Why do transaxles not need diff fluid?

[addyguy]

If you have a RWD, AWD, or 4WD vehicle, you need to replace the fluid in various differentials.

But, a front-wheen drive, automatic vehicle just needs the ATF changed/checked/topped off...why is there no separate fill for the actual gears that drive the wheels?

I understand they are lubed by ATF, BUT, ATF is way thinner and less additzed that gear fluid...haow to these gearsets not fail?

 

[SLCraig]

Probably differences in the metals/tolerances/application, etc.

On the Saturns, they use ATF even in their manual transmissions.

 

[Texan4Life]

Doesn't ATF have some EP additives?

kinda like on tractors of yesteryear. Used to be transmissions all got gear oil, and if you had a hydraulic system it had a separate reservoir. but now it is all a shared sump. so then you have transmission/hydraulic oil. basically a hydraulic oil with a beefed up add pack. Most (if not all) trans/hydraulic fluids also carry a GL-4 spec.

 

[addyguy]

Some, but I don't think as much as gear oil?

I KNOW gear oil is way thicker than ATF....so I just don't see how ATF does anything....

 

[mechtech2]

Most FWD auto transaxles use HELICAL, not HYPOID gearsets for the final drive.
ATF is adequate for these gears.
It is not good enough for HYPOID gears [like in an older car's rear differential].

FWD manual transmissions often spec ATF as well. But it is not our best choice for a bunch of reasons. It is only OK... adequate .

 

[Eddie]

+1 mechtech2. It is all about the design of the gearing.

 

[addyguy]

Thanks very much for that info, mechtech2!

 

[Texan4Life]

interesting... I found this to be an informative read: http://en.wikipedia.org/wiki/Gear_oil

 

[NHGUY]

The early A404/A413 Chrysler automatic transaxles 1978-1982 had a separate fill plug for the final drive/transfer gears.The large headed plug was in the wheelwell.There was no common area internally between the pan and the side cover.Starting in 1983 this extra maintenance was eliminated when they went with a common sump/feed arrangement.You also had to remove the diff cover to release the CV halfshafts with needlenose pliers.Once again starting in 1983 they just pulled out.

 

[addyguy]

Originally Posted By: Texan4Life
interesting... I found this to be an informative read: http://en.wikipedia.org/wiki/Gear_oil


Thanks, that was a very interesting read. Another bit of info I learned was about a transmission/transaxle being pressurized, while a gear set is not.

 

[tommygunn]

Wondered the same thing until I rebuilt my transmission. Its all run on ATF, differential is built into the same package so it makes more sense to integrate it all into one.

 

[addyguy]

Oh, I totally understyand the packaging thing - I just didn't understand HOW thin, low additive ATF protects gears that in a diff use thick, heavily additized gear oil.

 

[MolaKule]

Consider your ATF alone. It has one or more sun gear arrangements and the ATF AW package protects it just fine.

There are different differential designs for different drive arrangements.

The differential in the Transaxle is NOT the same type of design as in a rear drive hypoid differential where the loads on the drive pinion and ring gear are very high.

In a transaxle transmission, the differential is composed of spider gears and as mechtec2 said, "Helical cut gears" that do not require a high viscosity, EP loaded fluid.

 

[MolaKule]

From Wiki, take a look at this image:

http://en.wikipedia.org/wiki/File:Csonka_transaxle_1908.JPG

The right side is the differential and at about 9 oclock on the right cutout view you will see a drive gear driving the ring gear at 90 degrees.

Also see

http://www.dougfreeze.com/altima/manual/TM.pdf

page 32 for the differential assembly.

 

[artificialist]

Toyota had a separate differential and automatic transmission fluid sump during the 80s and part of the 1990s, but both used Dexron III ATF.

Also, there are automatic transaxles that have a separate sump for heavy sulfurous gear oil. However, they only appear in cars that have FWD, but have a longitudinal engine. Let me list a few:
Every Dodge intrepid/Chrysler LHS/300M
1979-1993 SAAB 900
B5 and B5.5 VW Passat
Any FWD Subaru
Early Acura models
60s, 70s, early 80s Olds Toronado

 

[dwcopple]

My Chrysler LHS 3.5L had a separate diff that took 80w90. It held just a hair under 1 qt.

 

[JZiggy]

If I recall correctly, hypoid gears involve tooth-to-tooth sliding contact while helical gears are involute (teeth merely "roll" on each other).