Most mechanical designers are familiar with the standard steel ball bearings used in everyday machinery. In some instances it is desirable to substitute ceramic instead of steel in place of either the ball bearings, the races or both. Understanding how bearings operate, how they are rated and the planned application all influence the suitability for ceramic materials.

The Annular Bearing Engineering Committee (ABEC) was established over 50 years ago to standardize the tolerances of roller bearings. Their intent was to help ensure more consistent manufacturing and performance of rotary ball bearings. Under the ABEC system, ratings are a function of the allowable tolerances.

The rating grades are ABEC 1, 3, 5, 7 and 9, with tighter tolerances assigned to the higher rating. Take for example, the tolerancing of an ABEC 1 bearing with an outside diameter less than 2 in. The allowable deviation in the width of the outer ring is 0.0050 in. For an ABEC 7 bearing of the same diameter, the width of the outer ring can deviate no more than 0.001 in. Most applications will work fine with an ABEC-1 or ABEC-3 rating. However, for applications that require precision, low runout and high speeds, ratings of ABEC 5, 7 or 9 are not unusual. The ABEC rating is a function both of dimensional accuracy and running accuracy. Equivalent regulations for other countries can be found in ISO 492 (Europe) and JIS B 1514 (Japan).

Special conditions

If the bearing application needs a high ABEC rating and also has to work in high temperature, high speeds for a long period of time, or ultra precise conditions, then it might benefit from ceramic bearing materials.

Ceramic bearings can be full ceramic (balls and races) or hybrid (usually ceramic balls and steel races). Full ceramic bearings have the advantage that all materials have a matched coefficient of thermal expansion. Since ceramics are non-porous they do not require lubricant for heat dissipation and can run at very high speeds. Ceramic balls are rounder, lighter, harder and smoother than steel balls. Because of this, even hybrid bearings (ceramic balls with steel inner and outer races) provide almost the same level of performance at a lower cost. However, they do require a lubricating oil.

Here is a table showing typical properties for ceramic bearing material versus steel used for ball bearings.

Aluminum oxide (Al₂O₃) or just plain alumina, is one of the most commonly used ceramic substitutes for steel bearings. The higher operating temperatures, low density and about double the hardness make it an attractive and cost effective substitute.

Common applications of ceramic/hybrid bearings are:

• Spindles
• Metal cutting and woodworking spindles
• Grinders
• Vacuum pumps
• Semiconductor components
• High-speed rolling mills
• Printers
• Race cars
• Turbochargers

In these specialized circumstances ceramic bearings typically have a life that is up to thrice as long as their steel counterpart.

Electrical isolation

Ceramic bearings can also be used for their isolation capability. This situation is most commonly seen with industrial electric motors driven by a variable frequency drive (VFD) and using some sort of feedback device, such as an encoder. Usually the encoder has an input shaft that is coupled to the motor stub shaft that sticks out of the back of the motor and rotates along with it. The motor’s VFD uses modulation of a PWM signal at a rate of approximately 20 kHz. This modulation simulates a sinusoidal drive. It also induces a series of high frequency pulses on the shaft of the motor. This pulsating voltage travels to the encoder shaft, through the encoder shaft bearings and then to ground through the case of the encoder. The end result is that the bearings have constant high voltage sparking going on within the bearings causing pitting of the races and oxidation of the lubricant. When installed in these systems without a mechanism to interrupt the electrical continuity, an encoder that should see a five to ten year life fails catastrophically in as little as three weeks. One solution is a hybrid bearing since the ball bearings are non-conductive. Another common solution is to couple the encoder shaft to the motor shaft via a non-conductive coupling – usually a high performance plastic sleeve is used.

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Limitations of ceramic bearings

Although ceramic bearings may seem like a “magic bullet” for difficult applications, there are a few notes of caution. Like all things mechanical, there are physical limitations and cautions about their use. To highlight these, here are the most common pitfalls to be aware of when using ceramic bearings.

Contamination

Ceramics are sensitive to contamination. Manufacturing and handling of ceramic bearings must be done in a clean environment to prevent dust, dirt or grit from getting into the bearing. Usually this is achieved by keeping any grinding and cleaning operation separate from the assembly and packaging operations. Also, best practice is to keep the bearing in its original packaging until it is ready to be installed.

Thermal shock

Despite their low coefficient of thermal expansion, high temperature gradients caused by thermal shock can cause stress fractures which will lead to failure. Symptoms of thermal shock damage are irregular wear patterns in the races and discoloration due to overheating. A common solution is to switch to a ceramic with a higher temperature rating.

Physical shock

Ceramic generally has a higher Young’s modulus when compared to steel. Consequently excessive physical shock can lead to brinelling. In bearings, evidence of brinelling is usually an indentation on the races. This indicates that the shock was sufficient to cause one or more of the balls to exceed the elastic limit of the race locally and leave a small indentation. This causes excessive vibration in the bearing which ultimately leads to destruction of the bearing. One of the common ways that brinelling occurs is by the use of a hammer to install the bearing by tapping it into place. Another cause would be to press the bearing in place by pushing on the outer race only, rather than both races simultaneously and evenly.

Summary

The high speed capability, temperature tolerance, electrical isolation and corrosion resistance make full ceramic bearings very desirable for a whole range of difficult environments. Compared to steel bearings in similar environments, ceramics can last two to three times longer. Trading off the higher cost of ceramics for fewer maintenance overhauls usually is worth it for those specialty applications which really need high performance.