Application characteristics of ceramic bearing materials

Ceramic bearings are a general term and can be subdivided into two types: full ceramic bearings and hybrid ceramic bearings (semi-ceramic bearings). Common ceramic materials in bearings include silicon nitride, aluminum oxide, zirconia and silicon carbide.

hybrid bearing

Hybrid ceramic bearings are generally assembled from rings made of bearing steel and bearing-grade silicon nitride rolling elements, which have electrical insulation properties. Compared with bearings with steel rolling elements, the advantages of hybrid ceramic bearings (silicon nitride rolling elements are mainly used as an example below) include:
       1. Prevent current corrosion
        When the motor is running, the potential difference generated between the two ends of the rotating shaft or between the shafts is called shaft voltage. If both ends of the shaft form a loop through the motor frame, etc., a shaft current will be generated under the action of the shaft voltage. The shaft current is generated by the shaft voltage through a closed loop formed by the motor, bearing, stator frame or auxiliary device. Under normal circumstances, the shaft voltage is low, and the lubricating oil film in the bearing can act as an insulator and suppress the shaft current; when the shaft voltage If the lubricating oil film is too high, or the oil film is not stably formed at the moment the motor is started, the shaft voltage will cause the lubricating oil film to discharge and breakdown, forming a path to generate shaft current.
        The high temperature generated by the release of partial discharge energy of the shaft current can melt many tiny areas on the inner ring, outer ring or balls of the bearing and form grooves, thereby producing noise and vibration. If it cannot be detected and dealt with in time, it will lead to bearing failure and bring negative consequences to production. Great impact. The most significant feature of the electrical corrosion of bearings caused by high-frequency shaft currents in variable frequency speed control systems is the generation of "washboard" dense groove stripes on the inner and outer rings and balls of motor bearings. What are the ways to fundamentally eliminate the shaft current of the motor? The first is to add a leakage device, that is, to install a bypass brush, so that the shaft current bypasses the bearing and is leaked; the second is to use insulated bearings to block the path of the shaft current through the bearing. In order to block the path of the shaft current, at least one end of the motor must Electrically insulate bearings.
        Shaft currents commonly occur in wheel set bearings and traction motors (rail vehicles), DC and AC motors (power transmission systems) and generators (wind power). Shaft current may cause damage to raceways and rolling elements and aging of lubricants, causing premature failure of motors or generators, increasing maintenance costs and causing losses such as shutdowns and production shutdowns. The best solution to these problems is to use electrically insulated bearings. Bearings with ceramic coatings on either the inner or outer rings are called insulated bearings. Ceramic coatings prevent the passage of electrical current and have insulating capabilities. The rolling elements of hybrid bearings are made of ceramics. The rolling elements prevent the passage of current, so they also have insulation capabilities and are more durable than ceramic coatings.
        2. Effectively increase the speed
        The density of silicon nitride rolling elements is 60% lower than that of bearing steel rolling elements of the same size. Lower weight and inertia result in higher speed performance, which gives the bearing outstanding ability to start and stop quickly. In addition, ceramic balls have oil-free self-lubricating properties, and the friction coefficient of ceramic balls is small, so ceramic ball bearings have high rotation speeds. According to statistics, the rotation speed of bearings using ceramic balls is more than 1.5 times that of ordinary bearings.

Hybrid ceramic bearings have been successfully used in the spindles of high-speed machine tools and have entered the practical stage, such as the HPM ultra-precision lathes produced by Makino and other companies in Japan. The spindle speed is 16000r/min, while the HSM700 produced by the American MIKRO company In high-speed machining centers, the spindle speed has reached 42,000r/min, and the cutting speed has increased by 5 to 10 times. In addition, hybrid ceramic bearings are also used in high-speed equipment such as electric spindles and eddy current molecular pumps.

3. Long service life
        The heat generated by friction in hybrid ceramic bearings is lower, especially at high speeds, which helps extend the service life of the bearing and extend the relubrication interval, which can greatly reduce downtime losses and increased maintenance costs caused by bearing maintenance.
       4. High hardness and high toughness
        Silicon nitride rolling elements have higher hardness and excellent toughness. The combination of these two characteristics can achieve better surface roughness, and can prevent damage from external hard particles and impacts, and has strong wear resistance.

5. Bearing stiffness is higher
The stiffness of the bearing refers to the degree of elastic deformation of the bearing under load. Stiffness affects various aspects such as bearing vibration, noise, life and rotation accuracy, and is an important performance parameter in bearing analysis. This deformation is generally small and can be ignored, but in some applications, such as machine tool spindle bearings or pinion bearings, stiffness is very important.
The hardness of silicon nitride ceramics is 1 times higher than that of bearing steel, and the elastic modulus is about 1/3 higher. Under the same load conditions, the elastic deformation of silicon nitride ceramics is small. Therefore, machine tool spindles using ceramic ball hybrid bearings have good performance. operating accuracy.
       6. Low sensitivity to temperature changes
Silicon nitride rolling elements have a low thermal expansion coefficient, which helps reduce sensitivity to temperature changes and prevents sticking. For hybrid roller bearings, the applicable operating speed range is wider.
Ceramic hybrid bearings are currently widely used in electric motors, aerospace applications, high-performance racing cars, laboratory equipment, underwater applications, etc. Ceramic hybrid bearings easily meet the requirements for higher speeds, lower friction and longer life.
       All ceramic bearings
       The races and balls of all-ceramic bearings are made of ceramic materials. Compared with hybrid bearing materials, all-ceramic bearings are more difficult to process. Although hybrid bearings have performed very well in many applications, they are in a very extreme few. All-ceramic bearings are divided into those with retainers and without retainers. All-ceramic bearings without retainers can continue to operate at extremely high temperatures. All-ceramic bearings have a high degree of corrosion resistance and can withstand most common acid, they do not corrode when exposed to water or salt water.