From off-line to running, the rotor of sleeve bearings move to a position slightly above the bottom and off-center, where the force from the oil wedge distribution balances the weight and other applied forces.

To my knowledge, for simple machine (I don't know about turbomachines), this fact is not taken into account in alignment procedures. Is it?

It does not seem like a big deal for machines where driver and driven are both sleeve bearings with similar shaft sizes, clearances, and loadings... we might expect the rotor movement from off-line to running to be the similar for both machines.

But we have two families of machines where a sleeve bearing motor drives an anti-friction bearing pump through a shim pack coupling. Should the expected change in position of the motor shaft within sleeve bearing clearances be taken into account during alignment on these machines?

Epete,

I too have a couple of these type machines. I have 2 anti friction bearing motors driving sleeve bearing Mycon NH3 compressors through a Thomas Shim Pack Coupling. This style coupling, by the way, is the only thing we've found that will stand the torque necessary to get the compressors off the bottom of the sleeve and up on the oil wedge.
To get to your question, I give the compressor 0.002" rise up onto the wedge, but don't give it anything to the side. Too much information for my old nogging to process. I put the info into rotolign, and let it tell me where to go. Even when hot aligning, I go ahead and take the rise into account.
I'm not even sure that 0.002" is right, I just picked the number, although I think it's somewhere inbetween 0.001 and 0.002 (I like 0.0015", but don't use it). I'm not sure it even matters, but seems to me to be something that I should account for, in the never ending process of trying to do the best one can.

Dave

One place where rotor movement should defintely be considered is in a gearbox with fluid film bearings.

In a horizontally split machine one of the rotors will move to the top of the bearing due to gear forces. If that happens to be the rotor on bearings with 20 mils diametral clearance this should be taken into account. I have seen many people get into trouble from not taking this into account. Typical gear drawings clearly show this movement.

John from PA

I've seen the rise considered. The rise may be harmless to the alignment on many machines over the span. On your big machines, the gravity sag of the equipment is very important, as well as the thermal growth.

Two fluid film bearing machines connected can have similar rises or not if the bearings are different, e.g. elliptical on one and tilt pad on the other, ignoring John's gears. This often helps to lessen the difference in alignment.

On an old application where you have historical data and using Thomas couplings - are you experiencing coupling failure? Are you experiencing bearing failure. And what model of coupling? So many machines when using dynamic methods of alignment (rolling the rotors together) you can observe the shafts rise ~3 mils generally via dial indicators. As Bill points out, what are the machines and their bearing configuration and are there variance is rotor loads or dynamics from end-to-end?

If failures are being experienced then it's time for hot alignment checks unless your initial setup is based on hot alignment data. One thing I'd do first is to determine the reason a Thomas coupling was selected and was it based on known running positions with known machine growths? Many times it's selected because you don't have to grease it.

Can you measure the shafts with the machines running? One method is eddy current probes to monitor shaft positions but you must have extreme good hot and cold alignment data. I've used laser beams to reflect back as well.

Bill
Can you explain the "gravity sag" for big machines. I'm not sure I understand.
If an alignment is performed, isnt gravity sag already part of the equation during the process?

Or are you saying that on flexible rotors supporting heavy mass between bearings that there is sag such that it results in artificial angularity at the coupling ??

Do you have an example?

Thanks
Jim Powers

Many machines have rotor bow that requires 'slow roll' to remove rotor bow prior to run-up to operating speed. There is also a storage problem with such rotors.

Coupling selection is very critical on such machines.

All horizontal rotors sag. The sag may not be significant in many instances. As Charlie Jackson once said, "Steel is just hard rubber."

I analyzed a compressor last week, and it showed over 1 mil sag (25 micrometres but less than 2 mils). On a large turbine generator this would be many multiples of this.

On large turbines the internal alignment considers the gravity sag of the rotor. This is not exactly the same thing as slow roll. The gravity sag remains in place during operation.

El Pete has some Nuke TG's as I recall. These are very large. The alignment of the entire train takes into account gravity sag and thermal growth to compromise the hot and cold alignment.

From offline to running , consideration has to be given if the driven equipment is a high temperature fluid handling pump, turbine etc. For example, boiler feed pumps. I have seen manuals of Byron Jackson boiler feed pumps prescribing motor to be kept up by 2 mils to take into account thermal growth of pump , both of then having sleeve bearings. In case of turbine , natural sag has to be maintained by keeping all bearings at relative heights such that it will maintain natural sag of the turbine. Now a days, many high temperature equipments are centre mounted such that due to thermal growth, there will be no change in alignment and hence no OL2R compensation has to be provided. I have seen this in M/S KSB , Germany make boiler feed pumps of 1600 KW, 55 bar head handling feedwater at a temperature of around 156 degree centigrade. Due to thermal growth, the pump rotor will grow equally on the upside as well as downside maintaining no change in alignment.

Sam / Bill
Our main engine turbines use jack gear to help prevent rotor bow...so I understand that pretty well....
And I also understand thermal growth issues.

What I'm still having difficulty in visualizing is how you account for the gravity sag during alignment process. During the alignment process the rotor is not on jack....right? So, a large flexible rotor with heavy mass between bearings, the rotor would bow and I can see how the coupling end of the shaft would have slight angle from horizontal ?? I would think this would create a angularity misalignment even though the shaft centerlines line up.
My "gut" feeling is that at speed, this gravity sag would not be an issue??
But Bill you did say its not the same as slow roll (sag).... so I guess I'm still confused (not that uncommon).
Also, not sure what you meant by "...internal alignment considers gravity sag..."

Thanks Jim P

p.s. Charlie said lots of things I will always remember. I haven't heard of that one before :-)

If the machines had no thermal growth aligning them to zero-zero would take into account the gravity sag (assuming measurement points aren't too far from the shaft ends. However, on a machinery train with multiple rotors, one must align all the pedestals, and this requires some offset.

If you do something equivalent to reverse alignment (including laser methods) you measure the projection of one shaft's position onto the other from one location to the next. This accounts for the angle and offset, not considering thermal growth or worrying about multiple casings.

Gravity sag is there whether the machine spins or not. Why would gravity sag not be an issue at speed? Gravity still pulls down when the rotor spins.

Suppose your rotor sags 40 mils or more in the middle. In this case you need to align the internals considering this. Otherwise, you get rubs.

One may consider the rise in the bearings when setting oil deflector seals or air seals. If not the rotor spins with less clearance on top and to one side (non-tilt pad).

If I understand you correctly, when you were talking "internal alignment" you are talking about accounting for rotor shaft centerline displacement when aligning the internal seals? That makes sense to me, I just never realized that was done for large machine train (my inexperience).
I guess my misunderstanding was your statement, "...internal alignment considers the gravity sag of the rotor. This is not exactly the same thing as slow roll." I just couldnt visualize the difference between the two.
Realizing gravity continues to "pull down" - I guess I was thinking it causes a center of mass offset that would = unbalance.

Bill, I appreciate your comments and patience. I need to digest a bit and see if I can get the light to come on :-)

Regards
Jim P