The SPM readings are used to ascertain bearing and lubrication condition. Here, the term SPM reading stands for ‘Shock Pulse Meter Reading’. These SPM readings are measured with the help of Shock pulse meters which works on a 32 kw resonant frequency. To know about shock pulse meter in detail, please visit https://electricalfundablog.com/blog/shock-pulse-tester/2521/. As the name indicates, the shock pulses generated from the bearing mount is analyzed in this technique.
One should keep it in mind that the shock pulses are available in every machine. These shock pulses are normal to a certain extent. However, the shock pulses above normal values of a machine are certainly an indication of some internal damage. The pattern in which these shock pulses appears can be used to identify the internal bearing condition and the state of lubrication. A bearing generates different shock pulses due to the rotation of internal bearing balls on the inner and outer cage. In addition, the cage itself rotates and produces a shock pulse. When they pass through a damaged area in the raceway, a shock pulse is generated. The value of the shock pulse depends on the following things:
- Physical condition of bearing surface
- Speed of rotation
- Thickness of lubrication
- Improper shaft alignment
The shock pulses generated by the bearing is picked by the transducer of SPM meter and thereafter converted into electrical values. These electrical values are further analyzed to give SPM readings. Depending upon the SPM readings, the SPM meter shows the bearing condition in Green (dBm upto 20), Yellow (dBm from 21 to 34) or Red (dBm 35 or more) zone. There are three types of SPM readings which we get from a SPM meter. They are :
- dbi (initial value)
- dbc (carpet value), and
- dbm(maximum/peak value).
Dbi (Initial Value)
When we feed the rpm and bore size of bearing in SPM meter, we get dbi. This is the initial standard shock pulse value of the normal bearing. It acts as the starting point for the zero reading on 0-60 db scale for green/yellow/red colour zones.
Dbc (Carpet value)
The dBc is carpet value of the SPM readings. By Carpet value, we are referring to the average of all the shock pulses in lower range in the complete time scale. Below mentioned figure will give you a better understanding of Carpet value.
The dBc value gives us information of lubrication state of bearing. The dBc of a bearing increases with decrease in lubrication layer. It is also known as Background noise of bearing.
The dBm indicates the maximum value to which shock pulses are peaking. The dBm gives the idea about the condition of bearing.
Delta (∆) (along with dBc) gives the idea about the lubrication state of bearing and bearing alignment. We can obtain Delta (∆) value by subtracting dBc from dBm.
Delta (∆) = dBm – dBc
Meaning of Various Delta (∆) observations :
- If Delta (∆) and dBc are small, it indicates that the condition of bearing and lubrication is good.
- If Delta (∆) is small and dBc is large, it indicates that the bearing’s lubrication has some issues. In this case, fresh lubrication is advised.