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
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 United States Patent No. 5,287,030
issued February 15, 1994 to Nutter.
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. Patent 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 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
Another self aligning bearing system for electric motors, comprising
a sliding bearing mounted in a bracket with interference fit and locked against
rotation, is known from US 4 368 931 A.
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.
According to 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 flange projecting radially from a hub, 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
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
BRIEF DESCRIPTION OF THE DRAWINGS
In drawings which illustrate by way of example only preferred embodiments
of the invention,
DETAILED DESCRIPTION OF THE INVENTION
- Figure 1 is a front elevation of a motor embodying the invention,
- Figure 2 is a side elevation of the motor of Figure 1,
- Figure 3 is a cross-sectional elevation of a bearing system according to the
- Figure 4 is a plan view of the bearing bracket in the system of Figure 3,
- Figure 5 is a cross-section of the bearing bracket along the line 5-5 in Figure
- Figure 6 is a plan view of the bearing in the bearing system of Figure 3,
- Figure 7 is a cross-section of the bearing of Figure 6, and
- Figure 8 is a bottom plan view of a further embodiment of the bearing system
of the invention.
Figures 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 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 Figure 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 34a. 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 Figures 3 to 5. Feet 34 are each provided with a hole 34a through which
bolt 34a 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
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 Figures 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 .001 inches
(0,000254mm) and .003 inches (0,000762mm). 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 .0005 inches (0,000127mm) 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 54a disposed about the outer surface of the hub 54, and complimentary
flats 40a distributed about the bearing receptacle 40 cooperating with the flats
54a, as best seen in Figure 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
40a and 54a 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
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 Figures 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 Figure 3,
and return to the rest position, shown in solid lines in Figure 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 Figure
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 54a, 40a 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 Figure 3. When the flange 52 has cleared the tips 64 the
arms 66 snap back to the rest position, shown in solid lines in Figure 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 60 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 34a. 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 Figure 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 52a 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.