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.
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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Hybrid roller bearings have longer service life and higher speed capabilities than conventional bearings. Higher speed is achieved in part through the lower density of silicon nitride. At just 40% the density of similar steel rollers, silicon nitride balls offer lower inertia for faster stops and startups. To discuss bearing speed it is necessary to understand the relationship between rpms and the size of the bearing.
DmN = N x Dm
Where DmN = Relative bearing speed pitch diameter
Dm = (Do + Di)/2 = Actual speed
N = Relative speed, rpm
The construct Dm is used to defined relative bearing speed and is given by the pitch diameter (ball path) times the bearing rpm. This dimensionless parameter can be used to compare bearings of different dimensions. Dm is the pitch diameter or ball of the bearing and is determined by the average distance between the outside diameter, Do and the inside diameter, Di, in millimeters. Relative bearing speed, DmN is calculated as the pitch diameter of the bearing multiplied by the speed in rpms. For example, a bearing with a pitch diameter of 10 mm and a speed of 10,000 rpm would have a DmN value of 100,000. A bearing with a pitch diameter of 100-mm bearing is 10 times that of the 10- mm bearing.
Frequently asked is whether load capability affects the L10 life calculation of a hybrid bearing. This is an important issue and has been given significant attention over the past several years. Silicon nitride balls are very stiff, and so the contact patch on the raceway is quite small. For any given load, this means that the stress in the raceway at the contact patch is increased and theoretically the L10 life is reduced in a hybrid bearing. Indeed applications that require bearings to perform at high load levels demonstrate lower life characteristics when a hybrid bearing is substituted for a traditional bearing. Fatiguing is usually to blame. However, if a designer adjusts bearing raceway curvatures and the number of balls, the load-carrying capability can be improved.
Special thanks to Saint-Gobain Ceramics for technical information and insight. For more information, visit www.cerbec.com.
Other bearing materials
Chemical makeup is what imparts different characteristics to different bearing balls. For all materials, atoms are everseeking to gain electrical neutrality (with equal number of protons and electrons) to reach the best equilibrium state. Metals are predominantly bonded by non-directional electron sharing between neighboring atoms, known as metallic or electronic bonding. At any moment in time, metal atoms must satisfy only overall electrical neutrality with their surrounding neighborhood; they do not require “ownership” of the electrons surrounding them. Non-directional electron sharing between atoms allows for tightly packed structures (which results in high density) with numerous slip planes (which allows for ductility). Since electrons are not tightly bound to specific neighboring atoms, they are also free to move through the metallic lattice (which results in electrical and thermal conductivity).
Polymers are made of individual molecules attached in long chains at specific, discrete points. Typically, the molecules in a polymer bond are also bound together very tightly. This makes for an interesting combination of properties. Because individual molecules do not allow their electrons to move, corrosion and electrical resistance result. Because the chains can stretch and move within the material, the bonding allows for elasticity and lubricity.
Unlike polymers in chains, ceramics are predominantly bonded by directional bonds between neighboring atoms in expansive lattice structures. Because every atom in a ceramic directionally shares its electrons within its lattice neighborhood through either or both ionic and covalent bonding, ceramics are not tightly packed like metals. Ceramics tend to be very non-reactive or inert because their atoms are essentially electrically neutral through strong directional bonds within a very fixed lattice neighborhood; they have no need to react with surroundings for electrons to satisfy neutrality.
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In the wind
Wind energy companies are beginning to realize the advantages of ceramic hybrid bearings, particularly for increasing speed. On a windy day, the blades of a mill can be rotated up to 30 rpm. But to generate electricity, this slower motion of the rotor shaft must be sped up to 2,000 rpm. Bearings that include ceramic balls are beneficial because they eliminate electrical arcing and last longer than steel bearings in these harsh conditions. Challenges include debris and temperature extremes. Maintaining lubricity is also an issue. A long-life bearing is especially attractive because of the high cost of bearing replacement in windmills; a bearing replacement can cost up to $10,000 because of the need for cranes and crews.
Jeff McLaughlin is general manager of Machine Building Specialists of Manitowoc, Wis., a company that provides engineering and manufacturing solutions to the wind turbine, power generation, and construction equipment industries. According to McLaughlin, “the use of hybrid ceramic bearings has helped us achieve and surpass our goals of improved machine performance, reduced maintenance costs, and higher availability for our customers.” Machine Building Specialists use Cerbec ceramic balls in their designs.
“The differential cost over standard bearings is easily justified in wind turbine generator applications, especially for high speed and generator shafting. The lower mass, thermal stability, reduced friction, and electrical isolation — when used as part of a comprehensive remediation plan — have resulted in ‘better-than-new’ OEM performance.”
The birth of a ceramic ball
Like most ceramics, silicon nitride isn’t easily fabricated into single crystals. Instead, silicon nitride balls are polycrystallines with micron-sized, needle-like grains (characterized by acicularity) bonded together by a secondary glassy phase. These two main crystalline polymorphs are called alpha and beta.
1. Through liquid phase sintering by solution-reprecipatation, a sintering aid (usually magnesium or yttrium oxide) forms an inter-granular glass. Alpha silicon nitride dissolves into it, while the beta silicon nitride precipitates out. (Typically the more liquid phase, the lower the temperature and pressure required for densification and microstructural development. But along with that comes increased difficulty to gain appropriate properties for bearing-grade silicon nitride.) Better ceramic varieties possess lower amounts of the liquid phase — under 15 volume percent — and need high temperatures and pressures for enough kinetic and thermodynamic driving force to promote alphato- beta transformation.
2. Ceramic ball fabrication begins with making a pressable powder. Slurry with binders is then spray-dried to make flowable, compactable powder, which is pressed into blanks using uniaxial or isostatic methods. Though other methods do exist, in one common method ball blanks are air-fired to remove binders, loaded into graphite crucibles with encapsulant glass, and hot isostatically pressed at extremely high temperature and pressure. This optimizes densification and microstructure. Both magnesia and yttria-doped materials benefit.
3. At the end of the process, densified ball blanks are de-encapsulated, inspected, and finished. Once qualified for further processing, the balls are roughed and finished in a series of steps utilizing free abrasive lapping with diamond. This is required because silicon nitride is such a hard and tough material.
The sintering aid is the controlling factor concerning corrosion resistance. Both magnesia and yttria-based silicon nitrides are highly inert in most liquids and gases. No organic solvent is known to corrode silicon nitride. In general, magnesia and yttria-based silicon nitrides behave the same in various environments. Some exceptions do exist. For example, yttria-based silicon nitrides are more resistant to hydrofluoric acid solutions than magnesia-based silicon nitrides.