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Read our primer articles on Vibration Analysis and Laser Alignment Tools.

Res Sirs,

I am presently engaged in the field of VA of rotating machines and bearings for a few years. I have following doubts. Please clarify them


1) If in the bearings, both inner race and outer race rotates , Rotating speed of shaft, inner race , outer race and number of balls are given. How to find BPFI?

2) If BPFI or BPFO in the bearing is accompanied by 1 x RPM side band what does it mean? . Could it mean the defect is wide enough to include 1 x RPM side band or bearing is loose on the shaft?

3)Suppose I have to measure 70.000 Hz frequency With 40,000 HZ Accelerometer, How amplitude sensitivity and frequency sensitivity will be affected? We know as the frequency of the accelerometer increases, size of crystal become smaller and hence amplitude sensitivity become smaller , but the problem is how the frequency accuracy is affected?

Again thanking you profusely for having clarified my doubts

Original Post
Hello K.
1) THe frequencies must be calculated by substituting (Rpm of Inner ring - Rpm of Outer) for Rpm in the traditional formulas i.e.:

BPFO frequency= (Nb/2)*(1-Bd*CosA/Pd)*(Rpm of Inner ring-Rpm of Outer ring)
where Nb= Number of Balls or Rollers, Bd= Ball or roller Diameter, A= Contact angle, Pd=Pitch diameter

2)1x Sidebands means that the 1x vibration is significant and coexistent when the BPFI or BPFO frequency is present. If BPFO or BPFI vibration intensity increases and decreases in synch with the shaft speed, sidebands will be created by the FFT. The cause of the 1x vibration can be numerous. "Looseness" is only possible reason.

3) Frequency of the vibration signal will be as accurate as the instrument and its setup will allow. For example, The accelerometer can not make a 70khz signal display as a 75khz signal or any thing other than 70khz. As you have said the intensity of the signal may be compromised greatly because of the physical characteristics of the accelerometer. You can use an accelerometer above its 3dB rolloff rating with no problem. Never use an accelerometer above its rated resonant frequency- the attenuation is above 90%.

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