Steel bearings and ceramic bearings are familiar engineering components, but what about a combination of the two? These so-called hybrid bearings typically use steel for the rings and ceramic for the rolling elements. In terms of application, hybrid bearings are popular where high-grade electrical isolation is necessary. Users enjoy extended service life because ceramic is an insulator that prevents electrical erosion. High-speed applications can also benefit, as ceramic balls are lighter than steel, resulting in less friction and power loss.
According to bearing manufacturer NSK, parasitic current can take various pathways through an electric motor, often due to the presence of an inverter that switches frequently across multiple phases. Specific electric motor architectures can cause different bearing current flows, such as rotor ground bearing currents, circulating bearing currents and EDM (electrical discharge machining) currents.
EDM currents present a severe risk to steel bearings, mainly because of arcing that takes place at certain voltages. This changes the morphology of the steel bearing balls and rings. Here, material melts and re-solidifies on the metal surface so that ripples several microns deep appear on the bearing raceways of the inner and outer ring. An indicator of this issue is often excessive noise.
Proven methods to help thwart electrical erosion comprise either insulation or conduction. According to NSK, if the charges are not overly high, it may be sufficient to use a conductive grease, seal or other grounding element. For higher charges, however, insulation is required in the form of ceramic bearing components.
Inverter motors deliver energy in pumps and blowers. As a general development trend, the frequency requiring control (known as the carrier frequency) is increasing, so the motor can operate with a higher degree of accuracy. However, as the carrier frequency increases, so does the risk of electrolytic erosion, due to high-frequency current in the bearing.
Some bearings for small motors feature ceramic balls that prevent current transfer to protect against electrolytic erosion, but there are issues with high-volume production of larger-diameter ceramic balls required for medium and large inverter motors. To deal with this issue, NSK has developed an anti-electrolytic-corrosion, ceramic-coated bearing specifically for use in these inverter motors, where the company sprays a ceramic coating on to the outer ring.
NSK tests show around 10 times more insulation performance (than general ceramic-coated bearings) when using a DC power supply, while also demonstrating equivalent or slightly better insulation with an AC power supply, satisfying an impedance of 100Ω or more at 1MHz frequency.
Insulation is also key in traction motor bearings. Here, NSK offers cylindrical roller bearings and deep-groove ball bearings featuring outer rings plasma-sprayed with an alumina-based ceramic coating. These bearings are currently in daily use on the N700S traction motors of Japan’s latest-generation high-speed passenger (bullet) train which began service in July 2020 on the Tokaido Shinkansen line between Tokyo and Osaka.
According to another bearing manufacturer, SKF, emerging technologies require special bearing properties, including suitability for use with variable speed drives, a reduction in maintenance requirements, enabling of increased power density and lower friction. SKF’s hybrid bearings make use of silicon nitride rolling elements.
Silicon nitride has a compressive strength far in excess of bearing steel, and its elastic modulus is about one-third higher, helping to reduce friction in the rolling contact. In addition, silicon nitride offers more than double the hardness of bearing steel, making it suitable for demanding operating and lubrication conditions. Its density is also significantly lower and its coefficient of thermal elongation is far less, which allows accurate clearance adjustment.
When used in rolling bearings, silicon nitride provides an upgrade to performance, not just regarding electrical insulation, but also weight reduction, tribology characteristics and wear resistance. SKF reports that a key benefit of hybrid bearings arrives in the shape of longer maintenance intervals. The use of ceramic rolling elements greatly increases lubricant life because they allow for improved lubricant supply to the rolling contact. With less friction and electrical erosion (and no subsequent thermal degradation), the bearing maintains lubricity for a longer period.
In SKF tests, the lifespan of the lubricant in a hybrid deep-groove ball bearing was at least twice and up to nearly seven times longer than for the same steel bearing. In similar tests involving cylindrical roller bearings, the lubricant in the hybrid bearing lasted at least twice as long and up to four times as long.
A reduction in friction also contributes to lower energy consumption. The ceramic rolling elements in hybrid bearings have a direct impact on friction generated from rolling and sliding torques. Energy losses due to rolling torque are lower in hybrid bearings because of the smaller contact ellipse, which results from a higher Young’s modulus. The better surface quality of ceramic rolling elements also helps. SKF tests show friction torque to be between 5% and 8% lower at high speeds with hybrid bearings, compared with steel bearings.
Further tests reveal that hybrid bearings cope extremely well with a reduction in lubricant volumes and viscosity, factors that can also help lower friction. By fine-tuning these lubrication parameters and optimising the application, a friction reduction of up to 50% can become reality, without compromising bearing service life.
BOX: AIRBORNE HYBRID BEARINGS
Timken engineers recently worked with defence aircraft builders to develop hybrid roller bearings for several applications, including an aircraft turbine engine offering significantly more power and efficiency than existing low-bypass turbofans. Timken tribology specialists played an important role in R&D, testing the strength of ceramic rollers against a variety of steel grades typically used to make roller-adjacent bearing components.
Melissa Kerney, aerospace programme manager, says that the silicon nitride used is the highest available grade and processed to very high standards: “Differentiation to achieve an aerospace-grade product occurs in two stages, both of them in manufacturing.”
The first stage involves forming a near-net shape ‘blank’ roller using silicon nitride powder. The powder formula determines how the material will withstand formation into ‘blanks’ that ultimately become ceramic rollers. Second, Timken finishes those blanks in-house to precise requirements using proprietary grinding and superfinishing processes.
“It’s very challenging,” states Kerney. “Silicon nitride is much stronger than steel and requires a completely different approach to finishing, gauging and inspection. We take what we know about material science and combine it with our manufacturing and tooling expertise. The result is durable, reliable bearings that meet industry inspection requirements.”
Timken intends to start full production of the new hybrid bearings for aerospace customers this year.