Bearings play a key role in the operation of virtually all types of
application. JMB Bearings Inc. can supply all types of bearings from
miniature bearings to bearings over seven meters in diameter.
Whether your application we
maybe able to fulfill your bearing requirement.
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 automobile.
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 fancy extras.
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.
There are four major parts to a standard ball bearing: the outer race,
the rolling balls, the inner race, and the cage.
- 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.
- 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.
- 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 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 the balls.
- 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 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.
- 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 size.
- 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.
- 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
- 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 shipping.
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 "space ball".
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 bearing.
Where To Learn More
Deere & Company Staff, eds. Bearings & Seals, 5th ed. R. R. Bowker,
Eschmann, Paul. 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. Rolling Bearings. Oxford University
Shigley, J. E. Bearings & Lubrication: A Mechanical Designer's Workbook. McGraw-Hill, Inc., 1990.
Gardner, Dana. "Ceramics Adds Life to Drives," Design News. March
23,1992, p. 63.
Hannoosh, J. G. "Ceramic Bearings Enter the Mainstream," Design News.
November 21, 1988, p. 224.
McCarty, Lyle H. "New Alloy Produces Quieter Ball Bearings," Design
News. May 20, 1991, p. 99.
[Article by: Steve Mathias]