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Gear Lube Temperature

Gear Lube Temperature

Gear Lube Temperature

I have a WW2 jeep, 1942 Ford GPW.  Recently I edited together a book based on articles from Army Motors.  An Army magazine for mechanics and drivers of WW2.  It was the precursor of PS magazine that is still being produced today.

One of the articles, "Sore as a Boil About Oil", August 1942, compares gear lube oils.  The major concern seems to be the operating temperature inside the transfer case.  The lubes discussed were 80-140wt  140-250wt vs 90 wt (0r 80wt for temperatures below zero).
Apparently, the thinking at the time by many was that the heaver oil wouldn't leak thru the leather oils like 80 or 90 wt oil.

The article claims the temp in the case when using 140wt oil will climb and the heat will thin out the heavier oil to the same viscosity as the lighter oil.  Thus leaks would still occur.  The temp using 140 produced disadvantages...

...tests show that 140 oil causes an operating temperature of 235F to 260F in the transfer case with a resultant viscosity of 90 Saybolt seconds.

...similar test show that 90 oil causes a temperature of only 190F to 210F with the same viscosity of 90 seconds.

...higher temperatures break down the 140 oil...ruins oil seals.

...90 oil, being made of refined oil with a fixed additive, remains stable up to a temperature of 265F.  140 oil is often made up of a poor base stock and separation at 230F is not uncommon - the result is a tarry mess at the bottom and a light, spindle oil at the top.  Bearing failure is just around the corner.

... SAE 90 (Federal Specification VV-L-761

...tests took place on a hot day - temperatures hovered around 100F.

So after this long winded post my questions are...

Are there any modern studies relating the operating temperatures of various weights of lube oils?

Modern ads for gear lube claim stability as a selling point in gear lubes like 80-140.  One would think that improvements to production and additives would have occurred during the 60 or so years that have passed since that article was written.  Is this the guess?

The bottom line?  Original specifications called for 80 or 90 wt lube is it safe (from an operating temperature standpoint and any other factors) to use 80-140wt lube?

Many in the WW2 jeep field claim ti use the 80-140 becuase it "keeps the gears quieter" or other reasons.  No one has offered any studies to prove using the 80-140 is better than  90.

And as a side question, other than manufacture specifications, how can you determine which engine oil to use?  During WW2, Willys and Ford specified 30wt for normal ambient temperatures and 10wt at low temperatures.  Today, many in the jeep field claim that 20w50 is best at least for summer time use.  Any studies comparing 30wt (or perhaps 10w30 vs 20w50 )?


RE: Gear Lube Temperature

Let's start by saying I'm not sufficiently acquainted with jeeps. However, I can say that those who warn against using heavier gear oil grades without warranted reasons are right. Viscosity is indeed one of the most important lube properties; the greater the viscosity the greater the protection against tooth failures. However, one must be cautious not to use thicker oils than necessary to avoid heat generation and power loss from churning and shearing by high speed gears or bearings. Right, the operating temperature of the gear drive will determine the oil viscosity. If it is too viscous, excessive heat will be generated, the heat will reduce the oil viscosity to a point of diminishing returns where by increasing the initial starting viscosity one leads to a higher operating temperature and higher oxidation and thermal degradation rates, without a significant gain in operating viscosity.

Unfortunately, hydrocarbons (most lubes are made of) cannot be effectively inhibited against thermally induced breakdown. At 205 oF a 90 grade would have the same absolute viscosity that a 140 grade would have at 260 oF. Some protective additives are even less stable than the base oil they are meant to protect! On the other hand, using too light an oil can bring about pitting fatigue and failure of the gear drives.

The pitch line of the gears is a good indicator of the required oil viscosity. An empirical equation to determine the right oil viscosity:

cS40 = 7000/v0.5 is recommended by experts.

cS40 is the kinematic viscosity of the oil at 40 deg C. A 90 grade is equivalent to an SAE 50, or AGMA No.5, or ISO 220. 220 are the mm2/s = cS of the oil @ 40 deg C. A 140 grade corresponds to AGMA No.7 and ISO 460; namely its viscosity at 40 deg C, is 460 cS.

v is the peripheral pitch-line velocity of the pinion in ft/min = 0.262 d.n    
where d is the operating pitch diameter of the pinion in inches, and n, the rotating speed in rpm; 0.262 is pi/12.

Assume d=1 in. and n=4000 rpm, v=0.262(1)(4000)=1048 fpm and the viscosity in cS=7000/(1048)0.5=216 cS. In other words, the 90 grade (220 cS) is suitable, not so the 140 grade.  

Of course, in cold climates, either the oil has a natural pour point at least 9oF lower than the lowest expected ambient temperatures, or pour point depressants are added to avoid high starting torques by ensuring free lube circulation. 75W, 80W, and 85W grades are in use. For automotive hypoid gears depressants are tailored to bring the lubes' pour point down to -40oF.

More sophisticated approaches, based on the specific film thickness are used for critical applications.

On the side question: monogrades are good enough, however, one mustn't forget that multigrades AW-B were created to provide an answer to required oil properties at the cold and hot extremes in temperatures that may be encountered in use. Right additive formulations may be the secret recipe for success. At the end one must decide on heeding OEM's advice, or on buying proven quality, or on sticking to reputable oil maker names.

By the way, under the SAE clasification, a 5W-30 engine oil and an 80W gear grade have equivalent viscosities at 40 deg C; so do a 10W-40 and an 85W, and a 20W-50 and an 80W-90.

RE: Gear Lube Temperature

Great information.  I have read it several times and don't pretend to understand it all.  But based on what you have written, no additives or lubes used today really change the outcome of that report from 1942.  Higher viscosity generally leads to higher heat, higher heat leads to "thinning" of the lube.

Is there ANY difference in using 80w140 (or any multi viscosity lube) in heat generation?  As I understand it the lube is 80wt at beginning of operation (or cooler temperatures) then as heat is generated, additives, which I believe are some sort of polymer, "uncoil" to increase viscosity (?) to equal that of 140.   It would seem then by your explanation that heat would generate; still end up with "too much heat"; and a kinetic viscosity that is lowered or the lube thinned.

On the engine oil question.  It comes down to the same issue as the gear lube? The manufacturer original specified 30wt oil.  One of the factors I don't know is at what temperature does oil begin to break down?  Using higher viscosity oil in an engine would seem to me to do the same thing as the gearboxes--allow the generation of more heat which affects viscosity.

Can you do/show me the comparison of the following:


The jeep engine:

60bhp engine (develops 60 bhp at 4000rpms - long stroking engine would not last long at those rpms! Without an overdrive installed the engine gets really loud at speeds over 45 mph)
134.2 cubic engine engine
4 cylinders

Operating temperature (water, coolant) - On my jeep, running with a 180F thermostat, seems to be 185 to 210F. Others claim lower temperatures when using a 160F thermostat.

Outside air temp - at least 90F (it's Florida and already nearly as hot as Hades)

My question(s) becomes given the 3 oil weights and all other factors being the same...would oil weight affect the operating temperature of the oil, would this in turn impact the operating temperature of the coolant?

I want to be able to run at the coolest operating temperatures possible given speed, outside air temperature, etc.

Finally, I would also like to share this information with other jeep nuts.

If you are interested in jeeps, the best site is www.G503.com
and my site has some jeep information as well www.42FordGPW.com

Thanks very much for explaining how viscosity impacts lube temperatures and the lube.

Otherwise I would still be sitting here with a puzzled look on my face.

RE: Gear Lube Temperature

Mineral undoped base mineral oils start their oxidative degradation, albeit very very slowly, at temperatures as low as 90oF by free-radical chain reactions.

Above 250oF hydroperoxide cleavage plays the most important role. Metal catalysts reduce the activation energy and oxidation starts earlier and advances at a quicker pace. The resulting acids act as accelerators.

Antioxidants are meant to inhibit ot stop oxidation chain reactions altogether and anticorrosives are used to neutralize the acidity (TBN values), as long as they themselves survive.

As for temperatures: the upper part of the piston ring and cylinder zone in passenger car engines can reach levels as high as 480oF; in heavy duty diesels up to 570oF. In these areas thin oil films (about 2x10-7in.) are exposed to blow-by gases during 2-5 minutes.
These gases, generated by combustion, contain free radicals, mainly OH.,NO, NO2, HNO3, oxygen, organic acids and olefins from partially oxidised hydrocarbons, metallic catalysts, and partially oxidised lube components.
Under these conditions the oil film undergoes rapid oxidation, initiated by the hydroxyl and NO2 radicals. The latter combines with olefins leading to the formation of "sludge precursors".

All degradation intermediates: nitro and nitroso compounds, radicals, nitrites, nitrates, acids, ketones, and oil-soluble polycondensation and polymerization products appear in the sump. With extended oil change intervals and temperatures of up to 300oF, oil thickening and sludge formation may result. Thus water cooling is very important. Enclosed gear lubricants must also carry away the heat generated by friction. It is now clear, I think, why a good additive doping is, to my grasping, the key to a succesful lubricant performance.
As an answer to your question, yes, oil temperatures tend to affect heat transfer properties. An HTC dependence on (Rem.Prn) would indicate an increase with temperatures in the range between 80 and 140oC as long as the oil doesn't suffer degradation of any sort. When thickened by polymerization and polycondensation heat transfer is bound to suffer.

VI improvers and thickeners include a large variety of polymers with differing chemistries. They are selected to incorporate pour depressancy and dispersancy. When these polymers are subjected to high shear stresses, and high temperatures, the random coil is severely distorted, bond energies can be exceeded and the polymer will break or shear, and molecular weights start to come down. Initial viscosity loss is rapid, it is followed by a slower loss and finally by a plateau, as the equilibrium, or terminal molecular weight is reached.

Besides, lubes used for combined engine and gearbox lubrication are formulated with additive packages showing a powerful EP activity.

All these items and more are studied by tribologists which I am not. I hope, however, having satisfied at least part of your queries.

RE: Gear Lube Temperature

Well, jeep, to ease your concerns about what lube to use in a long stroke slow turning engine (even splash lubed)---
My everyday driver is a 1930 Ford with over 9,000 miles on the engine since rebuild in 1999/2000 (Christmas present to myself) shows absolute zero wear when I had it out to add a high compression head.  I have checked the oil and water temp periodically with a pyrometer (car has no guages) just to satisfy myself--- ~200f for both has been the max so far.
The lubricant?  Mobil 1, 15W-50 changed every year regardless of mileage (usually less than 2700miles) I also use an 80W in the gearbox and diff instead of Fords 600W, leaks a bit but they are not a problem.

Then again, the original engine/gearbox went ~100,000miles on who knows what oil!!!

Tempest in a teapot?  Back home on the rance we had a surplus jeep much like yours that, as a kid, I ran the crap out of for years (mid '50's). I don't ever recall changing the oil!!! Overheated a bit in the summer but otherwise it never failed.

I use ATF in my competition manual gearboxes and an 80W synthetic in the hypoid diffs.


RE: Gear Lube Temperature

I was under the impression that Mobil doesn't advise on the mileage or time between oil changes for its synthetics, and leaves that to the OEM's recommendations. Otherwise it becomes your sole and entire resposibility. Am I right ?

RE: Gear Lube Temperature

25362, you may be correct as I recall no recommended change interval with the Mobil 1 that I am currently using.  In my Model A it gets changed once a year but in the "Money Pit Mini" vintage racing Mini Cooper it gets all NINE quarts changed after each event. A bit of overkill, yes. Piece of mind is worth it in my estimation.(I'm considering using this oil, properly re filtered, in the Model A as a cost saving effort. Just thinking at present, though).

FYI---I was in a conferance with several Ford regional reps this Saturday past and one of the major discussions was oil change interval vis a vis Lincoln.  Current recs are 5k for engine and 100k for transmission (sooner "if the consumer requests it"---???)  I have learned through these forums that long change intervals are common in Europe and I mentioned it.  Apparantly these longer change intervals and synthetic lubes have not gone unnoticed over here.  The reps stated that "soon" the oil change intervals for Lincoln (Ford?) will increase to 15,000 miles excepting the diesel engined vehicles!  (???)


RE: Gear Lube Temperature

one thing to bear in mind is that the vehicle was build in the forties and the oils specced were the types then available to the military. In other words: lubricants where problably more chosen on the basis of being in the army stocks anywhere in the world then on the basis of technical suitability. If the designer of the vehicle now had to spec lubricants, he would find available products of much improved quality and his choice most likely would be something like this:

engine oil: SAE xW-y, where x may be 10 or 15 and y may be 30, 40 or 50 and API SE or better.

transfer case: API GL5, SAE 80W 0r 90 - where most likely better seals would prevent leakage.

When the oil in a transfer case gets really hot, there are two possibilities: too high a viscosity and too high the oil level. Under both conditions excessive churning of the lubricant may lead to a substantial temperature rise that may  shorten the lubricant life substantially.

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