How many times have we seen the hold-down bolts being torqued by hand or by using pipes. There is a common misconception that more tight is always good. Our team recently had and a Generator set, we were called in to carry out vibration analysis and on-site balancing of the flywheel.

The challenge

• The first start-up of the machine showed very high vibrations, this was after major over-haul and bearing replacement of the entire set.
• The shafts were aligned using laser alignment kit and was well within the tolerances.
• Start-up vibration reached above 70 mm/s RMS, before reaching the full speed and was shut-down immediately to prevent any damage.
• As start-up data was taken during the run-up, the increase in amplitude and change in phase could clearly been seen. There was a critical speed very close to the full RPM.
• The critical speed combined with the un-balance prevented the machine from reaching its full RPM.
• The dominant vibration was on the Motor side. (Name of client crossed out to respect the confidentiality of the client)

• A trial weight was added, based on in-complete data collected during the run-up. In the next run, machine had vibration of around 40 mm/s at critical speed. But was allowed to run till it reached the full RPM to check the vibration. At full speed, vibration was steady around 23 mm/s RMS (with weight)
• A balancing weight was calculated and reduced the critical speed vibration to around 22 mm/s and full speed RPM vibrations to 7 mm/s.
• The challenge was not over yet, when intermittent load was given to the Generator there is a speed drop for this specific machine. This would mean that the machine would constantly moving in and out of the critical speed zone.
• Bump Test result was carried out in different zones to study the natural frequencies.