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4. LUBRICATION AND TRIBOLOGY

INTRODUCTION
Lubrication can be considered as vital part of a machine as any of the working parts. Of course
the various bearings, gears and cams which make up any machine today must be carefully
designed and precision made of the best materials to meet the demands of modern high speed
production. But without proper lubrication, these same working parts would soon develop rapid
wear and eventual failure. Then the machine would be useless as a production tool.
All of us in the plant have an important role to play in an effective lubrication programme. The
foreman and machine operator can be sure of 'getting out the goods' only if the lubrication
service man has properly lubricated the machine. In turn, the lubrication service man can
lubricate his machines properly only if the engineer has properly designed the machine and
specified the right lubricant for it. And in turn, the maintenance mechanic depends upon proper
lubrication to keep the machines running. It is a programme in which all of us have an important
role to play.
TRIBOLOGY
Lubrication is done to minimise friction between two interacting surfaces in relative motion.
Friction occurs because a solid surface never microscopically smooth. Even the best machined
surface has peeks and valleys called 'roughness'. When two such surfaces come into contact, it is
only the peaks on the surfaces that make actual contact. These contacts support the normal load
and deform plastically and get cold welded. Depending upon the magnitude of the normal load
more and more high spots or peaks come into contact and the 'real area' of contact increases in
contrast to the 'apparent area', which is the geometrical area of the surfaces in contact.
This phenomenon is called adhesion. Friction is believed to be caused by this adhesion. When
two such surfaces have to be moved in relation to each other, some force will be required to
sheer these contacts. This force is called frictional force. We study this in the subject called
TRIBOLOGY, which helps in better visualising conceptually the problems of friction, wear and
lubrication involved in relative motion between surfaces.
FRICTION: Friction can be defined as resistance to movement between any two surfaces in
contact with each other.
When friction occurs in machinery, it is not so desirable. It destroys the effectiveness of the
equipment through wear, heat and shortened life. We overcome this friction by doing
lubrication.

KINDS OF FRICTION

Friction can be classified into two types; solid friction which may be either sliding or rolling, and
fluid friction. Sliding friction occurs when two surfaces slide over each other without lubrication
as in a plain bearing or between a piston and a cylinder. Rolling friction occurs when a
cylindrical or spherical body rolls over another surface without lubrication as in the modern ball
and roller bearings. We require less force to overcome rolling friction than sliding friction.
Hence solid friction essentially occurs when there is no lubrication.
Now to compare fluid friction with solid friction, if a film of oil is introduced between the same
two surfaces, the peaks and valleys are filled up by the particles of oil. When a sufficient
number of these particles of oil are placed between the two surfaces to produce a thick strong
film, than the peaks and valleys slide by each other without inter-loading.
When such surfaces either flat, curved or spherical, are kept apart by a fluid film, we have what
we call fluid friction and these surfaces are said to be lubricated. Therefore, in lubrication we
actually reduce friction to a minimum by substituting fluid friction for solid friction.
Friction is governed by following two laws propounded by Amountons:
1. The frictional force is proportional to normal loads.
2. Friction is independent of the size of bodies.
WEAR
Wear can be defined as undesired removal of material due to mechanical action. It is
poorly understood in the scientific sense. By a conventional method wear is divided into
following main types:
1. Adhesive 2. Abrasive 3. Corrosive 4. Fatigue Adhesive wear means damage resulting when two metallic bodies rub together without the deliberate presence of an abrasive agent. Abrasive wear is characterized by damage to a surface by harder material introduced between two rubbing surfaces from outside. The severity of abrasive wear depends on size and angularity of abrasive particles and also the ratio between hardness of metal and the abrasive particles, more the tendency to wear. Fatigue wear occurs due to cyclic stresses in rolling and sliding contacts as in gears and rolling bearings. Corrosive wear occurs due to corrosion. Rusting is a well known example. The presence of moisture, oxygen availability and dusty conditions accelerate corrosive wear.
LUBRICATION
Lubrication is the reduction of friction to a minimum by replacing solid friction with fluid
friction
.
This is achieved by introducing between two surfaces in relative motion, an ideal film of oil or
sufficient amount of grease to keep the two metal surfaces separated under the speeds and loads
imposed on the bearings. The most important single factor that determines the effectiveness of
the oil is the viscosity of the oil.
FUNCTIONS OF LUBRICANTS:
Lubricants are agents introduced between two surfaces in relative motion to minimise friction.
Selection and application of lubricants are determined by the functions they are expected to
perform. The principal functions of lubricants are to :
a) control friction b) control wear c) control temperature d) control corrosion e) remove contaminants f) form a seal (grease)
TYPES OF LUBRICANTS
Following are the commonly known types.
1. Liquid Lubricants a) Plain mineral oil b) Mineral oil plus additive c) Synthetic lubricants 2. Quasi-solid Lubricants (Grease) 3. Solid Lubricants
Depending upon a typical application requirement a particular type of lubricant is chosen.
LIQUID LUBRICANTS
Liquids are generally preferred as lubricants because they can be drawn between moving parts by
hydraulic action. Apart from keeping the parts separated they also act as heat carriers. In the
choice of a liquid lubricant for a given application, primary consideration. Moreover effect of
temperature change on viscosity should be minimum Liquid Lubricants should in general be inert
toward metal surfaces and other components.
MINERAL OILS
Modern refining technology technicians has made it possible to produce lubricants of good
quality from a wide variety of crude oils. Refining crude oil is the process of separating the crude
oil into different fractions or cuts. These cuts are called naphtha, gasoline, kerosene, light and
heavy oils and residues. Each type of crude oil gives different amount of each 'cut'. Basically
crude oils are of two types namely paraffinic and napthanic.
MINERAL OIL PLUS ADDITIVE
A refinery makes only the base lube oil stocks of different viscosities. They are unsuitable for
direct consumption. Therefore, oils are mixed to attain right viscosity and additives are added to
improve other qualities.
SYNTHETIC LIQUID LUBRICANTS
Synthetic liquid lubricants can be characterised as oily and neutral liquids. They are not obtained
from petroleum crude oils. But they have almost similar properties as petroleum lubricants.
These find application in situations where petroleum oils cannot be used. Some specific chemical
classes of synthetic lubricants are Di-esters, organo-phosphate esters, silicone polymers etc.
LUBRICANT CHARACTERISTICS
SPECIFIC GRAVITY
Specific gravity is the ratio of the weight of a given volume of substance at 60 degree F. to that of
water.
VISCOSITY
Viscosity is a measure of the oil's resistance to flow. The more the viscosity of the oil more
will be it's resistance to flow, e.g. compare water and molasses. Water is less viscous and hence
flows freely. Where as molasses, which has a high viscosity, flows sluggishly.
An ideal oil film on a bearing depends on selecting an oil with the right viscosity to maintain
separation of two metal surfaces. The speed of the journal and viscosity are closely allied in
maintaining a good oil film in the bearing. The slower the journal speed, the higher viscosity or
thicker oil we must use. As journal speeds are increased, a thinner of lower viscosity oil is
needed.
Bearing loads must also be considered because the oil must have sufficient viscosity to maintain a
good oil film to support the load.
Technically speaking, it is defined as the force required to move a plane surface of one square
centimeter area over another plane surface at the rate of one centimeter per second, when the two
surfaces are separated by a layer of liquid one centimeter in thickness. The unit of this force is
poise and is called absolute viscosity.
Kinematic viscosity is the ratio of absolute viscosity to the specific gravity of the oil at the
temperature at which the viscosity is measured. Its unit is stokes.
For practical purposes, viscosity of petroleum oils is expressed in time in seconds taken by a
given quantity of oil to flow through a standard capillary tube. It is expressed as Saybolt
universal seconds at 100 degree F. or 210 degree F.
VISCOSITY INDEX
Viscosity index is an expression of effect of change of temperature on the viscosity of oils. This
change can be evaluated numerically and the result is expressed as V.I.
POUR POINT
Pour point of an oil is an important quality. It is a temperature at which oil will still remain fluid.
It reflects on the capability of the oil to work at low temperatures.
FLASH POINT
Flash point is the temperature at which the oil gives off sufficient vapours which can be ignited.
It reflects on the capability of the oil to work at higher temperature without any fire hazard.
The purification and manufacturing processes impact good qualities to lubricating oils. But still
they cannot be used directly. They will be prone to contamination and decomposition in the
exacting working conditions. Hence certain chemical compounds and other agents which are
termed as additives are added to the oil. Most modern lubricant additives can be classified as
follows:
1. Those designed to protect the lubricant in service by maintaining deterioration. 2. Those that protect the lubricant from harmful fuel combustion products. 3. Those which improve existing physical properties or impart new characteristics. Use of chemical additives in lubricants is very wide. They are used in the lightest instrument and spindle oils to the thickest gear lubricants; automotive lubricants; cutting oils; and hydraulic fluids. There are over 50 characteristics of lubricating base oils which can be improved by the additives. Generally speaking the additives must have the following properties: a) Solubility in base petroleum oil b) Insolubility in and lack of reaction with aqueous solution. c) Should not impart dark colour to the oil d) Low volatility e) Additives must be stable in blending, storage and use. f) Additives should not impart unfavourable odour. Table-1 outlines the various types of additives used and their purpose. ------------------------------------------------------------------------------------- Additive Used Purpose of Additives ------------------------------------------------------------------------------------- Anti-oxidant Detergents Cleanliness of lubricated surfaces. Rust Inhibitor improver with temperature change Anti-foam agent Extreme Pressure agent Improves film strength and load -----------------------------------------------------------------------------------
TYPES OF LUBRICATING OILS
Each major oil company will have over 300 different industrial and automotive types in its line of
oils. For simplicity following eleven classifications have been listed.
1. Spindle oils 2. Gear oils 3. General bearing oils 4. Electric motor oils 5. Steam cylinder oils 6. Turbine oils 7. Air compressor oils 8. Refrigeration compressor oils 9. Hydraulic oils 10. Cutting oils 11. Automotive oils Each type of oil listed has certain characteristics that make it well adapted for a given application.
SPINDLE OILS
It gets its name for its use on spindles. Spindles are small rotating shafts on upright drills which
have high speed and low load characteristics. The viscosity is the most important factor.
Temperatures are seldom high enough to make flash point critical, nor low enough to make the
pour point an important consideration.
GEAR OILS
Gear oils are of a heavier grade because of the rubbing action of the gear teeth and high pressure
on teeth. Gear oil viscosity usually ranges from 60 seconds to over 150 seconds at 210 degrees
F. Gear oil should have Anti-foam characteristics. Flash and pour points must be considered if
temperatures that will be encountered make these points critical.
TURBINE OIL
It is one of the highest refined oils that we use. These oils should be controlled to very close
tolerance in their physical properties. Flash point is very important as operating temperatures are
usually higher.
AIR COMPRESSOR OILS
These have to work under very difficult conditions. Under these conditions oil comes into
contact with air at high temperatures and pressures. This causes oxidation of oil. Flash point
must be high to guard against fire hazard.

REFRIGERATION COMPRESSOR OILS

These are usually straight mineral oils. Flash point is not so critical. However, pour point is an
important characteristic due to low temperature.
GENERAL BEARING OILS
Usually used in 'once through' systems, they go through the bearings and are wasted. These are
not used in circulating systems because they do not have the ability to stand up under extended
circulation and use. Viscosity is an important property speeds, loads and temperatures must be
considered to make viscosity selection.
We have only covered six of these classifications of oils. But it should be enough to make us
realize that the old saying oil is oil is not exactly true. There are many kinds of oils which have
specific use to meet different kind of service conditions.

SOLID LUBRICANTS

A solid lubricant is a thin film of a solid interposed between two rubbing surfaces to reduce friction and wear. The need for solid lubricants has grown rapidly with advance in technology. The solid lubricant should have following characteristics: 1. Low sheer strength 2. Low hardness 3. High adhesion to substrate material 4. Continuity 5. Self-healing ability (The film should reform immediately if broken) 6. Freedom from abrasive impurities 7. Thermal stability 8. Chemical inertness
Various inorganic compounds like graphite, molybdenum disulphide, tungsten disulphide, boron
nitride; and organic compounds like aluminum, zinc, sodium, lithium stearate and waxes are used
as solid lubricants. Solid lubricants have found wide application where conventional petroleum
oils have failed to work at extreme working conditions.
INDUSTRIAL GREASES
Our previous discussion was confined to various types of lubricating oils. There is another class
of lubricants which hold just as important a place in industrial lubrication as do oils. These are
greases.
A lubricating grease is a semi-solid lubricant. It is usually a mineral oil to which special soap is
added to produce a plastic mixture. The soap is called thickener. Certain additives are also added
as in the case of oils to impart special characteristics. Thus grease is, Oil or fluid + Thickener +
ADVANTAGES IN USING GREASES
1. Less frequent application necessary. This results in saving in lubricant cost and maintenance cost. 2. It acts as a seal against entrance of dirt and dust. 3. Dripping and splattering is almost eliminated. 4. Less expensive seals are required for grease lubricated bearings 5. Grease ensures some lubrication even when a bearing is neglected for a long period 6. Due to clinging property of grease, chances of rusting is considerably reduced in the bearings even when the machine is idle. COMPONENTS OF GREASE
Primary components of a grease are soaps and mineral oils. Soaps may be derived from animal or
vegetable fats or fatty acids. In addition certain additives are also present. Sometimes fillers are
also added to impart special characteristics.
TYPES OF GREASES
Greases are classified by the soap compound used in their manufacture. The properties of greases
are influenced considerably by the type of soap compound used in making the grease. The
following are the common types available.
1. Calcium base grease 2. Sodium base grease 3. Lithium base grease 4. Barium base grease A Calcium base in grease will give the grease a smooth battery appearance. This grease is highly resistant to water. Edible fats such as palm oil or cotton seed oil hydrated lime are used to make soap. This grease requires addition of water as stabilizer. This cannot withstand a temperature above 175 degrees F. It breaks down oil and soap get separated. The separated soap particles become hard and abrasive and cause scoring of bearings. Sodium base greases on the other hand, can be used where higher temperatures up to 250 degree F. are encountered. The sodium base grease is fibrous in structure. This enables the grease to withstand high loads on ball and roller bearings. However, sodium base grease is less resistant to water. Barium base grease is good up to 350 degree F. and above. This grease has good water resistance. Lithium base grease is also suitable for high temperature application and has excellent water resistant properties. For low temperature also this grease is suitable. Table-2 consolidates various characteristics of soap base greases.
Special greases:
To withstand very high temperatures and load conditions certain special greases are used as the
soap based greases are not able to withstand such conditions. These are called non-soap base
greases. Modified bentonites clay and silica gels are used with synthetic fluids. Some soap base
greases are used with synthetic fluids instead of mineral oils.

Table-2

PERFORMANCE CHARACTERISTICS OF DIFFERENT TYPES OF GREASES -------------------------------------------------------------------------------------------------- Type of Thickener -------------------------------------------------------------------------------------------------- SOAP BASE Aluminum -------------------------------------------------------------------------------------------------------
Bentonite:
These greases have a base made of modified Bentonite clay. They have a very high temperature
capability and they last much longer. Table-3 gives details and serviceable temperature ranges of
greases using synthetic lubricants.
As in the case of oils, additives also are added to greases to impart special characteristics.
Commonly used additives are antioxidants, corrosion inhibitors, E.P.agents, rust inhibitors and
Table-3
CHARACTERISTICS OF GREASE CONTAINING SYNTHETIC FLUIDS ----------------------------------------------------------------------------------------------- Fluid ----------------------------------------------------------------------------------------------- Diesters -----------------------------------------------------------------------------------------------
CHARACTERISTICS OF GREASE
The two most vital characteristics of grease are consistency and
drop point.
Consistency:
It is expressed in numbers in tenths of millimeter. Standard ASTM D217-52T test method is used
to determine this property. It is called penetration test. The National Lubricating Grease Institute
USA has classified grease into various classes based on their penetration readings determined
from the above test. Table-4 gives the NLGI classification.
Drop Point:
It is defined as temperature at which a grease changes from quasi-solid to a liquid state under
prescribed conditions of a test. ASTM D566-42 test is used to determine drop point. This is
used as a qualitative indicator of resistance to heat.
Apart from the above two characteristics following characteristics are also equally important.

CHARACTERISTICS OF GREASE CONTAINING SYNTHETIC FLUIDS ----------------------------------------------------------------------------------------------- Fluid ----------------------------------------------------------------------------------------------- Diesters -----------------------------------------------------------------------------------------------

Oil Viscosity:

Fluidized friction characteristics of grease at high rates of shear in an antifreeze bearings depends
on viscosity of oil used.
- For antifreeze bearings grease should offer minimum
fluid friction. This keeps the operating temperatures low.
- For normal ambient temperatures and for all speeds, grease
is made with light to medium body oil.
- Heavy bodied oils are used where high temperatures exist.
Chemical Stability:
A grease must have good oxidation resistance in antifriction bearings. Anti-friction bearings have
advantage of operating for long periods without re-lubrication. It, of course, depends on operating
conditions. Oxidation causes either hardening of grease or softening of grease thus rendering the
bearings unlubricated. Therefore grease must have chemical stability.
Structural Stability:
Grease should have the stability to resist softening. Softening may happen due to milling and
churning in bearings.
Pumpability:
This in influenced by temperature and ability to pump to the system. This is an important
property for in centralised greasing systems.
Table-5 gives various service types of lubricating greases.
Table-5
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Service
---------------------------------------------------------------------------------------------------- Axle Ball&Roller Long service life, Lithium, Sodium, to washing effect of water except water soluble ones grease containing graphite EP Grease Resists heavy pressures ------------------------------------------------------------------------------------------- LUBRICANT APPLICATION METHODS
Considerable care is usually exercised in machines. Sometimes, however, less attention is paid to
the consideration of method of application. As a matter of fact, the devices by which lubricant is
applied, and their manner of installation contribute greatly to the efficient and economical
lubrication of machinery. Therefore, these selection should be a matter of design consideration
and a bit of after thought.
The lubricating device or method selected must be economically compatible with the equipment
to be lubricated from cost and maintenance point of view. In order to competently select the most
suitable lubrication method for an application, it is better to have knowledge of the physical
design and basic characteristics of each device.
CHARACTERISTICS OF LUBRICATING METHODS
To evaluate a particular method for a specific application, certain characteristics should be
considered. Following evaluation criteria given in Table-6 can serve as a checklist to aid in
selection of lubricating devices.

CATEGORIES OF LUBRICATION METHODS

The methods for lubricating machine elements can be divided into
following categories.
- Manual Devices
- Drop-feed Devices
- Splash or bath lubrication
- Ring, chain, collar oilers
- Pad - and waste-type devices
- Positive force feed lubricators
- Air oil devices
- Pressure circulating systems
- Centralized lubricating systems
- built-in-lubrication

A. Manual Devices

Lubricating methods may require human action in one form or another. The term manual
lubrication applies to methods in which the operator is directly responsible for quantity of
lubricant and interval of lubrication. Although the initial cost of manual lubrication is low,
the maintenance costs can be high. Reliability may be owing to considerable dependence
on human action. The lubricant is quite prone in contamination.
Generally speaking, manual lubrication is satisfactory only for lightly loaded or low speed
bearings, typical applications include open gears, chains, wire rope, etc.
Table-6
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Factors Characteristics
---------------------------------------------------------------------------------- A. Delivery of 2. Adaptability Lubricator should adjust lubricant at required rate over long intervals of time 4. Continuity Continuous delivery prevents C. Cost 1. Initial Cost Low initial costs of Consideration 2. Maintenance The most obvious maintenance --------------------------------------------------------------------------------------
B. Drop-feed Devices :
Drop feed devices are gravity-flow lubricators. They are employed to deliver lubricant
drop-by-drop to individual bearings and other machine elements. They give the best
advantage when lubricant points are readily accessible.
Their cost is relatively low. Maintenance cost depends on type of service and location.
Depending on the lubricator, lubricant flow may or may not be stopped and started
automatically. Automatic operation increase reliability.
Typical service applications include journal and roller bearings, gears, chains, engine
guides, pumps and compressors.

C. Splash or Bath Lubrication :

This type of lubrication is commonly used for machinery having high speed moving parts.
These dip into oil and splash it on to the bearings or other machine elements. The splash
system requires enclosing the mechanism to be lubricated.
Initial cost of splash system depends on the expense incurred in enclosing the mechanism.
Maintenance costs are low. A splash system is reliable, prevents contamination. Typical
applications include internal-combustion engines, chain drives and enclosed gear sets.
D. Ring, Chain, Oilers :
These lubricators are applicable to horizontal rotating shafts. The ring or chain oiler
encircles the shaft and turns freely on it. Each provides an automatic oiling system by
bringing oil to the bearing clearance from the oil reservoir.
Initial cost depends on housing for the bearing that must be built to contain these
lubricators. Maintenance cost is usually low.
Typical applications include electric motors, fans, blowers, compressors, and line shaft
bearings.
E. Pad-and Waste-type Devices :
These lubricators use the oil-retaining properties of felt pads and waste packing to provide
the lubricant to a bearing. Oil is lifted from the reservoir by capillary action in the wicking
material. This system requires an appropriate housing, which accounts for a large initial
cost. Maintenance cost generally depends on the environment in which they are used.
They are generally low. This is often used for rail, road and traction motor bearings.
F. Positive Force feed Lubricators :
It consists of one or more plunger-type adjustable-stroke pumps mounted on a common
reservoir. The pumps are driven from a rotating shaft through a mechanical linkage. It
may have a separate drive motor.
Initial cost is high, but maintenance cost is low. The lubricant is free from contamination.
Typical applications include steam cylinders, bearings for diesel and gas engines,
oil-drilling rigs, etc.
G. Air-oil Devices :
Air-oil devices operate by injecting or pumping oil drop-by-drop into an air stream. The
oil is drawn by the aspiratory action of compressed air passing through an orifice or control
valve.
The initial-cost is very high. However, maintenance costs are low and efficiency of the
devices is high. These are well suited for high speed bearings, enclosed gears, slides and
table ways.
H. Pressure Circulating Systems :
Pressure circulating systems employ either gravity or pumps to develop the operating
pressures necessary. Generally these are designed to lubricate a number of parts on the
machine. Since oil is recirculated maximum economy is possible. Pressure circulating
systems are built into the machine. Therefore initial cost is high. Maintenance costs are
very low.
Typical applications include steam-turbine bearings, reduction gears, steel-mill gear drives,
mill bearings, paper-machine bearings and gears and internal-combustion engines.
I. Centralised Lubrication Systems :
Centralized Systems can be designed for oil or grease. A typical centralized system
requires centrally located reservoir and pump, and permanently installed piping and
distribution valves. These deliver measures quantities of lubricant at desired points. It can
be either operated manually or automatically.
The piping and intricate dispensing valves make initial cost very high, but maintenance
costs are very low. Initial cost is offset by dependability, durability, safety and resistance
of system to contamination.
Centralized Systems are ideally suited for steel and paper mills, machine tools etc.
J. Built-in-Lubrication :
Built-in lubrication refers to materials or components that do not require any external
lubricating device. Materials such as oil saturated porous metals, graphite materials,
PTFE, nylon can rub together without a lubricant. These materials may be used for sleeve
bearings, gears etc.
In this category ball and roller bearings are also included which are pre-lubricated by
manufacturers. These require no relubrication during their service life. Machine
components have built-in lubrication are well suited for use in inaccessible locations.
They can reduce maintenance costs, but should not be used indiscriminately.
The various categories of lubrication systems have been very briefly discussed in the above
paragraphs. There are, however, very many varieties in each finding specific applications.
Depending upon the severity of the working situation of machine elements the most
suitable means from cost, maintenance and efficiency point of view should be selected.
BEARINGS AND THEIR LUBRICATION
Bearings are one of the most vital parts in any rotating machinery. To a large extent proper
working of a machine depends on the type of bearing used and their lubrication and
application at various speeds. Rotating speeds vary from less than 100 rpm for large
bearings, such as found in calenders and those used on tool room grinders.
Bearing is a support for a moving part. It consists of a hole in a block inside which a shaft
can rotate. The part of the shaft which rests on the block is called a journal. The function
of a bearing is to hold the moving parts in the proper space relating to other moving or
stationary parts.
We know that friction is of two types namely, sliding friction and rolling friction.
Bearings are also of two types namely, plain or friction bearing and anti-friction bearing.
Ball bearings and roller bearings are called anti-friction bearings.
PLAIN BEARINGS
Plain bearings use in sliding friction. In these bearings one surface moves against another
with a sliding motion. Plain bearings are of three categories:
a) Journal Bearings are used to support a rotating shaft. The load on the bearing acts at
right angles to the axis of the shaft. There are four variations in the design of journal
bearings:
i) Solid bearings, usually referred to as 'sleeve bearings' or 'bushings'.
ii) Half bearings are used when the load on the shaft is always in one direction. A good
example is on the ordinary rail road car journal.
iii) Split bearings are used when the shaft cannot be easily pulled out of the bearings. The
shaft may have a flange that is too big to be pulled through the bearing, or, the shaft may
be so big and heavy that it is easier to move the bearing than the shaft.
iv) Multi-part bearings are used for same reason a split bearing is used - so it will be easy
to make bearing changes. The multi split allows to make adjustments of the bearing to
allow for wear and to maintain the necessary clearances.
b) Guide Bearings : It is used to support a shaft or other machine part that is moving in a
straight line. The movement to be guided is usually cross head on a steam engine and on
some air compressors.

c) Thrust Bearings: They are used to support a vertical rotating shaft and to keep a
horizontal rotating shaft from moving end ways along the shaft. Example - automobile
cooling fan, aircraft engine, worm and hypoid gears.
In each of the above types the bearing must meet certain requirements. They are
- Support the load
- Resist wear
- Resist deformation
- Reduce friction
- Accommodate a lubricant
BEARING MATERIAL REQUIREMENTS:
For satisfactory performance a bearing material should have
i) Compressive strength to withstand loads applied to it. It must also resist deformation.
ii) Low co-efficient of friction.
iii) Durability - the bearing should have sufficient strength to withstand repeated stresses.
iv) Embeddability - The bearing must be able to assimilate small abrasive particles
without
causing damage to bearing surface.
v) Workability - The metal must be easy to work and to machine.
vi) High Heat Transfer rate - Bearing material should be sufficiently heat conductive to
dissipate the heat.
vii) Corrosion resistance.
BEARING MATERIALS
There is no limit to the number of materials that have been used for the bearings.
Arbitrarily they can be broken down into several classes:
5. Miscellaneous metallic bearings 6. Miscellaneous non-metallic bearings SIZE OF BEARINGS
The diameter of the hole is kept slightly larger than the journal or shaft. The difference
between the size of the shaft and diameter of the bearing is called clearance. The clearance
helps in:
1. Rotation of the shaft with ease
2. To take up thermal expansion
3. To take up the defection or unevenness
4. Providing space for oil and to permit assembly.

Normally clearance is equal to .001" per inch diameter of the shaft plus .002". For
example a 3" diameter shaft should have clearance equal to 3x.001" + .002" = .005". This
may be taken as a guide. It can be less for precision spindle bearings and can be
considerably more for rough duty.
GROOVING OF PLAIN BEARINGS
Grooving of plain bearings is an important design factor. These are provided as a reserve
or lubricant. These can be used for either oil or grease depending on requirement of the
bearings. A groove is providing for the following:
i) To aid in distribution of oil to entire rubbing surface ii) To provide a reserve of oil within the bearing iii) To provide enough flow of oil for adequate cooling effect
A faulty grooving way make good lubrication impossible. Therefore correct design and
placement of grooves is important to proper lubrication of bearing. Factors to be
considered are location of pressure areas in the bearing, size of the grooves etc. The
location of grooves is very important. The oil must be conducted from low pressure area to
high pressure area. In a one - piece journal bearing, a longitudinal groove cut at the top,
and extending through the oil inlet hole to near each end will provide good distribution of
lubricant.
In a two-part bearing, proper oil distribution can be obtained by providing longitudinal
groove or chamfer on each side of the bearing. Both sides are chamfered as it is necessary
to provide for rotation in both directions. For grooving of vertical bearings, the oil inlet
should be placed near the top of bearing. This aids gravity distribution of oil.
Complicated grooving should be avoided. They are neither necessary nor desirable. In
many cases they are harmful to good lubrication. Complicated grooves may seriously
reduce the supporting area and effectiveness of oil film.

DESIGN OF GROOVES

When cutting oil grooves, we should always use a round nose tool. All edges should be
chamfered in the direction of rotation to allow the lubricant to be drawn out of the groove
by rotating shaft. Sharp edges of grooves should be scrapped round and smooth. Sharp
edges tend to act as scrappers and wipe off the lubricant from the shaft.
Dimensions of oil grooves vary according to the type of bearing. The length of groove is
determined by the length of the bearing. Cutting grooves too close to the ends of the
bearings will result in unnecessary oil leakage. The ratio between width and depth of the
groove is usually 2 to 4.

LUBRICATION OF PLAIN BEARINGS
Considered purely from lubrication point of view, oil can hardly be excelled. Nevertheless
there are a great many instance in which for practical reasons grease offers certain
advantages over oil. Grease can stay put longer and serve some extreme conditions. Plain
bearings are relatively leaky and require frequent application of grease. Correct grease
will maintain thick film for maximum length of time. It is important that the grease should
only be of correct consistency but also of adequate load carrying ability. Grease lubricated
plain bearings are generally lubricated by means of pressure guns or grease cups.
Table-7 gives some idea regarding selection of oils for plain bearing lubrication operating
between 60 degree F and 140 deg. F.

TABLE GIVES VISCOSITY OF OIL IN SSU @ 100 DEGREE F

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Speed Range
(Up to 100 Psi) (100-250 Psi) (above 250 Psi) ----------------------------------------------------------------------------------- 5000 - 10000 300 - 500 300 - 500 500 - 800 100 - 300 500 - 800 800 - 1200 1800 - 3000 50 - 100 -----------------------------------------------------------------------------------
ANTI-FRICTION BEARINGS
The anti-friction bearings are so called due to the fact that these utilise rolling friction of
rollers or balls instead of sliding friction as in the case of plain bearings. Anti-friction
bearings have much less friction during starting as compared to plain bearings.
The essential parts of such a bearing include a stationery race a rotating race, and rolling
elements that separate the raced while following free motion of the rotating race under
load. The rolling elements can be carefully matched balls, or may be cylindrical, tapered,
spherical or concave rollers. Broadly speaking anti-friction bearings are of three classes:
1. Ball bearing 2. Roller bearing 3. Needle bearing With in each class various types are available. For example in Ball bearing class following types are available: a. Deep groove ball bearing b. Angular contact ball bearing c. Double row ball bearing d. Self-aligning ball bearing e. Ball thrust bearing In roller bearing class following various types are available: a) Cylindrical roller bearing b) Tapered roller bearing c) Self-aligning roller bearing d) Concave roller self aligning bearing
Needle bearings can be considered as a type of roller bearing. These are used to carry
heavy radial loads when radial bearing space is severely limited.
BEARING SIZES
The sizes of an anti-friction bearing generally refers to the diameter of its inner race
namely to its bore. With any given bore, a given type of bearing may have smaller or
larger outer diameters, narrower or wider race ways, and smaller or larger rolling elements.
It depends on the duty it has to perform.

BEARING MATERIALS

The operating conditions in which rolling element bearings work require the use of
materials which can withstand high compressive stresses. These bearings are made of
special alloy steels, carefully heat-treated to give them a required degree of hardness and
Both case hardening and through hardening steel bearings are used. Through hardening is
extensively used in ball bearings. Depending on bearing type, size and material, the
hardness of balls, rollers and race falls within the range of 58 to 66 Rockwell C.
Radial internal clearance is the space between the ball, or roller, and the rolling tracks in the inner and out rings. This clearance permits a radial displacement between the inner and outer rings of the ball and roller bearings. Radial clearance must exist in free (unmounted) bearing because: a) the interference fits of the rings result in expansion of inner ring and contraction of the outer ring. b) differential expansions occur when the inner ring operates at a higher temperature than the outer ring. c) the ability of ball bearings to handle thrust loading is in terms of radial internal clearance. d) there are some machining inaccuracies in the shaft and housing seatings. Following four grades of radial internal clearance are available Group 2 Table-8 gives application of different grades of clearances.

Antifriction Bearing Lubrication

Lubrication requirements of rolling - element bearings are less critical than or
plain bearings. There are, however, certain sources of friction which might cause
immediate failures if bearing were to run without lubricants. These sources
include
a) Slipping between rolling elements and race ways b) Rubbing between rolling elements and their separators c) Rubbing between adjacent needles or rollers. While areas of sliding friction require positive lubrication, the critical lubrication
requirement is in providing adequate lubrication in heavily loaded contact areas.
Table-8
------------------------------------------------------------------------------------------------
Grade of clearance Conditions of application
------------------------------------------------------------------------------------------------
Group 2
Used when no radial or axial freedom must exist in the Any operating temperature change must not be sufficient to reduce the clearance ------------------------------------------------------------------------------------------------ Normal Group Small thrust loading exists. Only one race on interference seating fit. Very small operating temperature changes ------------------------------------------------------------------------------------------------ Group 3 ------------------------------------------------------------------------------------------------ Group 4 When maximum thrust loading exists combined with high operating temperature changes and both races are ------------------------------------------------------------------------------------------------
LUBRICATION PRACTICES
Both oils and greases are extremely used for all types of rolling elements over a
wide range of speeds and operating temperatures. Oil, because of its fluidity, has
advantages over grease in its ability to provide more positive lubrication, flush
away contaminants and remove heat from heavily loaded bearings. Grease,
however, is extensively used as it provides more effective sealing against dirt and
contaminants.
Speed and load are primary factors on the basis of which lubricant is selected.
Emphasis is also laid on operating temperature and other conditions.
1. Oil Lubrication:
Except for a few special requirements, petroleum oils satisfy most operating
conditions.
For severe duty applications of roller bearings exposed to high temperature, it is
customary to use heavier oils. The viscosity range for such applications will vary
from 50 to 175 SSU at 210 degree F. Oil circulating systems are commonly used
to carry away heat from bearings.
2. Grease Lubrication:
Grease lubrication has many advantages for rolling bearings. Foremost of these
are simplicity of housing and sealing designs, decreased attention from
relubrication and maintenance, and its overall economy. The major limitation on
grease lubrication is high speed application. It is not employed for speed factors
(bore in MM x rpm) above 2000,000. For effective results, grease selection must
be based on specific application.
Moderate temperature: General purpose greases are used for operating
temperature range 20 to 200 degree F. Greases for these applications are made
with petroleum oils having a viscosity range of about 200 degree to 500 SSU at
100 deg. F. They are classified according to the type of thickening agent.
Low temperature greases are based on synthetic fluids mostly. For example
di-ester, polyesters, silicones are usually used for operation at temperatures of 65
to 100 deg. F. Grease composed of silicon and various thickness are suitable for
temperatures of 100 to 450 degree F.
High temperature greases are made with petroleum oils of high viscosity (up to
3000 SSU at 100 degree F). In general use of petroleum greases in rolling
bearings is limited temperatures below 300 degree F. For temperatures much in
excess of 300 degree F., the choice is largely limited to silicon fluid greases.
Majority of such greases are thickened with lithium soap.
The majority of ball and roller bearings operating under conditions of moderate
load and speed may be lubricated satisfactorily with NLGI No.2 Grade greases
made with oils having viscosity 200 to 500 SSU at 100 degree F. For heavily
loaded and high temperature applications, greases made with oil viscosities up to
3000 SSU at 100 degree F have been recommended. Higher viscosities are
usually recommended for roller bearings than for ball bearings. for high speed
application at a speed factor of 150,000 to 200,000 NLGI No. 3 and No.4 grade
greases have been used successfully. These are made with oils of 200 to 500
SSU at 100 degree F.
On an overall basis, following factors must be taken into consideration for
bearing lubrication:
1. Speed 2. Temperature 3. Bearing Loads 4. Surrounding conditions.
Lubrication is not a cure-all for all bearing troubles. It must, however, be
understood that proper lubrication goes a long way in extending the operating life
of bearings.
*********

Source: http://www.productivity.in/knowledgebase/Plant%20Engineering/d.%20Lubrication%20and%20Tribology.pdf

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