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Gear Tribology and Lubrication - Part I #729255
10/07/03 03:02 AM
10/07/03 03:02 AM
Joined: Jun 2002
Posts: 19,230
Iowegia - USA
MolaKule Offline OP
MolaKule  Offline OP
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Posts: 19,230
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Gear Tribology and Lubrication - Part I

General
In this tech brief we will examine gear types and the lubrication requirements for manual transmissions, manual transaxles, and differentials.

Gears and gear boxes are the most often overlooked component when considering oil changes and periodic maintenance. Out of sight and out of mind seems to describe our maintenance attitudes. That is, they seem to function rather reliably in spite of our abuses.

Gears and gearboxes transfer power and transmit torque while sitting inside a closed box and bathed in a partial bath of oil. Gears transmit power by the meshing of gear teeth. Gears can reduce RPM and multiply torque, or reduce torque and multiply RPM, and transmit power at angles from 45 to 90 degrees.

Gears are machined from flat disks or cones. Gear teeth are formed either by casting, forging, or machining. The better gears are cut on special machines and then heat treated for strength. Sometimes, the gears and shaft are joined by splines or welded and then further machined and heat treated.

Gears types are classified as to spur, bevel, helical, double helical or herringbone, and worm gear. Hypoid gears are most common in automotive differentials.

The spur gear is probably the simplest of all gears. This gear is mainly used for changing ratios of RPM and torque in manual transmissions. The spur gear has teeth cut across the edge of the disk and are parallel to the shaft. Contact is limited to very few teeth. Spur gears are used where the shafts are parallel.

If you take a truncated cone, and cut teeth along the cone's main (shaft) axis, it becomes a bevel gear. These gears are mostly used for right angle transmission of power where the shafts intersect. A spiral bevel gear has the teeth cut in spirals along the longitudinal axis of the cone. The tooth contact area is greater with these gears than with spur gears.

Helical and spur gears are most often used in manual transmissions as "reduction" gearing to match loads between engine and rear wheels. Helical gears can transmit power between non-parallel shafts, whereas helical gears can only transmit power with parallel shafts. See gears at work: http://science.howstuffworks.com/gear.htm
http://science.howstuffworks.com/gear-ratio.htm and
http://science.howstuffworks.com/gear-ratio3.htm
http://auto.howstuffworks.com/transmission3.htm

A system of gears called planetary gears are often used to produce a compact design, as in Automatic Transmissions. http://science.howstuffworks.com/gear-ratio4.htm


Bevel gears are most common in differentials and transaxles. There five main types of bevel gears: straight bevel gears, spiral bevel gears, Zerol bevel gears, hypoid bevel gears, and spiriod gears; with each descriptor specifying the type of tooth geometry of the bevel gear.

Some great bevel gear graphics can be seen here: http://science.howstuffworks.com/gear4.htm

Most hypoid gears for differentials are the spiral bevel types with a drive pinion and ring gear, for which the hypoid gear and shaft intersect at other than 90 degrees to the face of the ring gear. http://auto.howstuffworks.com/differential.htm

[See also Joseph Edward Shigley and Charles R. Mischke, "Mechanical Engineering Design, chapter 15.]

Most gears transmit power via a "cam-ing" action, in which each tooth is profiled to allow a sliding action resembling a cam and a lever. In some gears, there is a small amount of roll as well, but the main motion is that of sliding or "cam-ing."

Lubrication

Lubrication of differentials is via an oil bath or "dipping and slinging" and rarely is the oil pressurized. The SAE and the American Gear Manufacturers Association (AGMA) has set viscosity grades or brackets. http://www.bobistheoilguy.com/visc.html

Even though an SAE 90 weight gear oil and a SAE 50 weight engine oil have a similar viscosity of 18.75 cSt, it is their additive packages that differ greatly for the most part. In some situations, the transmission manufacturer of heavy equipment may specify an SAE 50 weight engine oil, but this is the exception for daily driver automotive specifications. Most gear oils also contain thickeners and Viscosity Index Improvers.

As in engine oils, there are classifications for winter or cold weather use that carry the intermediate W symbol in between the grade range, with the 80W90's or the newer 75W90's and 75W140's being the most popular.

The API has specified gear oil service through the years with many obsolete services now on the charts. It is instructive to review those service classifications:

GL-1; Specified for spiral-bevel and worm gear axles and some manual transmissions under very mild service. Usually contains rust and oxidation inhibitors with pour point depressants and anti-foamants. Most R&O oils or AW hydraulic oils will suffice here.

GL-2; Specified for worm gear service more than can be satisfied by GL-1. Most R&O oils or AW hydraulic oils will suffice here.

GL-3; Specified for manual transmissions and spiral-bevel axles under moderately severe service. Most Tractor Hydraulic fluids (THF) or AW hydraulic fluids will suffice here.

GL-4; Specified for hypoid gear service under severe service but without shock loading. This classification is essentially obsolete but is still specified by some manual transmission/transaxle manufacturers. Implies an EP/AW additive package that contains 30% to 50% less S-P additives than the GL-5 service classification. Some Marine Gear Lubes fall into this classification, especially the full Synthetic Marine Gear lubes and specialty blenders MT lubes that use high levels of esters.

GL-5; Specified for hypoid gear service but with shock loads and severe service operation. Usually meets Mil-L-2105D and in most cases, is the multipurpose automotive gear oil. Most 75W90 to 75W140 grades meet the GL-5 classification. This grade has a high level of Extreme-Pressure additives that could be mildly corrosive to nonferrous parts, such as brass, bronze and aluminum parts. Most of the modern GL-5 lubes contain metal deactivators that prevents attacks by the extreme-pressure additives. In addition to EP additives, these lubes contain rust inhibitors, defoamants, friction modifiers, thickeners, and Viscosity Index Improvers.
http://theoildrop.server101.com/ubb/ultimatebb.php?ubb=get_topic;f=21;t=000029

GL-6; Although some manufacturers still specify this lube, it is obsolete as well and was never adopted by the API.

There are or were two new classifications proposed that have yet to surface:
PG-1 and PG-2.

PG-1 was to be designed for heavy, high temperature (to 300 F) transmissions with an L-60 cleanliness rating of 9.0+ (on a scale of 1 to 10, with 10 being the most clean), sludge protection, improved seal life, and synchromesh corriosion protection. This spec was to become the new GL-7 spec. (Robert W. Miller - Lubricants and their Applications, McGraw-Hill).

PG-2; This was the classification for heavy-duty, high temperature axles and has the same properties as PG-1, but with a lower cleanliness rating. PG-2 should have become the GL-8 specification. (Robert W. Miller - Lubricants and their Applications, McGraw-Hill).

To-date, neither of these specifications have come to life, with the result that many automotive manufacturers are specifying 'in-house" formulations, such as GM's "SynchroMesh" and Nissan and Honda's MT specialty fluids for manual transmissions.

Opinion
It is this author's opinion that the API needs to get off their "duff" and reclassify gear lubes according to the intended application(s), and to also recognize the differences between the old mineral-bases fluids and the new synthetic fluids.


In Part II, we will investigate the base oils and additive packages for manual transmissions and differential lubricants currently specified for GL-4 and GL-5 gear lubes.

[ October 07, 2003, 06:12 PM: Message edited by: MolaKule ]

Re: Gear Tribology and Lubrication - Part I #729256
10/07/03 04:27 AM
10/07/03 04:27 AM
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pgtr Offline
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Thanks for the breakdown - am looking forward to Part II. [HAIL 2 U!]

The API admits that GL-2 and GL-3 are dead as well as the GL-6 standard.

Things are nebulous with GL-4s status and future. API admits that the equipment no longer exists to validate GL-4 yet a variety of manufacturers do claim it. I've sent request to API regarding the status and future of GL-4 and have not received any sort of response. I don't know what the issue is but it could go beyond API and be a result of their relationship (or lack thereof?) between the API and auto manufacturers and lubricant manufacturers.

With the advent of proprietary transmission lubricants like those mentioned as well as ATF's in many manual transmissions - it seems the API may already be on the path to being marginalized as a player in the manual transmission lubricant arena. This may mean more confusion and questions on the part of consumers as well as expensive and limited choices for 'correct' lubricants. This could also further limit or reduce appropriate choices for consumers operating older or traditional transmissions looking for optimal performance levels w/ current technology lubricants. I hate to say it but each day the GL-4 standard languishes half dead and unpromoted and increasingly ignored and forgotten, the more likely it or another standardized manual transmission lubricant standards will not come about. [Mad]

I am a little in need of clarification on this:
quote:
Most gears transmit power via a "cam-ing" action, in which each tooth is profiled to allow a sliding action resembling a cam and a lever. In some gears, there is a small amount of roll as well, but the main motion is that of sliding or "cam-ing."
Do not helical transmission gears use an involute tooth profile that is nearly a full rolling action and provides a constant, smooth, rolling motion? That is, they are about 96-99% efficient w/ the remaining percent lost due to 'sliding'. This is somewhat less true in hypoid gears where I believe there is a bit more sliding action than a helical gear. The HowStuffWorks.com site is great as are all those links, however I would encourage folks to look further on "involute" tooth profiles as there are better discussions and animations and analogies out there. Unfortunately I don't have any of the links I've seen before at the moment.

With respect to Part II I would like to see some discussion on 'lubricity' or 'coefficient of friction' differences, if any, between a GL-4 and a GL-5. The reason for this is one of the most universally observed differences in these two types of lubricants in a manual transmission is the smoother shifting action found in at least some GL-4s compared to many GL-5s on average. Is this unique to a particular brand of GL-4 (e.g. Redline)? Or is this synchro friendly friction level common to GL-4s in general?

My own limited research and inference suggests this issue is an incidental by-product of GL-4s vs GL-5s in transmission applications. That is, the GL-5s w/ their EP additives are simply, by nature, 'slicker' and this it serves to provided less than optimal friction for the synchros in many transmission to operate at their smoothest.

Thus, while a 'modern' GL-5 or a GL-4/5 'combo' may indeed be safe for non-ferrous metals, a pure GL-4 may still yield the smoothest synchro operation in some cases. ... Can GL-4 vs GL-5 specifications 'validate' this antecdotal observation?

But as I said, I don't 'think' the GL-4 standard speaks specifically to friction and optimal synchro performance. Or is it possible to create a GL-4 specific lubricant that, due to it's lubricity, makes for a lousy lubricant w/ respect to syncho action. Just looking for some possible clarification or validation on this performance issue. It's an important issue considering this is probably one of the most notable areas a consumer can easily 'observe' lubrication performance 'in hand' so to speak.

...just saying a lubricant is non-ferrous 'safe' is not always enough - some demanding consumers want to improve the odds beforehand (by following a properly spec'd designed product) that the lubricant installed will provide good functional synchro performance.

thanks! [Big Grin]

[ October 07, 2003, 07:30 PM: Message edited by: pgtr ]

Re: Gear Tribology and Lubrication - Part I #729257
10/07/03 06:27 AM
10/07/03 06:27 AM
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Iowegia - USA
MolaKule Offline OP
MolaKule  Offline OP
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I couldn't agree more regarding the API's marginalization. You would think that among both the AGMA and the API, better definitions and service categories could be brought forth.

The article was a "Tech Brief" to give an overview and current status of gearing and fluid specifications.

The intent was not to describe the details of each and every tooth design, but to describe general gear designs; those details can be found in any basic engineering book such as the one sited. Tooth forces vary with the tooth type and involve some very complicated design/engineering equations.

The brief did state that some gears having a rolling action as well as a sliding action. While the gear (such as a worm gear) rotates around the longitudinal axis or "rolls" the action is still predominately one of sliding or cam-ing.

None of the oils speak specifically to friction modification for the synchros, which is why you need to experiment and do UOA's to determine which oil allows the smoothest shifting and lowest wear.

I have often found that thinner viscosity MT oils do not necessarily provide smoother shifting. Much depends on the correct viscosity and the right amount of friction modifiers. For example, in my own case, the Redline MTL (lower viscosity 5W30) was notchy, but still better than the factory oil. By adding MT-90 (15W50 weight), not only was friction modification done, but the viscosity increased as well. The result of the 50/50 mix was one of smoothest shifts (both up and down) and better shift performance over all temperatures. So the right viscosity combined with the correct amount of friction modification resulted in best shifting performance.

Re: Gear Tribology and Lubrication - Part I #729258
10/08/03 09:52 AM
10/08/03 09:52 AM
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tx
pgtr Offline
pgtr  Offline
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tx
As far as I know, (and I could be missing something) the only tooth profiles in automotive meshed gear applications for transmissions/differentials are involute. In theory, an involute has no sliding and is purely rolling. In reality there is some sliding at least in hypoid applications. In non-automotive applications I guess there are other traditional tooth profiles and such. The nerd in me finds the involute profile a fascinating, deceptively simple and elegant movement. [Wink]

Thanks again for a great article. Looking forward to Pt II!

---

...OK folks found some of my old links...
Here's a great general purpose one w/ good animations:
Gary's Clocks
Fancy Gears

But this is the BEST animation that clearly shows in detail the rolling action of an involute gear tooth profile.
uwa.edu.au class notes on gears

There are a lot of white papers, links and engineering class notes online w/ respect to gears and involute tooth profiles as it's a classic form that is commonly used in transportation. I just threw out a couple I'm familar w/ but a web search will yield plenty more.

[ October 08, 2003, 01:00 PM: Message edited by: pgtr ]

Re: Gear Tribology and Lubrication - Part I #729259
10/08/03 12:09 AM
10/08/03 12:09 AM
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MolaKule Offline OP
MolaKule  Offline OP
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The involute tooth profile produces a rotary motion which produces a constant angular-velocity ratio during meshing; this is called "conjugate action."

The angular velocity ratio between the two arms is inversely proportional to their radii, thus these profiles are capable of transmitting uniform rotary motion.

However, the surface action of tooth against tooth is still one of cam-ing or sliding, with the center of gear experiencing uniform rotary motion.

For more info see Joseph Edward Shigley and Charles R. Mischke, "Mechanical Engineering Design," chapter 13, McGraw-Hill. The authors,in Section 13-5, show engineers how to draw and layout involute curves for gear teeth and gear fillets in order to compute forces.

BTW, the gear tooth itself does roll (with respect to the gear center-line), but the gear surfaces slide past each other, with respect to the contact area.

[ October 08, 2003, 03:14 PM: Message edited by: MolaKule ]

Re: Gear Tribology and Lubrication - Part I #729260
10/13/03 06:10 AM
10/13/03 06:10 AM
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tx
pgtr Offline
pgtr  Offline
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tx
quote:
However, the surface action of tooth against tooth is still one of cam-ing or sliding,
quote:
Most gears transmit power via a "cam-ing" action, in which each tooth is profiled to allow a sliding action resembling a cam and a lever. In some gears, there is a small amount of roll as well, but the main motion is that of sliding or "cam-ing."
I dunno what to say... [I dont know] Maybe it's just semantics or gear applications in other purely non-automotive applications [I dont know] but those links as well as other white papers, textbooks & resources I'm familiar with are pretty clear that in classroom theory there is NO sliding whatsoever with pure involute gear tooth profiles. Further, in real world applications of an automotive helical (transmission) or hypoid (differential) gear there is 'some' sliding but certainly nothing remotely close to 'mostly sliding' as was stated. Regardless of whether the article is specific to automotive or generic to gears in general - Involute gear tooth profiles are nearly universally used particularly in auto trannies/diffies (I've even seen 'em in cheap toys). My understanding is diametrically the opposite on this one point in that I believe involute profiles are 'mostly' rolling with 'some' sliding (as opposed to mostly sliding w/ some rolling). I also believe it's a key point because a gear that mostly rolls (efficient) vs a gear that mostly slides (innefficient) has a big impact on lubrication needs.

To specifically summarize my 'different' take on gears and sliding vs rolling in real world applications:
A) ~1-2% sliding occurs in a helical (transmission) gear across it's involute tooth facing (98-99.5% efficient) and
B) ~4-7% sliding occurs in a hypoid (differential) gear across it's involute tooth facing (93-96% efficient).
Virtually all automotive gears and certainly those in transmissions and differentials utilize an involute tooth profile. Those percentages I've provided are certainly far from "mostly sliding".

As we can see, sliding is definately an issue w/ hypoids. But even so 'most' of the contact is indeed still a rolling action not 'sliding'. I presume this increase in sliding and loss of efficiency in a hypoid is precisely why we see EP additives in a GL-5.

Most any textbook dealing with gear efficiencies, sliding and such should be pretty explicity on this point. But if you have a reference that is explicit in stating modern helical or hypiod gears have a 'mostly sliding' action across their teeth surfaces - I'd like to hear more as it would obviously be news to me.

When I read your post restating that gears 'mostly slide' that troubled me and I scratched my head. After further thought I imagine there are some types of gears that are mostly sliding - perhaps some worm gear could approach this "mostly sliding" scenario? However the context of your article is clearly 'geared' (no pun intended [Smile] ) towards helical and hypoid gears in automotive applications utilizing GL 4 or 5 lubricants.

Anyway I've presented a differing view point on sliding vs rolling in gears so I suggest folks consider both perspectives on gear efficiences and decide for themselves. A close scrutiny of any good technical or engineering text regarding modern gears should validate the fact that modern gears used in automobile transmissions and differentials mostly roll w/ some sliding innefficiency (hypoids). I therefore humbly and respectively suggest you re-consider the reverse position being valid: that of mostly rolling w/ some sliding. Otherwise it appears to be a well written article beyond that exception and appreciate it very much.

Also, I don't fully understand why most gears have a 'camming' effect. Again, the whole point of the involute is to provide constant velocity. The term 'camming' is indicatetive of an eccentric movement or varrying velocity. I just don't understand that reference or it's need in the context of transmissions or hypoid diffies. [I dont know]

COnsidering this article will be around for a while and eventually more MEs and AEs will see it - just wanted to suggest you reconsider that one point on gears that 'mostly slide' - appreciate all the articles and hope you'll keep 'em coming [Big Grin]


Thanks,

Re: Gear Tribology and Lubrication - Part I #729261
10/15/03 08:58 PM
10/15/03 08:58 PM
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MolaKule Offline OP
MolaKule  Offline OP
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From the Book mentioned above, pages 530-531, Figures 13-6 to 13-7:

quote:
The following discussion assumes the teeth to tbe perfectly formed, perfectly smooth, and absolutely rigid. Such an assumption is, of course, unrealistic, because the application of forces will cause deflections.
Mating gear teeth acting against each other to produce rotary motion are similar to cams. When the tooth profiles, or cams, are designed so as to produce a constant angular-velocity ratio during meshing, they are said to have conjugate action. In theory, at least, it is possible arbitrarily to select any tooth profile for one tooth and then to find a profile for the meshing tooth which will give conjugate action. One of the solutions is the involute profile, which, with few exceptions, is in universal use for gear teeth and is the only one with which we shall be concerned.



[ October 16, 2003, 12:03 PM: Message edited by: MolaKule ]

Re: Gear Tribology and Lubrication - Part I #729262
10/16/03 02:37 AM
10/16/03 02:37 AM
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Deep South
krholm Offline
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MolaKule,
The manual on one of my vehicles specifies for my transmission and rear end a GL-4 or higher. Does that include GL-5? If it does include the GL-5 wouldn't I be better off to use it? You've written a lot here and I am just trying to comprehend some of it. Thank you so much. krholm

Re: Gear Tribology and Lubrication - Part I #729263
10/16/03 02:48 AM
10/16/03 02:48 AM
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MolaKule Offline OP
MolaKule  Offline OP
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I would try any one of these for the tranny: Pennzoil Marine Gear Lube, or Amsoil Marine Gear Lube, or Redline MT90. For the diffy, any good API rated GL5 (say 75W90).

[ October 16, 2003, 05:56 PM: Message edited by: MolaKule ]

Re: Gear Tribology and Lubrication - Part I #729264
04/29/05 03:50 AM
04/29/05 03:50 AM
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Winston Offline
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Molakule,
I am still confused. I read your old article about GL 1,2,3,4,5, and I still don't get it.
My manual for my 95 Mitsu truck says GL5 for Differentials and GL4 for the transfer case. I put Sta-Lube GL5 in both. (before I found this site) There is a synchronizer for the transfer case because I can shift in and out of 4WD while driving. Are the brass parts in the transfer case going to get ruined? Or will this "newer" GL5 be OK? [Confused]

Re: Gear Tribology and Lubrication - Part I #729265
04/30/05 05:02 AM
04/30/05 05:02 AM
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AEHaas Offline
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Mobil has stated not to use their GL-5 gear lubes in Ferrari applications as the lubricant is harmful to the "yellow" metals. This has generated a lot of discussion over at FerrariChat.com.

Only a select few gear lubes from Agip, Shell and Red Line are used routinely in Ferraris. I recommend you use only what they say is accepted.

aehaas

Re: Gear Tribology and Lubrication - Part I #729266
04/30/05 10:22 AM
04/30/05 10:22 AM
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Iowegia - USA
MolaKule Offline OP
MolaKule  Offline OP
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quote:
Molakule,
I am still confused. I read your old article about GL 1,2,3,4,5, and I still don't get it.
My manual for my 95 Mitsu truck says GL5 for Differentials and GL4 for the transfer case. I put Sta-Lube GL5 in both. (before I found this site) There is a synchronizer for the transfer case because I can shift in and out of 4WD while driving. Are the brass parts in the transfer case going to get ruined? Or will this "newer" GL5 be OK?

I cannot vouch for any lube except ours. In the past, manufactures speced GL4 because of the lower levels of the Sulfur-Phosphorous (S-P) additive package. The lower levels guaranteed less interaction (such as staining and corrosive pitting) with copper alloys such as brass and bronze.

However, and since about 2000, most additive packages now contain "inactive" sulfur which is mediated by metal deactivators and which do not allow the sulfur to interact with the copper-alloy metals, but still allows the S-P additive to protect the gear teeth and bearings.

The problem with most OTS GL5 lubes in MT's is their viscoity and friction modifiers. While the OTS GL5 gear lubes are great for differentials, they do not possess the correct viscosity or friction modification for smooth cold weather shifting.

Some newer MT lubes, such as our MTL-P and MTL-R, are dual-rated GL4/5 for maximum protection in manual transmissions and transaxles, and contain special friction modifiers for synchros.

Re: Gear Tribology and Lubrication - Part I #729267
03/09/06 12:41 AM
03/09/06 12:41 AM
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MolaKule Offline OP
MolaKule  Offline OP
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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.

Re: Gear Tribology and Lubrication - Part I [Re: MolaKule] #3902078
10/30/15 10:42 AM
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Last edited by MolaKule; 10/30/15 10:56 AM.

Reading is fundamental; understanding is not a given ability.
Re: Gear Tribology and Lubrication - Part I [Re: MolaKule] #4547067
10/18/17 11:05 AM
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MolaKule Offline OP
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Updated 10/18/2017

Gear Tribology and Lubrication - Part I

General
In this tech brief we will examine gear types and the lubrication requirements for manual transmissions, manual transaxles, and differentials.

Gears and gear boxes are the most often overlooked component when considering oil changes and periodic maintenance. Out of sight and out of mind seems to describe our maintenance attitudes. That is, they seem to function rather reliably in spite of our abuses.

Gears and gearboxes transfer power and transmit torque while sitting inside a closed box and bathed in a partial bath of oil. Gears transmit power by the meshing of gear teeth. Gears can reduce RPM and multiply torque, or reduce torque and multiply RPM, and transmit power at angles from 45 to 90 degrees.

Gears are machined from flat disks or cones. Gear teeth are formed either by casting, forging, or machining. The better gears are cut on special machines and then heat treated for strength. Sometimes, the gears and shaft are joined by splines or welded and then further machined and heat treated.

Gears types are classified as to spur, bevel, helical, double helical or herringbone, and worm gear. Hypoid gears are most common in automotive differentials.

The spur gear is probably the simplest of all gears. This gear is mainly used for changing ratios of RPM and torque in manual transmissions. The spur gear has teeth cut across the edge of the disk and are parallel to the shaft. Contact is limited to very few teeth. Spur gears are used where the shafts are parallel.

If you take a truncated cone, and cut teeth along the cone's main (shaft) axis, it becomes a bevel gear. These gears are mostly used for right angle transmission of power where the shafts intersect. A spiral bevel gear has the teeth cut in spirals along the longitudinal axis of the cone. The tooth contact area is greater with these gears than with spur gears.

Helical and spur gears are most often used in manual transmissions as "reduction" gearing to match loads between engine and rear wheels. Helical gears can transmit power between non-parallel shafts, whereas helical gears can only transmit power with parallel shafts.

A system of gears called planetary gears are often used to produce a compact design, as in Automatic Transmissions.

Bevel gears are most common in differentials and transaxles. There five main types of bevel gears: straight bevel gears, spiral bevel gears, Zerol bevel gears, hypoid bevel gears, and spiriod gears; with each descriptor specifying the type of tooth geometry of the bevel gear.

Most hypoid gears for differentials are the spiral bevel types with a drive pinion and ring gear, for which the hypoid gear and shaft intersect at other than 90 degrees to the face of the ring gear.

[See also the text by Joseph Edward Shigley and Charles R. Mischke, Mechanical Engineering Design, chapter 15.]

Most gears transmit power via a "cam-ing" action, in which each tooth is profiled to allow a sliding action resembling a cam and a lever. In some gears, there is a small amount of roll as well, but the main motion is that of sliding or "cam-ing."

Lubrication

Lubrication of differentials is via an oil bath or "dipping and slinging" and rarely is the oil pressurized. The SAE and the American Gear Manufacturers Association (AGMA) has set viscosity grades or brackets.

https://bobistheoilguy.com/viscosity-charts/

Even though an SAE 90 weight gear oil and a SAE 50 weight engine oil have a similar viscosity of 18.75 cSt, it is their additive packages that differ greatly for the most part. In some situations, the transmission manufacturer of heavy equipment may specify an SAE 50 weight engine oil, but this is the exception for daily driver automotive specifications. Most gear oils also contain thickeners and Viscosity Index Improvers.

As in engine oils, there are classifications for winter or cold weather use that carry the intermediate W symbol in between the grade range, with the 80W90's or the newer 75W90's and 75W140's being the most popular.

The SAE has specified gear oil service through the years with many obsolete services now on the charts. It is instructive to review those service classifications:

GL-1; Specified for spiral-bevel and worm gear axles and some manual transmissions under very mild service. Usually contains rust and oxidation inhibitors with pour point depressants and anti-foamants. Most R&O oils or AW hydraulic oils will suffice here if the same viscosity is used.

GL-2; Specified for worm gear service more than can be satisfied by GL-1. Most R&O oils or AW hydraulic oils will suffice here if the same viscosity is used.

GL-3; Specified for manual transmissions and spiral-bevel axles under moderately severe service. Most Tractor Hydraulic fluids (THF) or AW hydraulic fluids will suffice here.

GL-4; Specified for hypoid gear service under severe service but without shock loading. This classification is essentially obsolete but is still specified for Light Truck and car manual transmission and transaxle manufacturers. Some Marine Gear Lubes fall into this classification, especially the full Synthetic Marine Gear lubes and specialty blenders MT lubes. In addition to AW additives, these lubes contain rust inhibitors, defoamants, friction modifiers, thickeners, and Viscosity Index Improvers. Specialty friction modifiers are used for Dedicated MTF fluids that carry the GL-4 rating.

GL-5; Specified for hypoid gear service but with shock loads and severe service operation. Usually meets Mil-L-2105D and in most cases, is the multipurpose automotive gear oil. Most 75W90 to 75W140 grades meet the GL-5 classification. This grade has a high level of Extreme-Pressure additives that could be mildly corrosive to nonferrous parts, such as brass, bronze and aluminum parts. Most of the modern GL-5 lubes contain metal deactivators that prevents attacks by the extreme-pressure additives. In addition to EP additives, these lubes contain rust inhibitors, defoamants, friction modifiers, thickeners, and Viscosity Index Improvers.


GL-6; Although some manufacturers still specify this lube, it is obsolete as well and was never adopted by the SAE.

There are or were two new classifications proposed that have yet to surface:
PG-1 and PG-2.

PG-1 was to be designed for heavy, high temperature (to 300 F) transmissions with an L-60 cleanliness rating of 9.0+ (on a scale of 1 to 10, with 10 being the most clean), sludge protection, improved seal life, and synchromesh corriosion protection. This spec was to become the new GL-7 spec. (Robert W. Miller - Lubricants and their Applications, McGraw-Hill).

PG-2; This was the classification for heavy-duty, high temperature axles and has the same properties as PG-1, but with a lower cleanliness rating. PG-2 should have become the GL-8 specification. (Robert W. Miller - Lubricants and their Applications, McGraw-Hill).

To-date, neither of these specifications have come to life, with the result that many automotive manufacturers are specifying 'in-house" formulations, such as GM's "SynchroMesh" and Nissan and Honda's MT specialty fluids for manual transmissions.

Opinion
It is this author's opinion that the SAE needs to get off their "duff" and reclassify gear lubes according to the intended application(s), and to also recognize the differences between the old mineral-bases fluids and the new synthetic fluids.


In Part II, we will investigate the base oils and additive packages for manual transmissions and differential lubricants currently specified for GL-4 and GL-5 gear lubes.

https://www.bobistheoilguy.com/forums/ub...tion#Post729289

Last edited by MolaKule; 10/18/17 11:12 AM.

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