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High speed bearings

wallpapers Tech 2020-10-28
The constant pursuit of machine tool performance has led to the further development of bearings. In some cases, the requirements may conflict, but by carefully studying the performance of the rolling elements in the bearing, a solution can be found to improve each performance parameter at the same time. Improvements in bearing performance will increase machine productivity and increase production cost-effectiveness.
Centrifugal load
When the bearing rotates at high speed, the rolling elements in the rolling bearing will bear considerable centrifugal load. The load on the outer ring raceway will increase, and the load on the inner ring raceway will decrease. In addition, in the case of angular contact ball bearings, this will cause the working contact angle of the outer ring to decrease and the contact angle of the inner ring to increase. Since the contact angles on the rings become different, the ball must be slid relative to one ring. Since the load of the outer ring ball is heavier than the load of the inner ring ball, the balls become the outer ring guide and slide on the inner ring. Due to this change in contact angle, bearing stiffness is usually affected at increased speeds. The difference in contact angle is also affected by the radius of the raceway, so these radii and their relationships need to be evaluated.
Lighter rolling element
In order to keep the centrifugal load at a low level, the balls of angular contact ball bearings should have a smaller mass. A smaller mass can be obtained by using smaller balls or by making balls with lower density materials or by combining the two methods. By reducing the diameter of the ball to 75% (actually), the mass will be reduced to 42%.
Bearing 7014 CD with a ball diameter of 12.7 mm runs at 15,000 r/min. Thus, the centrifugal load of each ball is 174N. If the ball diameter is reduced to 7,938 mm (62.5%), the centrifugal load of each ball is 79 N, which is reduced to 45%.
When designing the new series of 719 CE and 70 CE SKF angular contact ball bearings, the advantages of using small balls have been taken into consideration. Figure 2 shows the cross-section comparison of angular contact ball bearings with "standard size" balls, 70 CD series and the new "small balls" 70 CE series.
Ceramic materials for rolling elements
By using a suitable material with a lower density than bearing steel, the same advantages as using smaller steel balls can be obtained. In fact, there are other advantages. Silicon nitride is a material with interesting properties that can meet demanding tasks in bearings. Silicon nitride is a ceramic material with a chemical formula of Si3N4. Due to technical and economic reasons, silicon nitride is the most commonly used ceramic material for rolling bearings today.
Bearings using only ceramic rolling elements are mainly used for high-speed applications.
The typical characteristics of silicon nitride are:
Low density, 40% of steel
Small thermal expansion, only 29% of steel
High elastic modulus, 151% of steel
Very high hardness, HV10 at 20°C is 1,700, while bearing steel is HV10
Low friction
Withstand high temperatures without losing hardness
Chemical resistant
Electrical insulator
These properties provide many performance advantages and in many cases will make up for the higher initial cost of hybrid bearings compared to all steel bearings.
Rolling elements made of silicon nitride will bear considerably lighter centrifugal loads than rolling elements made of steel of the same size.
Example 2
Observe the same bearing as the previous example. Its ball diameter is 12.7 mm and it runs at 15,000 r/min. The centrifugal load of the ball made of silicon nitride balls will be 71N. In comparison, the load is reduced by about 40% with the steel ball design. Obviously, the centrifugal load is lighter than the load when using smaller size steel balls. It can then be expected that the ceramic balls will have approximately the same dynamic properties as the smaller steel balls previously used.
In order to further reduce the mass of the ball, the ceramic ball can be made into a smaller diameter. Then naturally choose the same diameter as the smaller diameter steel ball previously used. With the same bearing size and speed as the previous example, the centrifugal load of the ball will now be 32 N, which is reduced to 18% of the original value of 174 N.
Actual tests have confirmed that the use of lighter balls (smaller steel balls or ceramic balls or smaller ceramic balls) can increase the speed and reduce the operating temperature.
Actual tests have confirmed that the use of lighter balls (smaller steel balls or ceramic balls or smaller ceramic balls) can increase the speed and reduce the operating temperature. The temperature rise of a spindle equipped with the same basic bearing configuration. There are two bearings in series on each side of the spindle and lightly loaded.
The high speed bearings needs higher performance in speed and high temperature resistance.

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