Lubricants play a vital role in the smooth functioning of various machinery and mechanical systems, serving as the unsung heroes that reduce friction and wear between moving components. These versatile substances are essential in a wide range of industries, from automotive and manufacturing to aerospace and energy production. First and foremost, lubricants are primarily known for their lubricity, the ability to reduce friction and minimize wear between contacting surfaces. This property ensures that moving parts can glide past each other with minimal resistance, reducing heat and preventing damage due to mechanical wear. The capacity to minimize friction is a core function of lubricants and one of their most essential properties.
In this blog, we will explore the fundamental properties of lubricants that make them indispensable in these applications.
Table of Contents
Properties of Lubricants
Viscosity
It is the property of liquid by which it offers resistance to its flow (the resistance to the flow of liquid is known as viscosity).
The unit of viscosity is poise. It is the most important single property of any lubricating oil because it is the main determinant of the operating characteristics of the lubricant. If the viscosity of the oil is too low, a liquid oil film cannot be maintained between two moving/sliding surfaces.
On the other hand, if the viscosity of the oil is too high, excessive friction will result. Effect of temperature on viscosity: The viscosity of liquids decreases with increasing temperature and, consequently, the lubricating oil becomes thinner as the operating temperature increases.
Hence, the viscosity of good lubricating oil should not change much with a temperature change, so that it can be used continuously, under varying conditions of temperature. The rate at which the viscosity of lubricating oil changes with temperature is measured by an arbitrary scale, known as the Viscosity Index (V. I).
If the viscosity of lubricating oil falls rapidly as the temperature is raised, it has a low viscosity index. On the other hand, if the viscosity of lubricating oil is only slightly affected by raising the temperature, its viscosity index is high.
Flash Point and Fire Point
Flash point is the lowest temperature at which the lubricant oil gives off enough vapors that ignite for a moment when a tiny flame is brought near it.
The fire point is the lowest temperature at which the vapors of the lubricant oil burn continuously for at least five seconds when a tiny flame is brought near it.
In most cases, the fire points are 5° C to 40° C higher than the flash points. The flash and fire do not have any bearing on the lubricating property of the oil, but these are important when oil is exposed to high-temperature service.
A good lubricant should have a flash point at least above the temperature at which it is to be used. This safeguards against the risk of fire, during the use of lubricant.
Cloud Point and Pour Point
When the lubricant oil is cooled slowly, the temperature at which it becomes cloudy or hazy in appearance is called its cloud point.
The temperature at which the lubricant oil ceases to flow or pour is called its pour point.
Cloud and pour points indicate the suitability of lubricant oil in cold conditions.
Lubricant oil used in a machine working at low temperatures should possess a low pour point; otherwise, solidification of lubricant oil will cause jamming of the machine. It has been found that the presence of waxes in the lubricant oil raises the pour point.
Aniline Point
The aniline point of the lubricant oil is defined as the minimum equilibrium solution temperature for equal volumes of aniline and lubricant oil samples. It indicates the possible deterioration of the lubricant oil in contact with rubber sealing; packing, etc.
Aromatic hydrocarbons tend to dissolve natural rubber and certain types of synthetic rubbers. Consequently, low aromatic content in the lubricant oil is desirable. A higher aniline point means a higher percentage of paraffinic hydrocarbons and hence, a lower percentage of aromatic hydrocarbons.
Aniline point is determined by mixing mechanically equal volumes of the lubricant oil samples and aniline in a test tube. The mixture is heated, till the homogenous solution is obtained.
Then, the tube is allowed to cool at a controlled rate. The temperature at which the two phases (the lubricant oil and aniline) separate is recorded at the aniline point.
Corrosion Stability
The corrosion stability of the lubricant oil is estimated by carrying out a corrosion test. A polished copper strip is placed in the lubricant oil for a specified time at a particular temperature.
After the stipulated time, the strip is taken out and examined for corrosion effects. If the copper strip has tarnished, it shows that the lubricant oil contains any chemically active substances that cause the corrosion of the copper strip.
Good lubricant oil should not affect the copper strip. To retard the corrosion effects of the lubricant oil, certain inhibitors are added to them. Commonly used inhibitors are organic compounds containing P, As, Cr, Bi, or Pb.
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Characteristics of Lubricants
Essential requirements or characteristics of a good lubricant are as follows:
- It should have a high viscosity index.
- It should have flash and fire points higher than the operating temperature of the machine.
- It should have high oiliness.
- The cloud and pour points of a good lubricant should always be lower than the operating
- the temperature of the machine.
- The volatility of the lubricating oil should be low.
- It should deposit the least amount of carbon during use.
- It should have a higher aniline point.
- It should possess a higher resistance to oxidation and corrosion.
- It should have good detergent quality.