|< Vibration Analysis – Basics||Vibration Analyzers >|
Finding the best Vibration Analyzer for your application can be quite tricky. Moreover if you don’t know how to identify the functionality that you need. Take some time to read this post and find out your best option.
Vibration analyzers are instruments used to measure, store and and diagnose the vibration produced by machinery. Unlike common vibration meters that only give a numerical value, vibration analyzers use FFT based tools to measure frequencies and identify the causes that originate them.
The Best vibration analyzers are capable of saving measurements and organizing them for future analysis, thus, generating trends for each machine. Devices that also have this function are also known as a data collectors.
Do you have time? Knowing what to look for in a Vibration Analyzer is very important, so grab a cup of coffee and – here it goes!
The main objective of a vibration analyzer is to detect early faults in the machinery and to identify the cause of the vibration avoiding invasive diagnostic means. This way, if for example, the fault is the internal race of a bearing, vibration analysis will avoid having to disassemble the machine to diagnose it. Anticipating unscheduled stoppage avoids the economic costs in production.
There is no such thing as “types of vibration analyzers”, however, we can identify differences in:
|Wireless Accelerometer||Vibration Analyzer|
You can find vibration analyzers in the market with different number of input channels, the most common being 2 to 4 channels.
Recording 2 channels simultaneously is usually enough for functions like balancing, phase analysis, Bode graph and ODS (these functions will be explained in another article). On the contrary, the use of a triaxial accelerometers as well as o 2 simultaneous planes balancing require 4 channels.
Single channel analyzers are not common anymore since they are unable to perform necessary techniques necessary for some of the most important faults, like misalignment, Bode and balancing.
The number of lines of resolution defines how many points will integrate the spectrum. Although the correct term should be “Points”, the word “Lines” is still used probably because it was created for in audio FFT which were plotted with bars (or lines).
The lines of resolution are sometimes wrongly associated with the resolution of a spectrum; however, this is not entirely correct. The LR do not distinguish between a large or short frequency range, thus, having a lower frequency range will show a better resolution a higher range with the same amount of lines. Observe the following example:
Suppose we select 6400 LR, let’s see these 2 examples:
Max Frequency of 10,000 Hz: 10,000 / 6,400 = 1.56 Hz (93 CPM)
Max Frequency of 1,000 Hz: 1,000 / 6,400 = 0.156 Hz (9.3 CPM)
The second example will show 10 times the resolution of the first one.
In other words, you will look for precision in a spectrum when you need to differentiate between two frequencies that are close to each other.
Imagine a fan with pulley transmission in which the pulleys have very similar diameters and thus work at few RPMs between each other.
To distinguish the imbalance of the motor from the fan, you will need enough resolution to get the two frequencies to show in independent peaks in the FFT and therefore measure the amplitude of each one. In case your maximum resolution lines were of 6,400 you would have to sacrifice your maximum frequency having to set a range of 200 to 400 Hz and thus not seeing the full range of the accelerometer.
In conclusion, the lines of resolution are a very important factor in the choice of the best Vibration Analyzer since your resolution and frequency range are extremely related to it. Hence, the analyzers capable of calculating millions of lines of resolution will rarely have to sacrifice frequency range to achieve excellent resolution.
The frequency range of a vibration analyzer is determined by two factors: the highest sample rate of the analyzer and the maximum frequency that the accelerometer can measure. The Sample Rate is the amount of measurements that a system can perform within 1 second. Thus, the maximum frequency that this device can see will be half of its sampling rate.
As an example, a system that samples at 20 kHz will have a maximum frequency of 10 kHz[i]. Think of it this way, if we had a signal of 1 Hz (1 cycle per second), to be able to see its wave form, that is to say in its negative part and in the positive part we would have to make a measurement at least 2 times per cycle.
Accelerometers, on the other hand, have a maximum and minimum frequency (usually 1 Hz to 10 kHz) that they can measure accurately. Outside of these limits, the sensitivity can be progressively different, so it is not recommended to make measurements outside their range.
Generally speaking, the frequency range of an analyzer is usually higher than the frequency range of conventional accelerometers, but having a wide range will give us the freedom to connect sensors with higher ranges.
Are you still here? Excellent now pamper yourself a little and go grab some cookies or gummy bears, because it gets better.
Finally! we got to the part that I like the most. The functions of the analyzers are probably the part that changes the most between one analyzer and another. I will start by listing the most important functions that every analyzer has, or at least should have and these are:
Spectrum, is the heart of an analyzer, almost all analysis functions need it inside their calculation. Not to mention FFT measurement tools and Window functions: Rectangular, Hann, Hamming, FlatTop.
Check out a FFT definition here:
Vibration analyzers draw this chart with the signal as it arrives from the sensor. It is not used very often, however some secondary functions like Circular Time Wave Form are useful to detect patterns in gearboxes or bearings.
Acceleration, Velocity, Displacement and Acceleration Envelope (Demodulation or equivalent, useful for early bearing fault detection)
A good vibration analyzer should also be a vibration data collector. At least it should have a mechanism to store and organize the recordings of the machines to store a history and trend of the machinery. This is one of the main factors to consider because consider that you will be dealing with this part of the system EVERY day. Having an intuitive and easy-to-use database interface is vital to avoid wasting time later.
will store all the information related to your machines and all your recordings over time.
The theory tells us the ideal vibration levels for some of the machinery in the industry, but what will you do if you do not know what the ideal value is? This tool will let you know the moment the vibration increases in any of its frequencies. It wraps the full spectrum and rings an alarm whenever any frequency grows more than the percentage you configured.
Reports can be very tedious if you create signal by signal, value by value. Automatic reporting saves considerable time.
Dynamic balancing is a functionality which is present in the vast majority of equipment usually as optional, however, consider that imbalance is one of the most common causes of vibration in the machinery, wouldn´t it be nice to correct the problem instantly?
Phase analysis requires you to record at least 2 channels simultaneously in order identify the direction of motion compared to each other. This function is mandatory to fully diagnose misalignment among some other faults.
This function is is important because bearing vibration analysis identifies fault frequencies related to bearing geometry. Thus, you a complete bearings database to obtain the manufacturer information to perform faults calculations.
The advanced functions may or may not be present on many systems, or could have different names than those I will mention.
(Operating deflection shapes) Simulates the movement of the whole machine in a 3D drawing. This is a tool that is not so common but is excellent for diagnosis and above all, it is very easy to understand for anyone, even people with little or no knowledge of vibration analysis. ODS is also undoubtedly a very attractive tool that sells for being so descriptive.
More than just a chart, it is a function to diagnose resonance and critical speeds during machine coast down.
It is a two-channel analysis in which 2 sensors are placed at 90º, orbits help to understand the behavior of the shaft within the bearing, as well as the modes of vibration and resonances if it is done within a run down test.
(Circular Time Wave Form) or circular waveform. This function is useful for vibration patterns interpretation in very low speed systems, like gearboxes. It is essentially the time domain graph plotted in a circular chart in which you can select the time to complete 360º thus, superimposing several cycles. Patterns can be easily seen when peaks overlap in the same position.
It is a 3D representation of the spectrum over time, very useful to detect resonance and natural frequencies.
(Frequency Response Function) or transfer function. This function helps measuring transferred vibration from a source in all its frequencies. For example, we know that a spring absorbs lower frequencies better, and rubber absorbs higher frequencies better thanks to this function.
Is meant to verify the origin of a vibration. Imagine a room with several machines, and a vibration outside of this room but generated inside of it. In order to determine which of the machines is responsible for this vibration you will need this function to gain certainty.
Some new vibration analyzers offer already a cloud based database to be shared with your customers. Sharing you vibration database with your customers, not only will it make your life easier reducing the amount of reports, but also it will give your customers the satisfaction of having the best technology available.
Look at an example here: EI Analytic Vibration Cloud
The output voltage is proportional to the acceleration of the vibration. Most of these sensors require power supply because they contain small amplifiers and filters to eliminate noise.
Vibration Analyzers most commonly use Accelerometers due to their excellent frequency and amplitude range as well as the low noise they generate. In return, they convert acceleration into other parameters like velocity and displacements which are more frequently used in vibration analysis.
Its signal is proportional to the velocity of the vibration. They are electromagnetic sensors, so they do not require power.
The output signal is proportional to the displacement of the vibration. They usually are non-contact sensors, so they are ideal for measuring vibrations and eccentricity of shafts.
It is convenient that the vibration analyzer you choose has the possibility to connect other types of sensors. There are machines for which simply the accelerometer is not the appropriate tool. Oil bearings is an example of the above because oil dampens the vibrations thus making its measurement with accelerometers less reliable.
Take in account the portability of an analyzer since you will be dealing with this every day. On the other hand, you must also consider that a small device with little functionality will be a big limitation.
In terms of portability, the general types of analyzers are the following:
People use these usually only to measure vibration RMS, the functionality is usually very pour, although they have the great advantage that they fit everywhere, even in your pocket.
They are devices the size of a cell phone, have a larger screen, show the spectra and are well prepared for taking routes. They are very portable and comfortable for use. These vibration analyzers usually have slow processors, so the number of lines of resolution, memory and functions are reduced, thus requiring PC based software to complement functionality.
They range from the size of a cell phone to beyond the size of a tablet but with a greater thickness. They are the most common and are a fair choice for industrial use. There are many brands and with very variable functionality.
The disadvantage is that they are not universally used devices and, thus, the cost of manufacturing them is high. This makes them have slow processors and low memory. This condition can make these systems less functional.
It is complicated to know what kind of processors these devices have because it rarely appears in the technical file. Our best approach will be to evaluate trough the amount of memory and functions of which they presume. The good news is that most of these computers have PC software to make analysis of the data collected easier. We just need to be aware of the cost of this software.
They are becoming more popular due to the huge power and memory of new computers. Updating the computer generates no cost while installing the software and also portability in comercial tablets increases on every new model.
A small disadvantage is that most of them are not sweated for industrial environments, however, Windows and Android rugged tablets are available at very affordable prices without the need to by tied to a brand.
In the best vibration analyzer, support is a very important aspect because, let’s face it. Nothing is ever perfect and we always need a assistance for complex devices. Thus, you will need to be in contact with the distributor or the manufacturer of the product with certain frequency. The most important points to take into account for technical support are:
Last but not least, price is one of the main factors to take in account. Understand how you will be charged, this is vital to calculate our financial capacity for acquisition. Points to take in account:
Thierry Erbessd, Mexican entrepreneur who has revolutionized the field of Vibration Analysis and Dynamic Balancing worldwide.
Thierry is a graduate of the National Polytechnic Institute of Mexico and is also a passionate programmer, he is the creator of DigivibeMX software that competes among the best systems for vibration analysis. He is currently the President of the Erbessd Instruments group, a leading company in solutions for industrial maintenance.
ERBESSD INSTRUMENTS manufacturer of Vibration Analysis Equipment and Dynamic Balancing Machines with offices in Mexico and the United States with representatives around the world.
|Wireless Accelerometer||Vibration Analyzer|
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