SPM instruction for Atlas Copco GA units

The Shock Pulse Method (SPM) was developed by SPM Instrument AB in Sweden. Its exclusive purpose was – and still is – to monitor the operating condition of rolling bearings, to get advanced warning of developing bearing damage and enable planned bearing replacements with a minimum of downtime and risk for machine failure. Over the years, the method was refined to evaluate the lubrication condition of undamaged bearings, so that the user can not only detect existing damage on rollers and raceways but prevent the most common cause of early bearing failures by optimizing bearing lubrication in any given application.

Throughout their lifetime, bearings generate shocks in the interface between the loaded rolling element and the inner and outer raceway. These shocks ‘ring’ the SPM transducer which outputs electric pulses proportional to the shock magnitude.

A shock pulse pattern is a sequence of either random or rhythmical strong pulses (dBm level) above a carpet of very rapid weaker pulses (dBc level). You have to be aware of:

• the dBm value
• the difference between dBm and dBc

Shock pulse amplitude is due to three basic factors:
• Rolling velocity (bearing size and rpm)
• Oil film thickness (separation between the metal surfaces in the rolling interface). The oil film depends on lubricant supply and also on alignment and pre-load.
• The mechanical state of the bearing surfaces (roughness, stress, damage, loose metal particle).

Input data (initial value, dBi):

The effect of rolling velocity on the signal is neutralized by giving rpm and shaft diameter as input data, with ‘reasonable accuracy’. This sets an initial value (dBi), the start of the ´normalized’ condition scale.

Before every single measurement the correct dBi-value has to be introduced in the measurement device.

The carpet (dBc) and maximum (dBm) shock pulse levels are measured during FULL Load operation (max. effective working pressure) and for VSD’s, these values are given by maximum speed. For VSD’s it is always preferable to measure at maximum speed, but if not possible in the application to run at maximum speed, a speed should be selected that is reproducible in time in order to enable follow-up of SPM levels at selected speed. The conditions during the measurement should be
stable.
To eliminate possible errors due to mis-positioning of the transducer, adapters are installed permanently at the measurement points. When doing SPM measurements, the measurement device (transducer) should be positioned on the adapter by means of a quick-connector.

Evaluation (according to SPM guidelines):

The initial value and the range of the three condition zones (green – yellow – red) was empirically established by testing bearings under variable operating conditions. The maximum value places the bearing into the condition zone. The height of the carpet value dBc and delta (dBm minus dBc) allows refined bearing condition analysis and indicate lubrication quality or problems with bearing installation and alignment.

Example of a shock pulse measurement in the figure below.

Shock pulse measurements, dBm/dBc

It should be noted that the real measured value is actually a dBsv value, which is normalised afterwards by subtracting the corresponding dBi-value afterwards: dBn = dBsv – dBi.

Calculation of the dBi-value

dBi = 20 ⋅ log(n) +12.5 ⋅ log(d) – 67.5[dBi]= dB: a norm value for bearings of the same kind, this is the value for a “perfect” bearing[n]= rpm: shaft speed[d]= mm: diameter of the bearingFor dBi values for the different machines contact us.

Evaluation & analysis of SPM measurements

For new units: never decide on the operating condition based on one single measurement. Instead make at least 3 measurements for each SPM point every 1000hrs or 2 months to determine an average “base line dBm and dBc” level. In case the first measurement is more than 15dB above the “normal dBm” values, confirm the high level after a shorter interval (500hrs/1 month).When the baseline dBm and dBc values are determined: as long as the measured dBm levels do not exceed the “normal dBm” (base line dBm / dBc) values by more than 15dB (“ATTENTION”), the operating condition is to be considered as acceptable and measurement intervals may be kept at the maximum possible: 2000hrs or 3 months, whatever comes first.dBm levels exceeding the “normal dBm” values by more than 15dB might indicate light but visible bearing wear. Shorten measurement intervals to 500hrs/1 month.dBm levels exceeding the “normal dBm” values by more than 25dB indicate developed bearing wear. Prepare overhaul.It is recommended to exchange the element as soon as measured dBm levels reach 30dB above “normal dBm” value. This is the “ALARM” level.When the dBc value increases, while the dBm value is almost constant, the lubrication of the bearings is getting worse, either quality of lubricant or amount of lubricant. Notes: 1. In case the list with dBi values is not on hand, measurements can still be done (and are useful) by setting dBi = 0 dB. In this case the absolute shock pulse value is measured and the normalised value can be derived from the measurement by subtracting the correct dBi value from the measured absolute shock pulse level.e.g.: dBm = 50 dBsv (measured with dBi = 0 dB) => dBm = 30 dBn (normalised) when dBi =20 dB2. Also in case wrong dBi value is set when doing the measurement, the measurement is not lost. The correct normalised value can be derived from the measurement by adding the wrongly set dBi value and then subtracting the correct dBi value.e.g.: dBm = 30 dBsv (measured with dBi = 10 dB) => dBm = 20 dBn (correct normalised level) when correct dBi =20 dB3. It is advised to do the first SPM measurement at start-up of the machine.

Calculation of data not mentioned on the datasheet

For VSD’s, data for other element speeds can be interpolated to obtain an “indicative” value (*). Also for other pressures, interpolation can be used to get an “indicative” value (*).Rpm1,2: Given motor speed on the datasheetRpm: at which motor speed the dB-value must be calculated, this value should be lying betweenRpm1 and Rpm2dBx1,2: the dB-value at respective motor speed (dBm or dBc)dBx: the wanted dB-value at a certain motor speed, Rpm Pressure1,2: Given pressure on the datasheetPressure: at which pressure the dB-value must be calculated, this value should be lying betweenPressure1 and Pressure2(*) Note: Relationship between speed or pressure and dB values is not linear and therefore using above equation will only give approximate values. Therefore it is advised to measure SPM at full load operation as stated before and for VSD’s at full speed. Especially for VSD’s, if customer installation does not allow to operate at full speed, it is advised to consult us.

Nomenclature

MOBH Male outlet bearing housingFOBH Female outlet bearing housingMIBH Male inlet bearing housingFIBH Female inlet bearing housingOBH Outlet bearing housingMDS Motor drive sideMNDS Motor non-drive side