Bearings – Rotary bearing – Antifriction bearing
Reexamination Certificate
2001-11-21
2003-12-02
Hannon, Thomas R. (Department: 3682)
Bearings
Rotary bearing
Antifriction bearing
C384S504000
Reexamination Certificate
active
06655846
ABSTRACT:
The invention concerns a ball bearing and a bearing assembly, intended especially for high-speed devices such as dental turbines, dental angle handpieces, turbomolecular pumps, etc.
Up to now, high-speed devices such as dental turbines, dental angle handpieces, turbomolecular pumps, etc., have used high-precision ball bearings which are mounted on a shaft with the drive element between them. These bearings or bearing assemblies are installed with a fixed prestress which is applied by metal or elastomer spring elements. The action of the prestress makes the ball bearings free from play.
The prestress of the bearing causes a track offset. This track offset is achieved, for example in a double-row ball bearing, by giving the outer and inner rings different axial track separations. In bearing assemblies with two similar single-row ball bearings, the track offset is achieved especially by means of a corresponding shape of the ball bearing rings or by means of mechanical adjustment of the ball bearing rings and/or by means of spring elastic elements whose force flows through the balls and is received from the support, so that no resultant outward-acting force is produced.
However, such ball bearings or bearing assemblies have a disadvantage, which is that installation which puts a fixed prestress on the support produces a moment of friction in the bearing which acts even at the smallest speeds, that is in the startup range, and has a negative effect on it. Deviations in axial geometry of the bearing seat and/or imprecise assembly can significantly change the prestress force and thus can cause the bearing to have an excessively large or small prestress.
OBJECTS AND ADVANTAGES OF THE INVENTION
An object of the invention is to provide a ball bearing or a bearing assembly intended especially for high-speed devices such as dental turbines, dental angle handpieces, turbomolecular pumps, etc., which has substantially better anti-friction properties in rotating operation than conventional ones do, especially during a startup phase, while simultaneously preventing the ball bearing or the bearing assembly from being overloaded by the prestress or due to deviations in the geometry and or imprecision in assembly.
This object is solved, in a ball bearing and a bearing assembly of the type mentioned at the beginning, by the characterizing features of claims
1
and
12
.
The measures mentioned in the subordinate claims make possible advantageous embodiments and further developments of the invention.
Thus, a ball bearing according to the invention is characterized in that at least one device is provided to produce a tension force that is speed-dependent.
A device according to the invention can be used, for example, to produce a ball bearing which has tensions of different magnitudes for different operating states. In rotating operation the ball bearing can have an adjustable tension force which changes continuously or incrementally, as a function of specified speed ranges. In particular this makes the ball bearing run in a more uniform manner.
It is advantageous to improve the startup behavior, that is, to shorten the startup phase, to provide a comparatively small tension force during the startup phase, so that a comparatively small drive force is needed. Among other things, this is advantageous for pneumatic or hydraulic drives such as are used for dental turbines, for example. The tension force of the required magnitude is achieved when operating speed is reached.
A defined axial displacement of the two rings relative to one another, i.e. axial play of the bearing is advantageously provided in the installed idle state. Establishing a defined axial play in the idle state represents a departure from the high-precision ball bearings developed up to now. However, the axial play makes possible a very small moment of friction at startup, which in turn makes it possible to reach operating speed especially quickly.
By contrast, a comparatively large tension on the ball bearing is also conceivable, especially to shorten a stopping phase of the bearing. Moreover, it can be advantageous to specify an appropriately large tension force in the idle state to hold the ball bearing.
It is advantageous for a maximum tension force to be provided when the maximum operating speed is reached. This makes the bearing comparatively stiff, especially at the beginning of a load. The comparatively large tension force simultaneously prevents, to the maximum possible extent, tilting of the outer ring, which produces an advantageous reduction in bearing noise. It is also possible to change the tension force as a function of the load.
It is preferable for the device according to the invention to be implemented by means of at least one tension element which can be tensioned to different extents, several connectable springs, etc. In particular, to determine the speed a sensor can be provided, e.g. an eddy current sensor, an optical sensor, etc.
In an advantageous further development of the invention an actuator is provided, especially a centrifugal governor, for centrifugally dependent open-loop or closed-loop control. This ensures that an advantageous continuous adjustment or control of the tension force to changing operating speeds can be achieved. This might also make it possible to do without other sensors.
In a particular embodiment of the invention, the device comprises at least part of the balls in the bearing. Thus, the balls rotating in the bearing can be used not only as rolling bodies, but also as a sensor and actuator element, due to the speed-dependent centrifugal forces acting on the balls. This means that the balls together with their tracks form a centrifugal governor and can simultaneously serve as tension elements, which makes it possible to implement an advantageous self-regulating and at the same time self-reinforcing tensioning of the ball bearing with comparatively small expense.
It is advantageous for a ball material to have a density greater than 5 g/cm
3
. It is known in the art that the centrifugal force of a body is especially dependent on the density. Using a ball material with a density greater than 5 g/cm
3
produces, according to the invention, a comparatively large centrifugal force per ball. The centrifugal force can also be changed by advantageous adjustment of the ball diameter.
Here the tension force of the ball bearing at operating speed depends on the sum of the individual centrifugal forces. Performing numerous elaborate experiments made it possible to establish that especially a density greater than 5 g/cm
3
in corresponding high-speed ball bearings, such as, e.g., in miniature ball bearings for dental turbines, etc., makes it possible to produce prestress forces which substantially improve the stiffness of the support.
It is preferable for at least one ball in a bearing to consist essentially of steel which has a density of 7.7 g/cm
3
. This has the advantage of using common ball bearing materials, which makes it possible to implement an embodiment of the device according to the invention that is especially economical.
In an advantageous further development of the invention, at least one ball consists essentially of zirconiumdioxide. Zirconiumdioxide has a density of about 6 g/cm
3
, so that corresponding balls also can be provided to produce a tension force according to the invention.
In a special embodiment of the invention, each bearing row has at least one zirconiumdioxide ball, with the other balls in the bearing row consisting of steel. This combination, or a comparable combination of zirconiumdioxide balls and steel balls produces all different kinds of advantages in the operation of the bearing, due to the comparable physical properties of the materials zirconiumdioxide and steel. For example, zirconiumdioxide and steel have comparable moduli of elasticity and almost equal coefficients of linear expansion. Thus, outside and inside rings can be assembled into ball bearings with steel or ZrO
2
balls, without changing the track radii. The almost equal coefficients of linear expan
Beckers Johannes
Engler Martin
Foehr Edmund
Gaile Otto
Moesle Peter
Breneman & Georges
MKL-Miniaturkugellager GmbH
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