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MANUFACTURING PROCESS OF ROLLING ELEMENT BEARING

Ramakanth P Joshi100922003M.Tech (CAMDA)

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OUTLINE

Parts or rolling element bearing

Raw materials

Manufacturing process of steel balls, RACES and cage.

Observations relating to manufacturing

Assembly

Inspection

Bibliography

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PARTS OR ROLLING ELEMENT BEARING

Courtesy SKF industries

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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.

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.

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STEEL FOR BEARING MANUFACTURING Production of wrought or pig Iron : Furnace

Degassing : Removal of gaseous molecules

Casting an ingot

Soaking : holding at certain temperature

Rolling : ingot to billet

Normalizing : To refine the grains, accomplished by heating at appx. 55 to 85 degree above the upper critical temperature( for 52100 its around 750) ans allowed to coll in air( austenitising)

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BEARING MANUFACTURING

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.

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.

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MANUFACTURING PROCESS OF STEEL BALLS

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DEFLASHING MACHINE

Photo courtesy of Noonan Machine Co.Ball machine

.Rill plates for ball machine

As the ball travels through the groove, it spins and tumbles, the rough edges get broken off, and the ball gets squeezed into a spherical shape, a little like rolling a ball of dough between your hands. This squeezing of the balls compresses the metal, giving the balls a very hard surface. Because the balls are metal, this operation generates a lot of heat, so water pours over the balls and plates to cool them.

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DEFLASHING, HEAT TREATMENT AND LAPPING

1550 & 300

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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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MANUFACTURING PROCESS OF RACES

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.

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HEAT TREATMENT PROCESS

Annealing : 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

Tempering : This is done by heating them in a second oven to about 300 degrees Fahrenheit (148.8 degrees Celsius), and then letting them cool in air

Consequence : Hard and tough races

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GRINDING

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.

very fine abrasive slurry is used to polish the races for several hours to get almost a mirror finish.

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CAGES

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.

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MANUFACTURING PROCESS OF CAGES The materials used include yellow metals, mild steel, tool steel, duralium

Steel cages are stamped out of fairly thin sheet metal

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.

Hemispherical die

Metal sheet

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PLASTIC CAGES

Plastic cages are usually made by a process called Injection molding or by permanent moulding. 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.

Eg. paper bonded with synthetic resin (phenolic), or plastics reinforced with disintegrated fabric

The cage is formed by riveting both

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OBSERVATIONS RELATING TO MANUFACTURING

During rotation of the bearing, those rolling elements under load push against the cage and rotate the cage, and the cage in turn pushes those rollers which are not under a load.

Unfortunately, the cage does not tend to remain concentric in the bearing but instead tends to move in an eccentric path due to dynamic imbalance;

To address this problem, the cage is usually piloted to limit eccentric movement of the cage within the bearing.

The cage may be piloted either on the outer race, the inner race or on the rolling elements themselves.

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CONTINUED…

During rotation of the bearing, "centrifugal force" maintains the rolling elements in firm contact with the outer race so that the rolling elements tend to run at the outer race speed (which may be zero for a static outer race) with any skid then being more likely to occur between the rolling elements and the inner race.

In such cases it is preferable to pilot the cage on the inner race ie to provide a closer clearance between the inner race and the cage than between the outer race and the cage, so that the lubricating oil in between the cage and the inner race introduces a net viscous drag between the inner race and the cage, tending to cause the cage (and hence the rolling elements) to run closer to the inner race speed.

Unfortunately, in practice centrifugal effects make it difficult to retain lubricating oil in-between the cage and the inner race, and this has a limiting effect on the `viscous coupling` between the cage and the inner race.

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ASSEMBLY

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ASSEMBLY TYPE 1

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.

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ASSEMBLY TYPE 2

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.

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INSPECTION ( BASED ON MASTER GAUGES)

After the final polishing operation the steel balls undergo ocular examination of diffused light for the purpose of detecting flaws or grinding marks

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.

Hardness inspection : BHN

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RACES INSPECTION

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.

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BIBLIOGRAPHY

Books

Rolling bearing,R.K.Allan, 2nd edition,Sir Isaac Pitman & Sons ltd.,1960.Bearing Design & Application, Donald.F.Wilcock &E.Richard Booser,

1st edition, McGraw-hill book company-1957.

Websites

www.howstuffwork.com www.wikepedia.com www.scribed.com 

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Thank you