Vibration analysis gears – frequency spectral components
When does the analysis vibrações gears, even if the frequency spectrum of the vibrations is often very complicated and seemingly, It can usually be decomposed in the combination of the following effects:
a) Frequency of gearing and Harmonics
Represent the average deviation from the ideal profile meshing tooth profile, that is, the perfect tooth profile.
b) Side bands
Components usually originated in the modulation of meshing, caused either by slow variations (by eccentricity) or sudden changes caused by occasional irregularities (bites, slots, etc.).
In addition to the modulation of engagement, the sidebands can also arise modulate a Natural Frequency. In this case indicate the shock repetition rate which excites the Natural Frequency.
c) Ghosts components
They appear to be a Frequency gearing, but with a different number of teeth that the gear has.
Generally they can be related to the number of teeth of the indexing gear cutter which carried out the machining, and are due to errors in this gear.
d) Mounting frequency
Component created a pair of gears used when it changed its relative position.
e) Teeth repetition frequency
Component created when the same pair of gear teeth.
f) Natural frequencies
Components that arise when a damaged gear begins to cause shock.
g) Rotation speed and harmonic
Vibration analysis gears – Frequency of gearing
The ideal tooth profile deviation, constant every time it makes gear, is that, therefore, causes a periodic vibration frequency gearing, You can have various causes. On the one hand, there is a deflection of the teeth under load, which varies each time that is shared among a different number of teeth in each gear cycle, and on the other, There deviations resulting from wear uniform, machining and assembly faults.
Figure 1 – Vibration analysis gears – Deflection of the teeth of a load of gear for action
Due to deformation of the teeth is essentially dependent on the load, to obtain comparable spectra, measurements have to always make the same charge. The loading must also be sufficient to ensure that the teeth are in permanent contact.
Yet, with constant load, any change in the amplitude frequency and harmonic gear should come from the wear. The Figure 2. shows a typical wear profile.
Figure 2 – Vibration analysis gears – Typical wear profile
Wear is greater on both sides of the pitch circle due to slipping, while at the midpoint of the tooth pure rolling occurs. This profile error will tend to cause considerable distortion of frequency gearing, with the result that the wear of the teeth is more visible in harmonics than the fundamental. General rule, it is advisable to control at least the first three harmonics of the frequency of engagement, when you want to follow this type of defect.
Figure 3 – Vibration analysis gears – Typical changes in harmonic frequency spectrum meshing frequency due to wear
Misalignment of wheels
The mounting problems, normally, They give rise to high amplitude components of the frequency gearing, It is the misalignment and incorrect clearances.
The misalignment causes an eccentricity gear creating a variable gap, originating, therefore, a deviation of the position of the teeth of its ideal position.
Figure 4 – Vibration analysis gears – misaligned shaft
Another consequence of the eccentricity is the creation of a reaction between the gears, when the clearance is insufficient. so there arises a component to the speed of rotation to monitor the frequency of gearing.
Like this, any cause that provokes an eccentricity of a gear can cause the rotation speed components and high frequency range gear. As possible causes of eccentricity has a poor manufacturing or imbalance, in high-speed gears.
In spur gears, misalignment often axial vibration generator.
Case study misalignment spur gears
In the figure 5 can be seen the Global Vibration Levels, vibrations caused by the meshing frequency, the bearings of the drive sprockets (with Spurs), a cement kiln, misaligned crown. Note the high amplitudes in the horizontal direction on the free side bearings.
Figure 5 – Vibration analysis gears – Vibrations in the bearings of the drive of a driven gear mill with Spurs
Incorrect clearances between gears also cause vibrations at frequency gearing.
Vibration analysis gears – Side bands
Most vibration components at different frequencies of meshing frequency can be explained by their modulations. For example, due to the dependence of the deflection of teeth, load, any fluctuation in this (for example caused by misalignments) will tend to cause a corresponding variation in amplitude, thus causing an amplitude modulation (see Figure. 1).
At the same time these fluctuations in charge of the teeth will cause variations in angular velocity, thereby causing frequency modulations. The modulation amplitude and frequency causes the appearance of sidebands around the basic frequency (frequency of engagement and its harmonics), with a spacing equal to the modulating frequency (generally the speed of rotation of the gears), thus containing important information for making a diagnosis as to which of the gear that is causing.
The following figure shows the effect of a defect in a tooth point and an eccentricity.
Fig. 6 – Vibration analysis gears – Effect of defect distribution pattern of the sidebands
A broken tooth produces a pulse on each rotation (amplitude modulation). This type of malfunction gives rise to a spectrum dominated by engagement with frequency small amplitude side bands, spaced Impulse frequency.
If there are many defects located, such as for example the case of multiple stings stocks, vibrational effects become evident then.
In the case of eccentricity, appear important sidebands around the frequency of engagement. A misalignment causes this effect. It is distinguished from bites phenomenon by cause significant vibration components at the speed of rotation of misaligned gear.
Even in gears in perfect condition, It is normal to find sidebands, they not being itself a malfunction. Its growth or the appearance of new sidebands, is that problem is evidence of evolving.
Vibration analysis gears – Ghosts components
Figure 7 – Vibration analysis gears – creating ghost components
As already mentioned, these arise from errors in the teeth of gear acionante indexing table which is mounted on the gear machining stage. The vibration frequency, later generated by gear while on duty, corresponds to the number of teeth and, therefore, It must be an integer multiple of the gear rotation speed. This provides an indication that an unknown frequency may be a ghost component, when you can not meet the manufacturer's data.
Another indication can be obtained through its behavior in function of load. Since this component represents a constant geometric error, It should not be greatly influenced by the load.
Once recognized, phantom component, usually do not cause any problems, there is a tendency for it to decrease with time (wear).
Vibration analysis gears – Mounting frequency
The teeth of a pair of gears, when in operation will create a “bed”. It turns out, unless the number of teeth of the two gears, are relatively prime, a given tooth of a gear will not contact all the other teeth. They are created so families of engagement that give rise to “Beds”. If after a disassembly of gears, these do not come back to be placed in exactly the same relative position, They give rise to a vibration whose frequency can be calculated as follows:
Mounting frequency f =
Fc = frequency gearing
Greatest Common Divisor N = the number of teeth of the two gears
Vibration analysis gears – teeth repetition frequency
If there is a defect in a pair of teeth of two gears, Each time you give the meshing of this pair will occur a boost.
This vibration occurs, therefore, the gearing repetition frequency of a pair of teeth, can be calculated as follows:
F = (Fe x N) / (Np x NC)
F – Repetition Frequency
N – Greatest common divisor of the number of teeth of the two gears
For example, – Number of pinion teeth
Nc – Number of teeth crown
Vibration analysis gears – Rotation speed and harmonic
The components of the rotation speed and its harmonics have come from the same causes as the other machines: imbalances, desalinhamentos, days off, eccentricities, etc.
In the spectrum of PeakVue represent shocks repetition frequencies.
From what has been said underscores the importance of having a vibration analyzer with high resolution of the frequency spectrum lines.
Case 1 – Vibration analysis gears - bearing failure
An apparatus comprising a motor with variable speed and a gear box, He began to develop an uncharacteristic noise normal operation. Fearing one equipment failure, maintenance services have chosen to replace the electric motor. After changing the engine, it was found that the noise remained unchanged.
Case 1 – first measurement
Faced with this situation, we chose to take a measurement of vibration equipment, with a view to detecting the noise source. In vibration measurement made, found that the presence of frequencies associated with degradation of the inner ring raceway bearing supporting shaft housing inlet, opposite the engine.
No spectrum PeakVue shown in Figure 8, the presence of frequencies can be seen from 131,46 Hz and harmonics.
Figure 8 - Vibration Analysis gears PeakVue spectrum collected before the replacement bearing
The presence of these frequencies concluded that the bearing was in degradation phase, being the noise associated with its operating status.
Depending on the collected data and the analysis performed, It was recommended the replacement of the bearings supporting the shaft housing inlet.
Case 1 - measurement after repair
After replacing, It performed a new vibration measurement equipment, It is shown in Figure 9, The spectrum collected PeakVue.
In then performed analysis, associated frequencies were not identified the development of anomalies in the bearings and the noise disappeared completely.
Figure 9 - Vibration Analysis gears PeakVue spectrum taken after replacing the bearing
Subsequently, in order to confirm the diagnosis, It was asked to dismantle the degraded bearing. Analyzing the respective bearing components, found that the degradation of the inner ring raceway, As can be seen by the photograph shown in Figure 10.
Figure 10 - Photograph of the inner race of the bearing degraded
Case 2 – Vibration Analysis gears -folgas
Routine measurement performed to drive a conveyor belt, constituted by a gearmotor whose electric motor that runs 1492 rpm, whose output shaft rotates 60 rpm, found, vibrométrica during inspection, vibration levels exceeding the limits established based on ISO 10816.
Figure 11 – Drive conveyor
Case 2 - initial situation
The highest global level value was 6,7 mm/s RMS, registered in the motor support opposite the drive. Despite the higher levels vibrométricos have been registered in the motor support, opposite the drive, the collection of spectra with smaller frequency ranges (up until 50 Hz) identified the presence of the operating frequency of the reducer output shaft (1 Hz) and harmonic.
The presence of these frequencies is symptomatic of excessive clearances.
The frequency spectrum shown in Figure 12 It is indicative of what was previously mentioned.
Figure 12 – Vibration analysis gears – Spectrum collected at reducing support, output shaft housing
Case 2 - made recommendation
Despite vibrométricos levels have higher amplitudes in the engine mounts, given the spectra obtained, It was made the recommendation to the intervention of the reducer, namely, for checking the gaps between the "sleeve" to the output shaft and the shaft of the conveyor belt drive drum. In the performed inspection found it was indeed, the presence of clearances between said components.
Case 2 - measurement after repair
After the intervention for the anomaly correction, a new measurement was made where it could observe a significant improvement in the characteristic of the spectra obtained (Figure 14).
Figure 13. Vibration analysis gears – Spectrum collected at reducing support, output shaft housing, after intervention
In vibrométrica inspection performed after the maintenance work on equipment, found a generalized decrease in the levels vibrométricos that the equipment is subjected. Particularly in engine mounts (points where the highest amplitude registavam), it was possible to observe a significant reduction in amplitude vibrométricas, As can be seen in the trend chart shown in Figure 14.
Figure 14 – Vibration Analysis on Trend data-graph gears Vibration Level Global registered in supporting the drive side of the motor
Also note that, with the intervention made, observes the disappearance of the harmonic amplitude peaks of the operating frequency of the reducer output shaft, as can be seen through the frequency spectra shown in Figure 15.
Figure 15 – Vibration analysis gears – Spectra recorded at frequencies support shaft gear unit output, before and after maintenance work
Case 2 – Vibration analysis gears - Conclusion
Select the range of frequencies appropriate to the type of equipment, as well as the correct number of lines for the spectral resolution, (among other factors), It allows you to diagnose faults successfully, even in cases where low speeds make the less obvious diagnosis. Therefore, during the parameterization of the equipment in Database, should be especially careful in measuring points of settings, in order to timely detecting possible malfunctions.
Case 3 – Vibration analysis gears – cardan
A drive unit of a cylinder "Yankee", comprising a gear unit driven by two (2) motors as illustrated in Figure 16, Predictive Maintenance is included in the program defined by the customer, It is subject to regular inspections vibrométricas.
Figure 16 - Vibration Analysis gears – drive unit photography
Case 3 – cardan – initial situation
Inspection carried out in June, there was a significant increase in levels of vibrométricos presence in reducing, as can be seen in the trend chart shown in Figure 18.
Figure 17 - Vibration Analysis gears – Global Level Trend Graph Vibration registered in support of the gear unit input shafts
The frequency analysis of the recorded spectra revealed that in the presence vibrométricos in reducing levels were, about everything, influenced by the amplitude of the second harmonic of the operating frequency of input shaft (Figure 18).
Figure 18 - Vibration Analysis gears – Frequency Spectrum recorded in support of the gear unit input shaft
Case 3 – cardan – recommendation made
From this analysis, Also the fact that the protruding extent of operating frequency of the shaft gear unit input display greatly reduced amplitudes. For this facto, It was discarded as a cause misalignment first and foremost to the high levels recorded vibrométricos. That way, was recommended to the customer replacement / repair of unions "gimbal".
Case 3 – cardan – measurements after repair
After replacement of joints "cardan", carried out by the technical services customer, during one of the scheduled installation stops, vibrométrica performed the inspection revealed a very significant reduction of the vibration levels is found that reducing subjected, as can be seen in the trend chart shown in Figure 19.
Figure 19 - Vibration Analysis gears – Global Level Trend Graph Vibration registered in support of the gear unit input shaft after replacing the "Cardans"
The decrease in severity originated vibrométrica, mainly, by decreasing the second harmonic of the operating frequency of the input shafts of the reducer, as can be seen in Figure 20.
Figure 20 - Vibration Analysis gears – Frequency spectra recorded before and after replacing the "Cardans"
Case 3 – cardan – Repaired equipment
The inspection revealed the replaced components of accentuated degradation of the bearings and hangers, As can be seen in Figure 21.
Figure 21 - Vibration Analysis gears – Crosshead one of the unions "cardan" substituted
Case 3 – cardan – Conclusion
A iMPLEMENTING a Predictive Maintenance Program, based on the measurement and analysis of vibration, It allowed on time and without production losses, make it possible to intervene in the equipment minimizing intervention costs.