“How to Analyze Data”
By
Kevin R. Guy
Senior Field Analyst
Delaware Analysis Services, Inc.
P.O. Box 365
Francisco, Indiana 47649
(krguy@delawareanalysis.com)
Abstract: Individuals assigned to vibration analysis programs struggle with analyzing collected vibration data for various reasons including, training and lack of confidence. The problem is not that people don’t know how to analyze data, but, rather they do not have a plan or procedure on how to systematically go about analyzing the data. In many cases, the vibration analyst should rule out what is not the problem, instead of looking for what is the problem. This paper will present a “plan of attack” so that an individual will have some guidance on what to look for when analyzing vibration data. Following the plan set forth in this paper will give the inexperienced vibration analyst a procedure on how they should proceed when analyzing vibration data.
Key Words: Amplitude, Frequency, Time, Spectrum, Harmonic Vibration, Periodic Vibration; Impulsive Vibration, Pulsating, Random Vibration, Sidebands, Synchronous, Non Synchronous, Subsynchronous, Electrical, Balance, Alignment, Looseness, Resonance, Vane Pass, Blade Pass, Gearmesh
Introduction: Analyzing data appears to be a daunting task. Many do not have experience examining at vibration data. The key is to go about the task with a plan to assist with the analysis. While each analyst develops their own plan of how they analyze data, each analyst’s plan contains many of the same procedures. Many inexperienced analysts only want to look at data in the spectrum plot because many analysis charts and tables present data in the frequency domain (Figure 1).
Figure 1 – Spectrum Plot – Synchronous Vibration
1X
2X
These same inexperienced analysts try to match the spectrum plot to a wall chart and expect the answer to jump out at them. A wall chart or other forms of troubleshooting tables are guidelines. The expectation is the exact same spectrum plot will be found and they will, therefore, have the solution to their analysis.
Data from the time plot will indicate what type of vibration is present. The five types of vibration are harmonic (Figure 2), periodic (Figure 3), beating (Figure 4), impulsive (Figure 5), or random (Figure 6).
Figure 2 – Time Plot – periodic Vibration
Figure 3 – Time Plot – Periodic Vibration
Figure 4 – Time Plot – Pulsating or Beating Vibration
Figure 5 – Time Plot – Impulsive Vibration
Figure 6 – Time Plot – Random Vibration
When one determines the type of vibration from the time plot, they use the spectrum plot to find out what group or frequency content the vibration frequencies in the spectrum fall under, they are: synchronous, non-synchronous, subsynchronous or electrical. While much of vibration analysis is pattern recognition, knowing the type of vibration from the time plot, and what groups the frequencies fall under in the spectrum plot, points the analyst to the vibration cause.
Ninety percent of our vibration problems are the big three: balance, alignment and looseness issues. If a forth problem was added to the list, it would be rolling element bearing faults. These four items are probably ninety – five percent of all vibration issues.
This paper will try to lay the foundation of rules for analyzing vibration issues by using techniques developed over the past thirty-three years of analyzing data. Vibration analysis does not have to be difficult if you know how to read the data. Also, the vibration instrumentation does an excellent job at providing information to make the task easier. The analyst has to have a plan and they have to know how to extract the information from the data.
Basics of Vibration Analysis: Before any discussion of analyzing data can begin, a discussion of vocabulary must take place. The two most important words in the analyst’s vocabulary are amplitude and frequency. Amplitude tells us the condition of the equipment and frequencies identify the problem. You, as an analyst, look at the frequencies present in the time and spectrum plots and determine the cause of the vibration. The type of frequencies present, help the analyst narrow down the cause of the vibration.
The word frequency can be broken down into four additional critical words that help the analyst indentify the frequency content in the spectrum plot. Synchronous vibration is vibratory frequencies that are related to the operating frequency of the shaft (i.e. balance alignment, looseness, vane pass, blade pass, gearmesh). Subsynchronous vibration is vibration frequencies that occur at a frequency that is less than the frequency of the shaft (i.e. oil whirl, oil whip, fundamental train frequency, rubs). Non-synchronous vibration is vibration that is not related to the frequency of the shaft (i.e. rolling element bearing defect frequencies, pump flow issues). Electrical vibration is vibration that is caused by line frequency, 60Hz in North American and 50 Hz around the rest of the world.
The basis for mechanical vibration issues is shaft speed. The rotational (operating) speed of the shaft is expressed in terms of revolutions per minute (rpm). The term frequency relates to cycles per unit time. Frequency can be expressed in cycles per minute (cpm) or cycles per second (cps – Hz).
Using the following example, a machine operating at 3575 rpm (Figure 7) this vocabulary can be tied together. If the operating speed of the shaft is 3575 rpm, then the shaft frequency is 3575 cpm or 59.58 Hz. This is also the first order of the shaft. Any frequency that is an even multiple of the shaft frequency would be synchronous vibration (Figure 7 - items 1 - 6). Vibration frequencies that occur at a frequency less than the operating frequency are subsynchronous (Figure 7 – item 7). Frequencies that are not an even multiple or order of shaft frequency is non-synchronous vibration (Figure 7 – item 8). Any vibration that is related to line frequency, 60 Hz (3600 cpm) or 50 Hz (3000 cpm) is electrically generated and is electrical vibration (Figure 7 – item 9). Table I contains a listing of vibration frequency groups and their problems.
When looking at how frequencies are identified in the spectrum plot, it is the choice of the individual analyst on how they would like to view the frequency axis: cpm, Hz or orders. There is no right or wrong way to look at frequency in the spectrum. Many analysts like cycles per minute (cpm) because they relate it to the shaft speed in rpm. Seasoned and higher certified analysts tend to use cycles per second (Hz) due to needing the second unit when performing many vibration calculations. It is recommended to many new inexperienced analysts that they use orders to assist with determining if the vibration is synchronous, non-synchronous, subsynchronous or electrical. How your frequency span is setup is personnel. Many swap back and forth between cpm, Hz and orders depending on what type of data is being analyzed. Order analysis, for the frequency units, works well when looking for rolling element defect frequencies. Defect frequencies will not be integers of operating speed.
One other important term is sidebands. Sidebands are evenly spaced frequencies that occur above and below a center frequency (Figure 7 – Item 10). Sidebands are mainly found in three places, with motors having broken rotor bars, rolling element bearing frequencies signaling an imminent failure, and gearmesh. The sidebands for motor broken rotor bars are spaced at the number of poles times the slip speed. Sidebands around gearmesh frequency are spaced at the frequency of the shaft, with the vibration problem. Rolling element bearing frequency sidebands are usually spaced at the frequency of the shaft or cage frequency. Table II has a list of locations where sidebands are found.
Figure 7 – Example of Spectral Frequency Terms
Table I – Generated Frequencies versus Shaft Frequency
Subsynchronous
Synchronous
Non Synchronous
Electrical
FTF – Gage Frequency
Alignment
Ball Spin Frequency
Stator Problems
Oil Whirl
Balance
Ball Pass Outer Race
Oil Whip
Looseness
Ball Pass Inner Race
Belt Frequency
Vane Pass
Pump Flow Issue
Blade Pass
Gearmesh
Table II – Sideband Frequency Locations
Sidebands
Frequency
Indication
Motors
Number of Poles x Slip Speed
Broken Rotor Bars
Gearboxes
Spaced at Shaft Speed
Indicates shaft with vibration problem
Rolling Element Bearings
Bearing Defects
Indicates imminent bearing failure
It is stated in many papers, that vibration analysis is nothing more than frequency matching of the spectral data – matching frequencies, in the spectrum to specific components, of the machine train (Figure 8) and grouping the frequencies, into synchronous, subsynchronous, non-subsynchronous and electrical. Vibration analysis is also evaluating the type of vibration from the time plots, in addition, to looking at the grouping of the frequencies.
Figure 8 – Spectrum Plot – Frequency Matching
The type of vibration found in the time plot also helps the analyst identify vibration problems. Harmonic vibration (Figure 2) is indicative of unbalance problems and surprisingly looseness issues in sleeve bearings.
Periodic vibration (Figure 3) is an indication of vibration that repeats over and over in the same time interval such as misalignment, vane pass, and gearmesh.
Pulsating or beating vibration (Figure 4) indicates two closely spaced frequencies that are adding and subtracting in a beat cycle. In some cases, this can be more of a nuisance than a major issue. An
2X
1X
Vane Pass
Rotor Bar Pass
Slot Pass
example would be the mechanical and electrical frequencies (2X running speed & 2X line frequency) beating in a two pole motor.
Impulsive vibration (Figure 5) indicates impacting and is found with rolling element bearing defects and gears having broken or cracked teeth. In severe looseness cases, impulsive vibration can also be present.
Random vibration (Figure 6) is vibration that comes and goes. It is not periodic and is usually associated with flow issues in piping or pumps.
Table III – Types of Time Based Vibration and Locations
Type of Time Based Vibration
Location Found
Harmonic
Balance / Natural Frequency
Periodic
Alignment / Looseness / Vane Pass / Blade Pass / Gearmesh
Pulsating or Beating
Closely Spaced Frequencies – Mechanical / Electrical or Both
Impulsive
Roller Bearings / Broken Teeth Gears / Severe Looseness
Random
Flow Problems in Pumps & Fans
Fault Analysis: While each analyst needs to develop their own thought process when analyzing vibration problems, several steps can assist with the process.
Steps to Analysis:
1) When notified about a problem, talk to those who found it, and get his or her view on the circumstances that lead up to the problem. Learn as much information about the equipment as possible. It is best to talk to someone from operations or the shop floor as they normally have “first hand” knowledge of problems. Managers and supervisors normally obtain second or third hand information, and may not have all the technical information required to assist with the analysis. Find the operating conditions during the vibration problems, which should include, but not be limited to: shaft speed, temperatures, pressures, flows, or process changes.
2) Draw a schematic design of the equipment (Figure 9), and investigate its internals. Find out what types of bearings are used. This is important because the same type of bearing is normally not used throughout each equipment train. The number of teeth on the gears (Gearmesh), number of blades on the impeller (Blade Pass), and bearing frequencies should be calculated. The location of any shaft critical or structural resonance points should be identified. The analyst should have a mental picture of the machine internals and know what the required function of each machine component.
3) The analyst should also have some knowledge of the equipment’s application in the system. This information should include suction and discharge pressures (BEP), motor amps, flow, and any other parameters that might point to a system operations problem rather than a mechanical vibration problem.
4) When collecting data used the “right hand rule (Figure 9).” Collect the horizontal data on the right
side of the machine and the vertical data 90 degrees left of the horizontal data or “top dead
center.” If using prox probes (Figure #10) collect the vertical data (Y) then the horizontal data (X).
Driver Coupling Driven
Axial
Horizontal
Vertical
Figure 9 – Equipment Schematic
Prox Probe
Prox Probe
Shaft
V H
Figure 10 – Prox Probe Locations
5) Collect data on the machine when it is out of service. This may sound odd; however, an analyst
needs to know what frequencies are present when the machine is not operating. Any frequencies
present when the machine is not operating, are not coming from the machine. They are being
generated by another machine and can be ignored during the analysis.
6) From the time plot (Figure 11) determine if five to eight rotations of the shaft are present and what
type of vibration is present. Based on the operating frequency and the time plot, there are 14.97
cycles in the time plot. The time plot indicates impulsive vibration that is periodic in nature.
7) Determine if the frequency content present in the spectrum plot are synchronous, nonsynchronous,
subsynchronous or electrical. Almost all data collection systems will provide this
information in tabular form (Figure 12). The spectral data in figure 10 is 85.8% synchronous,
meaning the vibration is order multiples of shaft speed.
Figure – 11 – Time & Spectrum Plot
Figure 12 – Frequency Content Breakdown
Vibration Problems and Symptoms: The remainder of this paper will discuss the dominate vibration issues that a vibration analyst will encounter along with the symptoms of each problem. Unfortunately not every vibration issue can be covered in a short paper; but, the most common vibration problems encountered will be discussed.
Imbalance is a Synchronous Vibration: The problem of imbalance (Figure 13) is characterized by a high operating speed vibration (1X) in the softest direction on the machine which is normally horizontal. The symptom is referred to as “one per rev” due to the unbalance force passing the vibration sensor once every revolution. Horizontal and vertical data will be 90 degrees apart due to the positioning of the sensors (Figure 14). The time plot data will be a fairly harmonic signal. Correction of the problem requires balancing of the machine component.
One issue many have with balancing, is no one wants to believe a new piece of equipment or a recently balanced piece of equipment can have a balance issue when it is installed and put back in service. Equipment that is balanced in a low speed balance machine is balanced for shaft rigid modes. If the shaft, when installed runs near or above the first critical, the shaft will become flexible and may still need to be balanced.
Many times, plants will have a motor and a fan balanced separately. The motor and fan will be installed, coupled up, aligned and placed in service. Analysis of this installed equipment will show a balance problem because the motor and pump were balanced separately. Now they are running as a unit, they exhibit imbalance issues because they need to be balanced as one component.
Impulsive Vibration
Additionally, many pieces of equipment are driven with variable frequency drives (VFD). These machines may operate with low levels of vibration at low speeds and then have imbalance issue when operating at full speed. Vibration analysts must understand that whenever the speed of a machine doubles the force of the vibration, not the vibration amplitude, goes up by a factor of four. This causes the rotor to operate with a balance problem. When balancing machinery driven by a VFD, balance it at the top of the speed range. If it is balanced at full speed it will be balance at low speed.
Figure 13 – Spectrum – Imbalance Problem – High 1X Vibration Frequency
Figure 14 – Time – Imbalance Problem – Horizontal & Vertical Data
1X
Horizontal & Vertical signals 90 degrees apart
Misalignment is a Synchronous Vibration: Alignment issues are maybe the number one cause of machinery vibration problems and they are also the toughest to convince people they have alignment problems. The problem is not that people don’t know how to align equipment, the problem is people do not use the correct cold offset.
Let’s face it, when equipment is aligned, it is actually misaligned, so that when it thermally grows the shaft will grow so that it is in a straight line. To get accurate alignment, the thermal growth needs to be calculated and plotted (Figure 15) or the equipment needs to be optically measured from cold to hot or hot to cold so the actual thermal growth can be determined. Once the actual growth is known or calculated, the correct cold offset can be determined.
Figure 15 – Time – Thermal Growth for Shaft Elevations – 5 Inches to 25 inches
Alignments issues are indicated by operating speed (1X) and twice operating speed (2X) vibration in the horizontal and vertical directions (Figure 16). Axial vibration will only show operating speed (1X) vibration components (Figure 17).
Figure 16 – Spectrum Plot – Horizontal – 1X & 2X Vibration Frequencies
1X
2X
Figure 17 – Spectrum Plot – Axial – 1X Vibration Frequencies
One additional symptom of severe misalignment is a figure eight (“8”) orbit (Figure #18) which is data collected from dual prox probes. Normally orbits are collected from two prox probes; however, orbits can also be collected from two accelerometers. Figure 19 is an orbit from a horizontal and vertical accelerometer mounted on a 4000 horsepower four pole motor, also showing severe misalignment.
Figure 18 – Prox Probe Orbit – “Figure 8 – Severe Misalignment
Figure
