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The description of the solution of defections identification through the vibration analysis
For the collection of vibration spectra it was used a collector-analyser produced by SKF, type MICROLOG CMVA 60, which, through some portable magnetic translators, measures, collects and records automatically or with the intervention of the operator the vibration spectra of the machines and allows their analysis in a thorough manner.

After collecting and storing the spectra with the collector-analyser MICROLOG CMVA 60, they are transferred in a calculation unit, where, with the help of the PRISM4 soft they are analysed and the technical condition of the machine is being diagnosed.
When following the vibration level of a machine in time, an increase of the fundamental frequency indicates a modification in the machine`s mechanism, which determines the emergence of that fundamental frequency, but does not indicate a certain problem in the functioning of the machine. For example, an increase in 1X at the bearing of an engine indicates an increase of the misalignement, but if harmonics start to appear at 1X it means that the bearing and its surrounding structure must be verified in order to track down the eventual defections.

Applying methods of specific analysis, defects had been identified such as:
looseness between the shaft and the bearing;
looseness in the frame;
defective lubrication;
misalignement;
looseness on the shaft of the half-coupling/driving pullei;
eccentic rotor;
looseness of the terminal plate connections;
etc.

In the vibration spectra, the general characteristic of the mechanical loosenesses on the shafts is represented by the increase of the amplitude in the fundamental frequency 1X (the rotation frequency of the shaft) and its harmonics.

This kind of example is represented by an electrical engine of 1500 rot/min speed, used for the driving of a rotary pump. It has been noticed the increase of the vibration level in the functioning of the electropump, fact which imposed the collection of the vibration parameters on the engine and on the pump, information which had been transferred in the computer for the analysis. The vibration spectra had been analysed and it turned out that at a frequency of 25 Hz (corresponding to the speed of 1500 rot/min) there are multiple of hereof (until the 16th multiple). Consequently the recommendation of repairing the engine was expressed, with the specification that there is a clearance between the shaft of the engine and the bearing (insufficient fastening between the shaft and the bearing). After the dismantling of the engine it turned out that the shaft was worn out, the inner ring of the bearing being loose on the shaft.

The clearance of the bearing in the frame is also part of the mechanical clearances category, the main characteristic being kept, respectively the fundamental frequency 1X and the multiple harmonics.

Indications regarding the type of the clearance rise from the amplitude of the multiples 2, 3 and 4 of the fundamental (higher than the ones of the fundamental with the dominant multiples in the spectrum), fact which localizes this type of looseness in the bearing area.

If the odd harmonic is dominant and its multiples also, the looseness is between the bearing and the shaft. In the case of the bearing looseness in the frame it is relevant the fact that the fourth harmonic and its multiples are dominant.

In both situations the initiation and the development of mechanical clearances (on the shaft or on the frame) is under favor of the emergence of frictions in the bearings because of the inappropriate lubrication.

The presence of the lubricant film between the rolling elements is measured and indicated graphically through the HFD coefficient, the alarm limits of which are being established with the help of the Prism 4 soft. The outrun of the alarm limits indicates the necessity of lubrication, even in the case when the lubrication graphic is not known.

The monitoring by following the HFD coefficient is very important, but when the functioning is inadequate, the analysis of speed and acceleration is done for determining the type of defection and especially the opportunity of repairing the machine.

If the machine has functioned with a short lubrication, the mobile parts have suffered wears which are reflected in the increase of clearances.

This also holds true for the bearings which fit the rotation axles from the machines. The bearing producers have determined in laboratory conditions specific frequencies emitted by the bearing parts. If the bearings are new, correctly set and have optimal functioning conditions, they determine in the spectrum individual frequencies of low values (reported to the fundamental and its harmonics). If the integrity of the bearing is affected, the amplitudes of the defect frequencies of the bearing elements grow and their multiples appear.

For example, it has been monitored the support bearing of a suction roll from the tissue paper machine, equipped with a bearing 23148K series. In the vibration spectrum, the dominant is the defect frequency for the exterior ring and its multiples. It was recommended the stopping of the machine and the inspection of the technical conditions of the 23148K bearing. After the dismantling of the bearing cap we noticed that, from the exterior ring a piece of approximately 30cm2 was missing. Information regarding the existence of the bearing defection existed from the day before, but the stopping of the machine for the technical inspection was not possible. We mention that, at that moment the defect frequency was present in the spectrum, but it was not dominant it was possibly the time when the defection was in the crack stage.

We present forward a case regarding the deficient functioning of a conical reductor with circural arc cogging.

The vibrations on the gear reductor have been measured, because it was noticed the increase of the noise level in its functioning. At the analysis of the acceleration spectra we noticed the modification of the fundamental frequency amplitude corresponding to the gearing (GMF) and its multiples. Thus it was formulated the recommendation for the verification of the gearing, with the specification that there are broken teeth, fact confirmed once with the stop of the reductor and the possibility of viewing the gearing through the manhole. The monitoring of the reducer must be done very carefully, especially if the cogging is very rough there is the danger that the parts which resulted from the breaking of the teeth to be driven in the gearing which determines the quick distruction of the gearing (a couple of hours of functioning).

In the following article we will continue the presentation of other concrete cases of vibrodiagnosys in which electrical defections have been noticed on the electrical engines (the misalignement of the machines, eccentric rotor, looseness of the terminal plate connections, etc.), cases validated after their dismantling for repair.
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Tracing the defects
As we mentioned in the previous article, we will continue the presentation of other concrete cases o
 
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The description of the solution of defections identification through the vibration analysis
For the collection of vibration spectra it was used a collector-analyser produced by SKF, type MICRO
 
  • Read...  
 
Vibrodiagnosis 2
As we mentioned in the previous article, the parameters of the vibrations that are collected from th
 
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