CHAPTER 1INTRODUCTION TO LIGHT AND MEDIUM MERCHANT MILLS (LMMM)
The Light and Medium Merchant Mills (LMMM) of VSP has been
envisaged to produce: 1. 7,10,000 Tons/year of LMMM products.2.
2,46,000 Tons/year of Billets for sale.3. 8,85,000 Tons/year of
Billets for WRM.
Entire mill has been laid out at an elevation of +5 meter except
the charging grids and their associated roller tables which are
located in the cross bays at +0.01 level.
1.1 BILLETS AND BLOOMS
A billet is a length of metal that has a round or square
cross-section, with an area less than 36sqin (230cm2). Billets are
created directly via continuous castingor extrusionor indirectly
viarollingan ingot.Billets are further processed via profile
rollinganddrawing. Final products includebar stockand wires.Blooms
are similar to billets except the cross sectional area is greater
than 36sqin (230cm2).Blooms are usually further processed via
structural shape rollingand profile rolling. Common final products
includestructural shapes,rails, rods, andseamless pipes.
1.2 CHARGING, FURNACE AND DISCHARGING AREAS
The feed material for the mill is continuous cast bloom of
250mmX320mm size having a normal bloom length of 6.3 meter. Cold
blooms are charged on the charging grid by magnet crane according
to the rolling sequence list. Each walking beam furnace has defined
track numbers and charging and discharging of blooms from charging
grids and furnaces respectively will be according to the rolling
mode i.e., each bloom destined for its defined track number will be
discharged from charging grid in the sequential order. Normally
furnace 1 is intended to feed hot blooms for production of billets
and deposition on to billet cooling bed while furnace 2 is intended
to feed hot blooms for onward transmission to bar mill. This
implies that the cold blooms are to charge into charging grid in a
definite pattern to suit the particular rolling mode adopted and
particular grid can be operated for discharging of blooms on to
roller table upon ON light flashing and the light will go OFF when
preselected number of blooms have been discharged. In order to
identify and check the correct charging pattern the bloom of a
changed heat number is given a colour marking. Rectangular blooms
will be placed on the charging grids (3no.) in flat position (320mm
flat) by means of magnet crane. The cranes are capable of loading 3
blooms at a time and will form a gapless bloom group of maximum 6
blooms each. Each grid is capable of receiving maximum of 6 groups
which are appropriately spaced apart for the folding dogs to
restore the upright position and engage.Two blooms are discharged
on to roller tables one by one in the sequential manner as
mentioned earlier. The respective grid will go OFF when the
preselected number of blooms has been discharged on to roller
table. When the blooms have passed the roller table section in
front of another grid, the light of that grid will be ON permitting
that grid to discharge blooms. Blooms are discharged on roller
table and conveyed to the elevator. Defective blooms, if any, on
its travel to the elevator will be detected manually by the
operator and positioned in front of defective bloom grids and will
be pushed out of the roller table. The roller table transports the
blooms normally under sequence control of their respective destined
elevators 1 or 2. The blooms are pushed off the roller table by
lever type off on to a ramp from where they are picked up carrier
links of elevators and pulled up. They carry off device downstream
and pick up the bloom and places on furnace approach roller table.
The furnace approach roller table working on sequential control,
positions the blooms while on motion on various sections of this
roller by means of light barrier. Since the walking beam system are
to be uniformly loaded different bloom lengths will be classified
into two categories and positioned in front of furnaces
automatically. The bloom on its passage is stopped and its actual
weight is recorded. The material tracking computer system takes
over the actual weight, checks with reference weight and then sets
the bloom free for further transport. As soon as the blooms are
positioned correctly and the walking beam furnaces are ready to
receive the blooms, the blooms are pushed into the furnace by
furnace pusher. 1.3 WALKING BEAM FURNACEEach furnace is loaded in
two rows and each row is meant for its defined destination. Two
furnaces have four tracks and materials are discharged by
discharging units (4 nos.) as per the discharging pattern envisaged
for the particular rolling mode. At the delivery side of the
furnace, actual position of blooms is detected by means of a sensor
and its actual position is given out as a coded series of pulses
referred to the theoretical position. Walking beam furnace has
capacity of 200tons/hr. the fuel used in the furnace is a mixture
of Coke oven gas, Blast furnace gas and LD converter gas. The fuel
calorific value is 2000kcal/m3. In general the calorific value of
Coke oven gas is greater than LD converter gas and Blast furnace
gas.
Fig 1.1 Walking Beam FurnaceThe walking beam furnace is divided
into three zones namely: Pre-heating zone, Heating zone and soaking
zone. The flue gases are sent into recuperators and the bloom is
pre heated to a certain temperature. This is pre-heating zone. In
the Heating zone the bloom is heated to the required temperature.
This temperature has to be maintained and this is done in the
soaking zone.The blooms have to be moved from the charging side to
the discharging side and this is done by skids. Skids are of two
types: Moving skid and fixed skid. These skids are cylindrical
tubes which are insulated on the top surface and through which
steam flows in order to cool the skids. The blooms are rested upon
the fixed skids. The moving skids lift the bloom from the fixed
skids, move forward and place the bloom on the next fixed skid. Now
the moving skid is lowered. The motion of the moving skids is
controlled by the pneumatic cylinders.When the furnace walking beam
system has completed its horizontal movement, the sensor gives out
the actual position of bloom. This information is utilized by the
corresponding discharging machine (the mobile beam of the machine
has already reached to the discharging line) to complete its
remaining stroke and to lift the material off the furnace and place
it on delivery roller table provided the respective roller table
section is free. If it is already occupied the discharging machine
is held in waiting position and subsequently deposits the bloom as
soon as the respective roller table sections are free. If the
discharged hot blooms are found to be defective the blooms are
conveyed along the roller table in the reversed direction and are
pushed over defective bloom grid by bloom pusher. Rejected blooms
are picked up by crane in layers of three and deposited on to
transfer car. The transfer car transfers the rejected blooms in the
cross bay R-Q.
1.4 BREAKDOWN MILL AREAHot blooms travel forward over the roller
table under sequence control and is freed from scale on the move by
the de-scaler unit and is finally positioned over the roller table
section having tilter unit. The tilting (under manual control) is
done to make the bloom upright. The upright bloom now enters the
first stand of Billet mill. As soon as the tail end of this bloom
clears stand no. 1, the upstream tilter is again ready to receive
the next bloom under sequence control. There are seven stands (four
horizontal and three vertical) in the breakdown mill. In five box
passes the bloom is reduced from 320mm X 250mm to 125mm square of
33m length (referred to nominal bloom length of 6.3m) with a
finishing temperature of 11000C-12000C.The draw-in speed of bloom
varies from 0.256m/sec to 0.315m/sec depending upon the discharge
bloom temperature.The stand arrangement is as shown below:Stand
1Stand 2Stand 3Stand 4Stand 5Stand 6Stand 7
HorizontalHorizontalVerticalHorizontalVerticalHorizontalVertical
3 Pass3 Pass3 Pass3 Pass3 Pass3 Pass4 Pass
Table 1.1 Table showing the arrangement of stands in LMMM
Fig 1.2 A Vertical Stand in Breakdown MillA four crank shear
installed behind the mill stand is designed to crop both ends and
to cut fixed billet length between 5.0m to 12.2m or to perform
optimum yielding dividing. When cutting fixed lengths, rest ends up
to 1.5m are guided into a scrap bucket (recoil roller table section
open). Rest ends above 1.5m are transported (recoil roller table
section closed) to a location where they are diverted into a short
length disposal cradle. Detection of a rest end below or above 1.5m
is automatically done before rest end cut is performed.Whenever a
bloom meant for a bar mill enters BDM, checks are made whether the
pendulum shear at the entry of bar mill is in the process of
performing an emergency out of the proceeding bloom in the very
strand for which the new bloom is being rolled in BDM. If it is so,
the new bloom being rolled at BDM must be out into pieces of
specified length by the 4-crank shear. Those pieces are then
deposited into the short length disposal cradle as described
earlier. If optimum yield cutting is performed, for example, for
billets, for wire rod mill, the total finishing length will be cut
to get three equal pieces with one minimum crop cut at each end.
Computer calculation of the first billet length is made on the
basis of bloom weight, already stored in the first billet in
conjunction with the reduction in the BDM stands, permits to assess
the billets length with substantial accuracy. However the exact
billet length measured by the measuring roll on the four-crank
shear is used to calculate the corrective factor. This factor is
also a parameter to calculate the exact discharging cycle for the
succeeding BM blooms.
1.5 EVAPORATIVE COOLING SYSTEMThe evaporating cooling system is
the main type of the cooling system that is at present being used
in the Vishakapatnam Steel Plant. In this the compressed air is
being passed through the pipes into the furnace and this cooling
fluid is being collected again into the drum. The de-mineralized
de-aerated water is being used for this purpose so as to prevent
corrosion of the pipe lines etc.1.6 COOLING BEDSThe cooling beds
are used to uniformly air-cool the bars and profiles and
transporting the same in a phased manner from the entry to
discharge side. The cooling beds are specifically designed
considering the smallest and the maximum size of the bars being
rolled. These cooling system provides adjustability to the size,
shape, and alloy of the profiles with appropriate cooling rate and
minimized distortion. Turn over type cooling bed for alloy steel
rotates the bars by one revolution every time the rake moves by a
pitch. These cooling beds impart better straightness in material
with improvised metallurgical properties, as the bars are uniformly
cooled.
1.7 SALIENT FEATURES OF LMMMa. High capacity and high speed.b.
Automatic minimum tension control in standsc. Double sided cooling
beds of walking beam type.d. High capacity and high productive
sawing lines.e. Automatic bundling machines.f. Computerization at
the sequential process control and material trackingg. Adoption of
closed circuit TV at furnaces.h. evaporative cooling system and
waste heat recovery.These features help to optimize the production
and assure quality products from the mill.
CHAPTER 2INTRODUCTION TO TURN OVER TYPE COOLING BEDS
A cooling bed device is provided for straight-edged rolled
material, such as billets, for example, for simultaneously
conveying and turning the billets over the bed. This is achieved by
a dual rake arrangement for engaging and moving the billets with a
stationary rake cooperating with a movable rake. Adjacent teeth in
the stationary rake form angles of about 90 with each other, and
adjacent teeth in the movable rake are spaced from each to form
horizontal bearing surfaces wider than the longest straight edge of
a billet to be handled. By utilizing a single movable rake movable
in a simple circular path, and with the incline of rising and
falling flanks of the teeth of both rakes being the same, the
device is much less expensive to construct and operate, while still
providing a full 90 turn-over of the billets in a single sequence
of movement of the device. The cooling beds are easily tilted by
the supporting mechanism of bearings, gear box, shafts,
coupling.
2.1 DESIGN AND DESCRIPTONThe turnover type cooling bed consists
of1. Supporting rakes with fixed and pendulum support turnover
rakes with toggle lever supports.2. The fixed supporting rakes are
mounted on single and double supports which are not cross-connected
and thus makes the bed unsusceptible to thermal expansion in cross
direction.3. In longitudinal direction, thermal expansions of the
supporting rakes are accommodated by pendulum supports.4. Both the
main and secondary swivel shafts are jointly moved by a spur gear
via separate cranks which are staggered under a certain angle.
2.2 FUNCTIONAL DESCRIPTION1. Their purpose is to cross transfer
billets being used as starting stock for the wire rod mill or for
sale to the loading station and to cool them down during this
transport. The two cooling beds are subdivided into two
mechanically independent sections as viewed in material flow
direction. 2. The cooling beds substantially comprise a system of
fixed rakes, so-called supporting rakes, and a system of moving
rakes, so-called turnover rakes. 3. The two drives of each system
are mechanically synchronized disposed on each gear output shaft
are two cranks which via push rods impart an oscillating movement
to the main and the secondary swivel shaft. 4. The main swivel
shaft and the toggle lever support coupled there to produce the
vertical movement of the turnover type rake system while horizontal
motions are produced by the secondary swivel shaft and its
connecting rods to the turnover rake system.5. The teeth or notches
of the supporting and turnover rakes are shaped in such a way that
during one turnover rake revolutions all billets lying on the
supporting rakes are twice turned through 90.The cranks on the
input shaft to the gears cover an angle of 360 in one cooling bed
cycle. 6. As soon as the first section of a cooling bed is filled
and the first billet is turned over to the second section, said
latter operates a flag switch releasing a signal for the drives of
the 2nd section.
Fig 2.1 Cooling Bed
2.3 ADVANTAGES AND DISADVANTAGES1 When blooms are tilting it
will be cooling uniformly2. High productivity, at a time we can get
maximum 60 billets3. It will be cooled at atmospheric air
temperature by the process of normalizing without any change of
properties.4. Easy maintenance5. One of the major drawback is that
it can only be used only for square type billets, it is the main
draw-back of the cooling beds.
CHAPTER 3INTRODUCTION TO BEARINGS
A bearing is a machine element which supports another moving
element (known as journal). It permits a relative motion between
the contact surfaces of the members, while carrying load. Due to
the relative motion between the contact surfaces, a certain amount
of power is wasted in overcoming frictional resistance and if the
rubbing surfaces are in direct contact, there will be rapid wear.
In order to reduce frictional resistance and wear and in some cases
to carry away the heat generated, a layer of fluid(known as
lubricant) may be provided. The lubricant used to separate the
journal and bearing is usually a mineral oil refined from
petroleum, but vegetable oils, silicon oils, greases etc., may be
used.Bearings are made of a variety of materials of which the most
common and important is stainless steel. Since bearings have to
bear a lot of stress, the steel with which it is made needs to be
very strong. The quality of steel used determines the durability of
the bearing. Cheap bearings are made of hardened carbon-steel or
pressed sheet metal, both of which are usually not recommended. The
standard industry classification for the steel in some bearings is
52100, which contains 1% chromium and 1% carbon. Such steels can be
made very hard and tough by heat treating. 440C stainless steel are
used to manufacture bearings which can face rusting threats.
3.1 BASIC PARTS OF BEARINGA bearings smooth performance is
assured by a combination of four basic working parts Outer race
(also called outer ring or cup) Inner race (also called inner ring
or cone) Rolling elements (either balls or rollers) Separator (also
called cage or retainer)The outer race, or cup, is the bearings
exterior ring. Since it protects the bearings internal parts, it
must be machined smoothly and accurately. The inner race, or cone,
is the part of the bearing that sits directly on the shaft. The
rolling elements, shaped as balls or rollers, provide the cushion
that eases the moving friction of the shaft within its housing.
These elements keep the outer and inner races separated and enable
them to move smoothly and freely. The shape of the rolling elements
depends on the type of load, operating conditions and particular
applications. It is the rolling elements that distinguish the two
basic bearing categories ball bearings and roller bearings. There
is a groove called the ball path on both the inner and outer races
of ball bearings in which the balls roll. For roller bearings, the
rollers roll on the flat surface of each race. This surface is
called the roller path. Finally, the separator is a metal retainer
that holds the balls or rollers. Positioned between the inner and
outer races, the separator keeps the rolling elements evenly
spaced.
Fig 3.1 Basic elements of bearing
3.2 CLASSIFICATION OF BEARINGS
3.2.1 CLASSIFICATION BASED ON DIRECTION OF LOAD
Based on the direction of load bearings are classified into two
types1. Radial Bearing: In this type of bearing, the load acts
perpendicular to the direction of the motion of the moving
elements.2. Thrust Bearing: In this type, the load acts along the
axis of rotation.
3.2.2 CLASSIFICATION BASED ON NATURE OF CONTACTBased on the type
of contact the bearings are classified into two types1. Sliding
contact Bearings: In sliding contact bearings, the sliding takes
place along the surfaces of contact between the moving element and
the fixed element. The sliding contact bearing are also known as
plain bearings. 2. Rolling contact bearings: In rolling contact
bearings, the steel balls or roller, are interposed between the
moving and fixed elements, the balls offer rolling friction at two
points for each ball or roller.
3.3 SLIDING CONTACT BEARINGSThe sliding contact bearings having
surface contact and are coming under lower kinematic pair.3.3.1
CHARACTERISTICS OF SLIDING CONTACT BEARINGa)Sliding contact
bearings are the oldest, simplest, least expensive bearing
technology, and they still have a wide range of applications, from
construction machinery to machines with atomic resolution.b)Sliding
contact bearings utilize a variety of different types of lubricants
between various interface materials. Lubricants range from light
oil to grease to a solid lubricant such as graphite or a PTFE
polymer. Because they often distribute loads over a large area,
contact stresses and space requirements are often low while
stiffness and damping are usually high. c) They are very robust and
reliable. d) They are speed limited and have friction-induced servo
limits.e) They are economical and for many applications will never
be replaced.3.3.2 SLIDING CONTACT BEARINGS - ADVANTAGES AND
DISADVANTAGESThese bearings have certain advantages over the
rolling contact bearings. They are: 1. The design of the bearing
and housing is simple. 2. They occupy less radial space and are
more compact. 3. They cost less. 4. The design of shaft is simple.
5. They operate more silently. 6. They have good shock load
capacity. 7. They are ideally suited for medium and high speed
operation. The disadvantages are: 1. The frictional power loss is
more. 2. They required good attention to lubrication. 3. They are
normally designed to carry radial load or axial load only.3.3.3
SLIDING CONTACT BEARINGS - CLASSIFICATION Sliding contact bearings
are classified in three ways. 1. Based on type of load carried 2.
Based on type of lubrication 3. Based on lubrication
mechanism3.3.3.1 Bearing classification based on type of load
carried a. Radial bearings: These bearings carry only radial loads
b. Thrust bearings or axial bearings: These bearings carry only
axial loads c. Radial thrust bearings: These bearings carry both
radial and thrust loads.3.3.3.2 Bearing classification based on
type of lubricationThe type of lubrication means the extent to
which the contacting surfaces are separated in a shaft bearing
combination. This classification includes (a) Thick film
lubrication: The surfaces are separated by thick film of lubricant
and there will not be any metal-to-metal contact. The film
thickness is anywhere from 8 to 20m. Typical values of coefficient
of friction are 0.002 to 0.010. (b) Thin film lubrication: Here
even though the surfaces are separated by thin film of lubricant,
at some high spots Metal-to-metal contact does exist. Because of
this intermittent contacts, it also known as mixed film
lubrication. Surface wear is mild. The coefficient of friction
commonly ranges from 0.004 to 0.10.(c) Boundary lubrication: Here
the surface contact is continuous and extensive. The lubricant is
continuously smeared over the surfaces and provides a continuously
renewed adsorbed surface film which reduces the friction and wear.
The typical coefficient of friction is 0.05 to 0.20.3.3.3.3 Bearing
classification based on lubrication mechanisma. Hydrodynamic
lubricated bearings b. Hydrostatic lubricated bearingsc.
Elasto-hydrodynamic lubricated bearings d. Boundary lubricated
bearings e. Solid film lubricated bearings
3.4 ROLLING CONTACT BEARINGSRolling contact bearings are also
called anti-friction bearing due to its low friction
characteristics. These bearings are used for radial load, thrust
load and combination of thrust and radial load. These bearings are
extensively used due to its relatively lower price, being almost
maintenance free and for its operational ease. However, friction
increases at high speeds for rolling contact bearings and it may be
noisy while running. These bearings are of two types, Ball bearing
and Roller bearing.3.4.1. BALL BEARINGThe bearing shown in the
figure is called Single row deep groove ball bearing. It is used to
carry radial load but it can also take up considerable amount of
axial load. The retainer keeps the steel balls in position and the
groove below the steel balls is the inner ring and over it is the
outer ring. The outer ring, called outer race, is normally placed
inside a bearing housing which is fixed, while the inner race holds
the rotating shaft. Therefore, a seat of diameter d and width B is
provided on the shaft to press fit the bearing. The arrangement for
housing a bearing is shown through a schematic diagram.
Fig 3.2 Single row deep groove ball bearing3.4.1.1. ADVANTAGES
AND DISADVANTAGES OF ROLLING-CONTACT BEARINGAdvantages1. Low
starting and good operating frictional torque.2. Ease of
lubrication3. Requiring less axial space4. Generally, taking both
radial and axial loads.5. Rapid replacement 6. Warning of impending
failure by increasing noisiness.7. Good low-temperature
starting.Disadvantages1. Greater diametrical space.2. More severe
alignment requirements.3. Higher initial cost.4. Noisier normal
operation.5. Finite life due to eventual failure by fatigue.6. Ease
of damage by foreign matter.7.Poor damping ability.3.4.1.2 TYPES OF
BALL BEARINGSSome types of ball bearings are:1. Deep groove
bearing2. Filing notch bearing3. Angular contact bearing4. Shielded
and sealed bearings5. Self-aligning Bearings6. Double row
bearings7. Thrust bearings
Fig 3.3 Types of ball bearings and roller bearings
3.4.2 ROLLER BEARINGSIn this type of bearings the spherical
balls are replaced by different shaped rollers. The types of roller
bearings are:1. Straight roller bearings: Takes higher radial load
than ball bearing (more contact area), but needs perfect geometry
& does not take thrust load.2. Spherical Roller thrust bearing:
Useful for heavy loads & misalignment (contact area increases
with load). 3. Thrust bearings4. Needle Bearings: Useful when
radial space is limited.5. Tappered-roller Bearings: Take both
radial & thrust loads (higher loads than ball bearings).
CHAPTER 4MODIFICATION OF BEARING4.1 PLAIN SPHERICAL BEARINGPlain
spherical bearings are particularly suitable for the accommodation
of the heavy radial loads. They can also carry a certain amount of
thrust in either direction. The designs of the radial Plain
spherical bearings differ by the material of the components in
sliding contact. Angular contact spherical bearings are an
excellent choice for applications subject to both radial and thrust
loads; plain spherical thrust bearings are intended for
applications where the loads are primarily thrustTo meet the
requirements of heavy machinery construction and the need for
spherical bearings whose load carrying capacity and size are beyond
the capabilities of the standard types of manufacture large
heavy-duty spherical bearings For angular contact spherical
bearings, plain spherical bearings and heavy duty spherical
bearings are used.
MATERIALS USED: Steel on composite material the inner ring
slides on the outer ring sliding surface which is made of composite
material. It consists of porous tin bronze layer sintered on to a
steel surface whose pores are impregnated with a mixture of PTFE
and lead. In addition, another layer consisting of the same
material is applied for running in purposes.Lubrication and
maintenance: Plain spherical bearings need no lubrication. It is
good practice, however, to improve corrosion resistance and sealing
by greasing the bearing. The lubricating grease ArcanolL71 is used
for this purpose. The permissible operating temperature depends on
the material of which the bearing is made of.
4.2 SPHERICAL ROLLER BEARINGSpherical roller bearings are
self-aligning bearings. The self-aligning feature is achieved by
grinding on of the races in the form of sphere. This bearings can
normally to tolerate angular misalignment which is in order of + or
one and half degree and when used with a double row of rollers,
this can carries thrust loads in either direction. Spherical roller
bearings have two rows of rollers with a common sphered raceway in
the outer ring and two inner ring raceways inclined at an angle to
the bearing axis. This gives them an attractive combination of
design features making them irreplaceable in many demanding
applications. They are self-aligning and consequently insensitive
to misalignment of the shaft relative to the housing and to shaft
deflection or bending.Spherical roller bearings can be of
ahydrostaticor mechanical construction. A spherical roller bearing
by itself consists of an outer ring and an inner ring and a locking
feature that makes the inner ring captive within the outer ring in
the axial direction only. The outer surface of the inner ring and
the inner surface of the outer ring are spherical (or more
correctly,toroidal) and are collectively considered the raceway and
they slide against each other, either with a lubricant, a
maintenance-free (typicallypolytetrafluoroethyleneor PTFE) based
liner, or they incorporate a rolling element such as a race of
ball-bearings, allowing lower friction 4.3 IMPORTANT
TERMINOLOGIESSome of the important terminologies which are required
for selection of rolling contact bearing are given below.4.3.1
Rating Life: Rating life is defined as the life of a group of
apparently identical ball or roller bearings, in number of
revolutions or hours, rotating at a given speed, so that 90% of the
bearings will complete or exceed before any indication of failure
occur.Suppose we consider 100 apparently identical bearings. All
the 100 bearings are put onto a shaft rotating at a given speed
while it is also acted upon by a load. After some time, one after
another, failure of bearings will be observed. When in this
process, the tenth bearing fails, then the number of revolutions or
hours lapsed is recorded. These figures recorded give the rating
life of the bearings or simply L10 life (10 % failure). Similarly,
L50 means, 50 % of the bearings are operational. It is known as
median life. 4.3.2 Bearing Load: If two groups of identical
bearings are tested under loads P1 and P2 for respective lives of
L1 and L2 , then, Equation (1) Where,L :life in millions of
revolution or life in houra :constant which is 3 for ball bearings
and 10/3 for roller bearings
4.3.3 Basic Load Rating: It is that load which a group of
apparently identical bearings can withstand for a rating life of
one million revolutions.Hence, in Equation (1), if say, L1 is taken
as one million then the corresponding load is, Equation (2)Where, C
is the basic or dynamic load rating.Therefore, for a given load and
a given life the value of C represents the load carrying capacity
of the bearing for one million revolutions. This value of C, for
the purpose of bearing selection, should be lower than that given
in the manufacturers catalogue. Normally the basic or the dynamic
load rating as prescribed in the manufacturers catalogue is a
conservative value, therefore the chances of failure of bearing is
very less.4.3.4 Equivalent Load: The load rating of a bearing is
given for radial loads only. Therefore, if a bearing is subjected
to both axial and radial load, then an equivalent radial load is
estimated as, or Kilo Newtons Equation (3)Where, Pe : Equivalent
radial load KNPr : Given radial load KNPa : Given axial load KNV :
Rotation factor (1.0, inner race rotating; 1.2, outer race
rotating) X : A radial factor Y : An axial factorThe values of X
and Y are found from the chart whose typical format and few
representative values are given below.eX YX Y
0.0210.211.00.00.562.15
0.1100.301.00.00.561.45
0.560.441.00.00.561.00
Table 4.1 Typical X and Y values for calculating Equivalent
Load
Depending on the shaft diameter and magnitude of radial and
axial load a suitable type of bearing is to be chosen from the
manufacturers catalogue, either a ball bearing or a roller bearing.
The equivalent radial load is to be determined from equation
(3).
4.4 TECHNICAL DATA1. For Plane Spherical BearingMaterial
Steel-on-steel spherical plain bearingLubrication SelfInner
diameter 180 mmOuter diameter 260 mmBasic load rating 2160Factor of
safety 1.33Equivalent Dynamic Load 360 KN
2. For Spherical Roller BearingsMaterial Steel-on-steel
spherical roller bearingLubrication oilsInner diameter 180 mmOuter
diameter 260 mmBasic load rating 88000Factor of safety
1.6Equivalent Dynamic Load 4860 KNNote: The technical Data has been
collected from the industry manual22
4.5 CALCULATIONSThe approximate rating (or service) life of
roller bearings is based on the fundamental equation,
revolutionsWhere, L = Rating life, C = Basic dynamic load rating,
Pe = Equivalent dynamic load, KN a= 3, for ball bearings, =10/3,
for roller bearings.Now calculating the life period of each bearing
1. Plain spherical bearings:
L= million revolutionsL=0.216 million revolutions2. Spherical
Roller Bearings:
million revolutionsL=1.56 million revolutions
4.6 CONCLUSIONIn evidence to the above made calculations it is
clear that the Spherical Roller Bearings have more life than Plane
Spherical Bearings. Spherical Roller Bearings have almost twice the
life of the Plane Spherical Bearings. More over the cost of Plane
Spherical Bearings is higher than the Spherical Roller Bearings.
Most damage prone part in cooling beds turns out to be the bearings
as they play a vital role in the power transmission system in
moving rakes. Therefore the bearing with higher life will also save
time for replacement of bearings. Therefore all the above points
leads us to the conclusion that the Plane Spherical Bearings are to
be replaced with Roller Spherical Bearings and the modification is
justified.
List of figures Page No.Fig 1.1 Walking Beam Furnace 3Fig 1.2 A
Vertical Stand in Breakdown Mill 5Fig 2.1 Cooling Bed 9Fig 3.1
Basic elements of bearing 11Fig 3.2 Single row deep groove ball
bearing 15Fig 3.3 Types of ball bearings and roller bearings 17
List of TablesTable 1.1 Table showing the arrangement of stands
in LMMM 5Table 4.1 Typical X and Y values for calculating
Equivalent Load 21
REFERENCES
1. J.E Shigley and C.R Mischke , Mechanical Engineering Design ,
McGraw Hill Publication, 5th Edition. 1989.
2.Khurmi, R.S. and Gupta J.K., Text book on Machine Design,
Eurasia Publishing House, New Delhi.
3. SKF bearing design catalogue.
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