A hybrid bearing is made up of metal rings and silicon nitride ceramic balls, with lubricant and retainer as required by the application.

Courtesy Koyo Corp.

Though increasingly affordable, there’s no getting around that silicon nitride balls are more costly than metal. Luckily, ceramic balls are often used in bearings that are otherwise pretty typical designs. When assembled into a hybrid bearing — ceramic balls in metal races — the price of the bearing assembly is actually quite competitive. On a total operating cost basis, hybrid bearings often pay for themselves many times over in extended life, enhanced performance, or increased durability. Additionally, ceramic balls have become increasingly common over the past fifteen years. As a result, they have steadily become affordable for more applications.

Ceramic bearing balls are made from the structural ceramic called silicon nitride. What makes the material appropriate for ball bearings is its crystalline structure, locked in place with covalent, directional bonds in a diamond-like arrangement. In these directional bonds every atom ionically shares its electrons with its lattice neighborhood. For this reason, ceramics are more loosely packed than metals, and (because they do not react with outside elements to absorb electrons) are electrically nonconductive.

Because they’re also lighter and stiffer, ceramic bearings are capable of higher operating speeds. And, quicker movements make for increased productivity. Another benefit is less required maintenance. The best bearing-grade silicon nitrides have attributes that maximize hardness, crack resistance and rolling contact fatigue life. With tiny (and scarce) pores with maximum diameters under one micron, the material has nearly 100% density. Typically over 85% of grains are in a micron-sized needle-like shape that makes for tough, strong, and fatigue-resistant bearings. Finally, superior rolling contact life (5 times) makes silicon nitride one of the best materials for increasing bearing durability and operational capabilities.

Controlling lube degradation and wear are other areas where ceramics excel. Sometimes frequent startstop cycles, high loads or vibration at low speeds, lack of lubricant, or low viscosity makes for marginally lubricated raceways. Loaded steel balls operating under these conditions quickly accelerate bearing wear. Micro-welding occurs, and micro-fractures of the steel ball and raceways follow. Then surface asperities form, causing metallic wear debris to contaminate the lubricant and degrade its chemistry. The final effect: bearing precision (as measured by vibration, non-repeating run out, and work-piece quality) deteriorates rapidly.

Because micro-welding and fracture does not occur at steel-silicon nitride contact points, ceramic balls help maintain long-term bearing precision. Even when rolling on marginally lubricated steel raceways, ceramic is structurally dissimilar, so adhesive wear is eliminated. Lubricant contamination and degradation is also reduced; smoother ball and raceway surfaces are maintained, resulting in lower internal friction and lower operating temperatures. The synergy of reduced wear and temperature means long-term high precision and extended bearing service life for the end user.

The most common ceramic ball bearing application today is for high-speed machine tool spindles. Other applications range from dental hand-pieces and surgical saws to wing-flap actuators and electric motors.

Courtesy Cerbec

In turn, reduced friction and operating temperatures allow for less lubricant. Here’s why: In typical designs, bearings need lubricant to separate balls from races. The thickness of the lubricant film is determined by factors including speed, bearing size and design, material, and operating temperature. One key guideline for determining the proper thickness of the lubrication layer is the composite surface roughness of the ball and raceway surfaces. In general, the film thickness of the lubricant needs to be 1.5 x CSR. Because ceramic balls have very fine surface finishes, lubricant needs are minimized. Often on hybrid bearings, grease completely replaces oil mist systems. On systems where grease is already specified, the amount used can often be reduced.

Just as other bearing balls, ceramic balls are classified by ABMA, JIS, and ASTM standards. Sphericity, surface finish, lot diameter variation, and other precisely defined factors fall into grade levels. Grade three is typically the highest; this denotes three-millionths sphericity or better. Other grades include five, ten, and so on. Though it is tough to say exactly how much longer hybrid bearings last than steel bearings, generally life is extended two to five times. Because of their inherent lower thermal expansion, smoother surface, increased hardness and corrosion and electricalresistant properties, ceramic balls last longer. It’s true a stiffer ball can increase contact stresses if raceway curvatures are not adjusted. (For extremely high load applications, silicon nitride balls may not be suitable since they may accelerate steel raceway fatigue.) Even so, ceramic balls are not brittle and fragile. While they are not as tough or ductile as steel, they are actually much more durable.

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