In this article we will discuss about:- 1. Definition of Lubricant 2. Types of Lubricants 3. Properties.
Definition of Lubricant:
A lubricant is a substance of high viscosity which promotes the lubrication process. For example, talc (soft mineral magnesium silicate), wax, oil, grease, French powder, etc. The substance is introduced in between two surfaces in contact having relative motion with each other.
The lubricant separates the two surfaces by making a film in between the surfaces and avoids the metal-to-metal contact directly. This lubrication process absorbs the frictional heat produced in between the two parts and hence the lubricant acts as a coolant which further reduces the loss of power by decreasing the frictional forces.
Types of Lubricants:
Lubricants can be classified as follows:
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(a) Solid lubricants and
(b) Liquid lubricants.
(a) Solid Lubricants:
Graphite, talc, French powder, etc., come under this category. Solid lubricants are applied on the surface where oil film cannot be retained due to excess temperature and pressure. They are used in the form of powder. For example, French powder is generally applied on carom board surfaces. The striker moves freely with the application of powder. Talc is a soft mineral magnesium silicate.
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(b) Liquid Lubricants:
Liquid lubricants which are commonly used in industrial application are obtained by refining crude petroleum. They have the ability to stick on the surface. Due to their high viscosity and fluidity, they are used in high-speed machines or engine parts and also in transmission system where high circulation is required. They are selected for an engine according to SAE (Society of Automotive Engineers) numbered as 10 SAE, 20 SAE, 30 SAE, and 40 SAE. Examples of liquid lubricants are mineral oil, synthetic oils, whale oil, etc.
Liquid lubricants are classified as follows:
i. Mineral Oils:
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Mineral oils were used earlier. Because of their high cost, they have not been considered very popular.
ii. Animal Oils:
They were used in early days. They are costlier than mineral oils. They are derived from animal fat and also from seeds such as olive oil, resin, etc. Animal oils are also derived from sea fish such as whales.
iii. Synthetic Oils:
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They are produced from sand and coal. Polyalkylene glycol and silicon can be used as lubricants. They possess properties of good lubricants.
iv. Semi-Liquid Lubricants:
Grease can be considered as a semi-liquid lubricant. It is prepared by compounding petroleum product with soap mixture. It has high viscosity and has the ability to adhere perfectly on the surface where it is applied. Semi-liquid lubricants are suitable for the machine components where large clearance is available between the two parts in contact.
Grease can be used when the operating temperature is found very high. It can be used when high pressure is available between two parts and difficult to maintain an oil film between the surfaces to be lubricated. Gear drive, chain drive are the examples where grease can be applied.
Properties of Good Lubricants:
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The specific properties of lubricants can be presented as follows:
i. Volatility:
It represents the ability of the lubricants to vaporize at certain temperature. If the temperature during the working operation of machine is high, the lubricant starts evaporating and hence starts spoiling the lubricant properties. A good lubricant should always have less volatility.
ii. Adhesiveness (Wettability):
Most of the lubricants have the properties to stick on the metal surface in order to keep the two surfaces separated from each other. A good lubricant should have the properties to stick to the surface.
iii. Oiliness:
A film of oil is formed in between the two moving metal surfaces when the lubricant is subjected to high pressure. The property of good lubricating oil is to stick on the surface even if the pressure is high.
iv. Flash Point:
It is the temperature at which lubricating oil gives off sufficient vapors to ignite spontaneously upon being shown to flame. A good lubricant should have high flash point.
v. Fire Point:
This represents the minimum temperature at which the vapor catches fire and starts igniting continuously if the flame is brought in this vicinity. A good lubricant should have high fire points.
vi. Surface Tension:
It represents the property of a lubricant to form bubbles on the upper surface due to cohesive forces. A good lubricant should not form bubbles.