GE 7EA Knocking Noise While on Ratchet


Thread Starter


While one of our units is on ratchet, we hear a loud knocking noise immediately before the shaft rotates and a much softer noise following that. The noise only happens while on ratchet and is loudest at the generator end. We cannot hear the noise at all in the accessory compartment where the ratchet mechanism is. Does anyone have any ideas what might be causing this?

How old is the generator?

Is the generator air- or hydrogen-cooled?

Does the unit have a brushed or brushless excitation system?

My best guess from the information provided is that there is something loose on the generator and/or exciter rotor. If it's brushless then it's possible the bolts which hold the shafts (generator rotor and exciter rotor) together are loose.

If it's air-cooled, it should be relatively easy to pull the upper half of the exciter end end-shield and inspect the rotor fan on the exciter end of the unit (which is what I presume you're calling the "generator end"). There may be some loose fan blades.

Also, pulling the turbine end generator end-shield would probably be a good idea.

Please write back to let us know what you find!

The generator is 20 years old, air cooled, and the excitation system is brushed.

I was refering to the exciter end, sorry about that. Combination of being new to the job and using colloquial terms.

We pulled the exciter end-shield off and found nothing. I think we also pulled the turbine end generator end-shield as well with the same results. I will check to make sure of that though.

What makes this problem so puzzling is that the sound only occurs while the unit is on ratchet, but we cannot hear it at all in the accessory compartment. The sound is audible but faint in the turbine compartment, and loud from the load tunnel onward.

Thank you for taking the time to help me out with this!

I'm stumped. I do know there was a phenomenon known as "bucket rock" which occurred on the last stage turbine buckets of 7E/EA units and could be heard as "tinkling" when the unit was on slowroll (30-50 RPM continous rotation method of cooldown, replaced with hydraulic ratchet when larger, air-cooled generators were used in place of hydrogen-cooled generators). If bucket rock was going on on this machine during ratcheting, I wouldn't think it would only be noticed during the start and stop of the ratchet cycle. Have you opened the manway access to the exhaust diffuser area and listened for the possible source of the noise?

Does this machine run very often? Is there any high vibration noticed on any of the bearings (turbine or generator) since this noise was first noticed? What happens when the unit is CRANKed, during the initial break-away from zero speed, and then when CRANKing is stopped (i.e., when the STOP command is initiated to shut down the starting motor)?

Is is warmer or colder at the location these days that normal winter periods?

Machines like Frame 5s and Frame 6Bs with load (reduction) gears between the turbine and generator often have "clunking" noises noted in the Load Compartment (where the load gear is located) during ratcheting, because of the different inertias of the turbine and generator rotors and normal gear lash that result in some noises when the unit starts and stops a ratchet cycle--but Frame 7E/EA units don't have a load gear, so this can't be an option for the source of the noise.

This doesn't really appear to be a controls-related problem (based on the information provided). Have you posted for help in the Frame 7EA Users' Group forum section? (I don't normally recommend other forums, but in this case, as the problem doesn't really seem to be controls-related it might be a better choice.)

Please write back with your findings!