Friction Reducing Technologies – Fact or “Friction”
Abstract
By John Schlobohm, Senior Global Technical Advisor – bp/Castrol
Do anti-friction additives actually enhance lubricant performance? Or is that just a myth circulated by the oil companies? This paper will explore the efforts of producers to enhance lubricants through the use of general additive systems and advanced technologies. It will examine additive technologies that reduce generated heat, increase equipment life, save energy, and reduce overall maintenance costs.
In a discussion of both non-solid and solid lubricant additives, this paper will examine how specifically engineered additives work to not only provide anti-friction protection, but also “repair” damaged gears and bearings.
Introduction
Tribology, the study of friction and sliding surfaces, is a relatively new scientific and engineering discipline. Yet the goals of Tribology – to reduce friction, to improve the design of bearings and other mechanisms, to extend the life of parts, and to develop superior lubricants – have challenged industry since making tools and using machines began.
Lubrication has three goals:
Reduce friction
Carry away heat
Reduce wear
The friction between working surfaces, a rotating shaft and journal for example, can be significantly reduced by separating those surfaces with a film of oil.
Asperities
Even the finest machining leaves a surface of peaks and valleys on bearings and gear teeth. Examined under a microscope, the working surface of a bearing takes on the rugged appearance of a lunar landscape (figure 1). Metallurgists and other specialists call these peaks and valleys asperities.
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Figure 1 Bearing rolling surface as seen under a microscope |
When a film of conventional lubricant is ruptured by heavy shock loads, contacting surfaces cannot be separated and opposing asperities tend to interlock or “cold weld.” As a result, the peaks fracture repeatedly under the forces of equipment operation, and an irreversible sequence of destructive wear begins.
When asperities interlock, energy is required to overcome the resistance. Besides squandering energy, uncontrolled friction generates excessive heat and wear. For the maintenance department, the bottom line is often premature parts failure, costly unscheduled downtime, frequent or expensive repairs and needlessly high energy bills. In fact, according to many authorities, over 1/3 of the world’s energy production is consumed in overcoming friction.
Despite its polished appearance, a microscope reveals pits, valleys and jagged peaks on even a finely machined bearing. Actual contact between working surfaces occurs only where opposing peaks touch. The contact area may be as little as 1/1,000 the apparent surface area (Figure 2).
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Figure 2 Typical bearing or gear work surface |
There are some advanced technology lubricants that offer a different dimension of friction-fighting capabilities for improved equipment performance.
Fluid Film
Although less than a few thousandths-of-an-inch thick, a layer of oil is sometimes called a thick film because its depth is several times greater than the height of the asperities on opposing surfaces. The combination of journal speed and lubricant viscosity tends to form a “wedge” of oil under the load. At moderate speeds and loads, the journal lifts up on this wedge and literally “floats.” Lubrication engineers call this “fluid film” or “hydrodynamic lubrication.”
Mixed Film
As loads and temperatures increase or as steady loads become shock loads or as operating speeds diminish, the lubricating oil film becomes thinner and thinner. Eventually, some asperities penetrate the oil film and come into contact with each other some of the time. This is known as “mixed film lubrication” (Figure 3).
Elastohydrodynamic Lubrication
Elastohydrodynamic lubrication occurs when concentrated loads produce high local pressure in the contact region. This will elastically deform the surfaces in an amount that can approach, and even exceed, the hydrodynamically generated lubrication film thickness. At the same time, due to the high local pressures, the lubricant viscosity increases many times over its normal value. The combustion of these two events result in the formation of a fluid film with greater load capacity and a shape that prevents asperity interaction between the surfaces. Several oil companies have propriety anti-wear and extreme-pressure additive technologies that synergistically work with the elastohydrodynamic process to impart increased surface smoothness, durability and protection against wear (Figure 3).
Boundary Lubrication
Conditions of sustained metal-to-metal contact are known as boundary conditions or “boundary lubrication.” Under conditions of ongoing and destructive metal-to-metal contact, friction is excessive. The abrasive wear particles that are formed can contaminate the oil and can rapidly deteriorate working surfaces, leading to parts failure and unscheduled down time for repairs.
However, the cost of parts, repair labor, and production losses can be controlled by specifying antiwear, EP (extreme pressure) or solid film high performance lubricants for machines operating under boundary conditions (Figure 3).
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