Maintaining Machinery Balance (test, format #3 - Word import via Google Drive)

Technical Paper

Fundamental Considerations for Maintaining Your (Machinery) Balance in an Unbalanced World

This paper is intended to provide useful information for in-place field balancing of turbomachinery (pumps, fans, turbines, compressors, etc.). It is assumed that some knowledge of the actual method details are already known or available. The method most commonly employed (and assumed here) is known by various names: the “three-run” method, the “trial weight” method, the Thearle method, the “influence coefficient” method, etc.

This information is basic, but nonetheless important to know and apply during field balancing. It is the result of countless troubleshooting and consulting experiences. This is a list of hints and reminders, not a “how-to” instruction.

1. Make sure that an unbalance problem exists, prior to expending time and effort in hopes of resolving “the balance problem”. All turbo-machinery operates in some state of unbalance – it is only a matter of degree or severity. Acceptable unbalance levels vary (sometimes as a result of subjective opinion) with machine, installation, operating conditions, etc. However, unbalance problems are almost always indicated by a higher than usual vibration amplitude at the running speed (or “1X”) frequency (divide the rotor r/min by 60 to get the 1X frequency in Hertz, abbreviated as “Hz”, which is cycles/second; for example, an 1 800 r/min machine has a running speed frequency of:

1 800 r/min ÷ 60 = 30 Hz). If there is not an increase in the 1X vibration amplitude, an unbalance problem is not likely.

Another indicator of unbalance is that the vibration amplitude should vary with changes in machine speed; for example, higher vibration amplitude at higher speeds. Correcting “high” vibration problems by

balancing is a reasonable “percentage shot”, since unbalance is the most frequent cause of vibration problems. However, many man-hours and plant-hours are wasted annually by assuming this is always true – it only takes a few minutes to confirm (unusually high 1X vibration and stable phase readings should be evident).

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2. Phase angle readings must be stable; if not, then it is unlikely that unbalance is the main cause of the excessive vibration. In any case, it will be difficult to impossible to improve balance condition by this method if phase angle readings are not stable. The phase angle reading should be stable and repeatable within about ±15° (for strobe readings), or better, during any given trial or operating run. The use of a “key-phasor” tach signal will yield superior results over a stroboscopic technique for measuring a phase reference angle; for example, the key-phasor is much faster, more precise (±3° or so), repeatable and stable.

3. A fixed orientation and sign convention must be adhered to while employing a given balance solution method. The direction of rotation (clockwise or counterclockwise), viewing position/orientation, direction of increasing/decreasing angles and key-phasor or strobe position must all remain constant and be consistent with the graphical or mathematical (vector) solution being employed. The key-phasor and strobe solutions cannot be interchanged for this reason. Although similar, there are small but crucial differences in resolving the vector solutions for key-phasor versus strobe balancing (sign convention, direction of rotation versus direction of increasing phase angles, vector orientation, etc.).

4. Vibration sensor location(s) must be the same for all (trial) runs during a given balance operation. Moving the sensor mounting location, even slightly, will alter the test conditions and readings (vibration phase and amplitude) unnecessarily. It is also a good practice to locate sen-sors (vibration, key-phasor/strobe) in the same locations for future balance operations, once good results are obtained. Sensor locations on a machine may be marked by ink, paint or a shallow drill-point indentation.

5. The machine speed should be in the same critical speed region during trial runs and normal operation. That is, if a machine oper-ates above “first critical” shaft speed, then the trial balance runs should be in this same speed region (above first critical, below second critical, in this case). This is not always possible, but is very desirable. Also, always run trial balance speeds as close to normal machine running speed as possible (or practical) and do not vary shaft speeds appreciably from one trial run to the next. If your machine operates under

1. 800 r/min, it is likely that “critical speeds” are not a consideration; for example; you are probably running below “first critical” speed. If you do not know what “critical speed” means or how it relates to your machinery, then find out before getting too involved with balancing a variety of turbomachinery.

6. have (trial and correction) weights available, as well as a practical and safe means of mounting. This sounds obvious, but is often overlooked. It is embarrassing to disrupt plant operations to balance a particular machine only to discover that it will take unexpected hard-ware is a must. It is preferred that the bolts thread-in along an axis parallel to the shaft longitudinal axis, not radial. A weight that dislodges is a lethal weapon and can seriously injure anyone in its path – safety is a major consideration!!!

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7. Some trial/correction weight location considerations are:

• Mount trial and correction weights at the same radial distance from the shaft, or else a (simple) correction factor^* must be applied.
• Mount trial and correction weights in the same plane (plane perpendicular to the shaft longitudinal, main, axis).

• If the desired correction weight position is not convenient or practical, there is a way to divide or split the required amount of weight into two or more parts^*. This enables mounting of weights at convenient locations such as blades, vanes, existing holes, etc.
• Weights should be mounted as close as possible to the (rotor) imbalance source.

* Contact SKF USA Inc. for description.

8. In cases of persistent balance method difficulty, key-phasor/tach signal integrity should be confirmed. The voltage level (amplitude), polarity, frequency and repeatability of any key-phasor/tach signals should be ascertained on an oscilloscope display. This can be a hidden source of difficulty in cases where the balance method is not yielding the proper results (other “hidden” problems can be a bothersome struc-tural resonance, harmonics, intermittent damping, etc.).

9. Two-plane balancing. Many balance problems cannot be eliminated by a single plane balance method, since dynamic and/or couple unbalance forces exist, and a two-plane balance method must be utilized. In general, a long, wide rotor is more likely to be a two-plane prob-lem than a tall, thin rotor. The actual test methods are similar (you employ two different locations/areas for mounting sensors and weights, instead of a single location), but the graphical or mathematical solution is much longer. It is suggested that a small, handheld programmable “pocket” calculator be used; this is almost a necessity for two-plane problems, and a great convenience for single-plane problems. They cost under a few hundred dollars and programs are readily available from SKF USA Inc. Again, note that strobe and key-phasor program solutions are not entirely interchangeable – they are slightly different from each other (see number 3).

10. One-run balance. After the balance is successfully performed, future balancing on that machine can usually be done with only one step to add (or remove) weights. The data from previous solutions will yield a “phase lag” and “sensitivity” (or influence coefficient) that enables determination of required correction weight (and location) directly from the initial (operating) vibration (amplitude and phase) readings. This assumes that the “system” (machine, bearings, mounts, foundation, piping and process conditions, etc.) have not changed significantly between the two balance efforts. Machine repairs and modifications, long-term structural response changes, drastic loading or erosion of the rotor mass, etc., can invalidate this technique. However, many machines are sufficiently stable as “systems” to permit successful use of this technique. Contact SKF USA Inc. for further information about this technique.

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11. Avoid “seat-of-pants” diagnostics and “committee” efforts, have confidence and understanding of the method. “High/low” or “heavy/light” spots can be determined from initial vibration/phase readings; however, the reasoning differs for strobe versus key-phasor, cer-tain types of vibration sensors (eddy probe), some balance instruments (and control settings), whether strobe phase angle readings are taken off a rotor shaft or stationary marker, etc. It is strongly suggested that, at least initially, you avoid attempting to reason out the “why” and stick to the “how”.

Audiences, invited and uninvited, tend to be liberal with dialogue and opinion, but usually lack relevant knowledge. Audiences also tend to disrupt the order and flow of the test/solution effort (a well-known professional recommends performing balancing at 3:00 AM, in the rain, to discourage the uninvited!). Plant personnel (mechanics, engineers, operators, etc.) need to be a part of the overall machine diagnosis process, as they often have relevant information, but they do not need to be a regular part of the actual balancing effort, as a group.

12. Balancing can be done without specialized equipment; however, it is usually not as convenient or effective. The necessary vibration amplitude and phase information can be obtained from an oscilloscope display (either orbit or time-domain waveform). There are also other methods ranging from a steady hand and chalk mark to a “four-run” method without phase. None of these methods are as accurate, conve-nient or reliable as the basic method assumed in this text. Other methods will work, but not as often and not as well.

13. “Runout” is a term (and scapegoat for many problems) that loosely means a portion of the signal response from a non-contact eddy probe that does not originate from shaft motion – it is a source of error. Vibration (phase and amplitude) data taken from eddy probe sensors may sometimes need to be corrected for runout error. Programs and methods exist to facilitate this correction process.

14. Shop balance versus field balance. It is common to remove a rotor from a machine and send it to a shop for balancing on a test-stand. In some cases, this may be the only practical means of balancing; in other cases, a shop balance may not be possible. Occasionally, a rotor that has been shop-balanced will yield poor (or even worse than original) balance-vibration results when reinstalled and operated. This does not necessarily mean that the rotor was incorrectly balanced in the shop – it may result from differences between test-stand conditions and oper-ating conditions. A shop test-stand will have different bearings, bearing spans, structural response, stiffness, mechanical impedance, etc. – the test-stand cannot duplicate the actual machine “system” and its response. The test-stand rotating speed may also, by necessity, differ significantly from machine operating speeds. Sometimes all these differences are not significant, at other times they are highly significant. For these reasons (and for reasons of time, cost and convenience), the field balance method is often preferred and may yield superior results.

15. Anyone capable of learning to time an automatic ignition system can learn this method of balancing. The equipment, methods and (modest) level of skill required are similar.

Please contact:

SKF USA Inc.

Condition Monitoring Center – San Diego

5271 Viewridge Court · San Diego, California 92123 USA Tel: [phone] · Fax: [phone]

Web: www.skf.com/cm

• SKF is a registered trademark of the SKF Group.

All other trademarks are the property of their respective owners.

© SKF Group 2012

The contents of this publication are the copyright of the publisher and may not be reproduced (even extracts) unless prior written permission is granted. Every care has been taken to ensure the accuracy of the information contained in this publication but no liability can be accepted for any loss or damage whether direct, indirect or consequential arising out of the use of the information contained herein.

PUB CM1004 EN · February 2012