CXM FFT Analyzer®

User guide for Balancing

Rev. 1.0 – 14.01.2014

Note:

Instrument Screens design can be changed in the future software version and can be changed without any notice.

The screen shots at the present Manual are for reference only.

MaintTech phone: +46 707 732274

Norrkoping e-mail: mainttech@live.se

Sweden website: www.mainttech.se

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Innehåll

1. Balancing with CXM ................................................................................................................................................. 3

2. CALCULATION OF TRIAL WEIGHT ............................................................................................................................. 5

3. Rotor Balancing Section ........................................................................................................................................... 9

4. USING SPLIT WEIGHT ............................................................................................................................................. 14

5. ADD VECTOR .......................................................................................................................................................... 17

6. “One Run” balancing ............................................................................................................................................. 19

7. THE REFERENCE POSITION FOR ANGLE MEASUREMENTS .................................................................................... 22

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Configure Balancing

Rotor Settings

Calibration runs

Balancing Popup Menu

Measure

1. Balancing with CXM

The balancing function is very simple and guides the user right through the proper sequence of steps: the initial run,

the introduction of a trial weight, the second run, and finally, the recommendation about the size and location of

the weight to be used for correction.

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SINGLE PLANE BALANCING FLOWCHART

Take readings in the

Analysis Menu

Is 1 ×RPM

significant? Quit

Use Analysis menu to

confirm the result

No

Yes

Balance &

Balancing settings

Rotor settings

Initial run

Add trial

weight

Trial run

Remove trial

weight

Add /remove

correction

weight(s)

Correction

run

Vibration

level is

acceptable?

No

Yes Quit

Balance is not the

problem

Trim #n weight

solution

Detection &

Analysis

Initial Setup

Unbalance

correction

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2. CALCULATION OF TRIAL WEIGHT

1. INTRODUCTION

Choosing the correct size of the trial weight is an important stage in rotor dynamic balancing.

Trial weight must be large enough for a significant change of the unbalance vector, but at the same time, it

must be small enough not to destroy the rotor.

The unbalance can be represented as shown in Fig. 1. Vector V1 represents the initial unbalance of the rotor

and V2 is the vector obtained by adding a trial weight.

Fig.1

Usually, a change of 20% of the unbalance vector is quite acceptable. This means that V2 vector must be

placed outside of a circle, as shown in Fig. 2

Fig.2

2. CALCULATION OF TRIAL WEIGHT

0⁰

90⁰

180⁰

270⁰

V1

V2

0⁰

90⁰

180⁰

270⁰

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ISO 1940/1 defines the permissible residual unbalance as being:

Uper [gmm] = 9549 × G × W/N [1]

Where:

G = Balance quality grade

W = Rotor weight [Kg]

N = Maximum service speed [RPM]

Most of industrial rotors can be balanced, considering a Balance quality grade of 6.3.

In formula [1], by replacing the maximum service speed N with the balancing speed Nb you can calculate the

trial weight (TW):

TW [g] = 9549 x 6.3 x W/Nb/R [2]

Where:

R = Trial weight radius

W = Rotor weight [Kg]

Nb = Balancing speed [RPM]

For a two plane balancing, the value of the trial weight will be half of Tw in each balancing plane (TW/2).

3. THE TRIAL WEIGHT ESTIMATION IN CXM

CXM software can estimate the value of the trial weight, based on the available rotor data.

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In Edit Weight menu, you can manually type the required trial weight or press F1 – Estimate key.

In the last case, the trial weight will be calculated according with the formula [2]:

The default suggested weight is always 10 grams.

By pressing the F1 key, the trial weight will be calculated using the existing

radius (R1), the rotor mass and the balancing speed:

If the selected trial weight is too low, a warning message will appear on the screen.

This means the selected trial weight doesn’t changed the unbalance vector with

more than 20%.

There might be a lot of reasons for this:

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- The response of the unbalance in vibration terms is very low (the machine is too rigid) - The balancing speed is too low - The initial unbalance of the rotor is too much.

In this last situation, even if the trial weight was chosen correctly, the resulting vector does not change with

at least 20%:

Fig.3

The initial unbalance (the red vector) is too much. In this case, even if the trial

weight was chosen correctly, it still cannot change enough the unbalance vector.

In this situation we strongly recommend you to proceed a roughly balance first,

then to complete the balancing with a final balancing procedure.

A general valid rule says: the initial unbalance of the rotor should be comparable

with the selected trial weight.

0⁰

90⁰

180⁰

270⁰

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3. Rotor Balancing Section

The balancing function is very simple and guides the user right through the proper sequence of steps: the initial

run, the introduction of a trial weight, the second run, and finally, the recommendation about the size and location

of the weight to be used for correction.

The balancing implementation should also have the provision for a bit of added functionality, like one or two

planes, adding or removing weights, and additional trim runs.

Now, we will begin a new balancing (1-channel or 2-channel) process.

Press 2 – New One Plane Balancing or 3 – New Two Plane Balancing.

First step is to Configure Balancing:

- Auto store. If it’s Enabled, the measurement process will be automatically stopped, when the readings are stable.

- Show unit. You may decide the vibration unit to be used to display the measurements result. - Detection. The above selected unit can be shown in RMS, Peak or Peak-to-peak amplitude. - Unbalance unit. Selection can be gmm/kg or ozin/lbs. - Speed unit can be Hz or RPM. Show unit, detection and unbalance unit cannot be changed in time of balancing session.

Press F4 – Continue and the Main Balancing screen will appear.

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You may now perform some dummy measurements to adjust the speed sensor or/and the transducer(s) position.

The measurements are shown only on the screen and they won’t be saved in the memory.

The Balancing Status shows a series of icons in red, because either rotor settings or trial runs weren’t performed,

yet.

Press F1 – Rotor (settings).

In this screen you must introduce the actual rotor radius, its mass, its service speed (maximum run speed for that

specific rotor) and optionally, you have to decide if the balancing tolerance will be used or not.

If the balancing tolerance (ISO 1940 Quality Grade) will be used, you may also need to introduce the Gravity Mass

Centre of that rotor, as follows:

The angle increases from zero position, in the reverse direction of shaft rotation!

The measurement begins by pressing the Enter key and can be finished with the

Store key.

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- For one plane balancing: not used

- For two plane symmetrical rotor: 50%

- For two plane non-symmetrical rotor: the gravity mass centre, as percentage, measured on the Left (VIB1)

bearing.

Press F4 to save the settings and to return to the Main Balancing screen.

Press F2 – Trial runs.

Depending on the balancing type (one or two plane balancing), you must complete two or three steps, as follows:

One plane Balancing:

- Initial run (free run measurement for vibration amplitude and phase) - Trial run (trial weight attached at the balancing radius R)

Two plane Balancing:

- Initial run (free run measurement for vibration amplitude and phase) - First trial run (trial weight on the left side, located at the balancing radius R1) - Second trial run (trial weight on the right side, located at the balancing radius R2)

On the second and on the third Trial Runs screen you can edit the trial weight:

If you want help with selecting the size of a trial weight, see page 14.

This chapter describes the Calculation of Trial Weight.

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Press ENTER to begin the measurements, only when the rotor reaches the balancing

speed.

Press STORE to finish the measurement, only when the measurements become stable!

NOTE: CXM proposes you a balancing weight, but you may change it.

To determine the influence coefficients, all the above steps must be performed.

Press ESC, to return in the Main Balancing screen.

Now, in this screen, F3 and F4 keys will be enabled and the rotor unbalance (in mass unit) will also appear.

Pressing F3 – Add/remove key, the correction mass and the angle will be shown positive (“+” indicates to add the

correction mass) or negative (“-“indicates to remove the correction mass).

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The unbalance can be shown in mass units (gram or oz), in gram ×mm (or oz × in) or in selected vibration units. Just

press several times F4 – Change unit key.

The correction mass must be placed at the appropriate angle into each plane, and a set of vibration measurements

must be done again, by pressing Enter key.

In this moment, we have the possibility of trimming the balance condition even further or to accept that we are

done. If we declare that the job is finished, we can save the Balancing session into a file. If not, we have to continue

by adding/removing correction mass, as shown on the screen.

Press Menu key and from the popup menu select Save

in file. The rotor can be saved in a file onto the internal

memory Card.

Copy this file to your computer.

CXM is delivered with a USB memory stick. This stick

contains BalReport software. Read the manual (Help)

describing how to transfer the balancing file to this

software.

The popup Menu is available only if the influence coefficients exist (the trial runs were performed).

This menu offers some useful tools:

- Save in file - Balancing options - Split settings - Add vector - Restore Influence coefficients - Vibration history.

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4. USING SPLIT WEIGHT

1. Introduction

The split weight function can be useful to (divide) split the correction mass is some fixed position across rotor

circumference.

Split setting is available in the Balancing Main screen, in the popup menu, only if the calibration runs are done for

the rotor (influence coefficients were determined).

To access the popup menu from the Balancing Main menu, just press MENU key:

The split position is numbered from zero to N-1 (where N is the split position across rotor circumference.

As can be seen below, the numbering is done in the reverse direction of rotor rotation.

In the example below there are set 5 positions, counted from 0 to 4.

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2. Splitting weight for Single Plane Balancing

Press MENU key, select the Split Settings line and press ENTER:

Select No of position and type the number of desired position.

Select Angle of first position and the angle for the 0 position (normally, zero position can be considered the

reflective tape position: 0).

Select Auto split weight to Enabled.

Press F4 to save the settings.

Now, in the Main Balancing screen will also appear the calculate weight for the selected split position:

In the above example, instead to add a weight of 3175 grams at 180, you may split this weight in two. The first one

must be added in the position 2 and the second in the position 3.

You can Disable or Enable the split weight result anytime.

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2. Splitting weight for Two Plane Balancing.

Press MENU key, select the Split Settings line and press ENTER:

Now, you must set the splitting position, separately, for the first (left) and the second (right) balancing plane. The

number of splitting positions can be any, but not less than 3.

You may also activate individually the split weight show, for one or both balancing planes.

If the split weight is Enabled for both planes, the Main Balancing Menu will show the weight for the split positions,

as below:

If the split weight is Enabled for the first (left) plane, the Main Balancing Menu will show the weights for split

positions, as below:

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5. ADD VECTOR

1. Introduction

The Add vector function is a vector calculator and can be used to calculate the sum of a series of vectors.

The vectors can be expressed in vibration units or in mass unit.

2. Using Add vector menu

From the Balancing Main screen, press Menu key to access the popup menu.

Select the Add vector item and press ENTER.

The Add vector screen will be shown.

You can add up to four vectors (edit for each vector the Amplitude and the Phase).

The sum of the added vectors will appear on the bottom of the screen:

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If the list is full, as below, just press F3 – Continue and the vector’s sum will be shown instead of vector #1,

so you are able now to add more three vectors.

The number of vectors to be added is unlimited.

With F1- Clear you can delete the entire list.

With F2 – Delete you can delete only the selected vector.

Press F4 to exit from the Add vector menu.

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6. “One Run” balancing

1. Introduction

One Run Balancing can normally be achieved by restoring influence coefficients from the instrument memory.

The required balance weight is then normally achieved after the first run, thereby achieving a true “One Run

Balance”.

Preliminary condition

Before to Restore Influence Coefficients from any existing balancing file, is assumed that:

- The new rotor is identically with the rotor from the file.

- The previous rotor has a single reference mark, placed on the reflective tape position, valid for both balancing

planes.

- The relative positions between vibration transducers and the photocell remain unchanged.

If any of the above conditions is not true, the balancing of the new rotor will fail.

The Restore Coefficients technique is useful for a series of identical rotors.

The Calibration Runs are required only for the first rotor from the batch, so plenty of time can be saved.

Generally, the procedure works fine when the balancing is done using a Balancing Machine.

2. Procedure

From the Balancing screen, select Restore Influence Coeff. item and press ENTER or directly the key number 5:

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From the list, select the file from where you want to restore Influence coefficients:

Then, press ENTER key.

The Main Balancing screen will appear, as below:

The STATUS icon (in red) shows that any measurement hasn’t been done for this rotor.

Observe that the F2 - Trial runs is disabled (the Influence Coefficients already exist).

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Now press ENTER to take readings and when the readings are stable, press STORE key.

The initial unbalance for the new rotor (in mass unit) will be shown.

Make the unbalance correction, as indicate, only if it is necessary.

Finally, press MENU key and save the rotor data to a file, for the purpose of documentation.

NOTE. In the above example, the rotor unbalance isn’t within tolerances (the readings are coloured in yellow).

The unbalance correction is required, with the weights shown on the screen.

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7. THE REFERENCE POSITION FOR ANGLE MEASUREMENTS

1. Introduction

Zero position for angle measurement (or the reference position) is defined to be the position from where the angles

of the correction weights are measured.

The angle is measured always in the reverse direction of rotor rotation.

For a maximum flexibility, the CXM software allows a full editing of the trial weight, as follows:

Mass and radius can be easily understood, but many users do not fully understand the meaning of the trial weight

angle setting.

Below is explained in detail the meaning of the trial weight angle, to avoid any mistake during the balancing

procedure.

2. Trial weights defined at the zero angles (default setting)

Assuming that the trial weight is defined having the

angle at 0 and the reflective tape is on tape

position as shown:

If the trial weight will be attached on the same

position with the tape, the zero position for angle

measurement will be at tape position.

Zero position for angle

measurement will be at

tape position

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Assuming that the trial weight is defined having the angle at

0 and the reflective tape is on the shown position:

If the trial weight will be attached on 50 before the tape,

reverse to the rotation direction, the zero position for angle

measurement will be the trial weight position. Note in this

case the zero position for angle measurement is NOT the

same as the tape position.

Rule: When the weight position is set to 0, the zero position for angle measurement will be the trial weight

position. The position of the reflective tape is irrelevant.

3. Trial weights defined at non-zero angle (user defined angle).

Assuming that the trial weight is defined having the angle at a

non-zero value (e.g. 90) and the reflective tape is on the

shown position:

The zero position for the angle measurement will be on

weight position minus the defined angle (in our example will

be the weight position minus 90)

Rule: When the weight position is set to a non-zero value,

the zero position for the angle measurement will be the trial

weight position minus the defined angle. The position of the reflective tape is irrelevant.

Zero position for angle

measurement will be at

the trial weight position

Zero position for angle

measurement will be

here!

90

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4. Conclusion

To avoid confusions, it is a good practice to define the angle for the trial weight at zero (default setting).

In this condition, the zero position for the angle measurement will always be on the trial weight position. In other

words, the position of the trial weight determines the zero position for the angle measurement.

If you are a beginner in balancing, it is much better to place the reflective tape in the position where the trial weight

will be mounted. So, you will have a clear zero position, without confusion: the tape position.

Alternatively, mark the

position where you attach each the trial weight. This will be the reference for the angle measurement for that

balancing plane. It is not necessary to have the same zero position for both left and right balancing planes.

Any mistake in the determination of zero position for the angle measurement, will result into a wrong placement of

the correction weights and, for sure, in erroneous balancing results.

5. Best practice

1. When you edit the trial weights, do not change the default value for the angle (must be always 0).

2. When attach the trial weight, just mark the position. You can attach the trial weight anywhere, but just

mark the place. This will be the reference point for the angle measurements, for that balancing plane.

3. The left and right balancing plane reference can be any. If you mark both positions, there is no need to

take care about the reflective tape position or about the reference point into the conjugate balancing plane.

Trial weight

at 0

Tape position

at 0

Zero position for angle

measurement

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The Optical RPM Sensor for shaft measurement

Incorrect

Shaft

Incorrect

Shaft

The Optical RPM sensor produces a narrow lightbeam which is reflected back to the sensor by areflective tape fixed on the rotation shaft.

Shaft

Correct

ShaftCorrect

Fix the sensor at an angle towards the reflective tape.This makes the light bounce off in a different directionfrom the shaft and only the light hitting the reflectivetape is sent back to the sensor.

Measuring Distance 0.1 to 2m