I was recently asked, “Why are lubricants so different? Why can’t we simply buy a ‘one fits all’ oil and grease?” I explained that the old farmers axiom, “Oil is oil, any oil will do,” may have had some merit 100 years ago, but it doesn’t now. In today’s world of sophisticated machinery and the never-ending demand for asset reliability and availability, lubricants have evolved into multi-function, engineered products. They’re as much a part of a machine’s design as its electrical or mechanical systems. Choosing the correct lubricant has become an important and informed decision.
Whether in the form of a liquid, solid or gas, modern lubricants truly are pure liquid engineering. Through the blending of additives into different base stocks, lubricants can be designed to perform up to eight simultaneous functions, which allow them to operate in a host of differing environments and working conditions.
A LUBRICANT’S JOB
Webster’s Dictionary describes a lubricant as “a substance (e.g., oil, grease, or soap) which, when introduced between solid surfaces that move over one another, reduces resistance to movement, heat production, and wear by forming a fluid film between the surfaces.”
In short, the job of a lubricant is to separate, control and minimize the sacrificial and harmful effects of moving surfaces passing over one another under load, and at speed. It does this in the following eight definitive ways:
FUNCTION 1: Control and Minimize Friction
The primary function of every 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. If no lubricant is present to separate the moving surfaces from each another, the surfaces will quickly heat up to a temperature where they can weld or lock together, resulting in a mechanical “seize.” If, enough power is available, the two surfaces can be “torn” apart, resulting in surface degradation and wear debris. Movement will continue until, eventually, the surfaces weld together (seize) permanently, causing the power source to stall and the machine to catastrophically fail.
The introduction of a lubricating film between the two wear surfaces is intended to create a full fluid film barrier (with a thickness of approximately 5 microns), that’s designed to separate and prevent surface-to-surface contact. Although a small amount of fluid friction is generated within the lubricant film, the energy required to move the surfaces over one another is but a small fraction of that required to overcome the surface-to-surface friction developed in the absence of a lubricant.
FUNCTION 2: Control and Minimize Wear
Note that a full lubricant film may not always be possible, and some metal-to-metal contact may occur under slow-moving, oscillating, heavy load, and lubricant-loss conditions. For those reasons, anti-wear additives that act as chemical “softening” agents on the metal surfaces can be added to a lubricant.
The lubricant chemistry is designed to coat metal surfaces with soft layers of metallic salts that include sulfides and phosphate additives. As the surfaces slide over one another, alternating load cycles can cause the softened high points (asperities) on each surface to collide with one another when film thickness is reduced. If the unit loading exceeds the sulfur phosphide film, a rupture can occur, and lead to a small metal-to-metal contact area. Localized heat then builds up, causing the two surfaces at the high point to “weld and break” that result in a micro-metal-particulate break, or asperity release, into the lubricant film.
Many lubricants are designed to control wear by promoting micro-surface degradation that allows asperity “tips” to be sacrificed easily without “tearing” the parent metal. This controlled sacrificial process is designed to occur during the machine or bearing’s initial “break in” period, thereby minimizing any continued surface wear under varying lubricant-film conditions.
FUNCTION 3: Control and Minimize Heat
When friction and wear levels are controlled and minimized, the amount of frictional heat produced is significantly reduced. Excessive heat can “cook” most lubricants and cause them to oxidize and become less effective. To combat this, antioxidant additives are added to the lubricant base stock.
Recirculating oil-system and air/oil system designs take advantage of a lubricant’s ability to absorb and transfer localized heat buildup at a bearing load point and, therefore, prevent any thermal runaway at the bearing surfaces. To facilitate the heat transfer/cooling process in recirculating systems, the oil may be pumped through a heat exchange unit (oil cooler), and/or reservoir baffle system upon return to the reservoir.
FUNCTION 4: Control and Minimize Contamination
As described above (in Function 2), a lubricant can become contaminated when wear asperities are introduced into the lubricant. Other forms of contamination, such as silica (dirt), and water can be introduced through the reservoir filling process (when proper storage, transfer, and cleanliness practices aren’t followed), or enter the system when seals become compromised.
To combat solids contamination, a combination of detergent and dispersant additives can be added to the base oil. Detergents ensure that hot metal surfaces stay clean while neutralizing any acids that form within the oil; dispersants help keep particulates and engine-soot colloidal suspended in the lubricant ready to be extracted under pressure by an in-line system oil filter. Care must be taken to ensure oil filters are changed regularly, however. Regular filter changes prevent a contaminated lubricant from acting as a “lapping” paste, which can accelerate the wear process in bearing areas.
In the case of water contamination, emulsifier additives are added to allow the moisture to coalesce, separate from the oil, and settle in the reservoir. This allows easy drainage.
Lubricants are also used to seal out contamination ingress around shafts. A good example of this can be seen in a labyrinth seal, which uses the lubricant to fill a series of annular grooves cut into the non-moving shaft housing, thereby acting as a live shaft seal.
FUNCTION 5: Control and Minimize Corrosion
While oxygen may be a basic human life force, it can be fatal to a lubricant. When present in lubricants, oxygen acts as catalyst to combine certain metals and organics that generate corrosive acids harmful to the bearing surfaces. If the wear surfaces are ferrite (iron) based, the acids will attack the metal and form rust on the bearing surface.
Most lubricants are designed to “cling” to the metal surfaces and prevent moisture and oxygen from reacting with the surface. But all lubricants aren’t formulated the same way. Consequently, if bearing surfaces are iron-based, a lubricant with anti-corrosive additives must be employed to neutralize the corrosive acids and form a protective skin on the metal surfaces.
FUNCTION 6: Control and Minimize Shock
Most RAM professionals will be familiar with the quieting effect of adding a lubricant to a gear train. In such a case, the lubricant acts as a hydraulic shock absorber between mating gears as they mesh. Poorly lubricated meshing gears set up shock waves as they start to mesh, resulting in a “chattering” sound that can fracture the gear teeth.
The same issue can occur in the cars and trucks we drive: In fact, the word “shock absorber is synonymous with automobile suspension systems that employ hydraulic oil to dampen and absorb the effects of road shock on a 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 system components, including pistons and accumulators, among others. 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 the asset is using the least amount of energy where moving parts are concerned.
In studies conducted on behalf of various electric-power companies, effective use of lubricants, delivery systems, and lubrication methods resulted in an energy reduction of 7.5% was achieved when a synthetic-lubricant product replaced a standard compressor oil; and a 17.92% energy reduction was achieved on a stamping press when the automated-oil-delivery system was “tuned” and a more appropriate oil chosen.
FINAL WORD
Lubricants are much more than oil and grease. They’re complex, engineered products that can be tailored to meet many different working situations and ensure asset reliability and long lifecycles. If you haven’t done so in the last two to three years, make sure your plant’s equipment assets are getting the best protection lubricants can provide: Work with a professional lubrication consultant to perform a lubrication effectiveness/consolidation audit.TRR
ABOUT THE AUTHOR
Ken Bannister has 40+ years of experience in the RAM industry. For the past 30, he’s been a Managing Partner and Principal Asset Management Consultant with Engtech industries Inc., where he has specialized in helping clients implement best-practice asset-management programs worldwide. A founding member and past director of the Plant Engineering and Maintenance Association of Canada, he is the author of several books, including three on lubrication, one on predictive maintenance, and one on energy reduction strategies, and is currently writing one on planning and scheduling. Contact him directly at 519-469-9173 or [email protected].
Tags: reliability, availability, maintenance, RAM, lubricants, lubrication, oil, grease, hydraulic fluid, base stocks, additives, friction, contamination, corrosion, power transmission, energy consumption, bearings, gears
