Torsional limit of generator shaft

[Alex68]

Dear all,
I'm an electrical engineer but 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

 

[Tmoose]

http://assist.daps.dla.mil/quicksearch/

 

[Alex68]

Thank you Tmoose
maybe you can help me concerning the maximum shear stress of steel without loosing elasticity in case of alternating torque.

 

[Tmoose]

I think if stress at maximum torsional deflection remains less than yield then response is considered elastic, and won't permanently deform.

For most materials fatigue issues start at significantly lower stress than yield strength.

A number as low as 4800 psi sounds like it has a big factor of safety, but MIL std 167 is pretty conservative and from the era before widespread FEA