Does anyone have experience,ideas or information on the best way to collect true displacement readings from a large ID fan with Dodge Sleevoil bearings? We have considered prox probes but have yet to find anyone who actually installed them into this type bearing. The next question is are the results worth the trouble and expense? The highest vibration is consistently in the axial direction @ 1X.

Travis

Its pretty straightforward to do this – use an L shaped bracket, bolted to the casing. You could use expoxy resin or even a mag base, but you then face the possible safety issues due to the bracket or base falling off.

Most permanent installations use axial proximity probes to measure thrust or axial position rather than axial vibration. The biggest problem by far is the surface finish of the area of the shaft that the probe is ‘looking at’ – unless you are very lucky, then you are likely to find that the probe is seeing a huge amount of runout (scratches etc) and the measurement you are trying to make loses much of its value.

Before going to proximity probe installation, you should consider what sort of mechanisms can cause axial vibration

a)Is the housing resonant in an axial direction?
b)Is there a thrust bearing that is cocked or swashing
c)Is the shaft bent?
d)Coupling Assembly/axial runout problems?

Check a) with a bump test, b) and c) with a DTI d) also with a DTI and disassembly/inspection

What’s the history of this problem? Is it a recent thing or has it been around for a while?

This message has been edited. Last edited by: Buzz LightYear,

Buzz,

This has been an ongoing problem for several months now. The fan has been in service for approx. 5 years. In the past most axial vibration was linked in some way to unbalance. We would either clean the wheel or balance and vibration levels would return to acceptable levels.

To answer the questions you asked,

a)I have not done a bump test. I will do that, but I have not seen any indication of resonance in the vibration data.

b)The thrust bearing has been checked for run out with no problem found. I'm not sure I understand what you mean by "swashing"

c) The shaft run-out has been checked and found to be OK. If my memory is right it was in the range .001" to .0015"

We have also checked alignment, coupling run out, coupling gap, motor shaft run-out, anything that would cause axial movement. I have balanced the rotor, single and dual plane which lowered radial vibration down to <.4 mils. We have replaced the motor...twice, that's another issue for a different post.

Also the axial vibration increases or decreases as operating conditions change, but it's not predictable.

Ambient air temperature appears to effect axial vibration. The fan usually but not always runs slightly lower at night than during the day. Now, with cooler weather the axial vibration is consistently lower, but inlet air temperature is relatively constant.

Could it be that the bearing housing is resonant and the air temperature changes stiffness??? The oil for the bearings is feed from an external pump, filter, cooler which controls oil temperature. This morning the internal bearing temperature is 117f on the inboard and 108f on the outboard. Internal bearing temperature only varies by approx. 5 degrees farenheight even on the hottest days.

Also, I was wanting to look at axial displacement so simply bolting on a prox probe and polishing the shaft is out. The design of the bearing has the shaft thrust collar placed internally inside the bearing cartridge. I think the prox probe would have to be drilled and tapped into the bearing itself.


Didn't intend to get so wordy...thanks for your input.

Travis

This message has been edited. Last edited by: Thud,

Quote:
Ambient air temperature appears to effect axial vibration. The fan usually but not always runs slightly lower at night than during the day. Now, with cooler weather the axial vibration is consistently lower, but inlet air temperature is relatively constant

Sounds like it could be a process related issue - lower air temperatures increase the air density which changes how hard the system is working. Are there any louvres/dampers/vents in the discharge system that you can alter to see if this influences the vibration?

I'm thinking it is a process related issue, I just haven't got a handle on what's driving it yet.

In the past air density has been a problem on start-up of the fan when ambient temperatures are very low, but as heat is added to the kiln fan vibration goes down.
The air that the fan sees is pulled through a 400 foot long kiln and a scrubber system. The discharge side has a silencer but no other restrictions.
The vibration levels I'm seeing now started when ambient temperatures were near or maybe even above, 100 degrees fahrenheit.

If air flow is the problem...why would air flow issues show up in the vibration spectrum as 1X turning speed and not at blade pass frequency?

Travis

Try a little research on acoustic resonance, organ pipes and standing waves

Thud,

What are the vibration levels that concern you?

There is a possibility that with temperature and load change, the rotor bows. This could be detected with phase difference variation between IB and OB bearings in axial direction. A better way to detect possible phase change in all other directions due to bowing is using a keyphasor measurment. Have you tryed it?

Dave

The vibration levels have been as high as 4 mils, currently they're running around 2 mils in the axial direction.
I've not tried the keyphasor measurement. I have taken phase measurements with a 134 degree shift on the west side of the shaft and a 128 degree phase shift on the east side of the shaft, all in the axial direction at the shaft centerline.

How is the fan load controlled? Is there a single damper on the inlet side of the fan? Is this a double-suction fan with dual inlet dampers?

Upon startup with relatively cold air, the fan load will be highest since the air is denser (all things being equal... damper position, fan speed, etc).

Radial prox probes are not going to measure axial fan vibration. When you measure the axial vibration on the bearing housing, are you seeing any impacting in the waveform (using both acceleration and velocity)? There will be sharp peaks in the TWF with an elevated crest factor (peak/RMS), usually greater than 4.5 - 5.0.

Take axial phase measurements on both bearings. If they are 180 out of phase (accounting for sensor direction), this indicates a bowed rotor. This can sometimes be balanced out, but if this is the case, you should have a high radial 1x as well.

You should be getting the same axial readings at both bearings measured in the same relative locations. If it is considerably higher at one bearing, that bearing probably has an internal mechanical problem. If both bearings read and look the same (spectra and waveform) you probably have a load-related issue. Using a "trigger" to start your data collection will allow you to see if both bearings are moving the same, viewing the velocity time waveform.

Just some ideas...

Rusty,
The fan has a single damper on the inlet side. The fan is a single input.

As for the prox probes, that was really my question. Is it possible to install prox probes in this type of bearing in the axial direction reading from the thrust collar of the shaft? Is it feasible to do and are the results worth the effort?

To give you a better idea of the fan set-up, I've attached a picture of our spare rotor. The wheel is a 86 inch diameter, single input rotor. Rpm is 1800, driven by a 2000 HP motor.

The axial readings are only high on the thrust bearing end. The outboard end has a floating bearing so axial movement is low. The last phase readings I took on the fan showed an axial phase difference of approx. 130 degrees after accounting for sensor direction.

The waveform shows no sign of impacting, more sinusodal. The crest factor on readings I took on Nov. 14 is at 3.71

Thanks for your input
Travis

Wow! That is a totally unusual fan wheel! Looks like they extended the blade tips to get more pressure. Did he blade-rate sound pressure level go up a lot?

Regarding high axial vibrations, I don't think axial proximity probe would help that much. I would inspect and measure wheel wobble with a dial indicator. The new welds on the ribs at the shaft hub raises a red flag that the shape may be distorted. Other possible causes of high axial vibrations at 1xSS:
Rotor couple unbalance (see wobble)
Worn/loose/runout thrust collar (inspect and measure)
Fan wheel axial natural frequency (impact test)
Bearing housing and pedestal natural frequency (impact test)
Shaft coupling worn (poor lube) combined with misaligment (inspect and align)

Walt

Let the smart people explain why, but I am removing all of the proximity probes off of our fans and installing pedestal mounted accelerometers that double integrate to displacement (you should measure velocity but it is easier to give in than to teach an old monkey new tricks). Something about the fluid film being stiffer than the pedestal which results in little or no relative motion. I have found proximitors are good for picking up corrosion, glitches, and magnetism on these large sleeve bearing fans. Good luck!

Mark,

Maybe I am not so smart, but that does not sound right to me... We can argue accels vs prox probes, but I usually recommend against double integrating casing vibration for permanent monitoring applications as people get casing displacement mixed up with shaft displacement and try to apply shaft based vibration alarms and shutdowns to casing vibration...

quote:

Something about the fluid film being stiffer than the pedestal which results in little or no relative motion.

How do you know this? In most cases I would say the pedestal/support is too soft, or the bearing is too stiff.

For generally higher speed machines (read more difficult to obtain) API 617 likes the pedestal to be 3.5 times as stiff as the bearing oil film. Personally, I would prefer greater, but that would be tough to get an agreement on.

The fan wheel was tipped a couple of years ago. The wheel in the picture is a spare which has not been put into service yet.

The modifications were made to increase air flow. The wheel was increased from 82 inches in diameter to 86 inches and the motor from 1200 horsepower up to 2000 horsepower. (The bigger is better mentality). The changes were made by the manufacturer and we have been assured that all of the nesesary natural frequency and performance tests were done.

We've been running a modified wheel for about three years now with no major problems until recently.

Travis