Bearings – Rotary bearing – Antifriction bearing
Reexamination Certificate
2000-08-10
2002-11-19
Hannon, Thomas R. (Department: 3682)
Bearings
Rotary bearing
Antifriction bearing
C384S450000, C384S492000
Reexamination Certificate
active
06481898
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
The present invention relates to a ball bearing used for various types of machine tools.
In recent years, speed of main spindles in machine tools has been increased for the improvement of working efficiency and productivity and, correspondingly, the number of rotation of ball bearings for use in machine tools has also been increased.
In general, when the ball bearing rotates at a high speed, large sliding is caused at the contact point between a ball and a raceway surface by spin or gyro movement. Further, the clearance in the bearing is reduced under the effect of centrifugal force exerting on an inner ring or a ball and a contact pressure between the ball and the raceway surface increases, to increase the heat value which leads to various disadvantages such as temperature elevation or seizing and progress of excessive wearing in the ball bearing.
As a countermeasure for suppressing the disadvantages, it has been adopted to decrease the diameter of the ball or to make the material of the ball with light weight ceramics to thereby moderate the increase of the surface pressure due to the centrifugal force of the ball. Further, it has also been disclosed a method of suppressing the change of the clearance in the bearing by making the material of the inner ring with a material of lower linear expansion coefficient than the material for the outer ring (Japanese Unexamined Patent Publication No. 62-24025).
When a ball bearing applied with such a countermeasure is used for a main spindle for use in machine tools, an operation is possible up to maximum dmn value of 1,500,000 to 2,000,000 showing the limiting speed. However, only by individual countermeasures in the prior art such as decrease of the ball diameter or use of ceramic material for the ball the limit for the dmn value is up to 2,000,000, and seizing or wear due to increase of the contact pressure occurs if the value is exceeded.
Particularly, it is difficult to increase the speed under slight amount oil lubrication such as grease lubrication, oil air lubrication or oil mist lubrication used frequently in bearings for use in machine tools, in which seizing caused by breakage of the oil films and increase of the surface pressure in the bearing becomes remarkable.
As the counter measure, the effect of temperature elevation can not but be prevented by increasing the amount of lubrication oil or changing the preload system from a constant position preloading system tending to increase the contact pressure due to the temperature elevation to a constant pressure preloading system in which the contact pressure is kept substantially constant free from the effect of the temperature change but this inevitably increases the cost.
The present invention has been accomplished for overcoming such disadvantages in the prior art and it is an object thereof to provide a ball bearing of a reduced cost, generating less heat and having long life, capable of adopting a slight amount oil lubrication system and a constant position preloading system which is advantageous for reduction of the cost and capable of attaining high seizing resistance and wear resistance even at high speed rotation of dmn value of 2000,000 or higher.
The ball bearing of the present invention includes balls disposed so as to roll between an inner ring and an outer ring in which a groove of an arcuate cross section is formed in a circumferential direction to each of the surfaces of the inner ring and the outer ring in contact with the balls, wherein the arcuate groove of the inner ring is formed such that the radius of curvature thereof is within a range of 52.5% or more and 60.0% or less of the diameter of the ball, and the arcuate groove of the outer ring is formed such that the radius of curvature thereof is within a range of 50.5% or more and 53.0% or less of the diameter of the ball.
When the radius of curvature of the groove in the inner ring is defined as within a range from 52.5 to 60.0% and the radius of curvature of the groove in the outer ring is defined as within a range from 50.5 to 53.0%, the heat value in the inner ring can be decreased and the heat value in the outer ring can be made closer to that of the inner ring and, thus, the heat value in the entire ball bearing can be suppressed. Therefore, seizing of the ball bearing in a high speed rotational region can be prevented without adopting a large flow rate lubrication method or a preloading method such as constant pressure preloading.
In this case, the radius of curvature for the inner ring is preferably formed within a range of 52.5% or more and 57.0% or less of the ball diameter. This can reliably suppress the heat generation in the inner ring and the heat value in the entire ball bearing can be suppressed more reliably.
Further, the invention provides a ball bearing wherein the diameter of the ball is defined as 0.32 to 0.60 times the value for: (outer diameter of outer ring—inner diameter of inner ring)/2.
Assuming the ball diameter/{(outer diameter of outer ring—inner diameter of inner ring)/2}=&agr;, decrease of value &agr; means that the ball diameter is decreased relative to the radial cross section of the ball bearing, and this has a close concern with the heat value in the ball bearing. When the value &agr; is controlled within a range from 0.32 to 0.60, the heat value in the ball bearing can be suppressed. If &agr; is less than 0.32, the ball diameter is excessively small making it necessary to decrease the wall thickness of a cage and, accordingly, the collision force and the hoop stress exerting from the ball to the cage are increased during high speed rotation, making it difficult for the design of the case capable of withstanding them. Further since the ball diameter is small, the contact area with the groove of the outer and inner rings is excessively small to increase the contact pressure and thus increase the heat value, which causes oil film breakage or the like leading to deterioration of the bearing life. On the other hand, if &agr; exceeds 0.60, the ball diameter is excessively large making the centrifugal force exerted on the ball excessively large during high speed rotation and, particularly, surface pressure increase with respective to the outer ring groove to result in lowering of the life, as well as spin sliding increases at the contact point between the ball and the raceway groove and, accordingly, the heat value increases to reduce the bearing clearance, increase the surface pressure and result in oil film breakage to cause seizing and deteriorate the life. Accordingly, the allowable range for the diameter of the ball is defined within a range of &agr; from 0.32 to 0.60 at the maximum in the present invention.
Then, in a more preferred embodiment, the range is further narrowed such that the ball diameter is from 0.35 &agr; to 0.55 &agr; and, further, the ball diameter is from 0.39 &agr; to 0.48 &agr;.
Further, the invention to a restriction for the number of balls and provides a ball bearing of the invention wherein the number of balls satisfies a relation: 1.1≦{pitch circle diameter×sin (180
umber of balls)}/ball diameter≦2.0
Assuming {pitch circle diameter×sin (180
umber of balls)}/ball diameter=&bgr;, the value &bgr; is from 1.1 to 2.0.
Decrease of value &bgr; means that the number of balls is increased in view of the design since the ball pitch space is decreased on a pitch circle diameter of a ball bearing. If &bgr; is less than 1.1, since the number of balls increases, the circumferential width for the bar of the cage has to be narrowed and, as a result, impact from the ball increases more as the rotational speed is higher tending to break the bars of the cage. Further, increase in the number of balls increases the stirring resistance of the lubricant in the bearing to increase the heat value. On the other hand, the value &bgr; in excess of 2.0 corresponds to excessive decrease for the number of balls and, since the load on the ball increases to increase the contact
Aramaki Hirotoshi
Fujii Akio
Kinno Dai
Noguchi Shoji
Ooura Yukio
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