When two mechanical components are pressed or slide over each other, friction occurs and consequently heat is generated. This friction will cause wear and tear of the surfaces of the moving parts. To prolong the lifespan of a mechanical system and increase efficiency, lubrication is required.
What is Lubrication?
Lubrication is the process of keeping metal surfaces clean by carrying away possible deposits and providing a seal to keep it out of dirt. Its aim can be seen as it reduces wears and tears resulting from friction generated by rubbing moveable parts. It can also be seen as lubricant carry away heat generated in bearings, gears and other sledding surfaces preventing overheating, seizure and possible breakdown.
Classes of lubrication
Lubrication can be classified into 3 main groups, thus;
Hydrodynamic or film lubrication:
This involves a lubricating condition where the bearing surfaces are completely separated by an oil layer or film. The rotation of oil drags oil into the loaded zone taking completely the incurred load. The shaft speed, load and viscosity of lubricant therefore determine the increase In pressure and thickness of the film.
This class of lubrication is achieved in certain types of bearings like high speed journal bearings (Michell bearings) as in turbine bearings or plane surfaces whose pivotage all oil film wedge to accommodate load, speed and viscosity effects.
Certain heavy loaded contacts such as ball and roller bearings makes use of special hydrodynamic lubrication.
In most cases where the geometry and type of movement are suitable, lubricants become subjected to very high pressures when trapped in the entry zone as it is squeezed into the most highly loaded section. The high pressures increases the loaded carrying capacity (viscosity) as well as elastically deforming the load surfaces spreading the load over a greater area. This is called elastro-hydrodynamic lubrication.
In this type of lubrication, complete separation of the two rubbing surfaces by the full film of oil is not possible due to some lower reciprocating speed, low oil viscosity or high load. Both surface asperities (roughness) will start to contact each other leading to metal to metal contact.
In most boundary lubrication conditions, the high spots of the roughness surface have contact while the low areas between are separated by a finite layer of lubricant. This normally occurs in some top end bearings, guides etc. the primary requirement of a boundary lubrication is that it should adhere to the metal surface and have a good oiliness but should not be too viscous.
This is a lubrication class where high pressure oil is forced into the bearing clearances which separates the surfaces prior to starting.
Sources of Lubricant
- Paraffin base: has high lubricating oil content with high pour point with high viscosity index.
- Asphalt base: This has a low lub-oil content with a low pour point and low viscousity index.
It is therefore important to subject lubricating oils refined from Paraffin and Asphalt base to various treatment to improve their properties suitable for blending to produce wide range of lub-oils.
- Mineral oils: This is gotten by refining or through distillation of crude oil. (Paraffin or Asphalt base). It most important property is viscosity and must have it in the lowest value value for satisfactory under all conditions especially in load speed and temperature differences.
- Synthetic oils: Silicones are the particular properties of synthetic oils. They are useful where viscosity is almost independent of temperature. Example is gas turbine machine which is usually very expensive.
- Greases: greases are semi-solid lubricant which has high viscosity mineral aid together with filler and metallic soap. The fillers enable grease to withstand shock and heavy loads. The soap include metal base like calcium, sodium with fatty or vegetable oil fillers, lead, zinc, graphite or molybdenum disulphide.
Grease properties is seen as it act as a real lubricant useful in accessing difficult areas or parts and large clearances. It has a continuous tenure of lubricating ability.
- Vegetable and Animal oils: fallow, whale, cod-liver, castor and olive oils belong to this family but they are unsuitable at usual operating conditions especially temperature. They are used in greases and as additives to mineral oils to give improved boundry lubrication.
- Solids: graphite, talc, molybdenum disulphide are good source of the solid kind of lubricant. They are difficult to apply but may be suspended in a fluid when being used and are useful for high operating temperature.
- Water: They are used in steel-rubber or steel-plastic bearings e.g water lubricated stern bearing with rubber bearing surfaces or impregnated plastic resin compounds.
- Gases: gases like air and CO2 are used when liquids are not allowed. It has very low viscosity and more suitable for hydrostatic lubrication.