Lubricants and their Applications

In this tutorial, you will learn about lubricants and the lubrication process in general. You will gain a deeper understanding of why lubrication is required, how are lubricants selected and what properties are associated with a lubricant.

Contents:

  1. What is Viscosity?
  2. The Viscosity Index
  3. Why do we Need Lubricants?
  4. Properties of Lubrications
  5. Modes of Lubrications
  6. Selection of Lubricants
  7. What are Lubricating Oils?
  8. What is Grease?

What is Viscosity?

Viscosity is defined as the internal frictional resistance offered by a fluid to change its shape or relative motion of its parts. The following figure shows the oil film placed between two parallel plates.

Fluid film between parallel plates
  • The lower plate is stationary while the upper plate is moved with a velocity U using a force P.
  • The molecules of oil are visualized as small balls, which roll in layers between two plates. The oil will stick to both surfaces, and therefore the layer of molecules in contact with the stationary plate has zero velocity. Similarly, the layer of molecules in contact with the upper plate will move with a velocity U.
  • The tangential force per unit area (P/A) is shear stress, while the ratio (U/h) is the rate of shear.
  • According to Newton’s law of viscosity, the shear stress is proportional to the rate of shear at any point in the fluid.
    P= μA \((\frac{U}{h})\)
  • The constant of proportionality m in the above equations is called the absolute viscosity.
  • The popular unit of viscosity is the Poise. Poise is a large unit and viscosities of most of the lubricating oils are given in terms of centipoise (cP), which is one-hundredth of a Poise.

The Viscosity Index

The rate of change of viscosity with respect to temperature is indicated by a number called Viscosity Index (VI). The viscosity index is defined as an arbitrary number used to characterize the variation of the kinematic viscosity of lubricating oil with temperature.

  • The viscous resistance of lubricating oil is due to intermolecular forces. As the temperature increases, the oil expands and the molecules move further apart, decreasing the intermolecular force in consequence. Therefore, the viscosity of the lubricating oil decreases with increasing temperature.
  • In practice, it is difficult to experiment with two parallel plates for the measurement of viscosity. The popular method of determining viscosity is to measure the time required for a given volume of oil to pass through a capillary tube of standard dimensions.
  • To find out the viscosity index of the oil, two groups of reference oils are considered. One group consists of oils having VI = 100 and these oils have a very small change of viscosity with temperature. The other group consists of oils having VI = 0 and these oils have a very large change of viscosity with temperature.
  • The given oil is compared with these two reference oils, one with a viscosity index of 100 and the other of zero.
  • The viscosity index indicates the rate of change of viscosity with temperature, as compared to oils with very small or very large rates of change of viscosity with temperature.

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Why do we Need Lubricants?

Lubricants are widely used in all machinery in the industry and are important for three major reasons. The following figure shows the close-up of metal surfaces in contact.

Surface Contact
  • Lubricants reduce friction between contacting surfaces by creating a film of lubricating material between contacting surfaces. These surfaces slide over the film with less friction.
  • Lubricants also serve as cleaning agents for precision machinery. Flowing lubricants prevent the accumulation of contaminants and foreign particles in the contact region and thus reduces wear.
  • In certain applications, lubricants also serve as coolants. Flowing lubricants carry away the heat which may be generated during the surface contact, and disperse the heat in their cycles before re-entry.

Properties of Lubrications

Lubricants come with a wide range of possibilities, and it is the responsibility of the designer to designate a proper lubricant for any system. An ideal lubricant has the following characteristics.

  • The viscosity of a lubricant determines the thickness of the layer of oil between metallic surfaces in reciprocal movement.
  • The pour point refers to the minimum temperature at which a lubricant can flow. Below the pour point, the oil starts to thicken and stops flowing freely. Lubricants used in cold applications must have a pour point below the working temperature.
  • The flashpoint is the minimum temperature at which an oil-vapor-air mixture becomes inflammable. Lubricants get heated during application. The flashpoint must be greater than the maximum temperature the lubricant can attain in a system.
  • Good Lubricants have high thermal stability and resistance to oxidation.

Modes of Lubrications

Lubrication is the application of a friction-reducing coating between moving surfaces in contact to reduce friction and wear. Depending on the application, there are several modes of application for lubricants.

  • Hydrostatic Lubrication is the process of continuously supplying a lubricant to the sliding interface at high hydrostatic pressure. This necessitates the use of a reservoir (sump), a pump, and piping to disperse the lubricant. When done correctly and with the right bearing clearances, this method can eliminate any metal-to-metal contact.
  • Hydrodynamic Lubrication is the application of enough lubricant to the sliding interface to allow the mated surfaces’ relative velocity to pump the lubricant into the gap and separate the surfaces on a dynamic film of liquid. It works best in journal bearings, where the shaft and bearing form a narrow annulus inside their clearance that traps the lubricant and allows the shaft to pump it around it. Because there is a leakage channel at the ends, a constant supply of oil must be given to replace the losses.
  • Elastohydrodynamic lubrication (EHL) is the normal lubrication regime for friction pairs in unconformal contact which have elastic contact under very high pressure, such as ball bearings and gears. EHL is a form of hydrodynamic lubrication that takes the elastic deflection of solid contacting surfaces into account.

Selection of Lubricants

Lubricating oils are commercially available under different trade names. Indian Oil Corporation5 manufactures a wide range of lubricating oils and greases. Commercial lubricants contain a base oil and a group of additives, which are suitable for a given application.

  • There are five grades from SAE 10 to SAE 50. These base oils are blended with viscosity index improver, detergent additives, and oxidation inhibitors.
  • These oils are used for engine lubrication of petrol and diesel vehicles. They are also used for generators and pumping sets operating on diesel engines.
  • There are two different classes of crankcase oil—Servo Engine Oil and Servo Super. The second is superior and costly. It is used for heavy-duty internal combustion engines.
  • Commercial lubricating oils for gears consist of SAE 80, SAE 90, and SAE 140 as base oils and a mixture of extreme-pressure additives, oxidation inhibitors, and oiliness additives.

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What are Lubricating Oils?

Lubricating oil must be available in a wide range of viscosities and should have little change in viscosity of the oil with a temperature change. The oil should be chemically stable with the bearing material and atmosphere at all temperatures encountered in the application.

  • Lubricating oils are divided into two groups, mineral oils, and vegetable or animal oils.
  • Mineral oils consist of hydrocarbons, which are obtained by the distillation of crude oil. There are two different classes of mineral oils, those with a paraffinic series and those with a naphthenic series.
  • Mineral Oils are chemically inert and have a large range of viscosities. They have little tendency to oxidize or form corrosive acids.
  • Vegetable oils used for lubrication are castor oil, rapeseed oil, palm oil, and olive oil. Lubricating oils of animal origin are lard oil, tallow oil, and certain oils obtained from marine species, such as whales, sperm, or dolphin jaws.
  • These oils are sometimes referred to as fixed oils because they are non-volatile unless there is chemical decomposition.
  • They retain their viscosities at a high temperature much better than mineral oils.

What is Grease?

Grease is a semisolid substance, composed of mineral oil and soap. Sometimes additives are added to this mixture to achieve specific properties, such as chemical stability or oiliness. The soap is present in the form of fibers, which form a matrix for the oil by the swelling mechanism.

  • Grease is thixotropic and it undergoes a change in apparent viscosity with the amount of shearing. When the journal is stationary, the grease in the clearance space is quite rigid and immobile but when the journal starts rotating, the viscosity of the grease approaches that of the base oil in the grease.
  • Grease is normally recommended for inaccessible parts, where leakage of oil is objectionable. It is also used in applications where clearance is large due to rough machining.
  • Lime-base grease consists of calcium soap in mineral oils of grades SAE 10 to SAE 40. It is insoluble in water. It is buttery and resists flow. However, it tends to channel and separate from the machine component by centrifugal action.
  • Soda-base grease is produced from sodium soap. It has more resistance to decomposition at high temperatures and pressures. Soda-base grease is water-soluble and possesses a sponge-like structure.
  • Lithium soap grease has excellent oxidation resistance and is used for water pumps, wheel bearings, and chassis fittings.

Key Points to Remember

Here is the list of key points we need to remember about “Lubricants and their Applications”.

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  • Viscosity is the internal friction of a fluid that prevents it from flowing freely. Viscosity Index is a measure of the body’s viscosity.
  • Lubricants help reduce the friction between mating components, prevent contamination, and carry away any heat generated during relative motion.
  • A good lubricant must have a pour point below the system’s operating temperature.
  • A good lubricant must have a flash point above the system’s maximum temperature.
  • In Hydrostatic lubrication, the fluid film is maintained by continuously pumping the lubricant between the surfaces.
  • In Hydrodynamic lubrication, the fluid film is created by the mating surfaces when in relative motion.

If you find any mistake above, kindly email to [email protected]

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Manish Bhojasia - Founder & CTO at Sanfoundry
Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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