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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)

Posted in Articles & Papers, Basics / Foundation, Diagnostics, Training, Vibration, Vibration: References | No Comments »

Les conseils d’expert (1)

Wednesday, February 6th, 2008

Lorsqu’en présence d’un moteur assez puissant, prenez gare à l’interaction entre l’aimant du capteur et le champ magnétique environnant ; elle peut mener à l’altération des mesures et dans les pires cas, peut conduire à un faux diagnostic. La présence ou l’importance de l’interaction peut s’évaluer en rapprochant le capteur de sa « cible » (sans avoir de contact) et en regardant en temps réel la forme d’onde temporelle ou le spectre FFT.

L’expert saura travailler aussi bien en échelle linéaire qu’en échelle logarithmique. Le linéaire présente un portrait analytique plus compréhensible (particulièrement pour le débutant), mais l’échelle log prendra toute son importance dans le diagnostic avancé ou pour les cas d’éloignement entre la sonde et la source.

Gardez toujours en tête que le spectre FFT est une réduction de l’information contenue dans la forme d’onde temporelle, et que la forme d’onde elle-même ne contient PAS obligatoirement toute l’information pertinente ou désirable, selon ses paramètres (sélectionnés par l’usager) relativement au phénomène mesuré.

Nettoyez régulièrement les aimants et assurez vous de la propreté de la cible (endroit du montage). Ne laissez pas non plus le câble ballotter au vent. N’imposez pas au câble de tension, et si vous devez absolument le faire, assurez vous que cette tension soit uniforme / constante.

L’aimant ne sert pas toujours : des matériaux non-magnétiques peuvent demander l’utilisation d’un contact direct entre capteur et point de mesure. Il est important de se rappeler qu’il est préférable d’éviter de d’exposer ses mains à de fortes amplitudes de vibration qui peuvent à la longue susciter des problèmes. Le port d’un gant ou d’un gant de caoutchouc réduira cette incidence dans la plupart des cas. C’est le HAV ou Hand-Arm Vibration.

Certains systèmes offrent l’interpolation et d’autres pas. Dans le cas d’une fenêtre Hanning (le choix habituel), l’erreur d’amplitude sur une crête dans le spectre peut atteindre 16%. Selon la position de la crête par rapport aux « lignes » du spectre FFT, l’erreur se situera donc entre 0% (une crête parfaitement centrée par rapport à la ligne) et 16% (une crête à cheval sur le seuil de deux lignes avoisinantes). Le spectre FFT interpolé élimine cette erreur. (Voir le conseil suivant)

Un moteur asynchrone (le plus commun) n’est presque JAMAIS véritablement à RPM fixe puisque le glissement dépend de la charge et la charge n’est que rarement parfaitement fixe. Donc, les crêtes se promènent sur une « toile de fond » fixe (les paramètres du spectre FFT). En balayant ainsi une petite plage de fréquence (selon les variations), la crête se déplace par rapport aux lignes et une amplitude CONSTANTE se PERCEVRA comme altérée : la crête change de place, et selon sa position en fréquence, l’erreur sera plus ou moins importante. Donc, d’une collecte à l’autre, les amplitudes montent et/ou descendent au selon la charge. L’interpolation (si elle est présente et qu’elle est utilisée) règle la question.

Dans le cas d’un test d’acceptation, hormis la calibration qui prend alors une importance particulière, les considérations énoncées dans les deux conseils précédents revêtent une importance toute particulière. L’interpolation pourra nous débarrasser de l’erreur de la fenêtre Hanning. Sinon, il faudra utiliser une fenêtre Flat Top pour s’assurer que les amplitudes (qui autrement seraient souvent altérées à la baisse) ne dépassent pas les seuils tolérés.

Posted in Condition Monitoring, Diagnostics, French / Français, Machine Types, Predictive Maintenance, Program Management, Surveillance de machines, Vibration | No Comments »

This is NOT a Guessing Game…

Friday, December 28th, 2007

…or at least it shouldn’t be! Vibration analysis corresponds to a science. Of course, the art and science of getting the proper measurement from the pertinent location(s) with adequate instruments before performing knowledgeable signal processing (if needed) also come into play, but the analytical part mostly relies on relatively simple notions or assumptions while reviewing fairly simple data.

We regularly hear about what we would deem failed predictive approaches where a lack of training or a lack of expertise seek dismantling and inspection of machinery components instead of proper analysis leading to an accurate diagnosis of the problem at hand. This approach amounts to bad (and costly) predictive maintenance management. It also favors extraneous manipulation of machine components best left alone when considering the potential human-error factor linked to intrusion (equivalent to resetting the bathtub or other curve to the infant mortality phase). Precision maintenance mitigates the previous factor, but then, it is a rare case indeed to see precision practices in a context where condition-monitoring fails to go hand-in-hand with proper diagnostics.

Posted in Condition Monitoring, Diagnostics, Maintenance, Predictive Maintenance, Program Management, Vibration | No Comments »

Vibration “Problems”

Friday, December 28th, 2007

Confusion reigns with respect to machinery vibration issues. Most people still hang on to vibration as a problem, instead of the diagnostic science of vibration as an indication of a developing problem.

Where does the difference lie? Vibration as a problem per say falls more in the AMPLITUDE analysis category, and remains blatantly obvious in most cases. Excessive vibration levels can be directly linked to unbalance, severe misalignment, resonance (or rotor critical speed problems) and other perceptibly notable behaviors, such as what might be exhibited by a machine or component having failed or well-advanced in its failure mode. Vibration as an indication of a problem tends to be a FREQUENCY (or time waveform contents) analysis issue. Of course, the appearance or growth of a small peak may bring us back into an amplitude context, and small amplitudes often show severe problems. Our vibration measurement will reveal anomalies or abnormalities that could yield catastrophes. The apparent insignificance of some amplitudes may well be linked to the location of the problem relative to measurement point, slow speeds, difficultly transmitted phenomena (ex: an incipient inner race problem must send the vibration wave through two thicknesses of lubricating fluid, above and below the rotating element which must also be traversed to then transmit to the outer race and subsequently, the pillow block: a long trajectory for a tiny peak lost amidst other events and noise) and other barriers to our perception. We’ll remind the reader that we advocate learning to look at data both in linear and logarithmic scales to familiarize and sharpen the analytical sense when interpreting more difficult or “veiled” problems.

The seasoned analysis veteran likely knew all of the above. But the newcomer or the merely distant onlooker (receiving reports as opposed to performing predictive tasks) may now realize that vibration is not just about crankshaft-like motion.

Posted in Condition Monitoring, Diagnostics, Machine Components, Maintenance, Predictive Maintenance, Vibration | No Comments »

Non-Interpolation

Wednesday, August 8th, 2007

An interpolated FFT spectrum benefits from amplitude (and possibly frequency) recalculations based on the leakage to either side of the FFT line (or bin) containing a peak. A peak located left of center causes the neighboring line to have more “stray” or leaked amplitude than the right-hand side line would, and the proportions of leakage will change as a peak’s frequency moves within a line (the latter should hopefully be recognized as the result of a variation in speed).

For very small RPM changes, the principal measurement comparative problem will arise from the change in position of the peaks relative to the FFT lines. On non-interpolated systems using a Hanning window, this may mean a rise or drop in amplitude of as much as 16% on some peaks, leading the analyst astray in overanalyzing an otherwise perfectly stable machine behavior.

In and of itself, the Hanning window only causes artificial reductions of amplitude, never an artificial increase. Since an FFT spectrum’s specific peak may already be affected by a drop, displacing it (when the RPM changes minutely) can provoke a rise as significant as the stated 16%. Interpolation resolves many of these issues and saves considerable time in restraining artificial variations. Averaging will also deliver better stability, albeit in a different context than what this article considers.

The reader will draw his own conclusions as to the value of interpolated versus non-interpolated monitoring data, whilst keeping in mind that there can occasionally be a need to refer to the unaltered FFT.

Posted in Condition Monitoring, Diagnostics, Vibration, Vibration: References | No Comments »

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.

Posted in Articles & Papers, Condition Monitoring, Diagnostics, Vibration | No Comments »

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).

Posted in Articles & Papers, Diagnostics, Phase Analysis, Training, Vibration, Vibration: References | No Comments »

Phase Analysis Proliferation…

Thursday, April 12th, 2007

…and lack thereof…

When doing open sessions for various others, I have at times sought to establish statistics on the percentage of people who performed phase analysis when they encountered a dominant 1X RPM or even a 2X RPM or 3X RPM predominance within developing “problems”.

I’ll admit that my initial attempts at arriving at an assessment seemed to return too low a number, so I eventually moved the “instructor asking questions” to a latter moment in a course, to benefit from the now warm feeling that we might share (a little humor and a ready openness tend to facilitate communications). However, to my great dismay, the previous statistics held and returned an equal answer of 15-17%…

Why is this worrisome? Most instructors spend time demonstrating not only the usefulness but also the absolute necessity of phase analysis. Yet, only 1 in 6 analyst / technicians / engineers seek to use this method to establish or confirm their findings?

Many participants repeatedly point to the difficulties in completing a phase analysis as a cause for their failure to execute the procedure. Progressive protective covering of any potential reference point and the quasi inevitable loss of any glued reflective tape certainly do form hurdles to measuring phase. However, by now, many vendors offer relative phase measurements between two channels off analyzers and data collectors; the excuses slowly evaporate.

One repetitive problem can disappear when performing phase confirmation: unbalance WILL generate 1X RPM vibration, but 1X vibration does not necessarily come from unbalance.

Other problems might yield that same 1X RPM vibration… That fact should always stay close at hand to pop up whenever 1X RPM amplitudes perk up.

The morrow should allow us to include a short but useful (or at least well-liked) methodical approach to phase as well as a basic interpretation of results.
© 2007 by François Gagnon

Posted in Condition Monitoring, Diagnostics, Phase Analysis, Predictive Maintenance, Training, Vibration: References | No Comments »

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).

Posted in Advanced Diagnostics, Articles & Papers, Multi-Channel, Structural, Vibration, Vibration: References | No Comments »

Frequency Mismatch

Wednesday, March 21st, 2007

Something that got on the radar again in the past few days: formula to calculate frequencies from various mechanisms, and mismatches occurring when confronted to reality.

Several items come to mind, not the least of which are the “supposedly” accurate bearing frequency formula. Why deride the end result? For one, the formulae used presume metal-to-metal contact, or attempt to compensate the presence of lubrication through consideration of the pitch diameter or value of the diameter measured between the central position (middle points) between races (inner and outer).

Does the center of the load-bearing elements correspond exactly with that Pitch diameter? It does not. Does the thickness of the lubricant film amount to something? We may neglect it, but it sometimes can play a role. Oil is thinner than grease, and where grease is concerned, the soap-base plays an important role in film thickness (as demonstrated by SNR conducted research; send email for paper). Of course, the effect of any sliding will outweigh considerations of film-thickness: overly light loading or dynamically contributing factors occasionally resolved as light loading (hence, a temporary light-load condition) will cause elements to slide and frequency values will either shift (considerably) or fluctuate (between a minimum and maximum value threshold). Installed bearings may not always conform to our asset list.

Pitch diameter is again used when dealing with sheaves or pulleys: “diameter of a sheave or drum measured from the center line of the cable wrapped around it”. Is that an accurate measurement or a useful estimate of values needed to determine belt speed, belt length, driven rotor RPM? We like to say estimate. Sheves wear down. So do belts (or they deform). V-belts will slip, often continuously. Review the definition of Pitch Diameter given in this case, and you will realize that even this “reference” varies over time. On chain drives, where the equations become an issue of links and sprocket (acting as a gear).

Line frequency may vary. North America and parts of Europe are well served in that respect, but it still has “end of the line” locations (the last plant downstream may get slightly altered power), local generation issues (small in-plant generation or limited network more susceptible to variations), and other areas may experience wider fluctuations. Since all induction motor speeds will vary accordingly (unless a VFD acts as compensator), most machines will also experience RPM and commensurate linked frequency shifts.

Posted in Bearings, Condition Monitoring, Diagnostics, Machine Components, Vibration | No Comments »

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