Archive for the ‘Articles & Papers’ Category

Change Culture Club (1), or Consultancy and Consensus

Monday, February 15th, 2010

WARNING: the reader is forewarned that while revealing to some degree, this article may confront endemic attitudes and be difficult to accept. This being said, like many other issues, not talking about them doesn’t make them miraculously go away. So we hope you can enjoy this minor brainstorm in spite of the disturbances it may bring. Triggering the right questions or a little introspection may sometimes be helpful.

I was once told that the percentage of success of personal therapies lies close to 18%, although I’ll admit I have never sought the truth of the matter (I do recall conversations with a former “significant other” specialized in this area of endeavor, and the gist of these conversations seemed to concur with the appallingly low success rate). Apparently, tribal witch doctors embarked on a similar mission to help an individual seem to achieve a success rate of… 18%… Certain prescription drug assisted therapies are far more successful for specific conditions, but as a whole, the above seems to hold water.

It would be easy to conclude that there may be a repetitive pattern in this accomplishment: therapy seeks to trigger or support change, and in this respect, the therapist’s work parallels the approach of the business or engineering consultant.

How many psychiatrists does it take to change a light bulb? Only one, BUT… the light bulb REALLY, REALLY has to WANT to CHANGE!!! And with that relatively stale joke, we uncover one of the great secrets of reliability consultancy: your consultant(s) will not change anything, YOU will! Well, actually, you might! But ONLY if doggedly determined to achieve a continuous improvement loop and/or specific targeted goals.

It would be tempting to jump to the conclusion that the consulting firm therefore serves little or no purpose. Here, then, is another rule of consultancy: if you could, you would! The stern review of current activities, the guidelines to establish a road map for change, the assessment of best and quickest results to be obtained (also called low lying fruit), the benchmark for exemplary or best practices, the traps and pitfalls to avoid, the identification of the areas where MORE money should be spent (and conversely, areas where money can be saved because it is an ineffective or inefficient use of resources), all of those items pertain to the expertise of said consultant.

Many firms choose to sugar-coat everything. Here is another rule: if you HAVE to SUGAR COAT it, the organization likely has not attained the maturity that will allow it to break its previous bad habits!Some years back, one consulting firm’s VP put it succinctly when the author raised concerns in one project: “Doesn’t matter! We get out fat fee anyway, and may get to do it again at the same location in the future…” Perplexed? Dubious? I agree, because so was this wincing writer! We all need the money (the rent, the mortgage, the business expenses), but must we be so crass and uncaring about it? This being said, the blunt reality must be addressed, but doing so with good humor and a view to how best to promote change remains inherent to our mission.

A recent challenge to our consulting wisdom was issued abroad where management customarily relies on a consensus culture. The rule applies: if you could, you would! Therefore, if they had been able to promote needed changes, or had they been capable under the current hierarchy, organization, system and culture to deliver (product, improvement, KPI, other) on target, we would not have been involved.

And Consensus?
Too many cooks… Imagine if the purchase of a car were open to negotiation or discussion for each and every component of that car. The process would be laborious and tedious, and the end result would quite likely be a monstrosity.

Somewhere along the line, the “car” would surely become a “vehicle” and every possible function of that car within a family, business or industrial context would be considered by participants. The size and footprint of small cars is interesting and so is their energy efficiency, but that shovel on the payloader is “oh so very useful”… What do you mean by “the shovel is heavier than the car”. Never mind the hydraulics, we’ll just leave the shovel at ground level all the time. Or install hydraulic stabilizer legs on the car to stabilize it when we lift the shovel. This is the underlying cause to the “camel” joke: a horse designed by a committee. And perhaps this writer can raise a few smiles by latching on to one of the Hispanic definitions of camel: difficult, laborious, improbable to bring to term. Clearly not our target!

Consensus implies input from various parties, and discussion. Some people will lend tremendous ideas to the process and will definitely have an impact on the end result. Some participants around the table, or involved at arm’s length (teleconferencing, minutes of the meeting, email distribution) will lend a cursory opinion, and no more. And someone will have to choose a course of action and take a decision. Managing is not a democracy!

A brief parenthesis about democracies, since I mentioned cars earlier: one individual had several daughters and to cut short “consensus” issues on car color, each daughter in turn chose the color of the family car. That is how the poor man ended up in a lemon yellow car for a few years.

Back to our main topic:

Seeking consensus about change ignores some basic principles of human psychology. We tend to resist change. We tend to reject it if we can. There is strong criticism whenever change is proposed, and such may be brushed aside as an agent of resistance to change. Yet, change for the sake of change serves no purpose.

What do you seek to accomplish? As consultants, we have waltzed in so many places where metrics (KPI or otherwise) were weak, distorted or inexistent at the outset. Can we rely on bad data? Can we nail our targets down when the underpinning of information paints an inaccurate picture of what we want to change?

In fact, we can, but it also becomes part of the mission to get the numbers to truly reflect the reality of the situation. Often times, a client would like the current numbers to become the basis for payment. This is feasible, but rife with problems when the numbers have been manipulated.

To be continued…

© 2009 by François Gagnon

Sources of Vibration

Monday, March 3rd, 2008

Where does vibration come from? What causes it? Any force changing in direction, changing in magnitude or any active component interaction generating a rub even as minimal as friction within rolling element bearings or moving fluids within pipes will cause vibration movement of some small to intolerable amplitude. Any combination of the previous would generate more complex vibration patterns. In practical terms, this means a long list of potential sources whenever an analyst tries to quantify and/or decompose machinery and structure behavior.

The (presumably, unless something escapes this writer) full list breaks down into:

Rotor related sources (rotor behavior and forces at play)

- unbalance

- static or force, couple, dynamic or combined

- misalignment, residual, growth-related or dynamic (reactionary)

- angular, parallel and combined

- bent shaft

- rubs

- macro rub

- rotor-casing

- rotor-product or moved fluid (when dense or solid)

- shaft-seal

- shaft-sleeve bearing

- gear coupling-jaws

- improperly lubricated gears

- micro rub

- belt-pulley

- sliding (instead of rolling) elements

- rolling elements themselves onto raceways (through lubricant film)

- pumping action or film-creation action within the bearing

- rolling elements and retaining cage

- fluid and casing or piping

- eccentricity

- whirl (of shaft or of the shaft within a fluid-film bearing) and whip

- gyroscopic effects (fairly common in inertia wheels)

- torsional vibration


Process related sources (what the machine does)

- hydraulic or aerodynamic forces

- blade / vane pass (BPF) or blade rate (BRF) vibration

- eccentricity relative to casing

- rotor mesh (lobes or screws)

- cavitation, recirculation, turbulence, stonewalling, surging, stalling

- compressed fluid wave or pulse (pipes, etc), turbulent flow

- reciprocating forces (can come close to shock)

- shock (presses, etc)

Drive related sources
Electromagnetic torque pulses from a motor and the interaction between armature and stator components a.k.a. rotor-bar and slot pass frequency
Pulses used to recreate Variable Frequency altered electrical line frequency
Pole or coil pass frequency

Magnetic centering
Coupling problems (lock-up, wear, etc)
Belt or chain drive w. eccentricity, belt problems, belt slip, cogs
Gear mesh and gear problem related vibration
Couple transmission discontinuities or variability from load transmission from one tooth to the next (simplified to picture the matter)
Hydraulic or aerodynamic forces from gas, steam or water turbines
BPF (Blade Pass Frequency) from turbines
BRF (Blade Rate) interaction between blades and stationary elements
Reciprocating forces from engines, unbalanced cam action

Other factors may cause the appearance or perception of an excessive vibration problem, but they are REACTIONS to one or more of the above sources as opposed to being themselves a source of vibration. The most easily recognized of these are looseness and resonance. Critical speeds of a rotor could be assigned to resonance for purposes of this listing. Oil whirl on sleeve bearings also fit into this category: it is an instability reaction due to lack of static load or dynamic loads overcoming static load. In this latter case, increased tolerances facilitate overcoming the static load. From the point of view of reactions, we can also add:

Reactive Problems

- dynamic misalignment (occurring from loads moving component positions)

- resonance

- looseness (really a change in response due to lessened stiffness)

- instability (rotor, hydraulic, oil film, other)

- base and retaining / supporting mechanism or structural problems

- distortion from pipe strain

- transmitted vibration (structure borne transmission from other nearby machines)

- acoustic excitation (airborne or structure borne noise exciting a plate, casing or other component; structure borne usually or closely falls under transmitted vibration)

 

© 2005, 2006, 2007, 2008 by François Gagnon

PV = ZnRT, or Why Do So Many Oversimplify?

Thursday, October 4th, 2007

This example always stood out (in this writer’s view) as a prime example of what we should never do in teaching / training / instruction. We first come across PV = nRT at the age of 12 (or so it was in my case; no clue as to nowadays). It is only years later that we learn that there is a factor Z, usually equal to 1.0 but variable when pressures are extreme (at both ends, the low and the high), at which point the factor must be obtained from curves specific to each gas.

Was there a significant hurdle to telling us about this nuance? What was so difficult with “usually equal to 1 unless pressures are very low or very high, but those cases will be dealt with later in your lives”?

Blanket statements or misguided oversimplification can be equally annoying when receiving instruction. They provide “false advice” or “false statements” that may lead to error. In the meantime, yes, we should strive to lay down a proper foundation for comprehension, BUT instructors should simultaneously avoid patronizing their students (or seminar participants) through a misguided presumption of lack of understanding or shortsightedness. Without delving into all the details, we CAN and should tell it like it is, while concentrating on the building blocks that will lead to a comprehension of the issues at hand, while SOME participants called to more complex tasks will be better armed to face the music!

© 2007 by François Gagnon

Causal Relationships

Monday, June 25th, 2007

Formidable as our diagnostic sciences may be, we often sublimate statements necessary for a full comprehension of mechanisms at play. (This is likely a wonderful spot for readers to interject “What’s he rambling on about?”)

Everybody recognizes the half-order frequency inherent to a rub phenomenon. Subtle difference from the usual statement one might have heard, the previous sentence stipulates “half-ORDER”, not half-RPM. The reason why should strike us by its evidence and simplicity: analysts link the 1/2X consideration to rotors (and even flexible rotors). Yet, within a gear context, when might we suddenly see the appearance of 1/2X? Apply it to some other mechanism: a gear “contact” (through the lubricant film) can be described as sliding-rotating-sliding, while the presence of looseness at the bearing lodging or the shaft journal will cause a potential rub reaction at the bearing position itself or at the gears. Thus, 1/2X suddenly applies to the gearmesh. And 1/2 GMF manifests a presence in spectral data.

There are a number of other instances where the rub component of a phenomenon will cause a “half” frequency to appear. Looseness left to grow uncontrolled will allow rubbing of sorts, for one.

© 2007 by François Gagnon 

Machinery Condition Monitoring: the Beauty of Simplicity

Sunday, May 6th, 2007

More often than not, the levels of complexity achieved when defining monitoring parameters for any machine type have been excessive to achieve the minimum level of “sensitivity” we seek within a detection context. The previous should also bring to mind the historical aspects of machine asset CM management: this complexity extends to the collection sequence (whether it is visible or not), the programming of the sequence, the handheld instrument processing, and it also spreads to data review. 

A simple trend of one parameter or another typically fails to adequately paint a picture of machine health. Neither velocity nor acceleration will succeed in that respect. Changing units from peak to RMS or the reverse will also fail in providing an efficient detection net. What kind of SIMPLE safety net will mostly be up to the task? Do note the caveat: “mostly”, as some problems would still likely escape our notice…

Imagine a system delivering the following simultaneous trends:

Velocity pk (time waveform derived or true peak)

Acceleration pk (time waveform derived or true peak)

Crest Factor for either of the previous

High-Frequency / Ultrasonic “Estimator” (meaning Spike Energy, PeakVue or similar; these last are trademarks of their respective owners).

Such a system would not function for ALL machines and configurations, but on most common configurations, it would adequately cover the detection function for most problems occurring within the machine park. It still would not be perfect. We can find a number of cases for electric motors where the previous system would leave gaps. Certain machines would also benefit from a trend of displacement and/or a correlate to measurements off proximity probes.

For many years now, single parameter pen-like mini-instruments have been a popular item. While the intent was likely to quick check a machine just worked on, some pre CM “entry level” shops or plants may partly or wholly rely on such overly simplistic single-parameter trends. Vendors might provide a better “view” through the inclusion within their ”simplest” tool of multi-parameter trending as suggested above. It would come closer to adequate monitoring. This being said, the detection safety net based on correlated trends of multiple parameters provides helpful coverage in a “wide” net: this applies to a large number of machine assets such as what is found in a paper mill, and not so much to a power generation station where the assets are few, usually complex, and allow for wider man-hour resources in terms of analytical exploration.

© 2007 by François Gagnon  

Topic Suggestions & Requests

Tuesday, May 1st, 2007

Time tends to be much shorter or even altogether lacking these days to produce (or adapt) my writings to the website. Nonetheless, the field of reliability and vibration remains as wide as ever, and still largely unexplored for many. In that spirit, let us ask “Any specific topics you might like to see discussed in Reliability Bytes™ and / or in our next papers?”

Of course, if you have an answer but you fail to send it in, we shall remain in darkness and could not then aptly provide a discussion… Try fgagnon@vibra-k.com to send in any good ideas…

Phase Analysis: A Simple and Methodical Approach

Sunday, April 15th, 2007

For those who have experienced problems in performing the analytical potion of phase analysis, a simple document that has been popular over the years (many copies of this have been distributed over three continents).

phasethmb.jpg

Un Acercamiento Metódico Hacia la Fase

Friday, April 13th, 2007

Análisis de Fase Según Vibra-K

Zeroing In on a Reference: Complex Spectrum Derived ODS

Wednesday, April 11th, 2007

Operating Deflection Shape can be completed using a phase collection method such as a laser or photocell reference, but is usually best or easiest when the method relies on complex spectrum collection.

The underlying reason strikes us as remarkably simple: a system collecting amplitude and relative phase data for orders of running speed (typically 10)usually provides little data until download, whereas the complex spectra can be observed and readily reviewed for any anomaly potentially affecting the data and should any reactions be present relative to a frequency other than synchronous (perhaps excited from a nearby source), the relative responses can also be plotted out and/or animated.

Complex spectra force us to choose a reference location where one accelerometer will remain while the other(s) roam. Other(s): single when dual-channeled, but plural when using more than a two-channel instrument.

Where is the best spot? For the reference location to be useful throughout collection, it must obviously provide a usable reference for EACH frequency of interest. The best procedure therefore calls for an initial data collection (most likely only partial in scope) to determine where all frequencies are present with significant amplitudes and stable, repeatable relative phase. From such a reference, creating the animation will impose fewer hurdles in terms of accuracy.

An ODS remains naught but an animated representation of a phase analysis. In that sense, it is eye-candy… Admitedly useful eye-candy, but still… The animation does not contain any information above and beyond what can be extracted from the comparative study of the various signals / measurements (when carefully / adequately) collected on the structure or machine. It does provide a quick view of aberrant relative behavior, if the animation is observed at the frequency or frequencies for which abnormal responses are present. Essentially, an ODS renders visible and comprehensible phenomena which might otherwise mystify a client (in the large sense of the word: the plant manager might be maintenance’s client for purposes of comprehending structural work).

© 2007 by François Gagnon 

Maintenance & Misalignment: Vibration or Not?

Monday, March 19th, 2007

Misalignment is a functional condition: it is not just a matter of completing a perfect cold alignment, as we all know. Offsets need to be factored in to accommodate thermal growth or rather, thermal growth differential between two machine train components. If that last factor affects alignment precision, there are others.

Dynamic reaction, often neglected, can play an important role: a machine “sits” differently when it delivers (motor) a 500 HP (for example) couple or receives it (driven rotor). Working stresses change the equation: a contracting or expanding pipe will push/pull on one component, or twist it out of shape. And finally, a significant unbalance will create a dynamic, temporary, acting misalignment: the center of a shaft moving by 4 Mils p-p can hardly line up with its counterpart. Obviously, any condition resulting in driver or driven overreaction (in terms of vibration) also resolves itself into misalignment.

The short catalog of causes (which we will likely explain at length in future articles) often explain why the millwright staring at alignment correction with work order in hand may feel a sudden wrath when thinking about his colleague the analyst: with the best of intentions and the machine still warm, no alignment fault seems present, so our poor millwright feels like someone is not performing properly. This can lead to credibility issues within maintenance.

Another item tied to misalignment: the case of the misalignment-induced energy-loss. One paper reported there were no such mechanical losses from misalignment. We will protect the innocent, and prove them wrong: whenever an asynchronous AC electric-motor vibrates, two things occur, in varying degrees: the RPM DROPS slightly, and the amperage draw rises slightly. This is common knowledge, but not necessarily common to just anyone as extensive experience with balancing and other field procedures should likely be common trait of those who know this for a fact.

In the test for misalignment, the professor(s) who controlled the experiment did not propose to simultaneously monitor the vibration to the amount of misalignment. No one ever suggested that misalignment was the loss by itself: if misalignment fails to produce mechanical energy losses through vibration, the misalignment is resolved as an applied force without any movement. Instead, they merely monitored the output through a dynamometer, without measuring the input.

Whosoever says “force applied without any movement” quickly realizes that no mechanical work (our old friend W = F d) is being performed (since d = 0, or close enough) and thus, no great energy losses occur. Simple! But when will we have a misalignment incapable of producing vibration? Given a relatively stiff structure for a small test bench, the dynamic stiffness could be important enough (low mobility, or very small responses per force unit) as to prevent any such vibration and losses. As a note, yes, the author is aware of rotational equations. Just consider Work along the horizontal axis for the sake of simplicity.

It should thus become clear that as a protocol to confirm or invalidate any misalignment-borne losses, the chosen approach lacked scientific rigor. The previous did not consider the possible pernicious losses within the electro-magnetic system should any air-gap variation effect come into play. In fact, for a very small motor, the probability of such losses becomes negligible, and their cumulative effect would be trifling should they be manifest.

© 2007 by François Gagnon

Off the “Maintenance Forum”, “ElectricPete” had a question: “I have never heard this before and at first glance I’m skeptical, but always interested to learn. Do you have any additional information on this? (For example: Any example of how much change you have seen? Any reference to discuss this? Any possible explanation for the physical mechanism?)”

FG replies:
The observation is purely empirical, over the balancing of hundreds of rotors, and really ties in best with the reverse of my statement…

Let me explain: while balancing, the slight RPM variation is typically known in high-precision work as a source of error. Not a huge one by any account, but still present. If using a balancer with a photocell or lasertach, the increase in RPM can be notable as one alleviates the 1X vibration amplitude. In lighter cases, it may only be beyond the decimal point. Worst case I’ve ever noted: abnormally vibrating (0.7 ips pk or more)light rotor (boiler fan) mounted directly on 2-pole motor shaft, 45 RPM variation from start to finish. Usual variation will usually be between 2 and 10 RPM (4-5 is a good target).

Obviously, VFDs will obliterate signs of this if the control is sophisticated and the RPM fixed to a preset: you’ll go right back to “nominal” RPM = last setting (within control precision).

The mechanical energy wasted by unwanted motion must come from somewhere… For the motor, there are only two possible variations to consume energy away from the main task and dedicate it to “shaking”: RPM and power consumption.

For RPM, the “hit” is also power consumption. For purposes of this example, I’ll use “neverland” RPMs (reality never has 1800 RPM unless synchro or VFD, and this neglects losses): take a 200HP, 4-pole motor at 1800 RPM, reduced (gears) to 900 RPM. What is the power output? 400HP. A drop of 9 RPM on that motor considered in a different application would account for (close to) 10 HP, if the relationship were linear… (9 for 1800, or 5% of speed applied to 200HP). Loose terms. This is not a thesis… The previous merely demonstrates the link between total power and RPM x HP. And we’re still constructing the example.

Consider the total power delivered by the motor, and the total amount of slip: if the motor delivers 200 HP while slipping by 20 RPM (1780 RPM rotor in relation to 1800 RPM magnetic field), the amount of slip can serve as a quasi linear power scale: 2 RPM of slip corresponds (in this case, for this motor) to 20 HP. Thus, for this motor, an extra 9 RPM of slip would represent a considerable amount or 45% of total motor power: the motor running at 1771 RPM would obviously be under overloaded strain as it would be delivering 290 HP. No need to mention the presumption of all other factors being constant and adequate (stable 60Hz line, normal voltage fed to motor at lugs, no insanely inappropriate environment temperature, etc). In this case, unbalance responsible for a 2 RPM drop means that 20 HP are used up in shaking the structure.

Do take note that my observation predates the wide adoption of high-efficiency motors. Namely because no one has wanted to pay our fees to cover balancing tasks in quite some time :D Thus, I can not positively affirm same behavior, but one could suppose same principles apply.

Being aware of the clauses of industrial power supply contracts (expected levels, penalties for spikes above steady consumption, and the like) and given our care to often seek better documentation of exact costs, not to mention the sensitive units, we had monitored many motors during balancing procedures.

Observations often placed Amp draw difference at fractional to 5 A, depending on motor. One of these days, on a large synchro motor, I’d like to get the exact power consumption before and after a balance job, but it’ll likely be as an observer busy on other items.

© 2007 by François Gagnon