Loads resulting from locked rotor torque

[LobstaEata]

I'm hoping this is the correct forum to ask this question. I'm a structural engineer designing a pile supported foundation for a 10MW Westinghouse Steam Turbine Generator. I was wondering if anyone in this forum knows whether a locked rotor force couple initiated by the generator will also engage the mass of the steam turbine through the gearbox. My initial thought was that if the generator locks suddenly, that the momentum of the turbine blades would first strip the teeth of the gearbox (like a fuse would) before transferring rotational load to the turbine mass. Our mechanical engineers say that all masses (turbine, gearbox and generator) will be engaged in a locked rotor condition and to design the foundation to resist the force couple developed from each of the masses. I was hoping someone would help me understand why the gearbox does not serve as a fuse for the locked rotor condition. Any help on this subject would be appreciated.

 

[desertfox]

Hi

Having the teeth strip in the gearbox would be a rather expensive fuse!
I usually deal with electric motors and gearboxes and we always ensure the gear boxes can handle the motor starting torque or locked rotor torque.
When the motor starts from rest the motor can produce something in the order of 3 times running torque which maybe required to overcome inertia of shafts gears etc before it settles down to normal running speed and running torque.
So in a nutshell the gearbox needs to withstand the locked rotor torque during start up otherwise its kaput!

 

[mikekilroy]

Queer quesstion... leaves plenty of chance for misinterpretation I think, hence discrepancy between your thinking and the other engineers?

I read your question as "do I have to use all inertias in the whole system added together (properly) or just everything BUT not the generator rotor - if the generator rotor suddenly locks up and stops."

Am I reading it incorrectly?

If not, then you need to consider WHAT stopped the generator rotor in its tracks I think....

Is there a method where the generator rotor CAN be instantly stopped by some event such that its inertia would not contribute? If it throws coils or magnets into airgap and crashes to a stop, its rotor inertia seems certainly involved in the Torque produced. But if there is a way some big hand can come out of the sky and GRAB the rotor in its center and stop it immediately... then perhaps its inertia will not generate a torque to your base?

I really cannot think of any event that would cause the rotor to stop instantly that would allow you to NOT include its inertia in your calcs... simplifying the scenario, T=Jw/t.... any inertia rotating that is stopped from speed w rad/sec in t time (sec) will generate that T torque..... Thinking out loud, it probably does not matter WHERE in the rotating equipment string you put an instant stop, all inertias will still contribute to T=Jw/t.... Keep in mind of course J is reduced by the square of the gear ratio, so the T of the even will be different on each side of the gearbox....

http://www.KilroyWasHere<dot>com

 

[ScottyUK]

You should consider the transient forces which would result from a bad synchronisation, or an out of phase reclosure. This type of event is about as bad as it gets for a turbo-generator, and the forces involved can readily de-blade a steam turbine, fracture the shaft, or pull the machine from its foundations. You need to discuss this with the machine vendor. I'm sorry I can't help you with the calculation itself.

 

[LobstaEata]

Interesting question indeed mikekilroy and your interpretation is very nearly correct. What I should have said to be more clear is that since the generator is the more likely initiator of the rotational overturning force on the foundation, why wouldn't the gearbox have some type of fail safe (maybe a shear pin) to prevent further damage to the gear box itself and turbine. I do understand that there are other issues in play and appreciate now knowing that J gets reduced by the square of the gear ratio. That was something I didn't know.

I'm also trying to understand how the forces provided by the manufacturer for skid anchorages are being distributed. Most of the anchorage loading, oddly enough to me at least, is resisted by anchorages at the gearbox, which is the component that I least expected to see such large loads being transferred.

 

[mikekilroy]

We sell gearboxes too. We do not sell torque limiting devices such as shearpins, breakaway couplings, etc. Most applications do not require these so to include them in all gearboxes would be a big waste of money. Everything possible is done nowadays to lower cost of products.

Based on ScottyUK's comment, perhaps your most important addition to this design will be adding a properly sized shear pin somewhere....

As ScottyUK suggests, it seems like you should have close contact with manufacturers of ALL the equipment involved for more expert advice.... Nothing personal, but if you were not aware inertia changes by the square of the gear ratio, you may have 'engineered' a really bad mount? And it could well be I am off base making that suggestion as today a lot of folks use software to calculate such things and it may do it right. But just for follow up, know that the inertia either INCREASES or DECREASES by the ratio squared - depending on which side of the gearbox you look, so don't assume it goes down....

http://www.KilroyWasHere<dot>com

 

[LobstaEata]

Thanks mikekilroy. I really do appreciate your responses. Fortunately, as a structural engineer, I'm not in the business of calculating machine forces, so my ignorance of some things mechanical is not in question. I just use forces provided by machine manufacturers and apply them to supporting foundations or frames. I'm just a curious sort of fella and just was looking to understand why some less expensive component in the gearbox wouldn't be included to prevent ruining three pieces of equipment rather than only one.