Trip Shaft Break, Interanl Crack , Surface Crack,Heat Treatment

The switcgears are not likely to work for so many cycles (Assuming it is a fairly high voltage) that there is a possibility of a mechanical fatigue.

If I take of this possibility then there are the possibility of

- Thermal fatigue

- Vibration induced fatigue.

I doubt either of these two.

next part your failure rate is 4000PPM ie out of 5000 breakers you are producing 20 are failing at your bench or at customer end.

Question- At customer end are these in infant mortality stage ? I assume so.

I will interpret these as a manufacturing fault and not the basic design fault.

As KVS has pointed out the case hardening will help but they usually help in improving the fatigue life (case prevents initiation, core prevents growth)

BTW- howmany cycles are tested at your bench and after how many do they fail?

If it is sufficiently low, it will further strengthen the case of non-fatigue. I feel it has got soething to do with either misalignment of misengagement of latch of someone else that is creating a bending moment at your keyway (ie the latch engagement slot) that is breaking it. The crack must be initiating at the corner _{(wait but that is obvious since for both fatigue as well as brittle failure mode it would be at the weakest and hence most stressed point ).}

To confirm it you may take to any failure analysis lab. The SEM _{(Scanning electron Microscope since a lot of ouer members don't like acronyms)} will exactly bring out the crack initiation, growth and failure mechanism. In absence of SEM any high resolution optical microsope also will be able to guess.

The case hardening will help but not much if it is due to the causes I guessed, in case of these it will fail sooner or later.

Have you found any abnormal loading/ rubbing/ other marks in the failed parts and the parts having contact with it?

The model of KVS was helpful in visualising a bit. And though I am Electrical, but not in SG field and now in purely mechanical field so i am guessing mechanically.

Please again note this degree of PPM usually denote manufacturing defect (of course raw material is a part of it and could be ascertained by the failure zone microscopy).

the only fault of design may be it is not a fail-safe design and has not taken care of practical exxects of misalignments or something (which you will find causing the failure). In a certain of our designs we have found that the engaging is too fast and the damping is late so that in case of error it will hit before it slows down. Some cushioning helped in those. In other cases we redesigned to negate the negative effects generated without sacrificing the requirements.