Page 69 - Gear Technology Solutions

1,5mm depth is placed by a diffusion process. After the carburizing a quenching
in oil and an additional tempering takes place. The result is a surface hardness
that is commonly close to 60HRc and a core hardness in the 30HRC range.
Case hardening provides an ideal transition between surface and core hard-
ness that makes gears on the surface hard and wear resistant and in the core
ductile. This makes shock loads and certain small plastic deformations tolera-
ble without failure of the gearset. One major side effect of the heat treatment
process is the distortion of the gears that is caused by carburizing, the re-
crystallization of the steel and the quenching. In order to make a gearset after
heat treatment suitable for power transmissions, for example in cars and
trucks, a hard-finishing operation is required. The hard-finishing operating elim-
inates the heat treatment distortions hereby providing the flank surfaces with
the correct geometry from before the heat treatment back. In addition, hard-
finishing improves the surface finish to a low roughness and waviness which
enhances the hydrodynamic lubrication and reduces noise. Grinding and skiv-
ing are the preferred hard-finishing methods, creating a defined surface form
that duplicates the original designed surfaces in the single micron range.

In case of face hobbed bevel and hypoid gearsets, grinding is not possible,
because of the epicyclical flank lead function. Skiving can generate epicyclical
lead functions but is not yet accepted for the high production volumes in the
automotive and truck industry. This leaves only the lapping process for the
hard-finishing of face hobbed angular gearsets. However, the face hobbed sur-
face texture and the relative sliding between the flanks of hypoid gears make
lapping an ideal alternative. Lapping can remove the surface scale left from
heat-treatment and it re-matches two mating members by removing some
runout and flank form distortions. Lapping can reduce the transmission error in
many cases due to the fact that the major material removal is in the center re-
gion of the teeth where the tooth contact under light load is expected. In order
for the lapping to work well, more crowning than required in the hard-finished
gearset is used in the gearset design for the soft cutting. Lapping removes
about 30% of this crowning, such that the length and profile crowning is just
right after the lapping. Soft cutting of parts that are lapped after heat treatment
considers a stock allowance of .03mm in the pinion and 0.01mm in the gear. If
grinding is the hard finishing process (for face milled gearsets) then the design
crowning is identical to the desired crowning after hard finishing. Between soft
cutting and grinding, a uniform stock allowance of 0.10mm to 0.15mm is ap-
plied to the pinion and gear flanks.

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