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tooth, Till • in turn, limits the maximum contact
stresses developed. A deep case depth, which is spec-
ified to willistam:! high contact load , is therefore not
necessary if tho e load cannot be reached by the
bending properties of the gear. At the same time, a
thinner case depth, which would increase the allow- i
i
able bending stresses. does not impart the necessary i
resistance to contact that these stresses can generate. !
[11 the past a compromise was necessary in terms of I
choosing a case depth which would re ult in an :
acceptable fatigue life.
The maximum case depth in the root region
should be optimized for maximum bending fatigue,
while in the flank ,ofthe gear 'tooth, the case depth
should be as deep as possible without running the
risk of developing exten ive carbides or other nonmartensitic
phases. One way to achieve this is to use
a two step carbllrizing process, where the component
is initially carburized to the case depth desired in the
root, taken out, and a copper coating applied to the
root region, The component is then recarburized to
achieve the desired depth in the flank. This requires
carefuJ modeling of the diffusion of carbon in the
masked region, as the carbon profile could become
very flat due to inward diffu ion, A more elegant
method would 'entail. a partial oxidation treatment in
the root region to, reduce the carbon intake in that
region. If the controlled fonnation of Cr 2 0 3 or even
. orne type of 5i0'2 could be encouraged in the sur-
face of the root region, thenthe influx of carbon
could be controlled to achieve the dual case depth
suggested.
Inve ligation imn novel heat treatment technologies
hould a1 0 be considered, especially ill the li.ght
of the work. carried out by Davies and associates at the
Wesl.land Helicopter Corp. (Ref. 43-47) dealing wilh
duplex 'treatments. It shoald be pointed out that in the
manufacturing of ease hardened gears, the benefits
expected from the best engineering practice and the
highest level of accuracy do not materialize if the fleeessary
metallurgical input into gear production isnot
given the necessary attention and emphasis. 0
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March 199