Patent Application
20140232226
The invention relates to a machine comprising a rotor, a stator, main bearings for supporting the rotor rotatably with respect to the stator, and at least one safety bearing for supporting the rotor rotatably with respect to the stator in a situation where the main bearings are non-operating.
In some cases there is a need to provide a rotating machine with safety bearings in addition to main bearings. The main bearings are arranged to rotatably support the rotor of the machine during normal operation, and the safety bearings are arranged to rotatably support the rotor when the main bearings are non-operating. The machine can be, for example, a turbomachine which can be e.g. a turboblower, a turbocompressor or a pump, and the main bearings can be for example contactless magnetic bearings. When the magnetic bearings become non-operating, e.g. due to an electrical power cut, the rotor is dropped to be supported by the safety bearings. It is also possible that the magnetic bearings are non-operating in the sense that they are still active but their load capacity is exceeded. Also in this case, the safety bearings have to support the rotor. Typically, there are radial and axial clearances between the rotor and the safety bearings when the rotor is supported by the main bearings in order that the safety bearings would not disturb the normal operation of the machine.
The above-mentioned radial clearance allows the rotor to whirl when being supported by the safety bearings. For example, when the safety bearings are ball bearings, the rotor typically goes into cylindrical forward whirling motion when dropped to be supported by the safety bearings. This whirling motion produces high centrifugal forces which stress the mechanical structures of the machine.
U.S. Pat. No. 4,629,261 describes a safety and centering device for a rotor supported by contactless magnetic bearings. The safety and centering device provides centering support for the rotor if the contactless magnetic bearings loose power. The rotor shaft comprises a collar with a conical friction surface, and the safety bearing comprises a sleeve controlled by an electro magnet with counteracting springs and also having a conical friction surface. In a case of power failure, the springs force the two conical surfaces together and provide centering support for the rotor. The conical angle of the conical surfaces has to be relatively small so as to provide appropriate centering effect. In the examples shown in U.S. Pat. No. 4,629,261 the coning angle is about 30 degrees, where the coning angle is the angle between the conical surface and the axis of the rotational symmetry of the conical surface. A challenge related to the technical solution described in U.S. Pat. No. 4,629,261 is that it requires a good alignment between the rotational axis determined by the above-described centering effect and the principal moment of inertia of the rotor because otherwise high centrifugal forces may occur.
The following presents a simplified summary in order to provide a basic understanding of some aspects of the invention. The summary is not an extensive overview of the invention. It is neither intended to identify key or critical elements of the invention nor to delineate the scope of the invention. The following summary merely presents some concepts of the invention in a simplified form as a prelude to a more detailed description of exemplifying embodiments of the invention.
In accordance with the invention there is provided a machine comprising:
There are a radial clearance and an axial clearance between the rotor and the safety bearing when the rotor is supported by the main bearings. A contact surface of the rotor arranged to contact the safety bearing in response to closure of the axial clearance is oblique with respect to a spatial plane perpendicular to a rotational axis of the rotor so that an angle between the rotational axis of the rotor and the normal of the contact surface at a point of contact between the rotor and the safety bearing is greater than zero and at most 10 degrees, and more preferably at most 5 degrees. Therefore, in an exemplifying case where the contact surface of the rotor is conical, the coning angle is at least 80 degrees, and more preferably at least 85 degrees. Thus, the conical shape of the contact surface is so blunt that the centering effect provided by the conical shape when the rotor is pushed against the safety bearing is so small that the rotor is allowed, even though not forced, to rotate around its principal moment of inertia. In conjunction with the present invention, it has been surprisingly noticed that the above-described obliqueness of the contact surface eliminates at least partly an excitation for whirling motion when the rotor is supported by the safety bearings.
A number of non-limiting and exemplifying embodiments of the invention are described in accompanied dependent claims.
Various non-limiting and exemplifying embodiments of the invention both as to constructions and to methods of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific exemplifying embodiments when read in connection with the accompanying drawings.
The verbs “to comprise” and “to include” are used in this document as open limitations that neither exclude nor require the existence of unrecited features. The features recited in the dependent claims are mutually freely combinable unless otherwise explicitly stated.
Exemplifying embodiments of the invention and their advantages are explained in greater detail below in the sense of examples and with reference to the accompanying drawings, in which:
a illustrates behaviour of a safety bearing system according to the prior art, and
C1<α≦C2,
where C1 is one of the following: 0, 1, 2, 3, 4, 5, 6, 7, 8 or 9 degrees and C2 is one of the following so that C2>C1: 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 degrees. More preferably C1 is one of the following: 0, 1, 2, 3, or 4 degrees and C2 is one of the following so that C2>C1: 1, 2, 3, 4, or 5 degrees.
The angle α is selected so that the tapering shape of the contact surface 107 is so blunt that the centering effect provided by the tapering shape when the rotor is pushed against the safety bearing 104 is so small that the rotor is allowed to rotate around its principal moment of inertia. An advantageous value or range for the angle α can be found out with simulations and/or with experiments.
In the exemplifying case illustrated in
In a machine according to an exemplifying embodiment of the invention, the safety bearing 104 comprises a rolling contact bearing 108 comprising an outer ring supported by a frame of the machine, a rotatable inner ring, and rolling elements between the inner and outer rings. The rolling contact bearing 108 can comprise, for example, one or more ball bearings arranged to be capable of carrying axial load. In the exemplifying case illustrated in
In a machine according to an exemplifying embodiment of the invention, the safety bearing 104 comprises a sleeve element comprising a first part 109 being radially between the inner ring and the rotor and a second part 110 being axially between the rolling contact bearing and the contact surface 107 of the rotor. The second part 110 comprises a surface getting in contact with the rotor in response to closure of the axial clearance 106.
The above-described obliqueness of the contact surface 107 of the rotor eliminates at least partly an excitation for whirling motion when the rotor is supported by the safety bearing 104. This effect is illustrated below with reference to
a illustrates a case where a rotor 221 is pushed by force F in the axial direction against a safety bearing 224. The axial direction is parallel with the z-axis of a coordinate system 299. For illustrative purposes, we assume that the rotational axis 222 of the rotor has moved in the positive y-direction of the coordinate system 299 away from the rotational axis 223 of the safety bearing. Furthermore, we assume that the rotational speed of the rotor is ωR and the rotational speed of the safety bearing is ωB. In a contact point 225, a speed difference Δv1 between the rotor and the safety bearing is:
Δv1=ωRRR1−ωBRB, (1)
where RR1 is the distance from the rotational axis 222 of the rotor to the contact point 225 and RB is the distance from the rotational axis 223 of the safety bearing to the contact point 225. In a contact point 226, a speed difference Δv2 between the rotor and the safety bearing is:
Δv2=ωRRR2−ωBRB, (2)
where RR2 is the distance from the rotational axis 222 of the rotor to the contact point 226.
In a case where the rotational speeds ωR and ωB happen to be adapted so that there is practically no slip in the contact point 225, i.e. Δv1≈0, there is slip in the contact point 226, i.e. Δv2≠0. The slip in the contact point 226 causes a force parallel to the x-axis of the coordinate system 229. This force may represent excitation for whirling motion of the rotor 221. Correspondingly, when the rotational speeds ωR and ωB happen to be adapted so that there is practically no slip in the contact point 226, i.e. Δv2≈0, there is slip in the contact point 225, i.e. Δv1≈0. This slip causes a force that may represent excitation for the whirling motion. The direction of the force is typically such that it causes forward whirling motion. Actually, when RR1≠RR2, both Δv1 and Δv2 can be zero if and only if ωR=ωB=0, because the equation pair constituted by the equations (1) and (2) has a non-zero determinant.
b illustrates a case where a rotor 201 according to an exemplifying embodiment of the invention is pushed by force F in the axial direction against a safety bearing 204. For illustrative purposes we assume that the rotational axis 232 of the rotor has moved in the positive y-direction of the coordinate system 299 away from the rotational axis 233 of the safety bearing. Due to the conical shape of the contact surface 207, there is a clearance 206 between the contact surface 207 and the safety bearing. Therefore, the rotational speeds of the rotor 201 and the safety bearing 204 can adapt themselves so that there is no slip in the contact between the safety bearing and the rotor. Hence, the situation related to the prior art and illustrated with reference to
The contact surface of the rotor arranged to contact the safety bearing in response to closure of the axial clearance does not necessarily have to be conical. This is illustrated in
In the exemplifying case illustrated in
In a machine according to an exemplifying embodiment of the invention, the profile of the contact surface 307 of the rotor is an arc of a first circle having radius R.
In a machine according to an exemplifying embodiment of the invention, a profile of a contact surface 311 of the safety bearing 304 is an arc of a second circle having a radius smaller than R as illustrated in
During the normal operation when the rotor 401 is supported by the main bearings 403, there are a radial clearance and an axial clearance between the rotor and the safety bearings 404a and 404b in order that the safety bearings would not disturb the normal operation of the machine. A contact surface of the rotor arranged to contact the safety bearing in response to closure of the axial clearance is oblique with respect to a spatial plane perpendicular to a rotational axis of the rotor so that an angle between the rotational axis of the rotor and the normal of the contact surface at a point of contact between the rotor and the safety bearing is greater than zero and at most 10 degrees, and more preferably at most 5 degrees.
The specific examples provided in the description given above should not be construed as limiting the applicability and/or the interpretation of the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 20135151 | Feb 2013 | FI | national |
| 13157420.4 | Mar 2013 | EP | regional |
