Re: [T3] What oil is recommended?

[Jim Adney <jadney@vwtype3.org>]

On 15 Jun 2007 at 11:15, chris537 wrote:

> yeah. you put 80-90W hypoid in the read diff. and 80w non-hypoid. what 
> ever hypoid means.

The main gear in most differentials is a bevel gear, which changes 
the direction thru a right angle. To make quieter gears, these are 
usually cut on a kind of spiral, so that the meshing of a single 
tooth happens gradually rather than all at once. This is the same 
principle as that used in our cam gear, which is helical.

In the case of a normal spiral bevel gear, the 2 axes actually 
intersect, so that they fall on a common plane.

If I offset one axis so that the 2 never actually intersect, then the 
gears have to be cut in a different manner, and this is what is known 
as a hypoid gear. The bevel gear in a typical rear wheel drive 
differential is a hypoid gear. Hypoid gears are actually stronger, 
plus they have an additional advantage that the driveshaft can enter 
the differential below the plane of the axles in a typical car. This 
allows the drive shaft to be lower and not create as much of a hump 
down the center of that car.

The disadvantage of hypoid gears is that there is "slippage" between 
the surfaces of the gear teeth, so these gears need special 
lubrication, thus the hypoid gear oil was born. 

IIRC, Gleason was the first company to introduce hypoid gears. This 
was probably in the late 30s or sometime in the 40s.

In a normal transmission, there would be no hypoid gears, but there 
are in the VW transaxle, just because the differential is in there, 
too. To the best of my knowledge hypoid oil is fine for all kinds of 
gears, but the additives that make it a hypoid oil attack some kinds 
of synchro rings. Thus, the VW transaxles, which DO have a hypoid 
differential need some of this additive, but not too much, so that 
the synchros aren't damaged.  

The VW MT transaxles take GL-4, which is just mildly hypoid, but the 
AT final drives, which have no synchros, prefer the GL-5 with more of 
the hypoid additives.

I think there is even more (and probably better) info on this in 
Wikipedia.

-- 
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Jim Adney, jadney@vwtype3.org
Madison, Wisconsin, USA
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