The following are the basic concepts of vibration measurement.

And vibration measurement equipment

An ideal non-rotating machine produce vibrations. The absence of imbalances, desalinhamentos, days off, etc. would not exist causes for the vibrations happening. In practice this does not happen, then appearing Vibrations.

The vibrations are relevant in many ways; may cause noise, discomfort, breakdowns etc..

A well-designed project will result in a machine with levels of vibrations and noise usually quite low. However throughout the machine life, the clamping bolts loose, components deform, increase the clearances, not to mention in misalignments, imbalances, etc. All these factors will contribute to an increase in vibrations and resonances which can cause increased load on the bearings. In turn vibrations accelerate the degradation processes of machine components for directing, so that a malfunction.

measuring-the-machine vibrations and vibration-

The vibrations are indicative of the operating conditions of the machines

While the forces generated in operation machines are more or less constant, vibration levels will also remain substantially constant. Furthermore, on most machines, the vibration level has a normal value, and when the machine is in good condition, its frequency spectrum has a characteristic feature. The Frequency Spectrum, obtained when the machine is in good working condition, It is so often called “signature” of the machine, and it is obtained through frequency analysis of vibrations in.

When defects are beginning to develop, the vibrations begin to rise and increases the amplitude of certain spectral components.

Thus, the vibration measurement is widely used for maintenance. But also in the development of a machine, in their manufacture and quality control often uses the measurement and analysis of vibration.

2 Measurement of vibration – characterization of a periodic vibration

2.1 . The vibrations that are

It is said that a body is to vibrate when describing an oscillating motion relative to a point.

There are basically three types of vibrations:

  • random
  • transients
  • periodic

Random vibrations rarely occur in machines. The phenomena in the machinery that can give almost random vibrations are for example cavitation in pumps or fans aerodynamic phenomena.

Transient vibrations only occur during starting and stopping or when changing a procedural operating condition. They do not have so much importance to characterize the machine status.

Are periodic vibrations that are actually important to characterize the status of the machines. Each rotation cycle occurs a repetition of the occurrence of the phenomena in the machine, thus creating, functioning, periodic vibrations.

2.2 Periodic vibrations

The number of times a complete cycle takes place over a given time interval is called Frequency. Normally, in the case of the machines speaks on the number of cycles per minute, RPM. When it comes to the number of cycles per second the unit (1 cycle per second) it is called “Hertz”.

measuring the periodic vibrations vibration

Figure – Periodic vibrations

The body movement can be at a frequency, such as for example the case of a fan with a large imbalance, or several frequencies at the same time, such as for example the case of a gear.

the machines, which is more common, and vibration occur at many frequencies at the same time so that looking for an oscilloscope can not distinguish if the amplitude of vibration at each frequency.

This can be known an apparatus that presents the amplitude of the vibrations at various frequencies. This separation of components is called Frequency Analysis, which is a tool for the diagnosis of faults in the machines.

When making a Frequency Analysis normally are predominant peaks, which are directly related to the movements of the various parts of the machines. Like this, this type of analysis can determine which parts of the machines which give rise to vibrations.

2.3. The quantification of the level of vibration

The vibration amplitude, which is the characteristic that describes its severity, It can be measured in several ways. In the following figure, one can see the relationship between the peak-peak amplitude, or Pico, Media and the Effective Level (RMS).

Measurement-of-vibration peak and rms-

The peak-peak value is important in indicating the maximum vibration amplitude, which is an important parameter when it comes to knowing for example, maximum displacements in machine tools or measurements made with displacement transducers.

2.4 The use of the RMS value and the peak value

The Effective Value (RMS) It is most often used because it takes into account a certain measurement time interval and gives a value that is directly related to the energy of vibration, that is, its destructive capacity; is, therefore, an average value.

In the following figure one can see displayed three waveforms with the same peak amplitude and with different effective values. In a piggyback pulse comes to a sine, another is a sine, another is a truncated sine.

The pulses that can be observed in the first waveform, of the previous figure, if they occurred in a rotary machine correspond to shocks. The measurement of peak amplitude detects better shocks or any other kind of impulsive phenomenon, measuring the RMS amplitude, due to the latter being an average value in a given interval while the peak value is, by definition, the maximum signal in time.

vibration measuring the peak and rms

To measure vibrations without effective pulse amplitude (RMS) It is due most suitable to provide an average value.

To measure sinusoidal vibrations, whatever, because there is a fixed relationship between the peak amplitude and the RMS.

A sinusoidal vibration has the following relationship between the various ways of measuring range:

= Peak-peak amplitude 2 x Amplitude pico

Effective Amplitude = 0,707 x Amplitude pico

These relationships may in practice have a direct application. With a global level meter Vibration the relationship between the peak amplitude and the effective range is equal to 0,707 You are then in the presence of a sinusoidal vibration, with a single frequency, to be the machine's rotation speed. This deduction can then be used for diagnosis.

2.5 The displacement speed and acceleration of a vibration

The relationship between the amplitude of the displacement speed and acceleration of a sinusoidal vibration is:

  • d = offset
  • Speed ​​= d
  • acceleration = 2 d = v = 2f that I f- Frequency Hz

The formulas relating to the displacement speed and acceleration we see also that the speed is equal to the displacement times the frequency, and the acceleration is equal to the displacement times the square of the frequency. Thus it is expected that the higher frequencies are where the vibrations appear with greater acceleration.

Off vibration level measuring accel

Displacement Speed ​​Acceleration

In the spectra of figure, obtained in the same measurement point, one can see that effectively at high frequencies vibrations manifest themselves especially in acceleration.

In practice this translates into that, while if you want to control with a meter vibration phenomena that manifest themselves at high frequencies, such as the first signs of faults in bearings, the measurement parameter is the acceleration using.

2.6. What is frequency analysis

The vibration meter gives us only a measured level in a wide range of frequencies. In order to know is the individual value of each component is required to make a Frequency Analysis.

vibration measurement time and spectrum

frequency analyzers used in vibration measurement algorithm by using a designated Fast Fourier Transform (English Fast Fourier Transform).

The various components of an operating machine originate, each per se, a vibration at a particular frequency. All these vibrations are added to each other, thus obtaining a total that is the vibration over time.

It is the phenomenon that can be seen on an oscilloscope or what you might feel when you put your hand on a bearing.

vibration measurement time and spectrum 2

In the picture we can see the relationship between the signal at the Time and Frequency Spectrum.

The frequency spectrum allows breaking that total in individual portions, that give rise.

Thus it can be said that while the signal at the time is a total, the frequency spectrum shows the shares that give rise.

a vibration-measuring-and-spectrum-diagnostic

Figure – Through the frequency spectrum can be known which the machine component that causes the vibrations.

2.7. – Parameters measured - offset - Speed – Acceleration

With the meters based on the use of accelerometer, the user usually has the freedom to choose how measurement parameter, offset, the speed or acceleration. There are several possible choices and it is normal to find very different measurement procedures between the various users and according to the purpose of the measure.

It may be noted that there are numerous standards for evaluating the severity of the vibrations in machines using as a measure the speed parameter (ex. ISO 10816).

The experience shows that actual speed is the most appropriate parameter for controlling defects on most machines (imbalances, desalinhamentos, days off, desapertos, etc.).

The only exception is the bearing damage, generating pulses, which are easier to detect accelerating.

The offset is used when using displacement sensors or for example when the machine tool is at stake is the mounting tolerance.

3. Measurement of vibration – vibration sensors

Typically in the industry are three types of transducers:

  • displacement
  • velocity
  • Acceleration

3.1. displacement sensors for measuring vibrations

The Displacement Sensors (also known as proximity sensors or proximitors) Frequently in industry measure variations in the magnetic field and act as comparators contactless. The advantages and limitations arise from this fact.

Displacement Sensor pair

Benefits

– They measure the vibrations directly into the veins.

On machines with oil film bearings, It takes place a great vibration damping. Thus the shaft vibration measurements are often much larger than the measured vibrations in the bearings. In this kind of machines have sometimes place only phenomena that are detected by measuring the vibrations directly into the veins.

– Measured vibrations to DC ( 0 RPM).

Because the comparators function as contactless measuring vibrations almost to 0 RPM.

– When installed in pairs, by bearing, determining the position of the center of the shaft.

Disadvantages

– The measures are influenced by the finish of shafts.

The irregularities and vibrations are measured as ovalizações.

– Only measure vibrations up 1 KHz.

The frequencies above 1 KHz the amplitude of the displacements caused by physical phenomena in the materials is so small that irregularities blend with the surfaces of the shafts.

– Sensors are installed permanently.

For this reason it becomes a more significant investment that is justified only in larger machines.

3.2. speed sensors for measuring vibrations

Speed ​​sensors are constituted by a coil and a magnet. The voltage generated in the coil is proportional to the relative speed of the two.

Benefits

– are self-generators

– No need signal conditioning system

Disadvantages

– High frequency lower limit (10 Hz).

The natural frequency of these sensors, normally undertaken in time of 10 Hz. This means that measures the vibrations around this frequency are enlarged. Normally the gauges working with these sensors are devices for filtering vibrations at these frequencies.

– Reduced upper limit frequency (1000 Hz).

The oil damper having inside dampens the higher frequency vibrations 1 KHz. They are not well suited to detecting faults in bearings.

– It has moving parts.

Are subjected to, so the wear, malfunction, etc.

– side elevational Sensitivity.

This means that in addition to measuring vibrations according to its main axis also measure the second lateral directions.

Today are falling out of use due to the fact that accelerometers replace them with multiple advantages.

3.3. acceleration sensors for measuring vibrations

The most common type piezoelectric accelerometers are the. In them the generated electrical charge is proportional to the acceleration to which they are subject.

Benefits

– Measure high frequencies.

Typically the measured frequency upper limit is imposed by mounting the accelerometer and can go up to a few tens of KHz

– Measure low frequencies.

The lower frequency limit is imposed by the amplifier where the accelerometer, and can go up to hundredths of a Hertz

– Measure large and small levels of vibration.

– They are very robust.

– They are insensitive to lateral vibrations.

Disadvantages

– They need signal conditioning.

3.4. The assembly of accelerometers for measuring vibrations

The manner in which the probe is brought into contact with the measurement point significantly affect the results of measures. As a general rule it can be stated that the more rigid is the binding of the probe to the machine, As will be stricter. So the ideal situation would be the accelerometer is attached to a threaded stud. Of course this often, It is impractical, and day to day the most common methods are the fixing with a magnet and the placement of a ferrule on the probe that allows its easy measurement points back to.

This issue becomes critical when making vibration measurements at high frequencies.

Figure – Frequency response of different fixings of an accelerometer.

When using the tip and the frequency of the vibrations is in the range of 0.5 a 1 KHz easily perform higher measurement errors that 100%.

Table given below you can see a comparison of the various techniques.

3.5. Choose the measurement point for measuring vibration

The reason why a machine vibrations are measured the position of said measurement point. When you hold an accelerometer should choose the shortest path between the source of the vibrations (usually the rotor) and a point where it can make the measures. Usually it follows that during the measurements of the bearings in boxes or any rigid structure attached to them.

Another issue that often arises is the direction in which it should measure. It is impossible to give a general rule, but often, measuring in three directions; vertical, horizontal e axial.

The vibration behavior of machines, especially at high frequencies, It is quite complex. So be expected, that even in very together points vibration levels are different.

Figure - measuring points on an engine group – bomb

4 Vibration measurement techniques

4.1. The measurement of the overall level of vibration in accordance with ISO 10816-3

This type of measurement gives a simple reading Effective Speed. The measured vibration values They can be directly compared with the standard values ​​severity criteria vibration Standards.

That's how this type of equipment is used in quality control and monitoring of simple machines condition, that up are the most common, such as e.g., electric motors, bombs, fans.

The defects that are controlled with this measurement are usually imbalances, desalinhamentos, days off, desapertos

The bearing failures are the most common fault that this technique does not handle in a satisfactory manner.

Advantages of this technique:

  • simple to use
  • reduced investment

Disadvantages

  • limited sensitivity
  • only detects faults in bearings in the final stages of degradation

4.2. The status monitoring bearings (measuring the acceleration of vibrations to higher frequencies 1 KHz)

The vibration produced by a bearing on early deterioration is beyond the capacity of perception of the human senses. Indeed not only their amplitude is reduced but also the vibrations generated in it are submerged in the other vibrations generated by the machine.

The general problem of fault detection in a bearing is how to separate the minute vibrations produced by the collision of the rolling elements, the roll a well lubricated surface, with the edges of a microscopic crack, not detectable to the naked eye, other machine vibrations.

The fact that the Global Level Measurement of Vibrations, (10 Hz – 1000 Hz) often do not give satisfactory response to the detection of this type of damage is what led to the study of this subject.

To understand the solutions that they came for the detection of faults in bearings is necessary to know how the vibrations are manifested as bearing degradation evolves.

4.2.1 Vibratory symptoms of a bearing in degradation

Consider the case of a bearing fault degradation outer race, a machine that runs e.g. 3000 R.P.M..

1STAGE

The effect of fatigue phenomena are produced micro-cracks under the bearing surface. Have bursts place vibrations at very high frequencies (hundreds of Kilo Hertz) which are called Acoustic Emission. Typically these vibrations are lost in the machine background noise.

2second phase

The micro-cracks reach the surface of the track.

The edges of the slit are sharp. They occur when the impacts they produce shock waves very abrupt. These produce vibration shocks that extend 300 KHz. The vibrations produced are very small and less than the background vibrations produced by the machine until about a few kHz.

3second phase

The slit increases and the successive impacts of the rolling bodies round their edges. The vibrations produced now extend only to about 100 KHz and increases the amplitude of vibrations at low frequencies.

When there are vibrations at frequencies below 500 Hz defects are clearly visible.

4STAGE

The track surface degradation becomes significant and easily viewable. The starting material has the effect, completely round the edges of the fissure. The vibrating effect can be detected in the mid-range, and end at low frequencies.

4.2.2. The status monitoring bearings – Limitations of this technique

All methods that claim to detect faults in bearings at an early stage, They do it by measuring the vibrations at high frequencies.

As it can be easily inferred from that stated above, methods of detection of bearing faults through measurements at high frequencies, start from two principles:

  • When a roller breaks down vibrations are produced at high frequencies.
  • The only existing vibrations at high frequencies are produced by a degraded bearing.

In everyday life are many situations where this is not true, where the limitations of such measures.

The first limitation relates to the fact that this technique for bearings lose sensitivity to rotate at speeds below 1000 RPM, and be of questionable effectiveness at lower speeds 600 RPM. Actually this speed range no longer produce shock vibrations at high frequencies as described above.

The second limitation is the fact that high frequency vibrations are rapidly damped in materials and its amplitude is drastically reduced by the separation between the surfaces of machine components. It is thus that the measurement point is not located in proximity to the rolling technique loses sensitivity, or even crashes.

The third limitation comes from there are other sources of vibration at high frequencies.

These limitations have, therefore, to be considered when using this technique.

Possible causes of vibrations and high at high frequencies (shocks)

The diagram in Figure it appears that the measurement result from vibration at high frequencies and high amplitudes indicate to say that a bearing is degraded goes a long way.

Phenomena from outside of bearings that can generate vibrations at high frequencies are different:

  • cavitation
  • aerodynamic phenomena
  • Shocks gear in poor condition
  • Shocks of loose parts
  • Etc.

If the vibration originates in the bearing, still yet, it may be that he is not in poor condition. If the lubricant is not being done under suitable conditions will occur in the lubricant film breaks, you should separate the rolling elements of the tracks, that will give, therefore place the occurrence of shocks such as would occur if the bearing was run down.

Technicians with experience, when a first measured bearing high levels of vibration at high frequencies, They make a rule, with which to undertake a lubrication of the same. Immediately levels will fall. If after some time (for example: three days) the level has not returned to rise, then the problem was due to poor lubrication. If the level, on the contrary, return to the previous is actually up before a degraded bearing.

How then overcome these limitations?

4.2.3. The status monitoring of bearings - to overcome the limitations of this technique

effectively, if somehow they are insurmountable, technical utility can often be jeopardized.

Experience shows that an isolated measurement, few results can be drawn, due to limitations in.

However, if instead of a measurement, to carry out a sequence of measurements most limitations can be overcome.

In virtually all facilities where this technique is applied successfully, proceeds to the regular measurement of vibration levels of the machine. Not evaluating the state machines based on a single measure, but yes, based on a set of measures.

Through regular measurements determines a normal level, and the results of new measures then compared to this reference level.

4.3. Measurement of vibration – the frequency spectrum analysis with a vibration analyzer

simple vibration meters, such as the aforesaid, Global measure the level of vibration in a wide frequency band. The measured level reflects the amplitude of the principal components of the spectrum, which it is evident, It is important to control. But when this vibration is analyzed in frequency and spectrum placed in a graphic form, the level of many more components, possibly important, it is disclosed. This technique is called Vibration Analysis.

Not only the increase of the amplitudes of the components in the frequency spectrum gives an early indication of faults, but also the frequency at which they occur indicates which parts of the machine are deteriorating. For each measurement point will be characterizing frequency offset, misalignment, days off, problems gears, etc. that are, therefore, diagnosed with the help of Frequency Analysis.

Frequency spectrum analysis allows the diagnosis of faults

Advantages of use

  • Diagnosis
  • It has no rotational speed limits

limitations

  • Cost
  • qualification requirement of the operator

In the picture above have come up several vibration analyzers.

5. Evaluation of the results of the measures

5.1. Introduction

When, after stating that a given machine needs to be put out of service for maintenance work, it turns out that after all it is in good condition, this is a less happy situation from time to time may occur. If the alert was given under a condition of the control system of the machine, It is one of the worst things that can happen to your discredit.

The correct evaluation of the results of the measures is one of the key success factors of a system of inspection of the machines.

There are multiple criteria that can serve as a basis for evaluating the results of the measures.

5.2. Rating criteria

  • Rules
  • Figures provided by the manufacturers of measuring equipment
  • Values ​​provided by machine manufacturers to control
  • Comparison with values ​​measured under identical machines
  • Experience
  • Trend Tracking

5.2.1 – The ISO 10816-3

The rules relating to acceptable vibration levels are often used as a first guide to assess the operating condition of the machines. some standards, as ISO 10816 specify limits depending on several factors.

The ISO 10816 - Assessment of Vibrations in measuring machines for non-rotating parts – It recommends that the measuring range covers all the relevant frequencies of the machine, which of course will vary from machine to machine.

5.2.1.1 The ISO 10816-3 – the classification of the machines

In the 3 this standard, that is specifically dedicated to field measurements in industrial machines with rated power above 15 kW and nominal speeds between 120 r / min e 15.000 r / min machines are first classified according to their type, shaft power or height and stiffness of the support structure.

  • Group 1: machines with power more than 300 k ; electrical machines with shaft height H 315 mm
  • Group 2: machines with power between 15 kW 300 k ; electrical machines with shaft height 160 H 315 mm
  • Group 3 : multivuluta impeller pumps with separate drive with a power exceeding 15 kW
  • Group 4 : pumps with multivuluta impeller with integrated drive with power more than 15 kW

As for the support they are classified as rigid and flexible. A support is considered stiff in one direction, when the natural frequency of the combined machine and support, lower, in the direction of measurement, is at least 25% higher than the rotational speed of the machine.

They are considered two vibration assessment criteria:

  • whereas the amplitude of the vibrations
  • given variations of the amplitude of the vibrations

5.2.1.2 The ISO 10816-3 – the classification of

To evaluate the vibrations in machines based on their magnitude are considered four zones:

  • zone: the vibrations of a new machine generally fall in this area
  • zone B: machines with vibration levels in this area are considered that are normally able to be operated for extended periods without restrictions.
  • zone C: machines with vibration levels in this area are considered that are not normally able to be operated for extended periods without restrictions. Usually the machine can be operated for a limited period until a chance to take corrective actions.
  • zone D: this magnitude vibration levels normally are considered likely to cause machine damage will

This standard limits are for the levels that are valid for measurements carried out in radial directions and in axial thrust bearings.

The limits are applicable in terms of effective and efficient speed shift, applying the latter to machine at low rotational speeds.

Other criteria specified by this standard relates to the vibration level variations. In particular, that when an increased or decreased level of vibrations to higher 25% the upper value of the zone B, this variation must be considered significant, specially for sudden.

For the definition of alarm values ​​the standard recommends that this exceeds the reference values, by a level equal to 25% the upper value of the zone B.

For machine stop recommends using not greater than 25% values ​​of the upper zone C.

Where these values ​​can not be applied, are produced by vibrations of gears and bearings.

5.2.2 – Figures provided by the manufacturers of measuring equipment

Most vibration measurement equipment manufacturers provide tables with evaluation criteria of the results of measurements made by their equipment.

– Criteria based on standards

The values ​​provided for the evaluation of Level Global measurements of vibrations usually are based on the values ​​of standards.

– own criteria of meters

The values ​​provided for evaluating the condition of bearings are generally characteristic of each type of meter.

The former have the limitations mentioned above. The latter have the limitations inherent to this type and method of controlling bearings.

These devices work by measuring a particular band of vibrations at high frequencies, and assuming that these are exclusively originated in the bearings. When the bearing degrades the amplitude of the vibrations in this frequency band increases thereby detecting the fault. Thus a bearing in good produce, under normal conditions, vibrations with an amplitude determined, being able to build a table for assessing the status of bearings.

This would work fine if no other sources of vibrations at high frequencies. However, the reality is that there are other sources thus resulting in limitations in the immediate application of the tables provided by the manufacturers of this type of meters.

Quite often the meters indicate high values, according to the measuring table, and in the end it appears that the bearing is in good condition. Occur even situations where the measured values ​​are always extremely high making it impossible to apply these techniques.

This limitation is easily overcome if you already have previous experience on the machine in question because, in these circumstances, already in a position to know whether “shape is defective or”.

So a first step in a particular machine, obtaining high values, We can not ensure that the bearing is in poor condition.

5.2.3 – Provided values ​​by manufacturers of machine to control

these values, when there are, They are always a good basis for assessing the state of a machine. Unfortunately it is not very often be provided. When data is usually because the machines are already of a certain size, which is not the case of the overwhelming majority.

5.2.4 – Comparison with Measured Values ​​on Identical Machines

It's a rare machine that is the only of its kind; Most lying in more than one copy, even within the same plant. Thus comparison of results of measurements with one of them the other is one of the most obvious basis to construct a safe criterion of evaluation results.

5.2.5 – The experience

A good experience is, as in all, an excellent basis for evaluating the results of the measures. After all works very similarly to the previous criteria.

5.2.6 – Trend Tracking

In each machine there are numerous factors that can influence the absolute values ​​of the readings. So the safest method of evaluating the results of the measures, consisting of, after a series of measurements, during which it is known that the machine is in good condition, using the measured value, classified as normal, For reference, and define acceptability criteria / limits from it.

Assuming that the evolution of the trend (constant / growing) It is more important than the absolute values, is achieved thus have a criterion that eliminates the constant errors and takes into account the specific characteristics of each machine. IS, for example, the only way to build a reasonable judgment on machines that are not new.

The Trend Tracking

5.3. The definition of alert and alarm levels

5.3.1 – For those who start

Here finally arrived the first day they will make a move!

Make the measurements, note the results, and will be compared with those given in a table – They seem very high. What to do?

This is indeed a critical phase for start. At this point the reliability of the advice is very low. Besides there is no history in terms of the measurement results on the machine in question, probably also who is the measure has little experience in this technique.

And then, what to do?

In these circumstances is recommended DO NOTHING.

It is necessary to assume that, the reliability of the opinion, in this conditions, It is greatly reduced, and that the risk of error is excessively large. It can jeopardize the credit of future opinions when it misses miserably in the first.

If indeed seem very high values, back to the machine in question in the following days and try to ascertain the trend of the results of the measures is, effectively, growing. If this is confirmed, beyond doubt, You can then suggest an intervention.

When you start the implementation of an equipment of the Condition Monitoring system are other investments made in addition to the measuring equipment. We must also invest in the acquisition of a minimum history of the machines and the acquisition of experience who measures and interprets the results.

It must be considered that the acquisition of the results of the first measures, part of the initial investment and is not intended to provide immediate results. It is intended rather to allow the construction of safe evaluation criteria and gain experience.

5.3.2 – When no experience

When there is experience it tries to optimize. Indeed the warning and alarm thresholds are dynamic; are not static values ​​that once established remains so, properties, forever. In its definition experience has a key role and this increases with time.

We should avoid listings absurd alarms, no one believes, due to have been established at the time of implementation and after that no one had touched them.

5.4. Conclusion

The most reliable criterion for evaluating the results of the measures is that is based on Trend Tracking. Their use generates alert values ​​and reliable alarm to continuously review based on experience

Leave a Reply