UNIT 1

STUDY OF VARIOUS PARAMETERS

UNIT 5

AIR LUBRICATED BEARINGS

UNIT 8

BEARING MATERIALS

UNIT 6

FRICTION

Written by

Challa Sai Priyatham

Kolla Chaitanya Krishna

UNIT 1

TRIBOLOGY INTRODUCTION

Tribology, which focuses on friction, wear and lubrication of interacting surfaces in

relative motion, is a new field of science defined in 1967 by a committee of the Organization

for Economic Cooperation and Development. The word ‘Tribology’ is derived from the

Greek word ‘tribos’ meaning rubbing or sliding. Wear is the major cause of material wastage

and loss of mechanical performance and any reduction in wear can result in considerable

savings. Friction is a principal cause of wear and energy dissipation. Considerable savings

can be made by improved friction control. It is estimated that one third of the world's

energy resources in present use is needed to overcome friction in one form or another.

Lubrication is an effective means of controlling wear and reducing friction. Tribology

is a field of science which applies an operational analysis to problems of great economic

significance such as reliability, maintenance and wear of technical equipment ranging from

household appliances to spacecraft. The question is why ‘the interacting surfaces in relative

motion’ are so important to our economy and why they affect our standard of living. The

answer is that surface interaction controls the functioning of practically every device

developed by man. An analysis of machine break-downs shows that in the majority of cases

failures and stoppages are associated with interacting moving parts such as gears, bearings,

couplings, sealing’s, cams, clutches, etc. The majority of problems accounted for are

tribological. Our human body also contains interacting surfaces, e.g. human joints, which are

subjected to lubrication and wear. Despite our detailed knowledge covering many disciplines,

the lubrication of human joints is still far from fully understood.

Tribology affects our lives to a much greater degree than is commonly realized. It is

common knowledge that the human skin becomes sweaty as a response to stress or fear. It

has only recently been discovered that sweating on the palms of hands or soles of feet of

humans and dogs, but not rabbits, has the ability to raise friction between the palms or feet

and a solid surface. In other words, when an animal or human senses danger, sweating occurs

to promote either rapid flight from the scene of danger, or else the ability to firmly hold a

weapon or climb the nearest tree.

A general result or observation derived from innumerable experiments and theories is

that tribology comprises the study of:

The characteristics of films of dominant material between contacting bodies.

The consequences of either film failure or absence of a film which are usually

manifested by severe friction and wear.

The practical objective of tribology is to minimize the two main disadvantages of solid to

solid contact: friction and wear, but this is not always the case. In some situations minimizing

friction and maximizing wear or minimizing wear and maximizing friction or maximizing

both friction and wear is desirable. For example, reduction of wear but not friction is

desirable in brakes and lubricated clutches, reduction of friction but not wear is desirable in

pencils, increase in both friction and wear is desirable in erasers.

LUBRICATION

Lubrication is a process of applying lubricant between two rubbing surfaces which are

in contact to each other and to carry away the heat generated by friction. It can generally be

defined as the reduction of friction by using a fluid lubricant.

TYPES OF LUBRICANTS

Solid EX: GRAPHITE

Semi-solid GREASE

Liquid OILS

TYPES OF LUBRICATION

Hydrodynamic lubrication

When a fluid lubricant is present between two rolling or sliding surfaces, a thicker

pressurized film can be generated by the movement of the surfaces. The non compressible

nature of this film separates the surfaces resulting in no metal-to-metal contact. The condition

in which surfaces are completely separated by a continuous film of lubricating fluid is

commonly referred to as Hydrodynamic or Full Fluid Film Lubrication. It can be formed by

wedging the lubricant through a convergent gap with the tangential surface velocities. It often

occurs in components such as cylinders, gears and plain bearings.

Boundary lubrication or thin film lubrication

Boundary Lubrication is a condition in which the lubricant film becomes too thin to

provide total separation. This may be due to excessive loading, speeds or a change in the

fluid's characteristics. In such a situation, contact between surface peaks and valleys occurs.

Friction reduction and wear protection is then provided through chemical compounds rather

than properties of the lubricating fluid. Boundary lubrication often occurs during the start-up

and shutdown of equipment or when loading becomes excessive.

Mixed Film Lubrication

Mixed Film Lubrication is a combination of both hydrodynamic and boundary

lubrication. In such a situation only occasional asperity contact occurs. This condition can be

the result of lubricant breakdown or increased load placed upon the lubricant.

Elastohyrodynamic Lubrication

This Lubrication occurs as pressure or load increases to a level where the viscosity of

the lubricant provides higher shear strength than the metal surface it supports. This regime

can occur in roller bearings or gears as the lubricant is carried into the convergent zone

approaching a contact area or the intersection of two asperities. As a result, the metal surfaces

deform elastically in preference to the highly pressurized lubricant which increases the

contact area and thus increases the effectiveness of the lubricant.

The requirements that lubricants need to satisfy generally consist of the following

(1) High oil film strength

(2) Low friction

(3) High wear resistance

(4) High thermal stability

(5) Non-corrosive

(6) Highly anti-corrosive

(7) Minimal dust/water content

(8) Consistency of grease must not be altered to a significant extent even after it is repeatedly

stirred.

Wear

Wear occurs to the parts when the two mating surfaces are in contact with each other.

Film failure impairs the relative movement between solid bodies and inevitably causes

severe damage to the contacting surfaces. Wear in these circumstances is the result of

adhesion between contacting bodies and is termed ‘adhesive wear’. It creates the loss of

material from the materials used.

BASIC THEORY (DYNAMIC VISCOSITY)

Viscosity is defined as the property of fluid which offers resistance to the movement

of one layer of fluid to adjacent layer of another fluid. The relationship between the shear

stress and the velocity gradient can be obtained by considering two plates closely spaced at a

distance y, and separated by a homogeneous substance. Assuming that the plates are very

large, with a large area A, such that edge effects may be ignored and the velocities of lower

and upper plate are taken as u & u+dy and the thickness of the fluid films which are adjacent

to each other are taken as y and y+dy. The lower plate is fixed, let a force F be applied to the

upper plate which causes shear stress between the layers.

The applied force is proportional to the area and velocity gradient in the fluid and

inversely proportional to the distance between the plates. The viscosity together with relative

velocity causes shear stress acting between the fluid layers. The top surface causes a shear

stress on adjacent lower layer and vice versa. This shear stress is directly proportional to rate

of change of velocity with respect to y.

Mathematically

Represents the rate of shear strain or rate of shear deformation or velocity gradient

Thus viscosity is defined as shear stress required to produce unit rate of shear strain.

The SI physical unit of dynamic viscosity is the Pascal-second (Pa-s), if a fluid with a

viscosity of one Pa-s is placed between two plates and one plate is pushed sideways with a

shear stress of one Pascal, it moves a distance equal to the thickness of the layer between the

plates in one second.

The CGS physical unit for dynamic viscosity is the poise (P), named after Jean Louis

Marie Poiseuille. It is more commonly expressed, particularly in ASTM standards, as

centipoises (cP).

TEMPERATURE VARIATION OF VISCOSITY

Temperature variation has an opposite effect on the viscosities of liquids and gases.

The viscosity of liquid decreases with increase of temperature and viscosity of gases

increases with increase of temperature. This is due to reason that viscous forces in a fluid are

due to cohesive forces and molecular momentum transfer. In liquids the cohesive forces

predominates the molecular momentum transfer, due to closely packed molecules and with

increase of temperature the cohesive forces decreases with result of decreasing viscosity. In

cases of gases the molecular momentum transfer increases and hence viscosity increases.

For liquids

For gases

KINEMATIC VISCOSITY

Kinematic viscosity is defined as the ration of dynamic viscosity to density of fluid

and also defined as ratio of the inertial force to the viscous force. This ratio is characterized

by the kinematic viscosity defined as follows:

Units

The CGS physical unit for kinematic viscosity is stokes (St), named after George

Gabriel Stokes. It is sometimes expressed in terms of centistokes (CST). In U.S. usage, stoke

is sometimes used as the singular form.

The kinematic viscosity is sometimes referred to as diffusivity of momentum, because

it has the same unit as and is comparable to diffusivity of heat and diffusivity of mass. It is

therefore used in dimensionless numbers which compare the ratio of the diffusivities.

Shear viscosity

It is the ratio between the pressures exerted on the surface of a fluid, in the lateral or

horizontal direction, to the change in velocity of the fluid as you move down in the fluid (this

is what is referred to as a velocity gradient).

Volume viscosity

It is called as bulk viscosity or second viscosity; it becomes important only for such

effects where fluid compressibility is essential. Examples would include shock waves and

sound propagation. It appears in the Stokes' law (sound attenuation) that describes

propagation of sound in Newtonian liquid. Alternatively,

Extensional viscosity

A linear combination of shear and bulk viscosity, describes the reaction to

elongation, widely used for characterizing polymers. For example, at room temperature,

water has a dynamic shear viscosity of about 1.0 × 10−3 Pa-s and motor oil of about 250 ×

10−3 Pa-s.

VISCOSITY INDEX

The viscosity index (V.I) of oil is a number that indicates the effect of temperature

changes on the viscosity of the oil. Low V.I signifies a relatively large change of viscosity

with changes of temperature and high V.I signifies relatively little change in viscosity

over a wide temperature range.

The oil becomes extremely thin at high temperatures and extremely thick at low

temperatures. Ideal oil for most purposes is one that maintains a constant viscosity

throughout temperature changes. The importance of the V.I can be shown easily by

considering automotive lubricants. Oil having a high V.I resists excessive thickening when

the engine is cold and, consequently, promotes rapid starting and prompt circulation; it resists

excessive thinning when the motor is hot and thus provides full lubrication and prevents

excessive oil consumption.

The Viscosity index of an oil may be determined if its viscosity at any two

temperatures is known. Tables, based on a large number of tests, are issued by the

American Society for Testing and Materials (ASTM). These tables permit calculation of

the V.I from known viscosities. Fig below shows the viscosity chart with variation of

temperature. Different oils may have different ASTM slopes the viscosity index can be

calculated from the following formula:

VI = (L − U) / (L − H) × 100

H- Viscosity of standard 100% VI oil at 100oF

U - Viscosity of oil with unknown VI oil at 100oF

EXAMPLE PROBLEM

Table 2.2

It was proposed by Dean and Davis (1929) as an indication of an oil’s viscosity-

temperature characteristics in terms of its Say bolt viscosities at 311 K (100°F) and 372 K

(210°F). Two series of reference lubricating-oil fractions (H and L) were used for

comparison. Series H exhibited little change of viscosity with temperature while the

viscosities of series L oils exhibited large variation with temperature. Series H and L

represented, respectively, the best and worst oils available in 1929. Series H oils were

assigned a VI of 100, series L a Correspondence concerning this article should be addressed

to P. T. Cummings Value of 0. The VI of an oil under test ( T ) was calculated from the

equation

VI = (L − U) / (L − H) × 100

Where U is the kinematic viscosity at 311 K of the oil in question, L and H, respectively, are

the kinematic viscosities at 311 K of the series L and H having the same kinematic viscosity

at 372 K as the oil T. Thus, the higher the VI the less the viscosity of an oil is affected by

temperature and, therefore, the better the oil.

TYPES OF FLUID FLOWS

Steady and unsteady flow.

Uniform and non-uniform flow.

Laminar and turbulent flow.

Compressible and incompressible flow.

Rotational and irrotational flow.

One, two and three-dimensional flows.

Steady and Unsteady flow:

Steady flow is defined as that type of flow in which the fluid characteristics like

velocity, pressure, density, etc. at a point do not change with time.

Unsteady flow is that type of flow, in which the velocity, pressure or density at a

point changes with respect to time.

Uniform and Non-uniform flow:

Uniform flow is defined as that type of flow in which the velocity at any given

time does not change with respect to space.

Non-uniform flow is that type of flow in which the velocity at any given time

changes with respect to space.

Laminar and Turbulent flow:

Laminar flow is defined as that type of flow in which the fluid particles move

along well-defined paths or streamline and all the streamlines are straight and parallel.

Thus the particles move in laminas or layers gliding smoothly over the adjacent layers.

This type of flow is also called as streamline flow or viscous flow.

Turbulent flow is that type of flow in which the fluid particles move in zigzag

way. Due to the movement of fluid particles in a zigzag way, the eddies formation takes

place which are responsible for high-energy loss. For a pipe flow, the type of flow is

determined by a non-dimensional number called the Reynolds number.

If the Reynolds number is less than 2000, the flow is laminar. If the Reynolds number is

more than 4000, it is called turbulent flow. It the Reynolds number lies between 2000 and

4000, the flow may be laminar or turbulent.

Compressible and Incompressible flows:

Compressible flow is that type of flow in which the density of the fluid changes

from point to point or in other words the density is not constant for the fluid.

Incompressible flow is that type of flow in which the density is constant for the

fluid flow. Liquids are generally incompressible while gases are compressible.

Rotational and Irrotational flows:

Rotational flow is that type of flow in which the fluid particles while flowing

along streamlines also rotate about their own axis.

Irrotational flow is that type of fluid particles while flowing along the

streamlines do not rotate about their own axis.

One, Two and Three-Dimensional flows:

One-dimensional flow is that type of flow in which the flow parameter such as

velocity is a function of time and one space co-ordinate only. For a steady one-

dimensional flow, the velocity is a function of one-space-co-ordinate only. The variation

of velocities in other two mutually perpendicular directions is assumed negligible.

Two-dimensional flow is that type of flow in which the velocity is a function of

time and two rectangle space co-ordinates. For a steady two-dimensional flow the

velocity is a function of to space co-ordinates only. The variation of velocity in the third

direction is negligible.

Three-dimensional is that type of flow in which the velocity is a function of time and

three mutually perpendicular directions. But for a steady three-dimensional flow the fluid

parameters are functions of three space co-ordinates.

TYPES OF VISCOSITY FLUIDS

Newton's law of viscosity is not a fundamental law of nature but an approximation

that holds in some materials and fails in others. Non-Newtonian fluids exhibit a more

complicated relationship between shear stress and velocity gradient than simple linearity.

Thus there exist a number of forms of viscosity:

• Newtonian: fluids, such as water and most gases which have a constant viscosity.

• Shear thickening: viscosity increases with the rate of shear.

• Shear thinning: viscosity decreases with the rate of shear. Shear thinning liquids are very

commonly, but misleadingly, described as thixotropic.

• Thixotropic: materials which become less viscous over time when shaken, agitated, or

otherwise stressed.

• Rheopectic: materials which become more viscous over time when shaken, agitated, or

otherwise stressed.

• A Bingham plastic is a material that behaves as a solid at low stresses but flows as a viscous

fluid at high stresses.

• A magneto rheological fluid is a type of "smart fluid" which, when subjected to a magnetic

field, greatly increases its apparent viscosity, to the point of becoming a viscoelastic solid.

VISCOMETERS

A viscometer is an instrument used to measure the viscosity of a fluid. For liquids

with viscosities which vary with flow, an instrument called a rheometer is used. Viscometers

only measure less than 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 for there to be laminar flow.

TYPES OF VISCOMETERS

Capillary Viscometers

Capillary viscometers determine viscosity through measurement of the flow rate of the

fluid travelling through a capillary tube. A capillary tube is one with a large length to

diameter ratio.

Capillary viscometers are typically made of glass and consist of a bulb

reservoir connected to the capillary tube.

The theory of operation for a capillary tube viscometer is based on the

Poiseuille model of laminar flow which describes flow through a tube. The

volume flow rate, in a pipe can be derived from the Navier-Stokes equations

for steady, laminar, fully developed, incompressible flow as:

Where R is the pipe radius, is the viscosity and dP/dx is the

pressure gradient which is the driving head for the flow. In the case of a

vertical tube with both ends open to the ambient, the pressure gradient is

caused by the hydrostatic pressure gradient:

By rearranging: The user is instructed to measure the time for the fluid to

travel a specified distance and then the kinematic viscosity is calculated as:

Types of viscometers/options:

There are 3 primary types of capillary tube viscometers.

Original Ostwald

Suspended level

Reverse flow capillary viscometers

Original Ostwald

The Ostwald Viscometer is one of the simplest capillary tube

viscometer. The viscometer consists of a bulb connected to a long

capillary tube. To use the viscometer one partially fills it and then

draws the fluid to the upper mark above the right side bulb. The

fluid is released to flow through the capillary tube and the time for

the upper bulb to empty is measured. Some of the problems

associated with the use of the Ostwald viscometer include the need

to keep the viscometer vertical, the requirement for a specific

volume of fluid and the effect of temperature on the viscosity

measurement.

Suspended Level Viscometers

Reverse Flow Viscometers

Rotational Viscometers

Rotational viscometers are based on the principle that the fluid whose viscosity is

being measured is sheared between two surfaces (ASTM D2983). In these viscometers one of

the surfaces is stationary and the other is rotated by an external drive and the fluid fills the

space in between. The measurements are conducted by applying either a constant torque and

measuring the changes in the speed of rotation or applying a constant speed and measuring

the changes in the torque. These viscometers give the ‘dynamic viscosity’. There are two

main types of these viscometers: rotating cylinder and cone-on-plate viscometers.

To determine viscosity, the test liquid is loaded into the upper bulb and

then released. The liquid flowing through the capillary is separated

from the reservoir bulb at the bottom. The third tube which connects the

bottom of the capillary tube to the ambient ensures that the only

pressure difference between the top of the bulb and the bottom of the

capillary is that due to the hydrostatic pressure--i.e., the weight of the

liquid.

Reverse Flow Viscometers are used to measure the viscosity of opaque

fluids. They measure the flow rate through a ‘dry’ capillary tube so that the

leading edge of the opaque fluids can be easily identified. Reverse Flow

viscometers must be cleaned between each measurement. In addition to there

are a number of variations including small volume viscometers requiring one

mL or less of fluid dilution viscometers with extra large reservoirs for dilution

of the sample and vacuum viscometers for fluids with high viscosities such as

asphalt. There is generally a range of types available for each capillary

viscometer with capillary tubes of varying lengths to allow for the

measurement of a range of viscosities. There also exist more rugged capillary

tube viscometers that are used under continuous flow condition for industrial

applications.

Rotating Cylinder Viscometer

The rotating cylinder viscometer, also known as a ‘Couette viscometer’, consists of

two concentric cylinders with an annular clearance filled with fluid as shown in Figure 2.13.

The inside cylinder is stationary and the outside cylinder rotates at constant velocity. The

force necessary to shear the fluid between the cylinders is measured. The velocity of the

cylinder can be varied so that the changes in viscosity of the fluid with shear rate can be

assessed. Care needs to be taken with non-Newtonian fluids as these viscometers are

calibrated for Newtonian fluids. Different cylinders with a range of radial clearances are used

for different fluids. For Newtonian fluids the dynamic viscosity can be estimated from the

formula

F IGURE shows Schematic diagram of a rotating cylinder viscometer

When motor oils are used in European and North American conditions, the oil

viscosity data at -18°C is required in order to assess the ease with which the engine starts. A

specially adapted rotating cylinder viscometer, known in the literature as the ‘Cold Cranking

Simulator’ (CCS), is used for this purpose (ASTM D2602). The schematic diagram of this

viscometer is shown in Figure 2.14.

The inner cylinder is rotated at constant power in the cooled lubricant sample of

volume about 5 [ml]. The viscosity of the oil sample tested is assessed by comparing the

rotational speed of the test oil with the rotational speed of the reference oil under the same

conditions. The measurements provide an indication of the ease with which the engine will

turn at low temperatures and with limited available starting power. In the case of very viscous

fluids, two cylinder arrangements with a small clearance might be impractical because of the

very high viscous resistance; thus a single cylinder is rotated in a fluid and measurements are

calibrated against measurements obtained with reference fluids.

Cone on Plate Viscometer

The cone on plate viscometer consists of a conical surface and a flat plate. Either of these

surfaces can be rotated. The clearance between the cone and the plate is filled with the fluid

and the cone angle ensures a constant shear rate in the clearance space. The advantage of this

viscometer is that a very small sample volume of fluid is required for the test. In some of

these viscometers, the temperature of the fluid sample is controlled during tests. This is

achieved by circulating pre-heated or cooled external fluid through the plate of the

viscometer. These viscometers can be used with both Newtonian and non-Newtonian fluids

as the shear rate is approximately constant across the gap. The schematic diagram of this

viscometer is shown in Figure 2.15.

The dynamic viscosity can be estimated from the formula:

Schematic diagram of a cone on plate viscometer

Falling Ball Viscometer

Many other types of viscometers, based on different principles of measurement, are also

available. Most commonly used in many laboratories is the ‘Falling Ball Viscometer’. A

glass tube is filled with the fluid to be tested and then a steel ball is dropped into the tube.

The measurement is then made by timing the period required for the ball to fall from the first

to the second timing mark, etched on the tube. The time is measured with accuracy to within

0.1 [s]. This viscometer can also be used for the determination of viscosity changes under

pressure and its schematic diagram is shown in Figure 2.16. The dynamic viscosity can be

estimated from the formula:

UNIT 5

AIR LUBRICATED BEARINGS

AERODYNAMIC BEARINGS

Aerodynamic bearings, which are sometimes known as active gas bearings, function

depending on the relative motion between the bearing surfaces and usually some type of

spiral grooves to draw the air between the bearing lands. This bearing action is very similar to

hydroplaning on a puddle of water in the case of automobiles moving at high speeds. At a

lower speed, the tyre cuts through the water on the road. In a similar way, aerodynamic

bearings require a relative motion between surfaces, when there is no motion or when the

motion is not fast enough to generate an air film, the bearing surfaces will come into contact.

Aerodynamic bearings are often referred to as foil bearings or self-acting bearings, and they

generate pressure within the gas film by viscous shearing. This type of bearing is relatively

simple because it is independent of an external pressure source and mechanism. However, its

application is limited due to the fact that the surfaces require a very high standard of accuracy

and a low load capacity is also not suitable for applications where frequent starts and stops or

change of direction is required. The aerodynamic bearing system is however simpler and

cheaper to operate compared to the aerostatic system.

AEROSTATIC BEARINGS

In contrast to aerodynamic bearings, aerostatic bearings can bear loads at a zero

speed. Air bearings offer a solution for many high-tech applications where a high

performance and high accuracy are required. Aerostatic bearings require an external

pressurized air source due to which aerostatic bearings are also sometimes known as passive

air bearings. Pressurized air is introduced between the bearing surfaces through precision

holes, grooves, steps or by using porous compensation techniques and discharges through the

edges of the bearings. If the correct design is used, a very High stiffness can be obtained.

The aerostatic bearing is able to support a higher load than the aerodynamic bearing, but it

requires a continuous Source of power for supplying pressurized air. Overall, aerostatic

bearings perform well in most aspects such as having a long life, noise-free Operations and

are free from contamination. Since air has a very low viscosity, the bearing gaps need to be

small, of the order of 1–10 µm. As the object floats on a thin layer of air, the friction is

extremely small and even zero when stationary.

Fig shows the air lubricated bearing

Gas lubricated bearings advantages:

Gas viscosity increases with temperature thus reducing heating effects during

overload or abnormal operating conditions,

Some gases are chemically stable over a wider temperature range than hydrocarbon

lubricants,

A non-combustible gas eliminates the fire hazard associated with hydrocarbons,

If air is selected as the hydrostatic lubricant, then it is not necessary to purchase or

recycle the lubricant,

Gases can offer greater cleanliness and non-toxicity than fluid lubricants.

ADVANTAGES OF HYDRODYNAMIC BEARING

Low noise: Since an oil film separates the moving components in a hydrodynamic bearing,

very little noise is produced. Rolling element bearings, on the other hand, because they

consist of several balls or rollers that can vibrate, often create unacceptable levels of noise.

Size:

The advantage of the compact geometry of hydrodynamic bearings is that it takes

occupies less space when compared to the other bearings.

Conformability:

The ability of hydrodynamic bearing materials to conform to minor misalignments

resulting either from assembly or changes during service provides a favorable degree of

forgivingness to the components.

Embeddable:

It is the ease by which foreign particles may embed below the bearing surface in the

soft overlay, thus reducing abrasive damage to the mating surface.

Long Life:

Hydrodynamic bearings are not subject to contact fatigue under monotonic loading

conditions. Consequently, extremely long service life can be expected from well-designed

and maintained hydrodynamic bearings.

Shock load resistance:

Hydrodynamic oil films can adjust their thickness and pressure distribution in

response to shock loading. Therefore, hydrodynamic bearings can generally withstand shock

loading for extended periods without failure.

LIMITATION OF HYDRODYNAMIC BEARING

Limited low speed capability:

HDL films break down at low operating speeds. This leads to the onset of mixed

hydrodynamic and boundary film lubrication in which somewhat higher friction and wear

rates prevails.

Lubricant maintenance:

The performance characteristics of HDL components are dependent on the condition

of the lubricant employed. Lubricant maintenance recommendations must be carefully

observed to avoid loss of adequate oil supply or viscosity, chemical changes or excessive

contamination which affects performance.

Bearing material, geometry, and surface finish:

The type of material selected will have a direct impact on costs. While some materials

are expensive – e.g. tin, Teflon and graphite fibers others are comparatively less expensive –

e.g., lead and low tin bronze. The need for high tolerances will also boost costs, as will

intricate designs and very smooth surface finishes.

Precision requirements:

In certain applications – e.g., in high-speed bearings – a high level of precision is

often required. Fabrication procedures to achieve a more precise bearing could involve

grinding, lapping or burnishing operations and/or individual matching of components, thus

creating higher manufacturing costs. From all these basic considerations, initial cost

projections can be compiled.

Maintenance and replacement costs: The performance of hydro dynamically lubricated

components depends on the lubricant used. Thus, to achieve full benefit from the lubricant

the system must be constantly maintained to avoid loss of adequate oil supply, reduction in

viscosity, and to prevent chemical changes that affect performance. Inadequate system

monitoring and improper lubricant maintenance are probably the main causes of repair and

replacement expenditures. Fluid levels, oil filters, and various other critical parts need to be

inspected regularly. If neglected or inadequately checked, interrupted lubricant supply or

contamination can lead to wear – and eventually to component failure.

APPLICATIONS OF HYDRODYNAMIC JOURNAL BEARINGS:

For construction and farm equipment, this product is typically used in kingpins, rock

shafts, differentials, hinges, pedals and many other pivot points.

This bearing is designed as a direct replacement with conventional 1/16" wall

bushings. These bearings are used in self-lubricated chain, variable speed sheaves,

boom pivot points on forklifts and many similar applications.

Applications include suspension points on large trucks and railroad cars. These

products are also used in the boom foot pivot of large cranes.

These bearings are used in many harsh applications and in food handling machinery.

ADVANTAGES OF HYDRODYNAMIC THRUST BEARINGS

Thrust Bearings

Hydrodynamic, wear-free thrust bearings carry loads which act along the axis of the shaft.

They can be combined with journal bearings. Depending on the application thrust bearings

will be designed with fixed wedges or tilting pads.

Advantages

• Decrease friction & save electricity

• Vibration free & noise free due to dynamically balanced

• It can run high RPM due to best polymer material

• Self lubricating & reduce heating

• Excellent sliding and dry running properties

• Low co-efficient of friction

• Good thermal conductivity

• High chemical resistance

• Excellent dimensional stability

• High fatigue resistance

APPLICATIONS OF HYDRODYNAMIC THRUST BEARINGS

These bearings accommodate thrust in clutches, hospital beds, screw jacks, valve

actuators, vehicle suspensions, and many other applications.

These bearings are used in cam actuator arms, turntable support bearings, exercise

equipment, truck differentials and many other applications.

These bearings are used in articulated frame joints, pivot arm supports, kingpins and

many other applications.

These bearings are used in crane boom foot positions, wheels and pallet jacks, frame

supports for large trucks and other construction equipment and many other

applications.

Advantages and Disadvantages of Aerostatic Bearings

Low viscosity and hence low friction during shaft rotation

Low power loss and cool operations due to low friction

High rotational speed operations

Precise axis definition and a high accuracy over a wide speed range

Long life due to a virtually zero wear rate

Low noise and vibration levels

Virtually no necessity for periodic maintenance

Ample and clean lubricant. No necessity for oil or grease lubrication

No contamination of surfaces by the lubricant. Minimal contamination to the

surrounding environment

No necessity for a fluid-recovery system; these systems are clean

Good performance of the lubricant at extremely low and extremely high temperatures.

The very-high-temperature operations feasible are limited only by the less capabilities

of bearing and journal materials [5]

No breaking down of the film due to cavitation or ventilation [9]

Availability for both linear and rotary application.

Disadvantages

The surfaces must have an extremely fine finish

The alignment must be extremely good

Dimensions and clearances must be extremely accurate

The speed must be high

The loading must be low

Careful designing is required to avoid vibration due to compressibility of the fluid

Careful filtering is required to avoid scoring and binding

More power is required to pressurize a compressible fluid

The design is more empirical since the flow relationships are almost impossible to

solve

A very small film thickness is required to confine the fluid flow to reasonable values,

thus requiring very precise machining in manufacturing

The stability characteristics are poor

APPLICATIONS OF HYDROSTATIC THRUST BEARINGS

These bearings mostly find their applications in the field of marine engineering

As they are having high load carrying capacity, they are highly applicable in marine

turbo chargers, marine engine shafts and in similar applications.

These also find their applications in machine tools. In vertical milling machines,

boring machines, drilling machines etc, they are widely applicable

They are also used mainly in case of highly précised machine tools as in case of

spindle of lathe head stock etc.

In gas compressors, vertical turbines, electrical generators, motors and many other

applications involves the use of hydrostatic thrust bearings.

HYDROSTATIC BEARING ANALYSIS

Load Capacity

The total load supported by the bearing can be obtained by integrating the pressure

distribution over the specific bearing area

After eliminating the unnecessary terms the final frictional power is

UNIT 8

BEARING MATERIALS

The bearing industry uses different materials for the production of the various bearing

components. Each and every bearing is carefully designed by choosing appropriate bearing

materials. The selection of bearing material plays very crucial role in designing the bearing.

Since every material has its own significance there should be certain classification of

materials that gives the appropriate choice for selection of bearing materials.

The bearing materials should pocesses certain properties for the full function of

bearing. When the journal and the bearings are having proper lubrication separating the two

surfaces in contact, the only requirement of the bearing material is that they should have

sufficient strength and rigidity. The conditions under which bearings must operate in service

are generally far from ideal, so the other properties must be considered in selecting the best

material. Some of them are listed below.

Properties Bearing Materials

Compressive strength:

The maximum bearing pressure is considerably greater than the average pressure obtained

by dividing the load to the projected area. Therefore the bearing material should have high

compressive strength to withstand this maximum pressure so as to prevent extrusion or the

other permanent deformation of the bearing.

Fatigue strength:

The bearing material should have sufficient fatigue strength so that it can withstand

repeated loads without developing surface cracks. It is of major importance in aircraft and

automotive engines.

Conformability:

It is the ability of the bearing material to accommodate shaft deflections and bearing

inaccuracies by plastic deformation (or creep) without excessive wear and heating.

Embedability:

It is the ability of bearing material ti accommodate (or embed) small particles of dust ,

grit etc., without scoring the material of the journal.

Bondability:

Many high capacity bearings are made by bonding one or more thin layers of a bearing

material to a high strength steel shell. Thus, the strength of the bond i.e. Bondability is an

important consideration in selecting bearing material.

Corrosive Resistance:

The bearing material should not corrode away under the action of lubricating oil. This

property is of particular importance in internal combustion engines where the same oil is used

to lubricate the cylinder wall s and bearings. In the cylinder, the lubricating oil comes into

contact with hot cylinder walls and may oxidize and collect carbon deposits from the walls.

Thermal conductivity:

The bearing materials should be of high thermal conductivity so as to permit the rapid

removal of the heat generated by friction.

Thermal expansion:

The bearing materials should be of low coefficient of thermal expansion, so that when

the bearing operates over a wide range of temperature, there is no undue change in the

clearance.

Various materials are used in practice, depending on the requirement of the actual

service conditions.

Comparison of properties of bearing materials

Bearing

material

Fatigue

strength Conformability

Embed

ability

Anti

scoring

Corrosive

Resistance

Thermal

conductivity

Tin base

Babbitt Poor Good Excellent Excellent Excellent Poor

Lead base

Babbitt

Poor to

fair Good Good

Good to

Excellent

Fair to

good Poor

Lead

bronze Fair Poor Poor Poor Good Fair

Copper

lead Fair Poor

Poor to

fair

Poor to

fair Poor to fair Fair to good

Aluminum Good Poor to fair Poor Good Excellent Fair

Silver Excellent Almost none Poor Poor Excellent Excellent

Silver lead

deposited Excellent Excellent Poor

Fair to

good Excellent Excellent

Materials Used For Sliding Contact Bearings:

Babbitt metal:

The tin base and lead base babbits are widely used as a bearing material because they

satisfy most requirements for general applications. The babbits are recommended where the

maximum bearing pressure (on projected area) is not over 7 to 14 N/mm² .When applied in

automobiles, the babbit is generally used as thin layer , 0.05 mm to 0.15 mm thick, bonded to

an insert or steel shell. The composition of the metals is as follows:

Tin base babbits: Tin 90%; Copper 4.5%; Antimony 5%; lead0.5%.

Lead base babbits: lead 84%; Tin 6%; Antimony 9.5%; Copper 0.5%.

Bronzes:

The bronzes (alloys of copper, tin and zinc) are generally used in the form of

machined bushes pressed into the shell. The bush may be in one or two pieces. The bronzes

commonly used for bearing material are gun metal and phosphor bronzes.

Gun Metal: (copper 88%; Tin 10%; zinc2%) is used for high grade bearings

subjected to high pressures (not more than 10 N/mm² of projected area ) and high

speeds.

Phosphor Bronze: (copper 80%;Tin 10; Lead 9%; Phosphorus 1%) is used for

bearings subjected to very high pressures (not more than 14 N/mm² of projected area)

and speeds.

Cast iron:

The cast iron bearings are usually used with steel journals. Such type of bearings are

fairly successful where lubrication is adequate and the pressure is limited to 3.5 N/mm² and

speed to 40 meters per minute.

Silver:

The silver and silver lead bearings are mostly used in air craft engines where the

fatigue strength is the most important consideration.

Non-metallic bearings:

The various non-metallic bearings are made of carbon–graphite, rubber, wood and

plastics. The carbon–graphite bearings are self lubricating, dimensionally stable over a wide

range of operating conditions, chemically inert and can operate at higher temperatures than

other bearings. Such types of bearings are used in food processing and other equipment

where contamination by oil or grease must be prohibited. These are also used in applications

where the shaft speed is too low to maintain a hydrodynamic oil film. The soft rubber

bearings are used with water or other low viscosity lubricants, particularly where sand or

other large particles are present. In addition to the high degree of Embedability and

conformability, the rubber bearings are excellent for absorbing shock loads and vibrations.

These are used mainly on marine propeller shafts hydraulic turbines and pumps. The wood

bearings are used in many applications where low cost, cleanliness in attention to lubrication

and anti seizing is important.

Tin bronze

This covers a range of alloys of copper and tin containing between 5% and 12% tin.

The tin content improves strength at the expense of tribological bearing properties such as

conformability and Embedability. At tin contents below 5% there is no significant increase in

strength and wear resistance, and above 12% tin alloys are brittle and difficult to machine.

Phosphor bronze

Small additions of phosphorus in tin bronze, typically 0.4% to l%, improve the

castability of the alloy. The very hard copper phosphide phase is introduced, increasing the

hardness, wear resistance and strength of the alloys, again at the expense of bearing

properties. Hardened mating surfaces are essential. Small additions of lead can be added to

improve bearing properties, but will reduce strength.

Leaded bronze

Lead is added to bronze in small quantities of 1% - 2% to improve machinability.

Further additions of lead, up to about 30%, improve tribological properties significantly but

reduce the mechanical properties such as strength and fatigue resistance. Lead is insoluble in

the solid phases, and separates out during solidification. The cooling rate should be controlled

to ensure that it occurs as small isolated globules dispersed throughout the matrix.

Copper-lead

These are materials formed by adding lead to unalloyed soft copper or copper with

minor additions. They contain large quantities of lead, typically 20% to 35%, with sometimes

as much as 50%. They have a low load capacity relative to other copper alloys, but excellent

tribological properties. They are often cast onto steel backing to improve load capacity. Such

high contents of lead make these alloys difficult to cast by conventional techniques; very

rapid cooling is required. Small additions of alloying elements such as tin, zinc or nickel are

used to improve castability. These materials are sometimes confusingly also termed lead

bronze. The lead phase is susceptible to corrosion by weak organic acids and can therefore be

overlay plated to advantage with a very thin layer of lead-tin or lead-indium for protection.

Aluminum bronze

Basically, these are alloys of copper with up to 11% of aluminum but frequently

contain other additions such as iron, manganese and silicon to further improve strength,

hardness and impact resistance. The alloys usually contain very hard particles, resulting in

good mechanical properties and wear resistance, at the expense of bearing properties.

Hardened mating surfaces and good lubrication are essential. They have excellent resistance

to corrosion and erosion, especially in marine and similar aggressive environments

Gunmetal

The addition of zinc to tin bronze in quantities up to 6% improves the cast ability of

the alloys which are known as gunmetal’s. Besides improving the cast ability, the zinc

improves the retention of mechanical properties at elevated temperatures but it reduces the

tribological properties. Up to 8% lead can be added to improve bearing properties. A wide

range of gunmetal’s is available with differing additions of tin, zinc and lead suitable for a

variety of end user requirements and manufacturing techniques.

Brass

Brasses are alloys of copper and zinc, typically containing between 20% and 40%

zinc. They are available as cast and in all wrought forms such as plate, sheet, rod, section,

forgings and tube. Without further alloying additions, brass has moderate tribological

properties. Additions of lead give free-machining brasses that are easy to machine and have

potential economic advantages. These are ideal for components with non-critical bearing

applications involving light loadings. Other additions such as manganese, silicon, aluminum

and iron are made to give high-tensile brasses with improved load capacity and tribological

properties.

UNIT 6

FRICTION

DRY FRICTION REGIME

There is no lubrication film between the sealing end faces. The friction is mainly

decided by the solid interaction of the gliding plane. Under the general engineering condition,

the sealing surface may adsorb the gas (or the steam of the medium) or oxide layer. Now two

end faces directly contact, which leads to severe wear and tear. The load and the material of

the friction pairs have obvious influence on the friction process.

BOUNDARY FRICTION

When the friction between the sealing faces happens, On the surfaces, there is a layer

of boundary film of the fluid molecules. This fluid film is very thin and separate two end

faces. This friction is called the boundary friction. In boundary friction regime, the boundary

film has the lubrication function, and the liquid pressure is difficult to be measured [2].

Generally, the boundary film consists of 3-4 layers of the molecules, and its thickness is

about 200 A (1 A= I 0-1° m) [3]. The boundary film is partly discontinuous, and there are

solid contacts in some areas. The micro-convex bodies of the solid surface bear almost all the

load, as shown in Fig.2. The viscosity of the liquid film has no significant effect on the

friction properties. The frictional behavior depends largely on the lubricating properties of the

boundary film and the material of the friction pairs.

According to boundary friction theory Mayer studied the true state of the leak flowing

in the gap of the end faces of mechanical seals when exits no obvious pressure difference and

built up the flowing theory of the fluid exchange [2].The liquid mainly permeates through the

seal faces through the gap. There are many rough discontinuous maze caves along the whole

width of the seal faces, so while the scaling rings revolve, the liquid exchanges in the dinky

gaps and the caves of two contacting friction surfaces is under the residual pressure and the

centrifugal force. The gaps of friction surface seldom connect each other in the boundary

friction regime. The above-mentioned gaps are formed because of the separation of the solid.

When one of two rings revolves, the liquid is transferred from one gap to another, which

causes the leakage, as shown in fig.3.

FLUID FRICTION REGIME

There is a layer of stable lubricant film between the friction pairs of the end faces of

mechanical seals. This extremely thin lubricant film can separate one end face front the other

so that the sliding surfaces don't directly contact. At this time the friction force is generated

only by the sheer force of the viscous fluid, and it is much less than that in the dry friction

regime. And there is no wear and tear of the solid surfaces. This friction regime is called the

fluid friction. In completely fluid friction regime the dynamic viscosity of the lubricant

affects the frictional property. Now the lubricant fluid shows its volume property. The

friction happens in the interior of the lubricant.

MIXED FRICTION REGIME

With the wave of the seal end face reduced, the gap of friction pairs becomes smaller.

The highest peak of the surface roughness will contact,. This is called the mixed friction

regime. The end faces of mechanical seals are irregular rough surfaces. The fluid film

between the end faces is extremely thin, and it has the same order of magnitude as the surface

roughness. Therefore, the high-frequency roughness and low-frequency wave of the surface

topography and the radial taper of the overall form error have great influence on the

performance of mechanical seals. The lubricant film of hydrodynamic pressure or hydrostatic

pressure is formed between the end faces, that is, there are several mixed frictions between

the contact surfaces at the same time, such as the fluid friction and the boundary friction, the

boundary friction and the dry friction, the fluid friction and the dry friction, and the boundary

friction and the dry friction.

In mixed friction regime, the fluid film and the contact micro-convex body between

the end faces of mechanical seals bear the total external load caused by the force of the elastic

element and the sealed medium pressure. The total friction forces include two parts, namely,

one generated by the sheer force of the viscous fluid film in the lubricating regime and the

other generated by the cleformation of micro-convex bodies in the contact regime of micro-

convex bodies. The dynamic viscosity and the material of the friction pairs have obvious

influence on the frictional process. Now, there is minor wear, and the friction factor is also

very small.

METHODS FOR JUDGING FRICTION REGIMES OF EDN FACES OF

MECHANICAL SEALS

FRICTION FACTOR METHOD

The friction factor of the end faces is one of the main parameters characterizing the

friction regime. In different friction regime, the friction factor of the friction end faces is not

the same. Mayer presented the friction factors of the different friction regimes [2], which are

listed in Table 1

DUTY PARAMETER METHOD

The duty parameter G was put forward by Stribeck after his research on the bearing

lubricity in 1900-1902. Afterwards,

SommerleId, Gimbel, Hershey .etc applied this similarity number of the friction characteristic

in the field of the sealing technology. The duty parameter could express the friction

characteristic of mechanical seals. Its magnitude indicates the operating condition of

mechanical seals and the carrying power of the liquid film. The duty parameter of mechanical

seals was defined as the ratio of the viscosity force of the liquid film between the end faces to

the locking force of the end faces Pg. Where p is Fluid dynamic viscosity (Pass), v is Average

slide speed of the seal surfaces (m/s), b is Effective seal width of the seal ring (m), n is

Rotational speed (rpm), pg is Locking force of the end faces (Pa), A„ is Area of the seal

surface (m), p,p is Spring pressure (Pa), fi is Balance factor, ps is Medium pressure (Pa) .

Where µ is Fluid dynamic viscosity (Pa-s), v is Average slide speed of the seal

surfaces (m/s), b is Effective seal width of the seal ring (m), n is Rotational speed (rpm), pg is

Locking force of the end faces (Pa), Aa is Area of the seal surface (m), mg is Spring pressure

(Pa), β is Balance factor, Ps is Medium pressure (Pa).

The method that the friction regime is judged by the duty parameter G has been put

forward by Chen [5]. When G.>I x10-6, 2x10-8<G<5x le and 5x10-8<G<1 x10-6, the

frictional pairs work in fluid friction regime. Boundary friction regime and mixed friction,

respectively.

MAYER METHOD

The relationship between the contact pressure coefficient Kg of the end faces and the

clearance height h formed by the roughness of the end faces is shown in Fig.4 Which was

obtained from the experiment by Mayer. The boundary or mixed friction regime of

mechanical seals can be judged from Fig .4.

RELATIVE FILM THICKNESS METHOD

The friction regime can also be judged by the relative film thickness λ. The relative

film thickness was defined as the ratio of the average thickness of the liquid film 110 between

the end faces to the total surface roughness of the end faces σ.

The relationship among the average thickness of the liquid Min between the end faces of

mechanical seals he, the duty parameter G, the friction factor f and the average radius of the

end faces r,„ can be expressed by Eq.(4) [6].