When a second plane is balanced, it involves more than just two single plane balancing exercises. When balancing is “performed” in two or more planes, one must consider something known as “cross-effect” which is also sometimes known as “corrective plane interference”. This can be defined as the change of unbalance indication on one orrection plane caused by unbalance in the other correction plane. For example, if a rotor were perfectly balanced on both the left and right planes and a trial weight was placed on the left plane, the trial weight would cause vibration not only in the left plane, but also in the right. For example, it may cause 5 mils (127 microns) of vibration on the left plane and increase the vibration on the right from almost nothing up to say 1 or 2 mils (25 or 50 microns). Then, if the same trial weight is moved over to the right plane, it might cause 1 or 2 mils (25 or 50 microns) additional
vibration back to the left plane. This is known as cross-effect. Because of such cross-effect, unbalance indications observed on any one plane do not truly represent the unbalance in just that plane. Instead, each measurement will be a combination of the unbalance in that
particular plane plus the cross-effect transmitting into this plane from other planes. When one begins a balancing exercise, he does not know the amount and the phase of cross-effect, but he must take this into account if he is to successfully balance the machine. This will be
1 DETERMINE IF THE DOMINANT PROBLEM IS UNBALANCE
First, and foremost, determine if the real problem is unbalance using the analysis procedures outlined early in this text. It is not unusual to find that more than 50% of the times an analyst will be asked to balance a machine he will find the dominant problem to be something else. This procedure should include a complete set of measurements that can be used as “before” balance measurements and later compared to “after” measurements.
2 MOUNT TRANSDUCERS
Mount a transducer securely at each bearing in the radial direction of the highest 1X RPM vibration. This will normally be in the horizontal direction due to less support stiffness than that provided by the vertical direction. In any case, both transducers must face the same direction on both the outboard and inboard bearing housings. Two transducers are recommended to avoid having to move one transducer between the two bearings. For identification purposes they will be referred to as “left” and “right” transducers.
3 MOUNT PHOTO-TACH AND REFLECTIVE TAPE
Mount photo-tach and place reflective tape on the machine using the recommended directions given below. The three quantities needed to balance are the frequency, amplitude and phase at 1X RPM. The accelerometer will provide the amplitude and frequency information to the data collector. A photo-tach is connected to the data
collector to provide phase information by sensing the reflective tape each time it passes by the photo-tach during each shaft revolution. The photo-tach can be targeted to any portion of the rotating shaft supporting the rotor to be balanced. The reflective tape can be mounted on any exposed, clean surface which rotates with the shaft including a coupling, sheave, flywheel or the shaft itself. If there is an option, it is usually best to mount it on a part with a larger diameter with a continuous, unbroken surface (i.e., not on a broken surface such as a gear). Large diameters will give better accuracy for angular measurements.
The photo-tach itself can be targeted on the tape at right angles to the shaft, or it can even be targeted in a direction along the shaft axis, (for example, on the fan wheel itself in a direction parallel to the shaft). However, the best positioning choice for the photo- tach will normally be with it facing upwards looking at the target surface from below. Then, should the photo-tach fall off during operation, it will not likely strike the rotating member. In addition, if this convention is adopted, the analyst will be able to remember how the photocell was positioned (the photo-tach position must be the same in order to make meaningful comparative phase measurements). If the photo-tach is moved, it will change the resultant phase readings for all subsequent measurements by the angular amount it was altered when moved. Check for triggering of the photo-tach by slowly rotating the shaft by hand and noting if the red LED photo-tach indicator goes out as the
4 INSTRUMENT SETUP
From the main setup menu, select instrument configuration. Choose the units that you will use for balancing, i.e., g’s, in/sec or mils. Also make sure that the sensitivity of the transducer you are using in properly set. It is recommended that the same type of transducer with the same sensitivity is used on both locations. If you are not using a two-channel instrument, it is suggested that the cable from these transducers be
connected to a selector switch and then to the instrument. This will allow switching from the left and right transducer without switching cables.
5 ESTIMATE TRIAL WEIGHT
From the menu, select estimate trial weight. This selection will calculate a trial weight size when the rotor weight (weight of all rotating parts including the shaft, pulley,
impeller, etc.), the run speed (rotation speed of the rotor to be balanced), and the radius (distance from the center of the shaft to the point where the trial weight is to be placed) are entered, Obtain a trial weight as per this step and set aside for use during the balancing.
6 COLLECT ORIGINAL READINGS
From the main balancing menu select Begin New Balance. Enter a rotor description and notes if so desired. Start the rotor and select to begin original reading for the left plane. It is suggested that this reading be taken two or three times to make certain that you are getting repeatable data. Upon the completion of this data collection, switch to the right plane and take the data from the right plane.
7 ATTACH TRIAL WEIGHT
Since this is a two-plane balance, a trial weight must be put in both the leftand right correction planes, and vibration readings taken from both transducers while the trial weights are at each location. To accomplish this, first stop the rotor and attach the trial weight to the “left” rotor plane. Record the amount of the trial weight and its angular location, (with or against rotation depending on instrument), from the reflective tape. Note that even though the weight can be placed at any location on the rotor it is
recommended that the trial weight be placed at 0° making sure that you are reading the leading edge or trailing edge of the reflective tape depending on your instrument. If the trial weight cannot be placed at 0°, measure the angle between the reflective tape and the trial weight and enter that value. See Figure 41 for angular measurement
conventions. Use the trial weight from Step 5. Always look at the rotor from the same direction when measuring this trial weight angle and any correction weight angles. Start the rotor and measure the new “right” vibration using the right transducer. Insure
measurement repeatability. Now, switch to the left transducer and measure the new “left” vibration and phase. Store this information. Stop the rotor and remove the “left” plane trial weight. Attach a trial weight to the “right” rotor correction plane.
Enter the amount and location of this weight. It is recommended this weight be the same size as the left trial weight and be placed at the same angle that the left trial weight had been located. If this cannot be done, enter the amount and angular location of the actual position. Remember to always look at the rotor from the same direction when measuring angles. Making sure that you are switched to the “left” transducer, measure and store the new “left” vibration and phase. Then, switch to the “right” transducer and measure and store the new “right” vibration and phase. Note during this procedure a significant change of at least 30% amplitude and/or phase is needed for accurate correction
weight calculations. If this amount of change was not achieved, increase the trial weight size and repeat the procedure.
8 ORIGINAL CORRECTION WEIGHT
Shut the machine down and remove the “right” trial weight. Select correction weight readout on the instrument. Both the “left” and the “right” correction weight should be displayed. Obtain the proper correction weights and attach at their proper locations in the “left” and “right” correction planes. Remember to always view the angle from the same direction.
9 SPLITTING WEIGHTS
If the angular location for a single correction weight cannot be achieved due to an obstruction or void (as if between blades or spokes), the “Split Vector” portion of the software can “split” the single weight vectorially into two weights that can then be attached in two more convenient locations.
Enter the number of equally spaced positions where split weights could be attached, the amount and angular position of the weight to be split. The result is two weights (Amt 1 and Amt 2) with the weight and angular location of each noted. Note the reflective tape or reference position is considered Position 1, and both weights must be added to the rotor to replace the original un-split single weight. Repeat this procedure for both correction weights if needed.
10 MEASURE NEW VIBRATION WITH ORIGINAL CORRECTION WEIGHTS
Start the machine with correction weights from Step 8 in both the “left” and “right”
correction planes. Select the menu selection to measure “new” vibration making sure that the proper transducer has been selected. Measure both the “new” vibration and phase for both the “left” and “right” planes. Insure that these reading are stable. Stop the rotor. Compare this vibration to your balance criteria. If the vibration is within tolerance,
permanently attach the correction weights. If the vibration is not in tolerance, stop the rotor and select “Trim Balance” to access the new weight and location for further correction.
11 TRIM BALANCE
The trim balance screen list both the “left” and “right” corrections weight size and
angular location. Obtain trim weights equal to these and attach to the rotor in their correct angular location in their respective “left” and “right” correction planes at their correct angular location. Do not remove the original correction weights from Step 8 and use the weight splitting technique of Step 9 if needed. Start the rotor and measure the new “left” transducer and “right” transducer vibration with both correction and trim weights in place. Check for measurement repeatability. Stop the rotor. Compare the results to your balance criteria. If within tolerance, the balance is complete. If not within tolerance, repeat the trim procedures list above. This procedure can be repeated as often as necessary, however, always leave on the previous attached correction weights.
12 COMBINE CORRECTION WEIGHTS
If more than one weight is used in each (left and right) correction planes, the weights can be combined by using the “Add Vector” function. Note only the weights in each plane may be combined. That is, only the weights in the “left” plane can be combined to make one “left” correction weight and only the weights in the “right” plane can be combined to make a single “right” correction weight. The “left” and “right” correction weights cannot be combined.
Enter the weight amount and angular location (in degrees from the reference tape) for any two weights in the same plane. The combined vector sum and combined vector angle are noted as the sum on the screen. If any two correction weights were used, this single final correction weight will replace them and can be attached as one weight instead of two. If more than one correction weight was used, the weights can be combined in steps starting by combining two weights vectorially, then combining that combined vector sum with the next weight, etc., until all weights in the same plane are combined to a single weight. This process must be done very carefully as the final balance results can be
greatly affected if inaccurate weight amounts and angles are entered. After combining weights it is recommended that the rotor be re-tested for balance with the “left” and “right” combined weights in place. Additional balance corrections may be required.
13 SECURELY FASTEN CORRECTION WEIGHTS
If measurements are now in complince throughout the machine with balance and vibration specs, the analyst should take care to securely fasten permanent correction weights or, if he desires, remove weight at locations 180° away from the final
correction weights. If possible, secure these weights in such a manner that if they do happen to be thrown off, they will not be thrown into areas where personnel might be stationed.
14 REPEAT VIBRATION MEASUREMENTS AT ALL LOCATIONS (BOTH THE
DRIVER AND DRIVEN).
Complete vibration measurements should next be captured at each location on both the driver and driven machine components. Ensure not only that the unbalance problem has been resolved, but also that other problems are not now indicated. Also, ensure that balancing in one direction on Plane 1 has not now caused an increase in vibration at other locations and directions.