Ceramic bearings on a roll

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.