Plant Reliability Improvement — RAPID

A) The Background

Engineers have tried to address the issue of Reliability of equipment and systems in various ways. Therefore, many methods and techniques are in existence. The usual approach pivots around the concept of “failure modes” and how best to guard against those so as to prevent the consequences of failures. However, the existing methods do not take into account, flow of energy as an important factor that determines reliability of plant and machinery.

Taking this into consideration, I am putting together some rules and principles that hopefully point to a path that might enable engineers to improve plant reliability with minimum effort, resources and time.

But before I do that I would like to put forward an easy understanding of the term — reliability.

B) Reliability of a Machine:

The period of time a machine would run (fulfils its function) without a problem or trouble.

Longer the period of trouble free running better is the reliability of the plant.

It therefore addresses the heart of reliability improvement — i.e. enhancement of useful operating life of a machine or a system of machines.

C) What does that mean in terms of energy flow?

We may say that a machine that can continue with the smooth flow of energy, with the minimum wastage of energy for a long period of time is more reliable than a machine where energy flow is disrupted frequently in some manner or the energy wastage is high or the energy is pushed out of equilibrium condition, which invariably stops the machine from functioning effectively.

D) So what might be the job of engineers?

It is evident that the fundamental job of engineers (both operation and maintenance) is to run and maintain a machine or system in such a way so that the energy wastage is minimized and smooth flow is ensured for a long or desired period of time.

E) How can this be done?

This may be done in three fundamental ways, which are as follows:

  1. Observe the dynamics of a machine or system to ascertain energy flow patterns and the degree of energy wastage and the reasons for such wastage. Also determine the degree of stability or instability of the system and what affects a system’s stability.
  2. Adjust or maintain the conditions within its operating context to ensure smoother flow of energy with minimum wastage. In the process, learn what changes in a machine/system would disrupt energy flow or push it out of equilibrium conditions and prevent such disruptions.
  3. Change, monitor, modify the system (made up of physical asset and components, process, information flow, analysis and decision making, teams) as necessary, for smoother flow of energy to continue over longer period of time with minimum energy wastage.

F) How to apply it in a real plant?

I have found the following method useful in various plants where I implemented Reliability Improvement Programs.

  1. Make a list of critical machines.
  2. Select a critical machine along with its sub-systems (machines that support its functioning)
  3. Apply the three steps as outlined in Section E (above) — within the operating context.
  4. Improve and stabilize performance; record the learning.
  5. Create a custom made monitoring system to spot changes in time along with custom made expert system to guide engineers to quickly decide the course of actions to be undertaken.
  6. Record the decisions, actions and changes in the form of equipment history.
  7. Move to the next critical machine and its sub-systems.
  8. As we go along, first check whether the all consequences of failures have been taken care of. Next check whether overall plant/area/section MTBF (Mean Time Between Failures) is going up and whether MTTR (Mean Time to Repair) is going down along with consequential lowering of maintenance and operation costs. Lastly check the accuracy of the custom made expert system (usually made up of multivariate parameters) in is ability to forewarn and guide maintenance decisions.

G) General Rules:

Keeping the above in mind I formulated the following four rules that might help engineers managers stay on the path of plant reliability improvement:

  1. For any machine, energy tries to move in sync through all elements of a machine through various interfaces against many contradictions and constraints but always choosing the path of least resistance.
  2. Changes in contradictions, constraints and interfaces change the quality of energy flow forcing energy flow to go out of equilibrium (instability) to either cause degradation of performance or cause failures that lead to unwarranted plant stoppages, affecting costs and productivity.
  3. Changes and the causes of such changes are reflected in the dynamics of a machine in terms of interdependent parameters like vibrations, heat, flow, wear, humidity, temperature, pressure etc.
  4. Reliability of any machine or system can be improved by either maintaining the contradictions, interfaces and constraints to “just right conditions” or changing those to enable smoother flow of energy for a longer period of time with minimum wastage.

H) Applications:

Having applied these basic rules some industrial plants were able to gain on-going benefits for years. Here are some examples of — Plant Wide Reliability Improvement

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Fixing Organizational Problems

Every day, managers in different organizations face an array of problems. Usually, such problems keep repeating — either randomly or at regular intervals. After a while, it then becomes clear to the managers that such problems resist current ways of thinking and actions as practiced within the organization. 

The réponse to such problems is — “How this problem can be fixed permanently?” 

A manager would then try to apply known theories, methods, and tools to solve the problem. And in this process, the managers can also increase their skills. But the problem is that the problems simply don’t vanish. They have a bad habit of sneaking back through the backdoor. 

Why is that?

The short answer is — “No problem can be fixed, at least permanently.

This is because the nature of the problems keeps changing with time or the same problem comes back with different intensity or frequency. 

However, one can find and install new guiding ideas. And one can intently engage in redesigning an organization’s infrastructure, policies, rules, methods, and the tools presently used to find new ways of dealing with work and problems. 

The key is to closely observe what is going on in the present and then discover the organizational subconsciousness (mindset) that allows such events to happen with alarming regularity or randomly. Once that mindset is found, a manager can then find new ways of thinking and practices to replace the old governing mindset. 

If one keeps going in this way one can gradually evolve a new type of organizations that is responsive, agile and observant about the numerous interactions that go within an organizational environment to become a better and a fitter organization. 

It would then be able to deal with the problems and opportunities of today and invest in its capacity with the right resources and efforts to embrace a better future. This happens because its members are focused on enhancing and expanding their collective consciousness  — where individual members are able to observe, learn and change together.  

In other words, they collectively create, support and sustain a organization that continually learns from their present situation. 

Respect the minds of people by going to where the action is – a fundamental rule of change management

All through my professional life, I have been connected to improving something on the shop floor — be it a machine, process or a quality problem.

And I have seen that it is very difficult to improve anything on the shop floor by just passing down verbal instructions or by commanding someone to do something or by conducting a training program in a classroom or handing over a well-documented piece of paper complete with all instructions and a to-do list.

In most cases, people don’t get the idea. As a result, workers on the shop floor soon lose interest in the improvement process and don’t like engineers and managers who just pass down orders sitting at their desks. Respect for engineers and engineering is soon lost. And improvements don’t take place. The company suffers as a result.

Respect for engineers and engineering comes from respecting the minds of workers and supervisors.

This is best done by engineers going down to the area of the shop floor where the improvement is to be made and then explain what they want the workers and supervisors to do and what exactly is to be done. It can be explained verbally by physically touching the parts or equipment where the improvements are to be made or through rough sketches quickly drawn on scraps of paper to instantly clarify the points.

This is an important point in making a change, which is often forgotten by engineers. I call this rule — ‘Respect the minds of people by going to where the action is.”

Attention — the Essential Energy to Achieve & Improve Anything.

Information enters our consciousness either because we intend to focus attention on it or as a result of attentional habits based on biological or social instructions.

For example, driving down the extremely busy and often chaotic streets of Kolkata, we pass by hundreds of cars without actually being aware of them. Their shape, size and colours might register for a fraction of a second, and then they are immediately forgotten the next moment.

But our primary objective is to reach from one place to another without an accident or suffering a scratch. But how do we achieve that goal?

So while driving, we occasionally notice a particular vehicle, perhaps because it is moving unsteadily between lanes or because it is moving too slowly or because it looks strange in some way.

The image of the unusual vehicle enters our focus of consciousness and we become intensely aware of it unusual behaviour.

In our minds, such visual information about the car (the abnormal behaviour) gets related to information about other errant cars stored in our memory, which helps us determine into which category the present instance fits. Is this an inexperienced driver, a rash driver, a drunken driver, a momentarily distracted (talking on a mobile phone) but competent driver?

As soon as the event is matched to an already known class of events, it is identified. Now it has to be evaluated: Is this something to worry about? If the answer is yes, then we must immediately decide on an appropriate course of action: Should we speed up, overtake, slow down, change lanes, stop?

All these complex mental operations must be completed quickly and in real time. But it doesn’t happen automatically. There seems to be a distinct process that makes such reactions possible. This process is called attention. It is attention that selects the relevant bits of information from a potential of thousands of bits available.

It takes attention to retrieve the appropriate references from memory, to evaluate the real-life event and then choose the right thing to do.

Despite its great powers, attention can’t step beyond the limits as already described. It can’t notice or hold in focus more information that can be processed simultaneously. Retrieving information from memory and bringing it into the focus of awareness, comparing information, evaluating, deciding — all make demands on the mind’s limited processing capacity. For instance, the driver who notices an errant car will have to stop talking on his cell phone if he wants to avoid an accident, which is, in fact, his goal.

Some people learn to use this priceless resource very efficiently while others simply waste it. The mark of a person who is in control of his/her consciousness is the ability to focus attention at will, to stay away from distractions, to concentrate as long as it takes to achieve a goal and not longer. The person who can do this effortlessly usually enjoys the normal course of everyday life and can effectively meet the challenges of everyday life.

Improving reliability of industrial equipment needs such keen attentional energy which Reliability Centred Maintenance helps one to achieve. It, of course, depends on how well a Reliability Centred Maintenance System is designed, developed and implemented.

But what is essential is the development of memory bank, which can be only developed through comprehensively designed training and education system run over a long period of time.

Computerised Maintenance systems, Condition Based Maintenance technology, rigorously developed Maintenance Planning, Internet of Things, Artificial Intelligence can all help but without a broad-based deep memory bank of different types of failures, failure modes, interactions and mechanisms that create failures, methods to detect failures, interpretation and evaluation of relevant information and deciding the right course of action –improving reliability of industrial systems would remain as a desire only,

Attention is the key to achieving desired outcomes and improving any system. It can’t be ignored.

 

By Dibyendu De

Short Quiz on RCM

Max marks = 10
1 mark for one right answer
-1 mark for a wrong answer
1. Which of the following statements is true?
a)  RCM does not bother about the consequence of a failure
b)  RCM does not try to achieve the inherent reliability of a system
c). Time-based Preventive Maintenance is the cornerstone of the RCM output
d)  RCM does not believe that failure rate increases with the age of the equipment
2. Which of the following statements is true in case of RCM?
a) Age of individual components can not be determined in a statistical manner
b) RCM believes that run-to-failure strategy is often the best strategy
c) RCM is not bothered about consequences of a failure
d) RCM is not interested in achieving the inherent reliability of equipment
3. Waddington Effect states:
a) The performance of an equipment improves upon regular overhauls
b) The failure rate goes up if an equipment is regularly maintained by time-based preventive maintenance
c) The failure rate goes down if an equipment is regularly maintained by time-based preventive maintenance
d) The failure rate remains the same with regular time-based preventive maintenance
4. The most important piece of information in RCM is:
a) Number of critical equipment
b) Failure Modes
c) Mean Time Between Failures (MTTF)
d) Mean Time To Repair (MTTR)
5. Most failures in a plant are:
a) Early
b) Wear Out
c) Random
d) Constant
6) Which of the following is not a failure mode?
a) Bearing seized
b) Bearing problem
c) Shaft sheared
d) Circuit opened
7) Which of the following must not be included in the Function of a machine?
a) Design Data
b) What the user wants an equipment to do?
c) Parameters that define the standard of performance
d) Operating Context
8) Secondary Functions are:
a) Additional functions an equipment is supposed to do
b) Functions of other machinery that ensure performance of a critical machine
c) Functions that are undesirable
d) Functions that are desirable but not essential for the performance of a machine
9) Which strategy might be the cornerstone of an RCM strategy?
a) Time-based maintenance (scheduled replacement/repair)
b) Condition Monitoring or Condition Based Maintenance (CBM)
c) Detective Maintenance (Inspections of Hidden Failures)
d) Design Out Maintenance (DOM)
10) To improve the inherent reliability of a system the best strategy is:
a) Condition Based Maintenance
b) Design Out Maintenance
c) Time-based Maintenance
d) Run-to-Failure
@Dibyendu De
Answers:
1) d 2) a 3) b 4) b 5) c 6) b 7) a 8) b 9) b 10) b

A Movement towards RCM

29th December 2017, Kolkata

On 29th December 1978, F. Stanley Nowlan, Howard F. Heap, in their seminal work Reliability Centered Maintenance, revealed the fallacy of the two basic principles adopted by traditional PM (Preventive Maintenance) programs – a concept that started from World War II:

  •  A strong correlation exists between equipment age and failure rate. Older the equipment higher must be the failure rate.
  •  Individual component and equipment probability of failure can be determined statistically, and therefore components can be replaced or refurbished prior to failure.

However, the first person to reveal the fallacy was Waddington who conducted his research during World War II on British fighter planes. He found that failure rate of fighter planes always increased immediately upon time-based preventive maintenance, which for the fighter planes was scheduled after every 60 hours of operation or flying time.

By the 1980s, alternatives to traditional Preventive Maintenance (PM) programs began to migrate to the maintenance arena. While computer power first supported interval-based maintenance by specifying failure probabilities, continued advances in the 1990s began to change maintenance practices yet again. The development of affordable microprocessors and increased computer literacy in the workforce made it possible to improve upon interval-based maintenance techniques by distinguishing other equipment failure characteristics like a pattern of randomness exhibited by most failures. These included the precursors of failure, quantified equipment condition, and improved repair scheduling.

The emergence of new maintenance techniques called Condition Monitoring (CdM) or Condition-based Maintenance (CBM) supported the findings of Waddington, Nowlan and Heap.

Subsequently, industry emphasis on CBM increased, and the reliance upon PM decreased. However, CBM should not replace all time-based maintenance. Time-based or interval based maintenance is still appropriate for those failure cases, exhibiting a distinct time-based pattern (generally dominated by wear phenomena) where an abrasive, erosive, or corrosive wear takes place; or when material properties change due to fatigue, embrittlement, or similar processes. In short, PM (Time based or interval based maintenance) is still applicable when a clear correlation between age and functional reliability exists.

While many industrial organizations were expanding PM efforts to nearly all other assets, the airline industry, led by the efforts of Nowlan and Heap, took a different approach and developed a maintenance process based on system functions, the consequence of failure, and failure modes. Their work led to the development of Reliability-Centered Maintenance, first published on 29th December 1978 and sponsored by the Office of the Assistant Secretary of Defense (Manpower, Reserve Affairs, and Logistics). Additional independent studies confirmed their findings.

In 1982 the United States Navy expanded the scope of RCM beyond aircraft and addressed more down-to-earth equipment. These studies noted a difference existed between the perceived and intrinsic design life for the majority of equipment and components. For example, the intrinsic design life of anti-friction bearings is taken to be five years or two years. But as perceived in industries life of anti-friction bearings usually exhibit randomness over a large range. In most cases, bearings exhibit a life which either greatly exceeded the perceived or stated design life or fall short of the stated design life. Clearly in such cases, doing time directed interval-based preventive maintenance is neither effective (initiating unnecessarily forced outage) nor cost-effective.

The process of determining the difference between perceived and intrinsic design life is known as Age Exploration (AE). AE was used by the U.S. Submarine Force in the early 1970s to extend the time between periodic overhauls and to replace time-based tasks with condition-based tasks. The initial program was limited to Fleet Ballistic Missile submarines. The use of AE was expanded continually until it included all submarines, aircraft carriers, other major combatants, and ships of the Military Sealift Command. The Navy stated the requirements of RCM and Condition-based Monitoring as part of the design specifications.

Continual development of relatively affordable test equipment and computerized maintenance management software (CMMS like MIMIC developed by WM Engineering of the University of Manchester) during the1990s till date has made it possible to:

  •  Determine the actual condition of equipment without relying on traditional techniques which base the probability of failure on age and appearance instead of the actual condition of an equipment or item.
  •  Track and analyze equipment history as a means of determining failure patterns and life-cycle cost.

    RCM has long been accepted by the aircraft industry, the spacecraft industry, the nuclear industry, and the Department of Defense (DoD), but is a relatively new way of approaching maintenance for the majority of facilities outside of these four areas. The benefits of an RCM approach far exceed those of any one type of maintenance program.

    Fortunately, RCM was applied in India for a few Indian manufacturing Industries from 1990 onwards with relatively great success. I am particularly happy to have been involved in development and application of RCM in Indian industries, which has continually evolved in terms of techniques and method of application to meet contextual industrial needs.

    I am also happy to report that RCM for industrial use has now reached a mature stage of its development, which can be replicated for any manufacturing industry.

    I am of the opinion that this maturity would provide the necessary stepping stone to develop Industry 4.0 and develop meaningful IOT applications for manufacturing industries.

    Wish RCM a very happy birthday!

    by

    Dibyendu De

Case of Missing Gear Mesh Frequency

Question:

“Why don’t we see the Gear Mesh Frequency (GMF) on the output side of a splash lubricated slow speed gear box?”

This is quite puzzling since common sense dictates that such peaks should be present.

My Answer:

The principles involved are the following:

1. Air, water and oil produce turbulence when worked on by machines like pumps, gears, fans, propellers etc.
2. Such turbulence creates damping force.
3. This is proportional to the square of the velocity.
4. But this damping force acts in quite a funny manner.
5. For slow speed machines (say below 750 rpm; slower the better) damping is positive that is it goes against the motion and so neutralizes the entropy as seen by the decrease in the vibration levels. Hence the gear mesh frequencies vanish. Coriolis Effect on the output side of the gear box also helps in attenuating the vibration.
6. But for high speed machines damping is negative. That is it goes in the direction of the motion and therefore enhances the entropy as seen by the increase in the vibration levels.
7. So, for low speed machines it goes against the motion and suppresses the GMF. In some cases it suppresses the fundamental peak as is found in the case of the vertical Cooling Water Pumps of Power Plants. GMF is produced when the fundamental frequency is superimposed onto the vibration generated through gear impacts.
8. It therefore follows that for high speed gear boxes it magnifies both fundamental and GMF peaks.

Missing peaks therefore indicate fluid turbulence, which might also be indicated by other peaks like vane pass frequencies. The condition monitoring of such gear boxes might best be done through Wear Debris Analysis/Ferrography.

So, this is the mystery of the missing GMF in splash lubricated slow speed gear boxes.

Therefore, splash lubrication for a low speed gear box is a good idea. It enhances the life of the gear box since it balances the entropy in the system.

But at the same time, with higher oil level in a splash lubricated high speed gear box the vibration level would increase, specially the fundamental and the GMF. That would spell trouble.

Similarly, it is better to have a turbulent air flow in low speed fans and blowers. It suppresses the vibrations and therefore enhances the life of bearings.

Nature also uses these principles of fluid turbulence and damping? Applications?

1. Bird’s nest are made up of loosely placed twigs and leaves usually not bound to each other. But these don’t break up or fall off in turbulent winds. Damping keeps them in place and provides the necessary security to birds.

2. Swift flowing rivers allow fishes to grow bigger and better.

3. Winds, storms etc neutralize the increase in entropy.

Design Ideas for Reliability & Sustainability?

1. Low speed gear boxes might best be lubricated by splash lubrication.
2. High speed gear boxes might best be lubricated by spray lubrication
3. Hotter and turbulent air might best be handled by low speed fans and blowers.

Schematic to Understand and Resolve Vibration Problems.

Screen Shot 2017-10-23 at 9.44.29 AM

The above is a simple but comprehensive schematic to understand and resolve vibration problems of industries.

Applications:

  1. Resolving vibration problems
  2. Design
  3. Manufacturing
  4. Design Review
  5. Machine Testing
  6. Modeling

 

 

Fractional Gear Mesh Frequencies

Recently I received an email which asked me give my option on a phenomenon the analyst observed.

Quote

Observing high vibs on pressing and lifting pinion Drive End (DE) and Non Driven End (NDE) bearing on a ball mill. Motor and main Gear Box drive are OK. Clear predominant gear mesh frequency is appearing in the spectrum along with harmonics and side bands. But 1st GMF (Gear Mesh Frequency) is predominant. side bands with pinion speed is also seen. no Girth Gear speed side band was observed

Some of the vibration data, spectrums and photos shown in the attachment. Phase measurements indicate inconsistency in the readings near pressing pinion bearing. Impacts were also seen in time waveform data along with modulation.pinion speed 122 rpm.  On pressing side bearing 2.03 Hz side bands are seen, On lifting side i can see side bands spaced at 6.09 Hz (That is 3 times of pinion speed). Both Pinion lifting and pressing bearings are behaving differently. the vibs are high on DE as compared to NDE on both pinions. Can we suspect eccentric moment of the pinions with looseness. Why am i seeing 2.03 side bands on pressing and 6.09 side band on lifting side bearings. What is the significance of this. One sample of GG tooth photo shows uneven shining surface on either side (refer photo). In this case I am seeing  (30 T = 1 X 2 X 3 X 5) and (210 = 1 X 2 X 3 X 5 X 7) 2 X 3 X 5 as the common factor. Pinion 30 teeth and GG has 210 teeth. Will this create gear ratio issues uneven locking and releasing of 2 mating teeths. But no 1/2 or 1/3 or 1/5 GMF seen in the data.

Unquote

My reply was:

Quote:

But after a quick look this is what I see as the problem: –

1. We are seeing 1/2, 1/3 and 1/5 of GMF — these appear due to common factors 2, 3, 5 as you wrote.

This means that the pinion is badly worn out and as the common factor teeth mesh they generate these fractional frequencies.

It also means that the GMF and the natural frequency are not separated by 2.5 times. [The natural frequency in the horizontal direction = 28.5 Hz; natural frequency = 30.9 Hz; Gear Mesh Frequency = 60.6 Hz]

Looking at the signatures it is clear that the GMF falls within 2.5 times the natural frequencies.

Also note how the GMF (60.6 Hz) falls right between two natural frequencies in both the vertical and horizontal  directions. (31.1 Hz and 83 Hz). This makes the situation worse.

2. From the time waveform, we can see vibration relaxation waves. It means that the wear out or damage is towards the addendum region of the pinion/gear
3. This means that the spray nozzles are wrongly placed or jammed. The nozzles must be placed after the gear mesh not at or before the gear mesh. Also ask the client to check for jamming of the nozzles and the present viscosity of the grease/oil and the quantity that is fed per hour.
4. We can also suspect eccentricity of the pinion and looseness.
5. There is a strong resonance. This appears to have generated from the top cover.
Regards
Dibyendu De
dde@rgbwaves.com
9836466678
 Unquote
Deeper Lessons:

It is important to question as to what else we can do other than detect a problem or detect an incipient fault?

With the above analysis and information we can easily see the relationship between fractional GMF and lubrication and wear. It means we can build an algorithm that would warn us about an imperfect lubrication system that would in fact accelerate wear and put the system out of service.

Further, we can refine the specification of a purchase a gear box. The specification should state  — a) number of pinion teeth should be a prime number to prevent accelerated wear b) if a prime number can’t be achieved then the natural frequency in the three directions must be away from the GMF by at least 2.5 times the GMF.

Similarly, we can specify the gear box top cover natural frequency should be at least 4 times the GMF.

Scheduled running checks may include — a) rate of lubricant flow b) motor current c) placement of lubricant nozzles etc.

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Details of the case: (relevant data)

VIBRATION STUDIES OF CEMENT MILL

Steps for vibration measurements

Impact test was carried out at selected locations on the torsion bar to know its natural frequency
Normal vibration signatures were recorded with motor speed being 994 rpm and pinion speed 122 rpm

Vibration data was recorded on selected bearing locations of motor, gearbox and pinion bearings
Data was recorded along horizontal, vertical and axial direction with 90% load on the mill

Phase measurements were recorded to know the behavior of pinion DE with respect to pinion NDE of pressing and lifting side

OBSERVATIONS

Vibration signatures recorded on Pinion DE and NDE of both pressing and lifting side bearing shows predominant gear mesh frequency and its harmonics
Side bands were observed along with gear mesh frequency and its harmonics
Gear mesh frequency 60.9 Hz is appearing predominantly in all HVA direction

Time waveform recorded on pinion DE and NDE bearings clearly shows modulation which occurs due to above phenomena
Impacting of the gear teeth was also observed. Refer time plots provided in this report in subsequent pages

Only Side bands of pinion speed (2.03 Hz or 122 rpm) are seen, no side bands of Girth gear was seen in the data
The phase measurements recorded on pressing pinion DE and NDE along axial and horizontal direction shows the phase is not consistent with time suspecting looseness due to uneven movement of pinion

Vertical vibrations recorded on pinion DE bearing lifting side shows the vibrations are low (5 mm/sec) on one end while its high (11 mm/sec) on the other end even though it’s a common top cover of that bearing
For any normal 2 mating gears the selection of no. of teeth on each gear should be such that when factorizing is done no common factor should be found apart from 1

In this case pinion has 30 teeth and Girth gear has 210 teeth o Then as per calculations
Pinion 30=1x2x3x5,GG 210=1x2x3x5x7
So common factors are 2x3x5

 

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Would Lubrication Cause a Sudden Failure?

This case is about a sudden failure of cooling tower fan motor of a copper mine.

The motor failed almost immediately after Planned maintenance, which was just about lubricating the motor bearings.

What Happened?

Electrical department conducted a scheduled PM task on this piece of equipment on 25.05.17. After 3 hrs of running; motor Non Driven End (NDE) bearing was damaged.

When the motor was opened it was observed:

1. One of the poles was severely damaged.

2. Bearing cage was also found damaged and all roller elements were crushed.

 

Why did this happen? 
1. Sudden application of load or abrupt change in load. It happened when the machine was started after PM — i.e. starting the machine from rest under loaded condition.
2. This caused hunting of the motor in which a rotor starts seeking equilibrium position. Such an equilibrium is reached when the load torque is equal to the electromagnetic torque. This equilibrium position gets disturbed if a sudden change occurs in the load torque, which has been the case when the motor was started the after the motor was stopped to lubricate its bearings.
3. In this situation, the rotor slips too much, i.e. — the rotor moves around trying to find its steady state equilibrium state and in this process the rotor and stator touched and shorted — damaging one of the poles.
4. Point 1 to Point 3 describes the root cause of the case. A  broad at the base 1N (1 times running speed) peak was observed.  This indicates presence of rubbing and resonance.
5. Resonant frequency excited the resonant frequency of the NDE bearing  which caused complete collapse (crushing) of the motor NDE bearing.
6. Another point which is important to consider is the time taken for a freshly lubricated bearing to stabilise. After lubrication, an anti-friction bearing generally runs hot (temperature greater than 75 degrees C but lesser than 95 degrees C) for a few hours (5 to 6 hours at times) to stabilise to a normal operating condition; with a temperature around 65 degrees C. This phenomenon can abruptly and adversely affect vibration levels of the bearing.
Notes:

1. The vibration signature did not indicate lubrication starvation of the bearing.

Hence the question is — why stop a machine for re-lubrication when the activity isn’t needed at all?

2. In the future, if the system is stopped, then during start up it has to be ensured that the load is zero or near zero or it has to start at no-load condition.

If that isn’t possible, the system has to be started at low rpm and then the rpm can be gradually increased, all the while maintaining a steady state. It might take up to 6 hours for the system to stabilise after a bearing is lubricated.

By
Dibyendu De
dde@rgbwaves.com
9836466678