The term hybrid bearing is generally understood to refer to a bearing whose bearing rings and rolling elements are made of different materials. These can be variants made of plastic and glass. However, the most common type is a combination of rolling elements made out of a high-strength ceramic material, silicon nitride (Si3N4), and bearing rings made of bearing steel.

But when were hybrid bearings developed and to what purpose? Time to take a short trip into the past: The idea for hybrid bearings actually arose more than 50 years ago – chiefly in order to install them in gas turbines with very high rotational speeds and thus high bearing temperatures. These turbines actually reached more than 30,000 rpm and the bearing temperatures exceeded 650 °C. In order to cope with these conditions, a material was required that was harder than bearing steel, but also light, very strong and able to resist high temperatures. Back then the most common ceramic materials were indeed very heat resistant and had a low density, but with regard to their brittleness and strength properties they left a lot to be desired. Luckily, however, the ’60s and ’70s had more to offer than just Woodstock and the Bee Gees, as it was precisely during this era that the search for a material with improved characteristics led to growing interest in the synthetic ceramic silicon nitride, now the standard ceramic material used to manufacture rolling elements.

Silicon nitride represented a veritable revolution in the nascent field of hybrid bearings: Its use solves bearing problems that cannot be mastered using conventional materials. In addition, hybrid bearings with rolling elements made of silicon nitride exhibit better wear characteristics compared to bearings made completely of steel, which under difficult operating conditions suffer particularly from inadequate lubrication and contamination. Even at high speeds only a very small amount of frictional heat is produced in hybrid bearings, as the ceramic balls have a higher elastic modulus than steel balls and an exceedingly low degree of thermal expansion; as a consequence, lower centrifugal forces are produced at high rotational speeds due to their low density, thus increasing the bearing’s service life. Haven’t had enough yet? OK, here are a few more facts: Besides all the factors mentioned above, the use of ceramic balls also results in a longer grease life even under difficult operating conditions. In addition, rolling elements made of silicon nitride are resistant to corrosion, remain stable when subjected to temperature changes and are resistant to overrolling under high loads. Hybrid bearings are not magnetic and possess electrical insulation properties, and can thus also be used in applications in which conventional bearings can be potentially damaged by the passage of current – they are thus bona fide all-rounders.

All these properties make hybrid bearings the ideal choice, especially for applications involving demanding conditions. First and foremost, they are designed for applications in which a high degree of electrical insulation is required and/or high rotational speeds occur; as hybrid bearings only exhibit a very low amount of wear and the rolling elements are lighter than steel rolling elements, they are perfectly suited for use in rapidly rotating systems involving high rotational speeds. Typical applications for hybrid bearing can be, for example, electric motors, gearboxes, pumps and compressors, generators for wind turbines, high-speed applications such as machine tool spindles, traction drives for railway vehicles, engines and similar sophisticated power units in the aerospace industry as well as applications in the dental, medical and vacuum technology sectors – a small change with a major impact.

Despite all their positive characteristics, the reality does, of course, look slightly different. That’s because hybrid bearing can do a lot of things, but not everything. For instance, the ceramic material makes the rolling elements susceptible to damage caused by high loads, especially shock loads. Due to their hardness, they cause tiny indents in raceways made of softer materials such as stainless steel, meaning that as time goes by the bearings can no longer run freely. Furthermore, the robustness of the ceramic can be impaired by small impurities in the material or on its surface. For this reason, the manufacture of the ceramic bearing components is subject to strict quality control measures; even after assembly, the complete bearings undergo another special inspection. Although this process is necessary to ensure that safety and quality standards are met, it is also expensive. On account of this price situation, to date hybrid bearing have often only been used in applications where they can really bring their advantages to bear.

DESCRIPTIONS

Hybrid bearings can be recognised by their prefix or suffix. Every manufacturer used its own designations. A few examples are listed below: