The old adage “oil is oil, so any old oil will do!” may have had merit a hundred years ago, but in today’s world of sophisticated machinery and demand for asset reliability, choosing the correct lubricant is now an important and informed decision. Whether in the form of a liquid, solid or gas, modern lubricants are pure liquid engineering. Through the blending of additives into a variety of base stocks, they can be designed to perform up to eight functions simultaneously in a host of different environments.
Webster’s Dictionary defines a lubricant as “a substance (e.g. oil, grease or soap) that when introduced between solid surfaces which move over one another reduces resistance to movement, heat production and wear (i.e. friction and its effects) by forming a fluid film between the surfaces.”
Essentially, a lubricant’s job is to control and minimize the sacrificial harmful effects of moving surfaces passing over one another under load and at speed. It does this in eight ways.
Function 1: Control and minimize friction
The primary function of any lubricant is to control and minimize the effects of friction.
When two solid surfaces passing over one another are allowed to come into contact under load, they rub together and produce dry friction, requiring considerable energy to keep the surfaces moving. With no lubricant to separate the moving surfaces from one another, surfaces quickly degrade and can weld or lock together resulting in a “seize.” The indiscriminate sacrifice of wear surfaces produces rapid wear and loss of energy to heat, resulting in poor performance, reduced reliability and increased energy use.
The introduction of a lubricating film between the two wear surfaces creates a fluid barrier that prevents surface contact. Although a small amount of fluid friction is still present in the film, the energy required to move the surfaces over one another is but a small fraction of that required to overcome surface-to-surface dry friction.
Function 2: Control and minimize wear
Knowing that a full lubricant film may not always be possible and that some metal-to-metal contact may occur under slow-moving, heavy-load, lubricant-loss conditions, additives that act as chemical “softening” agents on the metal surfaces can be blended into the lubricant.
The lubricant coats the two surfaces with soft layers of metallic salts (sulfides and phosphate additives). As they slide over one another, alternating load cycles can cause the softened high points (asperities) on each surface to collide with one another due to reduced film thickness. When the unit loading exceeds the sulfur-phosphide film, a rupture occurs, creating a small area of metal-to-metal contact. Localized heat builds up, causing the two surfaces to weld and break, which leads to a small metal particulate or asperity release into the lubricant film.
Many lubricants are designed to control wear by promoting minute surface degradation to allow asperity “tips” to be sacrificed easily without "tearing” the parent metal, thereby minimizing surface wear under varying lubricant-film conditions.
Function 3: Control and minimize heat
Whenever friction and wear levels are controlled and minimized, the amount of heat is also reduced. Excessive heat can “cook” the lubricant and cause it to oxidize, rendering it less effective; to combat this, an anti-oxidant additive is added to the lubricant base stock.
Recirculating oil- and air/oil-system designs take advantage of a lubricant’s ability to transfer localized heat buildup at a bearing load point and prevent any thermal runaway at the bearing surfaces. To facilitate the heat transfer/cooling process, the oil may be pumped through a heat-exchange unit (oil cooler) and/or reservoir baffle system.
Function 4: Control and minimize contamination
As described above, a lubricant can become contaminated when wear asperities are introduced into it. Other forms of contamination, such as silica (dirt), can be introduced through the reservoir-filling process when proper storage, transfer and cleanliness practices are not observed, or through compromised sealing systems.
To combat contaminant solids, lubricant additives can be used to coagulate particulate matter, making them heavy enough to “drop out” into the sump. Other additives can attach to asperities and stay colloidal, suspended in the lubricant so they can be extracted under pressure by an in-line system oil filter. Failure to refresh oil filters on a regular basis will cause the contaminated lubricant to act as a “lapping” paste and accelerate the wear process in bearing areas.
In the case of water or glycol contamination, additives are added to facilitate release of moisture in the sump or filter. These additive types are more prevalent in automotive oils.
Lubricants can also act to seal out contamination ingress around shafts. This is the case with a labyrinth type of seal that depends on grease to fill up a series of annular grooves cut into a non-moving shaft housing designed to act as a live shaft seal.
Function 5: Control and minimize corrosion
Oxygen may be a basic human life force, but it is a mortal enemy of lubricants. When present, it acts as catalyst to combine certain metals and organics that generate corrosive acids harmful to the bearing surfaces. If the wear surfaces are ferritic (iron-based), the acids attack the metal and form rust on the bearing surface.
A lubricant is designed to cling to the metal surfaces and prevent moisture and oxygen from reacting with the surface. Given the fact that not all lubricants are created equal, if the bearing surfaces are iron-based, a lubricant with anti-corrosive additives must be employed.
Function 6: Control and minimize shock
Readers of this magazine are no doubt familiar with the quieting effect from adding lubricant to a gear train—wherein a lubricant acts as a hydraulic shock absorber between mating gears as they mesh. When they are poorly lubricated, those gears set up shock waves as they start to mesh, resulting in a “chattering” sound that can fracture the gear teeth.
FYI: The very phrase “shock absorber” is synonymous with automobile suspension systems that employ hydraulic oil to dampen and absorb the effects of road shock on the vehicle.
Function 7: Control and transmit power
In a typical hydraulic system, oil is used to transmit force and motion from a single source (usually a pump) into multiple sources, pistons, accumulators, etc.
Hydraulic oil is also used to transmit power in soft-start devices such as fluid couplings, automatic transmissions and torque converters.
Function 8: Control and minimize energy consumption
Effective lubrication practice dictates use of the Right lubricant, in the Right place, at the Right time, in the Right amount, using the Right method. Doing so will ensure that the lubricated equipment is using the least amount of energy in terms of moving parts.
In studies conducted on behalf of various electric power companies,* effective use of lubricants, delivery systems and methods were shown to significantly reduce energy consumption of lubricated equipment: For example, an energy reduction of 7.3% was documented when a synthetic replaced a standard compressor oil, and a reduction of 17.92% was achieved on a stamping press when the automated oil delivery system was “tuned” and a more appropriate oil was chosen.
The fluid film
To combat friction and wear successfully, a lubricant film must be present at all times between the mating bearing surfaces. The degree of protection—and subsequent bearing surface life—is directly related to the lubricant’s working film thickness, load, speed and lubricant viscosity or “stiffness” (to be discussed in a later installment). The minimum working film thickness required to achieve full surface separation is also known as the lamda l thickness ratio.
Because the degree of surface separation is dependent on the surface “roughness” (Ra), it must be determined by measuring the profile (peaks and valleys of the surface) of both mating surfaces and by defining a centerline through them so that the areas above and below the centerline are equal. The lamda ratio is then defined as the ratio of lubricating film thickness to surface roughness, which is a lubricant film thicker than the combined height of both surface asperities enough to completely separate both surfaces.
Figure 1 shows the lamda l ratio thickness curve that depicts the relationship between the working film thickness and the resulting life expectancy of the lubricated component. Note that once the lamda ratio is thicker than four times, life expectancy remains constant. The figure also references the different film types—or stages—known as Boundary Layer, Mixed Film and Hydrodynamic Film. These important film types will be discussed relative to the different types of wear conditions in the next installment of this series.
Today, there are three lubrication certifying bodies: STLE (Society of Tribologists and Lubrication Engineers); ICML (International Council of Machinery Lubrication); and ISO (International Organization for Standardization).
Originally designed for engineers, STLE's Certified Lubrication Specialist (CLS) program has been offered since 1993.
ICML now offers two certifications for “hands-on” lubrication practitioners: the MLT (Machine Lubrication Technician) and MLA (Machine Lubrication Analyst) designations.
A relative newcomer, ISO’s lubrication certification program has adopted the ICML model (and collaborated with that organization to use its domain of knowledge). Participants who attend the requisite preparatory formal training associated with ICML certification are also eligible to take corresponding ISO exams (upon payment of the appropriate examination fees).
Of these three programs, ICML’s (currently offered in nine languages) has issued the most certifications around the world. For more information on this program, visit: www.lubecouncil.org.