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DoctorKnow® Application Paper Title: Advanced Troubleshooting Source/Author: Tony Dematteo Product: RBMware Technology: AMS Machinery Manager Classification: Section One Phase Objective Review the concepts of Phase Phase Review Phase can be defined in two ways. First, phase is that part of a cycle (0 o to 360 o ) through which a particular part of a machine travels relative to a fixed reference point. Second, phase refers to the location in degrees that marks a machine's high vibration peak and its frequency relative to a fixed reference mark on a rotating component of the machine. This second type of phase is also known as synchronous phase.
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Page 1: Advanced machinery Troubleshooting

DoctorKnow® Application Paper

Title: Advanced Troubleshooting Source/Author: Tony Dematteo Product: RBMware Technology: AMS Machinery Manager Classification:

Section One

Phase

Objective

Review the concepts of Phase

Phase Review Phase can be defined in two ways. First, phase is that part of a cycle (0o

to 360 o ) through which a particular part of a machine travels relative to a fixed reference point. Second, phase refers to the location in degrees that marks a machine's high vibration peak and its frequency relative to a fixed reference mark on a rotating component of the machine. This second type of phase is also known as synchronous phase.

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Phase varies with the monitored frequency. For machinery diagnostic work, synchronous phase refers to 1xTS or any harmonic of turning speed. CSI measures phase counter to the rotation of the shaft

Specific Fault Types In the following examples, the use of phase to analyze faults is explained.

Unbalance

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Use Phase to confirm unbalance. Static Unbalance shows a zero degree phase shift across the rotor radial to radial or horizontal to horizontal and a 90 o (within 20 o ) phase shift from vertical to horizontal at the same bearing location. Dynamic unbalance shows a phase shift across the rotor radial to radial or horizontal to horizontal that is related to the heavy spots on each end of the rotor. If the heavy spots are 180 o out of phase on each end, then the phase measurements will also be 180 o out of phase.

Reactionary Forces

Use phase to find problems that look like unbalance but are really caused by something else. In the following example, the predominant frequency is turning speed of the large pulley. Comparative horizontal to vertical phase readings indicates a zero or 180o phase shift from horizontal to vertical. It looks like unbalance but it' s really an eccentric sheave -- a well balanced, eccentric sheave. An orbit of this data would indicate an elliptical shape in line with the drive belt.

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Misalignment

Angular misalignment will typically show a 180 o (within 30 o ) phase shift across the coupling in the axial direction. Parallel misalignment will tend to show a 180 o (within 30 o ) phase shift across the coupling in a radial direction. Phase measurements, made on all bearings in the horizontal, vertical and axial directions will confirm the misalignment type.

Looseness and Soft Foot

Phase reading with looseness will be erratic from point to point around the machine train. A soft or loose component usually shows a phase shift between the tight and loose joints. Often this shift will be greater than 90 o and as much as 180 o . To identify the source of looseness on a machine, measure phase across all bolted or welded joints. When the phase shifts, the looseness has been found. For soft foot, measure phase across the bolted joint and compare to the other machine feet.

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Resonance

Through resonance, phase shifts 180 o . At resonance, a 90 o phase shift will be present. A Bode plot of coast-down data is an excellent test to verify resonance. As the amplitude peaks, the phase shifts 180 o

Bent Shafts/Bearing Twist

Phase can easily identify a shaft bent through its bearing or a self aligning bearing where the outer race and housing are not perpendicular with the shaft. To test for this condition, take phase measurements around the face of the bearing. If the phase is steady (within 30 o ) the bearing is not twisting. If the phase is constantly changing at each position measured, it is an indication of twist in the bearing or bend in the shaft through the bearing.

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Operational Deflection Shapes

Operational deflection shapes (ODS) use phase and magnitude data to animate the motions of machines and structures during normal operation.

Modal Analysis

The transfer function response to a known input force is used to animate the shapes of machines and structures at its natural frequencies.

Section Two

Phase and Magnitude Collection Techniques

Objectives

Review four different ways of making Phase and Magnitude measurements using a 2120 analyzer

Phase and Magnitude Measurements

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A vibration sensor gives magnitude and frequency information. When phase data is also measured the vibration direction is known.

Phase is the position of a part (at any instant) with respect to a fixed point.

Phase can be defined in two ways. First, phase is that part of a cycle (0° to 360 ° ) through which a particular part of a machine travels relative to a fixed reference point. Second, phase refers to the location in degrees that marks a machine's high vibration peak and its frequency relative to a fixed reference mark on a rotating component of the machine. This second type of phase is also known as synchronous phase.

If you've ever given your car's engine a tune-up with a strobe light, you have used phase. During an engine tune-up, a strobe light is used to freeze the image of the flywheel so that the position of a mark, scribed on the flywheel, is viewed and compared to a fixed gauge on the engine block. That's phase!

In vibration analysis, phase describes the direction of vibration. Many vibration problems have similar frequency patterns. For example, a high axial vibration at turning speed may be an indication of coupling misalignment, unbalance on an overhung rotor, a bent shaft or a misaligned bearing.

Phase analysis is probably the most important "tool" that an analyst has to confirm defects suggested by spectra and waveform data.

A phase analysis job consists of a collection of phase and magnitude values from points on a machine. The phase relationship, between points on a machine, identifies specific defect types such as misalignment, unbalance, looseness, bending and weakness. Once the

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data has been collected, it must be analyzed. Operational Deflection Shape testing (ODS) simplifies the analysis of phase and magnitude data through software animation.

This chapter describes four different methods of measuring phase and magnitude data for the purpose of ODS testing. The four methods include two single channel techniques that can be done with any CSI 21xx analyzer and two, dual channel techniques that require a CSI 2120-2 analyzer.

1. Monitor Peak/Phase This method is part of the Analyze functions on all 21xx analyzers. It is a single channel technique that requires a tachometer trigger signal and one vibration sensor. The figures below show the set-up screens for the monitor peak/phase measurement.

On the analyzer, press ANALYZE, MONITOR then MONITOR PEAK/PHASE . Set the order of rotation to monitor, bandwidth and active channel. Press ENTER to begin the measurement.

Measurements are made during normal machine operation. The tachometer reflects light off a piece of reflective tape placed on a rotating shaft. Magnitude and phase are measured at any synchronous frequency.

Phase is the angle measured between the tape, as it passes in front of the tachometer and the high spot on the rotor.

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Move the sensor to each measurement point and direction. Press CLEAR to begin the measurement. The 21xx analyzer display shows the speed of the machine, magnitude and phase at the triggered synchronous frequency.

As the sensor is moved from point to point on the machine, the phase and magnitude data must be written down since there is no way of storing the values. A typical phase analysis data sheet includes phase and magnitude values at each position measured. A table is needed for each synchronous frequency measured. On ODS jobs where many points are measured on the machine and structure, the points can be recorded as numbers and directions such as 1X , 1Y, 1Z, 2X, 2Y etc.

An alternative method of recording phase and magnitude data is to use a bubble diagram. An example is shown below.

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Monitor Peak/Phase Summary

Method Type: Single Channel

Analyzer Model: 21xx

Firmware: Standard – Data collector Program

Sensors: one required

Tachometer: one required

Advantages:

Measurements can be made with any 21xx analyzer Monitor Peak/Phase is a standard feature of the analyzer

Disadvantages:

Tachometer required Limited to synchronous frequencies Manual data recording (can't save data) Manual (typed) input in ME'scope for ODS

2. MasterTrend or AMS Suite: Machinery Health Manager Route

This method uses a route database with analysis parameters configured to measure phase and magnitude. Like method #1, this is a single channel collection technique requiring a tachometer signal and vibration sensor. The method is limited to synchronous frequencies. The collection points are predefined in a route. After measuring the points,

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the data is downloaded to MasterTrend or AMS Machinery Manager and imported directly into ME'scope ODS software.

This method of collecting phase and magnitude data is useful when ODS measurements will be made several times on a particular type of machine.

In order to automatically transfer MasterTrend or AMS Machinery Manager databases to the ODS software, measurement point descriptions must be defined in a specific format. CSI provides template databases that are already configured for certain machine types. If your ODS software was purchased through CSI, a floppy disk was provided containing MT or AMS Machinery Manager templates.

In a MT or AMS Machinery Manager database, the point ID's (limited to three characters) identify the measurement point number and direction. For example, 01H identifies measurement position 01 in the horizontal direction. Do not use ID's like MOH. Use point numbers followed by directions. Each point on the machine must have a unique measurement point ID. Some examples of acceptable measurement point ID's are:

01H

01V

01A

01X

01Y

01Z

Machine coordinate directions H, V and A can be used, although it is recommended to use X, Y and Z coordinates for point directions.

In a ODS route, the point description field is the most important field and insures proper transfer of MT or AMS Machinery Manager data to the ODS software. Measurement point descriptions must have the following format:

DOF[ABBBC]d

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where: DOF = degree of freedom

A = A minus sign (-) if the sensor was inverted

B = the measurement point number (01 or 001...999)

C = the measurement direction (H, V, A or X, Y, Z)

d = any text description of the point (optional field)

A few examples of point descriptions follow:

DOF[01H] Motor Outboard (right) Horizontal

DOF[-04H] Motor Inboard (left) Horizontal

DOF[010X] Pump Outboard Axial

DOF[-007X] Pump Inboard Horizontal

An example of the MasterTrend Database, measurement point set-up screen is shown below. The first eight points in the route are shown.

Notice the minus sign in front of some of the point descriptions signifying an inversion of the sensor. Think about measuring horizontal points along the left side of a I-beam frame under a machine. To measure the horizontal points on the right side of the frame, it is necessary to invert the sensor (i.e. the sensor is turned 180 degrees). The negative sign infers the sensor rotation. Since this is a route program, the sensor directions must be considered in advance of the measurements.

In the figure above, the analysis parameter set and alarm limit set numbers refer to the specific set configurations for measuring phase and magnitude data. A special analysis parameter set must be developed which contains the parameters for

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measuring phase and magnitude at each order of rotation of interest. An example parameter set is shown below.

The spectrum parameters tab of the parameter set identifies that Fmax, lines of resolution and number of averages.

The next page of the parameter set lists the twelve parameter slots available. Not all of the parameter slots need to be used. Peak and Phase are two separate parameters that must be defined for each order of rotation of interest. If the only interest is phase and magnitude at one times turning speed then only two of the twelve parameter slots are needed. If interested in data at 2x and 3x rotating speed then additional pairs of parameters are needed. A total of six orders of rotation can be configures in one parameter set. Cross channel phase cannot be configured in a MT or AMS Machinery Manager route.

The screen shown below is an example of a route parameter set to collect peak and phase data at the first six orders of rotation. The parameter units type controls the measurement units (acceleration, velocity or displacement). Peak or Phase parameter types is selected in the "type of parameter column" ;. The last two columns on the right define the order of rotation and measurement bandwidth.

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The "Units Type" is selectable from a drop-down menu. Choose a parameter type of "N-rpm-A" for measuring the amplitude at peak and "N-rpm-P for measuring the phase at peak. The "Low Freq" column contains a number indicating the order of rotation and the "High Freq" column is the bandwidth around the peak. A bandwidth of 0.1 means that the filter around the peak has a width that is equal to 10% times the frequency of the order. For example, if 1x=1790 cpm is specified with a bandwidth of .1, the filter width would be 10% of 1790 or 179 cpm. The bandwidth is adjustable between .02 - 1.0 and is used to exclude other vibrations close to shaft turning speed.

Alarms are not needed for ODS testing, however an alarm set number is required. To deactivated the alarms in a set used for ODS testing, change all numerical entry fields to zero.

MT and AMS Machinery Manager Summary

Method Type: Single Channel

Analyzer Model: 21xx (2120 for AMS Machinery Manager)

Firmware: Standard – Data collector Program

Sensors: one required

Tachometer: one required

Advantages:

Measurements made with predefined routes Measurements with 21xx analyzers Data stored and downloaded

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Automatic transfer to ODS program

Disadvantages:

Tachometer required Limited to synchronous frequencies

3. Cross Phase (standard feature of 2120-2) This method requires the CSI 2120-2 (dual channel analyzer) and two accelerometers. The benefit of using cross phase is that no tachometer or reflective tape is needed. Phase is measured between the two accelerometers.

A phase study is made by leaving one accelerometer at a fixed position on the machine while the second accelerometer is moved to other positions and directions. The position and direction of the fixed accelerometer is arbitrary. It is recommended to use channel "A" as the fixed sensor. Phase is measured relative to the fixed and roving sensors. The figure below shows an example of a cross-phase measurement set-up.

Cross phase is a standard feature of the 2120-2 analyzer and is found under the "ANALYZE" function button. The item labeled "CROSS PHASE" is active on the 2120-2 analyzer when both channels are enabled in the UTILITY FUNCTIONS menu.

Two cross phase measurement modes are available: SINGLE FREQUENCY MONITOR and FULL PLOT ACQUIRE . In both modes, the data must be written down in a table or on a bubble diagram as the measurement cannot be saved to analyzer memory.

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Analyze Function Menu – to measure Cross-Phase Single Frequency

In the single frequency monitor mode, the Fmax, Frequency to monitor and lines of resolution are entered. Any frequency may be monitored. The monitor screen shows the cross phase between the two sensors, coherence and channel A and B magnitudes. The channel "B" magnitude and cross phase is recorded for each measurement point.

Cross Phase Mode – Full Plot Acquire

In the full plot acquire mode, a full plot of phase is presented. Move the cursor to each frequency of interest and record the phase and magnitude.

Data cannot be saved using this method. All information must be written down in a bubble diagram or table.

Cross Channel Phase (standard) Summary

Method Type: Cross Channel

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Analyzer Model: 2120-2

Firmware: Standard – Data collector Program

Sensors: two required

Tachometer: none

Advantages:

Standard feature of the 2120-2 analyzer No tachometer required Phase at any frequency

Disadvantages:

Manual record keeping Manual input to ODS software

4. Advanced 2-channel DLP Cross-Phase A DLP is additional (optional) firmware that is loaded into the 2120-2 analyzer to increase analyzer capability. DLP stands for downloadable program. Advanced Transient, Cascade, FastBal II and Pro-Align are other examples of the many different DLP's available for the 2120 analyzer. Each DLP enables the 2120 to do special test functions. The Advanced 2-channel DLP enables the 2120-2 analyzer with cross channel measurement capability. Just as in method #3, one sensor is kept at a fixed position on the machine and the other sensor is moved to each measurement position/direction. Unlike method #3, the completed measurement is saved to analyzer memory. Data collected using the Advanced 2-channel DLP is transferred to ODS or Modal Analysis software.

Once loaded into the 2120-2 analyzer, the Advanced 2-channel DLP is selected by pressing the "Program Select" button on the top of the analyzer. A list of all DLP's loaded into the analyzer will be shown.

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Beginning with firmware version 7.43, the advanced 2-channel DLP firmware has four modes of operation. One of the modes, "ODS" has specifically configured menus for ODS measurements. The ODS mode is a very easy to use program that makes short work of measuring large ODS jobs.

The automated collection of cross-channel data results in a cross phase plot that is identical to the data measured in method #3. It is not necessary to move the cursor and read values at each frequency of interest since the entire plot can be saved. No manual data recording is necessary.

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Once collected and saved to analyzer memory, the advanced 2-channel DLP downloads data to a CSI software program called VibPro. VibPro is a standalone program. VibPro is a display and analyze program for the 2120 advanced transient and advanced 2-channel DLP's.

Cross Channel Phase (standard) Summary

Method Type: Cross Channel

Analyzer Model: 2120-2

Firmware: Advanced 2-channel DLP

Sensors: two required

Tachometer: none

Other software: VibPro

Advantages:

No tachometer required Phase at any frequency Data stored and downloaded Automatic transfer of data to ODS software

Disadvantages:

Requires a 2120-2 analyzer with Advanced 2-channel DLP

Review In this section we have discussed four ways to measure phase and magnitude data for ODS measurements. Of the two single channel methods, the route based method is more automated but requires preparation time to configure the route. Of the two dual channel methods, the Advanced 2-channel method is more efficient and provides automatic data transfer to the ODS software.

Section Three

ODS

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Objectives

Learn about Operational Deflection Shape Measurements, analysis of ODS data and how ODS can be used as a tool to solve complicated vibration problems.

Provide a basic overview of the four methods of acquiring ODS data with a 2120 and 2120-2 analyzer.

What is an ODS? An Operational Deflection Shape (ODS) is a measurement technique used to analyze the motions of rotating equipment and structures. An ODS is an extension of another technique that has been around for many years called phase analysis. In an ODS, a computer generated model of the machine comes alive with motion as the phase and magnitude data, for each structure point is animated. ODS is a non-intrusive test made on machines during normal operation.

The ability to measure and study ODS data is beneficial to everyone involved in vibration measurement and analysis. A common misconception in the predictive vibration business is that route vibration measurements are the only data needed to analyze machine vibration problems. Not so! Nobody ever said that an analyst must be able to completely explain machine vibrations after collecting one piece of route data. Think about it for a moment ¼ the route was designed to be fast and economical. It is based on a series of assumptions about how the data should be collected including: Fmax, lines of resolution, weighting, averaging, and so on. Once collected, the analyst may be able to understand the defects in the machine. On the other hand, he may not. Additional testing could be required before a problem is understood. ODS testing is not as quick and inexpensive as predictive vibration route measurements. ODS can, however, lead an analyst to a thorough and accurate diagnosis.

Uses for ODS Testing ODS testing has many practical uses. As analysts, we spend most of our time trying to analyze machine vibration problems by measuring only bearing positions. Many machine vibration problems result from defects in foundations, base plates, machine feet and other structural components. If we were to attempt to analyze spectra and waveforms, measured at 25, 50 or more positions on the machine, it would be a difficult task.

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An ODS animation says volumes about the source of mechanical vibrations. A picture truly is worth a thousand words. The following list identifies some of the used for ODS measurements.

Animate the shape of an operating and vibrating machine Confirm fault type Identify weak structures Suggest resonance Troubleshoot problem machines Evaluate transient responses.

ODS Data Acquisition Methods ODS testing is accomplished with a single or dual channel analyzer. The key requirements for an ODS are phase and magnitude data. If a single channel analyzer is used to collect ODS data, a tachometer is required for phase data, thus limiting the frequencies that can be analyzed to synchronous peaks.

There are four different methods of measuring phase and magnitude data for the purpose of ODS testing. The four methods include two single channel techniques that can be done with any 21xx analyzer and two, dual channel techniques that require a 2120-2 analyzer. The four methods are:

1. Monitor Peak/Phase

2. MT or AMS Machinery Manager Route

3. Standard Cross Channel Phase

4. Advanced 2-Channel DLP

General Steps in an ODS The following list outlines the general steps in an ODS job.

1. Evaluate the machine to test 2. Choose measurement points and coordinate axes 3. Measure data 4. Transfer data to VibPro Software 5. Export data to ME'scope ODS software 6. Draw structure in ME'scope software

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7. Import data into ME'scope software 8. Assign data to structure points 9. Animate the results 10. Create Shape table 11. Create AVI movie files of the animation results 12. Analyze the animations

Section Four

ODS Case Examples

Objectives

Review Case Studies

Interpreting ODS Results After viewing the animations, the ODS results need to be interpreted. In case you didn't know it, the picture does not talk. The ODS analysis involves studying the views, analyzing the motions and determining what is wrong with the machine. Some mechanical faults will be more obvious than others. For example, misalignment or looseness between bolted joints is easily spotted. Some analysis tips are listed below:

Look for global motions -- where the entire machine is moving together with no relative motion between components. This could be a result of a machine mounted on isolators or floor and building vibrations

Look for relative motions between bearing housings or shafts (if shaft data was taken) -- an indication of misalignment

Look for phase lag and relative motion between bolted or welded joints -- indicating looseness

Look for twisting of the machine base -- indicating torsional bending modes or structural weakness

Look for bending of structural components such as I-beams – an indication of resonance (note: ODS does not prove resonance)

Look for localized motion on machine feet or bases – an indication of soft-foot

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Being a good vibration analyst means being a good detective. Study the ODS animation and look for clues that will help solve the problem. Case Studies Case 1 -- Direct Coupled Fan Problem: A motor is direct coupled to a fan. Rotational speed is 29.8 Hertz (1788 cpm). A vibration analysis of the machine indicated that the problem was on the motor – possibly on the steel base supporting the motor that bolts to the concrete. Both motor bearing horizontal readings were over 1.0 inches/second – peak (IPS) at turning speed. The vertical and axial readings were less than 0.2 IPS. The fan bearing vibrations were less than 0.15 IPS.

Figure 1-1 – Motor Structure

Because of the large difference between horizontal and vertical readings on the motor, resonance was suspected. Impact testing on the motor did not show any natural frequencies near turning speed. An ODS test was made on the motor bearings, motor feet, motor base, concrete base and concrete floor.

Figure 1-2 – As Found Vibration Data

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Figure 1-3 – ODS Structure

Case 1 Results: The ODS animation showed that there was a soft-foot condition between the steel motor base and the concrete. It also indicated that the motor base was weak – flexing from side to side horizontally. The rocking motion of the base resulted in high horizontal vibrations. The vertical vibration levels were low because center of the motor was a pivot point for the motion. With the fan still in operation, each corner of the steel motor base was loosened (one bolt at a time) while watching for a change in vibration level. Shims were added or removed as needed until the soft foot was reduced as much as possible. The outcome resulted in a 74% reduction in vibration at the horizontal positions on the motor (see figure 1-4). The vertical and axial positions on the motor increased slightly. The motor base was not strong enough to support the motor side-to-side. It was recommended that the steel motor base be replaced before additional improvement could be gained.

Figure 1-4 – Before and After Motor Vibration Data

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Case 2 -- Direct Coupled Blower

Figure 2-1 – Direct Coupled Blower

Problem: Figure 2-1 shows a new blower that was installed at a water reclamation facility. The 1500 HP motor was direct coupled to a blower. The entire machine was supported by an I-beam frame that was isolated from the concrete base with springs. During installation, the machine was aligned to the manufactures specifications. The vibration analyst at the facility believed that the machine was not properly aligned. His readings showed high, vertical, turning speed vibration on the motor and blower outboard bearings. The largest reading was 0.39 inches/second. The manufacturer insisted that the machine was aligned properly and warned that changing the alignment would void the warranty. To reduce the vibration on the machine, the manufacturer installed a tuned absorber on the outboard bearing of the motor (figure 2-2). A tuned absorber soaks-up vibration at a particular frequency. The steering column in your car probably has a tuned absorber installed inside of it to prevent the column from vibrating. The tuned absorber installed on this machine was continuously failing due to fatigue. The flat steel bars were replaced several times within a few months.

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Figure 2-2 – Tuned Absorber on OB Motor Bearing

The analyst wanted to prove to the manufacturer that the machine was misaligned. He decided to do an ODS test. The ODS test was completed using a single channel CSI analyzer. Phase and magnitude data was collected in the Analyze mode using the monitor, peak/phase function. Measurements were made on the motor and blower bearings, steel I-beam support and at the tops and bottoms of the isolators.

Figure 2-3 – ODSStructure

Case 2 Results: The animation clearly showed misalignment between the motor and blower. Weakness in the front rail at the motor outboard end is visible as well. Is it possible that the rails are resonating in the vertical direction causing the misalignment?

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The ODS does not make that distinction. It would take a bump test on the rails to confirm a resonance. The machine has not been realigned as the manufacturer said that doing so would void the warranty. Case 3 -- Misaligned Boiler Feed Pump Problem: Figure 3-1 is a boiler feed pump. The electric motor is direct coupled to a fluid drive which is direct coupled to the feed pump. All six bearings are sleeve type. The motor had failed three inboard bearings one year.

Figure 3-1 Boiler Feed Pump

The vibration group believed that the problem was misalignment. The spectral patterns indicated misalignment. They couldn't convince the mechanical group, who actually did the alignment, that the machine was misaligned. CSI services was called in to diagnose the problem. Vibration measurements on the bearing housings showed the first three harmonics of turning speed. The highest levels were at 1x on the motor bearing. The values were only about .2 IPS -- not severe levels at all. Figure 3-2 shows the data for many of the bearing positions and directions.

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figure 3-2 Bearing Housing Vibration

Shaft reading were also measured at every possible location. The shaft data indicated severe horizontal shaft vibration at the inboard motor and inboard flywheel positions. The shaft readings were over 1.0 IPS (almost 6 mils p-p) at the inboard motor horizontal position. The shaft readings were up to five times higher than the housing readings at the inboard motor position. The vertical shaft readings were much lower. Figure 3-3 shows the shaft readings compared to the housing readings.

Figure 3-3 Bearing housing and Shaft Vibration

Figure 3-4 shows the ODS model that was constructed. Measurements were made on the bearing hosings, shafts, machine frames and sole plate.

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Figure 3-4

A heat rise study was made using a CSI 510 Infrared temperature gun and an UltraSpec analyzer loaded with the Thermal Growth Firmware. Temperature measurements were made on every bearing of the machine. Points were measured from the sole plate up to the center of the bearing (figure 3-5). The hot and cold temperature readings are listed in figure 3-6.

Figure 3-5 Temp Points

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figure 3-6 Hot & Cold Thermal Growth Temperatures

Figure 3-7 Thermal Growth Results

Case 3 Results The ODS indicated severe side to side motion of the motor and flywheel shafts -- an

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indication of vertical misalignment. The results of the heat rise study indicated that the pump and motor changed very little in the vertical direction. The fluid drive, on the other hand, grows about 10-11 mils figure 3-7). It was determined that the wrong thermal growth compensation was used. The flywheel alignment needed to be changed. It should be set about 10 mils low so that the fluid drive grows into alignment with the motor and pump when the machine heats up. The machine was eventually realigned – but not to the recommendation given. The spectrums of the bearing housings show a different picture before and after alignment. Both sets of data show misalignment. The distribution of amplitude between the first three harmonics changed as a result of the alignment. The overall velocity did not change significantly.

Figure 3-8 Before and After Alignment Vibration

Case 4 – Misaligned Motor Generator Set – Or was it? Problem: Figure 4-1 is a motor generator set. The machine is a motor, flywheel and generator -- all were direct coupled. Based on the vibration spectra that were collected and analyzed, the vibration people at the power plant believed that the machine was misaligned. Two separate attempts at laser alignment had not resulted in decreased vibration levels.

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Figure 4-1

MG set The vibration group at the plant did not own ODS software. They did, however, take phase and magnitude readings on the six bearings using a 2120-2 analyzer. CSI services first involvement on this problem was by phone. The phase and magnitude data was sent to CSI via email. The data was entered into ME'scope SHAPE software and animated. The animation results indicated that the machine was misaligned. Figure 4-2 shows the ODS structure file. The motor is to the right and the MG set is to the left. Figure 4-3 shows the shape of the machine at turning speed (3592 rpm).

Figure 4-2

ODS Structure File

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Figure 4-3

ODS Animation at 1x Since two laser alignments had not fixed the problem, the misalignment might be caused by something besides shaft to shaft alignment. It was believed that weakness, looseness or twisting of the machine base was creating the misalignment condition. A more complete ODS was scheduled. The job included phase and magnitude measurements on the bearings, mounting feet, machine base, sole plate and concrete floor. A total of 156 measurements were made on the machine. Every bolted or welded joint was measured. As a result, the ODS structure was more complex. Every fastened joint was represented and measured. Figure 4-4 shows the ODS structure file. Figure 4-5 shows the twisting of the machine base at turning speed.

Figure 4-4

ODS Structure File of Machine and Base

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Figure 4-5

ODS Animation of Machine and Base Case 4 Results The animation of the phase and magnitude data showed that the machine base was twisting. The twist was occurring between the flywheel and MG set. The motor and flywheel were in-phase and out of phase with the MG set.

Section Five

Resonance Testing Objectives

Review the single-channel analysis tools available for diagnosing resonance

Measuring Resonance Resonance testing should be performed whenever vibration levels or spectral patterns cannot be explained by forcing frequencies. When diagnosing a high amplitude vibration problem, the analyst needs to consider the possibility of acceptable vibration exciting a resonance and causing unacceptable levels of vibration. Several techniques can be used to detect resonance. Most are single channel techniques. The most common single channel resonance tests are described below. Negative Averaging Negative averaging is a very powerful technique that has the capability to subtract energy from a previously collected, normally averaged spectrum. Negative averaging is the only good way of detecting resonance on an operating machine. Negative

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averaging subtracts the "noise" (as defined for the job) from two spectral measurements. The "noise" is basically, any signal that appears in both spectrums. For example: In the first measurement, a machine is operating normally and is also impacted for resonance. In the second spectrum, the machine is operating normally. The negative averaging process subtracts the two measurements -- leaving only the data resulting from the impacts. The overall reduction of those amplitudes defined as noise is proportional to the square root of the number of averages. Normal Operation + Impacts - Normal Operation = Data from Impacts To perform Negative Averaging, follow these steps:

Collect data in the acquire spectrum mode with negative averaging selected. The analyzer will take the first data set in normal averaging. (All data receive the same weight.) Ten averages should be enough. During the averaging, the machine is operating normally. In addition, impact the machine with a rubber mallet or block of wood. If hanning weighting is used, impact the structure several times to be certain that the impact has been properly measured. Use 1-2 second intervals for the impact. Just be sure the machine has enough time to "ring down" before striking again. If uniform weighting is used, it is only necessary to impact the structure 1-2 (or more) times during the averaging.

2120 Set-up for Negative Averaging

The first data set in normal averaging is stored in a buffer until the second data set is subtracted from the data contained in the buffer.

At the end of the predefined number of averages, the analyzer will stop and display the message, Begin the negative averaging process by pressing Enter. At this time, the machine is operating normally with no impacting. As the averaging begins, any signal that was present in both sets of averages will begin to average out of the spectrum. The averager will not stop until the enter button is pressed. Continue averaging until no additional change is seen in the spectrum. Press Enter to stop averaging. Store the final spectrum if desired.

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The above data comparison shows:

Top - Impact. Middle - Linear Minus. Bottom - Impact during operation.

Monitoring Peak and Phase Data (Bode Plots) The monitor peak/phase data defines the location (in degrees) of a machine's vibration peak with respect to a fixed reference mark on the rotor. The general characteristics associated with monitoring peak and phase are different depending on the data to be gathered. Within the context of this lesson, we will discuss two types of measurements.

Monitoring Peak and Phase for Phase Analysis Monitoring Peak and Phase during coastdown or ramp-up

Vibration alone indicates how much and what frequency a machine is vibrating. Adding a phase measurement tells the direction of vibration. If the vibration magnitude and direction are known, a phase analysis can be performed. Phase analysis reveals the directions components are moving in relation to each other. Phase also reveals information about specific mechanical faults. For example, phase can confirm suspected unbalance or misalignment and looseness. Resonance may be identified due to unstable phase readings, unexplainable phase relationships, and significant phase shifts during startup or coastdowns. To complete a phase analysis, it is necessary to measure, record and analyze the phase and magnitude values from the monitor peak/phase screen. Data must be measured on each bearing in the horizontal, vertical and axial planes. If monitor peak/phase is measured during a coast-down or ramp-up, the changes in

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magnitude and phase can be studied and evaluated for resonance. The steps outlined below describe using monitor peak/phase for resonance detection.

2115/2120 Monitor Peak/Phase set-up screens

Monitor Peak/Phase Measurement and Display

The data plot below is a Bode plot. A Bode plot is a rectangular plot of peak vibration magnitude and phase vs. speed. The data below indicates that during coastdown, the vibration magnitude peaked out at 1134 rpm. At the same time, the phase changed about 180 degrees through the area of amplification. This combination of events proves, without doubt, that a resonance is present at 1134 rpm. The phase reading should change by approximately 180o through the resonance area. The phase reading always becomes unstable at the resonant frequency. Phase at resonance should, however, differ by roughly 90o from measurements off of the amplification curve.

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A Nyquist plot is the same peak and phase data viewed in a polar plot format. As the phase changes rapidly at resonance, it traces out a circle on the polar plot. Every loop in the plot is another resonant frequency.

Bode plots (Peak and Phase vs. RPM) yield important information about resonance. The presence of run-out or a bow in the shaft, however, can significantly alter the appearance of the plot. Nyquist plots, on the other hand, remain unaffected by run-out and bowed shafts. Always use Nyquist plots to confirm any conclusion based on Bode plots. Peak-Hold Data Collection Peak-hold data collection retains the highest amplitude at every frequency from all the averages acquired. It is most commonly used for coastdown data for all frequencies when a once-per-revolution (tach) event marker is not available. You can also use peak-hold averaging when amplitudes are unstable from sample to sample.

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Set up the Acquire Spectrum menus as shown below. The number of averages will depend on the time it takes the machine to coast down and the configuration of the analyzer. Three items control how long it takes the analyzer to process data. They are:

The frequency span of the measurement Lines of resolution Signal Overlap.

Without optimizing the analyzer's processing speed, the peak hold coastdown plot could look like what is called "picket fencing". This condition is simply missed data during the coast-down. The spectrum consists of a series of peaks rather than a smooth trace of the coast-down. In the Utility menu/Change set-up/Measurement Control, change the overlap from the default value of 67% to 99 percent. This means that after the first average, the analyzer will use 99% old data and 1% new data for every average. This results in a faster processing speed. Choose a frequency span that places the frequency of interest away from the left edge of the spectrum without sacrificing analyzer speed (lower frequency spans mean longer data acquisition time). Use 100 - 400 lines of resolution. Usually, for peak hold coast-down testing, it is not important to have high resolution to identify resonance. Select Peak Hold averaging and no trigger on page 2. Press Enter twice to acquire the data. Shut down the machine when your analyzer starts displaying a spectrum.

An example Peak Hold Averaged Spectrum is shown below.

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If any vibration frequency passes through a resonance during coastdown, its amplitude will peak – suggesting a resonance. Resonance is not proved completely unless phase is measured as in a Bode plot. Cascade Plots Cascade or Waterfall Plots provide a three-dimensional view of the coastdown or startup data. A finite number of spectra are stacked over time. The vertical axis may be Time or RPM. If the cascade data is collected without the aid of a tachometer input, then Time becomes the only available option. The cascade shown below demonstrates the coastdown of a 100+ megawatt gas turbine that passed through a resonance (critical) during its shutdown.

Single Channel Impact Testing In single channel impact testing, a sensor placed on the machine, measures the result of impacting the structure with a rubber tipped mallet or block of wood. The

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amount of input energy and the frequency response of the impact device determines how many natural frequencies are excited. An impact test identifies resonance. It does not indicate the shape of the structure at resonance nor reveal how to correct a resonance problem. Resonance is directional and can be localized. It is important to impact different locations on the structure. There are many ways to measure impact data on the 2115 or 2120 analyzer. The method shown below triggers the analyzer when the amplitude on channel "A" exceeds the trigger level value. In other words, the analyzer will take a measurement when it senses the impact.

The trigger function provides the analyst with the ability to collect data based on a specified input to the analyzer. The pre-trigger of 10% moves the impact away from the left edge of the time window by 10% of the total time.

When performing impact testing, we should establish a full-scale (FS) range amplitude. The purpose of setting this full-scale amplitude is to prevent the data collector from wasting time during the impact test procedure. The FS setting provides the analyst the ability to maintain control of the data acquisition process. When impacting (bumping) the machine, the person performing the impacts should attempt to maintain consistent force with the hammer. The FS setting may help in achieving this goal. As a second option, the analyst can enter a zero in the FS range to tell the analyzer to autorange. However, during the impacting process, a full-scale amplitude should be used. This will prevent the need for excessive impacts to the

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machine. When determining this full scale, using the analyzer's monitor mode allows the analyst to monitor the impacts as they occur to determine the amount of energy being placed in the machine structure. The monitor waveform option can be used to evaluate the machine's response to the impacts.

The waveform shown above displays the impact as seen from the response transducer. The analyst is able to set the trigger amplitude by viewing the waveform. Additionally, the monitored waveform provides the ability to see the decay of the response. This enables the analyst to select an appropriate time to ensure the data is not cut off prior to the completion of the impact decay. Dual Channel Impact Testing Only the response to the impact is being measured with a single-channel analyzer, so the machinery must be shut off to do this properly, unless the data collection is being performed with negative linear averaging. Phase is a test that clearly identifies resonance. Phase cannot be measured during an impact test with a single-channel analyzer. In order to confirm the presence of a shaft resonance, phase must be collected using other methods. For the most useful information from a single channel analyzer, it is a good practice to have both bump test data and startup or coastdown information. Resonance is best measured using a multichannel analyzer to measure impact and response data at the same time. Phase, coherence and the transfer function are products of a cross-channel measurement. (Coherence is a dual-channel function that relates how much of the input signal caused the output signal.) This means that resonance frequencies can be identified more rapidly.

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Hammer Considerations for Impact testing Force level and frequency content are important considerations when choosing a hammer. An improperly sized impact hammer results in missed data. The hammer must input enough force to excite the natural frequencies to measurable amounts. For example, when you test the springs in your car, do you tap the bumper with a steel hammer? Of course not….you stomp on the bumper with your foot forcing the springs to bounce up and down. The hammer tip hardness determines how much frequency is delivered in the blow. Most impact hammers have replaceable tips of different hardness. The graph below demonstrates the relationship between frequency and force. Softer tips deliver more force but less frequency. Harder tips deliver more frequency but less force. Harder hammer heads also tend to bounce multiple times on the surface, making the data invalid due to multiple impacts in each time window.

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Section Six

Filtered Orbits

Objectives

Measure Filtered Orbits

Orbit Measurements In vibration analysis, an orbit plot is the trace of the relative movement of the centerline of a rotating shaft within the clearance of a plain (journal) bearing. Orbit plots are used to detect and investigate abnormal movements of the shaft in a bearing. This movement often characterizes a developing fault, such as unbalance, misalignment, bearing rub, shaft or rotor whirl, etc. Two probes mounted at 90 degrees to each other are required for making shaft orbits. Shaft orbits are typically made with displacement probes such as proximity probes. A proximity probe emits an eddy current field at the tip of the probe. The probe is spaced away from the rotating shaft by a small amount (typically 0.060"). The probe's output voltage is proportional to the gap.

Orbit measurements can also be made using case-mounted accelerometers on a bearing housing. An orbit, measured with two accelerometers mounted 90 degrees to each other on a motor housing, indicates the vibrational pattern of the motor housing. Typically, the measurement is made by using the output of two non-contacting displacement transducers (proximity probes). According to the American Petroleum Institute (API) Standard 610, the first probe to sense the vibratory energy is considered the vertical probe, Y. The trailing probe is considered to be the horizontal, X. The probes must be mounted 90o from each other. This mounting may be a true vertical and horizontal relationship as shown below, or in an X and Y

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configuration 45o both sides of true vertical. Typically, the two signals are taken as outputs from a supervisory panel and fed into the inputs of an oscilloscope. The signals produce a trace on the screen corresponding to the total shaft motion, which is the orbit of the shaft in the bearing.

A tachometer signal is not required for an orbit. If a tachometer signal is present, the pulse provides both frequency and phase information. On an oscilloscope display, a reference pulse appears as a bright or blank spot on the orbit plot. On the 2120, phase is indicated as a line radiating out from the center of the orbit. An unfiltered orbit refers to vibration energy at all frequencies measured in the set-up. A filtered orbit is a trace of vibration at one particular frequency (usually 1x or harmonic). A sample orbit plot is shown below.

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In the 2120-2 analyzer, orbit plots can be measured four different ways:

1. Analyze / Monitor Waveform 2. Analyze / Acquire Spectrum 3. Analyze / Monitor Orbit 4. As part of a predictive route.

The Analyze, Monitor Orbits feature was added in 7.43 firmware. It is an easy to use, automated function for measuring orbits. Measuring Orbits using Analyze/Monitor Orbits The Monitor Orbit function was implemented in 7.43 firmware. This function offers filtered orbits. Bandpass and Lowpass filtering are available as set-up options. Monitor Orbits is found under Analyze/Monitor Mode.

Monitor Orbit is easy to use and does not require any calculation.

Two filtering options are available: Bandpass and Lowpass. The Bandpass option filters out the signals above and below the bandpass frequency and passes the

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signal for the order specified. A Bandwidth parameter specifies the width of the band that is passed. The Bandwidth parameter is adjustable between .02 and 1.0 (2-100%) of the order specified. It determines how much of the signal around the order specified passed. For example: If a 1X order (1800 rpm) is measured, using a bandpass filter of 0.1, the width of the frequency band that is "passed" is 180 cpm (1800 x .1 = 180). The shape of a Bandpass filter is shown below. All data above and below the filter is removed from the signal. Only the data within the specified band is allowed to pass. Bandpass filtering requires a tachometer signal.

The other filter option is Lowpass. Lowpass filtering removes all signal above the specified filter setting and passes the signal below the filter value. For example: If a 1X order (1800 rpm) is measured, using a lowpass filter, the signal that is "passed" includes all frequencies at or below the filter value – in this example, the orbit includes all frequencies at or below 1800 cpm.

The shape of a Bandpass filter is shown below. All data above the specified order are removed from the signal. Only the data at or below the specified order is allowed to pass.

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A tachometer signal is optional when using Lowpass filtering. If a tach signal is not available, the orbit frequency is manually entered into the set-up. The exact frequency must be entered Filtered Orbits

Filtered orbits, measured using the ANALYZE/MONITOR/ORBITS function, can be saved if a dual measurement point from a route or OFFROUTE is currently active on the 2120-2 analyzer. What can an Orbit do for me? An orbit display provides a visual representation of the shaft centerline rotation. This information may provide a number of different fault characteristics. Orbits are said to be good only when using non-contact eddy current probes (proximity probes). However, this has been proven incorrect. If performed correctly, orbit data may provide some additional insight into the condition of a machine. The illustration below displays orbit characteristics of typical faults.

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Orbit Lab Follow the instructor's directions to measure orbits on a motor demonstration machine.

Section Seven

Modal Testing Objectives

Define modal analysis and describe when/how to use it Discuss methods for correcting resonance

What is Modal Analysis? Vibration analysts use the Time and Frequency domains to analyze machine vibrations. Many vibration problems are traced back to mechanical defects. Some vibration problems are a result of resonance. Identifying resonance problems may be as simple as performing an impact test or coast-down study. Correcting resonance requires analysis of data in the Modal Domain. Modal analysis is the process of characterizing the dynamic properties of a structure in terms of its modes of vibration. Modal analysis derives the system properties from experimental testing. Modal analysis is used for simulation, troubleshooting and design. It is performed with the machine shut down.

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Modal animation shows the shape of a structure at each of its natural frequencies. Dynamic Properties

Mass Stiffness Damping

Modal analysis is experimental – it involves physical measurement (testing) of a system and derives its properties. Modal analysis is used for troubleshooting, simulation and design. Finite Element Analysis (FEA) uses a mathematical model of a structure to estimates its modal parameters. FEA is a theoretical prediction used in machine design. Terminology Natural Frequency – Every part of a machine has natural frequencies. A natural frequency is the frequency at which a part likes to vibrate at when excited by a single input force. For example, when a bell is struck, it vibrates at its natural frequencies. Remember the Memorex commercial on TV? The singer sings a note that causes the crystal glass to vibrate and shatter. The note was close to or at the natural frequency of the glass. The glass shattered because it vibrated more than its tensile strength would allow. Machines and structures have many natural frequencies. Each one has a distinctive shape. When parts of a machine are assembled, the machine takes on new natural frequencies with characteristics based on the mass, stiffness and damping of the assembled machine. If a machine is exposed to a force, momentary or periodic, with energy near a natural frequency, the machine will begin to vibrate. The closer the forced vibration is to a natural frequency, the more amplification there will be. Resonance – is defined as a natural frequency that is excited by a forcing function, like unbalance. All mechanical systems have natural frequencies which, if excited by a forcing frequency, will result in greatly amplified vibration on the machine. Several factors work together allowing resonance to occur, such as low stiffness and/or low damping at the resonant frequency. Resonance is not necessarily a problem unless machine defects create vibration or nearby machinery transmits vibration at the same frequency as the resonant frequency. Resonance does not create vibration; it only amplifies it. Resonance is not itself a defect, but it is a property of the whole mechanical system. The mass, stiffness, and damping of the system at each frequency determine how the system will respond to the forces acting on it. If the natural frequency is not excited by some forcing function, resonance will not be a problem.

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Critical – A critical is a special case a resonant frequency that occurs when a rotor's rotational speed is the forced vibration coinciding with one of the rotor's natural frequencies. Modal Testing Modal testing is performed with the machine shut down. Modal testing is composed of a series of multi-channel impact tests using an impact hammer and one or more response accelerometers. The data is fed into a modal analysis program like ME'scope where the machine motions, at the natural frequencies, are animated. Impact hammers are instrumented with load cells. The load cell measures the impact force in the hammer when the machine is struck. The amount of energy transferred into a structure depends on the size of the hammer. The amount of frequency put into the structure is determined by the hammer tip hardness.

When a machine is hit with an impact hammer, the impact puts broad band spectral energy into the machine exciting its natural frequencies.

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The machine's response to the impact is measured with one or more accelerometers. The vibration on the machine is amplified for a period of time as it "rings" at its natural frequencies. The vibration decays quickly and disappears. The machine's internal damping determines how long the machine responds to the input force. The picture below shows waveform examples of the impact hammer hit and machine response accelerometer. The impact, measured from the load cell on the hammer, is a sharp spike. An impact produces low level energy over a broad frequency range. Any natural frequencies on the machine are excited and amplified by the impact. Impact testing can be done without using an instrumented hammer. Striking a machine with a block of wood will excite the natural frequencies. A single accelerometer is all that is required to measure machine's response. Without measuring both the input and response, the system properties are unknown and therefore, correcting a resonance problem will be difficult if not impossible. By measuring both the input force and output response, the mass, stiffness, and damping properties of the machine may be estimated. Both channels of data are related mathematically in the form of a transfer function (output / input).

The plot shown above shows the transfer function of the accelerometer response divided by the hammer input. The units are acceleration divided by pounds force (G's / LbF). The display shows three natural frequencies excited by the impact. When transfer function data is fed into a modal analysis program, the shapes of each natural frequency are seen and can be analyzed for a solution to a resonance problem. Put another way, single channel "bump" testing may reveal natural frequencies in a machine, but only multi-channel impact testing reveals the shape of the machine at each natural frequency. Mode Shapes

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When a machine or component vibrates at one of its natural frequencies, it has a unique and predictable shape called a Mode Shape or Bending Mode. A mechanical engineer can predict mode shapes using finite element analysis. There are mechanical engineering books that show mode shapes for typical structures. The important point to remember is that when something is vibrating at one of its natural frequencies (resonating) it is not straight – it is bent. At resonance, a machine or structure is no longer a rigid body. It is flexible.

Take the case of a shaft supported at both ends. The shaft will have many natural frequencies. Each one has a unique mode shape. The first three bending modes are shown above. Notice that the bearing positions are stationary in each mode and that the points of maximum motion change in each mode.

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The figure above shows the first two bending modes of a cantilevered shaft. Although machinery has many more than three bending modes, at least the first three modes are evaluated in a modal analysis for two reasons:

1. the frequencies of the first three modes are more likely to coincide with forced vibrations and result in resonance 2. the displacements are higher for the lower bending modes

Every mode shape has Nodes and Anti-nodes. Nodal points are points of no motion and anti-nodes are points of maximum motion. It is necessary to know where these points are when correcting resonance problems. Who needs Modal Analysis? Anyone involved in vibration analysis knows that analyzing frequency spectra and time waveforms is not easy job. Too many mechanical defects have similar looking frequency spectrums. To make matters worse, all machines have natural frequencies. Usually, we don't know where or when to expect these natural frequencies to come into play. One thing is certain – when they do, solving vibration problems becomes more difficult. Some experts suggest that more than 60% of vibration problems are the result of natural frequencies. Do you need modal analysis? Probably. It is one of several advanced diagnostic

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tests that confirm suspected faults in machinery. Other advanced diagnostic tests include

Operational Deflection Shapes Phase Analysis Impact Testing Run-up and Coast-down testing Negative Averaging

These diagnostic tests are usually not as quick and inexpensive as predictive vibration route measurements. They can, however, lead an analyst to a thorough and accurate diagnosis. Modal testing is used to identify natural frequencies, mode shapes and determine damping levels. It can also be used to verify FEA designs. Mostly, modal analysis is used to troubleshoot problem machines and structures. Modal testing should be performed after a problem has been detected and resonance has been found using one of the many resonance testing methods described in the previous chapter. A modal test is the first step towards correcting a resonance problem. How to do a Modal Analysis The following CSI products are needed to do modal testing using:

Impact hammer 2120-2 with Advanced 2-channel DLP VibPro Software Accelerometer ME'scope Modal Software

The 2120-2 analyzer, in its standard form, has the ability to make cross channel measurements but it cannot save the data. The Advanced 2-channel downloadable program (DLP) is required for modal testing. The Advanced 2-channel DLP stores measurement data to analyzer memory. The stored data is dumped to VibPro Software. VibPro is the CSI software product for the Advanced 2-channel and Advanced Transient DLP's. VibPro transfers data from the 2120-2 analyzer. Planning and record keeping are key to a successful modal test. Consider exactly what needs to be resolved in the modal animation and remember that resonance

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may be directional. Steps involved in Modal Analysis

1. Plan the job 2. Sketch the machine and number points 3. Perform impact tests 4. Transfer data to Modal Software 5. Create a structure picture in the modal software 6. Link measurement data to the structure 7. Curve fit and animate 8. Analyze results

Correcting Resonance Problems Planning a correction for the resonance problems is the next step of the modal job. A natural frequency can not be eliminated. The effect of resonance may be diminished or a natural frequency may be shifted up or down in the frequency range. Changing one natural frequency changes all of the natural frequencies on a structure. It is important to consider the effect of a correction on all natural frequencies. It is very likely that correcting one resonance problem results in the creation of another. It is always a good practice to involve a structural engineer in correcting resonance problems. Safety, cost and a successful repair are the main objectives. There are several options to consider when correcting resonance. One solution does not fit every resonance problem. Structural modifications are very expensive and take time to plan and execute. Reducing the exciting force or changing the speed of the machine may be more economical solutions. The resonance correction methods are discussed below.

Reduce the exciting force – Nothing resonates without an excitation force. Forcing frequencies (mechanical vibrations) are most often the excitation for resonance. Reducing the exciting force by any amount diminishes the effect of the resonance. It's usually less expensive to reduce or eliminate the exciting force than it is to modify the structure. Some examples of reducing the excitation include

· Balance to precision levels · Precision alignment of shafts and belts · Use precision parts · Replace worn or broken isolators

Change the speed -- Move the exciting force away from the

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natural frequency. The rule of thumb is to change the speed 10%-15% on either side of the natural frequency. Depending on the damping value for a given bending mode, more or less speed change will be required.

Change the Mass -- Increasing the mass of a structure decreases the natural frequency. Consider the simplified natural frequency formula below:

If "K" remains constant and "M" is increased then "Fn", decreases If "K" remains constant and "M" is decreased then "Fn", increases Examples of changing the mass are filling a steel base frame with concrete or adding a steel plate to the top of a steel base frame.

Change the Stiffness -- Increasing the stiffness of a structure increases the natural frequency. Consider the simplified natural frequency formula below:

If "K" is increased and "M" remains constant then "Fn",

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increases If "K" is decreased and "M" remains constant then "Fn", decreases Examples of changing the stiffness are adding corner bracing and gussets to a base frame. Saw cutting a portion of a beam is an example of decreasing stiffness.

Finite Element Analysis -- (FEA) is a structural engineering tool used to theoretically estimate the natural frequencies of a structure and predict the response to a known input. FEA should always be considered before making structural modifications to machinery. Remember that all natural frequencies are shifted with mass and stiffness changes. If FEA is not used before structural modifications are made, there is a good chance that the problem frequency will be moved away from the forced vibration and a different natural frequency will take its place. Note: ME'scope SDM has some FEA tools that are used to estimate the effect of structural changes.

Modal Example 1 Case 1 – Direct Drive Fan

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Figure 2-1 – Direct Drive Fan

Problem: This direct drive, 3586 rpm fan was diagnosed as an unbalance problem. The vibration spectrum showed a large, 0.7 inch/second vibration at turning speed. Fan balancing attempts were unsuccessfully, during a production shutdown. The vibration at shaft speed could not

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be lowered through balancing. During the next outage, modal analysis testing was done on machine. Frequency response functions were measured from 0-200 Hertz.

Figure 2-2 – Shape Table of Natural Freq.

Figure 2-3 – Modal Structure

Click Icon to Animate

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Results: The results of the modal test identified twelve natural frequencies on the machine under 140 Hertz. The one at 56.4 Hertz was the closest to fan speed showed a horizontal bending mode of the motor base. As seen in the picture in figure 2-1, the motor base had an open front for access to the motor base bolts. The base design was weak. The solution to this problem was obvious. Cross bracing between the sides of the base was needed. The maintenance department didn't want to permanently close off the opening to the base. A "U" shaped plate was fabricated and bolted to the sides of the motor base. Three bolts were used on each side (figures 2-4 and 2-5). The plate increased the stiffness of the base and shifted the 56.4 Hertz natural frequency well above operating speed.

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The data in figure 2-6 shows a before and after comparison of the vibration on the fan. The only modification made was the plate that was added to the front of the motor base.

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