Tribological Systems – An Overview

Tribology is the science of lubrication, friction and wear in objects which rub against each other in motion.

A tribological system is the collection of four main parts. They are, the main body which moves, the opposing body which resists the motion, the intermediate material and the surrouding medium.

Consider a shaft supported by a bearing. In this example, the main body is the shaft, the opposing body is the bearing shell, the intermediate material is the grease and the surrouding medium are the other components on which the bearing and the shaft are mounted.

Tribological systems are classified into open and closed systems.

Bearings and seals are examples of closed tribological systems

Examples of open tribological systems are channels, pipes, etc. In these system, the intermediate material acts as the opposing body.


Fretting is the wear that occurs in two surfaces which are supposed to be stationary.  In certain conditions, very slight motion may occur between surfaces which are otherwise stationary.  This can be due to vibration or due to dimensional changes due to temperature variations.

If the change is repetitive, there can be wear along the surfaces.  This can, sometimes, be accompanied by chemical reactions, such as oxidation, which may occur along the surface.  This is known as fretting corrosion.

The worn out materials may get trapped between the surfaces and can cause further damage.

Fretting can be prevented by reducing micro-motion between the surfaces.  Special additives added to lubricants can also minimize fretting.

Total Acid Number (TAN)

The Total Acid Number (TAN) is the number of milligrams of Potassium Hydroxide (mgKOH) required to neutralize the acidic compounds present in one gram of oil.  It is represented in mgKOH/gram.

The TAN is an important parameter in lubricating oils and in hydraulic oils.  At high temperatures, the rate of oxidation in oils increases.  This can increase the TAN of the oil. The TAN can also increase with age.

Higher the TAN, greater is the deterioration in the oil.  While analysing the Total Acid Number (TAN), both trend and absolute values should be considered.



Total Base Number (TBN)

The Total Base Number (TBN) indicates the basic(alkaline) nature of an lubricating oil used in engines.  Higher the TBN, more basic is the oil.  Such an oil can withstand highly acidic impurities.

TBN is measured in mgKOH/gram.  This indicates the milligrams of Potassium Hydroxide that each gram of the oil is equal to.  Oils used in diesel engines will require a higher TBN as they have to withstand aggressive acidic residues.  Gasoline engine oils have a lower TBN.

As the engine runs, the TBN of the oil gradually falls due to reaction with the combustion residues.  The TBN has to be periodically checked.  A sample of the oil is sent to laboratories for checks.

Hydraulic Coefficients

In Fluid dynamics, the following four coefficients
are known as Hydraulic Coefficients.  They are

Coefficient of Contraction

This is the ratio of the area of the jet at Vena Contracta to the area of the orifice.

Coefficient of Velocity

This is the ratio of the actual velocity of the jet at vena contracta to the theoretical velocity of the jet

Coefficient of resistance

This is the ratio of loss of head in the orifice to the actual head.  This loss of head is due to the frictional losses when the fluid passes through the orifice.

Coefficient of Discharge

This is the ratio of the actual discharge to the theoretical discharge.

Hydrostatic Pumps

A hydrostatic pump is also known as a positive displacement pump.  In a hydrostatic pump, the volume of liquid discharged per cycle is constant.  In hydrostatic pumps, the liquid is drawn in to a chamber and then discharged. 

A reciprocating pump with a piston and a cylinder is an example for a hydrostatic pump.

Velocimetry – An Overview

Velocimetry refers to the study of the flow of fluids.  It is very important in the field of fluid dynamics and in the design of equipments, such as, pumps and turbines. 

The flow of liquids can be studied by adding small materials such as bits of paper or styrofoam in water.  This helps the observer to track the movement of the flow. 

The particles for observations used should have the same density as the fluids. 

Velocimetry can also be done using non contact methods such as with the use of lasers.  Laser Doppler velocimetry is a method, which uses two beams of lasers to analyze the flow of a fluid.

lbs. the symbol for pound

The pound, which is a unit of weight, is indicated by the letters lbs.  lbs comes from the latin Libra which is a short form of Libra Pondo (trans. a pound by weight).  This was a unit of weight used in the Roman empire.  

Over time, the libra got shortened to lbs.

Bar in pressure measurement

The bar is a very common unit of pressure.  It is used in the industry for normal operational measurements. It is popular as one bar is approximately equal to the normal atmospheric pressure (slightly lower, 0.987 atmosphere).  This helps people to visualize the pressure easily.

A millibar is 1/1000th of a bar.  A bar is equal to 1.01972 kg/cm2

Pounds per square inch (psi)

Pounds per square inch is a very common unit of pressure.  It belongs to the Imperial System of measurement.  
It is widely used in industries, tyres and other applications.  The unit indicates that the pressure is equivalent to a certain number of pounds applied to a square inch of area.  
There are two variations of the unit, the absolute and the gauge versions. 
psia (pounds per square inch absolute) indicates that the pressure value is made vacuum as the reference while psig (pounds per square inch gauge) indicates that the atmospheric pressure is the reference.
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