Design News is part of the Informa Markets Division of Informa PLC

This site is operated by a business or businesses owned by Informa PLC and all copyright resides with them. Informa PLC's registered office is 5 Howick Place, London SW1P 1WG. Registered in England and Wales. Number 8860726.

Super Bearings

Super Bearings

As machinery speeds climb ever higher, loads increase, and operational environments become more harsh and demanding, many engineers have found their old friend, the all-steel ball bearing, to be a limit to their designs. The solution: hybrids.

Hybrid ceramic ball bearings consistof a combination of materials--silicon nitride (Si3N4) balls and steel races. They exploit thebest features of both materials. Ceramics, extremely hard and possessing ex-ceptional compres-sive strength, make excellent balls. However, silicon nitride offers poor tensile strength, and thus the bearing races are best left made of steel.

The hybrid concept dates back 20 years, but silicon nitride didn't be-come available in commercial quantities until about 1990. The wide acceptance of hybrids into industries like machine tools is even more recent, occurring in the past two years.

Experts differ on the size of the current market for hybrids, but agree that the demand for them is growing strongly. Dr. Thomas Abraham, director of the ceramic group at Business Communications Co., Norwalk, CT, a market analysis firm, says that $2.75 million worth of hybrids were sold in 1991. Since then, their share has grown at 20.5% a year, and he expects that pace to continue into the next century.

By contrast, Tony Taglialavore, product manager at Norton Advanced Ceramics values today's world hybrid-bearing market at $100 million. The ball market alone he estimates at $25 million and growing at 40% a year. "We think balls could become a $100 million business within seven years," he predicts. "One project we're working on with a computer disk drive manufacturer would require 60 million balls alone." By comparison, only 15 to 20 million balls will be manufactured in 1996 for all applications.

Norton supplies finished silicon-nitride balls and unfinished blanks, one of only four major companies to do so throughout the world. Its line of CERBEC(R) silicon nitride bearing products compete with those of Enceratec (Columbus, IN), Kyocera (Elk Grove Village, IL), and ESK, (Norwalk, CT).

Why should engineers look at hybrids? Bearing companies and customers claim numerous advantages, including:

HIGH SPEED AND ACCELERATION. Silicon nitride balls are just 40% as dense as steel. This significantly reduces centrifugal loading and skidding, allowing 30 to 50% higher running speeds. With oil mist lubrication, hybrids run up to 3 million DN (bearing ID times the RPM); grease-lubricated hybrids reach one million DN.

  • INCREASED STIFFNESS. The modulus of elasticity for Si3N4 is 50% greater than steel, which translates to a 15 to 20% increase in rigidity.

  • EXTENDED OPERATING LIFE. Extremely hard--about twice that of bearing steel--and possessing extraordinary fatigue life--ten times that of M-50--ceramic balls endow hybrids with three to five times the useful life of all-steel designs. "We have applications where it's almost impossible to keep the bearings from failing early," says Robert Hanson, chief engineer at Miniature Precision Bearings, a division of MPB, Keene, NH. "Put in ceramic balls, and the problems just go away."

  • REDUCED LUBRICATION REQUIREMENTS. Silicon nitride's coefficient of friction is 20% that of steel, and the material takes a better finish. "They run noticeably smoother than all-steel bearings," says Tom O'Brien, director of engineering at Setco, a Cincinnati, OH, a machine-tool manufacturer. Even at extremely high temperatures with low lubrication, the material won't weld to the races--a common failure mode in many high-speed, high-load applications.

  • LOW THERMAL EXPANSION. Only one fifth that of steel, the thermal expansion of ceramic balls results in less change in bearing preload during spindle operation. And though the races are still steel, the balls can withstand up to 2,000F. A complete hybrid bearing: 1,000F.

The list could continue and include such properties as corrosion resistance, magnetic immunity, and electrical insulation. But often the best way to discover the advantages of a product is through examples. Here are three projectsin which engineers found hybrid-ceramic bearings to be not only the best solution, but theonly solution.

Currently, hybrids find their greatest application in machine tools. "Half the world market is in this area," says Taglialavore. Why? Higher cutting speeds and increased product warranties drive the need for greater reliability.

For example, an automotive components manufacturer recently approached Whitnon Spindle, Farmington, CT, with a request for longer life and greater stiffness. Spindles that the components manufacturer had been using, bought from another supplier, failed regularly. The cause: inability of conventional steel-on-steel bearings to provide acceptable life.

Whitnon, with help from The Torrington Co., embarked on a custom design to meet the component manufacturer's machine-tool needs. Critical to the project's success was Torrington's new 99100 series of bearings, a high-performing upgrade of the company's line of Fafnir hybrid ceramic bearings.

"In the past, there's always been somewhat of a tradeoff between load capability and speed," says Matt Boylan, Torrington's marketing manager for the machine-tool industry. "With our 99100 series of bearings, we've created a fine balance between the two."

Boylan credits three design features for the bearing's contribution to Whitnon's custom project. A smaller ball size, he explains, offers less resistance to rotation and a lower centrifugal force. More open race curvatures also lower rotational resistance, as well as reduce ball skidding. This, in turn, lowers the amount of heat the ball generates.

The biggest contribution, however, comes from Norton's CERBEC(R) silicon-nitride balls. "The bearing is certainly one of the critical components making up the spindle head," says Lawrence Hermanowski, sales manager at Whitnon. "With the 99100 series of bearings, it takes over 4.5 million pounds of force to move that spindle head less than half the thickness of a human hair."

That kind of rigidity, adds Hermanowski, means quality machining for customers, such as aerospace and automotive components manufacturers.

Hybrid provides newfound agility. One such automobile manufacturer--a member of Detroit's "Big Three"--approached engineers at Giddings & Lewis, Fraser, MI, with a request for a flexible, high-speed machining center. "They're going to make an aluminum bell housing with very thin walls, cutting at 9,000 or 10,000 rpm," says a Giddings & Lewis project engineer. "We're providing them with four of our AGILE three-axis machines set up with spindles using ceramic bearings."

The AGILE line of machine tools addresses an increasing trend at auto manufacturers for flexible tooling. Small machines can drill multiple holes in one pass at a single station, and then pass the part to the next station. "It used to be you'd have one machine with a 10-spindle head," says the engineer, "but then you can't do any changing; the agility of this unit is better."

Setco supplied the high-speed spindles with a pair of hybrid bearings from SKF, Bethlehem, PA, mounted to each. Powered by 25-hp motors, they cover a broad operating spectrum--from 1,500 to 10,000 rpm.

Giddings & Lewis had used spindles similar to this in the past, and they felt with the extended warranty on this project--three years--and the ever-present risk of lubrication problems, hybrids would give a margin of insurance. Says the engineer, "If you have a glitch to the lube system, ceramic-on-steel is more forgiving than steel-on-steel."

"It's not unusual to see hybrids run 10 to 15 degrees cooler than conventional bearings," says Setco's O'Brien. "In some cases, it's the difference between running and not running."

With only the first two of the four machines delivered, it's too soon to measure any additional life. The company expects the spindles to last from 30 to 50% longer. An initial concern was the lengthy lead time to obtain the bearings, and the price. Yet the potential benefits outweighed these disadvantages. "You're not going to survive if you're conservative on everything," the engineer explains. "The customer wants more, and you have to find a way to give it to him."

Butterfly valve goes the distance. Not all ceramic bearings find themselves on machine tools. MPB supplies high-precision full- and hybrid-ceramic bearings for everything from dental drills to the Sidewinder missile.

One aerospace application involves a butterfly valve that regulates air flow for the environmental control system on Boeing's new 777 twinjet. While the valve only rotates through 90 degrees--a seemingly easy requirement--the air it processes varies from 700 to 900F. And to meet the FAA's stringent ETOPS (Extended-range, Twin-engine Operations) standards, it would have to function through 625,000 cycles--four aircraft lifetimes--without replacement.

George Houston, senior bearing engineer for AlliedSignal Fluid Systems, manufacturer of the valve, turned to hybrid bearings to handle the 600 ksi stress and low-lube operation. "We try to have as many bearing elements as we can, but we also need some lubrication," he explains. The company's patented design for such applications intersperses four carbon-graphite lubrication pellets among 14 balls. But during an early endurance test, this solution produced significant problems.

A flutter condition in which the valve oscillates through a 4-degree arc caused the balls to develop a flat spot. The wear mechanism: ball-on-ball contact. "Ceramic balls are fantastic, but you can't have them touching one another," says Houston.

To eliminate the wear, he decided to place a graphite pellet between each ceramic ball. But doing so cut four balls from the bearing and upped the load stresses into the red zone. "We had to change from an R8 bearing to a larger R12," Houston says. The subsequent modifications to the valve housing extended a typical three-week design process to four months and added cost. The results were worth it.

"The triple seven is, well, the triple seven," Houston tries to explain. "Cost has been an issue before, but on this project we just had to do whatever it took."

A bearing for all reasons? Are hybrids a panacea, addressing all bearing woes? Not exactly. The ceramic balls are expensive. Complete bearings cost from two to five times that of all-steel designs. Silicon nitride's higher elastic modulus and hardness decrease the ball's footprint on the races, increasing static- and low-speed stress and reducing load capacity from 20 to 40%. "The balls are so hard that shock loads may Brinell the races," says Setco's O'Brien.

A hybrid's high precision may expose weaknesses in other areas of the design as well. "Everything around the bearing has to be better," explains Andy Fletcher, a spokesman for SKF. Setco, for instance, designs surrounding rotating parts and supporting components to Class 9 standards when using hybrids, but only Class 7 with conventional bearings.

But for many applications, a hybrid's strengths far outweigh its weaknesses. Says Hanson, "You need to look at the overall value of the application. Frequently, the cost of replacing the bearing is much higher than the cost of the bearing itself."

"If you have a performance problem, cost suddenly becomes much less of an issue," adds Harold Robinson, SKF applications engineer. "If cost were not a factor, I'd put them in all precision bearings."

Comparison of Ceramic and Steel Properties

PROPERTY

Si3N4

BEARING STEEL

Density

0.117 LB/IN3

0.28 LB/IN3

Hardness

HV10KG 1,700
HRC78

HV10KG 655-763
HRC58-63

Elastic Modulus

46 @ 106 psi

29 @ 106 psi

Temperature capability

1,800 degrees F

350-1,000 degrees F

Corrosion resistance

excellent

poor

Magnetism

nonmagnetic

ferromagnetic

TAGS: Materials
Hide comments
account-default-image

Comments

  • Allowed HTML tags: <em> <strong> <blockquote> <br> <p>

Plain text

  • No HTML tags allowed.
  • Web page addresses and e-mail addresses turn into links automatically.
  • Lines and paragraphs break automatically.
Publish