Strange Requirements

300 BHN and 350 BHN are 32 RC and 38 RC respectively which is the limit for gear machining (hobbing).

This also the hardness that is not yet sensitive to hydrogen embrittlement according to the MIL-STD's.

What is the designed use for these gears?

Don't know what the intended use is yet - but does it matter for an enclosed gear box?
As for the hobbing issue, that may once have been true but with current machines we are hobbing gears up to 415BHN with no problem.

About the hobbing, I know that for fine pitch gears the 38 RC used to be a limit. If this is not the case any more then this is an important issue for me.

I believe that if the gear box is subjected to inpact loads or is very critical it is better to have a high ductility material and suffer some tooth distorsion than having a brittle fracture of tooth.

I would assume that the 300 BHN and 350 BHN would be used for concernes with tensile and yield strength of the gear teeth.

Many enclosed drives are designed for a specific application. Getting the teeth any harder may make them too brittle for this particualr application and cause premature failure in the drive.

Another reason that many designers specify a MAX BHN is to cause a failure in a particular area of the equipment. This acts as a fuse to protect more costley repair items.

The expression of max hardness smacks of failure analysis experience. Could be that your contact had bad experience with failed parts with high hardness.

I worked on an interesting project where we hushed down gear noise by a helical combination of soft pinions and gears with high internal damping. We also used extended addendum on pinions. However, the upper stages were heat treated. You couldn't hear any gear noise after that.