In the October/November issue of GEARS, Dennis Madden’s informative article on understanding the systems of shift feel and shift quality brought back so many memories of testing transmission fluid and shift quality 30 years ago. We all tend to forget how the automatic transmissions of that day were given a specific “calibration” using all the seven factors listed that went into the specific mechanical setups used for each transmission calibration of that time. There was no computer or solenoid-controlled shifting. It was all figured out by a dedicated engineer using orifices, accumulators, springs, and valves with the calibration of the whole system based on the performance of a reference fluid at the test track/proving grounds of the OEMs of that time. If I may, I would like to expand on the seventh factor in his article, Fluid Type.
Having been actively engaged in the transmission fluid business at one capacity or another for the last four decades, I have become very familiar with the frictional problems of automatic transmissions, in particular the way used transmission fluid can contribute to common frictional problems. This has initiated so many conversations about shift quality and T/C shudder, (including mechanical, software and fluid solutions for both issues) over the years. So very often the conversation turns to the engineering answer to the question (which Dennis covered very well as far as older transmissions are concerned) and it becomes difficult to explain the why and wherefore of the fluid element if the listener has no background in fundamental lubrication theory or is not directly involved in the transmission fluid business. I will take a shot at explaining the importance and science of the frictional interactions that occur between steel reaction plates, friction clutch plates and the wet clutch lubrication fluids that make it all possible. It is the interaction between the controlling hydraulic functions and these three frictional components that allows smoother shifts, resistance to wear, increased torque carrying capacity, improved low-speed frictional performance (to prevent problems like T/C shudder), improved efficiency, and long-term frictional stability.
Understanding friction is a start. The force known as friction can be defined as the resistance encountered by one body in moving over another. There are basically two types of relative motion, sliding and rolling. In sliding (like clutch plates) a tangential (plate goes in one direction) apply force is needed to move the upper body (friction clutch) over its stationary opposite (steel reaction plates). The ratio between this frictional force and its normal load is known as the coefficient of friction. The magnitude of the frictional force involved in compression of these steel and friction clutch plates is described by the value of the coefficient of friction. For this discussion, sliding friction clutch plates, steel reaction plates in a wet (lubricated with ATF) environment. The coefficient of friction for best shift quality ideally should fall between 0.8 to 1, with endpoint vs midpoint toque ratio closest to 1:1 providing the best quality shift.
Under some conditions of sliding, the coefficient of friction for friction clutch plates and steel reaction plates, (with fixed conditions and proper lubrication), can be very stable. This observation leads to the laws of sliding friction. The laws of friction may be stated as follows:
- The force of friction is proportional to the normal load.
- The force of friction is independent of the area of contact.
- The friction force is independent of the sliding velocity.
Unfortunately, the three laws of friction are of varying reliability when predicting the frictional properties as they apply to automatic transmissions with wet clutches. Most metals and many other materials obey the first law well, while clutch reaction plates often do not, and the lubrication required becomes a controllable variable.
The second law of friction has not been so widely explored as the first, but it is well known for most materials, with some reservations the same is true of friction clutch plates and the materials bonded to them. The new high-energy friction materials used currently to line the friction clutch plates and torque convertor clutches provide smoother shifts, increased torque capacity, reduced torque drag and long-term durability. In addition to the high-performance fiber compounds now being utilized on friction clutch plates a great deal of engineering and research has gone into cutting groove patterns that will control the fluid flow dynamics at the clutch surface, on apply and release of the clutch pack. The design and placement of these grooves are designed to address three needs:
- Adequate lubrication.
- Reduce clutch interface temperatures.
- Reduce drag torque.
This carefully engineered friction clutch plate will create the balance between oil flow, groove design and clutch clearance that is necessary to keep the friction and steel plates separated while disengaged, yet not allow so much oil to flow between the plates that the parasitic drag has a negative effect on fuel efficiency.
The third law of friction is not as well understood as the first two. It is a well know observed phenomenon that the frictional force needed to initiate sliding is usually greater than that necessary to maintain it, meaning that the coefficient of static friction is greater than the coefficient of dynamic friction. But once sliding is initiated the coefficient of friction can be nearly independent of sliding velocity, especially at higher sliding speeds. The problems only start when sliding speeds become very low and the phenomenon known as “stick slip” or shudder becomes a problem of varying degree heavily influenced by oil and additives. These additive technologies are designed to slide properly at very low speeds without compromising the torque carrying capacity of the fluid. The breakdown of the lubricant film with decreasing film thickness results in a sharp rise in friction as the fluid quantity falls off. The mechanism of these effects and film breakdown are the subject of much research these days with thermal effects playing a big role under these extreme conditions. At extremely high loading or low sliding speeds when film thickness drops, increasingly severe surface damage (scuffing or glazing) occurs, and the behavior of the event depends critically on the properties of boundary lubricant additives present or not present in the transmission fluid.
When you have friction clutch plates faced with high-performance fiber compounds sliding on steel reaction plates under very high contact pressures, or a very low sliding speeds (like in torque convertors) hydrodynamic forces are insufficient to maintain even a thin EHL (elasto hydrodynamic lubrication) film between the sliding surfaces, and direct contact will occur between the friction clutch plate and the steel reaction plate or stator (if it’s a torque converter). High friction occurs, that glazes friction fiber plates and accelerates wear on steel surfaces unless the surfaces are protected by a suitable boundary lubricant additive. Boundary lubricants work by forming adsorbed molecular films (adsorption is the adhesion of molecules from a liquid or dissolved solid to a surface) on the surface of the plates causing repulsive forces between them to carry much of the load. Damaging contact between the fiber clutch plate and the steel reaction surface is prevented or limited.
Many oils naturally contain long chain hydrocarbon molecules with boundary lubricating properties; for example: vegetable oils like castor oil and rapeseed oil, the original friction modifier “sperm whale oil”, all contained more natural boundary lubricants than mineral oils. The boundary lubricating action of an oil is often associated with the terms’ oiliness and lubricity. In practice, boundary lubricant additives are incorporated into the transmission fluid based on the performance and engineering requirements of the transmission and clutch plates. Complete coverage of a steel or fiber surface by a layer of boundary lubricant only one or two molecules thick can be sufficient for protection and friction modification. Boundary lubricants are generally effective in transmission fluids at very low overall concentrations (0.5% -2.0% is typical).
Another type of molecule with boundary lubricating properties are the “extreme pressure” (EP) and antiwear additives. These additives react with the sliding surfaces of the clutch plates under severe conditions and high load in the contact zone allowing compounds with low shear strength to form a lubricating film right where it is needed. For many years it was thought the high pressures in the contact zone played a part in the way these additives worked, hence the name “extreme pressure”. Current testing has concluded that the transient high temperatures associated with sliding on a thin EHL film, and the composition of the friction fiber plate material, are more important.
Additives of this type usually contain sulfur, phosphorus or (less frequently these days) chlorine. They function by adsorbing on the surface of the steel reaction plates and friction clutch plates then reacting in the contact zone to form local sulfide or phosphide films which are easily sheared as the clutch plates slide to a stop (it is the “sacrificial” depletion of these additives over time that result in additive depletion). Probably the most widely used EP additives are zinc (ZDDP), Phosphorus and Sulfur compounds. Modern transmission fluid no longer contains zinc (zinc compounds form undesirable abrasive ash deposits as they oxidize) in general, with phosphorus, and sulfur-based additives as the most common EP additives found in automatic transmission fluid today.
Frictional properties and extreme pressure protection are not the only thing we look at when we select a fluid these days, but it often is at the heart of performance problems we encounter every day. There is no replacement for good fully formulated ATF designed for the transmission technology of yesterday and today serviced at intervals appropriate to duty cycle.
We hear so many comments these days about cutting-edge programming, world-class engineering and improved manufacturing processes eliminating the traditional problems we in the business have experienced, lived with, and corrected through the years. Unless you accept the fact that properly formulated lubricants (changed at regular service intervals based on how the vehicle is used) are critical to providing fluid-based solutions to automatic transmission problems like T/C shudder, frictional stability and long-term reliability, these types of problems will never go away. Quality hard parts, proper programming, and high-quality ATF with the required performance additives are the tenets of long-term success when building or servicing modern and older transmissions. Transmission fluid choices will be one of the few things the service technician or builder will have control of that will help determine the success or failure of their business in the future.
