Please help me identify the vibration peak details of which are as under. It may not be possible for me to attach the vibration spectra.

Shaft speed is 402 RPM
Measure for vibration is velocity and the units are inch per second RMS
Recorded vibration is extremely low -- 0.05 inch / sec RMS
At 360 Hz, I see a vibration peak having an amplitude of 0.01 inch

Can you guys identify this peak based on above information?

Thanks in advance

Riyaz, first of all this may be a total misapplication of transducer type (which you don't mention). If you are using a velocity coil then care must be taken as typically there are three different types, a 900, 450 and 270 cpm version, where the numbers refer to the lower end frequency response. Obviously if you are using the 600 or 900 cpm coil(typical with a lot of general purpose kits) then you can't do a great job investigating a machine that operates at 402 RPM. Also, the 600 cpm and 270 cpm coils are often orientation specific, in other words a coil might operate at 0° ± 2.5°. Mount it at 90° and you are likely to get severely attentuated levels.

Beyond that, it would be very difficult to identify a 360 Hz peak based on what you have provided without very specific details of the machine and even surrounding machines. Since the level is 0.01 inch (I assume you mean in/sec) it is extremely small and could come from some adjacent machine. The fact that it is so small (assuming you've made a reliable measurement) is also a reason not to worry about it. You can get terribly tied up and waste time in identifying peaks that are meaningless. In the instance of 0.01 in/sec, if you are using a 500 mV/in/sec velocity coil (again pretty much the standard) then you are attempting to measure something at 5 mV, a difficult task at best with a velocity coil.

John from Pa

This message has been edited. Last edited by: John from PA,

Here is some additional information:

Vibrations were recorded on the upper bearing of a vertical pump. No other pumps were in operation during vibration test on this pump. The spectrum shows 0 to 400 Hz frequency range. My understanding is that all the signals in this frequency range is picked up by the sensor.

It makes me happy to see peaks at 1X and at VPF and no or very little energy at high frequency. 360 Hz means 54X.

Would this mean some problem with either the VFD and / or the motor. This motor has a maximum speed of 435 RPM as per the nameplate. At 60 Hz I recorded only 402 RPM at motor shaft.

As John said, the amplitudes are very low, so why are you concerned?

2 potential sources come to my mind:

1. Rotor bar pass frequency
1. 6 x Line Frequency from full wave rectifier, which may be in the drive, depending on what kind of VFD you have.

In any case, unless it suddenly appeared or is trending up, don't worry about it.

New pump was installed with existing motor. Vibrations were recorded at the time of start-up.

Riyaz, critical that we know the transducer type. Even if you have an accelerometer that is being integrated 0.01 in/sec (RPM) is about 0.08 g peak. This points out the misconception that accelerometers are great "universal" measurement devices. 0.08 g is 8 mV with most accelerometers, again pushing the noise floor of the device. Let us know what you are using..."My understanding is that all the signals in this frequency range is picked up by the sensor" could be a bad assumption.

John from PA

I agree with Bill. If it is EXACTLY 360 it is more than likely comming from a DC drive. I have saw 360 on my machine frame more than once.
Also on other equipment, if its strong enough it can be almost anywhere ..
I never worry about my 360 it comes and goes with motor loading.

Mike Brooks

I thank everyone for participating in this discussion.

Have a great week end.

Hi Mike,

I'm also getting SCR frequencies in most of my DC Drives. We have 6 SCRs in our drives and that makes a peak at 360Hz in all our Motor bearings. Harmonics of these peaks were also seen. We normally neglect these peaks and will care about it only when it grows up.

But did anybody have an acceptable range for these peaks? What it indiactes if there is sidebands of running speed around the SCR frequencies?

Have a nice day!!

My amplitudes of 360 go up and down with load. If one section is pulling another or light loaded or however they are running th emachine. I never have seen sidebands around my 360. I normally don't worry about it.

Mike