Lubricants are often used in machines where they come into contact with rubber or plastic seal. The strength and degree of swell of these seals may be affected by interaction with the oil. Various tests have been devised to measure the effect of base oils different seals and under different test conditions [ 13 ].
Various tests measure the effects of base oils on different seals and under different test conditions. Selection and application of lubricating oil are determined by the functions which are expected for performance.
In one application, such as delicate instrument bearing, the reduction friction is paramount and in another, such as metal cutting, the temperature control may be most important. A lubricating oil performance or requirement for a modern high speed engine should fulfill the following five important functions:.
Reduction of the frictional resistance:. The reduction of engine resistance to minimum is necessary to ensure maximum mechanical efficiency running costs of a vehicle or engines are influenced by the lubricant viscosity.
Protection of the engine against all types of wear:. All users wants minimum maintenance costs, longer engine life and increased usefulness. Modern oil has allowed longer intervals between engines over hauls. Reduction of gas and oil leakages:. The reduction of gas and oil leakages in an efficient and lasting manner is necessary to maintain engine performance and to prevent the combustions products from adulterating the oil.
Contributing the thermal equilibrium of the engine:. In modern engines, the oil functions and more as a heat exchange medium, dissipating the heat is not converted into work. This is often associated with the first function in this list where the viscous oil give greater frictional resistance and its slow internal circulation leads to a rapid temperature raise of some vital part of the engine to cool efficiency, the oil must be able to circulated quickly.
Removal of all injurious impurities:. The lubricant give the function of protecting the engine against corrosive and mechanical wear which caused by all injurious impurities. So, the removal of these impurities by lubricants is very important for engine.
The function and the corresponding qualities required for engine lubricating oils are summarized in Table 1. Gaseous lubricants belong to the simplest, lowest viscosity lubricants known and include air, nitrogen, oxygen, and helium.
They are applied in aerodynamic and aerostatic bearings. Since the chemical properties and the aggregate state of most gases remain unchanged over a wide temperature range, gaseous lubricants offer several advantages over liquid lubricants.
First, they can be applied at both very high and very low temperatures. Their chemical stability eliminates any risk of contamination of the bearing by the lubricant, important for the machinery used in many branches of industry, primarily in the food, pharmaceutical and electronic industries.
A useful property of gases is that their viscosities increase with temperature, wheras the opposite is true of liquids, resulting in load — carrying capacity of gas — lubricated bearings increasing with temperature. However, the relatively low viscosity of gases generally limits the load-carrying capacity of self-acting, aerodynamic bearings to kPa. It is possible to achieve better bearing performance with gaseous lubricants than with liquid lubricants due to the very low viscosity of the gases which results in smaller heat generation by internal friction.
In some cases, such as in foil air bearings, sliding contact occurs during stops and starts [ 14 ], therefore solid lubricants such as PTFE are used to reduce friction. Mineral oils: As the hydrodynamic behaviour of plain bearings of plain bearings is totally dependent on the viscosity characteristics of the lubricant, typical liquid bearing lubricants are straight mineral oil raffinates of various viscosity grades.
The viscosity grade required is dependent upon bearing speed, oil temperature and load. The ISO grade number indicated is the preferred grade for the speed and temperature range.
ISO and Grade oils are commonly used in indoor, heated applications, with grade oils being used for high-speed, The higher the bearing speed, the lower the oil viscosity required and also that the higher the unit operating temperature, the higher the oil viscosity required.
If vibration or minor shock loading is possible, a higher grade of oil than the one indicated in table 2 should be considered. Other methods for determining the viscosity grade required in an application are to apply minimum and optimum viscosity criteria to a viscosity — temperature plot.
A third and more complex method is to calculate the oil viscosity needed to obtain a satisfactory oil film thickness.
On the other hand, increased viscosity also increases operating temperature. In practice, therefore, the extent to which lubrication can be improved in this way is often limited. The chemical compositions of these oils differ from typical base oils in that they contain somewhat more aromatic hydrocarbons and heterocyclic compounds, which act as natural oxidation inhibitors.
An increased viscosity for oils derived from the same crude oil does not significantly change their chemical composition; the difference generally lies with the increasing chain length of the paraffinic hydrocarbons, mostly isoparaffins, and in the aliphatic substituents of naphthenic and aromatic rings, together with a slight increase in the number of naphthenic and aromatic rings.
More highly refined mineral oils and oxidation inhibitors are used for applications where higher temperatures or longer service periods require better ageing stabilites. Synthetic lubricants: in practice, every synthetic oil of adequate viscosity and good viscosity-temperature behavior can be used as a bearing lubricant, e. However, in most cases the synthetic oils specifically developed for lubricating particular equipment are also used to lubricate its bearings.
Although synthetic oils do not form a lubricant film under pressure as well as mineral oils and may not be effective bearing lubricants despite their higher temperature viscosity. Biodegradable products: Biodegradable products of vegetable or animal origin are also considered for liquid lubrication, e. The use of vegetable oils as lubricants is likely to increase due to environmental and government requirements and is becoming increasingly important.
General description: bearings used under vacuum, at very high temperatures or under very high radiation cannot be lubricated by liquid lubricants or greases. For these and many other cases, solid lubricants are used, deemed to be any solid material used to reduce friction and wear between two moving surfaces.
Simply stated, an adequate solid material is required for the special lubrication requirements of extreme operating conditions, such as very high or very low temperatures over a wide range, e. Such materials normally have a layered crystalline structure which ensures low shear strength, thereby minimizing friction. The shear strength between the crystalline layers is weak and sets up a low and sets up a low friction mechanism by slippage of the crystalline layers under low shearing forces.
Examples of layer-lattice solids are molybdenum disulphide, graphite, boron nitride, cadmium iodide and borax. Solid lubricants are used mainly in the form of powders or as bonded solid films.
A good solid film lubricant has strong adhesion to the bearing substrate material, full surface coverage and good malleability. It should also be chemically stable and prevent corrosion, taking into account operational and environmental conditions.
Many solid film lubricants have poor wear resistance, since any breaks in the film are not self-healing, in contrast to the surface coating formed by a liquid lubricant.
Advanced solid film lubricants perform reliably in many specific applications and much experience has been gained to better understand their limitations.
The most commonly used disulphide, graphite, polytetrafluroethylene propylene. Another group of materials, the self-lubricated materials, are related to solid lubricants and are particulary important for bearings. Their self-lubricating characteristics eliminate the need of grease or other lubrication and gove improved performance under high temperature conditions. This phenomenon gives the material a self lubricating ability matched by few other materials and allows for the elimination of grease or oil that would evaporate, congeal or solidify, causing premature failure.
The graphite matrix can be filled with a variety of embedded lubricants to enhance chemical, mechanical and tribological properties to give a constant, low friction coefficient rather than just a surface layer, helping to protect against catastrophic failure. Lubrication is maintained during linear motion where lubricant is not frawn out and dust is not pulled in.
A recent development in solid bearing lubricants is micro —porous polymeric lubricants, MPL, where a polymer containing a continous microporous network has oil contained within the pores, which may include appropriate additives [ 14 ]. Water Content. It can be expressed as parts per million, percent by volume or percent by weight.
It can be measured by centrifuging, distillation and voltametry. The most popular, although least accurate, method of water content assessment is the centrifuge test. Apart from water, solids and other solubles are also separated and the results obtained do not correlate well with those obtained by the other two methods.
The distillation method is a little more accurate and involves distillation of oil mixed with xylene. Any water present in the sample condenses in a graduated receiver. Voltametry method is the most accurate. It employs electrometric titration, giving the water concentration in parts per million. Corrosion and oxidation behaviour of lubricants is critically related to water content.
An oil mixed with water gives an emulsion. An emulsion has a much lower load carrying capacity than pure oil and lubricant failure followed by damage to the operating surfaces can result. In general, in applications such as turbine oil systems, the limit on water content is 0.
In dielectric systems excessive water content has a significant effect on dielectric breakdown. Usually the water content in such systems should be kept below 35 [ppm]. Sulphur Content. It can have some beneficial, as well as some detrimental, effects on operating machinery. Sulphur is a very good boundary agent, which can effectively operate under extreme conditions of pressure and temperature.
On the other hand, it is very corrosive. A commonly used technique for the determination of sulphur content is the bomb oxidation technique. It involves the ignition and combustion of a small oil sample under pressurised oxygen. The sulphur from the products of combustion is extracted and weighed. Ash Content. There is some quantity of noncombustible material present in a lubricant which can be determined by measuring the amount of ash remaining after combustion of the oil ASTM D, D The contaminants may be wear products, solid decomposition products from a fuel or lubricant, atmospheric dust entering through a filter, etc.
Some of these contaminants are removed by an oil filter but some settle into the oil. To determine the amount of contaminant, the oil sample is burned in a specially designed vessel. The residue that remains is then ashed in a high temperature muffle furnace and the result displayed as a percentage of the original sample.
The ash content is used as a means of monitoring oils for undesirable impurities and sometimes additives. In used oils it can also indicate contaminants such as dirt, wear products, etc. Chlorine Content.
The amount of chlorine in a lubricant should be at an optimum level. Additives are also added to the base oil to optimize the properties of the lubricant. Another form of lubricant, grease, is made by combining a base oil with a thickening agent. It serves many purposes, but the ultimate goal of a lubricant is to reduce unwanted friction the resistance encountered when solid surfaces slide against each other.
This friction reduction is accomplished by separating two solid surfaces with a thin layer of lubricant. Friction, unabated, can cause many problems for machinery. Every metal surface within a machine, no matter how finely finished, will have some degree of roughness, with high points called asperities.
As the surfaces move past each other, these asperities come into contact and catch on one another. As the asperities catch and move, they rip each other apart, causing abrasions and adhesion, and in some scenarios, they can even weld and seize together. Additionally, friction produces heat which, if unchecked, can quickly degrade the lubricant and cause catastrophic machine failure. The right quantity of lubricant distributed to each point plays a major role. To under lubricate or to over lubricate can have a negative effect and must be avoided.
Over lubricating leads to excessive pressure on the bearings. Over lubricating increases your expenses linked to an enormous waste of grease.
On average, each dollar of lubricant purchased, the shop spends three dollars of expenses on lubricant waste. A good lubrication program permits to follow and control lubrication consumption and eliminate costs. On the other hand, a prolonged deficiency of lubricant under lubrication can cause extreme wear of non-lubricated surfaces, thus causing a failure to the machine along with elevated repair costs. Automatic lubrication insures a continuous lubrication at a regular intervals the greasing points and the special points which have difficult access.
Some lubricants come in tubes and applicators, making it easier to apply the lubricant exactly where you want it to go. And a final tip - squirt a small amount in your hand to warm it up first, rather than applying it directly onto the genitals.
Samantha Evans is a former nurse, sexpert and co-owner of online sex toy retailer www. Join thought-provoking conversations, follow other Independent readers and see their replies. Want to bookmark your favourite articles and stories to read or reference later? Start your Independent Premium subscription today. Recommended Why condom size matters The truth behind who really buys sex dolls What it's like to be addicted to sex.
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