Abstract:
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, to compensate for deviations in the axial alignment of the rotor shaft. A rotation lock 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 pitch of the axis of the bearing. In the preferred embodiment the bracket is composed of an engineering plastic and the bearing is composed of a high performance plastic polymer, so that the bearing system is non-lubricating. In the preferred embodiment the bearing bracket is provided with retaining fingers which hold the bearing in place during the assembly and working life of the motor.

Description:
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 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, in combination, a bearing and a bearing retainer, the bearing being composed of plastic and comprising a flange projecting radially from a hub, the bearing comprising an opening having at least one bearing surface and a first component of a rotation lock, and the bearing bracket comprising a receptacle dimensioned to support the bearing in a clearance fit for mounting the bearing on the bracket in substantially fixed relation, and a second component of a rotation lock such that the first component cooperates with the second component to restrain the bearing against substantial rotation relative to the bracket, whereby when a shaft is disposed through the bearing and rotates against the bearing surface, the bearing maintains the shaft in a substantially fixed radial position, wherein a clearance between the bearing and the bearing receptacle enables the bearing to maintain alignment with an axial orientation of the shaft. 
     In a further aspect of the invention the bearing is composed of a polymeric plastic and comprises a flange projecting radially from a hub. 
     In a still further aspect of the invention the bracket comprises a bearing retainer for retaining the bearing in the receptacle. The bearing retainer may comprise retaining fingers projecting from the bearing bracket about the receptacle and adapted to retain the flange of the bearing. In the preferred embodiment the bearing fingers each comprise an arm supported by a spring loop to increase a resilience of the retaining fingers, which terminate in barbed tips. 
     In a still further aspect of the invention the bearing bracket is composed of plastic and the retaining fingers are formed integrally with the bearing bracket. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In drawings which illustrate by way of example only preferred embodiments of the invention, 
     FIG. 1 is a front elevation of a motor embodying the invention, 
     FIG. 2 is a side elevation of the motor of FIG. 1, 
     FIG. 3 is a cross-sectional elevation of a bearing system according to the invention, 
     FIG. 4 is a plan view of the bearing bracket in the system of FIG. 3, 
     FIG. 5 is a cross-section of the bearing bracket along the line  5 — 5  in FIG. 3, 
     FIG. 6 is a plan view of the bearing in the bearing system of FIG. 3, 
     FIG. 7 is a cross-section of the bearing of FIG. 6, and 
     FIG. 8 is a bottom plan view of a further embodiment of the bearing system of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 and 2 illustrate an electric motor  10  embodying one preferred embodiment of the invention. The motor  10  is a fractional horsepower “shaded pole” type motor such as that used to provide air circulation in a refrigeration system. The motor  10  comprises a rotor  12  comprising a rotor body is  14  bearing a shaft  16  in rotationally fixed relation to the body  14 . The rotor body  14  is rotationally disposed within an opening in a magnetic stator assembly  20  formed from a stack of aligned annular stator laminations  22 . Electric field windings  24  wound around a portion of the stator  20  magnetize the stator laminations  22  to provide the required magnetic motive force for driving the rotor  12 . 
     According to the invention, the rotor shaft  16  extends through a bearing system, a preferred embodiment of which is illustrated in FIG. 3. A housing  30  comprises opposed bearing brackets  32  which extend radially across the ends of the rotor opening and are affixed to the stator  20 , for example by bolts  34   a . The bearing brackets  32  each support bearings  50  through which the rotor shaft  16  extends to stabilize and maintain proper alignment of the rotor  12  while allowing substantially free rotation of the rotor shaft  16  within the housing  30 . 
     The preferred embodiment of the bearing brackets  32  is illustrated in detail in FIGS. 3 to  5 . Feet  34  are each provided with a hole  34   a  through which bolt  34   a  is disposed to anchor the bracket  32  to the stator  20 . A bridge  36  is maintained spaced from the stator  20  by risers  38 , which may be oriented obliquely relative to the bridge  36 . The bridge  36  is provided with a bearing receptacle  40  comprising a hole extending through the bridge  36 , preferably centrally, and dimensioned to receive the bearing  50  in a clearance fit, as described in detail below. The bearing receptacle  40  may optionally include an annular extension  42 , as in the embodiment shown, to accommodate a larger bearing  50  and/or serve as a spacer for an impeller (not shown). 
     In the preferred embodiment the bearing bracket  32  is integrally molded from an engineering plastic. An engineering plastic suitable for the bearing bracket  32  is HTN ZYTEL® (Trademark)51G35 HSL nylon manufactured by DuPont (Trademark). Other suitable materials include PPA, PBT/PET/PTT polyesters, SPS, PPS, LCP, modified polyphenylene oxide, polycarbonates, polyethylene and polypropylene. In the engineering plastic embodiment illustrated, reinforcing ridges  31  are provided about the periphery of the bracket to impart rigidity to the bridge  36  and risers  38 . Other materials, for example metals conventionally used in shaded pole motor housings, are also suitable for the bearing bracket  32 . 
     Each bearing bracket  32  supports a bearing  50 , a preferred embodiment of which is illustrated in detail in FIGS. 6 and 7. The bearing  50  in the preferred embodiment comprises a flange  52  extending radially from a hub  54 . A hole  56  disposed axially through the bearing  50  is provided with one or more bearing surfaces  58  which contact the rotor shaft  16 . In the embodiment shown the bearing opening is “fluted”, comprising a plurality of truncated bearing surfaces  58  evenly distributed about the hole  56  and spaced apart by lobes  59  which are spaced from the shaft  16 . This minimizes the area of contact between the bearing  50  and the shaft  16  to reduce the degree of friction between the rotor shaft  16  and the bearing  50 , and thus reduce the heat generated during operation. The lobes  59  also provide a channel or pocket for the accumulation of debris during operation of the motor  10 . 
     In the preferred embodiment the bearings  50  are molded from a high performance polymeric plastic. One preferred bearing material is Vespel® (Trademark) SP-2624 grade manufactured by DuPont (Trademark), due to its superior wear characteristics and extremely low coefficient of thermal expansion properties. 
     The bearing  50  is dimensioned to nest in the bearing receptacle  40  with a small amount of clearance between the outer surface of the hub  54  and the inner surface of the receptacle  40 , to allow for self alignment of the bearing. The clearance between the hub  54  and the receptacle  40  may range between 0.001 inches and 0.003 inches. Too little clearance will interfere with self alignment of the bearing  50 , while excessive clearance can cause rattling of the bearing  50  in the bearing bracket  32 . Use of the Vespel® (Trademark) SP-2624 polymer is advantageous because it can be manufactured to very close tolerances (as low as 0.0005 inches for small diameters) with no machining required, thereby minimizing manufacturing costs. 
     A rotation lock is provided to restrain the bearing  50  against substantial rotation within the receptacle  40 . In the preferred embodiment the rotation lock comprises flats  54   a  disposed about the outer surface of the hub  54 , and complimentary flats  40   a  distributed about the bearing receptacle  40  cooperating with the flats  54   a , as best seen in FIG.  5 . The rotation lock may in alternate embodiments comprise tabs or grooves (not shown) in the hub  54  or the flange  52  with complimentary mating structures (not shown) formed into the bearing bracket  32 . However the use of flats  40   a  and  54   a  for the rotation lock is preferred for simplicity of design and reduction of opportunities for interference between the bearing  50  and the bracket  32  during self alignment. 
     Because of the clearance fit between the bearing hub  54  and the receptacle  40  a slight degree of rotational freedom is available to the bearing  50 , however the rotation lock substantially prevents the bearing from rotating during operation of the motor. 
     In the preferred embodiment a bearing retainer is provided to retain the bearing  50  in the receptacle  40 . In the embodiment illustrated in FIGS. 3 to  5  the bearing retainer comprises retaining fingers  60  formed integrally with and projecting from the bridge  36  of the bearing bracket  32 . Preferably the retaining fingers  60  each comprise a spring loop  62  supporting an arm  66  which terminates in a barbed tip  64  for retaining the bearing  50  against the bridge  36  of the bearing bracket  32 . The engineering plastic of the bearing bracket  32  is necessarily relatively rigid, in order to maintain stability of the rotor  12 , and the spring loop  62  is thus provided to impart to the retaining finger  60  sufficient resilience to displace radially (relative to the receptacle  40 ), as shown in phantom lines in FIG. 3, and return to the rest position, shown in solid lines in FIG. 3, after the bearing  50  has been mounted to the bearing bracket  32 . 
     The retaining fingers  60  prevent the bearing  50  from falling out of the receptacle  40  during the assembly of the motor  10 , as well as during operation of the motor  10 . The number of retaining fingers  60  can be selected according to the size of the components. It is anticipated that in most cases two retaining fingers  60  spaced in opposition about the receptacle  40  will be adequate to hold the bearing  50  in place, however other variations are possible, for example as shown in FIG.  8 . 
     As with the relationship of the bearing hub  54  to the receptacle  40 , in order for the bearing  50  to be self-aligning there should be a small amount of clearance between the bearing flange  52  and the tips  64  of the retaining fingers  60  when the bearing  50  is fully mounted into the receptacle  40 , to allow for self alignment of the bearing  50  during operation of the motor  10 . 
     In use, the bearing  50  is mounted to the bearing bracket  32  by aligning the hub  54  with the receptacle  40  so that the flats  54   a ,  40   a  are positioned in opposition, and depressing the bearing  50  into the receptacle  40 . As the flange  52  passes the barbed tips  64  of the retaining fingers  60  the arms  66  cam radially outwardly, as shown in phantom lines in FIG.  3 . When the flange  52  has cleared the tips  64  the arms  66  snap back to the rest position, shown in solid lines in FIG.  3 . The assembly of the bearing  50  into the bracket  32  can be performed by hand, or by automated equipment for high volume applications. The retaining fingers  66  retain the bearing  50  in the receptacle  40  as the brackets  32  are assembled to the motor  10 . 
     The rotor  12  is positioned within the opening in the stator  12 , and the bearing brackets  32  are assembled to the stator  20  by disposing the rotor shaft  16  through the bearings  50 , aligning the feet  34  with holes (not shown) through the stator laminations  20  and securing the housing  30  as by bolts  34   a . The motor  10  is mounted to an appliance in conventional fashion, and terminals  11  are connected to the local power supply. 
     In operation, as the rotor  12  rotates within the stator  20  the rotor shaft  16  rotates against the bearing surfaces  58 . In the preferred embodiment no lubrication is required due to the extremely low frictional resistance and coefficient of thermal expansion of the high performance polymer used for the bearing  50 . Because of the clearance fit the bearing  50  will shift to accommodate deviations in the axial pitch of the rotor  12 , thereby maintaining proper alignment between the bearing  50  and the rotor shaft  16  after assembly and during operation of the motor  10 . The use of plastics for both the bearing bracket  30  and the bearing  50  reduces opportunities wearing of the bearing system components, and also reduces noise and vibration levels. 
     A further embodiment of the invention is illustrated in FIG.  8 . In this embodiment the bearing retainer comprises three retaining fingers  60  evenly distributed about the flange  52  of the bearing  50 . The bearing retainer in this embodiment also provides the rotation lock, comprising in this case planar inner surfaces of the arms  66  cooperating with flats  52   a  disposed in complimentary relation about the periphery of the bearing flange  52 . In this embodiment the bearing  50  provides a single bearing surface  58  circumscribing the inner face of the flange  52 , although a fluted opening is equally available for this embodiment. As in the previous embodiment the retaining fingers  60  are spaced slightly from the bearing  50  to maintain the bearing  50  in a clearance fit within the receptacle  40 , and the operation of this embodiment is otherwise as previously described. 
     Preferred embodiments of the invention having been thus described by way of example, it will be apparent to those skilled in the art that modifications and adaptations may be made without departing from the scope of the invention, as set out in the appended claims.