Lubricant
From Wikipedia, the free encyclopedia
A lubricant (colloquially, lube, although this may also refer to personal lubricants) is a substance (usually a liquid) introduced between two moving surfaces to reduce the friction and wear between them. A lubricant provides a protective film which allows for two touching surfaces to be separated, thus lessening the friction between them.
Lubricants are an essential part of modern machinery. Everything from computer hard disk drives to the Airbus A380 requires lubrication of its moving parts.
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, silicone, 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.
Non-liquid lubricants include grease, powders (dry graphite, PTFE, Molybdenum disulfide, etc.), teflon tape used in plumbing, air cushion and others. Another approach to reducing friction and wear is to use bearings such as ball bearings, roller bearings or air bearings or to use sound, in the case of acoustic lubrication.
Lubricants such as 2-cycle oil are also added to some fuels. 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.
In addition to automotive and industrial applications, lubricants are used for many other purposes, including K-Y Jelly, often used as a personal lubricant, bio-medical applications (e.g. lubricants for artificial joints) and others.
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[edit] Purpose
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
Keep moving parts apart
Lubricans 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 ie 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.
Reduce friction
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.
Transfer heat
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. This circulatory 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 susceptile to catastrophic faillure during sudden system shut downs. 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 immidiately 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 & debris
Lubricant circulatory 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 removal and transport to the filter. Over time the filter will get clogged and require 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 will allow it to be thus it is unfortunate that there are no industry standards for consumers to readily assess the filtering ability of automotive filters. Poor filtration significantly reduces the life of the machine (engine) as well as making the system inefficient.
Transmit power
Pascal's law Blaise Pascal 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 antiwear or extreme pressure additives to bolster their performace against wear and fatigue.
Prevent corrosion
Quality lubricants are typically formulated with additives that form chemical bonds with surfaces to prevent corrosion and rust.
[edit] History
Romans used rags dipped in animal fat to lubricate wagon wheels; however the science of lubrication (tribology) really only took off with the industrial revolution in the nineteenth century.
[edit] General composition
Lubricants are generally composed of a majority of base oil and a minority of additives to impart desirable characteristics.
[edit] Types of lubricants
[edit] Liquid lubricants
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.
- Water
- Mineral oils
- Vegetable (natural oil)
- Synthetic oils
- Others
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.
[edit] Water
Water can be used on its own or as a major component in combination with one of the other base oils.
[edit] Mineral oil
This term is used to encompass lubricating base oil derived from crude oil. API designates several types of lubricant base oil identified [1] as:
- Group I - Saturates < 90% and/or Sulfur >0.03% and Viscosity Index >= 80 to <120
- Group II – Saturates >= 90% and Sulfur <=0.03% and Viscosity Index >= 80 to <120
- Group III – Saturates >= 90% Sulfur <=0.03% and Viscosity Index >= 120
- Group IV – Poly alpha olefins (PAO)
- Group V – All others not included above
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
[edit] 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.
[edit] Synthetic oils
- Polyalpha-olefin (PAO)
- Synthetic esters
- Polyalkylene glycols (PAG)
- Phosphate esters
- Alkylated naphthalenes (AN)
- Silicate esters
- Ionic fluids
Note: In the USA certain Group III base stocks may be designated as synthetic; typically hydrocracked oils.
[edit] Solid lubricants
[edit] Additives
A large number of additives are used to impart performance characteristics to the lubricants. The main families of additives are:
- Antioxidants
- Viscosity index improvers
- Anti-wear
- Metal deactivators
- Corrosion inhibitors
- Rust inhibitors
- Friction modifiers
- Extreme Pressure
- Anti-foaming
- Demulsifying / Emulsifying
[edit] Application by fluid types
- Automotive
- Engine oils
- Petrol (Gasoline) engine oils
- Diesel engine oils
- Automotive transmission fluids
- Gearbox fluids
- Brake fluids
- Hydraulic fluids
- Engine oils
- Other motors
- Industrial
- Hydraulic fluids
- Compressor oils
- Gear oils
- Aviation
- Gas turbine engine oils
- Piston engine oils
- Marine
- Top engine oil
- Crankcase fluids
- Stern tube lubricants
[edit] Disposal and environmental issues
It is estimated that 40% of all lubricants are released into the environment. (Note: 40% is a general number indicated in the industry and should not be relied upon without further referenced corroboration).
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 wastewaters 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. Typically various filtration systems remove all additives and oxidation products and recover the base oil. 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. 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 potential of the additives to poison the critical catalysts in the process is high. Cost prohibits carrying out both filtration and re-refining however the primary hindrance to recycling still remains the collection of fluids.
Occasionally, unused lubricant requires disposal. The best course of action in such situations is to return it to the manufacturer where typically it can be consumed as 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.
[edit] Societies and industry bodies
- API
- American Petroleum Institute
- STLE
- Society of Tribologists and Lubrication Engineers
- NLGI
- National Lubricating Grease institute
- SAE
- Society of Automotive Engineers
- ILMA
- Independent lubricant manufacturer association
[edit] Major publications
- Peer reviewed
- Tribology Transactions
- Journal of Synthetic Lubricants
- Trade periodicals
- Tribology and Lubrication Technology
- Lubes n’ Greases
- Compoundings
- Chemical Market Review
- Machinery lubrication
[edit] References
[1] API 1509, Engine Oil Licensing and Certification System, 15th Edition, April 2002; Appendix E, API Base Oil Interchangeability Guidelines for Passenger Car Motor Oils and Diesel Engine Oils, November 2004
[2] Boughton and Horvath, 2003, Environmental Assessment of Used Oil Management Methods, Environmental Science and Technology, V38(2) http://pubs.acs.org/cgi-bin/article.cgi/esthag/2004/38/i02/pdf/es034236p.pdf
