SPM HD is a further development of the Shock Pulse Method (SPM) used for fast, easy and reliable diagnosis of the operating condition of rolling element bearings.
Advanced digital technique and RPM-based sampling frequency makes SPM HD particularly well suited for measurement on low speed applications. Sophisticated mathematic algorithms makes the measuring technique less sensitive to interference from other signals. Extraordinary signal quality and 24 bit A/D conversion provides razor sharp resolution and exceptional detail in spectrums and time signals. Time signals are very easily interpreted, making the type and extent of the damage simple to determine.Throughout their lifetime, bearings generate shocks in the interface between the loaded rolling element and the raceway. These shocks ‘ring’ the SPM transducer which outputs electric pulses proportional to the shock magnitude. Unlike vibration transducers, the shock pulse transducer responds at its carefully tuned resonance frequency of about 32 kHz, enabling a calibrated measurement of the shock pulse amplitudes.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).
The effect of rolling velocity on the signal is neutralized by entering rpm and shaft diameter as input data, with ‘reasonable accuracy’. This sets an initial value (HDi), the start of the ‘normalized’ condition scale.
The SPM HD method produces four different types of results
HDm is a scalar value expressed in decibels. It is the primary value to use to determine the severity of a bearing damage. It represents the highest shock pulses found during the measuring cycle. This value is also used for triggering alarms.
HDc is a scalar value expressed in decibels. This value represents the level where 200 shocks/second are present. It is useful to determine lubrication condition.
Time Signal HD
Time signal HD is extremely useful to locate where in the bearing a possible damage is located. In many cases it is also possible to determine the nature of the damage (cracked inner race with spalling all around or a single crack etc.). The Time signal HD is a result of very advanced digital algorithms where repetitive shocks are enhanced and random signals are suppressed.
SPM Spectrum HD
SPM Spectrum HD is the result of applying FFT algorithms on the Time Signal HD. The SPM HD spectrum is useful to determine where a possible bearing damage is located. It is also useful for trending purposes (applying symptom and band values).
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 and delta (HDm minus HDc) indicated lubrication quality or problems with bearing installation and alignment.