Ivan Vegvary wrote:
I am posting this so I can get educated. Just finished taking apart an
automatic transaxle (1990 Nissan Maxima) for entertainment and "artsy"
parts. From the bearings and races I see a welding positioner in my future!

Anyway, the planetary gears are amazing as is the differential gear setup.
All of this runs in a transmission fluid bath, which makes me wonder, why
we use heavy gear oils for similar applications when not unitized with an
automatic tranny? Granted this unit was not as heavy duty as would be
required for a rear end of a truck, but, I am sure there are trucks with
front wheel drive automatic transaxles. (Maybe not?).

Educate me why heavy oils are used in gear boxes.
If you compare the size of the gear faces in the auto trans to
an old-style manual trans, for instance, you will likely find
the auto trans has a much larger load-bearing face, so as to
reduce the cross-sectional loading. This can also be
accomplished by making the diameter of the gears larger.

Different gear tooth profiles can also reduce the wiping action
of the gear teeth, reducing or increasing the wear. The optimum
amount of sliding action on the tooth face keeps the
hydrodynamic film working to keep the metal apart.

Auto transmissions use a light oil because the clutches and
bands need to slip freely when not engaged. Also, the torque
converter works efficiently with a lighter working fluid.

Jon