New technology can decrease heat and skidding.
by Ed Kaineg
Schaeffler Group USA, Inc.

Over the last 20 years, most people have come to associate the term “hybrid” with fuel-efficient cars that combine an internal combustion engine with an electric motor. It has become mainstream technology that has spread to six-figure luxury cars. Now, the world of pump bearings may be on the cusp of a similar seismic shift in technology, except that the “hybrid” in question does not involve regenerative braking or lithium-ion batteries. Rather, the hybrid technology that is set to revolutionize the pump industry is known as the “hybrid ceramic bearing” or simply “hybrid bearing.” This term refers to a bearing that combines inner and outer rings made from standard bearing steels with ceramic rolling elements. The most common ceramic material for these applications is silicon nitride (Si3N4), although other ceramics are possible as well. The combination of these two different materials creates certain advantages that are ideal for the challenging conditions under which bearings operate.

table 1 comparisonTable 1. Comparison of the material properties of steel vs. silicon nitride (Si3N4) (Image courtesy of Schaeffler Group USA, Inc.)

About Hybrid Bearings

Low Density:Silicon nitride has only about 40 percent of the material density of typical bearing steels. Lowering the material density reduces the mass of the rolling element, which decreases the centrifugal force imparted on the outer race. As bearing speeds increase, the value of this increases. Less mass reduces the rolling element’s tendency to skid, which is a source of heat generation. Many failures of the thrust bearing position are caused by excessive skidding. Modulus of Elasticity: Silicon nitride’s modulus of elasticity is 51 percent higher than the elastic modulus of bearing steels. Greater stiffness reduces the size of the pressure ellipse for a given roller or ball load. Because the contact area is smaller, the friction generated by the basic rolling process is significantly reduced. The actual stress level at the center of the ellipse is higher than it would be on a comparable steel-on-steel design. Lower Thermal Expansion: Hybrid bearings are characterized by lower thermal expansion rates, which means they are affected less by higher temperatures than conventional steel-on-steel bearings. Because ceramic rolling elements expand at a much slower rate (approximately 73 percent less) than their steel counterparts, the axial clearance settings for hybrid bearings can be maintained throughout a larger temperature range. Electrical Resistance: Replacing standard steel rolling elements with ceramic versions results in a tremendous increase in the bearing’s resistance to electrical current. From a practical standpoint, the bearing is now impervious to damage caused by electrical arcing. This is becoming more and more important as variable frequency drives (VFDs) are increasingly being installed to regulate pump and fan speeds for improved overall efficiency. Dissimilar Materials: By introducing a material that is different from steel, the opportunity for interaction and damage to the bearing contacts is reduced. Manufacturing techniques used on ceramics also impart lower finishes and better geometries. The hybrid ceramic bearing was developed in the 1990s in conjunction with the U.S. Department of Energy (DOE), where it was primarily limited to military applications. As the benefits became more widely known, hybrids were introduced into other aerospace and high-speed spindle applications. Today, hybrid bearings are used in railroad applications, wind turbines, pumps and even the basic electric motor. Hybrid Bearings in Electric Motor Applications: The No. 1 cause of electric motor failures can be traced to the bearings supporting the armature. In turn, lubrication is the number one cause of bearing failures. Most motors rated below 50 horsepower (hp) use sealed bearings with an initial grease pack of approximately one-third the void space inside the bearing. This initial grease volume has a finite life. And since the bearing is sealed or shielded, re-greasing is not only impractical, it is potentially hazardous. Re-greasing can introduce contaminants or cause over-lubrication, which can lead to overheating. Thanks to their smooth finish, improved geometry and dissimilar material composition, hybrid ceramic bearings offer lower operating temperatures. The correlation between reduced temperatures and grease life is well understood. Hybrids typically reduce heat generation by 30 percent at typical electric motor operating conditions. The resulting temperatures can extend grease service two to three times. The smoother finish also decreases asperity interaction, which can reduce the generation of metallic particles. These are the tiny material fragments that give fresh grease a dark, worn look as they collect and start to oxidize. These particles also eventually reenter the contact area, which, in turn, produces more metallic debris. And, their ceramic rolling elements can be good electrical insulators. This inherent characteristic protects the bearing from stray currents generated by motor asymmetries or VFD drives. It is important to keep in mind that “stray voltage” is still present and may move to another location within the system.

Hybrid Bearings in Pump Applications

All of the advantages of hybrid bearings outlined for electric motors also apply to pumps. The most interesting difference with respect to pump applications is the reliance on more complex lubrication systems. While the speeds experienced by bearings inside a pump are similar to those inside a motor, most pumps, especially centrifugal pumps, will be exposed to a wider range of loads. This is due to the hydrodynamic nature of this machine. At low flows, radial loads can spike, while during periods of high flow, thrust loads can increase. Pump solution temperatures also vary, which puts more stress on the lubrication system. Closer proximity to mechanical seals creates additional opportunities for ingress of contaminants. Opting for hybrid ceramic bearings inside a pump can provide a larger window for operation. By generating less heat, the self-contained sealed or shielded hybrid bearing may perform longer than its steel counterpart in an oil bath lube system. In a more extreme application, an open hybrid in an oil bath system may outperform a standard steel bearing requiring an air-oil or oil-mist system. In addition to improved performance, hybrid bearings also reduce the need for more complex lubrication systems. Of course, every technology has its limitations, and hybrid ceramic bearings are no exception. As mentioned above, hybrid bearings do produce smaller race contact areas. This means that the smaller footprint can lead to higher localized stress for a given load, and while fatigue failures are rare in the pump world, hybrids will not provide increased service life in these cases. Therefore, any new application for hybrid bearings should be evaluated carefully. This evaluation starts with a comprehensive failure analysis whenever a worn bearing is taken out of service. Assuming the failures are not fatigue-related, the hybrid ceramic bearing can become an invaluable tool in the never-ending quest to increase reliability and mean time between failures (MTBF).