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In vibration analysis, phase is measured using absolute or relative techniques.Absolute phase is measured with one sensor and one tachometer referencing a mark on the rotating shaft Fig

Case study: Pump vibration at an electrical utility

Tags: vibration analysis

A Reliable Plant reader, a maintenance planner at a major U.S utility, recently submitted this case study It offers invaluable information to all of you who work on pumps.

During monthly vibration data collection rounds, a power station’s 500-horsepower vertical turbine circulating water pump experienced a large increase in vibration compared to the previous month readings.

The running speed vibration trend showed the November 2004 reading went from a normal 0.257 inches/second velocity to 0.468 inches/second in February 2005, and then increased again to 0.637 inches/second in March Analyst review of the data found the largest response at the top of the motor, which is normal for most vertical pumps as they tend to pivot at the pump discharge head mounting to the floor Resonance is common with this design of pump, however, the natural frequency of this pump/motor system was found to be well above the 500 rpm operating speed of the pump through the use of a simple bump test The waveform pattern showing a predominant 1x (running speed) cycle supported an imbalance/wear condition.

Knowing this, shaft displacement readings were taken in an effort to judge the severity of the problem After using a strobe light on the exposed pump shaft below the coupling to ensure no keyway or protrusion would interfere, a shaft stick (fish tail) with the vibration sensor mounted on it was safely held against the shaft In the past, the shaft displacement had generally been around three to five mils Now the vibration exceeded 16 mils, all at running speed.

A spring outage was previously planned in three weeks for this unit Due to the rapid change in vibration and the severity, recommendations were made to pull the pump during this outage The pump was monitored on a bi-weekly basis until the outage to ensure it would not catastrophically fail Preparations were made for impeller replacement as this was likely the source of the high running speed vibration due to imbalance, possibly from a broken vane due to foreign object damage or excessive wear ring clearances This type of damage was suspected because this pump is in a pit fed

by a nearby river and has a history of damage due to debris River water is screened into the intake house, however, during the spring, high river levels tend to bypass debris and repetitive failures of the screen wash system have allowed debris past the screens which travels to the pit at the plant

However, plant management still needed convincing that this was the problem, so prior to the outage, divers were brought in to inspect the circulating water pit for debris and to look at the pump impeller The pit was surprisingly clean, but the diver found the pump impeller tips damaged and an obvious offset to the fit of the impeller to the bowl This likely meant that the impeller wear ring was worn excessively and/or the lower shaft was bent.

Armed with this information, the pump was then scheduled for repair during the outage Upon

inspection, the impeller wear ring was found worn, blade/vane tips were damaged/bent and the shaft was bent Parts were available for repair, but additional time was required to order a new shaft The condenser was found with excessive debris, so the hypothesis is that the pump had passed large debris that likely bent the shaft Repairs were completed within the timeframe of the scheduled outage and the pump was successfully returned to service

After the outage, an inspection of the remotely located river water intake house found damaged intake screens and inefficient screen washing In an effort to prevent any future failures to the circulating water pumps, repairs were made to the screen and a new screen wash pump with a stand-by unit were installed A new control system was added to sense differential pressure on the screen

improving the overall efficiency of this system Regular preventive maintenance inspections of the intake house system were developed and scheduled in the computerized maintenance management system to ensure this often neglected system is kept in good condition To date, less debris has been found in the condenser water boxes and the circulating water pumps have been running fine, but we are still fighting the high water levels during the spring rains.

Acoustic Emission: The Next Generation of Vibration Techniques

Martin Lucas, Kittiwake

Tags: vibration analysis, condition monitoring

History, experience and familiarity count for a lot where conditioning monitoring is

concerned, but that doesn’t negate the need for change, innovation and the advancement of tried, tested and trusted techniques The late Steve Jobs commented: “Innovation is the ability

to see change as an opportunity, not a threat.” Condition monitoring (CM) is transforming rapidly and so too must the mindset of CM practitioners and users It’s not good enough to simply disregard a disruptive technology in an effort to protect the “old guard.” When

combating downtime, there’s no place for historical sentiment

Steadily disrupting traditional vibration techniques is acoustic emission (AE) AE technology spawned from the aviation industry where vibration analysis simply couldn’t be easily

applied, short of a suicidal maintenance technician hanging off the wings AE technique is based on frequencies much higher than are monitored in the repetitive, synchronous

movement of vibration These frequencies are the result of shock, impact, friction and

cracking, for example By this means, it is possible to detect impending failure before

damage occurs, as well as monitoring its progress thereafter

With well-defined ISO standards, traditional vibration techniques including vibration

monitoring and vibration analysis have provided a trusted approach to condition monitoring for the past 30 years Yet, it remains a complex science and requires sophisticated knowledge and understanding from a seasoned expert In contrast, AE technology extends and simplifies the science, placing the power of vibration techniques directly into the hands of every

engineer Signals can be processed at the AE sensor into an easily understandable form

Of course, vibration analysis (VA) as a technique will have a place for many for years to come for many end users However, there is no escaping the fact that there is often a

requirement for a costly and unsustainable level of knowledge required to affect a good diagnosis For VA, the defect repetition frequencies are critically dependent upon the machine component design and geometry, as well as the precise running speed Vibration can occur independently in the X, Y or Z axis, and so orientation of the sensor is as important as

location For a detailed interpretation, it is also necessary to know internal machine

geometries, shaft speeds, meshing frequencies, etc., and to analyze the data before making a diagnosis So in summary, VA is valuable but too often overly complicated

With acoustic emission (AE), signal processing is undertaken automatically at the sensor level.

With vibration analysis (VA), the signal is processed downstream manually or

semi-automatically.

The areas in which vibration and AE both apply can be illustrated as overlapping circles However, AE provides an earlier warning, detecting wear and small defects, whereas with vibration, damage must have occurred to detect a signal AE will pick up a lack of

lubrication, friction and cracking, which vibration will not, although it must be acknowledged that the totality of information obtained from AE will be more limited than that derived from vibration

The signal processing required by AE is not something that can be performed by just anyone; it’s a high-frequency signal, so the user must have the knowledge to interpret the squiggly lines on a stethoscope However, recent developments have enabled this processing at the sensor level The sensor output can now provide pre-characterized numbers that tell you about the condition of the machine AE technology has been effectively de-skilled, enabling much wider application use

Suitable for continuously running machinery as well as machinery operating intermittently, slowly or for short durations, AE allows the user to diagnose problems with machinery at an early stage, carry out maintenance procedures and then monitor the improvement It provides real-time information with early sensitivity to faults and applicability to a wide range of rotational speeds

As awareness of the unique capabilities of AE increases, so too does the number of

applications that it is suited to, many of which have proven difficult for other forms of

condition monitoring to address For example, the analysis of signals, whether from AE sensors or accelerometers, requires a sufficiently long period of machine running at constant speed so that a statistically meaningful signal characterization can be made But that is where the similarity stops AE can be effective after around 10 seconds of measurements

For example, the algorithm used to derive the widely used acoustic emission parameters of Distress and dB Level in the MHC range of products from Kittiwake Holroyd requires a 10-second period of running at an approximately constant speed This compares favorably to Fast Fourier Transform (FFT) based vibration analysis, which typically needs 60 to 120 seconds of measurement time and tight tolerances on machine speed for an effective signal interpretation.

In cases where a hand-held instrument is used for periodic CM, it may be possible to interrupt normal machine operation and put it into a special continuously running mode for the

duration of CM measurements However, such disruption is not always possible and never convenient Furthermore, it is not compatible with the current trend toward CM automation, which requires continuous online monitoring with permanently installed sensors inputting

CM data or status into supervisory control and data acquisition (SCADA) systems or

programmable logic controllers (PLCs)

So why are many CM practitioners being so resistant to the benefits that AE brings to the table? It may be because many people have invested a lifetime in vibration and are

understandably wary of losing power and status After all, if you “dumb down” vibration, surely this reduces the perceived value that they bring to the organization Actually, it doesn’t Just because AE is disruptive as a technology, it in no way invalidates traditional vibration techniques but simply extends the impact way beyond what has been able to be achieved to date

For vibration techniques to be effective, you need equipment that’s far from cheap coupled with clever people to get the best from it Every result must be analyzed to understand what’s good and what’s bad For those who cannot afford the luxury of in-house vibration experts, there are many vibration specialists who offer a contract monitoring service, which is not an insignificant investment While for some, the criticality of certain applications coupled with the scale of some companies might justify this cost, others could still benefit from the

efficiencies realized by similar CM techniques

Furthermore, the “we don’t buy into one-month wonders/we’ve all been bitten by the latest whizz-bang technology” argument no longer rings true Indeed, AE techniques are only deemed disruptive because they are now mature with a proven track record

Ultimately, maintenance personnel are responsible for keeping machinery running If they are empowered to monitor condition themselves, identify where action is needed and then check that the action taken has solved the problem, then AE has significant advantages of cost, speed, flexibility and ease of field application in comparison to traditional vibration analysis techniques It is the efficient and effective approach to CM

To nurture the technology of a new era, a broader, longer-term view is required Surely it makes sense to embrace CM techniques that provide for the greatest protection or longest period of warning for potential damage and eventual failure By “de-skilling” technology, all maintenance professionals are empowered to make informed decisions quickly and with confidence, ultimately enabling them to positively and significantly impact a company’s bottom line Of course, there is room for sentiment in business but not at the expense of progress

Using Orbits for Condition Monitoring

Gary James, Ludeca, Inc

Tags: condition monitoring, vibration analysis

Orbits have historically been used to measure relative shaft movement within a journal-type bearing The shape of the orbit told the analyst how the shaft was behaving within the bearing

as well as the probable cause of the movement This was accomplished using proximity probes usually mounted through the bearings with a 90-degree separation and a tip clearance set to around 0.050 inches With today’s modern analyzers, it is possible to also collect an orbit using case-mounted velocity probes or accelerometers to see how the machine housing

is moving Another way of putting it would be the orbit represents the absolute path in space that the machine housing moves through (see Figure 1)

Figure 1 Figure 2

This is accomplished utilizing a two-channel instrument and collecting an orbit with the sensor of choice being a velocity probe or accelerometer This is what’s referred to as a poor man’s operating deflection shape or ODS (see Figure 2)

The analyst can interpret the data to determine machine movement at a particular

measurement location or a section of the machine if a tachometer trigger is used during orbit collection as a phase reference Analysts must keep in mind the exact location of each sensor

so that when they look at the shape of the orbit it is possible to tell the movement in

relationship to the sensor’s location The sensors should be placed 90 degrees apart or at least

as close as possible to 90 degrees Keep in mind that a properly wired sensor shows motion toward the sensor as a positive signal and motion away from the sensor as a negative signal

An orbit is usually collected while the machine is at its normal operating state or speed, but it can also be collected while the machine is increasing or decreasing in speed, such as during a coast-down or startup The data can be collected in a steady state, in what is known as an

unfiltered orbit, requiring no tachometer (see Figures 3 and 3A), or at multiples of running speeds such as first, second or third order to look for issues relating to that or another specific frequency (see Figures 4, 5, 6 and 7).

Figure 3 - Unfiltered displacement orbit Figure 3A - Unfiltered velocity orbit

Figure 4 – Second order setup Figure 5 – Second order results

Figure 6 – Third order setup Figure 7 – Third order results

How to identify, correct a resonance

condition

Alain Pellegrino, Laurentide Controls Ltd

Tags: vibration analysis, condition monitoring, predictive maintenance, maintenance and reliability

Many experts working in the field of vibration analysis will agree that resonance is a very common cause of excessive machine vibration

Resonance is the result of an external force vibrating at the same frequency as the natural frequency of a system Natural frequency is a characteristic of every machine, structure and even animals Often, resonance can be confused with the natural frequency or critical

frequency If equipment is operating in a state of resonance, the vibration levels will be amplified significantly, which can cause equipment failure and plant downtime It is,

therefore, important that the running speed of equipment be out of the resonance range

How to identify a resonance frequency

Many techniques can be used to identify and/or confirm a high vibration level caused by a resonance frequency It is very important to confirm a resonance phenomenon by at least two different types of tests before trying to correct it We will look at a few techniques commonly used in the industry

Techniques used to confirm a resonance

Impact test:One of the most commonly used methods for measuring a system’s natural

frequency is to strike it with a mass and measure the response This method is effective because the impact inputs a small amount of force in the equipment over a large frequency range When performing this technique, it is important to try impacting different locations on the structure since all of a structure’s resonant frequencies will always be measurable by impacting at one location and measuring at the same location Both drive point and transfer point measurements should be taken when attempting to identify machine resonances

This type of test must be performed with the equipment off This way you can easily identify the natural frequencies of the equipment (see Figure 1)

Figure 1 Impact Test, Equipment Off

Impact test using an instrumented hammer:This test is basically the same as a regular

impact test, except that an instrumented hammer is used to excite the system This hammer,

equipped with an accelerometer at one end, is used in tandem with the sensor used to measure the vibration A two-channel vibration analyzer is needed, in which one channel is connected

to the instrumented hammer and the other to the vibration sensor

Using this technique, you can effectively measure the force induced to the system by the instrumented hammer and the response at different frequencies When the phase shifts by 90 degrees, the frequency at which it occurs is a natural frequency (Figure 2) The advantage in using this method is that it allows you to monitor phase shifts and coherence With this

information, you can create operating deflection shapes to visualize the vibrating body

Figure 2 Impact Test with Force Hammer

Coast down peak hold:Another method used is to monitor the vibration level using a peak

hold function, while shutting down the equipment, as performed normally The vibration level should drop at a steady rate If the vibration levels start rising at any time while the

equipment is being shut down, the speed at which the amplitudes increase is a possible

natural frequency (Figure 3)

Figure 3 Coast Down Peak Hold

Coast down peak phase:Like the coast down peak hold, this test is to be conducted while

the equipment is being shut down By installing a photo tack and a piece of reflective tape on the rotating shaft of the equipment, you can monitor the vibration and its phase This will allow you to see the amplitude and phase shift at all running speeds of the equipment If there

is no resonance excited by the turning speed, the vibration levels should drop at a steady rate

If the vibration peaks at a certain speed and the phase shifts by 180 degrees, this indicates a natural frequency of the equipment or structure The actual natural frequency is the frequency situated in the middle of the phase shift (90 degrees) (Figure 4)

Figure 4 Coast Down Peak Phase Formula for natural frequency

The natural frequency is the frequency of free vibration of a system, in which a system

vibrates to dissipate its energy The natural frequency (ωn) of an equipment, expressed in

radian per second, is a function of its stiffness (k) and its mass (m), as shown by the

following equation:

If any of these two parameters are altered, the natural frequency will change

How do you modify a natural frequency?

If we want to modify the natural frequency of a body, we have to either change the stiffness

or the mass Increasing the mass or lowering the stiffness will lower the natural frequency while reducing mass or increasing stiffness will increase natural frequency

How can we operate critical equipment if we can’t change the natural frequency?

If we cannot change the stiffness or the mass of the equipment, two possible choices are offered to us One easy solution is to change the operating speed of the equipment by 20 to 30 percent, but this is not usually an option Another solution is to install a dynamic absorber on the equipment to significantly reduce the vibration levels of the equipment The dynamic absorber is a spring-mass system that is installed in series with the resonant system to create

an out-of-phase exciting force to effectively counteract the initial exciting force

Conclusion

Resonance is probably one of the five common causes of excessive machine vibration

Identifying a resonance frequency effectively can be challenging We need to positively identify the natural frequency by performing at least two different tests such as impact test, coast down peak hold, coast down peak phase or impact test using a force hammer

Once the resonance is confirmed, either change the mass or the stiffness of the equipment to change its natural frequency If it cannot be accomplished try to change the operating speed

of the equipment If that fails, consider installing a dynamic absorber to counteract the initial exciting force

Phase analysis: Making vibration analysis easier

Tony DeMatteo, 4X Diagnostics, LLC

Tags: vibration analysis, condition monitoring, predictive maintenance

Vibration analysis is mostly a learned skill It is based 70 percent on experience and 30

percent on classroom training and self study It takes years to become a confident and

competent vibration analyst When the analysis is wrong, the recommendations for repair also will be incorrect No vibration analyst wants to make the wrong call In this business,

credibility is gained in small steps and lost in large chunks

A vibration sensor placed on a bearing housing and connected to a vibration analyzer

provides time, frequency and amplitude information in the form of a waveform and a

spectrum (Figure 1) This data is the foundation for vibration analysis It contains the

signatures of nearly all mechanical and electrical defects present on the machine

Figure 1 Vibration Waveform and Spectrum

The vibration analysis process involves determining the vibration severity, identifying

frequencies and patterns, associating the peaks and patterns with mechanical or electrical components, forming conclusions and, if necessary, making recommendations for repair.Everybody involved in vibration analysis knows that analyzing vibration is not easy nor automated Have you ever wondered why? Here are a few reasons:

1) Machines Have Multiple Faults: The vibration patterns we learn in training and read

about in books just don’t look the same in the real world We learn how mechanical and electrical faults look in the purest form – as if there was always only that one problem on the machine causing vibration Machines usually have more than one vibration-producing fault

At a minimum, all machines have some unbalance and misalignment When other faults develop, the waveform and spectrum quickly become complicated and difficult to analyze The data no longer matches the fault patterns we have learned

2) Cause and Effect Vibration: For every action, there is a reaction Some of the vibration

we measure is the effect of other problems For example, the force caused by rotor unbalance can make the machine look like it is out of alignment, loose or rubbing Consider all of the things that shake and rattle on your car when one tire goes out of balance

3) Many Fault Types Have Similar Patterns: Because machine rotors rotate at a particular

speed, and vibration is a cyclical force, many mechanical and electrical faults exhibit similar frequency patterns that make it difficult to distinguish one fault from another

Learning to analyze vibration just takes time Training courses, technical publications and other resources such as online resources and commercial self teaching material are available that can improve analysis skills and shorten the learning curve

There is one diagnostic technique which quickly gets to the source of most vibration

problems It is possibly the most powerful of all vibration diagnostic techniques It has been around as long as vibration analysis itself yet hasn’t gotten a lot of attention, and it’s rare to find good information about the subject What is this technique? It’s called phase analysis

What is Phase?

Phase is the position of a rotating part at any instant with respect to a fixed point Phase gives

us the vibration direction Tuning a car engine using a timing light and inductive sensor is an application of phase analysis (Figure 2)

Figure 2 Engine tuning using a timing light is phase analysis.

A phase study is a collection of phase measurements made on a machine or structure and evaluated to reveal information about relative motion between components In vibration analysis, phase is measured using absolute or relative techniques.

Absolute phase is measured with one sensor and one tachometer referencing a mark on the

rotating shaft (Figure 3) At each measurement point, the analyzer calculates the time

between the tachometer trigger and the next positive waveform peak vibration This time interval is converted to degrees and displayed as the absolute phase (Figure 4) Phase can be measured at shaft rotational frequency or any whole number multiple of shaft speed

(synchronous frequencies) Absolute phase is required for rotor balancing

Figure 3 Absolute Phase Measurement

Figure 4 Absolute phase is calculated between the tach signal and vibration waveform

Relative phase is measured on a multi-channel vibration analyzer using two or more (similar

type) vibration sensors The analyzer must be able to measure cross-channel phase One single-axis sensor serves as the fixed reference and is placed somewhere on the machine (typically on a bearing housing) Another single-axis or triaxial sensor is moved sequentially

to all of the other test points (Figure 5) At each test point, the analyzer compares waveforms between the fixed and roving sensors Relative phase is the time difference between the waveforms at a specific frequency converted to degrees (Figure 6) Relative phase does not require a tachometer so phase can be measured at any frequency

Figure 5 Relative Phase Measurement

Figure 6 Relative Phase Calculated Between Two Vibration Waveforms

Both types of phase measurements are easy to make Relative phase is the most convenient way to measure phase on a machine because the machine does not need to be stopped to install reflective tape on the shaft Phase can be measured at any frequency Most single-channel vibration analyzers can measure absolute phase Multi-channel vibration analyzers like the Pruftechnik VibXpert illustrated in Figure 7 have standard functions for measuring both absolute and relative phase

Figure 7 Pruftechnik VibXpert 2-Channel Vibration Analyzer

When to use Phase Analysis

Everyone needs phase analysis A phase study should be made on problem machines when the source of the vibration is not clear or when it is necessary to confirm suspected sources of vibration A phase study might include points measured only on the machine bearings or it can include points over the entire machine from the foundation up to the bearings The

following are examples of how phase can help analyze vibration

Soft Foot

The term soft foot is used to describe machine frame distortion It can be caused by a

condition where the foot of a motor, pump or other component is not flat, square and tight to its mounting, or many other things, such as machining errors, bent or twisted feet and non-flat mounting surfaces Soft foot increases vibration and puts undue stress on bearings, seals and couplings Soft foot on a motor distorts the stator housing creating a non-uniform rotor to stator air gap resulting in vibration at two times line frequency

A good laser shaft alignment system should be used to verify soft foot by loosening the machine feet one at a time

Phase can be used to identify soft foot while the machine is in operation Measure vertical phase between the foot and its mounting surface If the joint is tight, the phase angle is the same between surfaces If the phase angle is different by more than 20 degrees, the foot is

loose or the machine frame is cracked or flimsy Figure 8 is an example of the phase shift across a soft foot.

Figure 8 A phase shift between the foot and mount may indicate soft foot.

Cocked Bearings and Bent Shafts

Phase is used to detect cocked bearings and bent shafts Measure phase at four axial locations around the bearing housing If the bearing is cocked or the shaft is bent through the bearing, the phase will be different at each location If the shaft is straight and the bearing is not twisting, the phase will be the same at each location (Figure 9)

Figure 9 Phase identifies in-plane or twisting bearing motion.

Figure 10 Horizontal to Vertical Phase Shift of about 90 Degrees Confirms Unbalance

Looseness, Bending or Twisting

Phase is used to detect loose joints on structures and bending or twisting due to weakness or resonance To check for looseness, measure the vertical phase at each mechanical joint as indicated by the arrows in Figure 11 When joints are loose, there will be a phase shift of approximately 180 degrees The phase angle will not change across a tight joint

Figure 11 A phase shift between bolted joints indicates looseness.

Shaft Misalignment

Shaft misalignment is easily verified with phase Measure each bearing in the horizontal, vertical and axial directions Record the values in a table or bubble diagram as shown in Figure 12 Compare the horizontal phase from bearing to bearing on each component and across the coupling Repeat the comparison using vertical then axial data Good alignment will show no substantial phase shift between bearings or across the coupling The machine in Figure 12 has a 180-degree phase shift across the coupling in the radial directions The axial directions are in-phase across the machine The data indicates parallel (offset) shaft

misalignment

Figure 12 Phase Data Indicates Parallel Shaft Misalignment

Operational Deflection Shapes

Instead of comparing the phase and magnitude numbers from a table or bubble diagram, operational deflection shape software (ODS) can be used to animate a machine drawing An ODS is a measurement technique used to analyze the motion of rotating equipment and structures during normal operation An ODS is an extension of phase analysis where a

computer-generated model of the machine is animated with phase and magnitude data or simultaneously measured time waveforms The animation is visually analyzed to diagnose problems ODS testing is able to identify a wide variety of mechanical faults and resonance issues such as looseness, soft foot, broken welds, misalignment, unbalance, bending or twisting from resonance, structural weakness and foundation problems

Figure 13 is a simple ODS of three direct-coupled shafts Phase and magnitude were

measured from permanently mounted X and Y displacement probes on a turbine generator The values listed in the table were used in ODS software to animate a stick figure drawing of the high- and low-pressure turbine shafts and the generator shaft The picture to the right of the table is a capture from the ODS animation showing the vibration pattern of each shaft and the relative motion between shafts at 3,600 cycles per minute (turning speed)

Figure 13 Shaft Operational Deflection Shape

Many machines vibrate due to deteriorated foundations, looseness, resonance of the support structure and other problems that occur below the machine bearings A phase study might include hundreds of test points measured all over the machine and foundation Good ODS software can make it easier to analyze phase and magnitude data from a large number of test points Analysis of an ODS involves observation and interpretation of the machine in motion Figure 14 is an ODS structure drawing of a vertical pump

Figure 14 Vertical Pump Operational Deflection Shape Structure Drawing

Conclusion

Condition-based vibration testing is a vital component of a reliability based maintenance program Vibration sensors, instruments and software are able to provide key information about machine health The weak link in the chain is the analyst’s ability to interpret the data, accurately diagnose the problem and trend the fault until it is time to recommend corrective action Phase analysis is a very powerful diagnostic tool Every vibration analyst should be using phase to improve vibration analysis accuracy

"Diagnostics of Rotating Machines in Power Plants"

Vibration Phase Analysis The first lesson

Posted on:Wednesday, June 19, 2013 comments:No Responses By: yabdo

category:Mechanical Engineering Views 221

Vibration Phase Analysis The first lesson

Phase Analysis The first lesson

Life for many vibration analysts revolves around the spectrum If the fault is not obvious in the spectrum then the fault may not be detected And in some cases, the fault condition is

misdiagnosed because a number of conditions present themselves in very similar ways The use

of phase readings can help you to differentiate between these conditions If you master phase analysis, your ability to diagnose faults correctly will be enhanced greatly

Phase analysis is a very powerful tool The perception may be that phase measurements are difficult to collect or possibly that the readings are difficult to understand or interpret Some may even believe that phase measurements do not offer any useful information They are wrong The aim of this article is to show that phase measurements are neither difficult to collect nor difficult to understand

We will start by revising the fundamentals of phase, and then look at how you can measure phase with a single-channel data collector, a dual-channel data collector, and with a strobe

In part two of this article, we will look at how these readings can be used to diagnose a wide range of fault conditions: unbalance, misalignment, looseness, bent shaft, cocked bearing, eccentricity and resonance We will review how comparing phase readings can reveal so much about the machine, and we will take a quick look at Operating Deflection Shape (ODS) and modal analysis

What is phase

Let’s first go through a review of phase

Phase is all about timing

Phase is all about the relative timing of related events Here are a few examples:

1 When balancing we are interested in the timing between the heavy spot on the rotor and

a reference point on the shaft We need to determine where that heavy spot is located, and the amount of weight required to counteract the rotational forces.

2 When we look at fault conditions such as unbalance, misalignment, eccentricity, and foundation problems, we are interested in the dynamic forces inside the machine, and as

a result, the movement of one point in relation to another point.

3 We can use phase to understand the motion of the machine or structure when we

suspect a machine of structural resonance, where the whole machine may be swaying from side to side, twisting this way and that, or bouncing up and down.

So, phase is very helpful when balancing, and when trying to understand the motion of a

machine or structure But phase is also very useful when trying to diagnose machine fault

conditions If your attitude is “the vibration levels are high – it needs to be overhauled”, then you probably don’t care about phase But if you want to make an accurate diagnosis, and correctly distinguish between faults such as unbalance, misalignment and bent shaft, then phase is an essential tool

Phase fundamentals

If you measure the vibration from a machine and filter out all sources of vibration leaving only the vibration at the frequency corresponding to the running speed (i.e 1X vibration) then the time waveform is a sine wave The vibration level will be dictated by a number of factors, but let’s just focus on the forces due to unbalance

Let’s use a simple fan as our reference machine There is a gold coin attached to one of the blades which generates the unbalance force We see a sine wave with the corresponding angles

of rotation as illustrated in Figure 1

Figure 1: A sine wave with the corresponding angles of rotation

But this information by itself does not tell us very much Phase is a relative measure, so we need

to compare one source of vibration to either another source of vibration or a reference of some kind

First we’ll try to understand phase by comparing two sources of vibration If we had two identical fans, each with coins on a blade (to generate an unbalance force), we would expect to see sine waves from each fan as shown in Figure 2 If the fans were perfectly synchronized such that the coins were both at the 12:00 position at the same time, they would be said to be “in-phase”

Figure 2: The two fans are in-phase.

However, if one coin was at the top (12:00) when the other was at the bottom (6:00), they would

be “180° out-of-phase”, as shown in Figure 3 Why 180°? Because one rotation is 360°, so half

a rotation is 180°

Figure 3: The two fans are 180° out-of-phase.

And if one coin was at the top, and the other was a quarter of a rotation around, they would be 90° (or 270°) out of phase, as shown in Figure 4

Figure 4: The two fans are 90° out-of-phase.

Comparing two waveforms

If you look at the previous examples you can see two waveforms with the same frequency (the fans are running at exactly the same speed) By comparing the two time waveforms we can see the time difference between them In our example the waveforms have come from two different fans We are normally interested in two sources of vibration from the same machine

We can determine the phase difference by first measuring the period (i.e time) of one complete cycle (remember, one cycle is 360°) and comparing that to the difference in time between the waves, as illustrated in Figure 5

Figure 5: Two waveforms, highlighting the time delay between them

The result is a voltage signal that provides a “TTL” pulse once-per-revolution as shown in Figure

6 The time between pulses is the period of the machine speed To keep the numbers simple, let’s assume the fan was rotating at 1500 RPM, or 25 Hz Therefore the time between the pulses would be 0.04 seconds (1/25 = 0.04)

Figure 6: Tachometer signal from a fan

As before, we can compare the vibration from the machine to the reference signal as illustrated

in Figure 7 The time between pulses is 0.04 seconds, and the time between the peaks of the wave would be 0.04 seconds If there is 0.01 seconds between the pulse and the peak of the wave, then the phase difference would be 90° Note: ¼ of 0.04 seconds is 0.01 seconds ¼ of 360° is 90°

Figure 7: Sine wave and tachometer signal showing the time and phase difference

Fortunately the data collector has the electronics and software necessary to utilize tachometer signals or signals from accelerometers in order to determine the phase angle, so these

calculations are performed automatically

Collecting phase readings

Let’s take a closer look at how we measure phase In the previous section we described two basic methods: using a tachometer reference, and using the vibration from another sensor There is a third method that utilizes a strobe, but we’ll get to that later

Using a tachometer

There are a number of ways to obtain a once-per-revolution tachometer signal The most common involves the use of reflective tape and an optical (or laser) tachometer as illustrated in Figure 8

Figure 8: Accelerometer and laser-tachometer installed on a fan

There are a number of products available that can use reflected light, including laser light, to generate the tachometer signal Some will work without reflective tape, as long as there is an area of high contrast – for example, a paint spot

The photocell shines a light (visible or laser) on to the shaft Due to the surface texture and color, the light does not normally reflect When the tape passes underneath, the light reflects The tachometer generates a “TTL” signal that is fed into the data collector

Another way to generate a once-per-revolution signal is to use a displacement (proximity) probe which is aimed at a keyway or setscrew The change in displacement provides the step in voltage which is used as the reference This is commercially known as a “keyphasor” (by Bently Nevada)

The output from the tachometer is fed into the tachometer input of the data collector; it may be labelled “EXT” or “TACH” or “TRIG” or by some other label You will need to refer to the

operating manual of your data collector to understand where to connect the tachometer signal and how to use it to collect phase readings Figure 9 is an example of one such data collector, used by DI, SKF, DLI and Rockwell (Entek)

Figure 10: Filtered 1X vibration signal and tachometer signal

The data collector can determine the phase angle in a number of different ways It can apply the two-channel method that will be discussed next, or it can use the tachometer to trigger the data acquisition process and acquire the phase angle from the FFT process

Using a two-channel data collector

Did you know that when your data collector takes a measurement on a machine and computes the FFT (spectrum), it actually computes the magnitude (amplitude) spectrum and “phase

spectrum”? But because you do not have a reference signal (the data collector starts sampling when you press the button, not according to any pre-defined reference on the shaft) the phase data does not have a lot of value So it is discarded and we only keep the magnitude spectrum However, there are two possibilities available to us If the data collection was synchronised to the tachometer reference, the phase data would be relevant We could look at the phase at the running speed and use that information This is one of the ways that data collectors measure phase when using the tachometer But there is another way

If we connect one accelerometer to one channel of a two channel data collector, and we connect another accelerometer to the second channel, the data collector can sample them

simultaneously (this is essential) and compare the phase spectra We would place one sensor

at a reference location, and the second sensor at the point of interest, as shown in Figure 11

We can also move that sensor around to different locations to see how the phase angle changes (while leaving the reference sensor in the same location the whole time) In Figure 11 we are measuring the difference in phase between the vertical and horizontal axis

Figure 11: Two accelerometers attached to the bearing so that we may measure the phase difference between the vertical and horizontal axes

As you can see, two-channel phase readings (or “cross-channel phase” as it is widely known) is easy to collect A great many analysts own two-channel data collectors but do not utilize it full potential

Using a strobe

Stroboscopes can be used to collect phase readings in two ways

Strobe as a tachometer

If we tune the strobe to the running speed of the machine (so that the shaft or coupling appears

to have stopped rotating), the output of the strobe can be connected to the tachometer input of the data collector The data collector would treat the signal from the strobe as if it were a normal tachometer input

However, if the machine speed varies slightly, the signal from the strobe will no longer represent the exact speed of the machine – the phase reading will be inaccurate If you set up the strobe

so as to freeze a keyway, setscrew or some other point on the shaft or coupling, then you should use that as your reference before you record the amplitude and phase reading If the speed varies then you will see the keyway/setscrew begin to rotate forward or backwards You can then adjust the flash rate so that it again freezes

Data collector driving the strobe or visa versa

There is another way to use a strobe that is very effective, however not all strobes or data

collectors have this capability

The vibration sensor is connected to the strobe and it is placed in “EXT” mode You control the flash rate of the strobe until you freeze the motion of the shaft Switch to “LOCK/TRACK” and the strobe will now use its internal circuitry to filter the vibration signal and extract the vibration at the running speed The strobe can now track any changes in speed The strobe will typically

have a TTL output signal that can be connected to the tachometer input of the data collector A sample strobe is shown in Figure 12

When the strobe or data collector is set to track the running speed you can perform “visual phase measurements” The strobe will flash at the running speed of the machine, thus the shaft (or coupling) will appear to freeze (Of course, you must be very careful – the shaft has not stopped and you must be careful not to touch it.) You should then set a visual reference, like a keyway or setscrew, and use the “Relative Phase” knob on the strobe to adjust the keyway/setscrew so that

it is at the 12:00 position

If you watch the shaft/coupling while you move the accelerometer, it will appear as if the

shaft/coupling rotates The amount of rotation is dictated by the phase difference between the original sensor position and the new position For example, if the machine was out of balance and you move the accelerometer 90º, the shaft/coupling will appear to rotate 90º (a quarter turn),

as demonstrated in Figure 13

Figure 13: The pulley has rotated a quarter-turn when the accelerometer was moved 90º

This is a very effective phase analysis method As you move the sensor around the machine you can see how the phase changes without even looking at actual phase values It is best if you can use a setscrew, keyway, paint spot, or reflective tape as your visual reference You should start by adjusting the strobe so that the reference is at the top of the shaft As you move the sensor it is very easy to note the change in phase

Using the phase readings to diagnose fault conditions

In part 2 of this article we will investigate how to utilize the phase readings to diagnose fault conditions We can do this in a very simple way, by comparing the readings between two axes or two points on the machine (utilizing a bubble diagram to make it easier to keep track of the readings), or we can utilize more sophisticated software to animate the movement of the

machine and supporting structure

Suffice to say that the phase readings allow us to understand the relative motion of the machine

We investigate whether two points are in-phase, 90° out-of-phase, 180° out-of-phase, or some other relationship The two points being compared may be two points on either side of the coupling, two points at either end of a component, or between two axes (e.g horizontal and vertical) at the same location

Phase is a great diagnostic tool, and if you have a dual-channel data collector or a strobe it is very easy to acquire and interpret the readings

Introduction

Welcome to part two of the phase analysis article If you missed part one then go read it now!

After reading part one of the article, I hope you will agree that phase readings are not difficult

to collect or understand - in fact, if you have a two channel data collector, they are downright easy to collect

In this article we will discuss how phase readings can be utilized to help you to diagnose a wide range of faults conditions While spectra and time waveforms can provide an indication

of a fault condition, quite often phase readings can help you confirm the exact nature of the condition, helping you to distinguish different conditions that have similar vibration patterns

Quick overview

By collecting phase readings at different points on the machine we can determine if it is correctly balanced; if the shafts or pulleys are correctly aligned; if the bearings are cocked on the shaft; if there is runout or eccentricity; if a shaft is bent; if a foot is cracked or loose; and more We can also use phase readings to provide an indication of a resonance condition The correction of these problems will greatly improve the reliability of the machine;

extending its life, and in some cases, producing products of higher quality Missing the diagnosis will ultimately reduce profitability And misdiagnosing the condition will waste time, labor, parts, and increased downtime

Relative phase readings

We utilize relative phase readings to diagnose fault conditions We do not care what the

actual reading is on top of the motor; we are only interested in how it compares to the reading

on the side of the motor and the reading at the other end of the motor If they are in-phase, that tells us something If there is a 90° difference (approximately), then that tells us

something And if the difference between the readings is something else altogether, for example 132°, then that also provides useful information

We can do this by taking phase readings at each of the key locations on the machine relative

to a reference (typically the tachometer), and then compare the readings between each of the points Better yet, with a two-channel data collector we can perform relative measurements between each of the points and simply record the difference Depending upon what we suspect, we will compare the readings in the vertical, horizontal and/or axial direction

You see, what we are actually analyzing is the dynamic motion of the machine The forces due to mass unbalance cause the machine to move a certain way A misaligned shaft causes a machine to move a different way The same is true for a number of other conditions So we use phase to detect the telltale movements (It is also worth commenting that some faults do not generate characteristic forces, thus the phase readings do not provide a clear picture of the

dynamic movement of the machine – however this in itself provides a clue to the nature of the condition.)

Representing phase

Rather than recording the phase readings numerically, we can record them visually It can be difficult to look at a series of numbers and interpret the movement of the machine However using graphical symbols makes this task easier

We can do this by drawing a circle and a tail at the desired angle, it is easy to quickly

determine the angle with a quick glance, as shown in figure 1

Figure 1 – Sample phase readings displayed graphically

You don’t even need to write down the phase angle - you can just draw the tails; either inside

or outside the circle, as shown in Figure 2 You can easily see that these two readings are 180° out of phase (Often the angle is written above the horizontal line and the amplitude is written below the line, or visa versa.)

Figure 2 – Alternative methods of graphically displaying phase readings

This data can be used in a number of ways, but one common method is called the bubble

diagram (developed by Ralph T Buscarello), as illustrated in Figure 3 You can take readings

around the machine and enter them into the diagram, adding the tails according to the angle

Figure 3 – Sample bubble diagram sheet

Precautions when utilizing phase data

You must be careful when comparing phase readings taken at opposite ends of a machine, or when comparing phase readings taken across a coupling Phase readings are sensitive to direction Therefore you have to add 180° to your readings if the accelerometer is turned 180°

You must also be familiar with the phase convention used by your data collector Figure 4 illustrates one such convention

Figure 4 – One of the phase conventions used

Also note that when we talk about the phase relationships between certain points machine I may quote that the phase readings should be in-phase, 90° or 180° out of phase These are only approximate values The actual readings may be up to 30° higher or lower and the rule still holds For example, if the difference between two readings was between 150° and 210°, then you can consider the readings to be 180° out-of-phase

Also, if the difference between two readings is approximately 270°, then that is equivalent to

a 90° phase difference Likewise the phase difference of -180° is equivalent to a 180° phase difference It all depends upon the direction of rotation, the setup of the data collector, and the convention used by the data collector

Diagnosing fault conditions with phase

Sadly there is not enough room in this article to fully explain all of the amplitude and phase relationships that can be made in order to diagnose all of the fault conditions, or show the sample bubble diagrams – that’s what our training courses and iLearnVibration are for!

We will use a sample machine, shown in Figure 5, to look at how phase readings can help us

to diagnose faults conditions

Figure 5 – The machine we will use to illustrate a number of the key phase relationships

We can take a number of measurements in order to understand the motion of the machine

We can take readings vertically and horizontally at each end of the component We can compare the amplitude and phase of vertical versus horizontal; we can compare the vertical readings at both ends of the component, and we can compare the horizontal readings at both ends of the component For coupled machines, we can also take phase readings on either side

of the coupling and compare the readings

Axial readings are also very important Rather than a single reading, we can take readings on either side of the shaft; to compare the left side to the right side, and compare the top to the bottom reading And again we can compare axial readings taken on either side of the

coupling (for example on the motor and pump)

You might routinely collect a single axial vibration reading, but when you are collecting phase readings it is important to collect two axial readings, and in certain cases that we will discuss later, you may even collect four readings Due to restricted access, safety issues, and machine construction, you may only be able to take axial measurements at one end of the machine

Diagnosing unbalance

Although considered by some to be the most common and simplest fault to diagnose, it is actually quite easy to confuse unbalance with other fault conditions If you find a high 1X peak and assume it needs to be balanced, you may be quite wrong – and generate a lot of unnecessary work - and still not correct the fault

We need to go back and study the motion of a rotor when it is not balanced correctly If you understand the underlying motion you will be able to use phase data to prove that the rotor is

in fact out of balance, and rule out other possibilities

We will now quickly review the different forms of unbalance, and then look at how we can analyze the end-to-end phase readings and the vertical-to-horizontal phase readings (relative amplitude values are also very useful, but that is beyond the scope of this article)

Static unbalance

The simplest type of imbalance is equivalent to a heavy spot at a single point in the rotor This is called a static imbalance because it will show up even if the rotor is not turning - if placed in frictionless bearings the rotor will turn so the heavy spot is at the lowest position

We would expect that the motion at the two ends of the component would be in-phase (that is, the two vertical readings would be in phase, and the two horizontal readings would be in phase) Due to the circular motion, we would also expect that the phase angle between the vertical and horizontal axis would be approximately 90°, as illustrated in Figure 6

Figure 6 – Phase relationship when a fan is not balanced statically

Couple unbalance

A rotor with couple unbalance may be statically balanced (it may seem to be perfectly

balanced if placed in frictionless bearings), but when rotated, it will produce centrifugal forces on the bearings, and they will be of opposite phase

Therefore the phase angle between two vertical readings (taken from each end of the

component) will be similar to the phase angle between the two horizontal readings;

approximately 180°, as illustrated in Figure 7

Figure 7 – Phase relationship for couple unbalance

Dynamic unbalance

In reality the amount of unbalance will not be evenly distributed along the rotor (unless it is a very narrow rotor or axial fan, in which case it will approximate static unbalance) We are likely to have a combination of static and couple unbalance, as illustrated in Figure 8 The combination is called dynamic unbalance

Figure 8 – Phase relationship for dynamic unbalance

Vertical machines and overhung machines

Phase readings can also help us to diagnose unbalance in vertical machines and overhung machines

Vertical machines, such as vertical pumps, are usually cantilevered from their foundation, and they usually show maximum vibration levels (at the running speed) at the free end of the motor regardless of which component is actually out of balance Phase readings collected along the machine should be in-phase Because of the circular motion that results from unbalance, the phase readings taken 90° around from the reference measurements should be 90° higher or lower; depending upon the direction of rotation

The dynamics of an overhung machine are quite different; therefore our study of relative vibration levels and phase readings is quite different Overhung pumps and fans are common

in industry so you must examine the machine closely to ensure that you know whether a component is in fact overhung or supported on both sides by bearings

The phase readings will be in-phase in the axial direction, as shown in Figure 9 Because of the bending motion there will be between 0° and 180° difference between the two horizontal readings, likewise between the vertical readings The phase difference between the vertical readings will be similar to the phase difference between the two horizontal readings And because of the circular motion, there will be approximately 90° between the vertical

and horizontal readings

Misalignment

Misalignment is very common, however it can be difficult to detect misalignment with vibration spectra alone Misalignment can be easily confused with other fault conditions, including imbalance and looseness Phase analysis is a great aid

Figure 10 - Pure offset misalignment

Figure 11 - Pure angular misalignment

Figure 12 - Common misalignment

When a machine is misaligned there are characteristic forces at play in proportion to the degree of offset and angle between the “rotational centerlines” of the shafts These forces are very different to those observed when a machine is poorly balanced; therefore the phase relationships are quite different If you suspect imbalance or misalignment, and you perform the tests described in the previous section and find that the rules are not met (for example, the

phase angle between the vertical and horizontal axes is not between 110° and 70°), then there

is a very good chance that the machine is misaligned

1. The phase relationship between the vertical and horizontal readings taken at the ends

of the machine will not follow the rules that we described with unbalance Due to the motion created with angular and offset misalignment, and the affect that different coupling types will have on that motion, the phase angle between the ends of the machine will not be consistent in the vertical and horizontal directions

2. If a machine is misaligned, we would not expect to see 90° difference between the vertical and horizontal readings taken at the same bearing Instead they are likely to

phase difference between the measurement taken in the axis of the belt and at right angles to that direction of 0° or 180° Note that we are not taking phase measurements in the true

vertical and horizontal directions We are taking one measurement in line with the belts, and the other at right-angle to this direction

Figure 13 – Phase relationships for an eccentric pulley

Bent shaft

A bent shaft predominantly causes high 1X axial vibration The dominant vibration is

normally at 1X if the bend is near the center of the shaft, however you will see 2X vibration if the bend is closer to the coupling Vertical and horizontal measurements will also often reveal peaks at 1X and 2X, however the key is the axial measurement Phase is also a good test used to diagnose a bent shaft The phase at 1X measured in the axial directions at

opposite ends of the component will be 180° out of phase

It is also possible to take phase readings around the shaft – on both sides of the shaft, and above and below, as illustrated in Figure 14 We expect all of the readings to be in-phase

Figure 14 - Phase relationships for a bent shaft

If the inner race is cocked on the shaft, then the bearing will appear to “wobble” as it rotates, generating a rotating 180° phase difference There will be 90° difference as you move from top to right to bottom, to left (or 12:00 to 3:00 to 6:00 to 9:00) The phase relationships are illustrated in Figure 15