The above is a simple but comprehensive schematic to understand and resolve vibration problems of industries.
- Resolving vibration problems
- Design Review
- Machine Testing
The above is a simple but comprehensive schematic to understand and resolve vibration problems of industries.
Recently I received an email which asked me give my option on a phenomenon the analyst observed.
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.
My reply was:
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.
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.
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
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
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.
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.
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.
Report on Thaisen Fan (Scrubber)
After cleaning of the fan blades, vibration of the fan gradually increases during operation and in a span of 10 to 14 days vibration level reaches an unacceptable level, which necessitates the next cleaning cycle. However, for so long, this matched the scheduled production window provided by operation. However, after the recent changing of the rotor and the bearings, the fan now reaches unacceptable level of vibration within a short span of time that does not coincide with the scheduled “production window” of the operation, which causes “unplanned downtime.”
Goal of the investigation: To correct the imperfection in the system so that the fan cleaning cycle coincides with.the scheduled production window.
Result of the investigation:
1. The problem of rising vibration within a short period of time is an inherent problem (a birth defect) of the fan. The main reason is the Coriolis effect on the fan. Coriolis force is a force exerted by a moving fluid on the disc or impeller rotating in the fluid. If the rotation is CCW (counter clockwise) then the fluid moves to the right of the impeller and away from the centre. Similarly, when the impeller moves in the CW (clockwise direction) the fluid moves towards the left of the impeller and away from the centre.
In this case, with the fan moving in the CCW direction the Coriolis force moves toward the right of the impeller in the same direction as the damping force. This effect (the fan moves in the CCW direction) produces negative damping (since the two forces are in the same line of action).
Negative damping is a phenomenon, when damping force, which usually opposes the driving force, acts in the same direction as the driving force. In such a case the vibration of the system is amplified.
Combination of negative damping and Coriolis effect produces this phenomenon of gradually rising vibration of the fan in a short period of time, which goes away upon regular cleaning. In the present context nothing can be done to eliminate the phenomena of Coriolis Effect and Negative damping. However, if a similar system is to be installed in the future, we would be pleased to provide necessary suggestions and recommendations so that such phenomena are eliminated right from the start.
2. Present signatures indicate misalignment and dynamic imbalance
3. Weak foundations
Actions to be taken to increase the cleaning cycle to match scheduled “production window.”
1. Take care to align the rotor properly. Care to be taken while putting shims.
2. Dynamically balance the fan in two planes to eliminate the imbalance
3. Cleaning cycle can be initiated when vibration of the fan on the bearings reaches 7 mm/sec (rms). It is safe to run the fan upto this point.
4. Monitor the condition of the foundation by taking vibration measurements in displacement and acceleration modes. Displacement should be taken in the horizontal direction on the topmost accessible point of the columns and at the base. Acceleration should be taken in both vertical and horizontal directions. Displacement should not cross 50 microns in the horizontal direction or at the base of the columns. Similarly acceleration both in the vertical and horizontal directions must not cross 1.5 g. This would ensure safety of the equipment. In case it crosses corrective actions are to be taken to rectify the foundation.
The inherent reliability of a system is determined by the system’s design. It means that the design of the system would determine the upper limit of reliability the system exhibits during operation. Suppose, for example, a system, with the best possible maintenance is able to achieve availability of say 90% we can say that this is the upper limit of the system’s capability that is determined by its design. A good “preventive maintenance” plan can never improve a systems inherent reliability. In other words, preventive maintenance, contrary to what many believe, cannot make a system “better”. It may, at best, only help realise the inherent reliability as determined by the physical design.
Hence the suggested process to “improve” the inherent reliability of a system, may be framed as follows: –
Understand the dynamics through tools like vibration analysis
Monitor changes and rate of change
Eliminate unnecessary maintenance tasks
Change the design of the system interactions to eliminate inherent “imperfections” and revise the maintenance plan.
In most cases, this would be the general approach.
Until we can effectively undertake some design changes (Design Out Maintenance – DOM) or take measures to eliminate inappropriate maintenance actions (Review of Equipment Maintenance – REM) it would not be possible to go beyond inherent reliability of an equipment, specially if it is undesirable in the business context. For example, a vertical pump of a power plant kept failing very frequently or had had to be stopped quite often when vibration shot beyond the trip limits. This behaviour of the system is determined by the design of the system. Unless the design (specifically the interactions between components) is corrected for improvement; the system (vertical pump) would continue to behave in that manner for all times. Likewise if the MTBF of a machine is say 90 days, it would not be possible to considerably improve the MTBF way beyond 90 days unless some undesirable interactions (which I call system “imperfections”) are corrected for improvement and a proper review of existing maintenance system is carried out. Such “imperfections” can be both physical and non-physical. Design features, most importantly, the interactions between physical/non-physical components are arguably the most important characteristic of a system that determine a system’s inherent reliability.
In addition, there are many physical design features that influence reliability like redundancy, component selection and the overall integration of various pieces of the system.
In the context of RCM, design extends far beyond the physical makeup of the system. There are a number of non-physical design features that can affect, sometimes profoundly, the inherent reliability of a system. Among these are operating procedures, errors in manufacturing, training and technical documentation. When a proper RCM analysis is conducted on a system or sub-system, there’s a good chance that the resulting maintenance actions will enable the system to achieve its inherent reliability as determined by its physical design features. However, if the inherent reliability is below user’s expectation or need then the design features are to be improved to achieve the desired level of inherent reliability.
Moreover, if unwarranted maintenance tasks are eliminated as it will greatly reduce the risk of suffering the Waddington Effect. There is also a good chance that if operating procedures, training, technical documentation and so forth are found to negatively impact inherent reliability, these issues will be identified and corrected. As evidenced by the Waddington Effect. In virtually every case, less than optimal, non-physical design features almost always have a negative impact on inherent reliability. Therefore, in RCM analysis a through review of existing maintenance plan (REM) along with DOM is necessary to improve inherent reliability of a system.
In brief, right amount of Condition Based Maintenance (CBM) tasks, Scheduled Inspections (which is a part of CBM activity) REM and DOM would not only help us realise the inherent reliability as determined by the physical design but also improve it, if the original inherent reliability is below business expectation.
General symptom: 1x radial (Vertical and Horizontal direction of horizontal machines)
Usually a rotor bow in a motor looks like a static imbalance. Broken bars and loose connections (at motor terminals and at MCC) cause motors to heat up (localized) owing to uneven current flow through the phases causing rotor bow — uneven weight distribution around the rotor’s centreline. Hence we see high amplitude peak at 1x running speed in the radial and horizontal directions.
Localized overheating can be seen on the motor body through infrared thermal imaging.
The effect of can also be seen on the rotating air gap — a high peak at 2xLf with pole pass sidebands around 1x and 2x peaks. The 2x peak often comes up when the effect is more severe.
The time waveform would be sinusoidal when viewed in velocity.
Phase: expect 90 degree shift between vertical and horizontal axes. The inner race will move in and out once per revolution with a bent shaft
This is a HFO (Heavy Fuel Oil) screw pump used in Power Plant for running boilers. There was a catastrophic failure of the pump. Though this pump was regularly monitored by vibration (in velocity mode — mm/sec) it didn’t give any indication of the impending failure.
The screws of the pump rubbed against each other and the case hardened layers of both screws were crushed. The force was so great that the body of the pump also cracked. Evidence of corrosion was also noticed.
What caused it?
For want of HFO oil, the plant personnel were forced to pump LDO (Light Diesel Oil) through this HFO pump for the past one year.
Hence the I, A, R factors that contributed to this catastrophic failure are the following:
Initiator(s) — I — factor(s), which triggers the problem — low viscosity of LDO compared to that of HFO was the significant ‘initiator’ in this case. While viscosity of LDO ranges from 2.5 to 5 cSt, the viscosity of HFO varies between 30 to 50 cSt (depending on the additives used). Use of lower viscosity oil ensured metal to metal contact thereby increasing Hertz stress that led to collapse of the hardened layer of the screws.
Accelerator(s) — A — factor(s), which accelerates the process of failure — a) Indian HFO does not contain friction modifiers such as vanadium and magnesium. Their absence causes higher friction between the screws (approximately 70 times increase in friction), which accelerates the wear process. b) Moreover, presence of vanadium and magnesium additives in HFO and LDO acts as anti-corrosive agents. Notice that the failure happened a year after the management decided to pump LDO rather than HFO through the HFO pump — enough time for corrosion to take effect. So, we may say that there are at least two factors that accelerated the failure process. There are other effects too on system performance, which we shall discuss in a moment (refer “Note”).
Retarder(s) — R — factors that slow down the failure process — a) surface finish of the screws b) right clearance of the bearings c) presence of chromium in the screws.
Surface finish plays a very important role in reduction of metal to metal friction and also allows fluid film development. Ideally the surface finish should be between 3 to 6 microns CLA (Centre Line Average) for best effect. This can be introduced as a specification of the MOC (Material of Construction).
Similarly, excessive clearance in bearings would modify the hertz stress zone or profile — both in width and depth, which would cause shear of the hard layer (depth of which depends on the type of hardening and the type of steel used) and the soft layer (core material). Depth and type of hardening might also be specified in the MOC to prevent failures and extend life of the equipment. Presence of chromium in the metal would help formation of Vanadium – Oxygen – Chromium bond which would effectively enhance the life by providing better lubricating property which in turn would ensure a high level of reliability of the equipment.
Hence, once the I, A and R s are identified appropriate measures can be taken to modify maintenance plan, MOC etc to ensure long life of the equipment without negative safety consequences (heart of reliability improvement).
1. (Effect of IAR on system performance — i.e. the boiler – superheater – pipes):
Problems of high temperature corrosion and brittle deposits drastically impair the performance of high-capacity steam boiler of Power Plants, using HFO. Research* shows that heavy fuel oil (HFO) can be suitably burned in high capacity boilers. However, if HFO is chemically treated with an anticorrosive additives like Vanadium and Magnesium, it diminishes high temperature corrosion that affect some operational parameters such as the pressure in furnace and pressure drop in superheaters and pipe metal temperature, among others like atomization and combustion processes. Therefore, inclusion of right additives like Vanadium and Magnesium have been found to diminish high-temperature corrosion and improved system performance. It therefore makes sense to monitor these parameters, which can provide direct information on the degree of fouling, as well as of the effectiveness of the treatment during normal boiler operating conditions.
By Dibyendu De
As of now, maintenance strategy looks similar to strategy taken by the medical fraternity in themes, concepts and procedures.
If things go suddenly wrong we just fix the problem as quickly as possible. A person is healthy to the point when the person becomes unhealthy.
That might work fine for simple diseases like harmless flu, infections, wounds and fractures. And it is rather necessary to do so during such infrequent periods of crisis.
But that does not work for more serious diseases or chronic ones.
For such serious and chronic ones either we go for preventive measures like general cleanliness, hygiene, food and restoring normal living conditions or predictive measures through regular check ups that detects problems like high or low blood pressures, diabetes and cancer.
Once detected, we treat the symptoms post haste resorting to either prolonged doses of medication or surgery or both, like in the case of cancer. But unfortunately, the chance of survival or prolonging life of a patient is rather low.
However, it is time we rethink our strategy of maintaining health of a human being or any machine or system.
We may do so by orienting our strategy to understand the dynamics of a disease. By doing so, our approach changes radically. For example. let us take Type 2 diabetes, which is becoming a global epidemic. Acute or chronic stress initiates or triggers the disease (Initiator). Poor or inadequate nutrition or wrong choice of food accelerates the process (Accelerator) whereas taking regular physical exercise retards or slows down the process (Retarder). Worthwhile to mention that the Initiator(s), Accelerator (s) and Retarder (s) get together to produce changes that trigger of unhealthy or undesirable behavior or failure patterns. Such interactions, which I call ‘imperfections‘ between initiator (s), accelerator (s) and retarder (s) change the gene expression which gives rise to a disease, which often has to be treated over the entire lifecycle of a patient or system with a low probability of success.
The present strategy to fight diabetes is to modulate insulin levels through oral medication or injections to keep blood sugar to an acceptable level. It often proves to be a frustrating process for patients to maintain their blood sugar levels in this manner. But more importantly, the present strategy is not geared to reverse Type 2 diabetes or eliminate the disease.
The difference between the two approaches lies in the fact — “respond to the symptom” (high blood sugar) vs “respond to the “imperfection” — the interaction between Initiators, Accelerators and Retarders”. The response to symptom is done through constant monitoring and action based on the condition of the system, without attempting to take care of the inherent imperfections. On the other hand, the response to imperfections involve appropriate and adequate actions around the I, A, R s and monitoring their presence and levels of severity.
So a successful strategy to reverse diabetes would be to eliminate or avoid the initiator (or keep it as low as possible); weaken or eliminate the Accelerator and strengthen or improve the Retarder. A custom made successful strategy might be formulated by careful observation and analysis of the dynamics of the patient.
As a passing note, by following this simple strategy of addressing the “system imperfections“, I could successfully reverse my Type 2 Diabetes, which even doctors considered impossible. Moreover, the consequences of diabetes were also reversed.
Fixing diseases as and when they surface or appear is similar to Breakdown Maintenance strategy, which most industries adopt. Clearly, other than cases where the consequences of a failure is really low, adoption of this strategy is not beneficial in terms of maintenance effort, safety, availability and costs.
As a parallel in engineering, tackling a diseases through preventive measures is like Preventive Maintenance and Total Productive Maintenance — a highly evolved form of Preventive Maintenance. Though such a strategy can prove to be very useful to maintain basic operating conditions, the limitation, as in the case of human beings, is that it does not usually ensure successful ‘mission reliability’ (high chance of survival or prolonging healthy life to the maximum) as demonstrated by Waddington Effect. (You may refer to my posts on Waddington Effect here 1 and here 2)
Similarly, predictive strategy along with its follow up actions in medical science, is similar to Predictive Maintenance, Condition Based Maintenance and Reliability Centered Maintenance in engineering discipline. Though we can successfully avoid or eliminate the consequences of failures; improvement in reliability (extending MTBF — Mean Time Between Failures) or performance is limited to the degree of existing “imperfections” in the system (gene expression of the system), which the above strategies hardly address.
For the purpose of illustration of IAR method, you may like to visit my post on — Application of IAR technique
To summarize, a successful maintenance strategy that aims at zero breakdown and zero safety and performance failures and useful extension of MTBF of any system may be as follows: