Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
1999-10-15
2001-02-27
LaBalle, Clayton (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C384S102000
Reexamination Certificate
active
06194801
ABSTRACT:
This application is based on and claims priority under 35 U.S.C. § 119 with respect to Swedish Application No. 9803537-1 filed on Oct. 15, 1998, the entire content of which is incorporated herein by reference.
FIELD OF THE 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 auxiliary bearing is used to support a shaft in the event of a 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, and this leads to an acceleration of the inner ring of the bearing. These contacts affect the shaft, resulting in the frictional forces. The shaft starts to whirl in the clearance 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 centrifugal force associated with the shaft rotating steadily about its center axis without any whirling motion. The limited contact area and cooling area often also cause temperature problems.
EP-0 499 310 A1 discloses a magnetic bearing shaft assembly having a back-up bearing to support the shaft in the event of a failure of the magnetic bearing. If the bearing portion of the back-up bearing includes a ball race, the radially outer raceway of the bearing is secured to an anchorage, such as a housing for the assembly. A low stiffness damper member in the form of an annular shaped body of resilient material is also included in the back-up bearing at a position between the bearing portion of the bearing and the anchorage. When the magnetic bearing fails 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 while the shaft whirl 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. The stiffness damper member constitutes a crucial part of the assembly.
A need exists however for a way of limiting the whirling movement of the rotatory shaft after a failure of the magnetic bearing has occurred. The problem associated with such whirling movement is particularly accentuated in the case of a vertical shaft.
SUMMARY OF THE INVENTION
The present invention provides a device which decreases the whirling motion of a shaft during a failure of a magnetic bearing. Generally speaking, the whirling motion limiting device is in the from of a disk-like element provided with an outer peripheral surface and a hole with an inner peripheral surface. At least one of the peripheral or perimeter surfaces is designed to possess a varying distance in the circumferential direction with respect to the rotational center of the element. The device is arranged between the shaft and a circular element, with the circular element and the projecting element being positioned in a common plane perpendicular to the shaft axis. The device operates as a whirl speed or motion limiting mechanism which causes a contribution to the whirl velocity at impact between any of the projecting element, the device, the circular element and the shaft.
One aspect of the invention involves an arrangement that includes a shaft whirl limiting device for limiting whirl of the shaft after failure of a magnetic bearing, after which failure the shaft is supported by at least one auxiliary bearing. The shaft possesses an axis and is provided with a surface extending substantially perpendicular to the shaft axis, and a machine element is provided with a projecting element that extends from the machine element in a direction towards the surface. The surface is provided with at least one circular element that is concentric with the axis of the shaft and that axially overlaps the projecting element, with the shaft whirl limiting device being arranged between the shaft and the circular element. The shaft whirl limiting device is in the form of a disk-shaped element provided with a through hole and possessing an outer perimeter surface and an inner perimeter surface. At least one of the perimeter surfaces is configured to define a varying distance in a circumferential direction with respect to the rotational center of the disk-shaped element. The disk-shaped element affects the whirl motion of the shaft upon impact between any of the projecting element, the disk-shaped element, the circular element and the shaft.
Another aspect of the present invention involves an arrangement limiting whirling of a shaft after failure of a magnetic bearing supporting the shaft, and after which failure the shaft is supported by at least one auxiliary bearing. The shaft has a shaft axis and is provided with a surface extending radially outwardly with respect to the shaft axis, and a machine element is provided with a projecting element extending from the machine element in a direction towards said surface. The surface is provided with at least one circular element that is concentric with the shaft axis, and a portion of the circular element axially overlaps a portion of the projecting element in a plane perpendicular to the shaft axis. A mechanism is arranged between the shaft and the circular element that causes successive contacts with the shaft to limit whirling of the shaft resulting from failure of the magnetic bearing.
REFERENCES:
patent: 4629261 (1986-12-01), Eiermann
patent: 4934781 (1990-06-01), Kato
patent: 5126612 (1992-06-01), Girault
patent: 0 287 440 (1988-10-01), None
patent: 0 499 310 (1992-08-01), None
patent: 0 499 309 (1992-08-01), None
patent: 0 584 846 (1994-03-01), None
Addison Karen B
Burns Doane Swecker & Mathis L.L.P.
LaBalle Clayton
SKF Nova AB
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