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Grease - the Cinderella of Lubrication
Ask anyone to describe a lubricant and they will probably think of the liquid product used in an engine. There is however a large family of lubricants that are not liquid at all and often get overlooked for this reason. These Cinderella's of the oil industry are the greases used to lubricate bearings or slide ways and protect wires throughout the ship. Greases are usually made from mineral oil stock of quite wide range in viscosity but still similar to those used for more liquid lubricants . Synthetic oil stocks are sometimes used for extreme temperature applications.
Having selected a base stock it is then necessary to add thickeners to solidify the lubricant, to improve the temperature stability, retention and load carrying properties. In conventional greases these are metallic soaps of calcium, lithium, sodium etc. Sometimes other compounds such as clay, silica or more exotic synthetic compounds are used.
The next products to be included into the grease are the additives. These perform similar functions to those in more conventional lubricants and are used to impart specific beneficial properties to the grease. Oxidation and corrosion inhibitors, pour point depressants, extreme pressure additives, pigments, water repellents are all typically found to some extent. Solid lubricants such as molybdenum disulphide, graphite, polyethylene even PTFE for heavily loaded applications.
A few more greasy facts that may be of interest include :
Having selected a base stock it is then necessary to add thickeners to solidify the lubricant, to improve the temperature stability, retention and load carrying properties. In conventional greases these are metallic soaps of calcium, lithium, sodium etc. Sometimes other compounds such as clay, silica or more exotic synthetic compounds are used.
The next products to be included into the grease are the additives. These perform similar functions to those in more conventional lubricants and are used to impart specific beneficial properties to the grease. Oxidation and corrosion inhibitors, pour point depressants, extreme pressure additives, pigments, water repellents are all typically found to some extent. Solid lubricants such as molybdenum disulphide, graphite, polyethylene even PTFE for heavily loaded applications.
A few more greasy facts that may be of interest include :
- Over greasing a bearing can be just as damaging as under greasing. Correct application and flushing are the key to long bearing life.
- Greases of the same thickener are usually compatible; those of different thickeners may be incompatible.
- Greases are very good at absorbing water and can take between 40 and 100% of their own weight in water without failure.
- Heavier greases have better water wash resistance than lighter ones.
- The re-lubrication interval of a bearing should be halved for every 15 - 20oC increase in operating temperature.
- So it appears that the humble grease can contain quite a complex product even if it is often regarded otherwise.
