Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
1999-07-14
2002-04-23
Ramirez, Nestor (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C310S043000, C384S295000, C384S215000, C384S217000, C384S192000, C384S202000
Reexamination Certificate
active
06376952
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to bearings. In particular, this invention relates to a bearing system for a rotating shaft, and a bearing and bearing bracket therefor.
BACKGROUND OF THE INVENTION
Small fractional horsepower “shaded pole” type motors are used in many applications, for example to provide air circulation in refrigeration systems. As is well known, shaded pole electric induction motors have a rotor comprising a rotor body bearing a shaft in rotationally fixed relation to the body. The rotor body is rotationally disposed within an opening in a magnetic stator assembly typically formed from a stack of aligned annular stator laminations. Electric field windings surrounding a portion of the stator magnetize the stator laminations to provide the required magnetic motive force for driving the rotor. In an air circulation system an impeller is mounted on the rotor shaft to drive the air flow.
In a conventional shaded pole motor the rotor shaft extends through a housing comprising brackets extending over each end of the rotor opening and secured, usually bolted, to the stator. The housing restrains the rotor body against substantial axial displacement relative to the stator, and supports bearings which maintain the axial alignment of the rotor shaft. The bearings thus maintain stability and alignment of the rotor while allowing for substantially free rotation of the rotor shaft. One example of such a motor is described in U.S. Pat. No. 5,287,030 issued Feb. 15, 1994 to Nutter, which is incorporated herein by reference.
Such fractional horsepower motors are particularly suitable for applications in which the motor runs for extended intervals over a prolonged period, which may be many years. As such the motor must be extremely durable, highly resistant to failure and preferably requires little maintenance over its useful life. The components which tend to be most problematic in achieving these parameters are the bearings, which are subject to persistent frictional contact with the rotating shaft over the life of the motor.
To maintain proper alignment of the rotor shaft, shaded pole type motors typically utilize spherical diameter, oil impregnated powdered metal bearings or ball bearings held in place by die cast aluminum or zinc bearing brackets. These types of bearings require constant exposure to a lubricant, which substantially limits the life of the motor. This problem is particularly acute in high temperature environments in which the oil used to lubricate the bearings dissipates over time, eventually causing catastrophic failure of the bearing system.
It is also known to press fit journal bearings tightly to the bearing brackets. However, this type of bearing system requires machining after the press fitting operation, which significantly increases the manufacturing cost of the motor. Moreover, although a press fit journal bearing will remain in place in the bearing bracket during assembly, due to the interference fit between the bearing and the housing, the performance of the motor at times may be less than optimum because the fixed position of the bearing does not allow for even slight deviations in rotor shaft alignment. If the motor is jarred or bumped during operation, severe vibration and squealing can result because the bearing is not capable of self alignment.
These problems are particularly acute in the case of metal bearings supported by metal brackets, and precision machining of these components is therefore critical. There are bearing systems which use a plastic bracket to support a metal bearing tightly fitted to the bracket in an interference fit, however in these systems adequate lubrication of the bearing remains critical to the proper operation of the motor. It is also known to use a plastic bearing press fitted into a metal bracket, but as the bearing is mounted the bracket closes the bearing inside diameter by the extent of the interference fit, which then necessitates precision machining of the inside diameter to restore adequate clearance for the rotor shaft. Also, the press fit operation causes the bearing to lose alignment during installation.
The design described in U.S. Pat. No. 5,287,030 uses a plastic bearing press fitted to a plastic bracket. However, this design gives rise to the same disadvantages of other bearing systems in which the bearing is mounted in an in interference fit, most notably the inability of the bearing to self align, which reduces the useful life of the motor and generally causes the motor to operate less efficiently over time.
SUMMARY OF THE INVENTION
The present invention provides a floating bearing system comprising a bearing supported in a clearance fit within a bearing bracket, for example for use in fractional horsepower shaded pole type electric motors. The bearing system is self aligning, and thus compensates for deviations in the axial alignment of the rotor shaft to maintain the optimum efficiency of the motor and reduce wear on the bearing, extending the life of the bearing system.
In the preferred embodiment both the bracket and bearing are composed of a non-metallic material, preferably plastic. The bearing may be composed of a high performance plastic which does not require lubrication, to prolong the life of the motor. Other aspects of the invention may be implemented in a bearing system that utilizes a metal bracket and/or a metal bearing.
In the preferred embodiment of the invention the flange portion of a flanged or bushing type bearing is provided with an opening having a bearing surface complimentary to the rotor shaft. A bearing bracket is provided with a bearing receptacle adapted to receive the hub of the bearing in a clearance fit. A rotation lock, in the preferred embodiment flats distributed about the bearing receptacle cooperating with complimentary flats in the hub portion of the bearing, restrains the bearing against rotation within the bearing receptacle while allowing the bearing to settle into proper alignment with the rotor shaft by automatically shifting the centerline or pitch of the axis of the bearing. The bearing is thus retained in the bearing receptacle in “floating” relation and is able to self align to accommodate deviations in the axial pitch of the rotor shaft.
In the preferred embodiment the bearing bracket is provided with bearing retainers comprising retaining fingers that hold the bearing in place during the assembly and working life of the motor. The retaining fingers are preferably formed integrally with the bearing bracket and provided with barbed flanges that retain the bearing in the bearing receptacle in a clearance fit. This aspect of the invention simplifies the assembly of the bearings into the bearing brackets and assembly of the bearing brackets to the motor.
The present invention thus provides a bearing system for a rotating shaft, comprising a bearing comprising an opening having at least one bearing surface, a bearing bracket comprising a receptacle for mounting the bearing on the bracket in substantially fixed relation, the receptacle being dimensioned to support the bearing in a clearance fit, and a rotation lock cooperating between the bearing and the receptacle to restrain the bearing against substantial rotation relative to the bracket, whereby when the shaft is disposed through the bearing the shaft rotates against the bearing surface to maintain the shaft in a substantially fixed radial position, a clearance between the bearing and the bearing receptacle thereby enabling the bearing to maintain alignment with an axial orientation of the shaft.
The present invention further provides a fractional horsepower motor, comprising a rotor rotationally disposed in a stator, stator windings disposed about the stator for driving the rotor and a rotating shaft rotationally fixed to the rotor, and a bearing system comprising a bearing having an opening with at least one bearing surface, disposed in substantially fixed relation in a bearing receptacle supported by a bearing bracket, the receptacle being dimensioned to support the bearing in a cleara
Dinh Dang
E. I. du Pont de Nemours and Company
Ramirez Nestor
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