1. Measure the amplitude at the top and bottom part of the vibration mount and it should be same across all vibration mounts of the machine, so any deviation will give u the hint of a faulty mount
  2. Measure the overall height of all vibration mounts (ideally loaded ) any major difference is a telltale sign of spring or material deformation
  3. Look for the signs of high 1x peak in horizontal direction with comparatively thicker base (however this is not always a sign of bad vibration mount) 

It is common for a bad selection and placement of these feet if you as normally are select them from the machine weight and in the chart of the supplier of the feet. This is then for a vertical load normally and if you have a vibrating fan in one end and it makes the frame wiggle-waggle (see-saw?) it may be due to a resonance made up by the feet... so if you gave hi vibration in one end and not the other plug in a pine wedge to short those feet in the vibrating end to verify the problem. In periods a very standard problem here in new built plants. This nice consultancy work may in principle happen in any of the 6 degrees of freedom but the above are the most common for me. Just a tip. 

Are those measurements on the photo in horizontal or vertical direction? What kind of machine and size and speed?

Your OP question needs to specify the type of vibration mount/pad and the direction of vibration. Why do you suspect there is a vibration mount problem?


I'll ask again "Why do you suspect there is a vibration mount problem?"

What indications do you have (observations or measurements) that the vibration mounts are defective or suspect?

Rotary lobe blowers produce pulsation frequency and harmonics that can change with load (pressure differential) and may excite structural resonant frequencies.

There are many indications of defective or failed vibration mounts depending upon type and if correctly selected and sized such as: loose/broken bolts at feet or at machine base, broken spring, cracked/crumbling rubber, incorrect static height (too high or too low), and incorrect vibration transfer/isolation (too high or too low). There can also be a short circuit that allows vibration transfer such as a piece of wood or steel wedged between machine base and ground or pile of debris around mount. As mentioned, there are actually 6 natural frequencies (degrees of freedom) for a rigid mass (rare situation) on vibration mounts, and not just for vertical motion that mounts are commonly selected. Frankly I have not seen a simple "one size fits all" method to detect a defective vibration mount, but I am always willing to learn something new! I have worked on many lobe blowers up to about 600+ horsepower, and they can be a challenge.



Dear Walt, 

thanks for your comment,

the motor is about 14kW (50Hz, 1500 rpm), I have checked the severity chart  ISO 10816, and maximum permissible for normal operation condition is about 4,5 mm/sec for low power machine.

based on this standard we consider to do some actions.. 




If you follow ISO recommendations, collect data as the ISO describe, Hor, Vert, Axial on the bearings. In my book I follow ISO 10816-1 as it been mu follower for 40 years and the 2.8 is acceptance and 7.1 where you at least should do something. So verify the readings, H,V,A at both bearings and tell us the result. Next check if it is 1xRPM same both H and V, clean the fan and if it doesn't help, balance it if that is not feasible actually don't want to be balanced, look at the foundation.

Dear All,

Today I did Dump Test in base of blower & motor, please check that we found two natural frequencies at 12.5Hz and 25 Hz, with motor in OFF position .

may for that reason the system response at 1x rpm force excitation?

 now we can say the system has looseness with resonant vibration?







Photos (1)

Mr Ramon, where is your statement about the looseness coming from? Also at 5,7 mm/s I would not talk about any vibration excitation. Have I overlooked any bearings spectrum data? What are your motor and fan bearing values? Please send us all of your recorded spectrums otherwise you can't expect a quality answer.

Your post started with isolation pads, now continues with bump tests..... I think you began to start solving your machine problem from the wrong side. Let's start from the beginning. What bothers you with this machine? Did you record any spectrum data to share with us? Bump test is one of the last things to measure. Resonance issues are considered when the vibration values are high. 

I don't see any critical issues from your data sent. No obvious mechanical looseness, no critical vibration level. 1st peak amplitude is a little higher in horizontal and could indicate some residual unbalance on the motor-coupling-blower shaft line. I wouldn't say this machine needs some actions. It would be good to have some envelope/peakvue data for bearing evaluation.

However if the vibration level still bothers you, you could try field balancing on the motor fan and on the coupling. I am sure you would get some results.

"Today I did Dump Test in base of blower & motor, please check that we found two natural frequencies at 12.5Hz and 25 Hz, with motor in OFF position .

may for that reason the system response at 1x rpm force excitation?

 now we can say the system has looseness with resonant vibration?"

You did not indicate what direction and locations you did "Dump Test" (impact test), but I assume it was in horizontal direction. You have a curious choice of mixed vibration units mm/sec RMS velocity spectrum and in/sec P-P for velocity waveform.

You have measurements at 1xSS with the following amplitudes: motor bearing 5.0+, top of support 5.8, and bottom of support 4.3 mm/sec RMS. An operating deflection shape test (ODS) for 1xSS would reveal motion from the intake vessel (filter/silencer), shaft centerline, top of support, bottom of support, and concrete foundation. All of the locations may not be in-phase.

Your question-1: yes if 1xSS is near 25-Hz (1500-rpm), then alignment or unbalance is the excitation with resonant structural amplification.

Your question-2:  you offered no data to support looseness, and you certainly did not indicate where the looseness was located.

If your software has a cursor, then put it on 1xSS or on the dominant peak of interest in the spectrum, and put it on the peak in the waveform. I really dislike poorly presented data!


Well, if you answer some questions it is easier to reply. So how old are the pads? OEM normally state 10 years op time and shorter in sun and chemical environ then they get stiff "change properties". So maybe the customer don't like beams in the machine and if the machine previously was ok, so change pads to new, correctly selected pads? Or make your own but blower foundations are tricky, not funny. Have you checked the bearing condition, greasing? Just asking.

You have some cracks in the foundation also. are they serious, if you measure vibration each side of the cracks is it the same vibration or do the cracks separate parts of the concrete? Isolation pads "seismic mount" are defined to have one side mounted on a reasonably stiff foundation, if that is not the case anymore they may not work as intended. Inject something if needed maybe?

Thanks for your comments: Mr Walt , Mr Oli and Mr Becar.

for continues supporting in this case,

I asked to Operational people they told me that this pads have not been replaced since the plant started (more than 30 years).

and I checked vibration in foundation  and I found 0.39 mm/sec at down side of Vib pad.

to replace all vibration pads will be good recommendation for our customer.





Your "temporary" structural modifications was surprisingly effective! You demonstrated significant vibration reduction by adding mass to lower the natural frequency. I don't know if you thought the wood beam would add stiffness or not, but at least it worked quite well. I could make a suggestion for a more permanent modification, but I would like to see what you do. The blower spectrum now looks more "normal" with pulsation peaks higher than 1xSS. Good job!


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