Hi All,

This situation is likely familiar to many.
An overhung fan, belt driven. Radial vibration was balanced out on both bearings to acceptable levels, but axial remained high and in-phase on both bearings.

How to get rid of it? What kind of force is causing it (sheaves alignment was verified, no resonance in axial). I can think of possible rotor defects, such as bowed cantilevered end of the shaft, not squared impeller mounting, couple unbalance... But will all that cause axial? What is the cause and mechanism of such axial vibration for overhung rotors?

Your opinion will be appreciated.
Dave

Dave, did you check the runout on the face (end) of the sheave with a dial indicator? Sometimes the sheave is not pulled up evenly on the bushing.

There is probably is not a direct answer to your question that will fit all overhung systems.

In your case, or maybe I should say in cases like yours. I have balanced using the axial direction as the direction for the balancing. Works good. If there is something else wrong other than mostly imbalance, it might not work as well. Of course, imbalance is not always the total cause of a high 1X in many cases. We are counteracting other forces with the balance procedure.

Only my opinion and I could be totally wrong.

We had an experience similar to Ralph's. In our case, it was a center-hung squirel cage fan. For some reason, axial vibe increased after a bearing overhaul, without affecting radial very much. I still don't know why that happened. We theorized, however, that axial vibration may occur due to couple imbalance...creating a kind of axial rocking back and forth. So, we approached it kind of like a single plane balance correction...looking only at a single axial response. Our trial weights, however, were coupled pairs. We were able to find a correction that quieted the axial, without substantially changing the radial. I can't brag that I totally understand it. But in our case, it worked.

Sometimes a small amount of base looseness on overhung fans will produce much larger axial vibration compared to the radial vibration. Excess belt tension combined with pulley runout can also be a driving force for rocking motion.

The axial natural frequency may be close to the rotating speed. Also, in the past I have noticed locking collar bearings with a little extra clearance can behave like a wobble plate which will give you the axial movement. Let the fan coast down and watch the bearing housing closely. If you can see the bearing move back and forth even as it is close to stopping check that the shaft size is correct. Consult bearing manufacturer for shaft tolerances.