A Brief Treatise on Alignment Standards
Within the context of Reliable Manufacturing, precision maintenance is a key element. We must also remind ourselves that “precision alignment” is only a part of precision maintenance and there is a lot more to placing a machine in a precise state than just achieving a value of misalignment at the feet or the coupling. It includes proper identification and correction of a number of potential machinery errors including soft foot, thermal growth, base problems poor fits, assembly errors, etc.
There are a lot of misconceptions around alignment standards. The main arguments focus around whether we should use coupling values or foot values to determine if are within tolerance. There have been many statements published on the subject so let us review some…
1. “Use of foot values for alignment standards will not improve machine reliability”… a. This is quite simple. There is just one question you have to ask yourself…would the bearings and seals of a machine that is aligned to within precision foot tolerances (say 0.002”same sign) run smoother and last longer than one that is not. Yes or No?
I think most reasonable people would agree that the answer is yes.
We often see articles the state “the objective of alignment is to minimize radial forces at the coupling” we would disagree with that objective. When we study the actual causes of machinery failure, coupling failures do occur but they are nowhere near as common as bearings and seals. A better statement would be...”The objective of alignment is to extend machine life by minimizing the misalignment conditions at the bearings of the machine thereby extending the life of the machine”
2. “Companies will spend an enormous amount of time and labor going the extra mile to achieve this standard” a. This statement demonstrates a lack of understanding of the precision process and is an exaggeration of reality. We believe that we may have to spend some extra time performing pre-alignment checks and preparation work, but it is not an “enormous” amount of time. In most cases, much of the time expended in the preparation is returned when the actual alignment is performed in the form of repeatable readings and accurate moves.
For example, by taking time to clean and inspect the base and effectively identify and eliminate both short leg and angular soft foot so me major sources of frustration and error are “pre-eliminated” resulting in significant time savings. Another source of inconsistency is not using torque wrenches to tighten holddown bolts. Most reasonable people will agree that it does not take significantly longer to tighten a fastener with a torque wrench than it does with a regular wrench.
3. “The foot value method will frustrate and discourage aligners who are motivated to perform precision maintenance as very small measurement errors will result in large footvalue fluctuation” a. In this statement, we can find some agreement. Measurement errors will cause frustration and can certainly be discouraging. We should therefore do everything we can to minimize measurement errors.
Machine induced measurement errors can cause issues. As an example, let's adapt an old adage to our needs “measure twice move once.” In any alignment job, moving the machine is the most time-consuming part. So we should focus on making sure that the move we are about to make is the right move and when we make the move we move accurately. How many technicians sweep twice to verify repeatability before making a move on the machine? It is MUCH less time consuming to determine and rectify the cause of the inconsistency than move the machine based on “large foot value fluctuation.”
How many technicians adjust the sample rate and averaging of their device to minimize the effects of outside vibration? How many technicians take steps to minimize coupling backlash, excessive bearing clearance, etc.? How many technicians set up dial indicators at the feet to measure the actual move made at the feet rather than relying on a measurement device that is possibly several feet from where the move is being made? How many technicians routinely measure shims with a micrometer to validate the thickness and keep a register of the shims under each foot. In summary, we prefer to focus on eliminating the sources of measurement error rather than the effect that error is going to have on the alignment data.
To clarify the issue of precision tolerances…
To maximize the reliability of the equipment it is necessary to align the machines to a value at the feet within the bearing clearance tolerances. In the majority of equipment this will no more than 0.002” at the feet. It is equally important that the “sign” of the corrections be the same. This prevents any potential effects from offset at the coupling. This creates a precision alignment “zone of acceptance” as Illustrated by the green shading below.
The plot above illustrates the zone of acceptance. In this case the rear feet position is -2 and the front foot position is+1 therefore creating an off set of 3 mils at the coupling which would be unacceptable.
Of course it is also essential to understand that the above assumes that thermal growth has been compensated for.
In conclusion, it really comes down to the question of what is “acceptable,”“average,” or “ok” and what is considered “precision.”
We believe that the coupling tolerance values cited in various articles are the former and footvalues are the latter. When common machinery failures are investigated we find that bearings and seals are high on the list and couplings fairly low. By focusing our attention at the coupling by “minimizing radial forces at the coupling” we are ignoring the most common sources of machinery failure. As the machines run the shaft centerlines try to align themselves and the forces are felt at the bearings this is what reduces bearing life. Were it not for this inconvenient truth machines would rarely fail and we really would not need a laser alignment tool at all!
Let us not forget that achieving precision alignment is only part of placing a machine in a “precise” state. To truly ensure the best possible reliability performance we must also eliminate assembly errors, ensure the machine is balanced to a precision tolerance and designed and operated within its proper operating envelope.
Will you be able to achieve precision alignment tolerances every time you attempt an alignment?... of course not, but by making the attempt will you improve the machine?Absolutely
Placing a machine is a precise state is not always easy and we may not always achieve that goal but should we not attempt to do something because it is hard or difficult?
Reasonable people would agree that mediocrity should be the exception rather than the rule.