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Rotalign Tech Note 1 1

Overview

This technote is designed to facilitatethe alignment of single-bearing machineslike generators and exciters which haveonly one shaft bearing housed in the endbell, behind the back feet of the statorhousing. The objective is to align the shaftof the generator to the stationary shaftwhile at the same time preserving theproper clearances between the stator androtor assemblies. This is often a challeng-ing undertaking, since the shaft (and rotor)is only supported by the rigid coupling atone end and by the bearing on the otherend. The coupling must therefore be loos-ened prior to taking misalignmentreadings. The feet of the generator sup-port both the stator and the rotor bearing;therefore, moving and shimming the feetaffects both the alignment and the air gapbetween rotor and stator. The procedureto correct both the alignment and air gapsimultaneously is straightforward and in-volves simple graphing. All graphingwork must be executed carefully, sincethe corrections are derived from thegraphs. Always maximize the scale ofyour drawing to the extent possible anduse good quality graph paper with a pre-cision printed grid, available from almostany good art or office supply store.

Procedure

Alignment MeasurementThe idea is to take alignment readings

with the coupling slightly loosened topermit flexibility, then mike the air gapbetween rotor and stator at 12, 3, 6, and 9oclock, and plot the relative positions ofrotor, stator, and a stationary referencebaseline.

1) The first step is to loosen the rigid(usually rabbet fit) couplings boltsonly to the point where the shaft stillrests in the rabbet, in order to allowsufficient flexibility to accurately mea-sure the angularity of any existingmisalignment, while still keeping theoffset near zero. Run a feeler gagebetween the solid hubs. A gap of .010"or .015" is usually enough. If the cou-pling is loosened to the point wherethe coupling hubs disengage and areheld only by the bolts, the rotor shaftwill drop by the amount of clearancein the fit of the bolts. This will producea negative vertical offset which it ispreferable to avoid, since it adds onestep to the procedure for evaluatingthe quality of your alignment (see thediscussion concerning angularity instep 4). Taper fit rabbets will unavoid-ably produce offset as you slightlyseparate the hubs. Some single-flexplane couplings will also register off-set, so always be sure to check theactual measured alignment conditionsat the coupling center.

2) Now carefully measure the air gapbetween rotor and stator using feelergages at 12, 3, 6, and 9 oclock. Alsomeasure carefully the distance fromthis location to the coupling center, asyou will need it later.

3) In the ROTALIGNs Dimensionsscreen, select a single-bearing ma-chine as your right machine, by goingto the option softkey. Dimensionsare entered as follows: Diameter 10"(as usual), Coupling Center to Re-ceiver (as usual), and Receiver to FrontFoot as the distance from Receiver tothe location of the bearing. Do not

Copyright 1995 by LUDECA, INC.

TECH NOTE 1

Single-Bearing MTBMs with Bearing in the End Bell(Graphical Method)

Revision 0.2

LUDECA, INC.1425 NW 88th Ave.Miami, Florida 33172

Phone: (305) 591-8935Fax: (305) 591-1537

2 Rotalign Tech Note 1

Plot the rotor centerline as being astraight line from the coupling center(C) (if you had no offset) to thebearings actual position (from theROTALIGNs foot position result) atlocation (X). The coupling center iswhere the rotor and baselines inter-sect (if we assume zero offset at thatpoint). If you did have a measuredvertical offset at the coupling center,then plot the rotor line as a line fromthis offset point to the bearing loca-tion. (See graph example 4 for anillustration of this situation). Eitherway, this line should coincide closelywith the ROTALIGNs graphical dis-play of the rotor alignment in theResults screen.

Now calculate the difference in airgap top to bottom (top minus bottom)for vertical position, or side to side (3oclock minus 9 oclock) for horizontalposition, and plot the stator centerlinerelative to the rotor centerline. To dothis, originate the stator line at the truebearing position at location (X), andposition the stator line one half of thedifference in the air gap away from therotorline, in the direction of the largergap. This is the amount by which thestator is shifted out of alignment withthe rotor at the axial location wherethe air gaps were measured (G). Whengraphing this stator line, you will findthat the shift in the direction of thelarger gap corresponds with the sign(+ or ) of the difference in air gap thatyou just calculated. For example: Ifgap at top is 20 mils, and at the bottomis 50 mils, then the gap difference

enter the distance to any of the actualfeet of the generator.

4) Now take alignment readings as usualand go to Results. Offset results shouldhopefully be near zero at the couplingcenter. Angularity should be comparedto tolerances to determine if anyalignment corrections need to be made.The foot result indicates the positionof the bearing relative to thestationary baseline. The graph on theROTALIGNs screen represents therotor shafts centerline relative to thestationary baseline, and is one of thelines that you will need to graph your-self. (If offset was not zero, assumethat it will be when the coupling ismade up again, so calculate the angu-larity manually, only for purposes ofcomparing to tolerances. Calculate an-gularity manually by dividing thebearing position by the distance fromcoupling center to bearing location (Cto X). Do not use the ROTALIGNsdisplayed angularity result, which doestake the offset into account.)

5) Now obtain the actual correction re-sults for the feet of the generator bygraphing, as indicated in the follow-ing section.

Graphing Begin by laying out your baseline as a

straight horizontal line, marking thefollowing locations: Coupling Center(C), Location where air gaps were mea-sured (G), Generators Front Feet (F),Generators Back Feet (B), and BearingLocation (X).

C FG B X

C = Coupling Center

G = Location where airgaps were measured

F = Front Foot ofSingle-BearingMachine (Generator)

B = Back Foot ofSingle-BearingMachine (Generator)

X = Location of Bearing


As always, evaluate the quality of youralignment and the need for performingcorrections by comparing the angularityat the coupling to proper alignment tol-erances. Remember to consider negativeoffset if it exists, and factor it out for thisevaluation, although not when graph-ing. Also keep in mind that the accuratemeasurement of the air gaps betweenrotor and stator is critical and is veryoften complicated by difficulty of access,or by differing thicknesses of varnish orpaint on the rotor and stator surfaces.

(top minus bottom) = 30 mils. Thusyou would plot the stator line belowthe rotor line, by one half of this differ-ence, 15 mils, at G.

Now derive the actual corrections forthe feet of the generator by measuringthe distance from the stator line to thebaseline at the foot locations (F & B).

See the four numerical and graphicalexamples following.

GRAPH EXAMPLE 1:

Scale:

Horizontal:1 square = 1"

Vertical:1 square = 0.002"

Locations:

C = Coupling CenterG = Location where air

gaps weremeasured

F = Front Foot of Single-Bearing Machine(Generator)

B = Back Foot of Single-Bearing Machine(Generator)


Dimensions:

C to X = 50"C to F = 15"C to G = 20"F to B = 25"

BASELINE

ROTOR

STATOR

F

+10

-15

GC B X

-15

+25

Measured Results (Vertical):

Vertical Offset: Zero.ROTALIGN Foot Position: +.025". (So actual bearing position is +.025" high)Gap at 12 oclock: .020"Gap at 6 oclock: .050"The gap difference is .030". Therefore, the stator is shifted one half of this difference(.015") towards 6 oclock, at G.

Calculated Corrections from Graph (Vertical):

F = +.010" (Shim up Front Foot of Generator 10 mils)B = .015" (Remove shims under Back Feet 15 mils)

These corrections will align the machine to the stationary machine and even out the air gap betweenrotor and stator simultaneously.

4 Rotalign Tech Note 1

GRAPH EXAMPLE 2:

Scale:



Locations:


gaps weremeasured




Dimensions:


Measured Results (Horizontal):

Horizontal Offset: Zero.ROTALIGN Foot Position: .020". (So bearing position is .020" towards 9 oclock)Gap at 3 oclock: .035"Gap at 9 oclock: .035"The gap difference is zero. Therefore, the stator is in line with the rotor.

Calculated Corrections from Graph (Horizontal):

F = +.006" (Move Front Feet of Generator 6 mils towards 3 oclock)B = +.016" (Move Back Feet of Generator 16 mils towards 3 oclock)

Since the rotor and stator are already aligned with each other, these corrections arenecessary only to align the whole machine to the stationary machine!

GRAPH EXAMPLE 3:

Scale:



Locations:


gaps weremeasured




Dimensions:


Measured Results (Vertical or Horizontal, generic example):

Offset: Zero.ROTALIGN Foot Position: .000" (Therefore the rotor is aligned with the stationaryreference baseline).Gap at 12 or 3 oclock: .045"Gap at 6 or 9 oclock: .017"

The gap difference is +.028". Therefore, the stator is shifted in the positive directionrelative to the rotor by one half of the difference (+.014").

Calculated Corrections from Graph:

F .016"B .005"Since the rotor is already aligned to the stationary machine, these corrections arenecessary only to align the stator to the rotor and even out the air gaps!

XB

+16-20

G

0

F

+6

C BASELINE

ROTOR & STATOR

X0

B

-5

ROTOR & BASELINE

+14

G

-16

FC

STATOR


GRAPH EXAMPLE 4:

Scale:



Locations:


gaps weremeasured




Dimensions:


Measured Results (Vertical):

Vertical Offset: .015"ROTALIGN Foot Position: +.018"Gap at 12 or 3 oclock: .012"Gap at 6 or 9 oclock: .064"

The gap difference is .052". Therefore, the stator is shifted down relative to the rotorby one half of the difference (.026").

Calculated Corrections from Graph:

F +.0355"B .0025"

The angularity for alignment purposes is considered to be only .018" over 50"(.36 mils per inch) since we assume zero offset when the coupling bolts are tight. If theangularity tolerance only allows .2 mils per inch you should perform the corrections.Note that in graphing we do take the offset into consideration since the air gaps weremeasured with the coupling bolts loosened. '

BASELINE

ROTOR

-2.5B

G

STAT

OR

-26

-15 (V0)

+18

XF

+35.5

C

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