Background of the Invention
This invention relates to means for and methods of assembling electrical
motors and, more particularly, to provide a means for precisely controlling ball
bearing preloading in motors, especially fractional and sub-fractional motors.
Electrical motors have rotors mounted on ball bearings in order to
turn within stators. The amount of axial and radial movement of the balls within
the inner and outer races of the ball bearing is reduced by an application of a
preload force, usually axially, transmitted by a spring.
Achieving a reasonably consistent ball bearing preload is essential
for minimizing bearing noise and extending the life expectancy of both the motor
and the bearing.
Prior art methods of designing ball bearing motors entail the establishment
of nominal manufacturing dimensions and tolerances for all of the components of
the motor which might have an effect upon the preload. Thereafter, the dimensions
across the rotor/shaft assembly, stator assembly and brackets are calculated to
determine a desired amount of preloading under the worst case conditions and when
there is a combination of the most extreme limits of otherwise acceptable tolerances.
Prior methods of assembling sleeve bearing motors are shown in U.S.
Patent Nos. 4,878,289; 4,944,055; and 5,051,632. However, a sleeve bearing assembly
method is not relevant to a method of assembling ball bearing motors since the
tolerance of sleeve bearings is so very great as compared to the very close tolerance
of the ball bearings. Sleeve bearing-equipped motors are associated with the technical
term "endplay" since a considerable amount of endplay is necessary for acceptable
operation of a sleeve bearing motor. However, the word "endplay," does not exist
in the ball bearing design vocabulary. The minimal amount of play necessarily manufactured
into ball bearings is known as "axial or radial play" and is measured in ten-thousandths
of an inch.
In an electric motor, the stator of the motor field assembly is the
component which may have the greatest variations in tolerance since the stator
is formed from a stack or series of laminates, each of which has a tolerance variation
of its own. Therefore, the tolerance of the stator is the sum of the tolerances
of all of the individual laminates which are stacked one on the other and then
fastened together in order to form the stator. Of course, the tightness of the
fastened stack also has an effect upon the total thickness tolerance. Hence, the
prior art has found it extremely difficult to form a stator having a thickness
tolerance of less than one-half of the thickness of a laminate. This variation
in the tolerance of the stator has been the cause of the greatest variance in the
ball bearing preload in motors because the bearing supports are mounted on the
In this respect, it has not been uncommon for electric motors within
a single production lot to exhibit considerable preload variation. Therefore, there
is a need to provide better manufacturing means for and methods of reducing the
preload variance of ball bearing electric motors and, more particularly, of fractional
and sub-fractional motors.
In the past, motor manufacturers have resorted to a wide variety
of assembly processes to obtain an optimum cost effective method. One practice
of the prior art has been to measure the thickness of the completed stator rather
than to count the laminations. Then, the rotor is selected to fit those measurements.
In U.S. Patent No. 5,051,632, describing a sleeve bearing design
spacer and retainer which are constructed to telescope somewhat. Therefore, each
individual stator could be used as its own custom gauge so that the dimension over
spacers would telescope to a length which matched the needs of that particular
stator. This assembly process reduces the sleeve bearing component tolerance problems
of the laminate stackup by using the stator or motor field assembly as an integral
part of an assembly fixture. However, realistically, the same procedures can not
be use to assemble ball bearing motors.
The traditional means of assembling ball bearings involves calculating
a preload fixing wave spring "working height" that can actually exceed the free
height of the spring (in other words, it is not compressed at all); or, at the
opposite extreme, the calculation might lead to a negative value indicating something
beyond a total spring collapse due to a compression of the spring. For this reason,
the prior art has used a rather lengthy coil spring with its generous working range
and has not been able to use a preferred wave spring.
The use of a coil spring for preloading requires an inner race slip
fit on at least one end of the motor. One bearing is inserted into each of a pair
of bearing end caps or housings which are then affixed to opposite ends of the
motor. The coil spring provides a light axial force for removing the clearance
between the inner and outer races and the rotating ball bearings. Then, at least
one ball bearing inner race is secured to the shaft by injecting an adhesive into
the joint between the shaft and the inner race of the ball bearing. This is a
complicated, complex and expensive manufacturing step. The other ball bearing may
be pressed on the motor shaft or may also be secured with an adhesive which provides
axial preload but only until the adhesive cures. After the adhesive cures, the
coil spring is no longer active.
Opposite the spring end of the assembly (the drive end), a solid
spacer is employed to position the other ball bearing. Since the spacer is in contact
with the rotor assembly, the contact complicates the spring compression tolerance
In keeping with an aspect of the present invention, a fractional
or sub-fractional horse power electrical motor is constructed by first placing
a spacer sleeve over the drive end of the drive shaft in order to fix the position
of a ball bearing on that end. Then, either an adhesive or a silicon rubber sleeve
is placed in each of the end caps or housings which receive the ball bearings
that support the shaft. Next, a wave spring is placed on the adhesive or rubber
sleeve of the end cap opposite or rear end of the drive shaft. A ball bearing is
pressed into each of the end caps. The ball bearing on the drive end is fastened
against the spacer sleeve. The other ball bearing is fitted over the rear end of
the drive shaft and drawn up by suitable tie bolts, at which time the wave sprint
preloads the rotor in order to establish the proper amount of end-play.
Brief Description of the Drawings
A preferred embodiment of the invention is shown in the attached
Detailed Description of the Preferred Embodiment
- FIG. 1 is a perspective and exploded view of an electric motor; and
- FIG. 2 is a partial cross-sectional view of the rotor and its mounts showing
the inventive method.
FIG. 1 shows the principal elements of an electric motor (especially
a fractional or sub-fractional horsepower motor) as including a motor field assembly
or stator 10, a rotor/shaft assembly 12, two end housings or caps
14, 16, and a coil 18. The rotor 12 has a central shaft
20 which is supported on its two ends by bearings mounted in the end housings
or caps 14, 16. The assembly is made by placing rotor/shaft assembly
in a hole 26 in the stator 10, fitting end housings or caps
14, 16 with their enclosed bearings onto the ends of the shaft
20 and then passing two bolts or other fasteners such as rivets (not shown)
through holes 28, 30, 32 and 34, 36, 38, respectively, to secure
all parts into a completed motor.
The problem is that the thickness dimension of the entire assembly
is fixed by the thickness A of the stator 10
since everything else
in the motor is bolted to it. The stator 10 is a laminate of steel plates
which are more or less tightly held together by a plurality of rivets or weldments,
three of which are seen at 40, 42, 44. Since nothing is perfect, every
manufactured item must have some tolerance dimensions. Therefore, some of these
steel laminate plates are thicker and some are thinner than the optimum thickness.
Sometimes, the rivets 40, 42, 44 clutch the plates more tightly and sometimes
more loosely. Thus, the stator thickness dimension A is subject to great
The rotor 12 must be loose enough to turn and therefore, is
subject to at least some degree of axial motion in directions B, C, the
freedom to slide being called "axial-play." If the amount of axial-play is too
great, bearing failure is accelerated and noise becomes objectionable. If the amount
of axial-play is too small, there is binding and the motor shaft will not rotate.
The trouble is that the limits of an acceptable amount of axial-play is less than
the necessary variations in the thickness A of the stator 10. Therefore,
it is not possible to make a quality motor simply by inserting a rotor into a stator
with no provisions for tolerance variances. As a result, the prior art has resorted
to a number of "fix-it" approaches, such as hand sorting, in order to accommodate
each rotor to its stator, on a more or less customized basis. This approach is
expensive, leads to a great variation in performance and may cause undesirable
side effects such as noise, heating or the like. These problems are solved by using
each stator itself as a gauge and as its own manufacturing tool or jig in order
to customize the positions of the bearings which are to be used with the particular
The inventive method has a solid spacer sleeve 52
(FIG. 2) inserted
over the drive end of the motor shaft 12 for establishing a bearing location.
It is possible to locate the drive end bearing by referencing the shaft end or
by using the solid spacer between the bearing the rotor assembly. However, positioning
the remaining back end bearing should be referenced to the actual stator thickness
and should eliminate the majority of the stackup tolerance variations. Hence, the
invention reduces six dimensional variables to only two variables, thereby making
the assembly easier and at a lower overall cost.
In greater details, outer ball bearing race may be secured to the
bearing housing or cap by one or a combination of the following three methods:
(1) press fit; (2) slip fit with adhesive; or (3) a snug fit in a resilient silicon
rubber sleeve. The following description and the claims are to be construed as
being applicable to all three of these securing methods.
The motor shaft is positioned and secured to the rotor assembly
12 by referencing the distance K between the rotor 12
drive end of shaft 20. The drive-end bearing 50 may or may not be
pressed into the end housing or cap 16. Then, the inner bearing race is
pressed over the motor shaft and positioned either by inserting a "stop" spacer
52 or by referencing the bearing to the end of shaft 20.
A resilient silicon rubber sleeve 54 may be inserted into
the end housing 16. Then, the end housing 16 and its silicon rubber
sleeve 54 assembly are pressed over the bearing 50
until it is seated.
Or, alternatively, an adhesive is applied to the outer surface of the outer race
of the bearing and then the end housing or cap is pressed over the bearing until
it is seated. The reference number 54 may identify either the adhesive
or the rubber sleeve.
The combined rotor/shaft/drive end bearing assembly is then inserted
into a fixture (not shown) supporting the inner race 55 of the drive bearing
50. The motor field assembly including stator 10 is positioned over
the rotor assembly.
A resilient elastomer (here silicon rubber) sleeve 54
into the rear end housing or cup 14. Then, the end housing/silicon rubber
sleeve 54 assembly is pressed over the bearing 58. Or, adhesive is
applied to the outer surface of the outer race of the bearing 58. Then
the end housing or cap 14 is pressed over the bearing. A wave spring is
positioned atop the rear bearing 58. The rear bearing 58 is pressed
over the motor shaft to a defined dimension relative to the rear surface
60 of the stator 10 of the motor field assembly.
Two tie bolts 66 are then driven through holes 28, 34
1) in the rear end bearing housing or cap 14 and holes 30, 36; in
the motor field assembly, and then into holes 32, 38 in the drive end bearing
housing 16. If the bearing outer races are secured by adhesive instead of
the resilient silicon rubber sleeve 54, the bolts 66 are tightened
as quickly as possible. The outer race press fit method cannot be employed on the
rear bearing 58 (FIG 2) and end housing or cap 14 assembly since
there must be at least one slip fit condition in order to apply a preload.
If the resilient silicon rubber sleeves 54 are employed in
both end caps or housings, the wave spring remains somewhat active throughout the
motor life since the assembly is not entirely rigid. The motor can later be at
least partially disassembled if this configuration is employed. If an adhesive
is employed to secure the outer race of one or both bearings, the wave spring
is no longer active after curing since the assembly is rigid. The motor cannot
be disassembled when an adhesive is used. In this case, the wave spring's function
is to provide a preload only until the adhesive cures.
Those who are skilled in the art will readily perceive how to modify
the invention. Therefore, the appended claims are to be construed to cover all
equivalent structures which fall within the true scope and spirit of the invention.