I found the following info on the net. In fact it was
the only discussion on this particular topic that I
could find. I always knew about the difference between
motor oil and "gear" oil and the advantages of hypoid
gears, but the difference (or in this case-the lack of
difference ) between EP, Hypoid and plain old gear oil
eluded me. I don't know if it is proper net ettiquite
to copy someone elses post, but he is credited at the
end, and his answer was intended for public
disemination.
James J.
--------------------
>Can someone explain the difference between Hypoid and
Non-Hypoid
>oil?
"Hypoid" is not really a question of oil, so much as a
question of
gearcutting. Old (1920's) rear axles used straight
bevel gears to form
the crownwheel and pinion. These had two disadvantage,
the pinion
shaft meets the crownwheel on its central axis, and the
straight cut
gears are noisy. By using a more complex "hypoid" gear
tooth shape (if
you look at a pinion, the teeth appear twisted) these
problems can be
addressed. The more gradual engagement of the teeth
along their length
reduces noise. By careful design of the geometry the
pinion can be
made to mesh _below_ the axis of the crownwheel. As the
centre height
of the crownwheel is fixed by the wheel height, this
allows the
propshaft to be lowered relative to the car body,
giving a clearer
floorpan and lower centre of gravity for better
cornering. Hypoid
bevels are now universal in this application.
Because of the sliding contact that hypoid gears make,
their
hydrodynamic contact pressure is higher. To be suitable
for use with
hypoid gears, a lubricant must be capable of resisting
high pressures.
Oils with "EP" ratings (Extreme Pressure) such as EP90
are required.
Some brands describe themselves as "hypoid" instead, a
term which is
synonymous with EP. GL-5 is a formal API standard for
this type of oil
(comparable to MIL-L-2105B/C/D)
> The book is telling me to use Non-Hypoid gear oil 80W
or
>80W/90 on the manual transmission and GL-5 hypoid gear
oil 90W on
>the rear axle.
A manual transmission won't usually contain hypoid
gears, so it
doesn't need an EP oil. Rare exceptions are those
transaxles where the
crownwheel and gearbox share the same lubricant.
Although an EP oil is
more complex to manufacture, it has no disadvantages
when used in
instances where the EP attribute isn't strictly
required. Manual
steering boxes and other slow-moving oil-containing
components are
often filled with 90 weight oil. It's usual to buy EP90
because that's
what the axle requires, then use the same oil for all
other
components.
There's little practical difference between 80 & 90
weights. I fill
everything with EP80 and I've never had a problem.
There's an increasing trend amongst manufacturers to
reduce the number
of different lubricant types required. My own gearbox
(5 speed Range
Rover) runs on ATF, but 20W/50 engine oil or EP90 axle
oil are equally
permissible.
--
Andy Dingley
dingbat@codesmth.demon.co.uk
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