A lubricant is a substance introduced to reduce friction between moving surfaces. It may also have the function of transporting foreign particles and of distributing heat. The property of reducing friction is known as lubricity.
A good lubricant possesses the following characteristics:
- High boiling point.
- Low freezing point.
- High viscosity index.
- Thermal stability.
- Corrosion prevention.
- High resistance to oxidation.
Typically lubricants contain 90% base oil (most often petroleum fractions, called mineral oils) and less than 10% additives. Vegetable oils or synthetic liquids such as hydrogenated polyolefins, esters, silicones, fluorocarbons and many others are sometimes used as base oils. Additives deliver reduced friction and wear, increased viscosity, improved viscosity index, resistance to corrosion and oxidation, aging or contamination, etc.
Lubricants such as 2-cycle oil are added to fuels like gasoline which has low lubricity. Sulfur impurities in fuels also provide some lubrication properties, which has to be taken in account when switching to a low-sulfur diesel; biodiesel is a popular diesel fuel additive providing additional lubricity.
Non-liquid lubricants include grease, powders (dry graphite, PTFE, Molybdenum disulfide, tungsten disulfide, etc.), teflon tape used in plumbing, air cushion and others. Dry lubricants such as graphite, molybdenum disulfide and tungsten disulfide also offer lubrication at temperatures (up to 350 °C) higher than liquid and oil-based lubricants are able to operate. Limited interest has been shown in low friction properties of compacted oxide glaze layers formed at several hundred degrees Celsius in metallic sliding systems, however, practical use is still many years away due to their physically unstable nature.
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.
In addition to industrial applications, lubricants are used for many other purposes. Other uses include cooking (oils and fats in use in frying pans, in baking to prevent food sticking), bio-medical applications on humans (e.g. lubricants for artificial joints), ultrasound examination, internal examinations for males and females, and the use of personal lubricant for sexual purposes.
Lubricants perform the following key functions.
- Keep moving parts apart
- Reduce friction
- Transfer heat
- Carry away contaminants & debris
- Transmit power
- Protect against wear
- Prevent corrosion
- Seal for gases
- Stop the risk of smoke and fire of objects
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 i.e., a thin layer of lubricant separates the moving parts. This is analogous to hydroplaning, the loss of friction observed when a car tire is separated from the road surface by moving through standing water. 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.
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, e.g. 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 i.e. 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 & pastes are not effective at 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 fines that get created.
It is apparent that in a circulatory system the oil will only be as clean as the filter can make it, thus it is 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.
Lubricants known as hydraulic fluid are used as the working fluid in hydrostatic power transmission. Hydraulic fluids comprise a large portion of all lubricants produced in the world. The automatic transmission's torque converter is another important application for power transmission with lubricants.
Protect against wear
Lubricants prevent wear by keeping the moving parts apart. Lubricants may also contain anti-wear or extreme pressure additives to boost their performance against wear and fatigue.
Good quality lubricants are typically formulated with additives that form chemical bonds with surfaces, or exclude moisture, to prevent corrosion and rust.
Seal for gases
Lubricants will occupy the clearance between moving parts through the capillary force, thus sealing the clearance. This effect can be used to seal pistons and shafts.
Types of lubricants
In 1999, an estimated 37,300,000 tons of lubricants were consumed worldwide. Automotive applications dominate, but other industrial, marine, and metal working applications are also big consumers of lubricants. Although air and other gas-based lubricants are known, e.g., in fluid bearings), liquid and solid lubricants dominate the market, especially the former.
Lubricants are generally composed of a majority of base oil plus a variety of additives to impart desirable characteristics. Although generally lubricants are based on one type of base oil, mixtures of the base oils also are used to meet performance requirements.
Base oil groups
- Group I – Saturates <90% and/or sulfur >0.03%, and Society of Automotive Engineers (SAE) viscosity index (VI) of 80 to 120
- Manufactured by solvent extraction, solvent or catalytic dewaxing, and hydro-finishing processes. Common Group I base oil are 150SN (solvent neutral), 500SN, and 150BS (brightstock)
- Group II – Saturates over 90% and sulfur under 0.03%, and SAE viscosity index of 80 to 120
- Manufactured by hydrocracking and solvent or catalytic dewaxing processes. Group II base oil has superior anti-oxidation properties since virtually all hydrocarbon molecules are saturated. It has water-white color.
- Group III – Saturates > 90%, sulfur <0.03%, and SAE viscosity index over 120
- Manufactured by special processes such as isohydromerization. Can be manufactured from base oil or slax wax from dewaxing process.
- Group IV – Polyalphaolefins (PAO)
- Group V – All others not included above such as naphthenics, PAG, 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 at least 140
Can also be classified into three categories depending on the prevailing compositions:
- Lubricants for internal combustion engines contain additives to reduce oxidation and improve lubrication. The main constituent of such lubricant product is called the base oil, base stock. While it is advantageous to have a high-grade base oil in a lubricant, proper selection of the lubricant additives is equally as important. Thus some poorly selected formulation of PAO lubricant may not last as long as more expensive formulation of Group III+ lubricant.
Biolubricants made from vegetable oils and other renewable sources
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, castor oil, palm oil, sunflower seed oil and rapeseed oil from vegetable, and Tall oil from tree sources. Many vegetable oils are often hydrolyzed to yield the acids which are subsequently combined selectively to form specialist synthetic esters. Other naturally derived lubricants include lanolin (wool grease, a natural water repellent).
In 2008, the biolubricant market was around 1% of UK lubricant sales in a total lubricant market of 840,000 tonnes/year.
Lanolin is a natural water repellent, derived from sheep wool grease, and is an alternative to the more common petro-chemical based lubricants. This lubricant is also a corrosion inhibitor, protecting against rust, salts, and acids.
Water can also be used on its own, or as a major component in combination with one of the other base oils. Commonly used in engineering processes, such as milling and lathe turning.
- Polyalpha-olefin (PAO)
- Synthetic esters
- Polyalkylene glycols (PAG)
- Phosphate esters
- Alkylated naphthalenes (AN)
- Silicate esters
- Ionic fluids
Teflon or PTFE: Teflon (PTFE) is typically used as a coating layer on, for example, cooking utensils to provide a non-stick surface. Its usable temperature range up to 350 °C and chemical inertness make it a useful additive in special greases. Under extreme pressures, teflon powder or solids is of little value as it is soft and flows away from the area of contact. Ceramic or metal or alloy lubricants must be used then.
Inorganic solids: Graphite, hexagonal boron nitride, molybdenum disulfide and tungsten disulfide are examples of materials that can be used as solid lubricants, often to very high temperature. The use of some such materials is sometimes restricted by their poor resistance to oxidation (e.g., molybdenum disulfide can only be used up to 350°C in air, but 1100°C in reducing environments).
Metal/alloy: Metal alloys, composites and pure metals can be used as grease additives or the sole constituents of sliding surfaces and bearings. Cadmium and Gold are used for plating surfaces which gives them good corrosion resistance and sliding properties, Lead, Tin, Zinc alloys and various Bronze alloys are used as sliding bearings, or their powder can be used to lubricate sliding surfaces alone, or as additives to greases.
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 a compacted oxide layer glaze formation. This is the generation of a compacted oxide layer which 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 is 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 oxidising 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, 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
- Gas turbine engine oils
- Piston engine oils
- Crosshead cylinder oils
- Crosshead Crankcase 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 perceived 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-4. 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 (4,828–9,656 km). The lubricant marketer may offer a lubricant that lasts for 12,000 miles (19,312 km) 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. 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 proved 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, un-affordable 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”; USA “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 trademark.
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.
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. 
Disposal and environmental issues
It is estimated that 40% of all lubricants are released into the environment.
Disposal: 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 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 man-made 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.
Environment: 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 which is safe for both users and the environment.
Societies and industry bodies
- American Petroleum Institute (API)
- Society of Tribologists and Lubrication Engineers (STLE)
- National Lubricating Grease Institute (NLGI)
- Society of Automotive Engineers (SAE)
- Independent Lubricant Manufacturer Association (ILMA)
- European Automobile Manufacturers Association (ACEA)
- Japanese Automotive Standards Organization (JASO)
- Peer reviewed
- Tribology Transactions
- Journal of Synthetic Lubricants
- Tribology Letters
- Lubrication Science
- Trade periodicals
- Tribology and Lubrication Technology
- Fuels & Lubes International
- Lubes n’ Greases
- Chemical Market Review
- Machinery lubrication
- Castor oil
- Grease (lubricant)
- Mineral oils
- Motor oil
- Oil analysis
- Penetrating oil
- Personal lubricant
- Society of Tribologists and Lubrication Engineers
- ^ Thorsten Bartels et al. "Lubricants and Lubrication" in Ullmann's Encyclopedia of Industrial Chemistry, 2005, Weinheim. doi:10.1002/14356007.a15_423
- ^ Engine Oil Publications
- ^ Turbo hydra-matic 350 By Ron Sessions, page 20.
- ^ National Non-Food Crops Centre. NNFCC Conference Poster. Improved winter rape varieties for biolubricants
- ^ millbrook.co.uk
- 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)
- Boughton and Horvath, 2003, Environmental Assessment of Used Oil Management Methods, Environmental Science and Technology, V38
- I.A. Inman. Compacted Oxide Layer Formation under Conditions of Limited Debris Retention at the Wear Interface during High Temperature Sliding Wear of Superalloys, Ph.D. Thesis (2003), Northumbria University, ISBN 1-58112-321-3
- Mercedes-Benz oil recommendations, extracted from factory manuals and personal research
- Measuring reserve alkalinity and evaluation of wear dependence
- Testing used oil quality, list of possible measurements
- Lubricant Additives: Chemistry and Applications, Leslie R. Rudnick, CRC Press.
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