i ) Flash Point:-

• It can be defined as, the temperature at which the oil gives out enough vapours that ignite for a moment when a small flame is brought near it. It is a very important property of an oil, because it helps in knowing the highest temperature upto which an oil can be used as lubricant. This constant is usually determined by using Pensky-Marten’s flash apparatus.

• Significance: Any lubricating oil selected for a job should have a flash point which is reasonably above its working temperature. This ensures safety against fire hazards during storage, transport and use of the lubricating oil. It is also useful for identification and detection of contaminants in the oil.

   

ii ) Pour Point:-

• It can be defined as, the temperature at which the oil ceases to flow or pour. These characteristic physical constant indicate the suitability of oils at lower temperature, or in cold condition. The lubricating oils used in machines working at low temperature

• Significance: It is significant as they help us to know the lowest temperature upto which the oil can be suitable as a liquid lubricant. By knowing this, the machines to be prevented from getting jammed in places from cold region or during winter season in some areas in India.

   

iii) Viscosity Index:-

• It can be defined as rate of change of viscosity with respect ro temperature. If the decrease in viscosity is rapid the oil is said to have a low viscosity index and vice versa. A good lubricant should have high viscosity index. It is related to molecular weight of oils. Generally oils with higher molecular weights show higher viscosity.
• Significance: A lubricating oilselected for a job should have viscosity index as high ass possible. This helps in achieving desired results to control wear and tear of machine parts. It also helps to decide about the addition of blending agents to improve the property of lubricating oil.