Gear Tribology and Lubrication - Part II

MolaKule

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Gear Tribology and Lubrication – 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 (VII’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 “fats.” 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 “mil-spec” did not address the use of synthetic base oils, since the specification was originally developed out of the use of mineral oils of the 1950’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 Redline’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 80W140’s. In partial synthetic gear oils or “blends,” as we shall see, mineral oils, PAO’s and even esters are used, where the predominate oil is a mineral oil. In synthetic gear oils, synthetic hydrocarbons such as polybutenes and PAO’s, along with esters dominate. Synthetic Gear Lubes shear little as compared to a mineral oil formulation with VII’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 VII’s at all. Gear Lube Formulations Here we will look at various gear lube formulations for mineral, “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 “Synthetic Luibricants and High-Performance Functional Fluids,” 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 VII’s whereas the mineral oils may have up to 29% VII’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 – 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. [ October 16, 2003, 07:30 PM: Message edited by: Patman ]
 
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Molakule, By chance do you have any dates available for precisely when the various GL- standards were first released (or retired)? Particularly GL-4 or GL-5 since those are areally the biggees we are mostly concerned with. I'll fire off a request to API but I don't usually find them to be very responsive. Also do you happen to have a list of the ASTM tests #s and their titles that comprise the GL4 and GL5 standards? I'm not 100% sure but I'm thinking that the GL spcs nowadays are a composition of a bunch of specific ASTM tests. I want to say 2105 was still commonly recommended by car makers into the early 60s before being replaced by GL-5 by and large around that time frame (it may have taken car makers a while to catch on to the API GL spec). 2105 still continues to prominently appear on many labels to this day! thanks for all the continued insights!
 

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Clarification on S-P additives and the GL4/GL5 quandry: The Zinc-Sulfur-Phosphorus package in GL4 lubes is 1/2 to 2/3 the level of the GL5 lubes. The difference in add levels is due to the fact that manual transmissions don't need as high a level of EP adds for the spur gearing, whereas hypoid differentials need extra EP because of their high loads and extensive shearing actions. Most gear lubes today use the "non-reactive" sulfur-phosphate packages. By "non-reactive," we mean that the sulfur is not just 'elemental' sulfur dissolved in oil, which would react with the metal to stain and corrode copper-containing metals such as brass and bronze. Rather, the new S-P add packages are esterified (incorporated in fats) S-P's. These only react under pressure/heat of shearing and loads to form FeP and FeS EP surfaces films to prevent galling and wear. TBN boosters (TAN reducers), antioxidants, metal deactivators, and rust inhibitors are also incorporated to prevent acid buildup and to protect from rust. Special Friction Modifiers insure proper engagement of the synchros. [ March 13, 2004, 12:50 PM: Message edited by: MolaKule ]
 

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GL5 dual rated products simply means that it is supposed to be backward compatible and that the GL5 Z-S-P add pack is more than sufficient for GL4. The problem is, the GL5 usually contains thickeners and tackifiers that might make shifting more difficult in cold weather for a purely rated GL4. In my view, only a full synthetic, properly formulated GL5 could have a dual rating.
 
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quote:
Originally posted by MolaKule: Rather, the new S-P add packages are esterified (incorporated in fats) S-P's. These only react under pressure/heat of shearing and loads to form FeP and FeS EP surfaces films to prevent galling and wear. ... Special Friction Modifiers insure proper engagement of the synchros.
Are there any GL-4/GL-5 that we SHOULD look out for and stay away from? JohnBrowning mentioned a much more pronounced sulfur smell with the Royal Purple stuff. I guess i'm still in the 'sky is falling' crowd for GL-5 in GL-4 boxes. ferb!
 

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El Cheapo mineral-based lubes.
quote:
In my view, only a full synthetic, properly formulated GL5 could have a dual rating.
 

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The sulfur smell in any gear lube is the smell of protection, the Sulfur-Phosphorus EP package. You're actually smelling the sulfur and phosphorus, with the sulfur smell dominating. To make this package, they usually start with something similar to phosphorus pentasulfide and react it with olefins or esterify it using a multi-step process. In a heavy viscosity synth lube, the base PAO's and esters, along with boron and sulfurized zinc, add secondary AW and EP protection. My technical sources state that chlorine compounds have not been used in gear lubes since about 1981. Chlorine compounds are also being phased out of metal-cutting fluids as well.
 

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Most differential lube manufacturers list the GL5 rating because the majority of differentials require that protection rating. Let's clarify that Manual Transmission Fluids (MT's) and Differential lubes are two entirely different lubricants and their formulations reflect that fact. GL4 is sometimes specified for components such as TC's and MT's because many mechanical engineers lack any knowledge about gear lube formulations. GL4 refers to the level of EP protection provided by the lubricant. Some people think that GL4 offer a less aggressive "attitude" toward non-ferrous parts such as the copper alloys of brass and bronze commonly used in sychronizer assemblies. And let me state this for the nth time, modern GL5 lubricant formulations are safe in most components/units. When there is a mismatch, it is usually a mismatch between viscosity and friction modification.
 

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For some of our newer members, welcome to BITOG. The difference between motor oils and hypoid differential lubricants: Gear lubes have primary and secondary Extreme Pressure additives to assist the drive pinion and ring gears when the mating teeth transfer torque and slide and roll on each other. This localized torque at the gear teeth causes heating and very high contact forces (pressures) which, without the proper lubricant, can cause tooth breakage, galling, scuffing, etc. A motor oil has Anti-wear additives for the engine and uses lighter cuts of base oils. A hypoid differential lubricant has Extreme Pressure additives to avoid those failures mention above, and contains heavier cuts or viscosities of base oils to provide thicker films for the increased loading. And we haven't even discussed the differential and axle bearings and their potential failure with the use of motor oils.
 
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Molakule, I have seen a lot of 80W90s and 85W140s around with a VI of 97-105...I always assumed that they were not VIIed, but your recipes indicate otherwise. How compatible are these adds and basestocks with PCMOs...one poster on BITOG stated that he'd rather add 85W140 to his engine (and does add gear oil) rather than 10W30 ?
 

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Originally Posted By: Shannow
Molakule, I have seen a lot of 80W90s and 85W140s around with a VI of 97-105...I always assumed that they were not VIIed, but your recipes indicate otherwise. How compatible are these adds and basestocks with PCMOs...one poster on BITOG stated that he'd rather add 85W140 to his engine (and does add gear oil) rather than 10W30 ?
The EP additives contained in gear lubes are NOT applicable for engine oils for the following reason: 1. with gear lube EP additives, the high temperatures in engines would create higher levels of acids being produced, resulting in accelerated corrosion. In engines you want anti-wear additives with thermally stable zinc, phosphorus, and sulfur combinations. The comment by the poster who used gear lubes in engines simply shows a lack of knowledge about engine oil/gear lube formulations and their proper applications.
 

pbm

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Molakule:
I wanted to ask your thoughts on a product called 'Torco MTF' which is GL-5 rated but states "is yellow metal safe and can be used in transmissions, transaxles, and transfer cases where an ATF, GL-5 or GL-4 light viscosity oil is recommended..." It also states that it is "recommended for use in Mercury Dry Sump 6 Drive Outdrives, Atlas Transfer Cases, Lenco Transmissions, Honda Transaxles and many like applications that call for a high performance light viscosity gear oil."

I'm considering this for the next change of MTF in my 2014 Focus which calls for a 75W GL-4 fluid. I realize the viscosity is a little higher than the Ford stuff but Torco recommends it.
 
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"I have seen a lot of 80W90s and 85W140s around with a VI of 97-105...I always assumed that they were not VIIed, but your recipes indicate otherwise."

These are attainable without VII just but a big dose of PPD.

wider range gear oils like a 75/140 will need a PAO base stock or VII.
 
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Back in the day when I did some heavy equipment work. I remember all of the power train of a certain bulldozer being lubricated by the same SAE 30 series 3 engine oil. That is the engine, transmission - bevel gear, and even the finals, no special gear oil was used at all. Only the hydraulic system used hydraulic oil or if not available SAE 10 oil could be substituted. All specified by the manufacture, and never a problem with gear wear. So that engine oil must have some decent EP properties to protect the gear sets as well as it did.
 
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