ALIGNMENT PITFALLS - HOW TO IDENTIFY AND ELIMINATE THEM
Robert D. Skeirik
Vibration Group Marketing Manager for
Computational Systems, Inc
alignment job into an all day affair - frequently with ABSTRACT
In theory, machine alignment is a straightforward unsatisfactory results despite conscientious effort and a process, but in real world applications, it is often considerable investment in manpower and downtime. For compounded by structural faults such as „soft foot‟, piping this reason, it is crucial for the personnel performing strain, induced frame distortion, excessive bearing alignments to be aware of the kinds of structural faults clearance or shaft rubs. These pitfalls can turn a simple that can complicate the alignment process and that they job into an all day affair - frequently with unsatisfactory learn to recognize the tell-tale signs of bad measurements results despite a conscientious effort and a considerable before they invest valuable downtime in an unproductive investment in manpower and downtime. This paper will exercise. This paper will examine several of the most examine how to eliminate some of the typical reasons why typical reasons why bad alignment measurements are alignments are unsuccessful. obtained, how to identify them, and finally how to
In theory, machine alignment is a very COLLECTING VALID DATA
straightforward process. With some type of measuring Some fairly simple yet powerful techniques can be device extended across the coupling, the shafts are rotated applied to determine the validity of alignment readings to several positions (at least three) to determine the before investing time executing a machine move that may relative position between them. Since alignment is an be wrong. If using a dial indicator set, it is useful to apply iterative process (meaning that the misalignment should the data validity rule to each set of readings. The data continuously decrease with each machine move), it is validity rule shown in Equation 1 compares the readings theoretically only a matter of sufficiently repeating taken at the four cardinal positions:
alignment corrections until an acceptable solution is
achieved. In fact, quality alignment is not dependent on Top + Bottom = Left + Right (1) the type of measurement system used. Any good dial
indicator set or laser system should be sufficient to It provides a quick way to determine the validity of an perform quality alignments. Therefore, in heavy alignment solution before moving the machine. industrial applications, where the cost of downtown can be
in excess of $10,000 per hour, the fundamental question
0for an alignment program is not simply: 0
T T “Can I successfully align the machine?” LR517LR1415
Bbut rather: B
2220“Which method will provide the fastest GOOD DATABAD DATAalignment solution so that I can start MACHINE PROBLEM production again?”
Figure 1: Examples of good and bad alignment data
Furthermore, since misalignment is often This simple check is able to catch many set-up errors and compounded by structural faults such as „soft foot‟, piping (1)mechanical faults such as : strain, induced frame distortion, excessive bearing ； loose brackets clearance, shaft rub, etc., it may not be possible to align ； sticking indicators the machine without first addressing these additional ； indicators set too high or too low problems. These pitfalls can turn an otherwise simple
automatically apply the validity rule to the obtained ； improperly recorded data values and/or signs
readings and indicate whether acceptable levels for ； sleeve bearing float
deviation have been exceeded.) ； surface irregularities or eccentricities
； excessive bearing clearance
SWEEP TECHNOLOGY Small deviations from the validity rule are to be expected.
The above example demonstrates the power of If the difference is more than 10%, it is possible that the
applying the validity rule, however, it is not always coupling may be loose enough to provide excess torsional
possible to obtain readings at all four cardinal positions. play (“backlash”). To reduce the effects of torsional play
In such cases, alternative measurement techniques must be keep the coupling engaged while rotating the shafts from
applied. For instance, laser alignment systems are now the driven machine in the normal direction of rotation.
available that can calculate the misalignment based on a If the error is greater that 20%, the cause should be
full or partial shaft rotation. These sweep systems make determined. This could be a problem with the alignment
use of internal inclinometers to automatically collect fixture(s) or a concern with the machine being aligned.
required readings during shaft rotation and then Alignment problems occur from loose fixtures or improper
mathematically calculate the misalignment. For a machine use of fixtures. Possible machine concerns include locked
in good working order, the sweep curve will look like a couplings, spalled bearings, machine binds, etc. If the
perfect sine wave. Figure 2 shows examples of sweep data validity rule is not checked when such a problem
curves for machines with various mechanical conditions. exists, these potential machine faults will remain
undetected and substantially complicate the alignment process. Even worse, the objective of increasing machine reliability through quality alignment will not be (2) accomplished.
When using a laser alignment system, the potential for user error is greatly reduced due to the automatic measurement and recording of readings. However, the 0 90 180 270 3600 90 180 270 360Full Sweep - Laser Fixtures Loosedata validity rule can still be very useful to identify Full Sweep - Good Machinestructural faults such as excessive bearing clearance and other forms of structural looseness. To apply the validity rule with a laser system, it is necessary to record ALL FOUR cardinal readings (top, bottom, left, right) and plug them into the formula. If, however, the alignment solution is based on only three of the four cardinal
0 90 180 270 3600 90 180 270 360readings, the user will not have the ability to check the
Full Sweep - Loose BearingFull Sweep - Shaft Bindvalidity of the solution. In one such example involving a feed water pump in a power plant, an alignment was Figure 2: Examples of sweep curves attempted using only three of the four cardinal measurement (top, left and right - the bottom reading was Such systems are even able to calculate a correction omitted). The machine was moved as indicated by the with as little as 35-70º of rotation by extrapolating the laser system but no improvement in the alignment remainder of the shaft rotation (see Figure 3). condition was achieved. Numerous readings and machine moves were implemented but failed to result in any improvement in the alignment condition. When manually collected the reading for the fourth position (on the bottom) and plugging the values into the equation, it was clear that the validity rule was being violated. Visual inspection of the machine train indicated that one of the feet on the gearbox had been bolted down with the wrong size bolt head - thereby substantially reducing the hold-down force at this foot. This allowed the foot to lift slightly during Figure 3: 70º sweep showing extrapolated calculation of shaft rotation creating substantial error in the readings. full shaft rotation After replacing it with the proper size bolt, the operator
While this approach has now made it possible to was able to align the machine in just a few moves. (Note:
accurately measure machinery misalignment in more advanced systems are currently available that will
applications that were previously untouchable with dial problem with an unsecured machine foot. Another indicators or even conventional laser alignment equipment, mechanical problem that frequently can be diagnosed it does not necessarily address the issue of data validity. from the sweep curve - when present - is a shaft bind. In fact, even with a sweep measurement, it is impossible
to verify data validity based on only three measurement BUILDING ON GOOD FOUNDATIONS
points. To fully utilize the potential of the sweep Another major pitfall in real world alignment occurs technology, it is advisable to collect a large number of when there is a gap between a machine foot and the readings over as large a sweep angle as possible - a simple foundation (so called “frame distortion” or “soft foot”). requirement to fulfill as advanced sweep systems The machine frame will actually distort from its resting automatically collect data as often as every degree during position as the hold-down bolts are tightened to secure the shaft rotation. The only requirement of the user is to machine in place. This distortion puts the shaft in a bind rotate the shaft smoothly through whatever portion is and pre-loads the bearings (Figures 5a - 5c). accessible. With as little as 12 readings collected over one
half of a shaft rotation (180º), it is now possible to apply Figure 5a: Soft foot creates
statistical sampling techniques to automatically determine stress lines in the frame as a the validity of the data. short foot is pinned to the
foundation by the hold down
Figure 5b: Internal bearing
substantial pre-loading on
Figure 4: Sweep data for motor/pump pair Figure 5c: Pre-loading on the bearings is one of the Figure 4 shows sweep data for a motor/pump pair leading causes of premature where the readings on the pump (Machine B) show high bearing failure data validity, while the readings on the motor (Machine A) show clear signs of structural damage and a correspondingly lower confidence level as determined by A classic example of soft foot - like the bar table with the automatic statistical processing. In fact, Machine A one short leg - occurs when the machine naturally rests on had excessive internal looseness due to a developing three legs and the fourth leg is short. If not properly bearing fault. In this case, however, the user chose to corrected before beginning the actual alignment, it may be ignore the warning from the data validity graph and difficult, even impossible, to achieve acceptable results. proceeded with the job. The bearing did not prevent him
from completing the alignment, but it did require him to
make several extra moves and to use hours of precious 21downtime to bring the misalignment into acceptable levels.
Ironically, the motor bearing failed two days after start-up,
so the user was faced with a much larger maintenance task
and repeated installation and alignment just a few days
The information provided by the sweep method of
alignment is both powerful and conclusive. When Figure 6: Machine with one "soft foot" performed with a dual laser system, the sweep actually distinguishes between faults on Machine A and Machine The technician may implement the exact machine B. Furthermore, the shape of the sweep curve can often be move as required by the alignment calculation, only to used to determine the nature of the machine fault. The find that the frame distorts when he tightens down the „double hump‟ curve in Figure 4 is indicative of structural hold down bolts. Verification measurements may show looseness. This could be either a damaged bearing or a
little or no achieved improvement. In addition to the This approach works fairly well in the case of a parallel direct impact on alignment, the frame distortion resulting foot but may give an incorrect indication of bent foot. from a soft foot can also lead directly to unnecessary Figure 8: Usage of dial indicator for soft foot vibration and premature component failure. In fact, soft
foot has been observed to increase machine vibration More recently, lasers have also been used to locate levels by as much as ten times. In such cases, by not soft foot. There are two basic methods of checking soft recognizing the contribution made by the soft foot, the foot with a laser that measure different parameters and technician may have tried to lower the vibration levels by have fundamentally different goals: Frame Distortion better balancing, better alignment and so on - but obtained Index and the Laser Soft Foot Locator. In each case, the (3). very little improvementlasers are mounted on the machine shaft on either side of
coupling and live readings are taken while individual hold
These facts clearly establish that a soft foot check is a down bolts are loosened and tightened. As the name vital part of the pre-alignment process. Unfortunately, suggestions, the goal of the Frame Distortion Index due to misconceptions and pressure from production to get method is to determine the total amount of frame “back on line”, it is still viewed as an unnecessary or distortion that occurs due to a soft foot. In contrast, the expendable step in many plants today. goal of Laser Soft Foot Locator is to locate potential soft
foot problems and quantify whether the problem must first
be addressed in order to be able to successfully complete
the alignment job. As such, the goal of the Laser Soft Foot BentBentLocator is to provide the technician with the UpDownrecommended approach for the “quickest path back to
Frame Distortion Index
The Frame Distortion Index (FDI) measures the ParallelBaseplatevertical movement of the machine shaft caused by
loosening any individual hold down bolt. The calculation
is listed below in Equation 2.
Figure 7: Common Types of Soft Foot Frame Distortion Index = 2(Y)(FB) (2) One must also understand that there are different types of soft foot. In its simplest form, a foot can either be Where: Y = Vertical angularity of laser beam parallel or bent (Figure 7). It is relatively simple to locate FB = Distance between front and back feet and correct for a parallel foot. It is substantially more complex to determine the profile of and correct for a bent This calculation is based on an old millwright‟s rule of foot. thumb but replaces the traditional dial indicator with a set of laser fixtures. The result is a numerical value that The traditional way to locate a soft foot is by quantifies the movement of the shaft. This value does not, installing a dial indicator on the foot and watching the however, correlate directly to the actual movement of the movement as you loosen the hold down bolt (Figure 8). machine foot.
Figure 9: Measurement of vertical shift ("Y"): Y = (vertical shift in laser) / (distance between laser Table 1: Laser Soft Foot Locator Results fixtures) Reading in Result Interpretation
(mils/in)(mradTherefore, the user must firmly resist the temptation ) to use this value as a shortcut to determine the appropriate Machine in good condition; shim correction for that foot. “[FDI] cannot distinguish conscientious effort should between shaft movement caused by a bent foot and the 0.0 – 0.5 OK achieve excellent alignment shaft movement caused by a parallel airgap. Yet the two condition. types of soft foot must be corrected entirely differently… Soft foot is present; may not be [FDI] does not produce suggested soft foot corrections. 0.5 – 1.0 X able to achieve or maintain The source of the soft foot must be determined by analysis (4)excellent alignment condition. Existing convention before corrections are made.”Substantial soft foot is present; dictates that a machine foot is soft if the FDI is above 2-3 may not be able to achieve or mils (50-75;m). maintain acceptable alignment Laser Soft Foot Locator 1.0 - 1.5 XX condition. Correct soft foot first The Laser Soft Foot Locator, a more recently if machine is critical or must developed method for quantifying soft foot, focuses run for an extended period specifically on the effect of the soft foot on the before next shutdown. technician‟s ability to successfully complete the alignment Serious soft foot is present; task. This advanced procedure can no longer be performed XXX don‟t waste time trying to align > 1.5 with dial indicators. In contrast to FDI, which only looks machine unless you have first at the vertical lift of the shaft, the Laser Soft Foot Locator corrected the soft foot. now looks at the total movement of the shaft - both Soft Foot Correction vertical and horizontal. Total angularity equals the vector Regardless of the method used to locate a soft foot on sum of the vertical angularity “Y”, as used in the FDI the machine, it is always required to actually measure the calculation, plus the horizontal angularity “X”. The total gap (size and shape) between the foot and the foundation. angularity is calculated for each machine shaft separately This is necessary to determine whether the soft foot is and the greater of the two values is used. For this reason, parallel or bent as well as verifying that the foundation is a dual axis, dual laser system is required to implement this level. With a parallel soft foot, the correction typically procedure. The resulting measurement represents the involves inserting one or two shims. In the case of a bent theoretical maximum possible misalignment that could be foot or uneven foundation, the technician will need to
create a “step shim” or “shim pack” to completely fill the
gap (Figure 10).
Shimsinduced at the coupling due to this “soft foot”.
5Established convention dictates that a machine foot is soft feeler gauge set5if the resulting shift in angularity is more than 0.5 mils/in (or 0.5 mrad), however, the Laser Soft Foot Locator Figure 10: Measure soft foot with feeler gauge and fill method does not express the results numerically. Instead, gap completely the result is expressed as a priority, or level of urgency,
using symbols that provide the technician with a OTHER STRESS FACTORS IN ALIGNMENT recommended course for the “quickest path back to Another structural condition that can effect the production” with the machine in acceptable operating alignment and the running condition of your machinery is condition. The results of the Laser Soft Foot Locator and piping strain (often referred to as “induced soft foot”). their interpretations are listed in Table 1.
Piping strain occurs when the pipe or conduit does not
mate up to within 2 mils (50;m) of the flange. This
condition can create equally damaging stress lines and bearing pre-loads as the conventional soft foot described above. However, in contrast to conventional soft foot, piping strain is equally likely to move the machine vertically or horizontally. This makes the Laser Soft Foot Locator method the preferred approach for quantifying piping strain.
It is important to realize that otherwise straight-forward alignment jobs can become highly complex and yield unacceptable results if the technician does not address the quality of the alignment measurement and potential frame stress conditions (frame distortion, soft foot, and piping strain) during the pre-alignment check. These steps should all be conducted before the technician ever begins to move the machine.
Data quality can be determined by using the validity rule for conventional dial indicator and laser alignment methods, while sweep laser system offer an added level of data qualification by measuring the shaft movement throughout the entire rotation. This added information could help identify not only the existence of a structural problem but also the nature of the problem.
Soft foot and piping strain can be quantified using either the Frame Distortion Index (FDI) or the Laser Soft Foot Locator. FDI looks at the total stress or distortion on the frame caused by vertical movement of the shaft, while the Laser Soft Foot Locator looks at both the vertical and horizontal movement to quantify the need (priority) to fix soft foot before proceeding with the alignment job.
(1) Murray, M.G., Alignment Manual for Horizontal,
Flexibly-Coupled Rotating Machines, p. 46A, Murray
& Garig Tool Works, Baytown Texas (1983).
(2) Nower, D.L., Multiple Data Acquisition Techniques
for Shaft Alignment, pp. 2-3, Computational Systems,
Inc., Knoxville, Tennessee, (1996)
(3) Buscarello, R.T., Practical Solutions to Machinery
and Maintenance Vibration Problems,
pp. 99-105, Ralph T. Buscarello, Lakewood, Colorado
(4) Evans, G. and Casanova, P., The Optalign Training
Book, p. 72, Ludeca, Miami, Florida (1990)
thPresented at Vibration Institute, New Orleans, June 17,