Defining Misalignment - Alignment and Coupling.pdf

7/23/2019 Defining Misalignment - Alignment and Coupling.pdf

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9 Defining Misalignment:Alignment and CouplingTolerances

9.1 WHAT EXACTLY IS SHAFT ALIGNMENT?

In very broad terms, shaft misalignment occurs when the centerlines of rotation of two (or

more) machinery shafts are not in line with each other. Therefore, in its purest definition,shaft alignment occurs when the centerlines of rotation of two (or more) shafts are collinear

when operating at normal conditions. As simple as that may sound, there still exists a

considerable amount of confusion to people who are just beginning to study this subject

when trying to precisely define the amount of misalignment that may exist between two shafts

flexibly or rigidly coupled together.

How do you measure misalignment when there are so many different coupling designs?

Where should the misalignment be measured? Is it measured in terms of mils, degrees,

millimeters of offset, arcseconds, radians? How accurate does the alignment have to be?

When should the alignment be measured, when the machines are off-line or when they are

running? Perhaps a commonly asked question needs to be addressed first.

9.2 DOES LEVEL AND ALIGNED MEAN THE SAME THING?

The level and aligned does not mean the same thing. The term ‘‘level’’ is related to Earth’s

gravitational pull. When an object is in a horizontal state or condition or points along thelength of the centerline of an object are at the same altitude, the object is considered to be

level. Another way of stating this is that an object is level if the surface of the object is

perpendicular to the lines of gravitational force. A level rotating machinery foundation

located in Boston would not be parallel to a level rotating machinery foundation located in

San Francisco as the Earth’s surface is curved The average diameter of the Earth is 7908 5



alig nment de vices wer e use d in the paper indust ry where extre mely long ‘‘line shaft s’’ wereinst alled to drive different pa rts of a paper mach ine. These line shaft s were constr ucted with

numero us sections of shafting that were co nnected end to end with rigid couplings an d

supported by a number of bearing pedesta ls along the lengt h of the drive syst em, whi ch

cou ld be 300 ft in length or more. Even if a mechani c carefully alig ned each section of shafting

at each rigid co upling connecti on alon g the lengt h of the line shaft and pe rfectly leveled each

shaft sectio n, the centerline of rotat ion at e ach en d of a 300-f t long line shaft would be out by

0.018 in. due to the cu rvature of the Ear th’s surface.Although level an d alig ned may not mean the same thing, proper level ing is impor tant as well

as having coplanar surfaces. Levelness refer s to a line or surface, which is perpend icular to

gravity; cop lanar surface refers to ‘‘flatness. ’’ Are the points where the machinery cases con tact

the basepla te (or solepl ates) in the same plane? If not, how much of a deviation is there?

It is very common to see baseplates where the machinery contact surfaces are not in the same

General processmachinery supportedin antifriction bearings

10 mils per foot 10 mils

General processmachinery supported

in sleeve bearings

(up to 500 hp)

5 mils per foot

Process machinerysupported in

antifriction bearings(500+ hp)

5 mils per foot

Process machinerysupported in sleevebearings (500+ hp)

2 mils per foot

Machine tools 1 mil per foot

Note : 1 mil = 0.001 in.

5 mils

5 mils

5 mils

2 mils

Machinery Type

Minimum

RecommendedLevelness

Coplanar

SurfaceDeviation

FIGURE 9.1 Recommended levelness and coplanar surface deviation for rotating machinery baseplates

or soleplates.



Another way of expressing circles is by use of radians. All circles are mathematically related

by an irrational number called pi (p), which is approximately equal to 3.14159. There are 2p

radians in a circle. Therefore one radian is equal to 57.2958288.Despite the fact that the expression ‘‘angular misalignment’’ is used frequently it comes as a

surprise to learn that no known shaft alignment measurement system actually uses an angular

measurement sensor or device.

9 4 TYPES OF MISALIGNMENT

Parallel misalignment

Angular misalignment

“Real world” misalignment usuallyexhibits a combination of bothparallel and angular conditions

FIGURE 9.2 How shafts can be misaligned.



Collinear means in the same line or in the same axis. If two shafts are collinear, then they

are aligned. The deviation of relative shaft position accounts for the measured differencebetween the actual centerline of rotation of one shaft and the projected centerline of rotation

of the other shaft.

There are literally dozens of different types of couplings. Rather than have guidelines for

each individual coupling, it is important to understand that there is one common design

parameter that applies to all flexible couplings:

For a flexible coupling to accept both parallel and angular misalignment there must be at least two

points along the projected shaft axes where the coupling can flex or articulate to accommodate themisalignment condition.

The rotational power from one shaft is transferred over to another shaft through these

flexing points. These flexing points are also referred to as flexing planes or points of power

transmission. Shaft alignment accuracy should be independent of the type of coupling used

and should be expressed as a function of the shaft positions, not the coupling design or the

mechanical flexing limits of the coupling. Figure 9.3 illustrates where the flexing points in avariety of different coupling designs are located. I have seen several instances where there is

only one flexing point in the coupling and have also seen more than two flexing points in the

coupling connecting two shafts together. If only one flexing point is present and there is an

offset between the shafts or a combination of an angle and an offset, there will be some very

high radial forces transmitted across the coupling into the bearings of the two machines. If

there are more than two flexing points, there will be a considerable amount of uncontrolled

motion in between the two connected shafts, usually resulting in very high vibration levels in

the machinery.

Why should misalignment be measured at the flexing points in the coupling? Simply

because that is where the coupling is forced to accommodate the misalignment condition

and that is where the action, wear, and power transfer across the coupling is occurring.

Flex points Flex points Flex points



Figure 9.4 shows a typical misalignment situation on a motor and a pump. By projecting the

axis of rotation of the motor shaft toward the pump shaft (and conversely the pump shaft

rotational axis toward the motor shaft) there is a measurable deviation between the projected

axes of rotation of each shaft and the actual shaft centerlines of each shaft where the power istransmitted through the coupling from one flexing point to another. As we measure misalign-

ment in two different planes (vertical and horizontal) there will be four deviations that occur at

each flexible coupling as shown in Figure 9.5. In a horizontally mounted drive train, two of these

deviations occur in the top view describing the amount of lateral (side to side) misalignment.

Two more deviations occur when viewing the drive train in the side view which describes the

Misalignment is the deviation of relative shaftposition from a colinear axis of rotation measuredat the points of power transmission when equipment

is running at normal operating conditions

Driver offset

(in mils)

Driven offset(in mils)

Maximum alignment

deviation occurs here

Driver shaft Driven shaft

FIGURE 9.4 Definition of shaft misalignment.



S i d

e

v i e

w

Top

vie w

Find the largest of the four deviations or gaps between the

centerlines of rotation of both shafts at each point of powertransmission (aka. the coupling “flex points”) and then dividethe largest deviation by the distance between these points

These two deviations dictate the alignment accuracyof the two shafts in the side-to-side direction

These two deviations dictatethe alignment accuracy ofthe two shafts in the up anddown direction

Points of power transmission or“flex points” in the coupling

Here is the distance between the pointsof power transmission (flex points)

For example:

If the maximum deviation of all fourpoints is 6 mils, and the distance betweenthe “flex points” is 4 in. then ...

6 mils4 in.

= 1.5 mils/in.is your maximummisalignment deviation

Here is the largest of thefour deviations

Centerlines of rotation

of each shaft

Remember, for a

flexible coupling toaccomodate bothparallel and angularmisalignment, it musthave two flexing points

FIGURE 9.5 How to find the maximum misalignment deviation?



9.6 CHECKING THE MISALIGNMENT TOLERANCE

The alig nment modeli ng techniq ues sho wn in Chapt er 8 enab le us to visually represen t the

relative pos itions of the centerlines of rotat ion of the tw o shaft s. Once the position s of the

shafts have been determ ined, the first step is to determ ine whet her the amount of misalign-

ment is within toler ance or not.

At any point in time, the machi nery shafts are somew hat mis aligned side to side and

misali gned up and down (or any other coordinates they hap pen to lie in). The ke y is to find

the largest of the four deviations at the points of power transmission (the flexing points) an ddivide it by the distance be tween poin ts of power trans mission (the flexing poin ts). Two of

these deviation s oc cur in the top view, which will show the amount of lateral (side to side)

misali gnment , two more deviat ions occu r in the side view, whi ch sh ow the a mount of vertical

(up=down) misalig nment as illustr ated in Figure 9.7 and Figure 9.8.

The side view in Figure 9 7 shows a 30 mil deviation at the coupling flex point by the motor

10 122 4 6 8 14 16 18 20 22 24 26

0.5

1.0

1.5

2.0

Maximumd

eviationat

eitherpointofpowertransmission

Mils per inch

Speed (RPM 1000)

Misalignment tolerance guideAngle

degrees

0.10

0.08

0.06

0.04

0.02

R e a l i g n m

e n t n e c e s s a r y

A c c e p t a b l e

E x c e l l e n t

FIGURE 9.6 (See color insert following page 322.) Recommended maximum misalignment of flexibly

connected rotating machinery.

http://dk4322_c008.pdf/




themselves, the misalignment deviation (remember, it is in mils=inch) will always be the same

amount (i.e., the deviation might be different at each of the points around the coupling area

but so is the distance between them).

9.7 SHAFT VERSUS COUPLING ALIGNMENT

Frequently people use the terms ‘‘shaft alignment’’ and ‘‘coupling alignment’’ interchange-

ably. Is there really a difference?

Yes, shaft alignment and coupling alignment do not necessarily mean the same thing.

Coupling tolerances typically define the maximum misalignment limit of the flexible coupling.

UpSide view

Scale: 5 in. 30 mils

Motor Fan

Flexing points or points ofpower transmission

30 mils

15 mils

6.5"

Motor shaftcenterline

Fan shaftcenterline

FIGURE 9.7 Misalignment deviations in the side view.

Top viewMotor FanWest

Flexing points or points ofpower transmission



In other words, at what poin t will the coupling ‘‘lock up’’ and stop worki ng or begin to

susta in rapid deteriorat ion in the coupling. It is not unco mmon to see 1=8 in. or more

allowabl e offs et ad vertised by a coupling manufa cturer . The cou pling may be able to accept

this amou nt of mis alignment but can the machi nery shafts, bearing s, and seals accept this for

long periods of time?

There may be other problem s wi th the coup ling that can co nfuse the person performing analign ment job. Noti ce in Figure 9.9 that the centerline of rotation of the shaft on the left is in

line with the centerline of the bore of the cou pling hub on the shaft on the right but it is not

in line with the centerline of rotat ion of the shaft on the right.

By its purest definition, shaft alignment occurs when the centerlines of rotation are collinear.

This is a very important point in aligning rotating machinery that a vast number of people

overlook. It is possible to align the centerlines of rotation of machinery shafts that are bent or that

have improperly bored coupling hubs and never know that these eccentricity problems exist.These eccentricities are referred to as runout problems and were discussed in Chapter 5. From the

inboard bearing out to the end of the shaft there are three basic types of eccentricity problems that

can occur: a coupling hub whose hole was bored off center or overbored and drawn off center

with the set screw, a skew (angle) bored hole in the coupling hub, or a bent shaft.

Notic e in Figu re 9.10 that when a bent sha ft is rotat ed, its center line of rotation is straight

Centerline of rotation Centerline of rotation andcenterline of coupling hub bore

Centerline of couplinghub outer perimiter

If you rotate this shaft only,

you will align the centerlineof rotation with thecenterline of the

improperly bored couplinghub, not the other shaft

centerline of rotation

To align the centerlines of rotation

(true shaft alignment) both shaftsshould be rotated together if a runoutcondition exists on either or both shafts.

If no runout exists (on the shaft youare measuring), then the shafts do

not have to be rotated together

0

50

10

40

20

30

+_

10

40

20

30

FIGURE 9.9 Aligning a centerline of rotation to the center of an improperly bored coupling.





Defi nitions:

Cat enary—the parabo lic cu rve assum ed by a perfectly flex ible inexten sible co rd of uniform

density and cross section suspen ded from two fixe d points.Cat enoid—t he surface descri bed by the rotation of a catenar y about its axis.

Figu re 9.11 shows an exagger ated view of the natural ly occu rring curvat ure of center

mounted and overhung shafts.

The amoun t of deflection dep ends on severa l fact ors such as the stiffne ss of the shaft, the

amoun t of weight between support points, the bearing design, and the distance betw een

support poin ts. For the vast major ity of rotating machi nery in existence, this catenar y bow

is negligible and for all practica l pur poses is ignored. On extre mely long drive trains howeve r

(e.g ., turbin e gen erators in power generat ing plants and motor -gener ator sets), this catenary

curve must be taken into consideration. Figure 9.12 shows the ideal catenary curve for a 1200

MW turbine generator unit. Notice that the difference in elevation between bearings 5 or 6

compared to bearing 10 is 1.25 in.

Centerline of rotationof bent shaft

Centerline of boreof coupling hub

Where should thisshaft be placed?

?FIGURE 9.10

Problems aligning a straight shaft to a bent one.





100 in.

Scale:

100 mils

Exciter Generator LPC Turbine LPB Turbine LPA Turbine HP Turbine

uplooking east

Zero elevation line

Side view

11 10 9 7 5 4 2 1

0.100

0.200

0.300

0.400

0.500

0.600

0.700

0.800

0.900

1.000

1.100

1.200

0

0.100

0.200

0.300

0.400

0.500

0.600

0.700

0.800

0.900

1.000

1.100

1.200

0

Best fit catenary curve

38 6

FIGURE 9.12 Catenary curve of a turbine-generator drive system.

2 0 0 6 b yT

a yl or &

F r an c i s Gr o u p ,L L C .

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Defining Misalignment - Alignment and Coupling.pdf