Note: this question is just for discussion e.g. I'm not looking for a solution to an existing problem. The purpose is to have general tips to extract information from the data, possibly under different scenarios.

A shaft average centerline plot shows the trend of the shaft center going down below (sometimes outside) the clearance circle. This happens while variable speed machine operates at higher speeds. The first a few gap voltage readings are collected while the machine is standstill (0 rpm before the start up). Assume the relative probes are well installed and the online system readings are verified. I'll try posting a plot of an actual case.

Possible interpretations:

1. There exists a downward (directional preload) force pushing the shaft down.
a. Sever coupling misalignment (the other bearing across the coupling experiences the opposite)
b. Internal misalignment beaten the machine's components

2. A change in the bearing clearance takes place during the run of the machine, specially the lower half of the bearing.
a. Abnormal thermal expansions
b. Casing distortions due to e.g. pipe stress

3. Nullification of an upward force pushing the shaft above the actual bearing bottom before machine startup.
a. Tight clearance of other components widened later with the start up.
b. Casing distortion adjusted by thermal expansion and load after the startup.

What is the interpretation of this data?
Comments on the above possibilities?

Regards- Ali M. Al-Shurafa

Attached is sample of a SACL plot where the shaft moves outside the clearance circle.

Your outer ring? What does it represent? Are you saying you have metal to metal contact? In what manner are you relating eddy current gap voltage to alignment data both cold and hot?

Possible interpretations:

1. There exists a downward (directional preload) force pushing the shaft down.
This is a shutdown (or startup). Gravity may push the shaft down. this is not atypical for a shutdown. The starting position does not appear correct.

How did yu establish the bottom position? The bottom should be at the low speed (or 0).

a. Sever coupling misalignment (the other bearing across the coupling experiences the opposite)
b. Internal misalignment beaten the machine's components

2. A change in the bearing clearance takes place during the run of the machine, specially the lower half of the bearing.

Not likely

a. Abnormal thermal expansions
b. Casing distortions due to e.g. pipe stress

3. Nullification of an upward force pushing the shaft above the actual bearing bottom before machine startup.

Do you have a reason that this could happen?
a. Tight clearance of other components widened later with the start up.

Not likely

b. Casing distortion adjusted by thermal expansion and load after the startup.


this looks like a coastdown or startup. What is unusual about this?

The comment says startup. Did you grab the first point as the bottom and then shut down the unit? Naturally, a horizontal machine will travel towards the bottom of the bearing during a shutdown, the opposite for a startup.

Sam/Bill,

Thanks for your reply.

The data is for a horizontal turbine outboard bearing with two relative probes mounted 45R and 45L w.r.t. the dead vertical line.
The circle shown on the plot is the bearing clearance circle (Boundary). The data shown is for startup, steady state and shutdown, from the same file. The reference is made to the first sample (0,0) when the shaft was stationed on the bearing with zero rpm (only gravity force is acting on the shaft). A few samples were collected under this condition to ensure capturing the reference point.

Not sure if there is a metal to metal contact.

The first (0 rpm before stat up ) and last point (0 rpm after the shutdown) are almost the same. A few points were collected after the machine stopped to ensure capturing the last point.

Why you have doubts on the starting position being correct?

Why not considering the preload due to some restrictions from internal components or coupling misalignment?

quote:

Why you have doubts on the starting position being correct?

Not so much the starting position, because that is from the data. This relates to the setup of the clearance circle. the starting point is outside of the clearance circle.

you mentioned taking a reading at 0 rpm after the shutdown. Many turbines have some type of turning mechanism. Did this not come down onto a turning mechanism? Large turbines may have lift oil which affects the 0 speed or even low speed centerline position when working.

It helps to know the details of the machinery train and operating condtions, but this looks like a tilt pad bearing startup and shutdown.

At the low speeds one will have a thin oil film and small lift on this plot. The 0 rpm point if indeed this is 0 rpm or there may be lift oil may not actually be on the bottom. Besides this is only approximately 0.0005 in off the plotted circle.

Correct the software input to get the cirlce to matchup with the lowest point or the 0 rpm point if that is on the bottom.

Are you sure that the probes or the probe brackets are not changing with thermal expansion?

Another point I was unclear on; are you trying to use these data from sleeve brgs to relate to shaft to shaft alignment?

To lessen confusion omit your clearance circle altogether (better to not have it than in the wrong position) and inject bearing types and clearances with say ~210 SSU at operating temp for your oil or type of oil and temps.

You are looking at <2 mils vibration. What may seem erratic may not be too far from normal or indicate a problem if more information is known, but as you suggested, downward movement is cause to 'want to know why'.

I have the utmost regard for Mr. Bill Foiles opinions and suggestions as they are noteworthy and valuable.

quote:

Are you sure that the probes or the probe brackets are not changing with thermal expansion?

This should always be of concern, particularly with turbines. In this case the startup and shutdown did come back to about the same position. This gives one some degree of assurance, although not proof of not thermal distortion of the measurement.

Sam,

This should be interpreted as position change in the bearing for good measurements. This looks like it could be a valid measurement.

Shurafa, The centerline plot has a lot of data and at times overlaps. Please post 3 seperate centerline plots: start-up, steady state and coastdown.

Thermal expansion of the bearing housing with probes is alweys of concern. I had a 650 deg charge pump change shaft centerline position ~1-2 mils before it started due to the warm-up line.

It is good to take pre-startup and steady state reference data. This will give you a good idea what to expect.

Also please give more machine information: bearing temps, type of bearings, type of turbine etc

If the circle is representative of the clearance, then it is obvious that the movement is not greater than the clearance. I agree with others that there is a software adjustment needed to put the movement inside the circle.

Why you have doubts on the starting position being correct?
The starting position will be at the 0 position but this may not be in the bottom of the bearing.