By Mark Stoneburner, Applied Industrial Technologies
Published August 2005, Plant Engineering magazine
At one manufacturing plant, bearings on a grinding spindle were wearing out much faster than it was thought they should, resulting in frequent downtime and costly replacements. The machine used oil-lubricated, precision all-steel bearings. The goal was to extend bearing service life without changing lubrication or reducing product quality.
The all-steel bearings were replaced with hybrid ceramic bearings of the same size and design (Fig. 1). In this application, the replacement bearings offered better kinematic behavior, reducing the applied preload without sacrificing stiffness. The new bearings also provided cooler operating temperatures. As a result, the service life of the machine's hybrid ceramic bearings was three times that of the all-steel bearings.
The rolling elements of ceramic bearings are made of silicon nitride. Unlike ceramics used in pottery and tableware, silicon nitride is a high-grade engineering ceramic known for hardness, strength and a highly favorable failure mode.
Hybrid ceramic bearings offer advantages in a number of applications:
Where bearings need to function under extreme conditions for a limited time, such as fire smoke exhaust fans and safety couplings, hybrid bearings hold up extremely well. Because adhesion between silicon nitride and steel is low, micro-welding does not occur and smearing resistance is very high. The chance of catastrophic failure is eliminated.
High power output
When used in electric drives and industrial machine tools, hybrid ceramic bearings offer high-speed operation with low friction. Since silicon nitride has only 40% of the weight of steel balls, centrifugal force is lower. Reduced friction and lower temperature rise allows increased speed of operation. In addition, the lower weight of hybrid balls enables rapid accelerations and decelerations.
Because the thermal expansion of hybrid ceramic bearings is approximately 30% less than steel, ceramic bearings are less sensitive to heat differences between races. Ceramic balls transfer less heat, as well. All of this translates to a smaller initial preload for cold ceramic bearings. This preload is not significantly affected by temperature increases.
Since hybrid bearings operate well in greased-for-life applications and usually don't require oil lubrication, opportunities for oil leakage into the environment are eliminated. Low-friction operation also requires less energy consumption.
Due to their smoothness (the coefficient of friction in hybrid bearings is approximately 20% of similar steel balls), hybrid bearings generate less vibration than all-steel bearings, reducing noise levels during operation. These benefits are an advantage when used in compressors, mixers, pumps and flow meters.
Low life cycle cost
Hybrid bearings' increased life, reduced operating and maintenance costs, increased production quality and simple handling and mounting contribute to a lower life cycle cost than all-steel bearings. This is particularly true when used in electric motors, stepping motors, encoders and pumps.
Longer life, less premature failure
Hybrid ceramic bearings often last longer than other bearing types. One reason is that, unlike all-steel bearings, ceramic balls have natural insulating properties that prevent electrical arcing, which can result in a washboard or fluting pattern on raceways (Fig. 2). This damage can create excessive noise and premature lubrication aging. Hybrid bearings also permit a wider range of speeds, allowing operators to meet the demands of specific jobs.
Because ceramic bearings are not prone to static vibration, a common cause of false brinelling, there is much less of a risk of spalling and premature failure. Spalling and flaking can occur with ceramic bearings, but the fatigue life of hybrid ceramics is generally much longer than that of steel.
All bearings, whether steel or ceramic, require lubrication. Grease and oil are common lubricants for hybrid bearings, but ceramic bearings are less sensitive to fluctuations in lubrication conditions. For example, compared to steel bearings, ceramic balls can operate under the same lubrication conditions at speeds up to 20% higher.
Except for those applications that operate at high speeds, grease is the recommended lubricant in most ceramic bearing applications. Grease is preferred because it is more easily retained at the bearing than oil and provides better protection against moisture and contaminants.
The most common grease used with ceramic bearings is lithium grease with a mineral oil base, suitable for precision bearings. For high-speed, high-temperature and prolonged service life applications, synthetic lubricants are preferred.
Regardless of the type of grease used, the quantity should never exceed 30% of the free space in the bearing. In high-speed applications, this quantity should be less than 30%.
As with any type of bearing, freshly greased ceramic bearings require a low-speed run-in period to ensure even distribution within the bearing. Excess grease is ejected during this period. Without a run-in period, premature bearing failure can result from sudden temperature rises.
A grease replenishment schedule should be calculated based upon the specific application. For high-speed applications, all grease should be removed and replaced with fresh grease on a fixed schedule.
Oil is typically used in applications where grease is either technically unsuitable or economically undesirable. Other situations where oil is preferred include those where grease relubrication cycles would be too short or when heat has to be removed from the bearing.
The combination of high operating speeds and low operating temperatures of spindle bearings require the use of circulating oil or oil spotting for lubrication. For other applications, oil bath, oil drop, oil jet and oil mist are suitable.
While oil must be changed at specific intervals, several lubrication methods offer slight exceptions to this rule. With oil-bath lubrication, an annual oil change is sufficient if bearing temperatures are generally lower than 122°F. At higher temperatures, oil should be changed more frequently. Where oil drop, oil mist or oil spot lubrication systems are in place, replacement generally is not necessary.
|Property*||Unit||Silicon nitride ||Bearing steel, hardened|
|Coefficient of thermal expansion||1/°C|| || |
|20 – 1,000°C|| ||3,2 x 10-6|| |
|20 – 300°C|| || ||11,5 x 10-6|
|Modulus of elasticity, E||N/mm2||315,000||206,000|
|Fracture toughness, Klc||Mpa — m½||7||25|
|Thermal conductivity||W/mK||30 – 40||40 – 50|
|Specific electrical resistance||Ù — mm2/m||1017 – 1018||10-1 – 1|
| || ||Insulator||Conductor|