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Induced Force & Freedom for Movement

While tackling vibration problems (most machinery problems are oscillatory in nature) it is important to grasp the idea — “What causes vibration?”

The answer in its simplest form consists of two parts, which are: –

  1. Induced Force
  2. Freedom for Movement

We can say, that when we put these two phenomena into a relationship or when we discover a pattern involving the two phenomena, we have effectively understood the essence of a vibration problem in order to solve it or improve the situation. Without the “induced force” a piece of machinery would not continue to vibrate. And without “freedom for movement” machines would not vibrate either. Both must be present for a machine to continue to vibrate.

However, I find that students of vibration analysis often face difficulty in understanding these two related phenomenon and have a hard time linking them into a coherent pattern exhibited by a vibration problem.

So, I would first try to explain the phenomenon of “induced force.”

There are many ways of classifying vibrations. Vibrations patterns are also described depending on how they are induced. This is an important way of classifying vibration since the cause of vibration can be easily understood from such classification.

For instance, a shop floor may vibrate when a machine is switched on. Or an adjacent machine or structure may vibrate when another machine on the same floor is running. This would be called machinery induced vibration.

Similarly, a bridge or a tower may be subjected to strong winds causing those to vibrate. In that case, it would be called wind induced vibration.

Or for example, a pipe carrying fluid in a power plant or a pump may be subjected to flow induced vibration. Common problems of pumps like cavitation, re-circulation, erosion and water hammer are all examples of flow induced vibration.

Likewise, unusual vibration of an anti-friction bearing may be induced by electromagnetic forces emanating from electrical cables. We would say that the bearing is subjected to electromagnetic induced vibration.

Similarly, vibration of machines, buildings, towers, bridges can be blast induced owing to sudden application of explosive forces, like the way it happens in mining industry.

In the case of earthquakes, bridges and towers are subjected to ground induced vibrations.

We may think of “induced force” as the necessary stimulus imposed on a structure that forces it to vibrate. Structure, from the vibration point of view, may be a piece of machine, building, tower, pipe, bearings, foundation — or simply anything that has stiffness and mass.

However, a structure would only vibrate or continue to vibrate if it has freedom to move. A machine can move in many directions provided it is allowed to do so. More the number of directions a machine is allowed to move more difficult it becomes to understand a problem. However, the question is “How do we know a machine’s Freedom to move?”

One easy way to find it out is by finding the number of natural frequencies exhibited by the machine. This may be effectively found out by conducting a “bump test” on the machine where the number of natural frequencies show up on the frequency spectrum. The number of natural frequencies is just equal to the number of directions a machine is free to move. For example, if a machine has five natural frequencies within the operating range that consists of the operating speed and its harmonics then the machine is free to move in five different directions.

So, when we know the nature of the induced force and the number of directions a machine is likely to move, we may then try to find the proper relationship between the two phenomena to complete our understanding of the essence of a vibration problem. Once such relationship is understood the solution(s) to a problem is self evident.

 

 

 

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Learning Vibration Analysis

Every year we gather at NTPC, Noida, for our animated dialog on real life Vibration problems. This year there were 39 of us happily engaged for four fun filled days. It is a type of annual conference where engineers and practicing vibration specialists across the country come together to interact, exchange and learn from each other.

This year, the workshop was designed differently. We gently moved away from the traditional methods of vibration analysis and instead emphasized the application of complexity science in analyzing system problems through vibration patterns. I think this approach is the first of its kind in the world.

So, what was new?

First, only cases from the real world of engineering were discussed and explored. Twenty cases were discussed. Each case was unique. They were something like Zen koans waiting to be cracked for enlightenment.

Why?

There are two sides to reality. One is the phenomenal one — what we can sense. The other is the essential one — what we can’t “see” through our senses. The phenomenal side manifest as events that we experience while the essential side provides the cause that precipitates such events. Problems of vibration offer us the opportunity to explore both sides of reality. Through measurements, we can easily see the phenomenal one (the degrees of freedom, amounts of vibration and their frequencies) — that is all about sensing oscillatory movement and its nature. But to understand the cause of vibration we must be able to “see” the essential part of reality – what induces vibration?

The cases forced the participants (practicing specialists) to take multiple takes and interpretations of the cause of vibration before the reason finally clicked. Initially, each case left the participants perplexed.They sort of provided the proverbial “whack” on the head for realization to dawn.

Why is this so? Cracking one problem does not ensure that the next problem can be solved by following the same method. If one tries to use the same method that helped one to solve a problem one has to use thoughts and concepts culled through previous experiences. By trying to apply a standard method and tactic one can’t see the essential part of the reality, which often proves to be a frustrating experience. Any effort to solve a vibration problem with a standard approach ties up a practitioner in knots. Not surprisingly, even vibration specialists find vibration problems paradoxical. They are paradoxical in the sense that seemingly logical, rational and conceptual thinking held in the minds of a practitioner are challenged when dealing with vibration problems.

Therefore, for each case, the essential part — the induced cause(s) — had to be built separately — bit by bit — connecting one bit to the other till the essential nature of the problem was self evident.

At the end of the four days the participants were left smiling, relieved to know that they need not remember any standard method or approach or a formula to tackle vibration problems — more so, for the most complex ones. They only need to see through a problem with patience or perseverance to develop deep intuitive capability, which would then help them see through the essential nature of any real life vibration problem quickly and accurately.

On the whole it was great fun and we all basked in the enjoyment.

 

Note: In conducting this course, I was helped by Mr. Anil Sahu, my co-facilitator. He had a bunch of paradoxical cases to share.

The Case of Burning BagHouse Filters

Recently I was invited to investigate a case of frequent burning of baghouse filter bags.

There were five such baghouses connected to five furnaces of a steel plant.

The client reasoned that the material of the bags was not suitable for the temperature of the gas it handled. However, with change of material the frequency of bag burning did not change. So it needed a different approach to home onto the reasons for the failures.

Hence, this is how I went about solving the case:

First I did a Weibull analysis of the failures. Engineers use Weibull distribution to quickly find out the failure pattern of a system. Once such a pattern is obtained an engineer can then go deeper in studying the probability distribution function (pdf). Such a pdf provides an engineer with many important clues. The most important clue it provides is the reason for such repeated failures, which are broadly classified as follows:

  1. Design related causes
  2. Operation and Maintenance related causes
  3. Age related causes.

In this case it turned out to be a combination of Design and Age related causes.

It was a vital clue that then guided me to look deeper to isolate the design and age related factors affecting the system.

I then did a modified FMEA (Failure Mode and Effect Analysis) for the two causes.

The FMEA revealed many inherent imperfections that were related to either design or aging.

Broadly, the causes were:

  1. Inability of the FD cooler (Forced Draft cooler) to take out excess heat up to the design limit before allowing the hot gas to enter the bag house.
  2. Inappropriate sequence of cleaning of the bag filters. It was out of sync with the operational sequence thus allowing relatively hot dust to build up on the surface of the bags.

Next, the maintenance plan was reviewed. The method used was Review of Equipment Maintenance (REM). The goal of such a review is to find maintenance tasks that are either missing or redundant for which new tasks are either added/deleted or modified. With such modification of the maintenance plan the aim is to achieve a balance between tasks that help find out incipient signals of deterioration and tasks that would help maintain longevity and stability of the system for a desired period of time.

Finally the investigation was wrapped up by formulating the Task Implementation Plan (TIP). It comprised of 13 broad tasks that were then broken up into more than 100 sub-tasks with scheduled dates for completion and accountability.

 

Failure Mode of SS 304 & its prevention

Chromium in concentrations above 12% renders steel stainless. 304 stainless steel (SS), contains 19% Cr and 9% Ni. The function of nickel is to strengthen the alloy and to provide greater corrosion resistance. Unfortunately, if the steel contains nearly 0.1% residual carbon from the iron and steel-making processes then the chromium in the SS has a strong affinity for carbon and slow cooling through the red heat range allows chromium carbides to nucleate heterogeneously on the grain boundaries. The adjacent regions in the grains are depleted of chromium to far below the 12% threshold and are no longer as corrosion resistant. Thus, the steel is said to be “sensitized” and is susceptible to intergranular corrosive attack. The slow cool after the welding of the 304 SS allows such precipitation and triggers sensitization. This is technically known as the sensitization temperature of SS which is around 650 degrees C (ball park figure). It can get activated during welding or due the application like using 304 SS in very hot working conditions.

 

Even when sensitized, the steel is adequate for many applications, such as household products (SS utensils that we use on the gas ovens) and even containers for less concentrated nitric acid. However, while sensitized steel is adequate for 75% nitric acid, it could not be used for the 90% solution.

Whenever corrosive attack on 304 grade of SS is the predominant failure mode it might be prevented in several ways. For example, low carbon stainless steel, designated as 304L could be used. Or, addition of niobium during steel making would tie up the carbon as fine, harmless intragranular niobium carbides. Alternatively, we can anneal, especially welded joints on 304 (if possible — size of the furnace often becomes a constraint) at a bright-red heat to dissolve the carbides and then water quench to prevent their re-nucleation. Any of these techniques can be effective, but the additional cost has to taken into account.

Negative Stiffness and Instability

The idea of negative stiffness might appear at first glance to be counter intuitive but that is what happens often to machinery in the real world. When such a phenomenon strikes, it triggers rapid deterioration of a machinery system.

Positive stiffness is a material property that tries to resist a force when applied on to a material, i.e. it tries to push back the force.

On the other hand, negative stiffness is a property that amplifies the deformation of a material when force is applied to a body.

Since such amplification is non-linear by nature it forces the system to go far away from its equilibrium position. When it does so, the system become unstable and as deformation quickly increases, the system fails.

The best possible way to detect the sudden appearance of negative stiffness is to monitor the displacement parameter of vibration. This is because displacement is related to stiffness. When displacement increases disproportionately without a corresponding increase in velocity parameter we would know that the phenomenon is that of negative stiffness. In addition, we might also notice a variation in the displacement readings. They don’t tend to settle to a steady state.

Negative stiffness might occur in many ways. It may happen when interference or push fitted elements come loose. It often happens to elements that are deformed over time like foundation supports or are pre-stressed like anti-friction bearings. It might happen when elements are worn out by a certain extent by different wear processes like corrosion or abrasive wear.

But in all cases, a very small change induces a system with positive stiffness to flip to negative stiffness, causing catastrophic damages and failures.

 

Note:

For a lucid understanding of the nature of negative stiffness you may refer to this article.

For an understanding of negative stiffness and isolation you may see this.

Natural Frequency – Whole is not equal to its Parts

The concept that a system behaves differently than its parts is a concept which many often find it difficult to understand. By our education and training we are naturally prone to look at parts rather than the system . The idea is that the property of a system is distinctly different to the individual properties of its parts.

Let us illustrate this concept that a system would always behave differently than its parts through a simple example. We would take a physical parameter and then see how this physical parameter, which is a distinct property of a part, would change as soon as two parts are made to work together. For our purpose we would take ‘natural frequency’ as a physical parameter and see how it changes. And we would take the turbine-generator combination — a prime example  to illustrate this phenomenon of system behaviour.

The first natural frequency of the turbine, which is a property of the turbine, when taken alone is 2433 cycles per minute (cpm). Similarly the first natural frequency of the generator, when taken alone is 2124 cycles per minute (cpm).

What happens when we connect the turbine and the generator together through a coupling and make them work together? The natural frequencies of both the turbine and that of the generator would change altogether! Remember that natural frequency is a distinct property of a part.

The combined natural frequency changes in the following manner:

The first natural frequency of the turbine becomes 2028 cpm (drops by 17%) while that of the generator becomes 1806 cpm (drops by 15%). So the first natural frequency of the parts have changed as soon as the parts were put together.

Hence, it is always true that the WHOLE is never equal to its PARTS. Therefore, while designing, maintaining or operating or examining a problem, fault or a failure we would always gain a better understanding by observing a system as a whole and not its parts.

The Greatest Crisis that we face today..

This morning the following email popped up in my inbox. I was completely surprised to get this from a colleague who is now at the peak of his career.

It is an honest assessment of his career over the last two decades. I find it simply beautiful. So smooth and easy. It makes me happy to have in some ways touched a life. It is so thrilling. I assure you — it is as thrilling, exciting and sensuous as a lover’s first kiss. It reaffirms my guess that we are essentially all connected and the greatest happiness comes when we take part, give and share whatever we have with fellow travelers.

But what I see as the greatest strength of Mr. Pathi is his sense of soft authentic gratitude. This letter overflows with that. I found him to be an authentic human being – so rare to find these days — a vanishing tribe or better – an endangered species to the point of extinction.

The greatest crisis that faces civilization at this point of time is — people losing faith in themselves, people losing hope and people forgetting to help, share and give to the happiness of others with empathy. It gives me great satisfaction and happiness when I am able to encourage a fellow human being to have faith in himself; not to lose hope and deal with life with great empathy. To me the greatest qualities that can take a human being anywhere in the world to do good for others and self are a) Simplicity b) Patience c) Compassion. If only I can help others see those qualities in themselves I would take my life to be fulfilled! What more do I want? Sure, there is nothing more to life than that.
Now over to Mr. Pathi’s introspection over a 20 year period.
Quote:

I write this mail on the occasion of completing 20 years of career.

Please read this. 

Thank you and warm regards

C.Lakshmi Pathi

My self apprisal after 20 years of  Career.

It is natural for humans to look back in time, when they are getting old.  As advancing in age, we recollect fond memories,  forget the bad ones and to pride over the achievements of the past. 

It is usual, that the hope for the future drives when we are young and sweet memories sail us thru. the old age.

With this preface, if I look back into my last 20 years, professionally, I found the  sweet memories.

 

1. Graduating in Mechanical Engineering from BITS, Pilani,  gave me a quest to learn more in Mechanical Engineering  to be called a Mechanical Engineer.

I thank my parents for their blessings and support and financing my education at BITS.    I had to cope with home sickness and new environment at BITS.  This phase was tough and taught me many a lessons.

 

2. Worked in Power plants in Operations, Commissioning and Maintenance for 3 years.    (first formative phase of my engineering capability)

I thank 3 persons who influenced me a lot.  Dr. B.L.Bihani (then the CEO of J.K.Paper), Engr. S.C.Garg, ( then the GM of Power Plant in J.K.Paper) and  Engr. Viswanadham ( a fine engineer and a humble human being who played an important role as my mentor in Power plant).

 

3. Worked as domain consultant in the field of Asset / Maintenance Management for 4 years.   (second formative phase of my engineering capability)

 

Though I was groomed as Power Plant Engineer, in my earlier phase,  I felt the urge to know a  better way of doing maintenance.  Thanks to Engr. Dibyendu De, who selected me in a job interview, knowing very well that I do not know the  ABC of Condition Monitoring and he was sure that I had an urge to learn and excel.  Under his guidance, I grew to call myself an Engineer.  Perhaps, I was not aware of my capabilities  fully, while I was working under him.  This is typical of Indian students.  Soon I realized how capable I am while serving as Head of the Department in Jindal Steel and Power and later in Saline Water Conversion Corporation. 

Engr. Dibyendu De is a genius and his understanding of  many  subjects (difficult to list them) is so profound that it is difficult to say, which are the subjects  he mastered.   Beyond the Engineering and Technical stuff, he is fine human being who helped me to get insights into life.  If I have to use only one word to describe him, it is guru.

 

4. Worked as Head of the Department in Power and Desalination plant and Steel & Power Plant for 6 years.

This 6 years, I call them  my performing years of Engineering.  The learning curve which was steep in the last 7 years, started to flatten.  

Such was the capability of Engr. De to shape me as an engineer,  I used to solve the maintenance problems faced in the plants of Jindal and SWCC, with effortless ease.   This boosted my confidence and gave me the satisfaction as an engineer.  I fondly remember, once Engr. De asked us( me and my colleagues) what you want to be.  I wrote, I want to be known in my field of expertise.  Thanks to Engr. Dibyendu De, this became true. 

5. Going for SAP Certification in Plant Maintenance.

I did certification in SAP PM with my own money(which was expensive), in 2005.  This study, preparation and clearing the exam were a challenge because I was re-doing the regular study after 16 years from passing from the college.    After clearing the certification exam, I wept uncontrollably.   The tears were the tears of joy and my emotions overwhelmed not because I cleared the exam, but because I proved again to myself that I did it.

I thank my friend D.Srinivas who motivated me to do SAP certification.  He also shifted from Marketing career in L&T  to SAP career.  The change was not smooth for him. 

6. Worked in SAP as PM consultant and Project Manager for 6.5 years in various companies including, Satyam, IBM, Wipro, CSC.

There was a career shift from Engineering domain to IT domain  perceived by recruiters.   I never felt the difference,  that I changed my career from Engineering to IT.   To me, the difference was minor, as it was a natural progression.

Thanks to Kishore Josyula, a senior (in BITS, L&T, Satyam and CSC) and a good friend, who guided  me in SAP jobs.  He was instrumental in Satyam job and CSC job.   He also guided me to take up the right kind of roles for fast track career growth in IT.   When I was interviewed for G50 position in CSC, he was in G60.   He exhibited rare qualities of humans, when he strongly recommended me for a higher pay scale though he was drawing lower than the one he recommended for me.  He also regularly suggests me on the personal financial management( I am poor at it).

The IT companies started utilizing my experience in  consulting and managerial skills to their advantage, though made me start my career again.  I played the same game of IT to change jobs frequently to gain my position and pay.    I took every challenge which came my way throughout my career.  But in CSC, I was reluctant to play the role of Project Manager, especially surrounded by hypocrisy.  I was not interested in the kind of stress, this role would bring me.   On Kishore’s advice, I took it up and delivered.      IT career also gave me a satisfaction, the highest being the role of Project Manager for Kiewit off-shore team of 80 members. 

This was  the time, I started looking back at the career.   For some reason the mind wanted to take the roles which are stress free.

 

I started to realize one thing that I need to prove to myself, what I am.  If I prove myself, it is proved to the rest of the world automatically.   I learnt this from Engr. De.  I asked him many times, how was my performance in seminar or training.   After a long time he replied with questions. What was my feeling?. Did I felt the joy of doing it?  Was there positive energy flowing from the participants?. I started realizing this slowly.  I fondly remember his appreciation after the problem solving session at IOC, Haldia.  

Today I am feeling satisfied in my career.   If  my financial situation allows me to retire and still fulfill my responsibilities,  I will be happy to retire from service, with a satisfaction.

I have one wish,   Maintenance Management in its true sense is a subject which is hardly taught anywhere.   After having experienced in this field want to transfer this knowledge to new engineering graduates, who aspire for a career in Maintenance Management.  Will I do it? and How? I do not have answers to this.  May be I will find one.

 

7.  I joined Khafji Joint Operations, Saudi Arabia as CMMS Engineer.

Doing this job on a relaxed note.  I am writing this my self-appraisal of 20 years, as I do not have to do anything here.

I thank my wife and son for bearing with my changes in career and the disturbance, I bring to them.  They are kind to bear with me always.

Unquote

Hope you enjoyed reading this as much as I have had.

At times an assessment like this over the ‘long now’ simply brings clarity and focus about the few essentials we need to effectively deal with in life. No?