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.
& 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?
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
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
- if the majority
of vibration is occurring at multiples of running speed, 'phase angle'
data is somewhat meaningless
|Detecting a 'Soft Foot'
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.
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
Figure 2. Infrared
thermal image of misaligned metal ribbon coupling. Photo courtesy Infraspection
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.
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
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'
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
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?
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
|'A Cost Effective, Pro-Active
Method to Find, Prioritize and Correct Coupling Misalignment Using Infrared
Thermography and Laser Alignment Technologies', Infraspection Institute,
|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.
to the Alignment / Balancing Reference Articles Index
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