failure analysis and lubrication. Reach him by email at firstname.lastname@example.org.
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