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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