Originally Posted by
Arnie Madsen
That is the type of info we need, thanks
I am a layman and speak as a layman but when a chip is produced in a MRGB it is not because a gear is too soft but because it is too hard (brittle)
Obviously soft steel gears would not be used because they would wear out quickly so they harden them to a point where they have a long life at the risk of being brittle and designers walk a fine line trying to find the best balance.
In an earlier post I gave some (poor) examples of witnessing bearing races shatter from impact and I did not make my point properly so I will try a different slant ...
If planetary gears were made of mild steel we could place them on an anvil and hammer them flat and they would never crack or break .... but if they were made of hardened steel they would never bend or distort , they would shatter into several pieces
The failed gears in these MRGB's are never deformed , they break into several pieces as the photos show.
Arnie you are right and I take your Point.
However, there are several types of heat treatment that can be done to
a steel.
When you say hardened and brittle. There are several types of hardening.
I assume you mean through-hardened.
If you take a normal Construction steel and harden it, that is about
what you would get a brittle steel.
but for gears there could be several heat-treatments depending on the type of application.
you have through hardening, case hardening, quenching, carburizing etc.
through hardening is normally not favourable on a gear,
allthough in low-cost Products in may also be used.
if you have a carburizing steel, you heat treat it to
make the core though and the outer Surface hard.
If you look at the Pictures of the broken gears from the preliminary report,
you can actually see that the gears are lightly oval from deformation Before they crack.
you can see from the reports I linked the different characteristics of the failure modes in a gear cog.
what is evident here is fatigue cracks.
from my understanding is not occuring due to overload of the gear
but after a number of repeated cycles Close to itīs yield strength
it will sooner or later start to spall or crack
anyway after looking again at the epi pics from AIBN,
with out any chip detection alarm. oil pressure?