Device for limiting a whirl speed of a shaft

Abstract: Device (1) for limiting a whirl speed of a shaft (5) after failure of a magnetic bearing. The device (1) is provided with a surface (7) extending essentially radially from the shaft (5) and a machine element (8) is provided with a projecting element (9) extending from the machine element (8) in direction towards the surface (7), the surface (7) being provided with a circular element (10) concentrical with the axis of the shaft (5). Characteristics are that the device (1) comprises a disk-like portion provided with an outer perimeter surface (2) and a hole (3). One of the perimeter surfaces (2, 4) is designed with a varying distance to the rotational center of the portion. The device (1) is arranged between the shaft (5) and the circular element (10). The circular element (10) and the projecting element (9) is positioned in a common plane perpendicular to the shaft (5), and the device (1) operates as a whirl speed limiting mechanism causing a contribution to the whirl velocity at impact between any of the projecting element (9), the device (1), the circular element (10) and the shaft (5).

Description: FIELD OF INVENTION The present invention relates to a device for limiting a whirl speed of a shaft after failure of a magnetic bearing, after which failure the shaft is supported by at least one auxiliary bearing. BACKGROUND OF THE INVENTION An object of an auxiliary bearing is to support a shaft during failure of an associated magnetic bearing. When a magnetic bearing fails, the rotating shaft comes in contact with the inside of the inner ring of the auxiliary bearing, which leads to an acceleration of the inner ring of the bearing. These contacts affects the shaft resulting in the frictional forces. The shaft starts to whirl in the clearence between the shaft and the inner ring of the auxiliary bearing. The friction forces are directed to increase the whirling speed until the auxiliary bearing has accelerated almost to the speed of the rotating shaft. The resulting whirl speed causes a centrifugal force which can be 30-50 times the force of a synchronous whirl equal to the shaft speed would have caused. The limited contact area and cooling area often also cause temperature problems. In EP-0 499 310 A1 a magnetic bearing-shaft assembly having a bearing to support the shaft in the event of failure of the magnetic bearing is presented. If the bearing portion of the back-up bearing comprises a ball race, the radially outer raceway thereof is secured to an anchorage, such as a housing for the assembly. A low stiffness damper member in the form of a annular shaped body of resilient material is also included in the back-up bearing, between the bearing portion thereof and the anchorage. Advantageously, when the magnetic bearing has failed, and the shaft is being brought to rest by being in contact with the back-up bearing, the resilient body is in shear, and the body has a lower stiffness than otherwise would be the case. Thus, any tendency for the back-up bearing, and/or the anchorage, to be damaged whilst the shaft is being brought to rest, is reduced, because the provision of the low stiffness damper causes the shaft to have a low natural frequency of oscillation. It should be noted that a crucial part of the assembly is constituted by the stiffness damper member. THE TECHNICAL PROBLEM The present invention deals with the problem of limiting a whirl movement of a rotatory shaft after a failure of a magnetic bearing and during the deceleration of the rotatory shaft. The problem with whirl is mainly accentuated in the case of a vertical shaft. THE PURPOSE OF THE INVENTION AND ITS MOST CHARACTERIZING PARTS A primary object of the present invention is to offer a device which decreases the whirling motion of a shaft during a failure of a magnetic bearing. Characteristics of the present invention are the device comprising a disk-like portion provided with an outer perimeter surface and a hole with an inner perimeter surface, at least one of the perimeter surfaces being designed with a varying distance in circumferential direction to the rotational center of the portion, the device being arranged between the shaft and the circular element, the circular element and the projecting element being positioned in a common plane perpendicular to an axis of the shaft, and the device operating as a whirl speed limiting mechanism causing a contribution to the whirl velocity at impact between any of the projecting element, the device, the circular element and the shaft. DESCRIPTION OF THE DRAWINGS Figure 1 shows an embodiment of a device according to the present invention, in which the device consists of a disk-like portion provided with an outer perimeter surface and a hole with an inner perimeter surface. Figure 2 shows an embodiment of a device according to the present invention in its operative environment. Figure 3 shows an embodiment of a device according to the present invention, comprising an asymmetric ring. Figure 4 shows an embodiment of a device according to the present invention in its operative environment, in which interacting surfaces are provided with conical surfaces. DESCRIPTION OF PREFERRED EMBODIMENTS Figure 1 shows a preferred embodiment of a device 1 according to the present invention. The device 1 consists of a disk-like portion provided with an outer perimeter surface 2 and a hole 3 with an inner perimeter surface 4. The disk-like portion can be arranged in different locations in order to accomplish the purpose of the present invention. One of these ways is presented in figure 2, in which a device 1 for limiting a whirl speed of a shaft 5 after failure of a magnetic bearing is presented. After such a failure the shaft 5 is normally supported by at least one auxiliary bearing 6. The present invention is primarily intended to deal with vertical shafts, but the scope of the appended claims encompasses arbitrary shaft arrangements. The shaft 5 is provided with a surface 7 extending essentially radially from the shaft 5 and a machine element 8 is provided with a projecting element 9 extending from the machine element 8 in direction towards the surface 7. The surface 7 is provided with at least one circular element 10 concentrical with the axis of the shaft 5 on the side facing the projecting element 9. Further, the device 1 comprises a disk-like portion as described in figure 1. At least one of the perimeter surfaces 2, 4 is designed with a varying distance in circumferential direction to the rotational center of the portion. The device 1 is arranged between the shaft 5 and the circular element 10 and the circular element 10 and the projecting element 9 is positioned in a common plane perpendicular to an axis of the shaft 5. The device 1 is arranged to operate as a whirl speed limiting mechanism causing a contribution to the whirl velocity at impact between any of the projecting element 9, the device 1, the circular element 10 and the shaft 5. In this preferred embodiment, the whirl speed is given a contribution when the projecting element 9 comes in contact with either the shaft 5 or the device 1. Thus, it is crucial to the function of this embodiment, that the projecting element 9 is arranged to be able to come in contact with the shaft 5 and the device 1. The present invention is based on the two possibilities of obtaining contact between the rotating parts (e.g. shaft 5) and the initially non rotating parts (e.g. projecting element 9). One of these possibilities results in a decreased whirl velocity and the other in an increased whirl velocity. The purpose of the present invention is obtained by the sequence of consecutive contacts of the above described type. As mentioned above, there are different locations possible for arranging the device 1 in its operative environment. It should be noted that it is possible to arrange a device 1 on one or more of these locations. The device 1 can be fixedly arranged to one of the surfaces either between an outer circular element 10 and the outside of the projecting element 9, or between the shaft 5 and the inside of the projecting element 9. In this first embodiment, the device 1 is arranged to the inside of a circular element 10 and the machine element 8 is constituted by the inner ring of the auxiliary bearing 6. As the shaft whirls, collisions appear randomly either between the inside of the device 1 and the outside of the projecting element 9, or between the shaft 5 and the inside of the projecting element 9. It should be noted that the projecting element 9 may be integrally manufactured with the inner ring of the auxiliary bearing 6. If the shaft 5 comes in contact with the inside of the projecting element 9 then there will be a negative contribution to the whirl velocity. In the other case, there will be a positive contribution to the whirl velocity. Thus, the whirl speed is limited. The whirl speed is the modulus of the whirl velocity. It should be noted that it is possible to use a plurality of devices 1 on several of the participating parts which are relevant for obtaining the purpose of the invention instead of one, since a key feature of the present invention is related to the asymmetrical relation between the parts. It is also possible that the projecting element 9 is arranged to engage with a free end of a shaft 5. In such a case, the projecting element 9 is arranged to engage between two concentrical circular elements 10, between which a device 1 according to the present invention is preferably arranged. Alternatively, the device 1 may be arranged on the projecting element 9. It should be noted that the ring-like portion can be reduced in its circumferential extent to at least two angular segments suitably arranged in circumferential direction. The object of the segments, which may resemble a fork, is to act as a mechanism for contributing to the whirl velocity. In the case of two angular segments, they are preferably arranged 90 degrees apart in angular direction. In figure 3, a second preferred embodiment of the present invention is presented. The means causing the desired eccentricity of the device 1 incorporates an asymmetric ring produced from a ring member which has been severed at two points, whereupon the severed ends have been displaced tangentially to form a space between them, and separate members 12 being fixedly connected between the severed ends. Needless to say, the ring member can be severed at a plurality of points. An alternative way of realizing the present invention is to incorporate a ring, which is eccentrically arranged in relation to the center of rotation. Another way of accomplishing the effect is to use an ellipse. A difference between the embodiments presented in figures 3 and 4 is that the embodiment presented in figure 3 is designed for two contacts per revolution for the purpose of faster limiting of the whirl compared to only one in the embodiment presented in figure 2. Also, it is possible to design the contacting parts so that there will be even more contacts per revolution. In order to limit the movement in axial direction, the projecting element 9 is arranged to be able to contact the surface 7. A contact between the surface 7 and the projecting element 9 limits the axial movement. Also, in order to prolong the operation of the bearing, at least one of the surfaces of the projecting element 9, the device 1, the circular element 10, the surface 7 and the shaft 5 are arranged with friction decreasing means, the purpose of which is to increase the service life by decreasing wear and friction and to extend the bearing speed-up time and thereby reduce the peak temperature and bearing clearing changes. Due to the asymmetrical features of the present invention as described in figure 2, the arrangement needs to be properly balanced. Balance can be achieved in a plurality of ways. One way is to provide balancing means on the radial outside of the circular element 10. Another way could be to arrange holes in the element 10. Several balancing options relevant in this context are available to the man skilled in the art. The present invention can be integrally manufactured in different ways. Examples are that the shaft 5 and the member to which the surface 7 is arranged are integrally manufactured. The asymmetric ring 11 can be integrally manufactured to the surface 7 and the circular element 10 and the device 1 can be integrally manufactured. Also, the device 1 is integrally manufactured to one of the radial surfaces of the projecting element 9, and the device 1 is integrally manufactured to the shaft 5. In figure 4, a third embodiment is disclosed. Since there are similarities between the first embodiment presented in figure 2 and this one, only differences will discussed. In order to be able to support the shaft 5 in axial direction after failure of the magnetic bearing, the interacting surfaces of the projecting element 9, the device 1, the circular element 10 or the shaft 5 are arranged with mating portions 13. The purpose of the these portions 13 is to act as a movement limiting mechanism in one axial direction. However, it is possible to arrange mating portions so that the movement limiting mechanism operates in both directions by introducing concave/convex mating portions.

Claims: Device (1) for limiting a whirl speed of a shaft (5) after failure of a magnetic bearing, after which failure the shaft (5) is supported by at least one auxiliary bearing, the shaft (5) being provided with a surface (7) extending essentially radially from the shaft (5), a machine element (8) being provided with a projecting element (9) extending from the machine element (8) in direction towards the surface (7), the surface (7) being provided with at least one circular element (10) concentrical with the axis of the shaft (5) on the side facing the projecting element (9), characterized by, the device (1) comprising a disk-like portion provided with an outer perimeter surface (2) and a hole (3) with an inner perimeter surface (4), at least one of the perimeter surfaces (2, 4) being designed with a varying distance in circumferential direction to the rotational center of the portion, the device (1) being arranged between the shaft (5) and the circular element (10), the circular element (10) and the projecting element (9) being positioned in a common plane perpendicular to an axis of the shaft (5), and the device (1) operating as a whirl speed limiting mechanism causing a contribution to the whirl velocity at impact between any of the projecting element (9), the device (1), the circular element (10) and the shaft (5). Device according to claim 1, characterized by, it being arranged between the shaft (5) and the inside of the projecting element (9). Device according to claim 1, characterized by, it being arranged between the inside of the circular element (10) and the outside of the projecting element (9). Device according to claim 1, characterized by, the reduction of the device (1) to at least two angular segments suitably arranged in circumferential direction, the segments acting as a mechanism for contributing to the whirl speed. Device according to claim 1, characterized by, the device (1) incorporating a ring, which is eccentrically arranged in relation to the center of rotation. Device according to claim 1, characterized by, the device (1) being an asymmetric ring produced from a ring member which has been severed at at least one point, whereupon the severed ends have been displaced tangentially to form a space between them, and at least one separate member (12) being fixedly connected between the severed ends. Device according to claim 1, characterized in, that the interacting surfaces of the projecting element (9), the device (1), the circular element (10) or the shaft (5) are arranged with mating portions (13) in order to act as a movement limiting mechanism in axial direction. Device according to claim 1, characterized in, that the machine element (8) is constituted by the inner ring of the auxiliary bearing (6). Device according to claim 1, characterized in, that the projecting element (9) is arranged to be able to contact the surface (7) in order to limit movement in axial direction. Device according to claim 1, characterized in, that at least one of the surfaces of the projecting element (9), the device (1), the circular element (10), the surface (7) and the shaft (5) are arranged with friction decreasing means. Device according to claim 1, characterized in, that the shaft (5) and the member to which the surface (7) is arranged are integrally manufactured. Device according to claim 1, characterized in, that the asymmetric ring (10) is integrally manufactured to the surface (7). Device according to claim 1, characterized in, that the circular element (10) and the device (1) are integrally manufactured. Device according to claim 1, characterized in, that the device (1) is integrally manufactured to one of the radial surfaces of the projecting element (9). Device according to claim 1, characterized in, that the device (1) is integrally manufactured to the shaft (5).

Inventors: GOERANSSON BO, GOERANSSON, BO, Göransson, Bo
Applicants: SKF NOVA AB
Application year: 1999
Main classification: F16C39
IPC codes: F16C32, F16C17, F16C39, F16C19