There are many types of machines where moving parts rub against each other. Mutual friction between parts provides resistance to movement. Friction is a term used to describe this resistance.
Moving parts are subjected to a great deal of wear and tear. Lubricants are substances that are introduced between moving or sliding surfaces to reduce friction.
By keeping the moving or sliding surfaces apart, lubricants minimize friction and consequent material destruction. Lubrication is a term that is used to refer to the process of introducing lubricant into a moving or sliding surface as a measure to reduce friction between the two.
Related Guide: The Selection of Lubricants in 2022 [An Ultimate Guide]
Functions of Lubricants
- The rubbing surfaces are reduced in wear and tear by introducing lubricant between them, so there is no direct metal-to-metal contact between them
- There is a reduction in metal expansion and damage caused by frictional heat
- Due to its heat transfer properties, it serves as a coolant for metal
- This reduces maintenance costs in the long run
- A reduction in power loss is also achieved in internal combustion engines
Classification of Lubricants with their Names
According to their physical state, lubricants are classified as follows:
- The liquid lubricant
- Semi-solid lubricants
- Solid lubricants
The three kinds of liquid lubricants include animal or vegetable oils, mineral oils, and blended oils.
Animal or Vegetable Oils
Various oil types are used in the production of animal oils, such as crude fats, vegetable oils such as cotton seed oil, as well as caster oils.
In addition to being oily, these oils contain good lubricity, which makes them excellent adhesives for metal surfaces, even under extreme temperatures and heavy loads.
It must be noted that they suffer from some disadvantages, such as being expensive, readily oxidized to produce gummy products, and hydrolyzing easily when in contact with moist air or water.
In order to reduce the risk of contamination, they are only rarely used today for lubrication purposes. But they are still used for improving oiliness in petroleum-based lubricants.
Mineral or Petroleum Oils
There are about 12 to 50 carbon atoms in these lower molecular-weight hydrocarbons. Due to their affordability, availability, and stability under service conditions, they are widely used.
However, mineral oils are less oily, thus adding compounds with a higher molecular weight such as oleic acid or stearic acid can increase their oiliness.
It is important to add additives to make single oils perform well since they possess all the properties needed for a good lubricant. Oils that contain additives like these are called blended oils.
The common additives used are oleic acid, stearic acid, palmitic acid, or vegetable oils like coconut oil, castor oil, etc.
Lubricating oils combined with thickeners make grease, which is a semi-solid lubricant. An essential component of the system is the lubricating oil, which can either be synthetic hydrocarbons of low to high viscosities or petroleum oils. Its main component is a chemical mixture of soap made from Li, Na, Ca, Ba, Al, etc., which is referred to as the thickener.
The most common types of thickeners are not soaps but other synthetic hydrocarbons and polymers, clays, etc, such as carbon black, silica gel, polyurea, etc. When grease is used at a slower speed, it can carry a much heavier load.
As grease has much higher internal resistance than lubricating oils, oil is a better alternative. Grease does not disperse heat from bearings as effectively as lubricating oils, so it works at a relatively lower temperature than lubricating oils.
Generally, they are used where lubricating oils or grease no longer provide a consistent lubricating layer. There is a chance for lubricating oils or grease to become contaminated. An excessive load or temperature might affect their functionality.
This kind of adhesive can be used either in the dry powder form or with binders to make it more sticky to metal surfaces while in use, depending on the type of application. They can be found as dispersions in non-volatile carriers such as soaps, fats, waxes, etc., or as soft metal films in soft plastics and rubbers.
Graphite, molybdenum disulfide, tungsten disulfide, and zinc oxide are some of the most common solid lubricants used today. Suitable for continuous operation, and can bear temperatures as high as 650° C. Suitable for increasing lubricant load carrying capacity as well.
This solid lubricant can be used either in powdered or suspension form. It is soft to the touch, non-flammable, and stable up to 375°C. There are layers of graphite sheets attached to each other by weak van Der Waal’s forces, giving graphite its flat plate-like structure.
As a result of these parallel layers, graphite makes a great lubricant. Graphite also absorbs oil and is easily wetted.
The structure consists of two layers of sulphur atoms sandwiching a layer of molybdenum atoms. These layers are easy to slide over one another due to poor interlaminar attraction. A temperature of 400°C does not affect its stability.
Compared to conventional mineral-based lubricants, synthetic lubricants combine synthetic base oil with thickeners and additives to provide several performance advantages. Synthetic oil is made of artificially synthesized compounds.
Rather than using whole crude oil, synthetic lubricants can be made from chemically modified petroleum components. In general, synthetic oils are preferred over mineral oils when operating in extreme temperatures, since they possess superior mechanical and chemical properties.
Synthetic oils are typically needed for aircraft jet engines, but not for piston engines. In addition to providing environmental benefits, synthetic lubricants are also used in metal stamping because they are more eco-friendly than conventional petroleum and animal-based lubricants.
Advantages of Synthetic Lubricants
Synthetic motor oils have the following technical advantages:
- Improved viscosity performance at low- and high-temperatures
- The viscosity index is better
- A higher degree of chemical and shear stability
- A reduction in evaporative loss
- Protection against oxidation, thermal breakdown, and oil sludge
- Environmental benefit: less waste generated from used oil, extended drain intervals
- Fuel economy improved in some configurations
- Improved lubrication in cold weather
- The engine may last longer
- A reduction in oil burn-off and less risk of clogging of oil passageways caused by “ash” and other deposits in turbochargers and superchargers
- A reduction in initial drag leads to an increase in horsepower and torque
- Fleet tests have documented an increase in fuel economy from 1.8% to up to 5%
Disadvantages of Synthetic Lubricants
These are some of the disadvantages associated with synthetic motor oils:
- Mineral oils are less expensive than synthetic oils
- A possible problem with decomposition can occur in certain chemical environments
1. What are the classifications of lubricants?
Lubricants are classified in several ways; these could be liquid, semisolid (greases), and solids such as graphite, molybdenum disulfide, boron nitride, tungsten disulfide, and polytetrafluoroethylene.
2. What is the SAE Classification of lubricants?
SAE number, code for specifying the viscosity of lubricating oil, established by the U.S. Society of Automotive Engineers. Oil flow is more readily achieved with a lower number of crankcase lubricants, like 5 to 50, and a higher number of transmission and axle oils.
3. Why is SAE called SAE?
SAE International formerly named the Society of Automotive Engineers, is a United States-based, globally active professional association and standards-developing organization for engineering professionals in various industries.
4. What is ISO oil and SAE oil?
In SAE 10W, the ISO 32 standard is represented, in SAE 20 the ISO 46 standard is represented, and in SAE 30 the ISO 100 standard is represented. As you can see, there is a bit of a difference between ISO 68 and SAE 30. The viscosity of the fluid largely determines the oil temperatures within which the hydraulic system can safely operate.