Preload control device of magnetic bearing

The present invention provides a preload control device of a magnetic bearing, which includes a main shaft, a housing, a first magnetic bearing mechanism, a second magnetic bearing mechanism, a pressing mechanism, and a sliding member. The main shaft is rotatably formed in the shape of a rod that is longitudinally long. The housing covers and is fixed outside the main shaft. The first magnetic bearing mechanism is disposed between the housing and the main shaft. The second magnetic bearing mechanism is spaced from the first magnetic bearing mechanism in the axial direction of the main shaft, between the housing and the main shaft. The pressing mechanism is disposed between the second magnetic bearing mechanism and the frusto-conical member. The sliding member is disposed between the pressing mechanism and the second magnetic bearing mechanism.

The present application is a National Stage of PCT/KR2011/002657, filed Apr. 14, 2011, which claims the benefit of Korean Patent Application No. 10-2010-0125503 filed Dec. 9, 2010. The disclosures of those applications are hereby incorporated in their entirety by reference as if fully set forth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a preload control device of a magnetic bearing, and more particularly, to a preload control device of a magnetic bearing that can automatically apply variable preload to a bearing while a main shaft rotates, by means of a pressing mechanism that circumferentially presses a sliding member due to a centrifugal force.

2. Description of the Related Art

It is required to apply appropriate preload to a ball bearing that supports a main shaft to be rotatable in a housing of machine tools. This is because, in one machine too, low-speed rough machining and high-speed finish machine are performed, large preload is needed to prevent vibration and increase rigidity of a spindle in the low-speed rough machining, and low preload is needed to avoid excessive heat generation due to friction instead of large rigidity in high-speed machining.

In general, switching preloading, multi-step regular position preloading, and automatic variable preloading have been known as a method of controlling preload of a bearing. The switching preloading is a method that changes preload from a regular position to constant pressure when the number of revolutions of a main shaft changes over a predetermined number of revolutions, which has a defect that the rigidity of the main shaft reduces at a middle speed after the preload is changed to the constant pressure. The multi-step preloading is difficult in designing because it is required to sufficiently consider deformation between bearing and deformation of a preloading mechanism in order to accurately set the preload. The automatic variable preloading makes it possible to control preload, using a centrifugal force due to rotation of a main shaft and pressure of a spring. Accordingly, it is possible to control preload without a complicated mechanical configuration or an electric device.

However, it is not enough to use only the elastic force of a spring in order to control preload against a centrifugal force over the elastic force of the spring, such that it is difficult to cope with a wide range of change in rotation speed of a main shaft.

SUMMARY OF THE INVENTION

The present invention relates to a preload control device of a magnetic bearing and an object of the present invention is to provide a preload control device of a magnetic bearing which can automatically apply variable preload to a bearing, using a pressing mechanism circumferentially pressing a sliding member by means of a centrifugal force, when a main shaft rotates.

The present invention provides a preload control device of a magnetic bearing, which includes: a main shaft rotatably formed in the shape of a rod that is longitudinally long and having a frusto-conical member fitted on the outer circumference; a housing covering and fixed outside the main shaft; a first magnetic bearing mechanism disposed between the housing and the main shaft and including first magnetic members allowing the main shaft to rotate at a distance in the housing; a second magnetic bearing mechanism spaced from the first magnetic bearing mechanism in the axial direction of the main shaft, between the housing and the main shaft, including a second magnetic member at a side to allow the main shaft to rotate at a distance in the housing, and providing early preload by sliding toward a frusto-conical member due to a repulsive force generated by the second magnetic member; a pressing mechanism disposed between the second magnetic bearing mechanism and the frusto-conical member, fitted on the outer circumferential surface of the frusto-conical member, and pressed in the circumferential direction of the main shaft by a centrifugal force due to a rotational force of the main shaft; and a sliding member disposed between the pressing mechanism and the second magnetic bearing mechanism and sliding the second magnetic bearing mechanism toward the first magnetic bearing mechanism by means of pressure generated by the pressing mechanism.

The pressing mechanism may include balls coming in contact with the frusto-conical member and the sliding member, and cages retaining the balls to prevent the balls from separating from the frusto-conical member, in which a gap may be formed to allow the ball to be moved in the case by a centrifugal force.

The sliding member may have an inclined surface corresponding to the frusto-conical member so that the second magnetic bearing mechanism is slid toward the first magnetic bearing mechanism by a centrifugal force applied to the balls.

The balls may be returned to the initial position by a repulsive force between the first magnetic bearing mechanism and the second magnetic bearing mechanism, when the centrifugal force applied to the balls reduces with a decrease in rotational force of the main shaft.

The first magnetic bearing mechanism may include a first thrust magnet levitated by a magnetic force between the pair of first magnetic members fixed at a distance from each other on the inner circumferential surface of the housing, and a first radial magnet inserted in the main shaft, at the position corresponding to the first thrust magnet, and the second magnetic bearing mechanism may include a second thrust magnet levitated by a magnetic force toward the frusto-conical member from a second magnetic member disposed on the inner circumferential surface of the housing, close to the first magnetic bearing mechanism, and a second radial magnet inserted in the main shaft, at the position corresponding to the second thrust magnet.

The first and second radial magnets and the first and second thrust magnets may be disposed to be levitated by a magnetic force, respectively.

According to the preload control device of a magnetic bearing of the present invention, since variable preload can be automatically applied to a bearing by a centrifugal force at a pressing mechanism which corresponds to a rotational speed of a main shaft, it is possible to cope with a wide variation of a rotational speed of a main shaft of machine tools, with a simple configuration and a low cost, and precise response is possible.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, exemplary embodiments of the present invention will be described with reference to the accompanying drawings. Prior to this, the terminologies or terms used in the specification and claims should not be construed as being limited to common or dictionary meanings, but be construed as meanings and conceptions that coincide with the spirit of the present invention, on the basis of the basic principle that the inventor(s) may appropriately define the conceptions of the terminologies to explain his/her (their) own invention in the best way.

FIG. 1is a cross-sectional view showing a preload control device of a magnetic bearing100according to an embodiment of the present invention andFIG. 2is a partial enlarged view of the preload control device of a magnetic bearing100shown inFIG. 1.

Referring toFIGS. 1 and 2, the preload control device of a magnetic bearing100includes: a rotatable main shaft110; a housing120covering and fixed outside the main shaft110; a first magnetic bearing mechanism130supporting rotation of the main shaft110; a second magnetic bearing mechanism140spaced from the first magnetic bearing mechanism130in the axial direction of the main shaft110; a pressing mechanism150applying a sliding pressure to the second magnetic bearing mechanism140; and a sliding member160carrying a centrifugal force generated by the pressing mechanism150to the second magnetic bearing mechanism140.

The main shaft110is formed in the shape of a rod that is longitudinally long and has a frusto-conical member111fitted on the outer circumference. The frusto-conical member111is formed such that the diameter gradually decreases toward the second magnetic bearing mechanism140. Accordingly, the frusto-conical member111increases in diameter from the side facing the second magnetic bearing mechanism140to the opposite side, protruding from the outer circumferential surface of the main shaft110.

The first magnetic bearing mechanism130includes a pair of first members131spaced from each other on the inner circumferential surface of the housing120, a first thrust magnet132levitated by a magnetic force between the pair of first magnetic members131, and a first radial magnet133inserted in the main shaft, at the position corresponding to the first thrust magnet132.

Since the first thrust magnet132is disposed at a predetermined distance between the first magnetic members131and the first radial magnet133is spaced from the first thrust magnet132by a repulsive force, the first magnetic bearing mechanism130has a function of supporting the main shaft110such that the main shaft110can rotate.

In this configuration, the first magnetic bearing mechanism130may be implemented by a magnetic member such as a permanent magnet or an electromagnet. Accordingly, friction resistance is never generated and thus a friction coefficient and noise can be absolutely reduced.

The second magnetic bearing mechanism140includes a second magnetic member141disposed on the inner circumferential surface of the housing120, close to the first magnetic bearing mechanism130, a second thrust magnet142levitated by a magnetic force toward the frusto-conical member111from the second magnetic member141, and a second radial magnet143inserted in the main shaft110, at the position corresponding to the second thrust magnet142. In this configuration, the second thrust magnet142can slide by a distance Δd from the outer circumferential surface of the second radial magnet143.

A repulsive force is generated between the second magnetic member141and the second thrust magnet142and the second radial magnet143is spaced from the second thrust magnet142by the repulsive force and has a function of supporting the main shaft110such that the main shaft can rotate. In this configuration, the second magnetic bearing mechanism140may be implemented by a magnetic member such as a permanent magnet or an electromagnet. Accordingly, friction resistance is never generated and thus a friction coefficient and noise can be absolutely reduced.

The second magnetic member141is disposed at a side of the second thrust magnet142and the sliding member160at the other side. The sliding member160slides with the second magnetic bearing mechanism140. The sliding member160provides early preload to the pressing mechanism150by means of the repulsive force between the second magnetic member141and the second thrust magnet142and is slid with the second thrust magnet142in the direction in which the early preload applied to the main shaft110reduces, by a centrifugal force applied to the pressing mechanism150due to a rotational force of the main shaft110.

The sliding member160has an inclined surface161corresponding to the angle of the frusto-conical member111and the pressing mechanism150is disposed between the inclined surface161and the frusto-conical member111.

The pressing mechanism150includes balls151that come in contact with the frusto-conical member111and the sliding member160and cages155retaining the balls151on the frusto-conical member111.

The balls151are metal balls and preferably made of metal having a relatively large specific gravity.

The cages155are arranged along the outer circumferential surface of the frusto-conical member111and rotate with the main shaft110, when the main shaft110rotates. Obviously, the cages155may be disposed in the inclination direction of the frusto-conical member111.

The cage155may be formed such that the ball151can protrude from the top and the bottom of the cage155, but it prevents the ball151from separating. Accordingly, a hole156in which the ball151can move is formed through the cage155.

The hole156is formed such that the vertical diameter of the cage155is smaller than the ball151and the maximum diameter at the center of the hole156is large than the diameter of the ball151, and accordingly, a gap that allows the ball151can move in the hole156in the circumferential or central direction of the main shaft110is defined.

Further, the hole156may be formed through the cage155in the direction of the centrifugal force applied to the ball151, that is, in the circumferential direction of the main shaft110, corresponding to the inclination angle of the frusto-conical member111.

Therefore, when a centrifugal force is applied to the ball151with rotation of the pressing mechanism150, pressure is transmitted to the inclined surface161, and the sliding member160and the second thrust magnet142slide toward the first magnet bearing mechanism130.

Hereinafter, a method of operating the preload control device of a magnetic bearing100according to an embodiment of the present invention is described with reference to the accompanying drawings.

FIGS. 3 and 4are cross-sectional views showing the preload control device of a magnetic bearing100shown inFIG. 1which is in operation.

Referring toFIGS. 3 and 4, with the main shaft110stopped, gaps are generated by the repulsive force between the first thrust magnet131and the first radial magnet132and between the second thrust magnet141and the second radial magnet142. The repulsive force between the second magnetic member141and the second thrust magnet142presses the sliding member160and thus early preload is applied to the pressing mechanism150.

When the main shaft110rotates at a low speed, a centrifugal force is applied to the balls151by the rotational force of the main shaft110and the balls151move to the inclined surface161, in the holes156, and as the rotational force of the main shaft110gradually increases, the force of the balls151which pushes the inclined surface161increases.

Therefore, while the main shaft110rotates at a low speed, the rotation of the main shaft110can be made stable by the early preload due to the repulsive force between the second magnetic member141and the second thrust magnet142.

When the main shaft110rotates at a speed over a predetermined RPM, the inclined surface161of the sliding member160is pushed by the centrifugal force applied to the balls161, such that the sliding member160and the second thrust magnet142slide by the distance Δd toward the first magnetic member131, and accordingly, the preload applied to the pressing mechanism150reduces.

Therefore, it is possible to reduce a friction force at the pressing mechanism150by reducing the preload at the second magnetic bearing mechanism140by means of the centrifugal force applied to the balls151while the main shaft110rotates at a high speed, and accordingly, it is possible to reduce maintenance cost and time.

When the centrifugal force applied to the balls151reduces with a decrease in rotational force of the main shaft110, the second thrust magnet142and the sliding member160is returned to the initial positions by the repulsive force between the second magnetic member141and the second thrust magnet142.

Therefore, the preload control device of a magnetic bearing100according to an embodiment of the present invention can automatically control preload at the second magnetic bearing mechanism140by means of a centrifugal force generated at the pressing mechanism150.

Although the present invention was described with reference to the embodiment shown in the drawings, but it is just an example and those skilled in the art would understand that various modifications and equivalents may be implemented from the present invention. Therefore, the protective range of the present invention should be determined by the spirit described in claims.

According to the preload control device of a magnetic bearing of the present invention, since the variable preload can be automatically applied to the bearing by the centrifugal force at the pressing mechanism which corresponds to the rotational speed of the main shaft, it is possible to cope with a wide range of rotational speed of a main shaft for machine tools, and the present invention can be used for the main shaft for machine tools in various fields by precise response.