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Torque those bolts! - The importance of torquing base bolts - Case study

How many times have we seen the hold-down bolts being torqued by hand or by using pipes. There is a common mis-conception 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 vbrations, 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 inermittent load was given to the Genarator 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.

  • Bump test from Motor showed dominant frequency at 28.5 Hz, and the machine was operating at 31.5 Hz. This would mean that the separation is only 10% and when machine speed drops even a bit during the load condition, the separation would even be lower.

  • The bump test carried out on the machine frame also showed the same frequecies (only near motor area).

  • The change is natural frequency occured only after the maintenance, so possible causes for this was investigates.

  • As normally seen with change in natural frequencies, no cracks were observed anywhere. There was no structural or machine modifications as well.

  • This led the vibration analyst to check for issues with installation.

The hold-down bolts?

+ The alignment was done within the limits, and soft-foot was checked to be within the tolerance. All hold-down bolts of the Motor was torqued with correct torque values.

+ The vibration dampers/ mounting was changed during the maintenance.

+ Since the tightening of this was not verified, all vibration damper hold-down bolts were loooesned and re-torqued.

+ Bump test was carried out again, which showed a reduction in the natural frequency.

+ Natural frequency of the motor and frame shifted to 26 Hz, which increased the separation to 20%. This also ensured that machine, even when speed drops does not go into the critical speed zone.

+ Data was taken again, the machine now operates with maximum vibration of 4.5 mm/s RMS. Which is well below the OEM tolerance of 7 mm/s.

+ A challenging balancing job, vibration troubleshooting was thus concluded successfully.

*Osborne Engineering provides dynamic balancing all accross the Middle East and Europe. Inlcuding UAE, Poland, Oman, Bahrain, Kuwait, Saudi Arabia, Qatar.


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