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Dear all,

please find attached image, where you can see

1. 25Hz (1X) frequency of amplitude 2mm/s ,

2. 125Hz (5X- let vane pass) frequency of amplitude 0.5mm/s

3. 1650Hz of some electrical frequency( rotor bar/ stator slot


i Just want to Know which frequency looks severe and why?

is there any way to calculate severity of different frequencies in a spectrum , also 1X of 2mm/s is severe or 5X of 0.5mm/s is severe



Images (1)
  • Spectrum
Original Post

In short, it depends.

Does an amplitude have a different evaluation if it is at different frequencies? Yes and no. It depends on your own standard.

For example, if you use an overall reading as the evaluation parameter, perhaps the difference in frequency will not matter that much. If you use frequency-based limits, the amplitude at different frequencies may have different evaluations. Also, most online VMSs do not consider the frequency factor to send alarms to the operators.

Do you have an actual case or this is only theoretical?

Regards- Ali M. Al-Shurafa

Dear Shurufa,

In normal training people teach that amplitude is the severity indicator, in practical one spectrum it self contains several number of forcing frequencies, by believing only overall amplitude trend some times it is not possible to predict coming fault, setting the frequency based alarm trends can help analyst to judge it easy,

i want to know like in the attached spectrum if overall vibrations are under limit, but which frequency making most impact on performance of the percular equipment?

Thanks& Regards

It depends on the machine, its faults, and its normal levels.

See this chart for example Techniques for Detailed Screening of Vibration Data.

Many good vibration analysis systems enable the user to add the band alarms based on the machine's characteristics (set points on filtered vibration components imposed on the spectrum plots). For example, machines with rolling element bearings would have tighter limits on higher frequencies compared to 1X and 2X. There are some guidelines on this technique.


Regards- Ali M. Al-Shurafa

Not sure of your intent in asking this question, but it seems to me that this attachment would appear to be a "hypothetical" test question.  Yes? No?

The specific 1X and 5X would appear to be fairly low in amplitude....and as Shurafa states, this would also be dependent on previous data from this machine.  Nothing alarms me with that "single" spectrum

The 3rd comment, "1650Hz of some electrical frequency( rotor bar/ stator slot" is certainly not clear (at least to me) of impending doom.....but modulation of rotor bar/stator slot would cause me to "watch" and "trend"....

Whenever I am presented with data like this... look at trends, take data more often, talk to the operators, consider electrical checks, etc.


Jim P


Greetings Venkatesh ,

We need to understand equipment behavior from whole vibration spectrum .For example 1X due to unbalance ,we can get other frequencies .Once you resolve the root cause problem other frequency peaks disappear .

For Vibration severity reference you have to follow concern /applicable standard guidelines .


Dear kishore,
I like your comment that Once you resolve the root cause problem other frequency peaks disappear .
I to seen in many cases


The question looks to me more of an assumption.

My contribution is.. It will not be wise to compare the frequencies among each other & know the severity, rather they should be compared with their previous trend in a waterfall plot. Which will show us the SEVERITY change.

An unbalance peak should be compared with last week's unbalance peak on the same equipment & same goes with Vane paas freq to last week's vane paas freq comparison.




Dear Venkatesh ,

1.1X dominant in spectrum  ,definitely it is the most damaging frequency component.

2.As per my experiences ,Low frequency events such as Unbalance ,Misalignment& Looseness are leads to generate high frequency event damages like bearing ,gear failure .

3.There are some exceptional cases ,where above criteria might not applicable .

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