Ever since man began to need to move things, he has used round rollers to
make the job easier. Probably the first rollers were sticks or logs, which
were a big improvement over dragging things across the ground, but still
pretty hard work. Egyptians used logs to roll their huge blocks of stone
for the pyramids. Eventually, someone came up with the idea of securing
the roller to whatever was being moved, and built the first
"vehicle" with "wheels." However, these still
had bearings made from materials rubbing on each other instead of rolling
on each other. It wasn't until the late eighteenth century that the
basic design for bearings was developed. In 1794, Welsh ironmaster Philip
Vaughan patented a design for ball bearings to support the axle of a
carriage. Development continued in the nineteenth and early twentieth
centuries, spurred by the advancement of the bicycle and the
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.
This article will describe only ball bearings. In these 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. In addition to these parts, there are a lot of optional parts
for special bearings, like seals to keep oil or grease in and dirt out, or
screws to hold a bearing in place. We won't worry here about these
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
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.
There are four major parts to a standard ball bearing: the outer race, the
rolling balls, the inner race, and the cage.
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, and the steel
usually warps during this process. They can be machined back to their
finished size after heat treating.
Surprisingly, the rolling balls start out as thick steel wire.
Then, in a cold heading process, the wire is cut into small pieces
smashed between two steel dies. The result is a ball that looks like
the planet Saturn, with a ring around its middle called
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 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
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 a 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
4 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. 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 proess. 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
5 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.
6 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 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
7 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.
The four parts of a finished ball bearing: inner race, outer race,
cage, and ball.
8 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
9 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.
10 Now that all of the parts are made, the bearing needs to be put
together. First, the inner race is put inside the outer race, only off
to one side as far as possible. This makes a space between them on the
opposite side large enough to insert balls between them. The required
number of balls is put in, then the races are moved so that they are
both centered, and the balls distributed evenly around the bearing. At
this point, the cage is installed to hold the balls apart from each
other. Plastic cages are usually just snapped in, while steel cages
usually have to be put in and riveted together. Now that the bearing is
assembled, it is coated with a rust preventative and packaged for
Bearing making is a very precise business. Tests are run on samples of the
steel coming to the factory to make sure that it has the
right amounts of the alloy metals in it. Hardness and toughness tests are
also done at several stages of the heat treating process. There are also
many inspections along the way to make sure that sizes and shapes are
correct. The surface of the balls and where they roll on the races must be
exceptionally smooth. The balls can't be out of round more than 25
millionths of an inch, even for an inexpensive bearing. High-speed or
precision bearings are allowed only five-millionths of an inch.
Ball bearings will be used for many years to come, because they are very
simple and have become very inexpensive to manufacture. Some companies
experimented with making balls in space on the space shuttle. In space,
molten blobs of steel can be spit out into the air, and the zero gravity
lets them float in the air. The blobs automatically make perfect spheres
while they cool and harden. However, space travel is still expensive, so a
lot of polishing can be done on the ground for the cost of one
Other kinds of bearings are on the horizon, though. Bearings where the two
objects never touch each other at all are efficient to run but difficult
to make. One kind uses magnets that push away from each other and can be
used to hold things apart. This is how the "mag-lev" (for
magnetic levitation) trains are built. Another kind forces air into a
space between two close-fitting surfaces, making them float apart from
each other on a cushion of compressed air. However, both of these bearings
are much more expensive to build and operate than the humble, trusted ball
Where To Learn More
Deere & Company Staff, eds.
Bearings & Seals,
5th ed. R. R. Bowker, 1992.
Ball & Roller Bearings: Theory, Design & Application,
Harris, Tedric A.
Rolling Bearing Analysis,
3rd ed. John Wiley & Sons, Inc., 1991.
Houghton, P. S.
Ball & Roller Bearings.
Elsevier Science Publishing Company, Inc., 1976.
Nisbet, T. S.
Oxford University Press, 1974.
Shigley, J. E.
Bearings & Lubrication: A Mechanical Designer's
McGraw-Hill, Inc., 1990.
Gardner, Dana. "Ceramics Adds Life to Drives,"
March 23,1992, p. 63.
Hannoosh, J. G. "Ceramic Bearings Enter the Mainstream,"
November 21, 1988, p. 224.
McCarty, Lyle H. "New Alloy Produces Quieter Ball
May 20, 1991, p. 99.