A lubricant (colloquially, lube) is a substance (often a liquid) introduced between two moving surfaces to reduce friction and wear between them. A lubricant provides a protective film that allows for two touching surfaces to be separated, thus lessening the friction between them. It also protects against corrosion and carries contaminants away.
Lubrication occurs when opposing surfaces are separated by a lubricant film. The applied load is carried by pressure generated within the fluid, and frictional resistance to motion arises entirely from shearing of the viscous fluid. The science of friction, lubrication, and wear is called tribology. The science of lubrication really took off with the Industrial Revolution in the nineteenth century.
Lubrication is required for correct operation of engines and many other mechanical systems where parts must slide over each other without seizing. For example, one of the single largest applications for lubricants, in the form of motor oil, is to protect the internal combustion engines in motor vehicles and powered equipment. Lubricants are also available for various other purposes, including for personal use and biomedical applications, such as for artificial joints.
Types and composition of lubricants
Typically, lubricants contain 90 percent base oil (most often petroleum fractions, called mineral oils) and less than 10 percent additives. Vegetable oils or synthetic liquid—such as hydrogenated polyolefins, esters, silicone, and fluorocarbons—are sometimes used as base oils. Additives deliver reduced friction and wear, increased viscosity, resistance to corrosion and oxidation, aging or contamination, and so forth.
Non-liquid lubricants include greases, powders (such as dry graphite, PTFE, and molybdenum disulfide), teflon tape used in plumbing, air cushions and others. Another approach to reducing friction and wear is to use bearings such as ball bearings, roller bearings, or air bearings, which in turn require internal lubrication themselves, or to use sound, in the case of acoustic lubrication.
Lubricants such as 2-cycle oil are added to some fuels. Sulfur impurities in fuels also provide some lubrication properties, which has to be taken into account when switching to a low-sulfur diesel. Biodiesel is a popular diesel fuel additive providing additional lubricity.
Functions of lubricants
Lubrication is required for correct operation of engines and many other mechanical systems where parts must slide over each other without seizing.
Lubricants perform the following key functions:
- Keep moving parts apart
- Reduce friction
- Transfer heat
- Carry away contaminants and debris
- Transmit power
- Protect against wear
- Prevent corrosion
Keep moving parts apart
Lubricants are typically used to separate moving parts in a system. This has the benefit of reducing friction and surface fatigue together with reduced heat generation, operating noise and vibrations. Lubricants achieve this by several ways. The most common is by forming a physical barrier—that is, a thick layer of lubricant separates the moving parts. This is termed hydrodynamic lubrication. In cases of high surface pressures or temperatures the fluid film is much thinner and some of the forces are transmitted between the surfaces through the lubricant. This is termed elasto-hydrodynamic lubrication.
Typically, the lubricant-to-surface friction is much less than surface-to-surface friction in a system without any lubrication. Thus use of a lubricant reduces the overall system friction. Reduced friction has the benefit of reducing heat generation and reduced formation of wear particles as well as improved efficiency. Lubricants may contain additives known as friction modifiers that chemically bind to metal surfaces to reduce surface friction even when there is insufficient bulk lubricant present for hydrodynamic lubrication, such as for protecting the valve train in a car engine at startup.
Both gas and liquid lubricants can transfer heat. However, liquid lubricants are much more effective on account of their high specific heat capacity. Typically, the liquid lubricant is constantly circulated to and from a cooler part of the system, although lubricants may be used to warm as well as to cool when a regulated temperature is required. This circulating flow also determines the amount of heat that is carried away in any given unit of time. High flow systems can carry away a lot of heat and have the additional benefit of reducing the thermal stress on the lubricant. Thus lower cost liquid lubricants may be used.
The primary drawback is that high flows typically require larger sumps and bigger cooling units. A secondary drawback is that a high flow system that relies on the flow rate to protect the lubricant from thermal stress is susceptible to catastrophic failure during sudden system shut downs. An automotive oil-cooled turbocharger is a typical example. Turbochargers get red hot during operation and the oil that is cooling them only survives as its residence time in the system is very short, that is, high flow rate.
If the system is shut down suddenly (pulling into a service area after a high speed drive and stopping the engine) the oil that is in the turbo charger immediately oxidizes and will clog the oil ways with deposits. Over time these deposits can completely block the oil ways, reducing the cooling with the result that the turbo charger experiences total failure typically with seized bearings. Non-flowing lubricants such as greases and pastes are not effective for heat transfer although they do contribute by reducing the generation of heat in the first place.
Carry away contaminants and debris
Lubricant circulation systems have the benefit of carrying away internally generated debris and external contaminants that get introduced into the system to a filter where they can be removed. Lubricants for machines that regularly generate debris or contaminants such as automotive engines typically contain detergent and dispersant additives to assist in debris and contaminant transport to the filter and removal. Over time the filter will get clogged and require cleaning or replacement, hence the recommendation to change a car's oil filter at the same time as changing the oil. In closed systems such as gear boxes, the filter may be supplemented by a magnet to attract any iron filings that are created.
It is apparent that in a circulatory system, the oil will be only as clean as the filter can make it. It is thus unfortunate that there are no industry standards by which consumers can readily assess the filtering ability of various automotive filters. Poor filtration significantly reduces the life of the machine (engine) as well as making the system inefficient.
Pascal's law is at the heart of hydrostatic power transmission. Hydraulic fluids comprise a large portion of all lubricants produced in the world.
Protect against wear
Lubricants prevent wear by keeping the moving parts apart. Lubricants may also contain anti-wear or extreme pressure additives to bolster their performance against wear and fatigue.
Quality lubricants are typically formulated with additives that form chemical bonds with surfaces to prevent corrosion and rust.
Liquid lubricants may be characterized in many different ways. One of the most common ways is by the type of base oil used. Following are the most common types.
- Lanolin (wool grease, natural water repellant)
- Mineral oils
- Vegetable (natural oil)
- Synthetic oils
Note that although generally lubricants are based on one type of base oil or another, it is quite possible to use mixtures of the base oils to meet performance requirements.
A natural, water-repellant lanolin is derived from wool grease and is a safe alternative to the more common petrochemical based lubricants (such as WD-40).
Water can be used on its own or as a major component in combination with one of the other base oils.
This term is used to encompass lubricating base oil derived from crude oil. API designates several types of lubricant base oil identified as:
- Group I - Saturates < 90 percent and/or Sulfur >0.03 percent and Viscosity Index >= 80 to <120
- Manufactured by solvent extraction, solvent or catalytic dewaxing, and hydro-finishing processes. Common Gr I base oil are 150SN (solvent neutral), 500SN, and 150BS (brightstok)
- Group II – Saturates >= 90 percent and Sulfur <=0.03 percent and Viscosity Index >= 80 to <120
- Manufactured by hydrocracking and solvent or catalytic dewaxing processes. Gr II base oil has superior anti-oxidation properties since virtually all hydrocarbon molecules are saturated. It has water-white color.
- Group III – Saturates >= 90 percent Sulfur <=0.03% and Viscosity Index >= 120
- Manufactured by special processes such as isohydromerization. Can be manufactured from base oil or slax wax from dewaxing process.
- Group IV – Poly alpha olefins (PAO)
- Group V – All others not included above
Such as naphthenics, PAG, and esters.
The lubricant industry commonly extends this group terminology to include:
- Group I+ with a Viscosity Index of 103 - 108
- Group II+ with a Viscosity Index of 113 - 119
- Group III+ with a Viscosity Index of >= 140
Can also be classified into three categories depending on the prevailing compositions: - Paraffinic - Naphthenic - Aromatic
Vegetable (natural) oils
These are primarily triglyceride esters derived from plants and animals. For lubricant base oil use the vegetable derived materials are preferred. Common ones include high oleic canola oil, palm oil, sunflower seed oil and rapeseed oil from vegetable and Tall oil from animal sources. Many vegetable oils are often hydrolyzed to yield the acids which are subsequently combined selectively to form specialist synthetic esters.
- Polyalpha-olefin (PAO)
- Synthetic esters
- Polyalkylene glycols (PAG)
- Phosphate esters
- Alkylated naphthalenes (AN)
- Silicate esters
- Ionic fluids
Note: Group III base stocks may be designated as synthetic oil.
Teflon or PTFE
Teflon or PTFE is typically used as a coating layer on, for example, cooking utensils to provide a non-stick surface.
Graphite, hexagonal boron nitride, and molybdenum disulfide are examples of materials that can be used as solid lubricants often to very high temperatures. The use of such materials is still restricted by their poor resistance to oxidation. For example, molybdenum disulfide can only be used up to 350 C in air, but 1,100 C in reducing environments.
Other relevant phenomena
'Glaze' formation (high temperature wear)
A further phenomenon that has undergone investigation in relation to high temperature wear prevention and lubrication, is that of 'glaze' formation. This is the generation of a compacted oxide layer that sinters together to form a crystalline 'glaze' (not the amorphous layer seen in pottery) generally at high temperatures, from metallic surfaces sliding against each other (or a metallic surface against a ceramic surface). Due to the elimination of metallic contact and adhesion by the generation of oxide, friction and wear are reduced. Effectively, such a surface is self-lubricating.
As the 'glaze' is already an oxide, it can survive to very high temperatures in air or oxidizing environments. However, it is disadvantaged by it being necessary for the base metal (or ceramic) having to undergo some wear first to generate sufficient oxide debris.
A large number of additives are used to impart performance characteristics to the lubricants. The main families of additives are:
- Metal deactivators
- Corrosion inhibitors, Rust inhibitors
- Friction modifiers
- Extreme Pressure
- Anti-foaming agents
- Viscosity index improvers
- Stickiness improver, provide adhesive property towards tool surface (in metalworking)
- Complexing agent (in case of greases)
Note that many of the basic chemical compounds used as detergents (example: calcium sulfonate) serve the purpose of the first seven items in the list as well. Usually it is not economically or technically feasible to use a single do-it-all additive compound. Oils for hypoid gear lubrication will contain high content of EP additives. Grease lubricants may contain large amount of solid particle friction modifiers, such as graphite or molybdenum sulfide.
Application by fluid types
- Tractor (one lubricant for all systems)
- Universal Tractor Transmission Oil - UTTO
- Super Tractor Oil Universal - STOU - includes engine
- Other motors
- 2-stroke engine oils
- Gas turbine engine oils
- Piston engine oils
- Crosshead cylinder oils
- CrossheadCrankcase oils
- Trunk piston engine oils
- Stern tube lubricants
The global lubricant market is generally competitive with numerous manufacturers and marketers. Overall the western market may be considered mature with a flat to declining overall volumes while there is strong growth in the emerging economies. The lubricant marketers generally pursue one or more of the following strategies when pursuing business.
The lubricant is said to meet a certain specification. In the consumer market, this is often supported by a logo, symbol or words that inform the consumer that the lubricant marketer has obtained independent verification of conformance to the specification. Examples of these include the API’s donut logo or the NSF tick mark. The most widely percepted is SAE viscosity specification, like SAE 10W-40. Lubricity specifications are institute and manufacturer based. In the U.S. institute: API S for petrol engines, API C for diesel engines. For 2007 the current specs are API SM and API CJ. Higher second letter marks better oil properties, like lower engine wear supported by tests. In EU the ACEA specifications are used. There are classes A,B,C,E with number following the letter. Japan introduced the JASO specification for motorbike engines. In the industrial market place the specification may take the form of a legal contract to supply a conforming fluid or purchasers may choose to buy on the basis of a manufacturers own published specification.
- Original Equipment Manufacturer (OEM) Approval:
Specifications often denote a minimum acceptable performance levels. Thus many equipment manufacturers add on their own particular requirements or tighten the tolerance on a general specification to meet their particular needs (or doing a different set of tests or using different/own testbed engine). This gives the lubricant marketer an avenue to differentiate their product by designing it to meet an OEM specification. Often, the OEM carries out extensive testing and maintains an active list of approved products. This is a powerful marketing tool in the lubricant marketplace. Text on the back of the motor oil label usually has a list of conformity to some OEM specifications, such as MB, MAN, Volvo, Cummins, VW, BMW, or others. Manufactures may have vastly different specifications for the range of engines they make; one may not be completely suitable for some other.
The lubricant marketer claims benefits for the customer based on the superior performance of the lubricant. Such marketing is supported by glamorous advertising, sponsorships of typically sporting events and endorsements. Unfortunately broad performance claims are common in the consumer marketplace, which are difficult or impossible for a typical consumer to verify. In the B2B market place the marketer is normally expected to show data that supports the claims, hence reducing the use of broad claims. Increasing performance, reducing wear and fuel consumption is also aim of the later API, ACEA and car manufacturer oil specifications, so lubricant marketers can back their claims by doing extensive (and expensive) testing.
The marketer claims that their lubricant maintains its performance over a longer period of time. For example in the consumer market, a typical motor oil change interval is around the 3,000-6,000 miles (or 7,500-1,5000 km in European cars). The lubricant marketer may offer a lubricant that lasts for 12,000 miles or more to convince a user to pay a premium. Typically, the consumer would need to check or balance the longer life and any warranties offered by the lubricant manufacturer with the possible loss of equipment manufacturer warranties by not following its schedule. Many car and engine manufacturers support extended drain intervals, but request extended drain interval certified oil used in that case; and sometimes a special oil filter. Example: In older Mercedes-Benz engines and in truck engines one can use engine oil MB 228.1 for basic drain interval (in Europe 15,000km). Engine oils conforming with higher specification MB 228.3 may be used twice as long, oil of MB 228.5 specification 3x longer. Note that the oil drain interval is valid for new engine with fuel conforming car manufacturer specification. When using lower grade fuel, or worn engine the oil change interval has to shorten accordingly. In general oils approved for extended use are of higher specification and reduce wear. In the industrial market place the longevity is generally measured in time units and the lubricant marketer can suffer large financial penalties if their claims are not substantiated.
The lubricant marketer claims improved equipment efficiency when compared to rival products or technologies, the claim is usually valid when comparing lubricant of higher specification with previous grade. Typically the efficiency is proven by showing a reduction in energy costs to operate the system. Guaranteeing improved efficiency is the goal of some oil test specifications such as API CI-4 Plus for diesel engines. Some car/engine manufacturers also specifically request certain higher efficiency level for lubricants for extended drain intervals.
- Operational tolerance:
The lubricant is claimed to cope with specific operational environment needs. Some common environments include dry, wet, cold, hot, fire risk, high load, high or low speed, chemical compatibility, atmospheric compatibility, pressure or vacuum and various combinations. The usual thermal characteristics is outlined with SAE viscosity given for 100°C, like SAE 30, SAE 40. For low temperature viscosity the SAE xxW mark is used. Both markings can be combined together to form a SAE 0W-60 for example. Viscosity index (VI) marks viscosity change with temperature, with higher VI numbers being more temperature stable.
The marketer offers a lubricant at a lower cost than rivals either in the same grade or a similar one that will fill the purpose for lesser price. (Stationary installations with short drain intervals.) Alternative may be offering a more expensive lubricant and promise return in lower wear, specific fuel consumption or longer drain intervals. (Expensive machinery, unaffordable downtimes.)
- Environment Friendly:
The lubricant is said to be environmentally friendly. Typically this is supported by qualifying statements or conformance to generally accepted approvals. Several organizations, typically government sponsored, exist globally to qualify and approve such lubricants by evaluating their potential for environmental harm. Typically, the lubricant manufacturer is allowed to indicate such approval by showing some special mark. Examples include the German “Blue Angel,” European “Daisy” Eco label, Global Eco-Label “GEN mark,” Nordic, “White Swan,” Japanese “Earth friendly mark”; United States “Green Seal,” Canadian “Environmental Choice,” Chinese “Huan,” Singapore “Green Label” and the French “NF Environment mark.”
The marketer claims novel composition of the lubricant which improves some tangible performance over its rivals. Typically the technology is protected via formal patents or other intellectual property protection mechanism to prevent rivals from copying. Lot of claims in this area are simple marketing buzzwords, since most of them are related to a manufacturer specific process naming (which achieves similar results than other ones) but the competition is prohibited from using a trademarked salesword.
The marketer claims broad superior quality of its lubricant with no factual evidence. The quality is “proven” by references to famous brand, sporting figure, racing team, some professional endorsement or some similar subjective claim. All motor oil labels wear mark similar to "of outstanding quality" or "quality additives," the actual comparative evidence is always lacking.
Disposal and environmental issues
It is estimated that 40 percent of all lubricants are released into the environment.
Recycling, burning, landfill and discharge into water may achieve disposal of used lubricant.
There are typically strict regulations in most countries regarding disposal in landfill and discharge into water as even a small amount of lubricant can contaminate a large amount of water. Most regulations permit a threshold level of lubricant that may be present in waste streams and companies spend hundreds of millions of dollars annually in treating their waste waters to get to acceptable levels.
Burning the lubricant as fuel, typically to generate electricity, is also governed by regulations mainly on account of the relatively high level of additives present. Burning generates both airborne pollutants and ash rich in toxic materials, mainly heavy metal compounds. Thus lubricant burning takes place in specialized facilities that have incorporated special scrubbers to remove airborne pollutants and have access to landfill sites with permits to handle the toxic ash.
Unfortunately, most lubricant that ends up directly in the environment is due to general public discharging it onto the ground, into drains and directly into landfills as trash. Other direct contamination sources include runoff from roadways, accidental spillages, natural or manmade disasters and pipeline leakages.
Improvement in filtration technologies and processes has now made recycling a viable option (with rising price of base stock and crude oil). Typically various filtration systems remove particulates, additives and oxidation products and recover the base oil. The oil may get refined during the process. This base oil is then treated much the same as virgin base oil however there is considerable reluctance to use recycled oils as they are generally considered inferior. Basestock fractionally vacuum distilled from used lubricants has superior properties to all natural oils, but cost effectiveness depends on many factors. Used lubricant may also be used as refinery feedstock to become part of crude oil. Again there is considerable reluctance to this use as the additives, soot and wear metals will seriously poison/deactivate the critical catalysts in the process. Cost prohibits carrying out both filtration (soot, additives removal) and re-refining (distilling, isomerisation, hydrocrack, etc.) however the primary hindrance to recycling still remains the collection of fluids as refineries need continuous supply in amounts measured in cisterns, rail tanks.
Occasionally, unused lubricant requires disposal. The best course of action in such situations is to return it to the manufacturer where it can be processed as a part of fresh batches.
Lubricants both fresh and used can cause considerable damage to the environment, mainly due to their high potential of serious water pollution. Further, the additives typically contained in lubricant can be toxic to flora and fauna. In used fluids, the oxidation products can be toxic as well. Lubricant persistence in the environment largely depends upon the base fluid however if very toxic additives are used they may negatively affect the persistence. Lanolin lubricants are non-toxic, making them the environmental alternative that is safe for both users and the environment.
- Engine Oil Publications. API. Retrieved November 25, 2007.
- 40 percent is a general number indicated in the industry (including agriculture and railways) and should not be relied upon without further referenced corroboration. In agriculture and railways operations, biodegradable lubricants have to be used—these come in direct contact with soil.
ReferencesISBN links support NWE through referral fees
- API 1509, Engine Oil Licensing and Certification System, 15th Edition, 2002. Appendix E, API Base Oil Interchangeability Guidelines for Passenger Car Motor Oils and Diesel Engine Oils (revised)
- Inman, I.A. 2003. Compacted Oxide Layer Formation under Conditions of Limited Debris Retention at the Wear Interface during High Temperature Sliding Wear of Superalloys. Northumbria University. ISBN 1581123213.
- Mang, Theo, and Wilfried Dresel, (eds.). 2007. Lubricants and Lubrication. 2nd ed. Weinheim: Wiley-VCH. ISBN 9783527314973.
- Pirro, D. M., A. A. Wessol, and J. George Wills. 2001. Lubrication Fundamentals. 2nd ed. New York: Marcel Dekker. ISBN 0824705742.
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