Dear colleagues,

my question is a little bit more theoretic and maybe basic. Does anybody know what is the reference value for the definition of decibel (dB) values derived with the SPM method?

We all know that dB values are defined relative to a given reference. For example, in acoustics, the sound pressure level in dB of a pressure p is defined as Lp=10log(p**2/p0**2), where p0=20 microPascal is the reference pressure for this dB definition.

What is then the reference value (velocity, acceleration?) in the definition of dBsv, dBm, dBc, or dBi used in the SPM method?

Thank you very much for your time.

Dear friends
dBm, dBc and dBi values in SPM method refer to Shock value generated by a “shock wave” in the bearing housing.

The wave is transient (quickly dampens out). When the wave front hits the shock pulse transducer, it will cause a dampened oscillation of the transducer's reference mass. The peak amplitude is a function of the impact velocity

For further information please see www.spminstrument.com
Best regards

Rolf Sjögren
Market support

SPM Instrument AB

Dear friend :
Wich you say is right but let me tell you that when you seek a failure in a bearing, this is between 20 Hz and 20 KHz and when you can detect, the bearing is near catastrofic failure. There is a technic, Shock Pulse Method (SPM), invented in Sweden. Please consider the following :
1.- The SPM instrument measure the mechanic shock speed, measuring the compression wave produced when ball and race has shock. When both contact there is a molecular aceleration wich produce a compression wave and travel at sound speed across material. The wave front shock contact with the piezoelectric acelerometer (SPM instrument)wich has a 32 KHz resonance frequency.
2.- A free defect bearing has an initial acoustic emission value (dbi). A bearing in service has an absolute value of shock pulse (dbsv). The measurement ofpiezoelectric acelerometer is the normalized value (dbn), then : dbn = dbsv - dbi , it mean that a piezoelectric acelerometer wich register a value of 20 dbn is telling us the bearing has a value 20 times greater than a free defect bearing.
3.- Next scale is usefulness :
- dbn lower than 20 mean very good bearing
- dbn between 20 and 33 mean deficient bearing
- dbn greater than 33 mean you have to replace bearing
4.- I suggest you a SPM instrument for bearing evaluation
I hope it help you

But there was still no answer of the question of the dB reference value was it? Olov

The db scale is a logarithmic scale relative to some reference. In sound measurement the db scale is referenced to a specific sound pressure level. In machinery analysis the db scale is used to express the ratio between two voltages, an output and an input, so
db=20 log voltage ratio.
SPM register mechanical shock speed wich produce impact, impact produce molecular aceleration, aceleration only dependof shock speed (no depend of mass). Molecular aceleration produces a wave compression wich travel across iron (or cast iron) at sound speed.
Sound speed is not always the same. It depend on middle across travel. For example sound speed across a gaseous fluid (air) is
M= root square(KRT). I dont know what is the sound speed across iron or cast iron (if you find it please e-mail me).
Now, speculating :

1.- Shock pulse emited for a ball bearing is an output sign produced for mechanical shock speed (Power emited wich is energy in the time)
P = F . V = m . ab . V
with Power , m:mass , ab:ballbearing aceleration , V:shock speed

2.- Molecular aceleration wich produce wave compression is an input sign (Power received wich is energy in the time)
P = m . am . M
with Power , m:mass ,
am:molecular aceleration=ballbearing aceleration
M:sound speed across iron or cast iron

3.- Dividing Power output/Power input we get the ratio: Ratio=V/M

4.-Then db = 20 log Ratio = 20 log (V/M)
with V: mechanical shock speed
M: sound speed acoss iron or cast iron

5.- In this way way you can find mechanical shock speed, while higher is V, then higher is dbsv, higher is dbn and nearest is the damage condition of the bearin. Another way to make bearing diagnostic is using cepstrum analysis wich is the power spectrum of the logarithmic power spectrum.
6.- Please explain me what is the usefullness to calc dbi, dbsv and dbn

So what you say is that the reference is the speed of sound in steel? OK fine. Olov

Yes,I believe it.
Do you know this value?

Hi all,

I'll try to explain some of the theory behind the SPM method. The most important value generated by the SPM method is the dBm value. The SPM method measures the voltage generated by the SPM transducer and the maximum voltage, Vmax, is recorded. This Vmax is converted to a dBsv value according to, dBsv=20xlog(Vmax/Vref) where Vref = 100 uV. But the dBsv value depends strongly on the rotational speed and the size of the bearing and can't be used alone to evaluate the condition. That is why a new parameter is introduced, dBi (i=initial), which is a function of the size of the shaft and the rotational speed.
The parameter used to evaluate the condition of a bearing is dBm and it is simply the difference, dBm=dBsv-dBi.
Based on a very large number of measurements the following color evaluation guide has been defined as,
dBm <21 => Green
20 < dBm < 36 => Yellow
dBm > 35 => Red

The SPM method also generates a dBc value which is derived in a more complicated way. In addition to this there is also the LR/HR value which is similar to the dBm/dBc value.
So if returning to the initial question, what is the reference value? As seen above, the reference value is just the voltage 100 uV. Translating this value into acceleration or velocity requires a detailed knowledge of the transducers sensitivity as a function of frequency but that is beyond this reply.

Regards,
Pontus Eriksson

Pontus a very informative reply, However if you have to introduce a new parameter dBi (i=initial), if I were to change the bearings and bearing blocks would the formula have to be recalculated to achieve?
dBm <21 => Green
20 < dBm < 36 => Yellow
dBm > 35 => Red
Or would the spm levels remain the same?

Hi,
The dBi value is a function of the shaft diameter and the speed of rotation. So, it is only a change of these parameters that requires a recalculation of the dBi value.
Regards,
Pontus

Hi Pontus
Thanks for the information;
However in my case alarms had broken “yellow” and “red” so we changed the fan bearings and blocks, after replacing them should the alarm indicators return to green?
Or would I have to reset the control parameters?
Mike