INTRODUCTION
Rolling element bearing became prominent when there was a requirement of such bearing
which can withstand heavy loads produced by even light loads. It was only the development of
case hardening steels from the about the year 1900 onwards, and subsequent progress in the
manufacture and heat treatment of alloy steel, that made it possible for the rolling bearing
industry to grow and by degrees assume its present proportions.
There are thousands of sizes, shapes, and kinds of rolling bearings; ball bearings, roller bearings,
needle bearings, and tapered roller bearings are the major kinds. Sizes run from small enough
to run miniature motors to huge bearings used to support rotating parts in hydroelectric power
plants; these large bearings can be ten feet (3.04 meters) in diameter and require a crane to
install. The most common sizes can easily be held in one hand and are used in things like
electric motors.
In rolling element bearings, the rolling part is a ball, which rolls between inner and outer rings
called races. The balls are held by a cage, which keeps them evenly spaced around the races.
Raw Materials
Almost all parts of all ball bearings are made of steel. Since the bearing has to stand up to a lot
of stress, it needs to be made of very strong steel. The standard industry classification for the
steel in these bearings is 52100, which means that it has one percent chromium and one
percent carbon (called alloys when added to the basic steel). This steel can be made very hard
and tough by heat treating. Where rusting might be a problem, bearings are made from 440C
stainless steel.
The cage for the balls is traditionally made of thin steel, but some bearings now use molded
plastic cages, because they cost less to make and cause less friction.
When running under load, the metal of the bearing with rolling elements is subjected to
stresses of great intensity, thus causing deformation, flexure, tension, sliding, and local heating
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of different surfaces in contact. In the course of bearing’s life the alternating stresses may recur
at a given point many millions of times.
Because of the very small contact between the rolling element and the raceways, the localized
maximum stresses are especially severe, for which reason the requirements as regards strength
and fatigue resisting properties are most exacting than those to be met by the material for
almost any other engineering work. In the contact areas of a ball bearing, the recurring stress
may be of the order of 100 to 200 tons per sq.in.
In-time – only too soon if the bearing is overloaded, or the steel of inferior quality – the steel of
inferior quality – the strains cause the surface to disintegrate , small particles of the material
becoming detached from the surface referred to as scaling or flaking. Which is an unmistakable
sign of fatigue of the metal.
The selection of suitable material, therefore, resolves itself into finding the steels with the
highest fatigue limit, subject to its meeting other practical requirements, such as suitability for
machining when in the annealed state, responsiveness to heat treatment, and it being
obtainable at an economic price. Experiments and experience combine to show that materials
with high elastic limit also have high fatigue limit. Practically speaking, a high elastic limit means
a great hardness, which is therefore a most valuable property , always provided that it is
accompanied by brittleness or lack of toughness, and uniform to an adequate depth.
All structural shortcomings, such as porosity, slag inclusions, and carbide segregations, must of
course be avoided and impurities like sulphur and phosphorous be practically nonexistent.
In the process of rolling, forging, annealing, turning, hardening, grinding etc., every care must
be exercised in order that the structure of steel may not suffer, and the performance of the
finished product be adversely affected in consequence.
The essential properties of a satisfactory steel may accordingly be summarized thus,
1) Great strength
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2) Great hardness
3) Deep penetration of the hardness
4) Uniform hardness
5) Uniform structure throughout
6) Fine-grained texture
7) Minimum brittleness
8) High resistance to fatigue
9) High resistance to fatigue
10) High resistance to wear
11) Freedom from soft spots
12) Freedom from segregations
13) Freedom from impurities, metallic and non metallic.
Some of the materials used are
1) Case – hardened steel : better results can be obtained with case hardening steel than
with poor- quality high carbon steel and alloy steels provided it is correctly heat treated.
Unfortunately, however its heat treatment is a complicated process beset with
difficulties.
Case hardening steel is a hypoeutectoid steel with a structure characterized by small
areas of pearlite and large ferrite crystals. It cannot be hardened direct , owing to its deficiency
in carbon and must be partly converted into eutectoid or hypereutectoid steel by carburization,
which increases the the carbon content. For this purpose three intricate heating operations are
necessary.
2) Chrome steel: Chrome steel containing about 1 percent carbon and 1.5 percent
chromium was developed specially for ball bearing, extensive research having proved
that chromium enabled steel to fulfil the essential requirements enumerated above
better than any other alloy.
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The following is a typical analysis, tough the percentages of carbon and chromium are
subject to slight variation to suit different ranges of bearing sizes.
Material Composition of Chrome Alloy Hardened Steel Ball(High Carbon High Chrome Steel, Bearing Steel - Thorough Hardened)
CarbonC
ManganeseMn
SiliconSi
PhosphorusP
SulphurS
NickelNi
ChromiumCr
HardnessRockwell
C
EN 31 0.90-1.10
1.10 max 0.10-0.35
0.05 max 0.05 max
Nil 1.00-1.50 58-63 HRc
AISI52100
0.95-1.10
0.20-0.50 0.35 max
0.025 max 0.025 max
Nil 1.30-1.60 58-63 HRc
Figure 1 High Carbon High Chrome balls
3) Rustless steel: The chrome steel used for rolling bearings is no different from ordinary
carbon- steel so far as susceptibility to rust under direct exposure to moisture or other
corrosive agents is concerned. Rust on the rolling elements or raceways increases
friction, causes wear with the attendant pitting, and provides the nuclei from which
fatigue cracks start and which ultimately develop into surface flaking. The following
represents a preferred composition.
Carbon 0.65 to .70%
Silicon 1.50 to 1.80%
Manganese 0.10 to 1.15%
Chromium 0.50 to 2.00%
With balance iron.
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PARTS OF ROLLING ELEMENT BEARING
Fig 2& 3: sketch and 3 D view of rolling element bearing with parts.
Description
A rolling-element bearing comprising an inner race , an outer race and an array of rolling
elements arranged within a bearing cage situated between the two races , the inner surface of
the bearing cage being piloted on the inner race for limiting eccentric movement of the cage
within the bearing and further having a reservoir formed between a pair of circumferential weirs
on the cage , the outer surface of the inner race comprising a raised land portion for contacting
oil in the reservoir thereby to control the relative speed of the cage and the inner race , wherein
the radial depth of each weir , is greater than the maximum radial clearance between the land
portion and the cage for maintaining said controlling contact between the land portion and the
oil during said eccentric movement of the cage.
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MANUFACTURING OPERATIONS
The overall manufacturing process can be represented by a simple flow chart as shown below
There are four major parts to a standard ball bearing: the outer race, the rolling balls, the inner race, and
the cage.
Bearing manufacturing
Some manufacturing processes increase bearing life. Their main disadvantage is additional cost.
Forging produces a fiber orientation in the race material that makes the races less sensitive to
variations in steel quality. Bearings with forged races can have dynamic capacities up to twice
as high as bearings with races cut from tubing.
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Cutting step (for balls,cages and races)
molding step(for cage and ball)
Pressing step( for cage and Ball)
Hardening and tempering step (all)
grinding step ( all parts)
Assembly
Inspection
Packing
Compressive residual stress reduces maximum shearing stress and increases fatigue life. For
bearings with light to medium loads, residual compressive stress can increase life. But for
bearings with heavy loads, the effect is significant.
Controlled-hardness bearings have rolling elements and races matched for hardness.
Generally, the rolling elements are 1 to 2 Rc harder than the races. Since fatigue life is related to
hardness, the matching process can result in order-of-magnitude improvements in fatigue life.
Races
1) Both races are made in almost the same way. Since they are both rings of steel, the
process starts with steel tubing of an appropriate size. Automatic machines similar
to lathes use cutting tools to cut the basic shape of the race, leaving all of the
dimensions slightly too large. The reason for leaving them too large is that the races
must be heat treated before being finished.
2) The rough cut races are put into a heat treating furnace at about 1,550 degrees
Fahrenheit (843 degrees Celsius) for up to several hours (depending on the size of
the parts), then dipped into an oil bath to cool them and make them very hard. This
hardening also makes them brittle, so the next step is to temper them. This is done
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by heating them in a second oven to about 300 degrees Fahrenheit (148.8 degrees
Celsius), and then letting them cool in air. This whole heat treatment process makes
parts which are both hard and tough.
3) After the heat treatment process, the races are ready for finishing. However, the
races are now too hard to cut with cutting tools, so the rest of the work must be
done with grinding wheels. These are a lot like what you would find in any shop for
sharpening drill bits and tools, except that several different kinds and shapes are
needed to finish the races. Almost every place on the race is finished by grinding,
which leaves a very smooth, accurate surface. The surfaces where the bearing fits
into the machine must be very round, and the sides must be flat. The surface that
the balls roll on is ground first, and then lapped. This means that very fine abrasive
slurry is used to polish the races for several hours to get almost a mirror finish. At
this point, the races are finished, and ready to be put together with the balls.
Balls
Fig : showing the manufacturing process of balls
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1) The balls are a little more difficult to make, even though their shape is very
simple. Surprisingly, the balls start out as thick wire. This wire is fed from a roll
into a machine that cuts off a short piece, and then smashes both ends in toward
the middle. This process is called cold heading. This process is called cold heading.
Its name comes from the fact that the wire is not heated before being smashed, and
that the original use for the process was to put the heads on nails (which is still
how that is done). At any rate, the balls now look like the planet Saturn, with a
ring around the middle called "flash."
The bulge around the middle of the rolling balls is removed in a machining process. The
balls are placed in rough grooves between two cast iron discs. One disc rotates while the
other one is stationary; the friction removes the flash. From here, the balls are heat
treated, ground, and lapped, which leaves the balls with a very smooth finish.
2) The first machining process removes this flash. The ball bearings are put
between the faces of two cast iron disks, where they ride in grooves. The inside
of the grooves are rough, which tears the flash off of the balls. One wheel
rotates, while the other one stays still. The stationary wheel has holes through it
so that the balls can be fed into and taken out of the grooves. A special conveyor
feeds balls into one hole, the balls rattle around the groove, and then come out
the other hole. They are then fed back into the conveyor for many trips through
the wheel grooves, until they have been cut down to being fairly round, almost
to the proper size, and the flash is completely gone. Once again, the balls are left
oversize so that they can be ground to their finished size after heat treatment.
The amount of steel left for finishing is not much; only about 8/1000 of an inch
(.02 centimeter), which is about as thick as two sheets of paper.
3) The heat treatment process for the balls is similar to that used for the races,
since the kind of steel is the same, and it is best to have all the parts wear at
about the same rate. Like the races, the balls become hard and tough after heat
treating and tempering. After heat treatment, the balls are put back into a
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machine that works the same way as the flash remover, except that the wheels
are grinding wheels instead of cutting wheels. These wheels grind the balls down
so that they are round and within a few ten thousandths of an inch of their
finished size.
4) After this, the balls are moved to a lapping machine, which has cast iron wheels
and uses the same abrasive lapping compound as is used on the races. Here,
they will be lapped for 8-10 hours, depending on how precise a bearing they are
being made for. Once again, the result is steel that is extremely smooth.
Fig : Various stages of Ball manufacturing
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Figure Grinding of steel balls
Production sequence of steel balls in brief
Wire Coil: Steel wire of specific material and size is checked for correctness.
Cutting: The wire is cut into required lengths.
Forging / Heading: Spherical shapes between dies are formed.
Deflashing: The flash line along the circumference is removed.
Heat Treatment: Chrome and bearing steel balls are thorough hardened.
Cleaning / Descaling: Basic surface cleaning is done.
Hard Grinding / Filing: An exacting process to achieve required sphericity and size of the steel
ball.
Lapping: The final surface finish is acquired through this process.
Polishing / Burnishing: Surface luster, work hardening, higher product life is achieved.
Passivation: Carryover iron and other contaminants are removed and a surface film prevents
atmospheric and water corrosion on stainless steel balls.
Inspection: For surface finish, size and tolerance.
Rust preventive oil: Applied on chrome alloy steel balls to save from rusting and corrosion.
Packing: Done as per requirement in numbers, pieces or by weight. VCI paper or bags are used
wherever necessary.
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Cages
The rolling element, whether balls or rollers, are seldom equally loaded in complete bearing; in
fact in a radial bearing the rollers furthest from the point of loading may be quite out of control
so that a suitable means of keeping them evenly spaced round their orbit is essential. The
auxiliary component used for this purpose is known variously as the cage, the retainer, and the
separator.
Cages are manufactured in different ways from a variety of materials. Although but an auxiliary
component, the cage has such an important part to play that the success, or failure. Of a
bearing under a given set of working conditions may well depend on its design and material
used in its construction. Experience is the only real guide to the correct choices of cages to suit
different types of bearing and the speeds and the loads met with under working conditions. The
user must therefore have recourse to the expert for guidance. Some of the more usual
combinations of materials and design are described below.
Cages made from solid: The material may be in the form of bars, forgings, or thick sheet. The
pockets for the rolling element may be drilled either radially or axially. The cage itself may be
centered on either the inner or the outer ring lands, or on the rolling elements. The materials
used include yellow metals, mild steel, tool steel, duralium, laminations of fabric or paper
bonded with synthetic resin (phenolic), or plastics reinforced with disintegrated fabric, paper,
or other fillers.
Cages made with press tools: The pockets are usually formed in the pressing operation. The
cage is usually centered on the rolling elements. The materials used include wire, sheet brass,
sheet steel, and sheet duralumin, sometimes in combinations with turned parts held together
by rivets.
Cages made by molding: The materials used include plastics with fabric or other fillers, and die
casting, but the latter are not very satisfactory.
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Manufacturing process for CAGE
Steel cages are stamped out of fairly thin sheet metal, much like a cookie cutter, and then bent
to their final shape in a die. A die is made up of two pieces of steel that fit together, with a hole
the shape of the finished part carved inside. When the cage is put in between and the die is
closed, the cage is bent to the shape of the hole inside. The die is then opened, and the finished
part is taken out, ready to be assembled.
Plastic cages are usually made by a process called injection molding. In this process, a hollow
metal mold is filled by squirting melted plastic into it, and letting it harden. The mold is opened
up, and the finished cage is taken out, ready for assembly.
ASSEMBLY AND INSPECTION
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The typical ball bearing, called a Conrad bearing. There is enough space between the balls that
if they're all pushed over to one side; the inner ring can be pushed to the opposite side, into the
space left by moving the balls. This increases the space on the side where the balls are, letting
them be removed. The bearing cage usually keeps the balls evenly spaced so this doesn't
happen by accident.
Fig : Bearing assembly
The other kind of ball bearing is called a maximum capacity bearing, and has a special notch cut
in the side of the rings, into which the balls are placed during assembly. As a result of this
notch, the axial loads this kind of bearing can take are quite small, and must be in combination
with a large radial load. However, the increased number of balls that can be fit into the bearing
means the maximum capacity type bearing can handle a larger radial load.
Fig : Maximum capacity bearing assembly
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Fig : The four parts of a finished ball bearing: inner race, outer race, cage, and ball
INSPECTION
After the final polishing operation the steel balls undergo ocular examination of diffused light for the purpose of detecting flaws or grinding marks. That is work requires skill may be gathered from the fact that a novice usually passes as perfect about 90 percent of the balls that would be rejected by an expert examiner.
Since the balls are manufactured by the million, to measure each one by ordinary method is obviously out of the question, so that some other means must be resorted to. That adopted consists in grading the balls in automatic grading machines of very ingenious design.
For balls up to about 5/8 in. in diameter the machine used is designed so that the hopper delivers the ball one by one to a pair of knife edges forming a narrow V and set on an incline. The balls roll down the incline and drop through the V – opening, at different points according to their diameter, into recepticles below. The ocular inspection, it should be noted, precedes the grading, since slight flaws in the surface of the balls might otherwise affect the accuracy of gauging.
Balls about 5/8 in. or more in diameter are fed through a grading machine in which the v opening is horizontal lengthwise and vertical as far as its width is concerned. Cylindrical and taper rollers are also gauged in machines constructed on this principle.
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The outer and the inner rings of the bearing are inspected at every step in their manufacture from the straightening of the steel bar preceding delivery to the automatic lathes to the final check up following assembly of the bearing. Some sixty to eighty separate checking operations are thus undergone by every bearing produced.
The basis of the entire system of gauging is a set of master block gauges of the first or second grade, which are kept for reference only. These and all the other gauges are used only at the standard temperature, 68 degrees F. Other sets of block- gauges are used in the gauge room in conjugation with a highly sensitive comparator, in which a discrepancy of 0.001mm, in the size of the test piece makes a difference of 3 mm. on the graduated scale, i.e. it is magnified 3000 times. The working gauges are the with the help of this device.
In the workshop, fool proof plug and snap gauges are generally employed for bores and outside diameters, but gauges of special design and sensitive dial- gauges that have maximum and minimum indicators, and are adapted to suit various kinds of gauging fixtures, are used for parts made to very close tolerances, and also for various tests conducted with completely assembled bearings. All inspection gauges are set to limits closer than the guaranteed limits.
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