Archive for March, 2008

Reliability Challenges & Fundamentals

Sunday, March 16th, 2008

Numerous experts report the undeniable absolute necessity of management conviction and dedication for reliability efforts to result in a proper and sustained “field” (that would be the plant floor) implementation. No argument could possibly otherwise convince us or our readers. We agree on something.

Why then do we see a generalized backtracking to antiquated methods or unwitting feeble returns from reliability approaches? Where is this management commitment when reports from the field indicate reliable methods get implemented and then seem to vanish into thin air or quickly get set aside? Why is apparent regression almost inevitable?

What some may fail to mention is the capital importance of fundamentals. And these essentials may escape most of the “business model improvement” proponents due to a simple and quite ugly fact: the “business” people rarely speak with the technical personnel who get the maintenance train on track and keep it there. And when they do, as they are often used to speaking very different languages, these parties may have limited exchanges or communications due to the different focus or interests they manifest.

The communication gap may be further aggravated by a largely false perception in head offices that the technical people remain uneducated. Nowadays, their credentials and pertinent experience may even at times exceed those of decision makers. This is not to criticize anyone: the preceding merely attempts to state occasionally occurring (and previously encountered) circumstances.

The predictive maintenance and condition-based approach to maintenance and reliability are vital to success. Yet, these facets of maintenance and engineering have been in place for some time, with often poor to moderate results (the author is being kind; so many PdM or CBM programs are hobbled structurally and technically that their returns can only be qualified as hobbled). The promise of fabulous returns can not be met when the program is not “fed” properly: investment, education (training at ALL levels) and sensible recognition of efforts, to name but a few factors too often missing from the final recipe…

What makes you think that doing the same thing in the same way will now yield a better result? What will now be done to properly adjust the targeting? The liabilities (or problems) and personnel are the same. Or personnel keeps changing in turnstile-mode.

Of course, critics might glumly reply “Have you not been part of the system that created the ineffectiveness we must now endure?”, a pertinent albeit biased question. We propose a viable working platform or product to our clients. We might express it as “we sell a car”. The client then plays pick and choose, dismantling the platform to purchase (allegorically) the two left-side wheels, one rear bumper, and parts of the engine, instead of buying the whole functional vehicle. The reasons are many and they feel justified in this purchasing approach: they already have two right-side wheels (of a different size), one bumper and engine parts (likely mixing centimeters and inches), so they feel they can reassemble a working whole. The truth seems to point to “wishful thinking”.

An example? Some years back as I instructed a Vibration Analysis 1 course for one of the main vendors in our field of endeavor, two technicians showed up from an automotive parts manufacturing plant. They expressed their concerns relative to the course, having discussed with their supervisor what they felt was their need for training (they would have been more comfortable with an introductory course instead of the one they were now to attend). I assured them of my availability over lunch and after regular hours should they face serious hurdles, and also told them I ran a very open classroom where questions were welcome at any time. This can be helpful, but if fear of ridicule and timidity become the governing concern of the puzzled, our best efforts to remain available yield little in the way of improved comprehension. At any rate, they never availed themselves of the repeatedly tendered “extra” support or semi-private sessions.

Most Level 1 courses clearly state a need for 6 months prior experience and/or having attended an introductory class. These two participants were lacking in both respects. Net result: come evaluation time, these two felt that the vendor offering this training was only interested in revenue and they never should have been “allowed” to attend that course.

Everybody is entitled to an opinion. Ours would differ slightly: in the hopes of saving some training dollars, their supervisor sent these participants to a course that was above their current reach. In so doing, he wasted more training dollars than he saved. He also exposed his technicians to a little discouragement, and lower motivation due to perceived difficulties.

Could the problem have been resolved right then and there by using a different approach? Course contents did not lend itself to a quick readjustment: other participants quite liked that course and got what they came for, but then, they had the necessary bases to absorb the curriculum.

To the previous example, we should also add the unpopularity of introductory courses in certain areas of the world. The “client” (purchasing or management) feel them to be a reach for more $$$, when in fact, the participant get swamped with too much material when attending the “chosen” (not by them) higher-level course.

Worse yet, in those same areas, when a certification exam is faced at the end of a week’s training, some participants have at times been warned that failure would equate dismissal. Sitting (or writing) an exam that is meant to cull the untrained may dauntingly challenge the neophyte.

Another sad hurdle to reliability excellence remains the discrepancy in pay rate and benefits afforded those who practice it adequately. Sooner or later, if competent technical personnel find they may be given better opportunities elsewhere, they will jump ship and seek out better climes. There are management or HR theories about pay scales and bonuses (this writer will purposely stay away from them as readers might be offended by psychological aspects of remuneration), but an occasional carrot, such as two checks waiting in the wings (gold and silver medals, if you wish) might yield surprising competitive benefits. Or setting PdM program goals and tying a bonus to meeting those targets.

Proactive engineering steps used hand in hand with precision maintenance yield better reliability. The latter is undeniable. Yet, without a strong predictive program, above and beyond any scheduled preventive replacements (likely arising from an RCM study) and inspections, production and asset reliability will remain out of reach.

PdM / CBM or Condition-Monitoring ARE the technical foundation that can be encouraged, but not managed at any level other than that of maintenance and engineering. In other words, if reliability is a fortification against trouble and decay and a guarantee of productivity, the walls or battlements depend on solid groundwork called predictive maintenance.

Simply put, Asset Reliability (and maintenance) can not be driven by failure-detection if predictive efforts remain incapable of perceiving faults.

© 2008 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