flexible rotor

8/10/2019 flexible rotor

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CHAPTER 13

DYNAMIC BALANCING OF ROTORS

Till last chapter, we concentrated on transverse and torsional vibration analyses of rotor-bearing

systems, e.g., free vibrations, forced responses and instability analysis. These analyses are very much

useful tool for designers of rotating machineries to predict the behaviour of such machineries before

actually their manufacturing and commissioning. These analyses help in the modification of design if

operating speed is close to critical speeds or in the instability zones. In the present and subsequent

chapters, we will address another class of practical and the most common problems related to rotating

machineries that practicing engineers face during the commissioning of new rotating machine, during

operation, or after every major overhaul of such machineries. Some of these fault are unbalances,

misalignments, rotor-stator rubs, bent or bowed shafts, fatigue cracks, the wear and tear of various

moving and stationary components, loose components, and faults related to components of bearings,

gears, coupling, blades, seals, etc. Among various faults, the most common fault is inherent

unbalances (or residual unbalances) in machineries occur due to manufacturing error (fits and

tolerances), material in-homogeneity, improper commissioning, thermal deformation, during

operation due to the wear and tear, residual stresses, and so on. Basic definition of the unbalance and

its type for rigid rotors has been introduced earlier in Chapter 2. To prevent vibration we must first

decrease this unbalance (or to balance the rotor), which is the major source of vibration.

In the present chapter, the procedure of static and dynamic balancing of rotor will be discussed in

great details. For dynamic balancing, rotors are classified in two major categories, e.g., the rigid and

flexible rotors. In fact, the same shaft of a rotor can be considered as rigid if it is operating much

below its first critical speed and the flexible when it is operating near or above the first critical speed.

That is why sometime it is also called the slow and high speed rotor balancing. Basic principles of

rigid and flexible rotor balancing are quite different. Necessary principles and theories for dynamic

balancing will be outlined before describing practical methods of balancing. For rigid rotor balancingtwo methods are described, e.g., the conversional cradle balancing machine method (off-site or off-

field balancing) and the modern influence coefficients method (on-site or field balancing). Similarly,

for the flexible rotor two basic methods are available, e.g., the modal balancing method and the

influence coefficient method. In general, the rigid rotor can be balanced by putting correction masses

in two balancing planes, however, in flexible rotor case it can be balanced by Nbalancing planes,

where Nis the number of flexible modes need to be balanced. Some time it is suggested to balance

flexible rotor by (N+2) balancing planes (i.e., to balance rigid rotor modes by 2 planes at low speeds

and flexible rotor modes byNplanes at high speeds).


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The unbalance in rotors will not only cause rotor vibrations, but also transmit rotating forces to the

bearings and to the foundation structure. The force thus transmitted may cause damage to the machine

parts and its foundation. If the transmitted force is large enough, it might affect even the neighbouring

machines and structures. Thus, it is necessary to remove the unbalance of a rotor, to as large an extend

as possible, for its smooth running. The residual unbalance estimation in rotor-bearing system is an

age-old problem. From the state of the art of the unbalance estimation, the unbalance can be obtained

with fairly good accuracy (Kellenburger, 1972; Drechsler, 1980; Gnilka, 1983; Krodkiewski et al.,

1994; Darlow, 1989). Now the trend in the unbalance estimation is to reduce the number of test runs

required, especially for the application of large turbogenerators where the downtime is very expensive

(Edwards et al., 2000; Tiwari,2005 ).

13.1 Unbalances in the Rigid and Flexible Rotors

Unbalance in a Single Plane: Such unbalance occurs in gear wheels, grinding wheels, single stage

compressor, propeller of aircraft engines, etc. Figure 13.1 shows a rigid thin disc with the single plane

unbalance. O is the centre of rotation of the disc and G is the centre of gravity of the rotor. The

eccentricity, e, is defined a distance between the centre of rotation and the centre of gravity. The

unbalance in the disc is defined as

meU= (13.1)

where Uis the unbalance with a unit of kg-m or g-mm, mis mass of disc, e is the eccentricity in the

disc (length OG in Fig. 13.1).

Unbalances in Two or More Planes:

Figure 13.2 shows two types of unbalance in a rigid rotor system. The rotor consists of a rigid rotor

and a massless elastic shaft. First (Fig. 13.2a) is the static unbalance, which is the state represented by

a geometric eccentricity eof the center of a gravity of a rotor from the centerline of the shaft. The

unbalance produces a centrifugal force proportional to the square of the rotational speed. This static

unbalance can be detected without operating the rotor since the unbalance is always directed

G

Ox

y

m

Figure 13.1 A unbalance in a single plane


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downward if the shaft is supported horizontally by bearings having little friction. Theoretically, it is

similar to the single plane unbalance described above, except the unbalance in uniformly distributed

along the length of the rigid rotor. Second type of unbalance is couple unbalance, which is the state

represented by the angular misalignment of the principal axis of moment of inertia of the rotor with

respect to the centerline of the shaft. The magnitude of the couple unbalance (

( ) 2 2d pM I I F rl = = ) is determined by the angle as shown in Figure 13.2b. This type of

unbalance cannot be detected without rotating the shaft. Figure 13.2(a and b) shows these unbalances

as models with one and two concentrated masses, respectively. That means static unbalance can be

balanced by a single plane balancing and couple unbalance has to balance with two balancing planes.

With the above definition now the dynamic unbalancein a rigid rotor means the state with the both

static and couple unbalances. (i.e., combination of Figures 13.2a and b). However, for such a case also

two-plane balancing will be enough. On the other extreme case would be the unbalances in a

continuous rotor (i.e., a flexible rotor with distributed mass) as shown in Figure 13.3. Here we require

Nplane balancing, where N 2 and generally for balancing up to mthmodeN= mfor m 2 .

(a) Static unbalance (b) Couple unbalance

Figure 13.2 Unbalances in a long rigid rotor system

Figure 13.3 Variation of the unbalance in a continuous flexible rotor


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13.2 Principle of Rigid Rotor Balancing

Now some basic principle of rigid rotor balancing will be outlined and this will pay the way to

understand balancing methods for practical rotors.

13.2.1 Static Balancing (Single plane balancing):

The unbalance force, for a single plane disc as shown in Fig. 13.1, is given as

2F m e= (13.2)

where is the spin speed of the rotor. If we want to know correction mass, mc, at a radius of r, it will

be given by

/ce r

mm

= (13.3)

The correction should be placed 1800 away from unbalance mass m. Such a correction is called a

single plane balancing of the rotor, which eliminates the inertia forces transmitted to the foundation

(or bearing).

13.2.1 Static Balancing (Two plane balancing):

We represent eccentricities and centrifugal forces as vectors, for which both magnitude and direction

are necessary.

(a) Actual system (b) Equivalent force model

Figure 13.4 Elimination of the static unbalance

Balancing is attained if the centrifugal force F me= 2 is cancelled by the other centrifugal forces,

due to balancing weights m1and m2 . In practical machines, the positions of the correction planes are

determined from the shape of the rotor. Balancing is done by removing parts of the rotor or by

attaching correction masses in plane I and II. In practice, removing some part is done by drilling,

milling or grinding. Addition of weight would require the use of wire solders, bolted or riveted

washers and welded weights. Let the masses m1and m2 are attached to the surface at radii a1 and


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a2 , respectively. To cancel the unbalance force F me=2

by centrifugal force IF m a =2

1 1 and

IIF m a =2

2 2 , the following relationship must hold

I IIF F F+ = and I IIF l F l=1 2 (13.4)

where l1and l2 are shown in Figure 13.4. Equation (13.4) can be solved as

21

2

ll

FlFI

+= and

21

1

ll

FlFII

+= (13.5)

13.4.2 Couple unbalance

Figure 13.5 Couple unbalance

The moment ( )d pM I I = 2 due to couple unbalance can be replaced equivalently by a couple of

forces P M d= , which is separated by the distance d. We add correction masses 1m and 2m to

cancel momentMby the centrifugal forces IP m a=2

1 1 and IIP m a=2

2 2 (Fig. 13.5). For this case

the following relationships must hold

I IIP l P l M + =1 2 and I IIP P= (13.6)

The latter is the condition to prevent a new static unbalance due to the addition of m1 and m2 .

Vectorially they should be P P= 1 2

. Equation (13.6) gives

I II

MP P

l l= =

+1 2 (13.7)

It is assumed here that we know the plane of couple unbalancing.


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13.4.3 Dynamic unbalance

Figure 13.6 Static and couple unbalance

The static and couple unbalances effects i.e. force and moments may not be in the same plane;

however, they will be perpendicular to the bearing axis (Fig. 13.6). The balancing is attained by

adding correction weights in the correction forcesI

R

andII

R

determined by the vector relationship

I I IR P F= +

and IIIIII FPR

+= (13.8)

The balancing method described above is called the two-plane balancing. On the contrary as we have

seen above, when a rotor is thin, the balancing is attained practically by adding a correction weight in

one plane. This method is called the single-plane balancing. Practical balancing machines are made

based on such principles. Basic principles of flexible rotor balancing will be described subsequently.

13.2.4 Various expressions of unbalance

In this section various terminologies used in industry related to unbalance is described. When a static

unbalance exists, a centrifugal force2

me exists. This unbalance force is eliminated if mass 1m ,

which satisfied the relationship2 2

1 0me m a + =

, is added at radius a in the same plane as the

center of gravity G. From this condition it is clear that the product me

or1m a

is more important than

the eccentricity itself. Therefore, the quantity

U me=

(g-mm) (13.9)


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is called an unbalance vectorand its magnitude U me= is called a magnitude of unbalance. These

quantities are sometimes called simply unbalance. Different types of expressions are described in the

general case where an eccentricity e and an inclination of the principal axis of moment of area

coexist.

(a)Resultant Unbalance U

and Resultant Unbalance momentV

:

Figure 13.7 Equivalent concentrated unbalances 1U and 2U

A thinly sliced disc with thickness dz, which is perpendicular to the rotor axis, is considered (Fig.

13.7). Let the mass of the disc is ( )dm z dz= , where ( )z is a line mass density of the rotor and the

eccentricity of the center of gravity be ( )e z

. Then its unbalance is represented by

( ) ( ) ( ) ( )dU z e z dm e z z dz= =

. Summation of such unbalance, called a resultant unbalance, is given

by

1 1

1 1

( ) ( ) ( )

l h l h

l l

U dU z e z z dz

+ +

= =

(13.10)

where1l is the distance between the origin oand the rotor and his the length of rotor. Multiplying

this by 21 , we get the resultant unbalance force of the centrifugal force.

UF

2= (13.11)

The quantity

( ) ( ) ( ) ( )1 1

1 1

( )

l h l h

l l

V zi dU z zi e z z dz

+ +

= =

(13.12)


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is called a resultant unbalance moment concerning point o, where i is a unit vector in the direction

of the bearing centerline and is the cross product of vectors. Multiplying this by 2 , we get the

moment N

produced by the centrifugal forces of all elements.

2N V=

(13.13)

This moment is called a resultant moment of the unbalance force. We can represent the unbalance

of a rigid rotor by using the resultant unbalance U

and the resultant unbalance moment V

.

(b)Dynamic unbalance 1 2,U U

: U

(Resultant unbalance) and V

(resultant unbalance moment) are

to be replaced by the concentrated unbalance IU

and IIU

in the correction plane I and II,

respectively. For this replacement, the following relationship must hold

1 2U U U+ =

and ( ) ( )1 1 2 2 z i U z i U V + =

(c)

where 1z and 2z are positions of the correction planes. The balancing is attained if we add 1U

and

2U

, which cancel 1U

and 2U

, respectively. This set 1 2,U U

is called the dynamic unbalance

represented at positions1z and 2z .

(c ) Static unbalanceU

and couple unbalance ,C C

U U

:

Figure 13.8 Replacements of unbalances


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The resultant unbalance 1 2U U U= +

is also called the static unbalancebecause it can be detected

without rotating the shaft. This expression shows that the dynamic unbalance mentioned in section (b)

contains the static unbalance quantitatively. Now, we replace the dynamic unbalance 1 2,U U

by the

static unbalance U

and a couple whose forces are located in the correction planes I and II,

respectively, as follows:

1. Suppose that the static unbalance U

and the dynamic unbalance U

are added at position,

3z , as shown in Figure 13.8(a). Since 0U U =

holds, balancing as a whole does not change

due to this addition.

2. Since U

is decomposed into 1U

and 2U

as 1 2U U U =

, the summation of the

dynamic unbalance 1 2,U U + +

at1z and 2z and a given unbalance U

at3z is equivalent

to two couples which are the set of unbalance 1 1,U U

at 1z and 3z and the set

,II II

U U +

at3z and 2z , as shown in Figure 13.8(b).

3. From the law of mechanics the effect of a couple is the same for any position of the rigid

body. The couple 1 1,U U

at1z and 3z can be replaced by 2 2,C CU U

at1z and 2z as

shown in Figure 13.8(c): ( ) ( )2 3 2 2 1 2Cz z U z z U =

.

4. Using 1 2c c cU U U + =

, the summation 1 1,c cU U

and 2 2,c cU U

is replaced by an

equivalent couple ,c c

U U

at the correction planes I and II as shown in Figure 13.8(d).

This set of unbalance is called the couple unbalance.

From such replacements, we know that the unbalance of a rigid rotor can be represented by the

static unbalance, U

, and the couple unbalance, ,c c

U U

.

Example 13.1 For a rigid rotor with constant eccentricity 0e in the right half of the rotor span as

shown in Fig. 13.8, obtain its equivalent (a) the resultant unbalance, U, and the resultant unbalance

moment, V, (b) the dynamic unbalance 1 2,U U

, (c) the static unbalance U

and couple unbalance

,C C

U U

.

Solution: Fig. 13.9a shows a rigid rotor with constant eccentricity 0e in the right half of the rotor

span. : Fig. 13.9b shows the positions of the two balancing planes.


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Fig. 13.9 A rotor unbalance distribution and its equivalent form

(a) If the line mass density ( )z is a constant, the magnitude of the resultant unbalanceis obtained

as

( ) ( )1 1

1 1

0 0

/2

10

2

l h l h

l l h

U e z z dz e dz e h

+ +

+

= = + = (a)

and the resultant unbalance momentabout point ois given by

( ) ( )1 1

1 1

2

0 0 1

/2

1 30

2 4

l h l h

l l h

V ze z z dz e zdz e l h h

+ +

+

= = + = +

(b)

This vector V

is perpendicular to the plane in which 0e is acting for the present case. Since equation

(b) contains1l , the magnitude of V

depends upon the origin o. From equation (b) N

can be obtained

by multiplying 2 to the V

.

(b) For the present example of Figure 13.9, we have (from equations c and a)

11 2 02U U e h+ = (d)


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and from equations (c and b), we have

( ) ( )2311 1 1 2 0 12 4l U l h U e l h h+ + = + (e)

On solving equations (d) and (e), we get

11 08

U e h= and 32 08U e h= (f)

(C) From Figure 13.9, we have the magnitude of the static unbalance U

is 1 2 0 / 2U U e h+ = . Two

unbalances with the magnitude 2/0he and pointing to the upper and lower directions at the position

3 1z l b= + are considered. We replace U

and the dynamic unbalance 1 2,U U

by the set 1 2,U U

in the correction planes I and II (Figure 13.8d). The following relationship hold between them

(Figures 13.9 and 13.8d)

1 2 1 2 0U U U U U + = + =

and

( ) ( ) ( )1 1 1 1 1I II I IIl U l h U l b U l U l h U + + + + = + + (g)

On substituting1,U U and 2U , we get

( )311 2 02 4 cU U e h b U = = (h)

The static unbalance Uand the couple unbalance ,c cU U

are illustrated in Figure 13.8(d).

Answer

Finally, we remark on the problem regarding to what extend we should balance rotors. The smaller

the residual unbalance attained the better. However, in a practical application, we must take into

account the time and expenses necessary to balance the rotor. Therefore, it is appropriate to vary the

permissibility of unbalance depending on the kind of rotating machinery. A quantity called

balance quantity is used to express the degree of balancing, where the quantity is a correction

plane eccentricity and is the maximum angular velocity in the operation range. The correction


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plane eccentricity is given by the specific unbalance, that is, ratio of the magnitude of unbalance

to the rotor mass. In the case of single plane balancing, the correction plane eccentricity is given by

s sU M = (13.14)

where sU is the static unbalance andMis the stator mass. In case of two-plane balancing, it is given

by the larger value of/ 2

II

U

M = and

/ 2

IIII

U

M = , where [ ],I IIU U is a dynamic unbalance. If the

quantities of two rotors are equal, the loads transmitted to the bearings are the same even if the

dimensions of the rotating machines are different. The permissibility of the unbalance is given by the

International Standardization Organization (ISO) or various national standards. For example, balance

quantity defined by the Bureau of Indian Standard (BIS) is given as

Balance quantity = mm/s (13.15)

where the unit of is mm and that of in rad/s. The balance quantity is classified into several

grades: G0.4, G1, G2.5, G2.6, G40, G100, G250, G630, G1600, and G4000. For example, G100

means that the maximum permissible value of balance quantity for this grade is 100 mm/s. Based on

experience, the specific balance quantity grade is recommended for individual rotating machinery. For

example: G0.4 is recommended for gyroscopes and G2.5 for gas and steam turbines. (Refer standards

on balancing e.g., IS 5172 (1969), IS 13274 (1992), IS 13275 (1992), IS 13277 (1992), IS 13278

(1999), IS 13280 (1992), IS 14280 (1995), IS 14734 (1999) and IS (14918)).

13.3 Balancing of Practical Rigid Rotor

In the present section now practical methods of the rigid rotor balancing will be described. In practical

rotors axial and radial locations residual unbalances and its orientations are unknown. Depending

upon the geometry of rigid rotors, a single plane or two plane balancing methods are employed. In the

present section, both the single plane balancing and the two plane balancing will be described in

detail.

13.3.1 Single plane balancing: In the actual practice location (radial as well as angular)) of centre of

gravity point Gis unknown in the single plane rotors. The orientation of point Gcan be obtained by

keeping the rotor on frictionless (knife edge) supports and gently allow it to rotate freely without any

external drive as such (may be a small toque by hand to initiate its rotation). The rotor becomes

stationary after some time with heavy spot (G) vertically downwards. It can be repeated to confirm the

orientation of the residual unbalance (or heavy spot) and it can be marked by chalk or any other


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means.Now we will place a correction mass at 1800to the heavy spot (i.e., at the light spot) and again

allow rotor to rotate freely by gentle push of the rotor. (i) If the marked heavy spot again comes

vertically downwards that means the correction mass mto be increased. (ii) If the marked heavy spot

comes vertically upward position, means correction mass is more, and it has to be decreased. (iii) If

heavy spot rests at some other position, means rotor is nearly balanced. This can be confirmed by

freely rotating the rotor again and finding whether it rests always at some indifferent equilibrium

position. Such a process is called the static balancing of rotor (disc) and it is valid for a rotor with

only one disc or balancing is required in the single plane only. For the single plane rotor, the static

balancing rotor will also be dynamically balanced.

13.3.2 Two plane balancing (Cradle balancing machines):The rotor has to be removed from the

installation and is placed on the bearings of a cradle balancing machine as shown in Figure 13.10.

Two procedures will be described (i) Hit and trial method: It requires large number of measurements

to obtain correction masses at two balancing planes (ii) A systematic method: It requires only eight

measurements to obtain correction masses at two balancing planes.

Figure 13.10 A schematic of the cradle balancing machine

Hit and trial balancing method: The cradle is placed on four springs and can be fulcrum about F1

or F2to form a simple vibrating system to oscillate about F2or F1, respectively. Two fulcrum can

be located at two chosen balance planes (i.e.IandII), where the correction mass to be added. The

rotor can be driven by a motor through a belt pulley arrangement. If the spring system is such that

the natural frequency of the system is in the range of motor speed, the phase angle or the location

of the mass in either plane can be determined as follows. Fulcrum the cradle in plane I, by fixing

F1and releasing F2. Run the rotor to resonance, observing the maximum amplitude to the right of

fulcrum F2.This vibration is due to all the unbalance in plane II, since the unbalance in plane I

has no moment about F1. Use a trial mass at a chosen location and determine the amplitude of

vibration.


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Figure 13.11(a) Variation of vibration amplitudes versus trial mass angular locations

Figure 13.11(b) Variation of vibration amplitudes versus trial mass magnitudes

Make a plot of this amplitude for different location of the same trial mass (see Fig. 13.11a). The trialmass for correction is added at the location where the amplitude of vibration is minimum. Increase or

decrease the trial mass at the same locations, until the desired level of balance is achived (see Fig.

13.11b). This will give correction mass for plane II. Similar procedure can be repeated by Fixing F2

and releasing F1 to get correction mass at plane I. This procedure is tedious and sometimes may be

time consuming.

Example 13.2 In a dynamic balancing of a rigid rotor by a cradle balancing machine the following

measurement were obtained when fulcrumed at F1(i) for a known trial mass at a fixed radius when

kept at different angular position at regular interval of 300the following vibration amplitudes (all in

mm) were obtained: 3, 5, 8, 6, 5, 3, 2, 4, 7, 8, 6, 5. 4 (ii) when at a constant angular position and at

fixed radius corresponding to the minimum vibration amplitude different masses were tried the

following vibration measurements were obtained (gm, mm): (1, 1.2), (1.2, 1.4), (1.4, 1.6), (1.6, 1.4),

(1.8, 0.9), (2.0, 0.8), (2.2, 1.2), (2.4, 1.5), (2.6, 1.8). Obtain the unbalance mass and its location in the

plane 2.


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Solution: From the first set of measurement the location of the angular position is 1800corresponding

to 2 mm displacement. From the second set of measurements the magnitude of the unbalance mass is

2.0 gm corresponding to the minimum vibration amplidue of 0.8 mm. Answer

A systematic balancing method: This method is aimed to reduce the number of measurements while

using the cradle balancing machine. This method also requires measurements in the cradle balancing

machine and correction masses at plane I and II are obtained by fulcruming at F2and F1, respectively.

A procedure to determine the correction mass and location at one plane at a time can be laid down as

follows, based on only four observations of amplitude : (i) without any addition of the trial mass to the

rotor (ii) with a trial mass at = 00, where is measured from a conveniently chosen location on the

balancing plane (iii) with the same trial mass at 1800and (iv) with same trial mass at = 90

0. This

procedure has to be repeated for two cases (e.g. when fulcruming at F1and then for F2). Let OA is

the amplitude measured with trial run (1) without a trial mass, OB is the amplitude measured in trial

run (2) by addition of a trial mass TR at 00(arbitrary chosen location on the rotor). Hence, vector AB

represents the effect of trial mass TR. (at this stage we do not know the location of vector OA on the

rotor). OC is the vibration measured in trial run (3), with the same trial mass at 1800. So we will have

AB = AC with 1800phase difference between them (since AC vector is also the effect of trial mass

Wtso the magnitudes AB = AC with a phase of 1800). We know only OA , OB and OC from test

runs (1), (2) and (3) respectively, and apart from this we have information that AB = AC with 1800

of the phase. From these information we have to construct and locate points O, A, B and C on a plane.

Figure 13.12 A construction procedure for finding the unbalance vector

The construction procedure for finding the unbalance vector is given now in detail. Erect a line OD

equal to 2 OA . With point O a center and OB & OC as radii and then point D as center and OC &


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OB as radii draw arcs to intersect at points B & C. Construct the parallelogram OBDC (points B and

C we will be obtained by above construction). Now AB represents 00position (i.e. reference line) and

AC represents 1800position on the rotor ( OA is actual unbalance). The angular measurement may

be clockwise or counter-clockwise and is determined from the fourth observation. Draw a circle with

BC as the diameter and point A as the center. On the circle the fourth observation observation could

be either OE or OE (corresponding to measurement while keeping the trial mass at 90

0). If the

value observed is in the vicinity of OE , then the angle to be measured in counter-clockwise direction.

However, it will be clockwise if OEis the reading observed in the test. The fourth run also checks

the validity of the linearity used in the balancing procedure. The magnitude of trial mass Wt is

proportional to AB . The unbalance OA can be obtained accordingly in terms of the mass. The

location of unbalance is OAB and the direction from Figure 13.13 (i.e. clockwise or counter-

clockwise).

Fig. 13.13 Procedure of obtaining the sense of the angular position of the unbalance mass

In Fig. 13.13(a) the left hand side figure represents the location of the intersection of fourth

measurement on the circle (i.e., at E) for trial mass kept at 900 on the disc. This gives the CCW

direction as positive for measurement of unbalance angular position, , from 0o location (i.e., AB).

The right hand side drawing of Fig. 13.13(a) represents the unbalance position, , on the disc location

that is CCW direction as positive from 0o. In case the for above measurement the intersection of

fourth measurement (i.e., point E) is as shown in Fig. 13.13(b) for trial mass kept at 900on the disc


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then this gives the CW direction as positive for measurement of unbalance angular position, , from

0olocation (i.e., AB). The right hand side drawing of Fig. 13.13(b) represents the unbalance position,

, on the disc location that is CW direction as positive from 0o. Similarly, Figs. 13.13 (c and d) can be

interpreted.

The test is repeated by making the cradle pivoted at FII and measurements made in plane I. This

procedure is little time consuming and also restricts the mass and size of the rotor. Modern balancing

machines use amplitude and phase measurement in two planes for the balancing a rotor.

Example 13.3 In the balancing process we make the following observations: (i) ao= amplitude of

vibration of the unbalanced rotor as is (ii) a1= amplitude with an additional one-unit correction at

the location 0 deg and (iii) a2= same as a1but now at 180 deg.

The ideal rotor, unbalanced only with a unit unbalance (and thus not containing the residual

unbalance), will have certain amplitude, which we cannot measure. Call that amplitude x. Let the

unknown location of the original unbalance be . Solvexand in terms of 0 1,a a and 2a to showthat

in this answer there is an ambiguity sign and prove that total four runs are necessary to solve the

problem completely.

Solution:Measurements are (i) =0a amplitude of vibration with residual unbalance RU , (ii) =1a

amplitude with unit trial mass at an angle of00 , (iii) =2a amplitude with unit trial mass at an angle

of0180 , and (iv)x= amplitude with unit trial mass at an angle of 00 and without residual unbalance

(i.e., 0RU = ). So that 0OA a= , 1OB a= and 2OC a= . Figure 13.14 shows various parameters

involved in the present problem. From OAB , we have

22 2

0 1

0

ABcos

2 AB

a a

a

+

= (a)

and

( )

22 2

0 2

0

ACcos

2 AC

a a

a

+ = (a)

Since AB AC= , we have

2

2 20 2

0

ABcos2 AB

a a

a + = (c)


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Figure 13.14 Geometrical constructions for determination of residual unbalance

On equating equations (a) and (c), we get

2 22 2 2 2

0 0 2 0 12 OBcos ( AB ) ( AB )a a a a a= + = +

which gives

22 2 2

0 1 22 2AB ( )a a a+ + (d)

AB x= , since AB (or AC ) are the effect of trial mass of unit magnitude. Hence, equation (d) gives

2 2 2 2

1 2 00.5( )x a a a= + or 1 2 2 21 2 02 ( )x a a a= + (e)

Equations (a) and (c) gives (noting that AB AC x= = ),

2 2 2

0 0 12 cosx a x a a= + (f)

and

2 2 2

0 0 22 cosx a x a a= + (g)

On equating equations (f) and (g), we get

2 2

2 1

0

( )cos

4

a a

a x

= (h)

Equation (e) gives the magnitude of the unbalance and equation (h) gives the magnitude of the phaseangle, the direction or sense of the phase cannot be obtained from only three measurements, a fourth


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measurement by keeping the unit unbalance at 900(or at 270

0) will be required. Let us take 3a be the

fourth measurement, now obtain the condition by which the direction angular position, , of the

unbalance,x, can be obtained. This is left to the reader as an exercise. Answer

Example 13.4A short rotor or flywheel has to be balanced. Observations of the vibration at one of

the bearings are made in four runs as follows: (i) Run 1; rotor as is : amplitude 6.0 m, (ii)Run

2; with 5gm. at 0 deg.: amplitude 5.0 m, (iii) Run 3; with 5 gm. at 180 deg.: amplitude 10.0 m,

and (iv) Run 4; with 5gm. at 90 deg.: amplitude 10.5 m. Find the weight and location of the

correction. Take the trial and balancing masses at the same radius.

Solution:

Figure 13.15 Geometrical constructions of unbalance vectors

Figure 13.15 shows the geometrical construction of unbalance vectors for the present problem.

Various lengths are given as: OA = AD = 6 cm 4.762 gm ( = a0), OB = CD = 5 cm 3.968 gm

(= a1), and OC = BD = 10 cm 7.937 gm ( = a2). From Fig. 13.15, the residual unbalance is given

as AB = 6.3 cm 5.14 cm ( 4.08 gm). AB is the reference line. The fourth observation intersects at

E, hence angle to be measured in the CW direction (i.e. BAE ). Thus, the unbalance position is

given as BAO = 308.7oCW or 51.3

0CCW direction. The unbalance magnitude and phase can be

also obtained from equations (e) and (h), we have

1 12 2 2 2 2 2

1 2 02 2( ) (3.968 7.937 ) 4.762x a a a= + = + = 4.09 gm

and

( )2 22 22 1

0

7.937 3.968( )cos 0.6065

4 4 4.762 4.09

a a

a x

= = =

, this gives

o52.66=

The difference in the values of unbalance and angular position is due to rounding off error and the

error due to the graphical construction.


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13.3.3 Two plane balancing (The influence coefficient method)

The influence coefficient method is also called the field balancing since the balancing can be

performed at site itself woithout taking out rotor from the machine as such. The number of trial runs

to obtain the residual unbalnces in two planes are also less with a minimum of three runs are required.

In Chapter 2 and 8, influence coefficients in detailed, here some relevant basic concept are briefed

again. In the present section, then procedure would be outlines to obtain these influence coefficients

experimnetally with the help of vibration amplitude and phase measurement in a rotor system.

Definition of Influence coefficients:Figure 13.16(a) shows that when a force F1is applied at station 1

and due to this force the beam deflections at stations 1 and 2 are given as

y11= displacement at station 1 due to force F1at station 1 = 111F

and

y21= displacement at station 2 due to force F1at station1 = 121F

where is the influence coefficient and its first subscript represents the displacement station and

second represents the force station. Similarly for Figure 13.16(b), we have

y12= 212F and y22= 222F

Figure 13.16 Definition of influence coefficients

In Figure 13.16(c), we have

1 12 12 11 1 12 2

2 21 22 21 1 22 2

y y y F F

y y y F F

= + = +

= + = +

1 11 12 1

2 21 22 2

y F

y F

=


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Influence coefficients can be obtained by experimentation or by strength of material formulae i.e.

11 11 1 21 21 1/ , /y F y F = = etc.

In soft support machines, the resonant frequency of the rotor support system is low and the rotor runs

at a speed above the resonance of the support system. Vibratory amplitudes are measured, which are

then converetd to forces. In hard support system, the support natural frequency is very high and they

measure the rotor unbalance forces directly, independent of rotor mass and configuration. The

balancing procedure is based on the influence coefficient measurement. We choose two convenient

planes L and R for trial mass and two measurement planes a and b (can be chosen as bearing

locations) as shown in Figure 13.17. Let Y1Land Y1Rbe the initial readings (i.e., without trial mass) of

vibration levels (the displacement, velocity or acceleration) measured with phase angle 1L and 1R ,

respectively.

Figure 13.17 Bearing response measurements and influence coefficients for a rigid rotor balancing

The phase angles are measured with the same reference during the test and their relative locations

with respect to rotor is initially known. In the second run, place a trial mass, TR, at a convenient

location in planeRand let the observations be Y2Land Y2Rwith phase 2L and 2R , respectively in the

a & bplanes. The difference between Y2Rand Y1Rwill be the effect of trial mass, TR, in right planeR

on the measurement made in plane b. We can denote this as an influence coefficient bR .

2 1( ) /bR R R RY Y T =

(13.16)


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where "" represent vector since displacement has magnitude and phase information. Hence, we

can be able to work with complex quantities. Similarly

2 1( ) /aR L L RY Y T =

(13.17)

We remove the trial mass from plane Rand place a trial mass, LT

, in plane L and repeat the test to

obtain the measured values

3 1( ) /bL R R LY Y T =

(13.18)

and

3 1( ) /aL L L LY Y T =

(13.19)

With the help of equations (13.16) to (13.19), we can obtain influence coefficient experimentally. Let

the correct balance masses beRW

andLW

in the right and left balancing planes, respectively. Since

the original unbalance response is R1and L1as measured in right and left planes, we can write

1 1 andR R bR L bL L R aR L aLY W W Y W W = + = +

(13.20)

Correction masses will produce vibration equal and opposite to the vibration due to residual

unbalance masses. Hence,

1

1

bR bL RR

aR aL LL

WY

WY

=

(13.21)

These can be calculated either by a graphical method or analytical method of vectors (complex

algebra). We know

=

ac

bd

dc

ba 11

)(where cbad=

which gives

1 1L bL R aLR

bR aL aR bL

Y YW

=

and 1 1R aR L bR

L

bR aL aR bL

Y YW

=

(13.22)

A schematic and a picture of the overall experimental for dynamic balancing of rotor setup are shown

in Figs. 13.18 and 13.19, respectively. Fig. 13.20 shows measurement of the phase of the vibrationsignal,y(t), with respect to the reference signal, r(t).

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