Gear Tribology and Lubrication ˇV Part II
Rheology and Chemistry of Gear Oils
By MolaKule

Gear oils lubricate the gearing teeth and the support bearings of transmissions and differentials as they transmit power, and react to forces such as shock (impulse loading), braking, and acceleration.

Basic Gear Lubricant Requirements
The primary functions of a gear lubricant are to reduce friction and provide cooling. Most of the friction reduction, and certainly the cooling, is provided by the base oils formulated into the gear lube. Secondary functions, such as protection and enhancement of the gear lube, are provided by the additives. Elemental chemicals and synthesized organics provide rust protection, oxidation inhibition, metal deactivation, and increase the Viscosity Index for wide temperature swings. Additional additives for foam suppression, water demulsifiability, and seal compatibility are also important. The most pronounced additives are the anti-wear protection (AW) additives and the anti-galling/anti-scuffing protection provided by the extreme-pressure (EP) additives. Some fluids use Viscosity Index Improvers (VIIˇ¦s) to broaden the temperature range of operation if the base fluid does not have its own inherent VI. In limited-slip differentials, with clutches based on inertial mechanisms, additional Friction Modification is provided by natural or synthetic esters generally called ˇ§fats.ˇ¨

History
Historically, the only requirement for gear oils was based on the proper oil viscosity for the prevailing temperature, which usually meant changing oil three or four times a year. As gear types grew and the lubrication requirements expanded, standardization and testing was needed to enhance gear oils. Military Specification MIL-L-2105 of 1951 started the ball rolling by laying down specifications for gears oils for military vendors. The military specifications included testing for rust protection, thermal and oxidative stability, as well as the standardization of the physical and chemical properties. The ˇ§mil-specˇ¨ did not address the use of synthetic base oils, since the specification was originally developed out of the use of mineral oils of the 1950ˇ¦s. In 1960, the API put forth a series of gear classifications and specifications which all gears oils had to meet. In the mean time, the American Society for the Testing of Materials (ASTM) developed a series of gear lubricant tests and published and catalogued their first procedures in 1987! The API is supposedly working on new gear lubricant categories PG-1 (GL-7 specification are slated for Transmissions only)) and PG-2 (GL-8 is slated for Differentials and Axles). To-date, these categories have yet to surface. Any new service category must take into account new base oil and additive developments, as well as new differential designs such as the the Torsen„· limited-slip differential. http://auto.howstuffworks.com/differential8.htm
as well as other designs: http://auto.howstuffworks.com/differential6.htm

Some manual transaxle manufacturers specify ATF due to the fact that pin and needle bearings are used with the gearing shafts and that with an average SAE 0W20 specification, this viscosity range is optimum for those types of bearings. In some cases, a light GL-4 rated gear lube such as Redlineˇ¦s MTL (or equivalent) will work as well or better.

Base Oil Chemistry

In mineral oil gear lubes, the Group I and II fluids dominate, since no high viscosity Group III has yet been produced. These fluids use bright stock and/or polybutlyene thickeners to thicken up oils for the higher viscosity oils such as the 80W140ˇ¦s.

In partial synthetic gear oils or ˇ§blends,ˇ¨ as we shall see, mineral oils, PAOˇ¦s and even esters are used, where the predominate oil is a mineral oil.

In synthetic gear oils, synthetic hydrocarbons such as polybutenes and PAOˇ¦s, along with esters dominate. Synthetic Gear Lubes shear little as compared to a mineral oil formulation with VIIˇ¦s. Full synthetics loose little viscosity as the oil is squeezed, sheared and churned in the differential case. In most cases, synthetic gear fluids use no VIIˇ¦s at all.

Gear Lube Formulations

Here we will look at various gear lube formulations for mineral, ˇ§Blends, and Synthetic oil lubes for a GL-5 rated gear lube.

A. A mineral 75W90 Gear Lube formulation will contain:

7% EP additive package
55% 100 neutral mineral oil
19% Viscosity Index Improver
1% Pour-Point Depressant

It will have a viscosity of 14.2 cSt and VI of 102.

B. A 75W90 Blend
9.5% EP additive package
20% 600 Neutral mineral oil
25% PAO of 4 cSt viscosity
20% PAO of 100 cSt viscosity
2% Pour-Point Depressant

It will have a viscosity of 14.45 cSt and VI of 157.

C. A 75W90 Synthetic

9.5% EP additive package
24% PAO of 8 cSt viscosity
52% PAO of 40 cSt viscosity
No Pour-Point Depressant

It will have a viscosity of 15.48 cSt and VI of 140.

D. A mineral 80W90

5.5% EP additive package
43% of 200 Neutral mineral oil
51% Brightstock
0.5% Pour-Point Depressant

It will have a viscosity of 14.2 cSt and VI of 101.


E. 75W-140 Synthetic

9.5% EP additive package
23% PAO of 4 cSt viscosity
51% PAO of 100 cSt viscosity
15% of diisodecyl ester (a diester or may contain a polyol ester)
1.5% Pour-Point Depressant

It will have a viscosity of 26.9 cSt and VI of 148.

F. 80W-140 Blend

6.5% EP additive package
13.5% 600 Neutral mineral oil
41% PAO of 40 cSt viscosity
17% PAO of 100 cSt viscosity
20% of diisodecyl ester (a diester or may contain a polyol ester)
No Pour-Point Depressant

It will have a viscosity of 25.2 cSt and VI of 153.

G. A 80W140 Synthetic

6.5% EP additive package
26% PAO of 40 cSt viscosity
50.5% PAO of 100 cSt viscosity
15% of ditridecyl phthalate ester (a diester or may contain a polyol ester)
2% Pour-Point Depressant

It will have a viscosity of 26.3 cSt and VI of 168.

H. A Mineral 80W140

5.5% EP additive package
64.5% 200 Neutralmineral oil
29% Viscosity Index Improver
1% Pour-Point Depressant

It will have a viscosity of 25.9 cSt and VI of 141.
(Data derived from ˇ§Synthetic Luibricants and High-Performance Functional Fluids,ˇ¨ Ch. 17, Marcel Dekker, Ed. Ronald Shubkin).

What you will note is that synthetic formulations have a greater VI overall. Also notice that the synthetics contain no VIIˇ¦s whereas the mineral oils may have up to 29% VIIˇ¦s to keep the oil functioning at temperature extremes. Some of the synthetic formulations have more EP additives than do the mineral gear lubes or blends, but this is because the synthetics, and some blends, are expected to go for longer drain intervals. Since the synthetic base oils will maintain their thermal and oxidative stability for longer periods, the EP adds will have to have more reserve EP over the life of the oil.

EP Additive Chemistry
Here is some data averaged over 3 conventional (mineral) Hypoid GL-5 gear oils of SAE 90 weight as to range of additives:

As we stated before, the additive package may occupy from 3.5% to 10% by weight of each quart of oil.

KV - 17.42 to 18.24 (100 C)
VI - 98-101

Additive EP Elements (% by weight)
Sulfur - 2.72 to 2.93%
Phosphorous - 0.11% to 0.12%
Chlorine - 0.02%
Nitrogen - 0.08 to 0.09%
Borons ˇV 0.5% TO 2%.


The Borons and Cacium carbonates are slowly replacing the chlorines.

Now for certain oils, you might see additional AW/EP components such as Moly and Antimony in less than 1%-3% concentrations.

GL-4 rated gear oils have a milder EP additive package which amounts to 30 to 50% less EP elements than do GL-5 oils.