Tribology_01

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STUDY OF VARIOUS PARAMETERS

INTRODUCTION:-

Tribology is the science and engineering of

interacting surfaces in relative motion. It includes the study and application

of the principles of friction, lubrication and wear. Tribology is a branch of

mechanical engineering. Tribology may have its origin from a greek word

Tribo Rubbing process, Ology The Study

The study of tribology is commonly applied

in bearing design but extends into almost all other aspects of modern

technology, even to such unlikely areas as hair conditioners and cosmetics

such as lipstick, powders and lipgloss.

Any product where one material slides or rubs

over another is affected by complex tribological interactions, whether

lubricated like hip implants and other artificial prosthesis or unlubricated as

in high temperature sliding wear in which conventional lubricants can not

be used but in which the formation of compacted oxide layer glazes have

been observed to protect against wear.

Tribology plays an important role in

manufacturing. In metal-forming operations, friction increases tool wear

and the power required to work a piece. This results in increased costs due

to more frequent tool replacement, loss of tolerance as tool dimensions

shift, and greater forces are required to shape a piece. A layer of lubricant

which eliminates surface contact virtually eliminates tool wear and

decreases needed power by one third.

VARIOUS PARAMETERS:-


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Viscosity:- Viscosity is a measure of

the resistance of a fluid which is being deformed

by either shear stress or tensile stress. In

everyday terms (and for fluids only), viscosity is

"thickness" or "internal friction". Thus, water is

"thin", having a lower viscosity, while honey is

"thick", having a higher viscosity. Put simply, the

less viscous the fluid is, the greater its ease of

movement (fluidity).

Viscosity describes a fluid's

internal resistance to flow and

may be thought of as a

measure of fluid friction. All

real fluids (except superfluids)

have some resistance

to stress and therefore are viscous, but a fluid which has no resistance to

shear stress is known as an idealfluid or inviscid fluid.

DYNAMIC VISCOSITY:-

The usual symbol for dynamic viscosity used by

mechanical and chemical engineers as well as fluid dynamicists is the

Greek letter mu ().

FIG: Schematic representation of the fluid

separating two surfaces

The SI physical unit of dynamic viscosity is

the pascal-second (Pas), (equivalent to Ns/m2, or kg/(ms)). If a fluid with a

viscosity of one Pas is placed between two plates, and one plate is pushed

sideways with a shear stress of onepascal, it moves a distance equal to the

thickness of the layer between the plates in one second.


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The CGS physical unit for dynamic viscosity

is thepoise (P), named after Jean Louis Marie Poiseuille. It is more

commonly expressed, particularly in ASTM standards, as CENTIPOISE (cP).

Water at 20 C has a viscosity of 1.0020 cP or 0.001002 kg/(ms).1 P = 1 gcm1s1.

1 Pas = 1 kgm1s1 = 10 P.

The relation to the SI unit is

1 P = 0.1 Pas,

1 cP = 1 mPas = 0.001 Pas.

Kinematic viscosity:-

In many situations, we are concerned with

the ratio of the inertial force to the viscous force (i.e. the Reynolds

number,Re = VD / ) , the former characterized by the fluid density . This

ratio is characterized by the kinematic viscosity(Greek letter nu, ), defined

as follows:

The SI unit ofis m2/s. The SI unit of is kg/m3.

The CGS physical unit for kinematic viscosity is the STOKES (St), named

after George Gabriel Stokes. It is sometimes expressed in terms

ofCENTISTOKES (cSt). In U.S. usage, STOKE is sometimes used as the

singular form.

1 St = 1 cm2s1 = 104 m2s1.

1 cSt = 1 mm2s1 = 106m2s1.

Water at 20 C has a kinematic viscosity of

about 1 cSt.

The

kinematic viscosity is sometimes referred

to as DIFFUSIVITY OF MOMENTUM,

because it has the same unit as and is

comparable to diffusivity of


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selected in such a way that one of them has the viscosity index equal to

zero (VI=0) and the other has the viscosity index equal to one hundred

(VI=100) at 100F (37.8C) but they both have the same viscosity as the oil

of interest at 210F (98.89C).Since Pennsylvania and Gulf Coast oils have

the same viscosity at 210F (98.9C) they were initially selected as

reference oils. Oils made from Pennsylvania crude were assigned the

viscosity index of100 whereas oils made from the Gulf Coast crude the

viscosity index of 0. The viscosity index can be calculated from the

following formula:

VI = (L U) / (L H) 100 (2.8)

Firstly the kinematic viscosity of the oil of

interest is measured at 40C (U) and at 100C.Then from (ASTM D2270),

looking at the viscosity at 100C of the oil of interest, the corresponding

values of the reference oils, L and H are read. Substituting the

obtainedvalues of U, L and H into the above equation yields the

viscosity index. Note that the viscosity index is an inverse measure of the

decline in oil viscosity with temperature. High values indicate that the oil

shows less relative decline in viscosity with temperature. The viscosity

index of most of the refined mineral oils available on the market is about

100, whereas multigrade and synthetic oils have higher viscosity indices of

about 150.


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VISCOMETERS

Viscometer :

A viscometer(also called viscosimeter) is an instrument

used to measure the viscosity of a fluid. For liquids with viscosities which vary with flow conditions, an

instrument called a rheometer is used. Viscometers only measure under one flow condition.

In general, either the fluid remains stationary and an object moves

through it, or the object is stationary and the fluid moves past it. The drag caused by relative motion of

the fluid and a surface is a measure of the viscosity. The flow conditions must have a sufficiently small

value of Reynolds number for there to be laminar flow.

At 20.00 degrees Celsius the viscosity of water is 1.002 mPas and its

kinematic viscosity (ratio of viscosity to density) is 1.0038 mm2/s. These values are used for calibrating

certain types of viscometer.


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TYPES OF VISCOMETERS:-

U-tube viscometers:-

These devices also are known as glass capillary viscometers orOstwald

viscometers, named after Wilhelm Ostwald. Another version is the Ubbelohde viscometer, which

consists of a U-shaped glass tube held vertically in a controlled temperature bath. In one arm of the U is

a vertical section of precise narrow bore (the capillary). Above this is a bulb, with it is another bulb lower

down on the other arm. In use, liquid is drawn into the upper bulb by suction, then allowed to flow down

through the capillary into the lower bulb. Two marks (one above and one below the upper bulb) indicate

a known volume. The time taken for the level of the liquid to pass between these marks is proportional

to the kinematic viscosity. Most commercial units are provided with a conversion factor, or can be

calibrated by a fluid of known properties.

The time required for the test liquid to flow through a

capillary of a known diameter of a certain factor between two marked points is

measured. By multiplying the time taken by the factor of the viscometer, the

kinematic viscosity is obtained.

Such viscometers are also classified as direct flow or reverse flow. Reverse flow

viscometers have the reservoir above the markings and direct flow are those with

the reservoir below the markings. Such classifications exists so that the level can be

determined even when opaque or staining liquids are measured, otherwise the liquid

will cover the markings and make it impossible to gauge the time the level passes the mark. This also

allows the viscometer to have more than 1 set of marks to allow for an immediate timing of the time it

takes to reach the 3rd mark, therefore yielding 2 timings and allowing for subsequent calculation of

Determinability to ensure accurate results.

Falling sphere viscometers:-

Stokes' law is the basis of the falling sphere viscometer, in which

the fluid is stationary in a vertical glass tube. A sphere of known size and density is allowed to descend

through the liquid. If correctly selected, it reaches terminal velocity, which can be measured by the time

it takes to pass two marks on the tube. Electronic sensing can be used for opaque fluids. Knowing the

terminal velocity, the size and density of the sphere, and the density of the liquid, Stokes' law can be

used to calculate the viscosity of the fluid.


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A series of steel ball bearings of different diameter is normally used in the classic

experiment to improve the accuracy of the calculation. The school experiment

uses glycerine as the fluid, and the technique is used industrially to check the

viscosity of fluids used in processes. It includes many different oils,

and polymer liquids such as solutions.

In 1851, George Gabriel Stokes derived an expression for the frictional force (also

called drag force) exerted on spherical objects with very small Reynolds

numbers(e.g., very small particles) in a continuous viscous fluid by changing the

small fluid-mass limit of the generally unsolvable Navier-Stokes equations:

where:

Fis the frictional force,

ris the radius of the spherical object,

is the fluid viscosity, and

vis the particle's velocity.

If the particles are falling in the viscous fluid by their own weight, then a terminal velocity,

also known as the settling velocity, is reached when this frictional force combined with

the buoyant force exactly balance the gravitational force. The resulting settling velocity

(or terminal velocity) is given by:

where:

Vs is the particles' settling velocity (m/s) (vertically downwards ifp > f, upwards

ifp < f),

ris the Stokes radius of the particle (m),

gis the gravitational acceleration (m/s2),

p is the density of the particles (kg/m3),

f is the density of the fluid (kg/m3), and

is the (dynamic) fluid viscosity (Pa s).

Note that Stokes flow is assumed, so the Reynolds number must be small.

A limiting factor on the validity of this result is the Roughness of the

sphere being used.A modification of the straight falling sphere viscometer is a rolling ball viscometerwhich times a ball roling down a slope whilst immersed in the test fluid. This can be further improved by


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using a patented V plate which increases the number of rotations to distance traveled, allowing smaller

more portable devices. This type of device is also suitable for ship board use.

Falling Piston Viscometer

Also known as Norcross viscometer due to inventor, Austin Norcross. Principle of viscosity

measurement in this rugged and sensitive industrial device is based on piston and cylinder assembly.

Piston is periodically raised by an air lifting mechanism, drawing the material being measured down

through the clearance(gap)between the piston and the wall of the cylinder into the space which is

formed below the piston as it is raised. The assembly is then typically held up for a few seconds, then

allowed to fall by gravity, expelling the sample out through the same path that it entered, creating a

shearing effect on the measured liquid, which makes this viscometer particularly sensitive and good for

measuring certain thixotropic liquids. The time of fall is a measure of viscosity, with the clearance

between the piston and inside of the cylinder forming the measuring orifice. The viscosity controller

measures the time of fall (Time-of-fall seconds being measure of viscosity) and displays the resulting

viscosity value. Controller can calibrate time-of-fall value to cup seconds SSU or

centipoise.

Industrial use is popular due to simplicity, repeatability,

low maintenance and longevity. This type of measurement is not affected by flow

rate or external vibrations. Principle of operation can be adopted for many different

conditions, making it ideal for process control environment.

Oscillating Piston Viscometer

Sometimes referred to as Electromagnetic Viscometer or EMV viscometer, was

invented at Cambridge Viscosity in 1986. The sensor (see figure below) comprises a measurement

chamber and magnetically influenced piston. Measurements are taken whereby a sample is first

introduced into the thermally controlled measurement chamber where the piston resides. Electronics

drive the piston into oscillatory motion within the measurement chamber with a controlled magnetic field.

A shear stress is imposed on the liquid (or gas) due to the piston travel and the viscosity is determined

by measuring the travel time of the piston. The construction parameters for the annular spacing

between the piston and measurement chamber, the strength of the electromagnetic field, and the travel

distance of the piston are used to calculate the viscosity according to Newtons Law of Viscosity.


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The Oscillating Piston Viscometer technology has been adapted for small sample viscosity and micro-

sample viscosity testing in laboratory applications. It has also been adapted to measure high pressure

viscosity and high temperature viscosity measurements in both laboratory and process environments.

The viscosity sensors have been scaled for a wide range of industrial applications such as small size

viscometers for use in compressors and engines, flow-through viscometers for dip coating processes,

in-line viscometers for use in refineries, and hundreds of other applications. Improvements in sensitivity

from modern electronics, is stimulating a growth in Oscillating Piston Viscometer popularity with

academic laboratories exploring gas viscosity.

Vibrational viscometers

Vibrational viscometers date back to the 1950s Bendix instrument, which is of a class that operates by

measuring the damping of an oscillating electromechanical resonator immersed in a fluid whose

viscosity is to be determined. The resonator generally oscillates in torsion or transversely (as a

cantilever beam or tuning fork). The higher the viscosity, the larger the damping imposed on the

resonator. The resonator's damping may be measured by one of several methods:

1. Measuring the power input necessary to keep the oscillator vibrating at a constant amplitude.

The higher the viscosity, the more power is needed to maintain the amplitude of oscillation.

2. Measuring the decay time of the oscillation once the excitation is switched off. The higher the

viscosity, the faster the signal decays.

3. Measuring the frequency of the resonator as a function of phase angle between excitation and

response waveforms. The higher the viscosity, the larger the frequency change for a given

phase change.
http://en.wikipedia.org/wiki/File:OPvisc1.JPG


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The vibrational instrument also suffers from a lack of a defined shear field, which makes it unsuited to

measuring the viscosity of a fluid whose flow behaviour is not known before hand.

Vibrating viscometers are rugged industrial systems used to measure viscosity in the process condition.

The active part of the sensor is a vibrating rod. The vibration amplitude varies according to the viscosityof the fluid in which the rod is immersed. These viscosity meters are suitable for measuring clogging

fluid and high-viscosity fluids, including those with fibers (up to 1,000 Pas). Currently, many industries

around the world consider these viscometers to be the most efficient system with which to measure the

viscosities of a wide range of fluids; by contrast, rotational viscometers require more maintenance, are

unable to measure clogging fluid, and require frequent calibration after intensive use. Vibrating

viscometers have no moving parts, no weak parts and the sensitive part is very small. Even very basic

or acidic fluids can be measured by adding a protective coating or by changing the material of the

sensor to a material such as 316L, SUS316, or enamel.

Rotational viscometers

Rotational viscometers use the idea that the torque required to turn

an object in a fluid is a function of the viscosity of that fluid. They measure the torque required to rotate

a disk or bob in a fluid at a known speed.

'Cup and bob' viscometers work by defining the exact volume of a

sample which is to be sheared within a test cell; the torque required to achieve a certain rotational

speed is measured and plotted. There are two classical geometries in "cup and bob" viscometers,

known as either the "Couette" or "Searle" systems - distinguished by whether the cup or bob rotates.

The rotating cup is preferred in some cases because it reduces the onset of Taylor vortices, but is more

difficult to measure accurately.

'Cone and Plate' viscometers use a cone of very shallow angle in bare

contact with a flat plate. With this system the shear rate beneath the plate is constant to a modest

degree of precision and deconvolution of a flow curve; a graph of shear stress (torque) against shear

rate (angular velocity) yields the viscosity in a straightforward manner.

Stabinger viscometer

By modifying the classic Couette rotational viscometer, an accuracy

comparable to that of kinematic viscosity determination is achieved. The internal cylinder in the

Stabinger Viscometer is hollow and specifically lighter than the sample, thus floats freely in the sample,


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centered by centrifugal forces. The formerly inevitable bearing friction is thus fully avoided.

The speed and torque measurement is implemented without direct contact by a rotating magnetic

field and an eddy current brake. This allows for a previously unprecedented torque resolution of

50 pNmand an exceedingly large measuring range from 0.2 to 20,000 mPas with a single measuring

system. A built-in density measurement based on the oscillating U-tube principle allows the

determination of kinematic viscosity from the measured dynamic viscosity employing the relation

The Stabinger Viscometer was presented for the first time by Anton Paar GmbH at the ACHEMA in the

year 2000. The measuring principle is named after its inventor Dr. Hans Stabinger.

Stormer viscometer

The Stormer viscometeris a rotation instrument used to determine the viscosity of paints, commonly

used in paint industries. It consists of a paddle-type rotor that is spun by an internal motor, submerged

into a cylinder of viscous substance. The rotor speed can be adjusted by changing the amount of load

supplied onto the rotor. For example, in one brand of viscometers, pushing the level upwards decreases

the load and speed, downwards increases the load and speed.

The viscosity can be found by adjusting the load until the rotation velocity is 200 rotations per minute.

By examining the load applied and comparing tables found on ASTM D 562, one can find the viscosity

in Krebs units (KU), unique only to the Stormer type viscometer.

This method is intended for paints applied by brush or roller.

Bubble viscometer

Bubble viscometers are used to quickly determine kinematic viscosity of known liquids such as resins

and varnishes. The time required for an air bubble to rise is inversely proportional to the visosity of the

liquid, so the faster the bubble rises, the lower the viscosity. The Alphabetical Comparison Method uses

4 sets of lettered reference tubes, A5 through Z10, of known viscosity to cover a viscosity range from
http://en.wikipedia.org/wiki/File:StabingerViskosimeter.jpg


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0.005 to 1,000 stokes. The Direct Time Method uses a single 3-line times tube for determining the

"bubble seconds", which may then be converted to stokes.

Miscellaneous viscometer types

Other viscometer types use balls or other objects. Viscometers that can

characterize non-Newtonian fluids are usually called rheometers or plastometers.

In the I.C.I "Oscar" viscometer, a sealed can of fluid was oscillated torsionally, and by clever

measurement techniques it was possible to measure both viscosity and elasticity in the sample.

The Marsh funnel viscometer measures viscosity from the time (efflux time) it takes a known volume of

liquid to flow from the base of a cone through a short tube. This is similar in principle to the flow cups

(efflux cups) like the Ford, Zahn and Shell cups which use different shapes to the cone and various

nozzle sizes.

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