Why Shaft Misalignment Continues to Befuddle and

Why Shaft Misalignment Continues to Befuddle and Undermine Even the Best CBM and Pro-Active Maintenance Programs

Detecting shaft misalignment using vibration analysis, infrared thermography, and oil analysis techniques can be difficult and somewhat deceptive unless you understand the true mechanism of misalignment and how it affects our rotating machinery. Correcting a known misalignment condition can be even more arduous and extremely frustrating if your alignment measurement system is unable to determine the true misalignment condition or offers corrections that would be difficult to perform. This article uncovers the most common pitfalls in detecting and correcting a shaft misalignment condition and how to increase the effectiveness of your CBM and Pro-Active Maintenance programs by understanding the common misconceptions of this damaging and highly prevalent machinery problem.

CBM & Pro-Active Maintenance Program Philosophies

Condition Based Maintenance (CBM) or Predictive Maintenance programs are based on the premise that rotating machinery should only be repaired or replaced if certain conditions have degraded to the point where a costly failure is about to occur of if it impacts on production quality. Vibration, temperature, or lubrication data is gathered and an assessment of the mechanical or operational condition of the machine is made. If certain unacceptable values have been met, recommendations are forwarded to correct the malady. Pro-Active or Prevention Based Maintenance Programs utilize predictive / preventive maintenance techniques with root cause failure analysis to detect and pinpoint the precise problems combined with advanced installation and repair techniques including potential equipment redesign or modification to avoid or eliminate problems from occurring. They're great programs and anyone who is not utilizing these forward looking agendas will eventually be undone by their competitors who are. Exactly how effective are these technologies in detecting and correcting shaft misalignment?

Detecting misalignment using vibration analysis

A large majority of people who actively participate in CBM and Pro-Active maintenance programs will tell you that shaft misalignment will be indicated by higher running speed and/or twice running speed vibration frequency components with high axial vibration and a 180 degree phase shift across the coupling.

This is true only some of the time. Figure 1 shows several vibration spectral patterns on rotating machinery operating under misalignment conditions.

Figure 1. Various misalignment vibration 'signatures' with different types of flexible couplings.

Notice that the patterns are different and don't always show running speed and/or twice running speed vibration frequency components (1x and 2x). Several controlled tests by several individuals over the past ten years have indicated that vibration spectral patterns can be different under similar misalignment conditions depending on the type of flexible coupling installed on the machinery and under certain conditions, virtually no vibration can be detected even under moderate to severe misalignment. There are several points that you should be aware of concerning the use of vibration analysis for misalignment detection :

- there typically is not a linear relationship between the overall vibration amplitude and the amount of misalignment (i.e. it is possible that the vibration could decrease as misalignment increases)

- it is possible for the vibration levels to increase after re-aligning a piece of rotating machinery

- if the majority of vibration is occurring at multiples of running speed, 'phase angle' data is somewhat meaningless

Detecting a 'Soft Foot' Condition

One of the most prevalent problems with aligning rotating machinery can be traced to machine case to baseplate interface problems. When rotating machinery is set in place on its base / frame / soleplate, one or more than one of the 'feet' are not making good contact at the 'foot points' on the frame. This can be attributed to warped or bowed frames, warped or bowed machine cases, improper machining of the equipment feet, improper machining of the baseplate, or a combination of a warped / uneven frame and warped / uneven machine case. This problem is commonly referred to as 'soft foot'. Soft foot generally describes any condition where poor surface contact is being made between the underside of the machine casing 'feet' and where they contact the baseplate or frame. Can 'soft foot' problems be detected using vibration analysis? Some daring individuals have loosened and then tightened foot bolts while the machinery is running and have noticed changes in the vibration amplitudes or shifts in the spectral patterns occurring. On electric motors, data has shown elevated levels of Rotor Bar Pass Frequency (RBPF) may increase. However 'soft foot' problems can occur on any type of machine. How can someone detect a 'soft foot' problem on centrifugal pumps, fans, steam turbines, etc. using vibration analysis? If the 'soft foot' condition is severe enough, rotating parts may actually contact stationary parts that will produce continuous or intermittent rubs that can be detected through vibration. Distortion of gearbox casings due to a 'soft foot' condition can be seen at gearmesh or multiples of gearmesh or sidebands straddling the gearmesh frequency. These vibration anomalies don't always occur however making for detection of 'soft foot' conditions very difficult if not impossible in the majority of cases.

Detecting misalignment using infrared thermographic techniques

A few years ago, two technicians ran a controlled test using infrared thermography techniques to detect changes in the temperature of flexible couplings operating under various misalignment conditions. Figure 2 shows just one of the couplings tested.

Figure 2. Infrared thermal image of misaligned metal ribbon coupling. Photo courtesy Infraspection Institute.

Technical papers were presented on the study and following statement was made. "This study has shown that a Certified Infrared Thermographer with an infrared camera can quickly find misaligned coupled equipment. The greater the heat, the greater the misalignment. "Will every type of flexible coupling get 'hot' under misalignment conditions? Sorry to say, but some coupling designs don't follow this rule (e.g. flexible disk or diaphragm couplings typically will not). The main problem with trying to use this technique is that most couplings are hidden from view since coupling guards are required for safety reasons.

Correcting misalignment

Once the machinery is off-line and safety tagged, the actual process of re-alignment can take place. How well can shaft alignment measurement systems determine the amount of correction needed? To test this hypothesis, an experiment was ran with an electric motor and centrifugal pump coupled together with a metal ribbon type flexible coupling. The 'soft foot' conditions were corrected and the unit was aligned using the reverse dial indicator technique. A laser shaft alignment system was then attached to the shafts and measurements were taken with the laser system to verify the reverse indicator readings (which it did). 50 mils of shim stock was then added under all four of the motor hold down bolts and measurements were taken with the laser system again. The results of the test are shown in figure 3.

Figure 3. Laser shaft alignment system test.

Notice that the proposed corrections at the inboard and outboard feet of the motor does not indicate to lower all four feet by 50 mils. Uh-oh, why did this happen? Figure 4 shows what actually happens to the machinery shafts when forced to operate under misalignment conditions.

Figure 4. Elastic shaft bending occurs with moderate to severe misalignment when the 'flexible coupling' is engaged.

Under moderate to severe misalignment conditions, the shafts will start to elastically bend with the flexible coupling engaged. The alignment measurement system will be capturing measurements at certain points along the length of each shaft that does not represent where the centerline of rotation would be if no misalignment forces were present.

Correcting a 'Soft Foot' Condition

Again, 'soft foot' generally describes any condition where poor surface contact is being made between the underside of the machine casing 'feet' and where they contact the baseplate or frame. It is important to recognize that our machinery feet are not making point contact. Instead, there are typically four (or more) supposedly flat foot surfaces on the underside of our machine case trying to mate up to four (or more) supposedly flat surfaces on the baseplate. Now, the chances of all four surface on the undersides of our machines cases being flat and in the same plane and all four surfaces on the baseplate being flat and in the same plane are rare indeed. Quite often when we try to mate the underside of a machine foot to the point of contact on the baseplate, a non-parallel, very complex, tapered, wedged shaped gap type of condition exists that cannot be corrected with a flat piece of shim stock. In addition, it is most probable that a soft foot condition exist at all of the foot points. Now I'm not saying that our machinery is levitating in free space, just that the feet are not making good contact on the baseplate. A wide variety of different conditions can exist. Machinery can 'rock' across two diagonal corners or can rock from side to side or end to end. It is not uncommon to see three of the feet 'toe up' and the fourth foot 'toe down'. It is possible to have 'edge contact' at the inboard side of a foot and the outboard edge at that foot exhibit a gap.

Take this pipe and shove it

Much of our rotating machinery is attached to piping, ductwork, or conduit. Can excessive piping strain affect shaft alignment? Figure 5 shows a motor and pump arrangement that was aligned to mils/inch. This drive system was used to transfer fatty acid from a holding tank to the batch process system. The fluid temperature while being pumped was 140 degrees F. The unit was operated for 6 hours after the alignment was completed. A period of 12 hours elapsed to allow the unit to attain ambient air temperature and the alignment was checked again.

Figure 5. Alignment shift after 12 hours of operation on motor pump unit.

Uh-oh. Notice that the shafts are no longer in alignment. Upon further investigation it was determined that the expansion and contraction of the poorly supported suction and discharge piping caused the pump to shift its vertical position.

OL2R movement ... something almost everyone neglects to consider

Virtually all rotating equipment will undergo a change in position during start-up and while running that affects the alignment of the shafts. In order for the shafts to run collinearly under normal operating conditions, it is desirable to know the amount and direction of this movement to properly position the machinery during what is commonly called the 'cold' alignment process (i.e. off-line or not running) to compensate for this change. The off-line to running (aka OL2R) movement characteristics of the vast majority of rotating machinery drive systems in existence have never been measured. On perhaps 80% of the drive systems in existence, the amount of movement is negligible and can pretty much be ignored. In the remaining cases, however, it can make all the difference between a smooth running drive system and one that is plagued with problems. It is important to know how much movement is occurring before you deem it insignificant and ignore it. There are several important issues that need to be mentioned concerning off-line to running measurements:

- Less than 10% of the people who are responsible for alignment of rotating machinery in industry have ever actually conducted off-line to running machinery movement surveys (1 out of 5 may be aware of this phenomenon but only half of them have actually tried to measure it).

- Many of the people who have conducted OL2R surveys or are aware of this phenomenon, believe that machinery only moves in the up and down direction disregarding the possibility (and probability) of sideways movement.

- Compared to the amount of time usually taken to align rotating machinery when off-line (which takes 3-6 hours on the average), off-line to running machinery movement surveys can take days and even months to complete. These types of surveys are frequently conducted on the more critical pieces of rotating machinery in a plant and these drive systems usually can't be turned on and turned off at the convenience of the people performing the test. So be prepared to spend a lot more time planning, designing, fabricating, installing, measuring, and analyzing this data than you typically would for 'off-line / in-line' shaft alignment jobs.

- In several cases where this data has been collected, and it was verified that the machinery was not in the right alignment position when off-line, there was a great reluctance (and in some cases, pure refusal) to change the existing positions of the machinery to reflect the newly discovered information. The reasoning typically expressed was ... "The machinery has been in this position for several years, why change it now?" or "If we move the machines, we will void our warranty." or "We have to wait until the next shutdown to change the alignment" (which seems to be forgotten at the designated shutdown time). It seems very silly to spend inordinate amounts of time, effort, and money on aligning machinery to extreme precision when the equipment is not running and ignore off-line to running machinery movement. What has been accomplished if the machinery has been aligned to 1/2 mil per inch or better in an off-line condition if the machinery moves 20 or 30 mils (or more) when it running? How much OL2R movement can actually occur on rotating machinery?

Conclusions

Detecting shaft misalignment can be difficult if not impossible on operating rotating machinery. Even for severe misalignment conditions, sometimes only slight changes in vibration or temperature occur making it difficult to asses the severity of the problem. Not until mechanical degradation actually occurs, can the root cause of misalignment be suspected but by then, the damage has already taken its toll on the equipment. Correcting a known misalignment condition can be one of the most frustrating tasks you could undertake. With the flexible coupling engaged, elastic deformation of the shafts being subjected to moderate to severe misalignment conditions can make even the most accurate shaft alignment measurement system look foolish (as well as the operator of the system). Other factors such as 'soft foot' conditions, excessive static or dynamic piping strain, and off-line to running machinery movement further compound the problem challenging even the best CBM and Pro-Active Maintenance programs to find and correct this problem. Shaft misalignment will continue to be one of, if not the most prevalent problem with rotating machinery until everyone involved with this process is properly trained, they are given the correct tools to do the job, are given enough time to complete all phases of the alignment, and they have the desire and inspiration to do a perform an outstanding job.

References

'A Cost Effective, Pro-Active Method to Find, Prioritize and Correct Coupling Misalignment Using Infrared Thermography and Laser Alignment Technologies', Infraspection Institute, 1994

Piotrowski, John D., Shaft Alignment Handbook - Second Edition, Marcel Dekker Inc., New York, NY, ISBN # 0-8247-9666-7.

Lynn, Daniel, "Soft Foot : A Fairy Tale?", P/PM Technology, Vol. 9, Issue 5, Oct. 1996.

This article is provided courtesy of Turvac Incorporated.

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