Torsional stress of synchr generator

[Alex68]

Dear all,
I have already posted this question to our mechanical collegues but no answer arrived.
I'm trying to understand which the torsional limit of a synchronous generator is.

I have got the "lateral and torsional vibrations study" of the rotating shaft of a gas turbine + generator system (250MW). In the discussion of results I can read that:

"The resonant alternating shear stresses are well within the limits of MIL-STD-167 which allows an alternating shear stress of 4800 psi for critical speeds within 10% of steady state speed.
The maximum torsional stress for the studied perturbations was 64ksi."

Some questions:
1- Is the MIL-STD-167 applicable for systems of 250MW (I haven't a copy)?
2- Is the limit of 4800 psi referred only to the resonant torsional mode?
3- What is the limit of the total torsional stress, compared to the calculated 64ksi?
4- Why were used different units for the stress: psi and ksi?

Any comment, suggestion or clarification will be gratefully accepted.

Regards
Alex68

 

[electricpete]

One small piece you may be already aware ksi is sometimes used as abbreviation for kpsi = 1000 psi

Critical speed normally refers to radial vibration. I'm not sure how that ties to the discussion of torsional.

64 kpsi sounds pretty darned high. Torsional/shear limits are usually a lot lower than tensile limits as I recall.

I'm not familiar with MIL-STD-167 but it is available for free. Google shows lots of results including

http://assist.daps.dla.mil/quicksearch/basic_profile.cfm?ident_number=35544

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