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6:00 am
April 1, 2008
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Don’t Look At Your Feet

0408_foot_11If you’re aligning equipment using foot value tolerances, you may need all the luck you can get and then some. “Modern” as that approach is, this author says it doesn’t stand up in the real world.

My company specializes in laser shaft alignment equipment. Recently, a prospective customer commented that he liked our equipment, but wondered when we were going to get “modern” about alignment tolerances. Generally accepted shaft alignment tolerances in industry use two criteria: Radial shaft centerline offsets at or near the coupling and angular shaft relationships. My customer’s opinion, based on an article he had read, is that shaft alignment tolerances should be based on offset values at the motor feet. If that’s the approach you’re taking, I would have to ask, “Do you feel lucky?”

0408_foot_fig11

Based on 28 years of experience in the alignment field, I have great concerns about using the foot value tolerance scheme. They include the following: 1. Companies will spend an enormous amount of time and labor going the “extra mile’ to achieve this standard. 2. Use of the foot value tolerance vs. angle and offset tolerances will NOT improve machinery reliability. 3. The foot value tolerance method will not improve bearing or seal life. 4. This foot value tolerance method will frustrate and discourage aligners who are motivated to perform precision maintenance, as very small measurement errors will result in large foot value fluctuations. 5. The foot value tolerance method breaks a basic measurement principle by amplifying measurement errors.

0408_foot_fig21Basic shaft alignment
Every shaft rotates about an axis. In shaft alignment (see Fig. 1), the driven machine is usually considered as a stationary machine element. The rotational axis of the driven machine serves as the measurement datum (reference line). The driver is usually considered to be movable. The movable rotational axis is compared to that of the stationary. Shafts are considered misaligned when the two rotational axes are not colinear.

As shown in Fig. 2, misalignment can be represented by expressing the horizontal (x) or vertical (y) position of the movable rotational axis in relation to stationary shaft at various axial positions (z).

  • Offset Misalignment is the actual radial position of the movable rotational center relative to the stationary center. If the shafts are not parallel, the offset misalignment is different at every axial position.
  • Angular Misalignment is the slope relationship of the two shafts. The slope has a positive value if the offset values are more positive at the rear feet than at the coupling.

For example, using the graph in Fig. 2, we can see that:

Offset Misalignment = -20 at the coupling center,
0 at the front motor feet and +30 at the rear motor feet
Angular Misalignment = 20 mils/10” = 2.0/1”.

0408_foot_fig31Measuring and correcting misalignment
Misalignment is measured with dial indicators or laser sensors as the shafts are rotated (see Fig. 3). The measurement planes are defined by the axial locations of the sensors. Correction planes are where shims can be added or removed.

Misalignment forces…
Misalignment, as illustrated in Fig. 4, creates forces at the coupling that are exerted on the shafts and, subsequently, on bearings.

The force effects of misalignment can be simplified by considering the misalignment as a simple lever. Misalignment at the coupling creates a moment of force acting on an effort arm. This will create a first class or second class lever with either the inboard or outboard bearing acting as a fulcrum. The length of the motor shaft between its bearings is the resistance arm.

0408_foot_fig41Alignment tolerances…
It is very unlikely that perfect alignment is achievable— or is really that important. The objective of shaft alignment is to minimize radial forces by minimizing the offset at the coupling where power is transmitted. Further, we minimize axial forces by minimizing the slope relationship of the two shafts. The tolerances we recommend (see Fig. 5) are based on angle and offset values. You can choose to be more or less permissive.

Zone of good alignment using angular and offset tolerances…

The graph in Fig. 6 shows a zone of acceptance for a 3600 RPM machine using angular and offset tolerances. When the movable shaft axis falls completely within the shaded “bowtie,” acceptable alignment is achieved. There are a range of foot values that are acceptable.

0408_foot_fig51The plotted line represents a shaft with offset misalignment of 1 mil (0.001”) at the coupling center. The slope is 0.1 mil/1”. This is a very good alignment!

Zone of good alignment using foot value tolerances…
The graph in Fig. 7 shows a zone of acceptance for a 3600 RPM machine using the foot value tolerances (inset box) referenced by the author of the article my customer had read—which is what compelled me to write this article. When the movable shaft axis falls completely within the shaded area, acceptable alignment is achieved. There is a very small range of acceptable alignments. Small measurement errors will make these tolerances hard to satisfy!

0408_foot_fig61Measuring errors
It is impossible to produce error-free shaft alignment measurements, even with very resolute laser systems. Bearings must have clearances to assure free shaft rotation at varying temperatures. Machine shafts shift slightly within those clearances as the shafts are rotated in the measurement process. Therefore, the machine shafts are not perfectly repeatable. The effects of measurement errors are always minimized when the planes of interest are between the measurement planes. The effects of errors are amplified when the planes of interest are external to the measurement planes.

The effect of a 0.5 mil measurement error in one measurement plane is shown in Fig. 8. This creates an angular error of 0.5 mil in 6”, or 0.08 mil/1”. When the offset tolerance is applied at the center of the coupling, the error is small because that plane of interest is between measurement planes. When the tolerance is applied at the feet, however, the error is amplified because those planes are external to the measurement planes.

0408_foot_fig71Conclusion
This article is intended to offer insight on shaft alignment tolerances for close coupled machines with flexible couplings. Precision alignment is important, but perfect alignment is not achievable—nor is it needed to reduce destructive coupling forces. Tolerances should be based on solid measurement principles and with the understanding that alignment measurements are only as repeatable as the machine shafts can repeat themselves during manual rotation.

0408_foot_fig81Final thoughts…

  • Understanding the rotational axes assists in making alignment corrections.
  • Shafts will only “seek” co-linearity if they are coupled.
  • Therefore, the objective of shaft alignment is to reduce coupling forces.
  • The angle and offset tolerance method meets this objective.
  • The actual angular and offset values that are acceptable are negotiable, but tolerances should not be made smaller than the machine shafts are capable of reproducing within bearing clearances.
  • Small measurement errors are amplified when misalignment is calculated at the feet.
  • Foot values should be used only to make alignment corrections.
  • The required bearing clearances are larger as shaft diameters are larger.
  • It is not that hard to achieve small foot values in a classroom with small demonstrators. In that environment, the bearing clearances are usually small and the amplified errors are also small because the feet are not that distant from the coupling.
  • In real-world situations, small measurement errors produce foot value fluctuations that are greater than the stated foot value tolerance.
  • Small measurement errors have little adverse effect when applying angular and offset tolerances.
  • Consequently, when applying foot value tolerances in real world situations, the aligner must be either lucky or a liar to achieve his goal.

Perhaps this article can launch a discussion on industrywide shaft alignment standards. If others want to weigh in on this perspective, we would welcome your thoughts. MT


David Zdrojewski is founder and CEO of VibrAlign, Inc., headquartered in Richmond, VA. Telephone: (804) 379-2250; e-mail: david.z@vibralign.com


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