The surface finishes of bearing balls and raceways mostly govern bearing internal friction. Cerbec silicon-nitride ceramic balls from Saint-Gobain, East Granby, Conn., have a <0.004-micron surface finish, four times smoother than steel balls used in most machine-tool bearings. Cerbec balls are said to lower internal friction by up to 25% when mated with typical raceway steels which have a 1 to 2 micron finish. Ceramic balls also weigh less than steel balls, which cuts centrifugal force and therefore raceway normal loads and related friction. And less friction generates less heat, boosts operating speeds, and extends bearing life. Moreover, ceramic ball roundness and harmonic specs often exceed those of steel, which lessens bearing vibration and noise.
Ceramic balls have other desirable properties as well. For example, their high modulus of elasticity raises bearing stiffness. Ceramic balls can also operate with less lubrication and at much higher temperatures than their all-steel counterparts, and they better resist corrosion. However, all this capability doesn't come cheap. Hybrid bearings tend to cost more initially than steel-ball bearings, though reduced equipment downtime often justifies the expense over time.
One company using Cerbec ceramic balls is bearing maker Torrington Co., Torrington, Conn. Its Fafnir hybrid THSS (True High-Speed Sealed Bearings) are said to run 20% cooler than equivalent steel-ball bearings. This is especially important when speeds approach 600,000 DN (700,000 DM) because excess heat at high speeds lowers grease viscosity and breaks molecular bonds between oil and thickeners, destroying lubricating properties and shortening bearing lifetimes.
Torrington also incorporates Cerbec balls in its WearEver superprecision hybrid bearings. The bearings use a special steel alloy for the raceway that is said to have higher compressive yield strength, a smoother surface finish, higher hardness, and better wear resistance and fatigue life than 52100 chrome steel and other high-nitrogen bearing steels. Torrington credits a higher volume of fine, evenly dispersed carbides with the material's improved properties. In particular, the alloy better resists "peeling" — a propagation of surface cracks in the rolling contact
zones — caused by denting from hard contaminants as found in machine-tool environments. But hard contaminants can also dent bearing balls. Torrington says tests show WearEver hybrid bearings can live 10 times longer than conventional all-steel bearings made of 52100 steel, even when run with marginal lubrication or in the presence of hard contaminants.
Another company using Cerbec bearing balls is New Hampshire Ball Bearings, Precision Div., Chatsworth, Calif. NHBB builds hybrid bearings with stainless raceways coated with a Dicronite dry-film lubricant for operation in highly corrosive environments.
One application for NHBB hybrid bearings is SpeedVac centrifugal concentrators from Thermo Savant, Holbrook, N.Y. The concentrators use a centrifuge and a vacuum pump to extract solvents from drug concentrate. Bearings are exposed to heated solvent vapors, including alcohol, strong acids and bases, and stripping organics, conditions that would challenge conventional steelball bearings. "NHBB hybrid bearings have nearly doubled service life of the SpeedVac product," says Bob Evans, Thermo Savant vice president of engineering.
Cerbec silicon nitride balls are densified by hot isostatic pressing, utilizing a glass-encapsulation technology licensed from ACC of Robertsfors, Sweden. The technique forms balls in diameters from 1 to >100 mm, rollers, and raceways, and is patterned after conventional pressed and sintered ceramic component manufacturing to minimize cost.
Powder compacts made from spray-dried raw materials are encapsulated in glass and hot isostatically pressed at pressures near 2,000 bar and temperatures exceeding 1,800°C. Removing the glass leaves a net-shape ball blank. The resulting material has a fine and uniform microstructure that lets bearings better resist wear and rolling-contact fatigue. Blanks are then ground and lapped to final dimensions with equipment similar to that used in metal-ball finishing. Sphericity of silicon-nitride balls is typically held to less than 1 in. (0.025 m) which helps lower bearing vibration and noise.