What are the properties of Good Lubricant
Properties of Lubricant:
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Viscosity is a measure of the resistance to flow or the internal friction of the lubricant.
-usually measured by Saybolt universal seconds (SUS) and Redwood viscometer. Also it is expressed with centistoke (unit of kinematic viscosity) and centipoise (unit of absolute viscosity)
-expressed in two temperature i.e. -18ᵒC (0ᵒF) and 99ᵒC (210ᵒF)
2. Viscosity Index:
It is used to grade lubricants. Viscosity is inversely proportional to temp.
-If temp. increases, the viscosity of the lubricant decreases and if temp. decreases, the viscosity of the lubricant increases.
-The variation of viscosity of oil with changes in temperature is measured by viscosity index
-oil to measure is compared with 2 reference oil having same viscosity at 99ᵒC. one is paraffinic base oil index of zero and another naphthenic base oil index of 100
-high viscosity index number indicates relatively smaller change in viscosity of the oil with temperature.
-low viscos oil is recommended for automobile engines in winter than summer. The viscosity of a lubricant should be just sufficient to ensure lubrication. If it is more than this value, power loss will be higher due to increased oil resistance.
-VI improver are added to improve viscosity index
It is the property of a lubricating oil to spread & attach itself firmly to the bearing surfaces as well as provide lubricity. Generally, the oiliness of the lubricating oil should be high particularly when it is to be used for mating surfaces subjected to a high intensity of pressure and smaller clearance portion to avoid the squeezing out of the oil. Such a way that the metal is protected by a thin layer of the oil and the wear is also considerably reduced. It is measured by co-efficient of friction at extreme operating condition.
4. Flash Point:
Flash point of oil is the min. temp. at which the vapours of lubricating oil will flash when a small flame is passed across its surface. It is of two type open flash point and closed flash point. The flash point of the lubricating oil must be higher than the temp. likely to be developed in the bearings in order to avoid the possibility of fire hazards.
Characteristics of Good Lubricant
5. Fire Point:
If the lubricating oil is further heated after the flash point has been reached, the lowest temp. at which the oil will burn continuously for 5 seconds is called fire point.
-usually 11ᵒC higher than open flash point and varies from 190ᵒC to 290ᵒC for the lubricants used for IC engines -The fire point of a lubricant also must be high so that the oil does not burn in service.
6. Cloud Point:
It is the temp. at which the lubricating oil changes its state from liquid to solid. Its temp. must high for the low temp. operability of the lubricating oil during winter.
7. Pour Point:
It is the lowest temp. at which the lubricating oil will not flow or totally form wax or solidify. This property must be considered because of its effect on starting an engine in cold weather. Oil derived from paraffinic crudes tends to have higher pour points than those derived from naphthenic crudes. The pour points can be lower by the addition of pour point depressant usually a polymerised phenol or ester. Pour point must be at least 15ᵒF lower than the operating temperature to ensure maximum circulation.
The present of acid (mineral acid, petroleum acid) is harmful to the metal surfaces. The lubrication oil should not attack chemically the materials of the engine. The lubricant should not be corrosive, but it should give protection against corrosion. New oil has low neutralisation number i.e. it maintains the alkaline and acid solution to make the oil neutral.
9. Oxidation stability:
It is resistance to oxidation. Due to oxidation the oil will form deposits on the piston rings and lose its lubricating property. Low temperature operation avoiding the hot-area contact and crankcase ventilation can help in preserving the stability of oil over longer periods. Oxidation inhibitors are used to improve oxidation stability. These are complex compounds of sulphur and phosphorus or amine and phenol derivatives.
Lubricating oil must be clean. It should not contain dust and dirt particles as well as water content which promote corrosion.
11. Carbon residue:
after evaporation of a mass sample of lubricating oil under specific condition may remain as carbonaceous residue. It indicates the deposit characteristics of oil. Paraffinic oil has higher carbon residues than the naphthenic base oil.
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