shaft Alignment of rotating mach..

[powerjunx]

hello!
id used laser alignment equipment from masterlign but i never read any related feature on how to align a machine which was equipped or uses Sleeve bearing(babbit)..
anyone who experience in aligning such machine? id aligned 4 units of about 1000 hp ac squirrel cage induction motor, id used the chart,suggested alignment tolerance table but it seems, i think, not very accurate since it never consider the "float tolerance" of the shaft which could affect the vertical offset reading when the machine is energized, correct me if im mislead.
my question is; does the Float of the shaft on the babbit must be consider? since were aligning the machine at rest.. that results to high vertical offset when the unit is running..


thanks

 

[jmack]

fbjavier,
You are correct in thinking this float has an effect. You are actually talking about the hydrodynamic oil film that causes the shaft to lift under rotation. This oil film separates the two parts (shaft and bearing surface) with this oil film and prevents metal to metal contact. You have to account for the "lift" of the shaft especially when dealing with a babbit bearing coupled to a roller bearing. The rule of thumb here is that you typically have 0.001" - 0.002" diametral clearance for every 1" of shaft diameter, depending upon loading, shaft deflection and other factors. For an 8" shaft you would therefore have 0.004" - 0.008" lift typically, check with the installation manual for actual numbers. You could also plastigage the bearing cap cold to give you a rough approximation of the diametral clearance and assume the bearing lift will be half of this (not likely, but a fair approx.). More importantly remember to do your soft foot and thermal expansion corrections first before beginning your final alignment.

J MacK

 

[Ralph2]

Typically I would ignore the clearance in the babbitt bearings, on the assumtion that the entire drive line is at rest and laying in the "bottom". But this does need to be thought about..... especially when aligning a gearbox pinion that will ride up on the bull gear under load.
There are more variables than that of the bearing clearance if one is aiming for perfection, The relative running location of the shaft, it will not be just up. And,thermal growth is seldom accurately factored in and will likely exceed that of the clearance.
Good luck
Ralph

 

[jmack]

fbjavier,
Ignoring babbit clearance is not standard practice by millwrights. These lift numbers are usually given by the manufacturer and are compensated for as such. Also, I have yet to see a pinion ride up a bull gear under load. Pinion and bull gears are designed so that the radial loading from the pinion is transfered through the bearings and into the foundation, you also will typically find that these arrangements use roller bearings and not babbit bearings. Ralph2 is correct in saying that that the running location of the shaft "will not be just up", there is an eccentricity. A good book to read is "Machine Design" by Norton. This will help you with this and many other issues you may run into.

J MacK.

 

[Ralph2]

I guess I stand to be corrected regarding babbitt clearance. Would you hazzard a guess as to how high (ie how thick the oil film) would be on a 12 inch journal that has .014" clearance? (500 MW turbine rotor running at 3600 RPM) If you had even (while running)a oil film of .0015" I would be surprised. Perhaps you use thicker oil? ;-)
An interesting question though, just how thick is that oil wedge?
Ralph

 

[sms]

First to J Mack's comment, in my industry, Petrochemical and refining, all high speed gears have babbit bearings (1800-3600 rpm input, 4000-12000 rpm out), and the vast majority have an up running pinion, so that under load the pinion actually rides in the top of the bearing due to the lifting action of the bull on the pinion. Depending on shaft size and bearing clearance you might want to take that into account in the alignment.

Second to Ralph2's comment, There is oil film thickness, and there is shaft position, and it is different on tilt pad bearings vs plain journal bearings. After many many years of measureing shaft radial position relative to the bearing clearance I can say that for a healthy tilt pad bearing the shaft centerline position moves from rest at the bottom of the bearing, straight up in the clearance, to a point just below the centerline of the bearing. About 60% of the radial clearance. For a .014 diametral clearance tilt pad load on pad bearing the shaft would move from rest in the bottom, straight up about 0.004-0.005. For the same clearance plain journal, the shaft would move up and over to one side depending on rotation, but the net upward movement would be about the same.

The question is, for that 500 MW turbine is 4-5 mil worth messing with, especially if we know that the generator is going to lift about the same? Typically I ignore it.

 

[jmack]

fbjavier,
First and foremost you would not treat an induction motor alignment the same as a turbine or a gearset. You also would not treat an 1800 or 3600 rpm motor the same as 4000 - 12000 output gearset, so i would ignore the advice of some who may think that this is a similar situation, as it is most definitely not. As for ignoring lift, do not, and I am glad my friends at my previous employer, a major power generating station did not when aligning the turbine and generator set or we would be all out buying portable generators in a hurry !! The long and the short of it is that one would require more details in order to make a proper assessment of what you should do, ie. what is the mechanical arrangment? Speeds? Shaft diameters? Oil viscosity? Thermal growth considerations etc...
As to SMS's comments on high speed gearboxes, I can tell you that these are the exceptions and not the rule in most industries, I have worked in the Automobile manufacturing, Tire manufacturing, Railcar manufacturing, Power generation, and Paper manufacturing as a millwright specializing in optical, laser, and reverse dial alignment and more recently as a Mechanical Engineer in Power generation and Paper manufacturing, so I would know a little about alignment and standard practice.

J MacK

 

[sms]

"As for ignoring lift, do not, and I am glad my friends at my previous employer, a major power generating station did not when aligning the turbine and generator set or we would be all out buying portable generators in a hurry !!"

JMacK, I think you are really overstating the consequences. We always shoot for perfection, but rarely achieve it. The equipment is designed to tolerate some misalignment for that very reason. A few mils off set over a 6 foot coupling span on a main power plant turbine is not going to wreck it.

 

[vgarzani]

You may consider making an effort to quantify thermal growth. Acculign (not sure of spelling) is a simple system where benchmarks are mounted on the bearing casings and on the foundation (or whatever your interested in really) and precise distance measurements are made in three dimensions at cold and then hot conditions. You work out the geometry and determine thermal growth. Structures that are tied to things like piping systems often move in interesting ways that may surprise you.