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Vibrodiagnosis 2
As we mentioned in the previous article, the parameters of the vibrations that are collected from the machines are:
- speed (FFT);

- acceleration (FFT and temporal variation);

- acceleration, with the 50-1000 Hz filters, 500-10000 Hz with demodulation of the signal, the parameter obtained being called by the specialty literature „enveloping”.

In each of these parameters certain types of faults are being followed. Thus, information regarding the weakenings in the fastening, the lacks of balance, the lacks of trimmings, the wrong coupling are being followed in the vibration speed.

In the acceleration there are information regarding the weakenings on the shaft, the workings on the power flow, teeth brake and wears, serious development of the bearings wear.

In the enveloping there are information regarding the starting of the bearings wear, workings in incipient phases, workings of the unrestricted rings on the shaft, leakinesses, weakened screws, noises in the machineries.

In the HFD we follow the functionings in conditions of short oiling.

Once the problem is identified through the vibration signature, the following question is "Is the problem that serious that it needs an intervention?" In the following paragraphs we will try to answer this question on the grounds of the practical experience.

The analysis of a machine`s vibration values is based on the fact that specifical elements of the machine`s parts, that rotate, will produce forces inside the machine that will cause the emerge of the vibrations at specifical frequencies.

One of the most important frequencies is the rotation speed of the axis (rpm) and it emerges from the fact that any rotor will always have a certain level of residual disequilibrium. This creates a radial centipetral force on the bearings, which determins a vibration of the structure at 1X (or the fundamenal frequency).


During the watching over of the vibration level of a machine in time, an increase of the fundamental frequency shows a modification in the machine`s mechanism, which produces the emergence of that fundamental frequency, but it does not necessarily 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 desequilibrium, but if oscilations start to appear at 1X, than we can say that defections appear, such as wearing in progress of the bearings, weakening or clefts in the structure. This is why a strong vibration at 1X means that a rotor must be equilibrated, but at the emergence of the oscilations at 1X, it means that the bearing and its surrounding structure must be verrified in order to find out the possible defections.

For each bearing there is in the database of the spectral analysis programme Prism4 «the defect markers», that is the defect frequencies for the constituent elements of the bearing, specific to each type of bearing (determined in the laboratory by the bearing producers). The defect frequencies are generated by the impact forces of the covers and the balls of the bearing itself, correlated with the fundamental frequency (the speed of the axis on which the bearing is set).

The frequencies of the cogged weels gearings are generated by the individual impact of a cog upon all the other cogs from the gearing, the frequency of the cogged gearing being equal with the number of cogs from the gearing multiplied with the speed of the shaft.
The frequencies of the paddles from a ventilator are similar to the cogged weels and are equal with the number of paddles multiplied by the rotation speed. Each fundamental frequency will create a top in the frequency spectrum, its amplitude depending on the gravity of the cause which produces it.

Thus, the frequency indicates the type of problem, and the amplitude indicates its gravity. In the standards, the equipments have been grouped according to their size and their working conditions and it is specified the allowable level of vibration. Such a standard is ISO 2372 - it ensures a range of acceptance of the vibration amplitude for rotatary machines with speeds situated between 600 and 12000 RPM. This standard specifies the vibration speed limits based on the power of the engines and covers a range of frequency between 10 and 1000 Hz.

The analysis of the functioning condition of the machines from the standpoint of their vibration is carries on according to the following algorithm:

- there are identified the the vibration tops in spectrum and they are associated with the fundamental frequency;

- the degree of severeness of the problem is determined by reading the amplitude, and also the connection between the tops from the vibration spectrum;
- it is recommended to carry out the repairs according to the seriousness of the problem.

In the following article, we will present concrete cases of vibrodiagnosis where we found defections of the functioning macines, cases validated after their dismount for repair.

(17.06.2009)
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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
 
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Vibrodiagnosis 2
As we mentioned in the previous article, the parameters of the vibrations that are collected from th
 
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