During the past few years, three factors have caused us to reexamine the accepted theories and generalizations regarding lubrication.
First, it has been established that wear and friction are caused principally by sliding contact. Wear and friction in rolling contact are so minimal that they are difficult to measure, and therefore, can be neglected in determining the performance of a machine.
Second, a class of polymers called matched molecular chemistry (MMC) has been discovered. These polymers form conjugate pairs and are attracted to the bearing surfaces themselves, resulting in much thicker boundary layers than those produced by viscous forces of conventional lubricants.
Third, a squeeze-out mathematical model has been devised which more accurately describes the thickness of boundary layers in low-speed, sliding contact than that derived by the generally accepted model of elastohydrodynamic (EHD) theory of lubrication.
Elastohydrodynamic Theory
Making little or no distinction between rolling and sliding contact, the EHD theory postulates that the bearing surfaces, under enormous pressure, deflect locally to form a pool of lubricant at the interface. The liquid, because of the higher pressure-viscosity coefficient, becomes almost a solid and, in some treatments of the theory, becomes a glass-like structure before returning to a fluid state once the pressure of contact is released. It is concluded that the asperities never touch and wear does not take place; wear is reduced to a nonentity. From this perspective, it is easy to believe that an increase in performance can result only from a reduction in the viscosity of the lubricant.
Squeeze-out Model
The squeeze-out model describes the behavior of the bearing surfaces in sliding contact. Most of the lubricant is squeezed out, leaving only a mono-molecular layer of lubricant between the surfaces (boundary layer) and small pools of lubricant in local recesses created by the irregular profiles of the bearing surfaces. This theory, derived from the Navier-Stokes equation, is in agreement with test data. It is clearly a better explanation of why the addition of a thicker, multimolecular boundary layer has such a profound effect on the friction and wear of sliding contact as demonstrated in a variety of tests and real-world applications.
Recently, a scientist at Akron University has described the molecular structure of MMC and has proposed a mathematical model of its behavior. Accordingly, the boundary layer using MMC is as much as four or five molecular diameters in thickness as opposed to the mono-molecular thicknesses using conventional lubricants. Replacing one molecule with four or more molecules with larger diameters makes a significant difference in the separation of asperities, reducing the number and severity of collisions by two to three orders of magnitude. This strategy accounts for the reduction of friction by ten percent or more instead of the one percent everyone is looking for.
Additives Combat Wear
The present method of dealing with wear in conventional lubrication, in spite of the nonwear benefits suggested by the EHD theory, is to add four different additive agents: a friction reducer, an antiwear agent, an extreme pressure (EP) agent and zinc dialkyl dithiophosphate (ZDDP). This is a phosphate coating that prevents metal-to-metal contact. Each of these, except perhaps the friction reducer, forms a chemical bond with the bearing surfaces. As they wear down, they produce precipitants and sludge - not ideal conditions in industrial applications.
Heavy industry has caused us to reexamine the theory of EDH lubrication. The problems of lubrication in a steel mill are so enormous that they are almost beyond belief. Equipment designed to last for years is failing prematurely. Maintenance has been reduced to fixing equipment that breaks, and accepts this as a cost of doing business. Some of these problems have existed since the plants opened.
An example of the EHD theory of lubrication is difficult to find here. The rates of wear experienced here could come only from boundary lubrication, with some of it approaching accelerated wear (scuffing). The standard antiwear agents in the lubricants in the cases that were examined were insufficient to establish an acceptable rate of wear. A thicker boundary layer was needed, without increasing the bulk viscosity of the lubricant. By adding the MMC, the rate of attrition was reduced, allowing the bearing surfaces to wear in more slowly. This permitted smaller particles to wear during break-in, providing a smoother bearing surface capable of being supported by a film of lubricant.
Contact
Rolling contact is always accompanied by some degree of sliding contact. Gear drives are in rolling contact at the pitch line only because of backlash. The rest of the contact is accompanied by sliding contact. Roller bearings are subject to sliding contact as well. For the rolls to roll, there must be traction. Traction can be generated by the traction of the lubricant or by a degree of sliding contact between rolls and races. These sliding contacts can easily be magnified by misalignments, bearing tolerances and variations in manufacturing tolerances, causing the points or lines of contact to have sliding transverse components of motion as well.
This is especially true in industry where the effects of scale become paramount. On a smaller scale, a roller bearing might be called frictionless. On a larger scale, it might be a wasteland. In the applications of MMC tried in heavy industry, friction has been reduced by as much as 25 percent and wear reduced by two to three orders of magnitude.
Consider this example: A gear set that lasts one day versus a gear set that lasts six months or longer.
180 to 1; 2 orders=100X; 3 orders=1,000X
This was first discovered in lab tests. For example:
7 x 10-7mm3/Nm vs. 9 x 10-9mm3/Nm
Cold Rolling Mill
The concept was recently demonstrated in a cold rolling mill application. A 5,000-horsepower motor was being used to drive the rolls through a gear-driven spindle lubricated with grease. The requirement that the spindle be able to reverse complicated its ability to be lubricated. Using conventional high-performance grease, the mill was able to run at only 80 percent capacity with the aid of water on the spindle to keep it cool. By replacing the conventional grease with one containing MMC, the spindle was able to transmit 100 percent load without external cooling. Spindle life was thereby increased from a few months to an indefinite life (possibly years) and the horsepower loss to friction was reduced by approximately 25 percent.
References
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P. Rountree, J. Luthern. Living Nano-bearings Utilizing Matched Molecular Chemistry: an Alternate Approach to the Lubrication Problem, in preparation.
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S. Zilberman. Boundary Lubrication: Dynamics of Squeeze-out, in preparation.
