Friction Modifiers (FMs)

MolaKule

Staff member
Joined
Jun 5, 2002
Messages
23,032
Location
Iowegia - USA
Remember, a Friction Modifier (FM) can be a friction reducer, a friction increaser, or one that controls friction in a specified manner. In an engine, we want to reduce kinetic friction to increase fuel mileage. In a CVT type AT we want a friction increaser 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 in relative motion 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. The pressure for the clutches is fed through passageways in the shafts. The hydraulic system controls which clutches are energized at any given moment. 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. 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.
 
Last edited:
Joined
Jul 16, 2003
Messages
779
Location
Palouse WA
Interesting My 19 Subaru CVT has two clutches one for front and the other for rear wheels. The new CVT uses a chain from LUK in Germany Wonder how this works Ken
 

MolaKule

Staff member
Thread starter
Joined
Jun 5, 2002
Messages
23,032
Location
Iowegia - USA
Originally Posted by Ken42
Interesting My 19 Subaru CVT has two clutches one for front and the other for rear wheels. The new CVT uses a chain from LUK in Germany Wonder how this works Ken
Do you have an exploded view of this system?
 
Last edited:
Joined
Sep 9, 2008
Messages
229
Location
Greece
Hello Molakule, The change in dynamic friction coefficient value can affect the static one and vice versa with the addition of the FM, or are these two parameters not related to each other? Thank you for your feedback.
 

MolaKule

Staff member
Thread starter
Joined
Jun 5, 2002
Messages
23,032
Location
Iowegia - USA
Let's start with the definition of Static Friction since friction is an often misunderstood topic. As with any definition of friction we must remember that friction is surface (interfacial) dependent, that is, friction is dependent on the types of surfaces under consideration for the surfaces in the system. Static Friction is that force that opposes movement, i.e., there is no movement and the static friction force holds an object motionless unless a greater force is applied to overcome it which then results in Dynamic Friction (friction due to motion). Every system has a Static COF. Dynamic Friction = Kinetic Friction = Sliding Friction which is a force opposing, but not preventing, the actual movement of objects between two surfaces. Every system has a Dynamic COF. Coefficients of Friction (u or Mu in the literature) are dimensionless values and are determined experimentally. Dry surfaces (no lubricant present) means we will have high Coefficients of Friction (COF). For example, a rectangular wood crate weighing 20 kg. on a wood floor will have to have a force greater than 70.8 N to move it horizontally since the opposing Friction force = u*Normalforce = (0.3)*236 N = 70.8 N; where the Normal force is that force acting vertically down and Mu is the COF. Apply 71.8 N of force and the crate will begin to move. Wet lubrication is the application of a substance known to reduce friction between two surfaces. Lubricated Friction results in motion of the crate with greater ease since the lubricant reduces the Dynamic COF. Apply some floor wax to the floor and the COF may be reduced to say 0.1, so now we can move the crate horizontally (from another room and a rope) ) with only 23.6 Newtons of force, Grandma may not appreciate the experiment and she may have you wax the whole floor. LOL In Step-Shift automatic transmissions and in limited-slip differentials with clutches, we want a wet lubrication Dynamic Coefficient of friction Mu(V) to overcome stick-slip phenomenon and it is Velocity and Force dependent. "Stick" means an opposing static force whereas "Slip" means an opposing dynamic force. Friction Modifiers in ATF and in limited-slip differentials with clutches provides a "compromise" solution between those two frictional forces. How they do this involves precise shearing forces at the molecular level of the FM chemistry and is beyond the scope of this short note. Added: Here is another view: The FM chemical compound modifies (changes) the Coefficient of Friction (COF) between the surfaces "on-the-fly" depending on the relative speeds of the rotating components and the applied pressure forces. That is, it decreases OR increases friction depending on the relative speeds of the rotating components and the applied pressure forces in order to prevent Stick-Slip which causes shudder. Below is an actual Mu(V) plot for some experimental ATF Friction Modification (FM) compounds.

Mu of V Plot for BITOG.jpg
 
Last edited:
Joined
Sep 9, 2008
Messages
229
Location
Greece
Hello MolaKule, thank you for your feedback the above is clear. As I see the FM cause primarily a dY change instead of a tanφ rotation of the curves. What is the behavior of the curves above in the neighborhood of zero sliding speed? Would somebody expect something like A or B (I suppose A). [Linked Image] thank you
 

MolaKule

Staff member
Thread starter
Joined
Jun 5, 2002
Messages
23,032
Location
Iowegia - USA
Originally Posted by berlyn
Hello MolaKule, thank you for your feedback the above is clear. As I see the FM cause primarily a dY change instead of a tanφ rotation of the curves. What is the behavior of the curves above in the neighborhood of zero sliding speed? Would somebody expect something like A or B (I suppose A).
I have no idea what you are asking wrt graphs since none of the graphs have any context, such as X, Y titles nor are the curves annotated. Friction modifiers in fluids such as AT's and LS differential FMs have characteristic shear moduli at the surfaces of the rotating parts which at the macro level dynamically changes (on-the-fly) the COF with respect to: 1) type of surface and its finish, 2) relative speeds of rotating parts, 3) applied forces, whether those forces are applied via springs (LDS) or via fluid pressure actuation.
 
Last edited:
Joined
Sep 9, 2008
Messages
229
Location
Greece
Hello MolaKule, The question is if the Mu(V) diagram has the shape A or B in the area where the X=0, Y=0 point. Vertical axis is friction coefficient, horizontal is Log of sliding speed.
 

MolaKule

Staff member
Thread starter
Joined
Jun 5, 2002
Messages
23,032
Location
Iowegia - USA
I copied and printed out the graphs and they are very out of focus and and I see no values for the X, Y coordinates nor do I see any data on applied force(s), whether this is dry friction, lubricated friction, wet clutches, etc. I still have no clue as to what you are asking.
 
Joined
Dec 12, 2006
Messages
196
Location
Europe
How different are the friction coefficients (dynamic and static) of Type A Suffix A vs. Dexron/Mercon (I tried hard to find any info but failed...)? If the latter are backwards-compatible why certain manufacturers keep on specifying TASA only (60y. old spec ! ) for certain applications? Thank you !
 

MolaKule

Staff member
Thread starter
Joined
Jun 5, 2002
Messages
23,032
Location
Iowegia - USA
Originally Posted by Rollins
How different are the friction coefficients (dynamic and static) of Type A Suffix A vs. Dexron/Mercon (I tried hard to find any info but failed...)? If the latter are backwards-compatible why certain manufacturers keep on specifying TASA only (60y. old spec ! ) for certain applications? Thank you !
Most wet clutches in Step-Shift transmissions today are cellulose-based composite types and have similar dynamic COF's IN COMBINATION with the proper ATF. I suppose they have their reasons (maybe price and they think they are saving their customers some cash?) but I have yet to see any logical reason for specifying an ancient fluid like the Type A suffix A. I can still acquire Type "F" DI additive packages but can not acquire any Type A suffix A DI packages. If a customer wants Type A suffix A I have to use another DI package and insert some friction modification into the formulation. The Dexron VI fluid has the most stable chemical components of any fluid to-date.
 
Last edited:
Joined
Feb 19, 2006
Messages
660
Location
Stewartstown PA
I love reading your posts MolaKule-I feel have learned so much despite not having aptitude for chemistry and mechanical information. Gives brain a workout. I often wonder if there is a table available differentiating the ATF specifications correlated with friction modification requirements. I imagine Chrysler ATF +4 is at one end of spectrum with static friction less slippery and Dextron 6 is at other end-more slippery-but again not sure am using terms correctly or thinking of them in correct way.
 

MolaKule

Staff member
Thread starter
Joined
Jun 5, 2002
Messages
23,032
Location
Iowegia - USA
Hi Spider, I would list the fluids from most friction modified to least as: ATF+4, Dexron, Type "F", One thing we have to realize is that it is the combination of the fluid's chemistry and the clutch's surface roughness and porosity that determines the dynamic coefficient of friction at the surfaces.
 
Last edited:
Joined
Dec 6, 2019
Messages
82
Location
Sweden
Originally Posted by MolaKule
Hi Spider, I would list the fluids from most friction modified to least as: ATF+4 Dexron Type "F"
As you stated in your first post in this thread, it can be FM for both lower and higher friction. At this list I quoted you, you say FM. Does that just mean degree of additives, or does it mean that ATF type F have least amount of FM and therefore the highest friction?
 
Joined
Feb 27, 2009
Messages
7,717
Location
down in the park
The friction modification is speed dependant, some types make more friction at slow speeds (to make the clutches grab) others make less friction just before coming to a stop, for smoothness. So amounts shouldn't be linked to more or less friction, in case of ATFs
 

MolaKule

Staff member
Thread starter
Joined
Jun 5, 2002
Messages
23,032
Location
Iowegia - USA
Originally Posted by Sveina
Originally Posted by MolaKule
Hi Spider, I would list the fluids from most friction modified to least as: ATF+4 Dexron Type "F"
As you stated in your first post in this thread, it can be FM for both lower and higher friction. At this list I quoted you, you say FM. Does that just mean degree of additives, or does it mean that ATF type F have least amount of FM and therefore the highest friction?
I am not quite sure what you are asking. You might want to re-read post #5154145. Friction for any wet clutch system is dynamical and is a function of fluid composition and the surfaces in question..
 
Last edited:

MolaKule

Staff member
Thread starter
Joined
Jun 5, 2002
Messages
23,032
Location
Iowegia - USA
Originally Posted by Brian553
Can you end up altering chemistry to where friction modification raises Mu(V) > Static COF ?
The Mu(v) curve on a linear-linear plot shows a slope where the slope (Dx/Dy) of the curve is the resulting COF for a specific load at a specific rotational speed. On a linear-Log plot as above, the COF verses Sliding speed shows almost straight lines. In either case, Mu(v) is not a static value, but rather a dynamical value meaning it "changes" with changing conditions.
 
Last edited:
Joined
Aug 23, 2010
Messages
3,803
Location
pa
thanks AGAIN Mola for taking the time to teach, learned something again
 
Top