I have had a belt driven overhung fan I monitor develop an increasing vibration at 2X rpm. The past history shows that this 2X was present at low levels in the past but then disappeared and a few months ago reappeared at high levels which are still increasing. I had the belts and sheaves checked and nothing unusual was found. I have not done phase analysis or bump tests yet but am planning to soon. I read an article that said increasing 2X accompanied by varying phase and amplitude at 1X, (I have seen variations of amplitude at 1X), can mean cracked rotor or shaft. I have quite a few very similar fans close by and none of them have shown this problem. All the references I have checked just show 2X as a misalignment or looseness problem but readings don't indiacte that either of those problems are present. The bearings, shaft, and belts were changed less than 3 years ago and the typical life of these fans is much longer than that. Has anyone had experience with a similar problem? Is there anything in particular I should be looking for in the diagnostic tets I am planning?

You might check for runout on the fan and motor pulleys. A cocked pulley would give a 2x signal.

Jon
Spintelligent Labs

Thanks Jon, I will add that to my list of checks for the next outage.

Randle,
Is the 2X of the motor or fan?

Thanks,
Bill

Bill Jones, did you work at Lamb Technicon once upon a time?

Bill,
The 2X is from the fan.

Dan,
Must be another Bill Jones, there are alot of us in the world .

Randle,
Just curious what your amplitudes are in relationship to the fan and the motor.

Thanks,
Bill

The fan shaft is tightened with a stationary force from the belt towards the motor. It will yield a bit twice per rev due to the slightly weaker shaft in the direction of the keyway. This could - if a fatigue sensitive design is present - start a crack that will cut the shaft sooner or later. An increasing trend is unfortunately a bad sign.

If the fan is stopped and you have a bump test capacity, I would suggest that you make that on the sheave in different directions to verify that the resonance is identical in keyway direction and 90 degrees from it. A normal keyway will weaken the shaft approx. 6 percent, so the difference when bumped should be very small, if any.

A crack, if developing, is tricky to detect easily, since the resdual material tends to cover up, being still strong. I have found crack when applying a known force and checking the stiffness across a suspect section using a dial gauge. A crack present will allow for an easier opening (less force for a given deflection). If the bending stiffness is really the same all the way around, you can at least be sure the crack is not yet learge.

Crack sensitive is for instance a shaft which has a sharp step in diameter.

Randle,

In addition to the possibilities suggested by knowledgable gentlemen above, I suggest the following:
If this is a fan with two bearing housings placed on a structure supporting the fan shaft then it is possible that those two housings are not perfectly in line. I have seen warping of the plate that supports the bearing housings causing the two bearing housing centers not to be collinear. This may come about due to improper mounting also. Presumably typical bearings used for this purpose should take up misalignment of the sort but that does not happen always. In such cases, the shaft acts as if there is shaft misalignment; effectively it is! At two locations along the shaft the shaft center is not coincident with housing center. Therefore I would verify that the base plate is flat and the two housing centers are actually collinear.

Regards
Ibrahim