Lubrication
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 the shearing of the viscous fluid. The scientific study of friction, lubrication, and wear is called tribology.
Lubrication is classified into three major categories:
- Hydrodynamic lubrication (HL): The opposing surfaces are completely separated, such as aquaplaning on a road.
- Elastohydrodynamic lubrication (EHL or EHD): The opposing surfaces are completely separated and the solid bodies deformed by a major load.
- Boundary lubrication: The bodies are not entirely separated.
Lubricants
Lubricants are an essential part of modern machinery. Everything from computer hard disk drives to the Airbus A380 requires lubrication of its moving parts.
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.
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.
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
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.
General composition
Lubricants are generally composed of a majority of base oil and a minority of additives to impart desirable characteristics.
Types of lubricants
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.
Water
Water can be used on its own or as a major component in combination with one of the other base oils.
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
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.
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.
Solid lubricants
Additives
A large number of additives are used to impart performance characteristics to the lubricants. The main families of additives are:
- Anti-oxidants
- Viscosity index improvers
- Anti-wear
- Metal deactivators
- Corrosion inhibitors
- Rust inhibitors
- Friction modifiers
- Extreme Pressure
- Anti-foaming
- Demulsifying / Emulsifying
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
- 2-stroke engine oils
- Industrial
- Hydraulic fluids
- Compressor oils
- Gear oils
- Aviation
- Gas turbine engine oils
- Piston engine oils
- Marine
- Top engine oil
- Crankcase fluids
- Stern tube lubricants
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.
See also
- Bearing (mechanical)
- Grease (lubricant)
- Oils
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
ReferencesISBN links support NWE through referral fees
- 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.
- Boughton and Horvath, 2003, Environmental Assessment of Used Oil Management Methods, California Environmental Protection Agency, Department of Toxic Substances Control.
- Surface Wear – Analysis, Treatment, and Prevention: R. Chattopadhyay, published by ASM-International, Materials Park, Ohio, 2001. ISBN 0-87170-702-0.
External links
- U.S. Army Corps of Engineers grease definition and application guide. (pdf file)
- Silicone grease (includes link to MSDS and application guide)
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