As demands for greater speeds, higher loads, and better adaptability to harsh
environments strain the imaginations of design engineers, many are finding
inspiration in the properties of ceramic materials. The ability of ceramics to
run at high speeds, live a long life, and operate with a minimum of lubrication
makes them ideal for bearing applications.
Although there are all-ceramic bearings, most ceramic bearings consist of
ceramic balls and steel races. Silicon nitride (Si(sub 3)N(sub 4)) is the
ceramic material of choice because of its failure mode. Rather than shattering
like other ceramics, Si(sub 3)N(sub 4) balls tend to spall in a manner very
similar to that of a steel bearing.
Falling prices. Historically, one of the barriers to rapid expansion
of the market was the high cost of ceramics relative to steel. In fact, it
wasn't too long ago that the price ratio between the two was more than 1,000:1,
says Tony Taglialavore, a product manager at Norton Advanced Ceramics.
But while the cost of steel balls has remained relatively constant, the cost
of ceramic balls has practically been in free fall (see chart). Two factors
account for plummeting prices: improvements in the manufacturing process and
higher volumes, says Taglialavore. In fact, volume has now expanded to the point
where ceramic balls represent about a $35 million market worldwide. To put that
figure into perspective, the total value in these bearing assemblies is about
three to five times that figure.
Back in the early 1990s, machine-tool spindles were the pioneering
application for ceramic bearings. As is still the case today, engineers valued
ceramic over steel for its higher speed capabilities and greater stiffness. The
reduced lubrication requirements also offered machine-tool manufacturers the
ability to convert from oil to grease lubrication and eliminate expensive oil
recirculation systems in machine-tool applications.
One of the earliest users of hybrid ceramic bearings was Bryant Grinder Corp.
(Springfield, VT). The company started using ceramic bearings from Torrington
(Torrington, CT) in its precision grinding machinery in 1988. Dr. Russell Kulas,
the company's manager of spindle technology, notes that, "the ceramic bearings
provided a marked improvement in stiffness and speed and enabled us to use a
larger bearing; this also allowed us to increase load capacity. The hybrid also
reduced noise and vibration, while the lighter balls significantly reduced
stress on the bearing." For the future, Bryant is planning to convert a portion
of its standard spindle product line to hybrid ceramic bearings.
Machine tools account for about 25% of the balls sold each year. However,
they actually represent about 50% of the dollar volume due to the fact that they
require a higher degree of precision, which costs more. Other leading
applications include turbomolecular pumps, accounting for approximately 19% of
the balls sold; high-speed dental tools (15%); and turbochargers (10%).
Turbomolecular pumps, used for high vacuum chambers, employ ceramic bearings
because of their ability to operate with little lubrication. The use of ceramics
on the main shaft has improved pump reliability and given designers flexibility
in how they orient the equipment.
Below-ground pumps used in oil fields are another type of pump application
for which ceramics have proven to be a plus. Only in this case it is the
extended life possible with ceramics that engineers value. That's because a pump
replacement in places like the North Sea oil fields can cost more than a
half-million dollars. Reda, a division of Camco International, is using ceramic
bearings in its electric submersible pumps (Design News 12/2/96). The swap
reduced bearing wear by a factor of 100 and extended the life of pumps from
several weeks to a year or more in the harshest environments.
Dental drills represent another ideal application for ceramics. Heat
sterilization of the drills increases the corrosive effects of the lubricants
used in the bearing assemblies, reducing bearing life from 20 years under normal
operating temperatures to less than two years when subjected to frequent
sterilization. By substituting hybrid ceramic bearings for all-steel units, one
manufacturer, Star Dental, was able to provide a tool that stands up to the
high-temperature sterilization environment (Design News 6/10/96).
New uses. Up-and-coming applications, according to Taglialavore,
include bearings for in-line skates and off-road bicycles. Large, high-speed
electric motors for medical applications and chemical processing are another
growing market, where the potential for longer life and safer operation is the
appeal. Designers also specify ceramic bearings in some fairly exotic
applications, including the space shuttle main engine fuel pump, where they are
lubricated by liquid oxygen. Designers expect that the ceramic balls will
outlast steel by a factor of ten in that application.
Ceramic bearings also hold promise for applications in dirty environments, as
evidenced by a single-ball hybrid bearing offered by SKF. The use of a single
ceramic ball keeps the steel raceway free of foreign particles and keeps costs
down. Possible applications include transmissions and gear boxes.
Designers of the British Thrust SSC, the first car to break the
sound-barrier, also turned to ceramic bearings, from SKF. The vehicle reached a
speed of 1,229.7 km/h in the Black Rock Desert in the fall of 1997. Engineers
chose ceramic wheel bearings because of their ability to withstand the high
centrifugal forces.
Although applications for ceramic bearings will continue to expand as prices
fall, they are not appropriate for all uses. In applications using pure water,
for example, ceramics may not be appropriate because the water tends to interact
and cause wear of the ceramic balls.
One controversial issue is the fatigue life of the bearing. The higher
stiffness of the ceramic balls leads to a reduced area of contact between the
balls and the race, which in turn increases stresses in the races by about 15%.
Ceramic balls are also usually smaller than steel balls, which again leads to
smaller contact areas and increased stresses. Higher stresses in turn lead to a
lower fatigue life for the steel bearings.
However, Taglialavore notes that bearings don't often fail due to
fatigue--more than 90% of bearings fail from lubrication problems, either too
much, too little, or some type of contamination. Ceramics, which require little
lubrication, and operate at lower temperatures, preserving grease life, can last
three to five times as long as steel bearings at the same loads. In some
applications, though, fatigue can be the limiter. One of the challenges for
bearing makers is to come up with some testimonials that would demonstrate the
longevity of ceramic bearings and boost design engineers' confidence in them.
All-ceramic bearings. Since ceramic material is much more expensive
than steel, an all-ceramic bearing is quite costly. In addition, it does not
have the tensile strength required for use in the race. Also, due to differing
rates of thermal expansion, all-ceramic bearings can end up with either a loose
fit on a steel shaft, or a shattered bearing as the shaft expands too fast for
the bearing race.
On the plus side, all-ceramic bearings roll more easily, weigh less,
accelerate faster, develop lower centrifugal forces than steel bearings,
and--most significantly--they resist corrosion. In applications where corrosion
is a serious problem, all-ceramic bearings have found a niche. Examples include
pumps for corrosive fluids, food processing equipment, chemical processing, and
semiconductor manufacturing equipment where debris from bearing wear might
contaminate the process.
Industry observers expect the market to continue to expand, particularly as
the cost of ceramic bearings keeps dropping. In fact, the goal of a project
underway at Oklahoma State University is to bring costs down by speeding up the
manufacturing process.
"We have developed a new process called magnetic field assisted polishing
that can polish ceramic balls to a sphericity of 0.15 to 0.25 microns and a
surface finish of about 4 nm Ra," says project leader Dr. Ranga Komanduri. "We
can polish the balls in about 20 hours, while conventional polishing may take
several weeks (6 to 12, depending on the requirements and available technology).
We also do not use an expensive diamond for polishing, and the process can be
used on balls of any size."
Funds for the project came from the Department of Defense and the National
Science Foundation. The university is currently negotiating to license the
process to a manufacturer, and Komanduri anticipates that it could be in use
within a year.
Ceramic bearings have lots on the ball
- A relatively low density. Small ceramic balls are about 40% as dense as
steel. This means less centrifugal force is developed on the bearings and they
can operate at higher speeds.
- High rigidity. The modulus of elasticity of silicon nitride (Si(sub
3)N(sub 4)), the ceramic material that is most often used in bearings, is about
50% higher than that of steel, leading to a 15 to 20% increase in rigidity.
- High hardness. The hardness of ceramic materials runs from about Rc75 to
80, compared to about Rc 58 to 64 for steel.
- High compressive strength. The compressive strength of ceramic materials
is about 5 to 7 times that of steel.
- Low coefficient of friction. A low coefficient of friction reduces
rolling resistance, again allowing higher speeds with lower operating
temperatures and less wear--and less need for lubrication.
- Low thermal expansion. The coefficient of thermal expansion of silicon
nitride is about 25% that of typical steel bearings. This property results in
less change in bearing preload during spindle operation.
- Low electrical conductivity. Since ceramic materials don't conduct
electricity, they are resistant to damage from arcing. Hybrid bearings have a
receptivity comparable to the ceramic alone, eliminating the need for special
bearing insulation in applications such as traction motors.
- Corrosion resistance. The chemically inert nature of ceramic balls makes
them resistant to corrosion. They also exhibit a resistance to galling and
adhesive wear problems that plague all-steel bearings that are marginally
lubricated.
Designers can take advantage of these properties to produce components
with superior capabilities.
The next frontier: larger diameters
Up to now, most ceramic balls have been in the small size ranges of 1/16
to 1/4 inches in diameter. If larger balls became available, up to 11/2 inches
in diameter, it would open up whole new areas for ceramic bearings, including
use in engines and certain machine tools. Manufacturers are making progress, and
ceramic balls up to 3/4 inch in diameter have been used in some machine tool
applications.
However manufacturing larger balls is not a simple matter of scaling up.
Cooling rates change dramatically for the larger balls and "the larger balls
have enough weight to hurt themselves during the aggressive finishing and
handling in the manufacturing process," as Frank Baker, general manager of MRC
Specialty Balls, puts it.
Also, if large ceramic bearings are to be used in gas turbine engines, one
of the target markets, manufacturers need to develop a non-destructive test to
make sure that there are no defects in the balls. Harold Burrier Jr., senior
research specialist for Timken, says that designers in the aerospace industry
are still concerned about the possible brittleness of ceramics and need testing
techniques that would build their confidence. Ultrasonic tests, used to check
steel bearings for subsurface defects haven't yet been effectively adapted to
ceramics. However MRC, for one, is working with the Department of Defense
Advanced Research Projects Agency to develop inspection testing procedures and
hopes to have results by early in 1999.
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