Greases or Semi-solid Lubricants
Lubricating grease is semi-solid contains soap dispersed throughout a liquid lubricating oil. The liquid lubricant might be petroleum oil or even synthetic oil and it might contain any of the additives for specific requirements. Greases are prepared by saponification of fat with alkali (as lime) followed by adding hot lubricating oil while under agitation. The total amount of mineral oil added finds out the consistency of the finished grease. The configuration of lubricating greases is that of a gel. Soaps are gelling agents that give an interconnected structure containing the added oil. At high temperatures, the soap dissolves in the oil, at which point the interconnected structures cease to exist and the grease liquefies. Consistency of greases might vary from a heavy viscous liquid to that of a stiff solid mass. To develop the heat resistance of grease, inorganic solid thickening agents are added up.
Greases have higher shear or frictional resistance than oils and, thus, can support much heavier loads at lower speeds. Greases are utilized
1. in situations where oil may not remain in place, because of high load, low speed, intermittent operation, sudden jerks, etc.,
2. in bearing and gears that work at high temperatures,
3. in situations where bearing ought to be sealed against entry of dirt, dust grit or moisture, because greases are less liable to contamination by these, and
4. in situations where dripping or spurting of oil is undesirable because unlike oils, grease if utilized do not splash or drip over articles being prepared by the machine.
The main function of soap is to behave as thickening agent, so that grease sticks firmly to the metal surfaces. Though, the nature of soaps decides
a. Temperature upto which the grease may be used.
b. Its consistency.
c. Its water and oxidation resistance.
The important greases are following:
(a) Calcium based greases or cup-greases are emulsion of petroleum oils with calcium soaps. Generally they are prepared by adding requisite amount of calcium hydroxide to a hot oil like fallow; whereas under agitation. These greases are cheapest and most frequently utilized. They are insoluble in water. However, they are satisfactory for use only at low temperatures because above 80oC, oil and soap begin to separate out.
(b) Soda based greases are petroleum oils, thickened by mixing sodium soaps. They are not water resistant, because the sodium soap content is soluble in water. But, they might be used upto 175oC. They are appropriate for use in ball bearings, while the lubricant gets heated because of friction.
(c) Lithium based greases are petroleum oils, thickened by mixing lithium soaps. They are water resistant and appropriate for use at low temperature (upto 15oC) only.
(d) Axle greases are extremely cheap resin greases, prepared by adding lime to resin and fatty oils. The mixture is comprehensively mixed and allowed to stand, whereas grease floats as shift mass. Filters as like mica and talc are also added up to them. They are water-resistant and appropriate for less delicate equipment working under high loads and at low speeds.
Besides the above, there are greases prepared by dispersing solids in mineral oils. They are mostly utilized in machine bearings, rail axle boxes, tractors, rollers, etc.