I have a 1800 rpm fan with sleeve bearings. Vibration is very directional, Horiz.4 ips and vert .08 ips @ 1x. Balance attempt went south quick, phase didn't react well and corrections were off. Addition phase analysis show 180 deg phase shift across bearing, fan is center hung.We finally took an educated guess and got the vibes at 1x's in the horz direction to .09ips. Now the axial phase across the fan bearings is 90 deg. We looked at an identical fan (sister unit) and got the same phase results. We also ruled out a cocked bearing with phase. We are doing all of this with a SKF Microlog cmxa 70 using the cross channel phase option.

I guess my question is, what is an acceptable axial phase shift between bearings?

Thanks

"I guess my question is, what is an acceptable axial phase shift between bearings?"

Mike - I'm not aware of acceptance criteria for axial phase shifts. Perhaps others on this forum may have an answer to this. I dont use phase as an acceptance criteria, simply a measurement to help diagnose root cause problems.
The one exception I'm aware of are the vector (i.e. amplitude and phase) criteria levels for detecting cracked shafts using non-contact probes proposed by B-N
I think the only acceptance criteria are the actual vibration levels and what effect they have on the long term health of your fan.
Certainly, 0.4 ips is high and it now seems you've field balanced so that both your hor and vert levels are similar. (0.08 ips not too bad -I assume similar to the other fan??). Good end result considering balancing of fans not always straightforward.

Regards
Jim P

Mike,

The fan stricture may be resonate near 1XSS in the horizontal direction. Have you conducted any tests to very this?

Walt

I concur with Jim P. Phase shift across the bearings is inconsequential (in this case) providing that amplitude is acceptable,

I guess I was not that clear, nothing unusual here, but the fan was showing 180 deg phase shift in the axial direction on the fan bearings. After balancing the AXIAL phase shift was 90 deg. I first suspected a bent shaft but now I am unsure.

A couple unbalance, especially for an over-hung rotor can case high axial vibration. Why are you so concerned with axial vibration (what's it?) when the horizontal is 0.4 in/sec velocity?

Walt

quote:

Originally posted by Walt Strong:
A couple unbalance, especially for an over-hung rotor can case high axial vibration. Why are you so concerned with axial vibration (what's it?) when the horizontal is 0.4 in/sec velocity?

Walt

Mike has it down to 0.09ips. But he mentions using cross-channel phase, so do you mean that you did not obtain phase using an optical pick-up or some other 1 pulse per rev. device? Sorry, but I've lost touch with the SKF products and don't know the one you mention.

I misread the first post where it was Stated that after balancing the Horizontal Vibration was reduced to 0.09 in/sec velocity. We still don't know what Her before-after axial vibration levels are.
Walt

The fan is not only center hung, but also rides on sleeves. Thus axial phase shift could certainly not be a result of a bent shaft between the bearings. It may be if bent outside the bearing, or if the drive sheave is sitting cockeyed on the taper adapter.

Yvan, thanks for the reply as well as noting that it is a center hung fan with sleeves. I am not sure why a 180 phase shift between the fan bearings axially could not be a result of a bent shaft???

My bad Mike, of course it could. (check the time of my post) . I was thinking that the sleeve bearings would simply allow the shaft to float on the oil wedge. But, I hadn't figured in the fact that there is a thrust face somewhere that locks the shaft axially.

Here is what I picture. The shaft puts a moment on the bearing. The bearing puts a moment on the frame. Hi L/D bearing with small clearance would have the least tolerance for shaft/bearing misalignment due to bow and would create the most moment as a result of a shaft bow. Something like attached. (I did not attempt to show shape of frame deformation)

BentShaft180.ppt (24 Kb, 45 downloads)

OK, so how much moment with a radial fluid film bearing take?

Yvan,
May be a bad assumption. Most sleeve bearing motors are NOT set up to take any thrust (horizontal). They rely on the driven equipment to hold thrust. This, of course, requires a limited end float coupling.

Mike,

quote:

what is an acceptable axial phase shift between bearings?

There generally is no correlation of phase, since one bearing has thrust restraint and the other does not. Have you corrected for phase orientation of accelerometer on each bearing housing in axial direction? What is axial vibration level @ 1xSS on each bearing housing?

Walt

Walt, thanks for the input, yes, we did correct for transducer orintation. I have included a few axial spectrums for consideration. The axials did drop a lot just from balancing but I continue to see hight 2x's on the outboard fan bearing. Fan speed is 1790.

b_fan.doc (84 Kb, 24 downloads)

Mike,

Possible natural frequency near 3600 cpm at fan outer bearing housing in axial direction. An impact test with fan running or during coast-down could confirm this. If the bearings are Dodge-type, then I would check the torque of the inner housing restraint bolt to correct possible looseness.
Walt

Doc,

I have run into a few things with similar stuff.

A bent center plate or shaft from the mechanics (not millwrights) hammering it onto the oversized shaft. (Never measured. Had to go out and get a 20# sledge when the 16 wouldn't do.) This one caused high axial (center hung), but I'm not sure if it was 1 and 2 x or just running speed. That and combination of a bad resonance at 57 hz made us take the easy route and lock out that frequency with the VFD since they didn't need that much air anyway.

A loose thrust plate in a Dodge sleevoil bearing showed pretty much the same spectrum as you have.

A loose plunger will cause harmonics, but probably not axial as will loose water jacket/connections.

I'm not sure what Walt is talking about but if there is an inner housing restraint bolt, I would check the torque to correct possible looseness.

Doc, I have seen exactly this situation. Was called to a steel mill because a large baghouse fan "suddenly started shaking". This was a large, center-hung fan, running in Dodge SleevOil bearings, about 1000 h.p. with a 10,000 lb. rotor. As is typical, vibration was high horizontally with fairly low vertical vibration. This is normal as the pedestals are almost infinitely stiff in the vertical direction.

Axial vibration on the fan bearings, measured at the top of the bearing, in line with the shaft centerline, was exactly 180 out of phase (corrected for sensor orientation, of course). This is exactly what you should see with a "bowed" shaft (i.e., 'bent' between the bearings). These bearings actually "follow" the shaft in this instance. Picture a banana shaped shaft.... with the "bow" at the bottom, the bearings both lean inward, toward the center. A half a revolution later, with the bow at the top, the bearings both lean outward. Thus they are 180 out of phase axially. This is nothing magical, it's just the way the bearing housings are physically moving in response to the shaft movement.

A bowed shaft on a center-hung fan balances pretty easily. Normally, it's a routine balance, with a very large weight. In this case, I used 10 lbs. and told them the balance might not "stick" and if the vibration went back up to previous levels, just to cut off the balance weight. This is exactly what happened after about a week. They cut the weight off, and all was normal again. This was simply a thermal bow caused by letting a fan get too hot, and then shutting it down. In this case the thermal bow eventually "relaxed" which threw it back out of balance. I did another, smaller fan a couple of years ago that had the same scenario, but it never relaxed. But it's still running good.

Your axial phase difference of 90 is probably just due to the shaft still having some bend in it, even though it's balanced.

You put a balance weight on the bent shaft (painful situation) and presumably reduced the vibration to acceptable levels. Do you think your weight straightened the shaft, or was it still bent?

If the shaft was still bent, which it should have been for the balance weight to work if only temporarily, why wouldn't you still expect large out of phase vibration axially at the bearings? Possibly, because the bearings don't support much of a moment without an extreme angle through them, like a cocked bearing where one should see a wear pattern.