November 1, 2017
Shock Factor and SFI
By Tim Sorensen
For years, vibration analysts have depended upon the ISO Vibration Severity Chart for the determination of machinery health. Typically, this chart provides a level of certainty that would allow the analyst to look at the vibration levels, and if acceptable, move onto the next machine for analysis.
Some defects are determined in the waveform by the presence of impacts (shocks) and determining the reason for these impacts can consume a great deal of the analyst’s time.
Unless, due to the nature of the machine, shocks are permissible, they then indicate an issue with the condition of the machine.
Take this machine for example:
A double reduction gearbox newly installed with approximately 6 months of operational time. The machine has exhibited low levels of vibration. This gearbox is driven by a 4-pole motor operating at approximately 1785 rpm. Output speed is around 70 cpm with an unknown intermediate shaft speed.
The highest level of vibration encountered on any of the gearbox bearing locations was .048 ips/rms. Using the ISO charts shown above, this level of vibration is acceptable. However, the Shock indicator told another story.
The Shock Indicator used by OneProd for both the Hawk and Falcon in the Automated Diagnostics, is an averaged value based on shocks found in the waveform. The values presented range from 0-5 with 0 meaning no shocks exist, while a value of 5 indicates many shocks exist.
On this gearbox the Shock Index was 2 or 3 on several of the measurement locations. From my perspective, these values, although not significant on some other machines exhibiting higher levels of vibration, seemed heightened for this machine with its very low levels of overall velocity vibration.
Looking at the measurement points displaying elevated Shock Index, this is what was found:
- Multiple harmonics of a speed that could be the intermediate shaft
2. Shocks in the Waveform
A subsequent visual inspection of this machine by a gearbox representative revealed severe wear on the intermediate shaft gear. This defect, found with the aid of the shock indicator and verified using SFI, would have been missed using Overall Velocity Vibration as the only determining factor for a machinery health.