ATF if a special hydraulic oil for the following purposes:
1. Lubrication for gears.
2. Cooling and heat transfer.
3. Transmitting torque via the Torque Converter.
4. 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 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.Â
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.Â
The friction characteristics, Mu verses Velocity, or Mu(v) of the clutch/fluid system are carefully monitored for each separate transmission and fluid formulation.Â
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.Â
The appropriate FM additive type and levels are then determined for the complete additive package.Remember, a Friction Modifier (FM) can be aÂ friction reducer, aÂ friction increaser, or one that controls friction in a specified manner.
In an engine, we want to reduce kinetic friction to increase fuel mileage.Â
In a CVT type AT we want a frictionÂ increaserÂ so the chain/belt/pulley system can "get a grip."
In an Automatic Transmission or Limited Slip (LS) Differential, we want a controlled and specific type of friction.
The frictional characteristics we are discussing here is called Kinetic Friction, Dynamic Friction or Sliding Friction, a special kind of friction. Dynamic friction is a friction that changes its "coefficient of friction" as two surfaces that are inÂ relative motionÂ come into or are in contact as in an AT or in LS differential clutch plates.
Recall that AT clutch plates have alternating layers of clutch friction material and steel plates. The friction material is splined on the inside, where it locks to one of the gears. The steel plate is splined on the outside, where it locks to the clutch housing.Â
The pressure for the clutches is fed through passageways in the shafts. The hydraulic system controls which clutches are energized at any given moment.Â
In AT's and LS Differentials, we want the fluid to create a specific dynamic friction coefficient (dependent upon the clutch materials used) such that we have smooth engagement and disengagement, so we don't have shudder or slippage. Shudder and slippage cause increased frictional material wear and increased heat.
It is this complex package of frictional modifier chemical compounds found in ATFs and LS additives that is important for smooth operation.
Again, in an engine, we primarily want friction reduction. In an AT or LS Differential, we want controlled friction modification called Mu(V) in the literature.
Mu is the Coefficient of Friction and V is the relative velocity of rotating or sliding machine elements. In special friction test machines, the resulting curve of Mu(V) gives us the resulting Dynamic Friction Coefficient and it tells us how the friction coefficient value varies with the relative speeds of components.Â
The fluid chemistry must assist in providing a specific friction versus velocity relationship for the AT or LS Differential.
There is a phenomenon commonly called "stick-slip" or "dynamic frictional vibration" and manifests itself as "shudder" or low speed vibration in the vehicle. Using friction modifiers in the ATF or differential fluids prevents this shudder.
A small sample of the ATF Friction Modifiers described 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.Â
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:
What I am saying is, friction modification for a specific ATF involves a very complicated and specific chemical matrix to obtain the needed Mu(v), and the ester type can affect friction coefficients and has to considered as to how it, the base oils, and other additives affect the overall friction properties.