Seeking advice on the causes of axial vibration in wet-air-cooled axial flow fans
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In the composite cooler, there are 3 air-cooled axial flow fans arranged in a row; two of them are driven by AC motors, while one is driven by an inverter-driven motor. (The order is not AC, AC, inverter.) The motors are located at the top, with the blades mounted vertically inside the fan casing. The inverter-driven motor is directly connected to the fan impeller; the impeller has a diameter of around 4 meters and consists of 6 blades, with each blade having an angle deviation of no more than 0.5 degrees, all at 9 degrees. The blades have no cracks. At loads below 50%, both axial and radial vibrations remain below 1.0; as the load increases beyond 50%, the axial vibration rises gradually, reaching 14.1 mm/s at a load of 100%, while the radial vibration remains low at 1.1 mm/s. There is one SKF 6314 deep groove ball bearing on each side of the motor, and these bearings have just been replaced with new ones. Could it be due to inverter parameters or other electrical issues? No dynamic balancing was done on the machinery – are there any other issues with it? I would appreciate it if experts could help analyze the cause of the vibration. Thank you!
It could be due to the frequency converter, as I heard from the electrical staff in our company that the failure rate of frequency converters is quite high
First, it is necessary to ensure the dynamic balance of the rotor, and then other factors can be considered…… This should be an analysis using the elimination method. For reference.
Please first check whether there are any differences in the weight of the 6 blades. Also, make sure to recheck that the angles of the blades are consistent and that the height of their apexes is the same
Whether the blades are connected rigidly or flexibly to the shaft: in our previous axial flow fans, the deviation in blade angles was within acceptable limits, and the height of the blade tips was more or less consistent. However, the blade roots were connected flexibly, which caused the blades to have different heights during rotation, resulting in excessive vibration
What was said upstairs is correct; moreover, deep groove ball bearings may not be suitable for use here as the axial thrust is relatively high – cone roller bearings could be used instead.
I agree with this view. With blades having a diameter of 4 meters and being mounted vertically in direct connection to the motor, the axial force should be very high. Using deep groove ball bearings at both ends of the motor may not be appropriate; it is necessary to discuss this with the designers to see if alternative types of bearings can be used
If the connection between the blade root and the hub is a rigid one, there’s not much of a problem; but if it’s a flexible connection, the elastic element at the root may be damaged, resulting in uneven heights of the blade during operation and affecting its dynamic balance. This was an issue we encountered in one of our previous projects. I drew a simple diagram, and the problem was resolved by adjusting the dynamic balance (of course, this was done after performing a spectrum analysis). I’m not sure if your situation is similar; this is just for reference
The wheel hub and motor shaft are rigidly connected via a tapered sleeve