Manual Gearbox Lubricants - Interesting Statement

[RustyJC]

The following statement was made in a lubrication newsletter I received today:

In gear contacts, low surface roughness and thin oil films produce much less wear than high surface roughness and thick oil films. Properly "running in" a new gear unit can play a critical role in reducing surface roughness.

Oil film thickness is a function, among other things, of the lubricant's viscosity. The statement that a thinner oil film (read, lower viscosity) results in much less wear on the gear contact surfaces is interesting and generally counterintuitive for most of us.



Rusty

 

[drrhoades]

More then likely the higher viscosity oil trap all of the wear particles whereas the low viscositiy oils let the wear particles disperse more and flow to a magnet or out of the wear path.

 

[klenger]

I think the key is "low surface roughness" vs "high surface roughness".

 

[nickleinonen]

the thicker oil can also channel when turning at faster speeds. . if the oil channels, there won't be much lube between the gear teeth. .

 

[amsoilman]

the thicker oil can also channel when turning at faster speeds. . if the oil channels, there won't be much lube between the gear teeth. .

If I'm reading the original post correctly, they are talking about "Manual" Gear Boxes. IF this is the case, a lighter (lower viscosity) fluid is a better choice as the gears inside a Manual gearbox generaly have "helical" gears which are almost a straight cut gear, but on an angle. There is spiral action, and very little sliding action compared to a "Hypoid" gear found in a differential.



Manual transmissions haveing "Helical" gears do not require GL-5 Performance lubes which have (EP) Extreme Pressure additives. A GL-5 lube will have twice the EP additives that a GL-4 will have.



Wayne

amsoilman

 

[ezcurra]

I think the roughness in the gear teeth surfaces allow the oil to escape into the "valleys", just like a tire evacuates rain water from underneath, while a finer surface keeps the oil film between the two surfaces.

 

[Texas Diesel]

Rusty, on the BMWMOA oilhead forum there was a thread on final drive bearing failures. Oil viscosity came up there are a possible problem. Hence the BMW change from 19 balls to 17 balls, more time for the oil to flow back into the bearing race.



Sounds like my choice over the years of a 75W-90 oil for the rear axles may have been the correct one.



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Wayne is correct on the helical cut explanination but the helical cut gears put quite a bit more side thrust on the gears then straight cut. I've seen several transmissions wear through the thrust washers and eat into the transmision cases. It would seem that thinner oil would be able to lube these throust washers better than a heavier oil would.



Personally I would rather have the 'whine' of straight cut gears over the quietness of helical cut. Remember the whine of the Jeeps in M*A*S*H? Those are straight cut gears.

 

[rbattelle]

Perhaps this is why Dodge specifies 75w-90 regardless of towing for the axles in 2003-up trucks.



Ryan

 

[Joe Mc]

If only they had double helical gears - the quietness of helicals and the reduced side to sid thrust of straight cut . . . the trans would probably have to be placed in the bed due to its' size. This would seem to leave open a good conversation on how to get power from the engine to the trans and back to the drive ends:-{}. Kind of like a Rube Goldberg for trucks, eh?:-laf