A tapered roller bearing from a manual
you ever wondered how things like inline skate wheels and
electric motors spin so smoothly and quietly? The answer can
be found in a neat little machine called a bearing.
The bearing makes many of the machines we use every day
possible. Without bearings we would be constantly replacing
parts that wore out from friction. In this edition of HowStuffWorks,
we'll learn how bearings work, we'll look at some different
kinds of bearings and explain their common uses, and we'll
explore some other interesting uses of bearings.
The Basics The concept behind a bearing is
very simple: Things roll better than they slide. The wheels on
your car are like big bearings. If you had something like skis
instead of wheels your car would be a lot more difficult to
push down the road.
That is because when things slide, the friction
between them causes a force that tends to slow them down. But
if the two surfaces can roll over each other, the friction is
A simple bearing, like the kind found in a skate
Bearings reduce friction by providing smooth metal balls or
rollers, and a smooth inner and outer metal surface for the
balls to roll against. These balls or rollers "bear" the load,
allowing the device to spin smoothly.
Bearing Loads Bearings typically have to
deal with two kinds of loading, radial and
thrust. Depending on where the bearing is being used it
may see all radial loading, all thrust loading or a
combination of both.
The bearings that support the shafts of
motors and pulleys are subject to a radial
The bearings in the electric
motor and the pulley
pictured above face only a radial load. In this case, most of
the load comes from the tension in the belt connecting the two
The bearings in this stool are subject to a
The bearing above is like the one in a barstool. It is
loaded purely in thrust, and the entire load comes from the
weight of the person sitting on the stool.
The bearings in a car wheel are subject to
both thrust and radial loads.
The bearing above is like the one in the hub of your car
wheel. This bearing has to support both a radial load and a
thrust load. The radial load comes from the weight of the car,
the thrust load comes from the cornering forces when you go
around a turn.
Types of Bearings There are many types of
bearings, each used for different purposes. These include ball
bearings, roller bearings, ball thrust bearings, roller thrust
bearings and tapered roller thrust bearings.
Ball Bearings Ball
bearings, as shown below, are probably the most common
type of bearing. They are found in everything from inline
skates to hard
drives. These bearings can handle both radial and thrust
loads, and are usually found in applications where the load is
In a ball bearing, the load is transmitted from the outer
race to the ball, and from the ball to the inner race. Since
the ball is a sphere, it only contacts the inner and
outer race at a very small point, which helps it spin very
smoothly. But it also means that there is not very much
contact area holding that load, so if the bearing is
overloaded the balls can deform or squish, ruining the
Roller Bearings Roller
bearings like the one illustrated below are used in
applications like conveyer belt rollers, where they must hold
heavy radial loads. In these bearings, the roller is a
cylinder, so the contact between the inner and outer
race is not a point but a line. This spreads the load out over
a larger area, allowing the bearing to handle much greater
loads than a ball bearing. However, this type of bearing is
not designed to handle much thrust loading.
A variation of this type of bearing, called a needle
bearing, uses cylinders with a very small diameter. This
allows the bearing to fit into tight places.
Bearing Ball thrust bearings like the one
shown below are mostly used for low-speed applications and
cannot handle much radial load. Barstools and Lazy Susan
turntables use this type of bearing.
Bearing Roller thrust bearings like the one
illustrated below can support large thrust loads. They are
often found in gearsets like car
transmissions between gears, and
between the housing and the rotating shafts. The helical
gears used in most transmissions have angled teeth -- this
causes a thrust load that must be supported by a bearing.
Bearings Tapered roller bearings can support
large radial and large thrust loads.
Photo courtesy The
Torrington Company Cutaway view of (left) a spherical roller
thrust bearing and (right) a radial tapered roller
Tapered roller bearings are used in car hubs, where they
are usually mounted in pairs facing opposite directions so
that they can handle thrust in both directions.
Some Interesting Uses of Bearings There are
several types of bearings, and each has its own interesting
uses, including magnetic bearings and giant roller bearings.
Magnetic Bearings Some
very high-speed devices, like advanced flywheel energy storage
systems, use magnet bearings. These bearings allow the
flywheel to float on a magnetic field created by the bearing.
Some of the flywheels run at speeds in excess of 50,000
revolutions per minute (rpm). Normal bearings with rollers or
balls would melt down or explode at these speeds. The magnetic
bearing has no moving parts so it can handle these incredible
Bearings Probably the first use of a bearing was
back when the Egyptians were building the pyramids. They put
round logs under the heavy stones so that they could roll them
to the building site.
Buildings The new San Francisco International
Airport uses many advanced building technologies to help it
One of these technologies involves giant ball bearings.
Click on "Earthquake!" to see the earthquake
bearing support system at work.
The 267 columns that support the weight of the airport
each ride on a 5-foot-diameter steel ball bearing. The ball
rests in a concave base that is connected to the ground. In
the event of an earthquake, the ground can move 20 inches in
any direction. The columns that rest on the balls move
somewhat less than this as they roll around in their bases,
which helps isolate the building from the motion of the
ground. When the earthquake is over, gravity pulls the columns
back to the center of their bases.
For more information on bearings and related topics, check
out the links on the next page!