US20060181168A1 - Pump motor with bearing preload - Google Patents

Abstract

A electrical machine includes a housing assembly, first and second bearings mounted in the housing assembly, and a rotor assembly mounted in the bearings such that it has a predetermined amount of axial and radial play relative to the housing. A biasing element is disposed between one of the rotor assembly or the housing and one of the bearings. The biasing element urges the rotor assembly to a preloaded position which eliminates the axial and radial play. Each of the first inner and outer races and the second inner and outer races are secured to one of the rotor assembly or to the housing, such that the rotor assembly is retained in the preloaded position. The components of the electrical machine are chosen such that this preload is maintained over the operating temperature range of the machine.

Description

BACKGROUND OF THE INVENTION
• This invention relates generally to electric motors and more particularly to an electric motor intended to be used with a reciprocating load such as a diaphragm pump. Electric motors often use bearings to reduce friction, particularly rolling element bearings such as ball bearings. Commercially available bearings have some clearance between their individual components, e.g. between the balls and the outer race or the inner race, thereby allowing some degree of radial and axial play. In an application where the motor is connected to a cyclic load, particularly a radial load (i.e. perpendicular to the motor shaft axis) such as that applied by a diaphragm pump, the interaction of the bearing play with the load may cause the motor life to be appreciably reduced through fatigue, fretting of the motor components, and rapid wear.
• Attempts have been made to apply a preload to motor bearing assemblies to remove play. However, in operation the motor will be subject to changing internal temperatures, resulting from heat generated by the motor itself or absorbed from the environment in which the motor operates. The parts of the motor responsible for creating the bearing preload condition have differing rates of thermal expansion. This varying thermal expansion may cause the preload on the bearings to be lost, resulting in the accelerated wear described above. The varying thermal expansion may also cause an excessive axial and/or radial load to be placed on the bearings thus also accelerating wear.
• Accordingly, it is an object of the invention to provide a motor in which the radial and axial play is eliminated from the bearings thereof.
• it is another object of the invention to provide a motor having a consistent preload under all operating conditions.
• It is another object of the invention to provide a method of assembling a motor which eliminates radial and axial play from the bearings.
BRIEF SUMMARY OF THE INVENTION
• These and other objects of the present invention are achieved in the preferred embodiments disclosed below by providing. an electrical machine, including: a housing assembly having first and second ends; a first bearing mounted in the housing, the first bearing having a plurality of rolling elements disposed between first inner and outer races; and a second bearing mounted in the housing and spaced away from the first bearing, the second bearing having a plurality of rolling elements disposed between second inner and outer races.
• A rotor assembly having first and second ends is mounted in the first and second bearings, respectively, such that the rotor has a predetermined amount of axial and radial play relative to the housing. A biasing element is disposed between one of the rotor assembly or the housing and one of the bearings. The biasing element urges the rotor assembly to a preloaded position which eliminates the axial and radial play. Each of the first inner and outer races and the second inner and outer races is secured to one of the rotor assembly or to the housing, such that the rotor assembly is retained in the preloaded position.
• According to another embodiment of the invention, the first and second outer races are secured to the housing, and the first and second inner races are secured to the shaft.
• According to another embodiment of the invention, the biasing element comprises a spring disposed between the rotor assembly and the first or second inner race.
• According to another embodiment of the invention, the biasing element is a spring disposed between the housing and the first or second outer race.
• According to another embodiment of the invention, the housing assembly includes a generally cylindrical housing including an axially extending portion with a front end plate connected to a front end thereof; and an end bell attached to a rear end of the housing.
• According to another embodiment of the invention, the coefficients of thermal expansion of the housing assembly, the bearings, and the rotor are selected so that the rotor assembly will be retained in the preloaded position over a temperature range of about −40° C. to about 105° C.
• According to another embodiment of the invention, the bearings are constructed from high carbon chromium steel and the housing assembly and the rotor assembly are constructed from 400 series stainless steel.
• According to another embodiment of the invention, a method of assembling an electrical machine includes providing a housing having first and second ends; disposing a first bearing in the housing, the first bearing having a plurality of rolling elements disposed between first inner and outer races; disposing a second bearing in the housing, the second bearing having a plurality of rolling elements disposed between second inner and outer races; and providing a rotor assembly having a longitudinally-extending shaft.
• The rotor assembly is rotatably mounted in the housing with the shaft received in the first and second bearings, such that the rotor is in a first position in which it has a predetermined amount of axial and radial play relative to the housing. A biasing element is installed between one of the rotor assembly or the housing and one of the bearings, such that the biasing element forces the rotor assembly to a second position in which the axial and radial play is eliminated. Each of the first inner and outer races and the second inner and outer races is secured to one of the rotor assembly or to the housing, such that the rotor assembly is retained in the second position.
• According to another embodiment of the invention, the first and second outer races are secured to the housing, and the first and second inner races are secured to the shaft
• According to another embodiment of the invention, the biasing element comprises a spring disposed between the housing and the first or second outer race.
• According to another embodiment of the invention, each of the first inner and outer races and the second inner and outer races is secured by a method selected from the group consisting of: press fitting, adhesive bonding, welding, or brazing
• According to another embodiment of the invention, an electric motor, includes a generally cylindrical housing assembly having first and second ends, the housing defining first and second spaced-apart bearing pockets; a first bearing having a plurality of rolling elements disposed between first inner and outer races, the first outer race being received in the first bearing pocket; a second bearing having a plurality of rolling elements disposed between second inner and outer races, the second outer race being received in the second bearing pocket; and a rotor assembly including a shaft received in the first and second inner races, such that the rotor has a predetermined amount of axial and radial play relative to the housing.
• A biasing element is disposed between one of the rotor assembly or the housing and one of the bearings which urges the rotor assembly to a preloaded position which eliminates the axial and radial play. The first inner and outer races are secured to the shaft, and the second inner and outer races are secured to the housing, such that the rotor assembly is retained in the preloaded position.
BRIEF DESCRIPTION OF THE DRAWINGS
• The subject matter that is regarded as the invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which:
• FIG. 1 is a side elevational view of a ball bearing in a rest condition.
• FIG. 2 is a side elevational view of the ball bearing ofFIG. 1 in a preloaded condition.
• FIG. 3 is enlarged view of a portion of the bearing ofFIG. 2.
• FIG. 4 is a side elevational view of a first embodiment of a motor constructed in accordance with the present invention.
• FIG. 5 is a side elevational view of a first alternative arrangement of the components of the motor ofFIG. 4.
• FIG. 6 is a side elevational view of a second alternative arrangement of the components of the motor ofFIG. 4.
• FIG. 7 is a side elevational view of a third alternative arrangement of the components of the motor ofFIG. 4.
• FIG. 8 is a side elevational view of a second embodiment of a motor constructed in accordance with the present invention.
• FIG. 9 is a side elevational view of a first alternative arrangement of the components of the motor ofFIG. 8.
• FIG. 10 is a side elevational view of a second alternative arrangement of the components of the motor ofFIG. 8.
• FIG. 11 is a side elevational view of a third alternative arrangement of the components of the motor ofFIG. 8.
DETAILED DESCRIPTION OF THE INVENTION
• Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,FIG. 1 shows a schematic view of a typical ball bearing1 including generally cylindrical, concentrically disposed inner andouter races2 and3. An array ofballs4 are mounted between the races. Theballs4 may be separated and located by acage5 as shown. Theballs4 are received inarcuate grooves6 and7 formed in the inner and outer races respectively. The grooves have a radius of curvature greater than the radius of theballs4, so that when assembled theballs4 will have a point contact with the races. Because of spacing between the various elements, the bearing1 has a radial clearance in the direction denoted “R”, and an axial clearance in the direction denoted “A”. These clearances allow relative radial and axial motion between theinner race2 and theouter race3.
• FIG. 2 depicts the bearing1 in a preloaded condition. An axial preload force is applied to the bearing1 in the direction of arrow P. This causes theinner race2 to shift axially with respect to theouter race3. As shown more clearly inFIG. 3, The axial motion is stopped by the interference of theballs4 with the grooves in the inner andouter races2 and3. Additionally, because of the arcuate shape of the grooves, relative axial motion of the bearing races causes a wedging effect which prevents relative radial motion between the inner and outer races. Thus, an axial preload may be used to remove both axial and radial play from a ball bearing.
• Turning now to the present invention,FIG. 4 shows a first embodiment of amotor10 constructed in accordance with the present invention. The illustrated example is of a brushless permanent magnet DC motor, but the operative principle of the present invention is equally application to other types of motors as well. The basic components of themotor10 are ahousing12, anend bell14, astator16, arotor assembly18, afront bearing20, arear bearing22, and aspring24. Thehousing12 is a generally cylindrical, open-ended member including anaxially extending portion26 and afront end plate28 which has afront bearing pocket30 formed therein. The front end plate portion of thehousing12 could also be a separate component attached by a variety of methods, for example, screws, press fit, welding, etc. Thehousing12 may be formed by any known method including casting, forging, machining, powder metallurgy, etc. Theend bell14 is a member adapted to close off the rear end of thehousing12 and is attached to the rear end of thehousing12, for example by themachine screws32 shown inFIG. 4. Theend bell14 has arear bearing pocket34 formed therein. Thestator16 is of a known type comprising an array of flat plates wound with coils of wire. Therotor assembly18 comprises ashaft36 having acentral portion38, an axially extendingfront shaft extension40, and an axially extendingrear shaft extension42. A plurality ofpermanent magnets44 are secured to the outer surface of the central portion, for example with an adhesive. Thefront bearing20 is of a known rolling-element type such as a ball bearing. Itsouter race46 is received in thefront bearing pocket30, and itsinner race48 receives thefront shaft extension40 of therotor assembly18. Therear bearing22 is also of a known rolling-element type such as a ball bearing. Itsouter race50 is received in therear bearing pocket34, and itsinner race52 receives a portion of therear shaft extension42. In the illustrated example the spring is a compression-type coil spring. However, thespring24 may be of any type which fits in the space provided for it and which provides the required preload force. A Belleville spring washer could be used, for example.
• Themotor10 is assembled so that a preload is applied to thebearings20 and22 which removes all axial and radial play in each bearing as described above. The preload is applied such that the inner races of the bearings are axially biased in opposite directions. An exemplary assembly sequence is as follows. Therear bearing22 is assembled to theend bell14. Theouter race50 of therear bearing22 is secured to theend bell14 so that it cannot move relative to theend bell14, for example by press fit, adhesive, tack welding, brazing, or the like. Thefront bearing20 is then assembled to thehousing12. Theouter race46 of thefront bearing20 is secured to thehousing12 so that it cannot move relative to thehousing12, in a manner similar to therear bearing22.
• Thespring24 is then assembled to thefront shaft extension40 of therotor assembly18, and therotor assembly18 is then inserted in thehousing12. One end of thespring24 bears against theinner race48 of thefront bearing20 and the other end of thespring24 bears against thecentral portion38 of therotor assembly18. Theend bell14 is subsequently attached to thehousing12 which places therear shaft extension42 into theinner race52 of therear bearing22. The action of thecompressed spring24 forces the inner races of each bearing outward into a condition where all axial and radial play is eliminated. This creates a preload force of a magnitude determined by the characteristics of thespring24.
• Finally, theinner race48 of thefront bearing20 is secured to thefront shaft extension40, and theinner race52 of therear bearing22 is secured torear shaft extension42, so that no relative motion can take place between either of the inner races and therotor assembly18. The inner races may be secured to therotor assembly18 by a variety of methods, as described above. Thus, the components of themotor10 are secured in a position which maintains the preload created by thespring24 during the assembly process. The arrangement eliminates all axial and radial play from the bearings and shaft.
• FIG. 5 illustrates amotor110 which is a variation of themotor10 depicted inFIG. 4. In this instance, thespring24 is placed over therear shaft extension42 of therotor assembly18, between thecentral portion38 of theshaft36 and theinner race52 of therear bearing22. The assembly and operation of themotor110 is otherwise similar to that of the example illustrated inFIG. 4 and described above.
• FIG. 6 illustrates anothervariation210 of themotor10. The construction is again generally similar to that illustrated inFIG. 4 above, the primary difference being that thespring24 bears on the outer race of the bearings, as described in detail below.
• Assembly of themotor210 starts with thefront bearing20 being assembled to thehousing12. Theouter race46 of thefront bearing20 is secured to thehousing12 so that it cannot move relative to thehousing12, for example by press fit, adhesive, tack welding, brazing, or the like. Therotor assembly18 is assembled to thehousing12. Theinner race48 of thefront bearing20 is secured to thefront shaft extension40 so that it cannot move relative to thefront shaft extension40.
• Therear bearing22 is then assembled to therotor assembly18. Theinner race52 of therear bearing22 is secured to the rear shaft extension so it cannot move relative to the rear shaft extension. Thespring24 is assembled to theend bell14, being inserted in the rear bearing pocket. Theend bell14 is then assembled to thehousing12 which inserts therear bearing22 into theend bell14. Thespring24 thus mates between theend bell14 and theouter race50 of therear bearing22.
• The action of thecompressed spring24 forces the inner races of each bearing outward into a condition where all axial and radial play is eliminated. This creates a preload force of a magnitude determined by the characteristics of thespring24.
• Finally, theouter race50 of therear bearing22 is secured to theend bell14, so that no relative motion can take place between theouter race50 and theend bell14. Theouter race50 may be secured to theend bell14 by a variety of methods, as described above. Thus, the components of themotor210 are secured in a position which maintains the preload provided by thespring24 during the assembly process. This arrangement eliminates all axial and radial play from the bearing/shaft mechanism.
• FIG. 7 illustrates avariation310 of themotor210. In this instance, thespring24 is placed over thefront shaft extension40 of therotor assembly18, between thehousing12 and theouter race50 of therear bearing22. The assembly and operation of this variation is otherwise similar to that of the example illustrated inFIG. 6 and described above.
• FIG. 8 shows a second embodiment of amotor410 constructed in accordance with the present invention. This type of motor is sometimes referred to as a cantilevered design because of the relationship of the rotor assembly to the bearings. Elements in common with the motors depicted inFIGS. 4-7 are shown in prime reference numerals. The basic components of themotor410 are ahousing12′, astator16′, arotor assembly18′, afront bearing20′, arear bearing22′, and apreload spring24′. Thehousing12′ is a generally cylindrical, open-ended member including outer axially extendingportion26′, an inneraxially extending portion27, and afront end plate28′. The inner axially extendingportion27 defines afront bearing pocket30′ and arear bearing pocket34′. Thehousing12′ may be formed by any known method including casting, forging, machining, powder metallurgy, etc. Thestator16′ is of a known type comprising an array of flat plates wound with coils of wire. Therotor assembly18′ comprises ashaft36′, amagnet hub37 attached to the rear end of theshaft36′, and a plurality ofpermanent magnets44′ secured to the outer surface of themagnet hub37, for example with an adhesive. Thefront bearing20′ is of a known rolling-element type such as a ball bearing. Itsouter race46′ is received in thefront bearing pocket30′, and itsinner race48′ receives thefront shaft extension40′ of therotor assembly18′. Therear bearing22′ is also of a known rolling-element type such as a ball bearing. Itsouter race50′ is received in therear bearing pocket34′, and itsinner race52′ receives a portion of theshaft36′. In the illustrated example the spring is a compression-type coil spring. However, thespring24′ may be of any type which fits in the space provided for it and which provides the required preload force. A Belleville spring washer could be used, for example.
• Themotor410 is assembled so that a preload is applied to thebearings20′ and22′ which removes all axial and radial play in each bearing as described above. The preload is applied such that the bearings are axially biased in opposite directions. An exemplary assembly sequence is as follows. First, thespring24′ is assembled to therotor assembly18′. Therear bearing22′ is assembled to thehousing12′. Theouter race50′ of therear bearing22′ is secured to thehousing12′ so that no relative motion can take place between theouter race50′ and thehousing12′, for example by press fit, tack welding, brazing, adhesive, etc.
• Thefront bearing20′ is assembled to thehousing12′. Theouter race46′ of thefront bearing20′ is secured to thehousing12′ so that no relative motion can take place between theouter race46′ and thehousing12′.
• Next, therotor assembly18′ is assembled to thehousing12′, placing theshaft36′ into the inner races of each bearing. Alock ring54 is then assembled to the front end of theshaft36′. This compresses thespring24′. The action of thecompressed spring24′ forces the inner races of each bearing inward into a condition where all axial and radial play is eliminated. This creates a preload force of a magnitude determined by the characteristics of thespring24′.
• Finally, the inner races of both thefront bearing20′ and the rear bearing are secured to theshaft36′ so that no relative motion can take place between the inner races and theshaft36′, in a manner described above. This arrangement eliminates all axial and radial play from the bearing and shaft mechanism.
• FIG. 9 illustrates amotor510 which is a variation of themotor410 depicted inFIG. 8. In this instance, thespring24′ is placed over the front end of theshaft36′ between thelock ring54 and theinner race48′ of thefront bearing20′. The assembly and operation of themotor510 is otherwise similar to that of the example illustrated inFIG. 8 and described above.
• FIG. 10 illustrates anothervariation610 of themotor410 The construction is again generally similar to that illustrated inFIG. 8 above, the primary difference being that thespring24′ bears on the outer race of the bearings, as described in detail below.
• First, thespring24′ is assembled to thehousing12′. Therear bearing22′ is then assembled to therotor assembly18′. Theinner race52′ of therear bearing22′ is secured to the shaft so that no relative motion can take place between theinner race52′ and theshaft36′, for example by press fit, tack welding, brazing, adhesive, etc.
• Thefront bearing20′ is assembled to thehousing12′. Theouter race46′ of the front bearing is secured to thehousing12′ so that no relative motion can take place between theouter race46′ and thehousing12′, in a manner described above.
• Therotor assembly18′ is assembled to thehousing12′. This places theshaft36′ into theinner race48′ of thefront bearing20′. Alock ring54 is then assembled to theshaft36′. This compresses thespring24′. The action of thecompressed spring24′ forces the inner races of each bearing inward into a condition where all axial and radial play is eliminated. This creates a preload force of a magnitude determined by the characteristics of thespring24′.
• Finally, the inner race of thefront bearing20′ is secured to theshaft36 and theouter race50′ of therear bearing22′ is secured to thehousing12′ so that no relative motion can take place between these components, in a manner described above. This arrangement eliminates all axial and radial play from the bearing and shaft mechanism.
• FIG. 11 illustrates amotor710 which is a variation of themotor610 depicted inFIG. 10. In this instance, thespring24′ is placed over the front end of theshaft36′ between end of thefront bearing pocket30′ and theouter race46′ of thefront bearing20′. The assembly and operation of themotor710 is otherwise similar to that of the example illustrated inFIG. 10 and described above.
• While several basic configurations and methods of assembly have been described above, it is noted that the specific configuration or assembly sequence is not critical to the present invention. Rather, it is important that a preload be applied to remove axial and radial play from the rotor and bearing assemblies, and that the inner and outer race of each of the bearings be secured such that no relative motion can take place between the race and the mating component. Furthermore, a preload must be maintained over the motor's operating temperature range adequate to preserve a zero-play condition in the axial and radial directions, under the expected loads. This is accomplished by the selection of materials used for the housing, rotor assembly, and bearings based on their coefficients of thermal expansion. The difference in coefficients of thermal expansion of the various components is minimized. Furthermore, the absolute value of the coefficient of linear thermal expansion of each component is minimized, because even if all of the components are of the same material, excessive thermal expansion will cause loss of the bearing preload if the coefficient of linear thermal expansion is too high. Examples of materials which are known to exceed the required coefficient of linear thermal expansion include brass, zinc, and aluminum.
• An example of a suitable combination of materials is as follows. The bearings may be made of a stainless steel alloy such as high carbon chromium steel, JIS G4805/SUJ2. This is consistent with the alloys used in commercially available ball bearings, and provides a baseline for the coefficient of linear thermal expansion to be matched by the other motor components. Accordingly, the housing, shaft and end bell may be made from a stainless steel alloy, such as a 400-series alloy. Alternatively, some of these parts could be made from a low-carbon steel. This combination of materials will preserve an adequate preload over the operating temperature of a typical motor, for example from about −40° C. (−40° F.) to about 105° C. (220° F.).
• The foregoing has described a motor assembly for use with a reciprocating load such as a diaphragm pump. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention. Accordingly, the foregoing description of the preferred embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation.

Claims (19)

1. A electrical machine, comprising:

a housing assembly having first and second ends;

a first bearing mounted in said housing, said first bearing having a plurality of rolling elements disposed between first inner and outer races;

a second bearing mounted in said housing and spaced away from said first bearing, said second bearing having a plurality of rolling elements disposed between second inner and outer races;

a rotor assembly having first and second ends mounted in said first and second bearings, respectively, such that said rotor has a predetermined amount of axial and radial play relative to said housing; and

a biasing element disposed between one of said rotor assembly or said housing and one of said bearings, said biasing element urging said rotor assembly to a preloaded position which eliminates said axial and radial play, wherein each of said first inner and outer races and said second inner and outer races is secured to one of said rotor assembly or to said housing, such that said rotor assembly is retained in said preloaded position.

2. The electrical machine ofclaim 1 wherein said first and second outer races are secured to said housing, and said first and second inner races are secured to said shaft.

3. The electrical machine ofclaim 1 wherein said biasing element comprises a spring disposed between said rotor assembly and said first or second inner race.

4. The electrical machine ofclaim 1 wherein said biasing element comprises a spring disposed between said housing and said first or second outer race.

5. The electrical machine ofclaim 1 wherein said housing assembly comprises:

a generally cylindrical housing including an axially extending portion with a front end plate connected to a front end thereof; and

an end bell attached to a rear end of said housing.

6. The electrical machine ofclaim 1 wherein the coefficients of thermal expansion of said housing assembly, said bearings, and said rotor are selected so that said rotor assembly will be retained in said preloaded position over a temperature range of about −40° C. to about 105° C.

7. The electrical machine ofclaim 6 wherein said bearings are constructed from high carbon chromium steel and said housing assembly and said rotor assembly are constructed from 400 series stainless steel.

8. A method of assembling an electrical machine, comprising:

providing a housing having first and second ends;

disposing a first bearing in said housing, said first bearing having a plurality of rolling elements disposed between first inner and outer races;

disposing a second bearing in said housing, said second bearing having a plurality of rolling elements disposed between second inner and outer races;

providing a rotor assembly having a longitudinally-extending shaft;

rotatably mounting said rotor assembly in said housing with said shaft received in said first and second bearings, such that said rotor is in a first position in which it has a predetermined amount of axial and radial play relative to said housing;

installing a biasing element between one of said rotor assembly or said housing and one of said bearings, such that said biasing element forces said rotor assembly to a second position in which said axial and radial play is eliminated; and

securing each of said first inner and outer races and said second inner and outer races to one of said rotor assembly or to said housing, such that said rotor assembly is retained in said second position.

9. The method ofclaim 8 wherein said first and second outer races are secured to said housing, and said first and second inner races are secured to said shaft.

10. The method ofclaim 8 wherein said biasing element comprises a spring disposed between said shaft and said first or second inner race.

11. The method ofclaim 8 wherein said biasing element comprises a spring disposed between said housing and said first or second outer race.

12. The method ofclaim 8 wherein each of said first inner and outer races and said second inner and outer races is secured by a method selected from the group consisting of: press fitting, adhesive bonding, welding, or brazing.

13. An electric motor, comprising:

a generally cylindrical housing assembly having first and second ends, said housing defining first and second spaced-apart bearing pockets;

a first bearing having a plurality of rolling elements disposed between first inner and outer races, said first outer race being received in said first bearing pocket;

a second bearing having a plurality of rolling elements disposed between second inner and outer races, said second outer race being received in said second bearing pocket;

a rotor assembly including a shaft received in said first and second inner races, such that said rotor has a predetermined amount of axial and radial play relative to said housing; and

a biasing element disposed between one of said rotor assembly or said housing and one of said bearings which urges said rotor assembly to a preloaded position which eliminates said axial and radial play, wherein said first inner and outer races are secured to said shaft, and said second inner and outer races are secured to said housing, such that said rotor assembly is retained in said preloaded position.

14. The electric motor ofclaim 13 wherein said first and second outer races are secured to said housing, and said first and second inner races are secured to said shaft.

15. The electric motor ofclaim 13 wherein said biasing element comprises a spring disposed between said shaft and said first or second inner race.

16. The electric motor ofclaim 13 wherein said biasing element comprises a spring disposed between said housing and said first or second outer race.

17. The electric motor ofclaim 13 wherein said housing assembly comprises:

a generally cylindrical housing including an axially extending portion with a front end plate connected to a front end thereof; and

an end bell attached to a rear end of said housing.

18. The electric motor ofclaim 13 wherein the coefficients of thermal expansion of said housing assembly, said bearings, and said rotor are selected so that said rotor assembly will be retained in said preloaded position over a temperature range of about −40° C. to about 105° C.

19. The electric motor ofclaim 18 wherein said bearings are constructed from high carbon chromium steel and said housing assembly and said rotor assembly are constructed from 400 series stainless steel.

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