Gear Tribology and Lubrication - Part 1

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/...;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.