Originally Posted by MolaKule
ATF if a special hydraulic oil for the following purposes:
Lubrication for gears.
Cooling and heat transfer.
Transmitting torque via the Torque Converter.
Friction modification for clutch packs, bands, Torque Converter Clutch.
One of its more important duties of ATF is to make sure the fluid's coefficient of friction is matched to the friction materials inside the tranny. I.e., to ensure the proper static and dynamic coefficients of friction during engagement/disengagement.
In the early days of AT's the clutch material was composed of cellulose material with cotton and other soft materials in a phenolic resin binder. Today, hybrid materials are used.
Hybrid (or composite) materials are typically manufactured using a process similar to that used for paper-based materials, but using carbon fibers in combination with organic or synthetic fibers such as Kevlar and Aramid fibers. So Friction Materials may include cellulose, carbon fiber, Aramids, fiberglass, or a combination of materials (composites). Today, composites are the norm.
Many of the clutch materials from various manufacturers are close in terms of static and dynamic friction coefficients.
The clutch disc material of Automatic Transmissions, and other wet clutch applications, are made of porous friction materials which are bonded to steel core plates.
Note: Sintered metal composites may be found in HDD applications.
This material must exhibit:
1.) Mu(o), low speed dynamic coefficient; affected by friction material ingredients and ATF additives adsorption
2.) Mu(i), initial dynamic coefficient at high speed; affected by hydrodynamic effects/porosity/compression/roughness.
When an additive company tests its ATF additive package, many transmissions are run through the SAE2 and other friction test machines (JASO M349-98, R-H Friction Apparatus, ZF GK Test Bench, Low Velocity Friction Apparatus, etc) to determine if the Friction Modifier set is appropriate.
The friction characteristics (Mu verses Velocity) of the clutch/fluid system are carefully monitored for each separate transmission and fluid formulation.
Mu is coefficient of friction plotted on the vertical axis, V is the relative speed of rotating components plotted on the horizontal axis. Mu(v) then is the resulting curve of the frictional characteristics of the clutch/fluid system.
The appropriate FM additive type and levels are then determined for the complete additive package.
A sample of the ATF Friction Modifiers are shown below:
fatty phosphites, fatty acid amides, fatty epoxides, borated fatty epoxides, fatty amines, glycerol esters, borated glycerol esters, alkoxylated fatty amines, and borated alkoxylated fatty amines, metal salts of fatty acids, sulfurized olefins, fatty imidazolines, non-hydroxyl fatty tertiary amines, and any mixtures of the above.
The exact composition of and specific chemistry (molecular structure) of the Friction Modifier(s) are closely guarded secrets.
With the many different chemistries and potential mixes available, I think one can now see why Mutli-Vehicle fluids can be manufactured.
Ford Type "F" ATF is a fluid with very low levels of friction modification and has a Mu(V) curve separate and apart from either the Dexron Series or the Chrysler ATF+ series.
Ford Type "F" ATF is often used today in racing transmissions such as the PowerGlide and TH400 Turbohydramatics because of the positive lockup, IE, little to no slip in the clutch packs during engagement.
Chrysler ATF+ fluids are on the other end of the spectrum and exhibit a completely different Mu(V) characteristic. The ATF+ fluids are highly friction modified.
For a more Technical Study on ATs and heat transfer one can go to:
https://www.bobistheoilguy.com/automatic-transmissions-study/
