Abstract:
An electric motor has a wound rotor having a shaft, a rotor core, a commutator and windings wound about the rotor core and connected to the commutator. The motor has a stator confronting the rotor; brush gear electrically connecting the commutator to motor terminals; first and second bearings for rotatably supporting the rotor, and an oil collector fitted to the shaft between the commutator and the first bearing for preventing oil migrating along the shaft from the first bearing reaching the commutator. The oil collector is of high temperature material, preferably a metal such as brass, aluminum and steel. The oil collector returns the collected oil to the first bearing.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This non-provisional patent application claims priority under 35 U.S.C. §119(a) from Patent Application No. 0720599.0 filed in Great Britain on 22 Oct. 2007. 
   FIELD OF THE INVENTION 
   This invention relates to an electric motor with a commutator and in particular to an oil migration barrier for such a motor. 
   BACKGROUND OF THE INVENTION 
   In small, fractional and subfractional horsepower, permanent magnet direct current motors, brush gear and a commutator are typically used to transfer electrical power from motor terminals to rotor windings. The commutator, which consists of copper segments laid on a commutator base, is located adjacent one of the bearings rotationally supporting the rotor. Such bearings are typically oil impregnated sintered bushings due to wear and cost considerations. One disadvantage of these bearings is that the oil is not sealed within the bushing and tends to migrate along the shaft. Such migration, if not checked, can extend to the commutator segments where the oil contaminates the brush/segment interface and mixes with brush dust ultimately rendering the motor inoperable by short circuiting commutator segments. Carbon brushes are hydroscopic and readily soak up oil. The brushes then become abrasive and quickly grind down the commutator segments. Oil migration is also a problem for ball and roller bearings, albeit not as great, especially in high temperature applications such as within the engine compartment of a vehicle. 
   In motors used over a wide range of temperatures, for example, the motors used in the engine compartment of a motor vehicle, such as the electronic throttle control motor, the motor for adjusting the vane angle in a turbocharger or supercharger, suction intake pipe actuator, swirl actuator, air flaps, and bypass flaps, etc, which must operate satisfactorily over a wide ambient temperature range, for example, from −40° C. to 180° C., the management of oil in the sintered oil impregnated bushings becomes problematic due to a difference in the coefficient of thermal expansion of the lubricating oil and the material of the bushings. Typical lubricating oils used in sintered bushings have a coefficient of thermal expansion between 2.2 to 3.4×10 −4 , whereas typical sintered bushings have a coefficient of thermal expansion between 12 to 18×10 −6 . At 180° C., the oil volume expands far greater than the volume for the oil in the bushing resulting in oil being pushed out of the bushing. Some of this oil migrates along the shaft. For the bushing at the commutator end of the motor, this oil migration causes a serious life issue. Should the migrating oil reach the commutator and contaminate the commutator segments, the motor will fail. 
   In the past slingers have been used to fling the migrating oil from the shaft on to the surrounding motor casing. The slinger looks like a washer fixed to the shaft. See, for example, GB2207956. However, this results in loss of oil and when the motor is then asked to operate in a low-temperature situation, e.g. at startup after a night in snow country, the oil volume which has been already reduced due to the loss of oil is now further reduced due to the thermal contraction. Thus resulting in the bushing running dry at the beginning, possibly producing an annoying squeaking noise. However, the squeaking noise is indicating a period of high metal on metal wear occurring at the shaft/bearing interface. 
   Also, in the modern engine compartment, these motors are often operated through a very limited range of motion and are expected to work first time, every time. By limited range of motion, typically less than 20 revolutions, maximum, in any one direction, which often is not enough to warm up the bearing oil enough to establish a hydraulic lubricating oil film. Indeed, some motors operate for just a few milli-seconds or less than one complete revolution. In such situations, the use of the standard oil slinger is not helpful as the oil is not slung off, and merely delays migration of the oil to the commutator but does not stop the oil. 
   GB2192312 address this problem by providing a rubber oil collector disc which traps the oil. However, being rubber, care and space is required to prevent the disc from coming into direct contact with the bearing. Also, the motor is not suitable for use in very high temperature applications. It is also not suitable for use in motors subjected to high vibration operating conditions. In addition, there may be problems associated with a possible chemical reaction between the oil and the rubber, out gassing of the rubber and the breakdown or aging of the rubber material over time. 
   Hence, there is a desire or need for a oil collector, which is more suitable for use over a wide temperature range. 
   There is also a secondary desire for such an improved oil collector, which can return collected oil to the bushing thereby creating an oil recirculating system. Thus retaining a high percentage of the oil. 
   SUMMARY OF THE INVENTION 
   Accordingly, the present invention provides an electric motor comprising: a wound rotor having a shaft, a rotor core, a commutator and windings wound about the rotor core and connected to the commutator; a stator confronting the rotor; brush gear connecting the commutator to motor terminals; first and second bearings for rotatably supporting the rotor; and an oil collector fitted to the shaft between the commutator and the first bearing for preventing oil migrating along the shaft from the first bearing reaching the commutator; wherein the oil collector is of high temperature material. 
   Preferably, the oil collector is of metal. 
   Preferably, the oil collector has a body portion for fixedly mounting to the shaft and an oil collecting portion. 
   Preferably, the oil collector is fixed to the shaft by an oil tight press fit. Most preferably, this press fit remains oil tight over the full range of operating temperatures, namely from −40° C. to +180° C. 
   Preferably, the body portion of the oil collector is adapted to make direct contact with an axial face of the first bearing. 
   Preferably, the body portion of the oil collector is adapted to function as a spacer and is fixed fast to the shaft to limit movement of the shaft through the first bearing in a first direction. 
   Preferably, the first bearing is an oil impregnated sintered bushing. 
   Preferably, the bushing is a self aligning bushing. Alternatively, the bushing may be a sleeve bushing. 
   Preferably, the oil collecting portion of the oil collector extends radially outwardly from the body portion and has an annular cavity open towards the first bearing. 
   Preferably, the annular cavity has a radially inwardly extending lip on the radially outer edge of the opening. 
   Preferably, the opening is spaced from the shaft and the diameter of the outer edge of the opening is greater than the diameter of the bearing. 
   Preferably, the bushing has a body portion and an axially extending cylindrical portion having a lesser diameter than the body portion. 
   Preferably, the lip of the oil collector locates radially about the cylindrical portion of the bushing. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
       FIG. 1  is a section view of a subfractional horsepower PMDC motor according to a preferred embodiment of the present invention; 
       FIG. 2  is an enlarged sectional view of a bearing assembly, being a part of the motor of  FIG. 1 ; 
       FIG. 3  is a cross sectional view, taken along line X-X, of the assembly of  FIG. 2 ; 
       FIG. 4  is an enlarged sectional view of a modified bearing assembly; and 
       FIG. 5  is an enlarged sectional view of an alternative bearing assembly. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The motor of  FIG. 1  is a typical the PMDC motor  10  which has been modified to operate in a wide temperature environment, such as in the engine compartment of a motor vehicle. In such an environment the temperature may vary within a range of −40° C. to +180° C. The motor may be used, for example, to drive the throttle valve or butterfly valve in a carburetor system of the engine. 
   The motor  10  has a deep drawn cup shaped metal housing  11 , supporting permanent magnets  12  forming the stator for the motor. A plastics material end cap  13  closes off the open end of the housing. A rotor  14 , comprising a rotor core  15  mounted on a shaft  16  is disposed within the housing  11  such that the rotor core  15  confronts the magnets  12  across an air gap  17 . The shaft  16  extends through the housing  11  and the end cap  13  and is journalled in bushings  18 ,  28 , fitted to the end cap  13  and the closed-end of the housing  11 . The rotor  14  also comprises the commutator  19  fitted to the shaft  16  adjacent a rotor core  15  and windings (not shown for clarity) wound around poles of the core and electrically connected to segments of the commutator  19 . The end cap  13  supports brush gear  20  for transferring electrical power to the windings. The brush gear comprises a pair of brush cages  21  slidably securing a respective cage brush  22  electrically connected to a respective motor terminal  23 . 
   The bushing of the end cap  13 , hereinafter referred to as the first bushing  18 , is of the self aligning type, which means that it is pivotably fixed to the end cap. The first bushing  18  has a body portion  24  which is spherical and an axially extending cylindrical portion  25 , forming a shank, which projects towards the commutator  19 . 
   It would be readily realized by persons skilled in the art that a sleeve bushing, with or without the shank could be used in place of the self aligning first bushing  18 . 
   The bushing of the housing  11 , hereinafter referred to as the second bushing  28 , is also a self aligning bushing having a substantially spherical appearance, which is pressed into engagement with a bearing hub  29  formed in or fitted to the closed end of the housing  11  by a spring  30 . A collar  26  is fitted to the shaft  16  to limit axial movement of the rotor and forms an abutment which contacts the second bushing  28  through a low friction washer  29 . 
   It would be readily realized by persons skilled in the art that a sleeve bushing or a ball bearing could be used in place of the self aligning second bushing  28 . 
   A portion of the end cap assembly is shown enlarged in  FIG. 2 . The spherical portion  24  of the first bushing  18  mates with a partly spherical bearing recess  31  in the end cap  13  and is urged into engagement by a spring, known as a bearing retainer  32 , which is press fitted into an outer portion of the bearing recess  31 . The bearing retainer  32  has resilient fingers  33 , which engage the spherical portion  24  of the first bushing  18  to retain it. The cylindrical portion  25  projects axially inwardly passed the fingers  33  of the bearing retainer  32  towards the commutator  19 . 
   Between the commutator  19  and the first bushing  18  is an oil collector  35 . The oil collector  35  is pressed onto the shaft  16  in an oil tight manner. In the preferred embodiment shown, the oil collector  35  is located abutting an axial face of the commutator through an insulating washer  36 , with a washer  34  of metal or other friction reducing material located between the oil collector  35  and the first bushing  18 . However, the proximity of the oil collector  35  to the commutator  19  is not material to the working of the oil collector  35  and the insulating washer  36  is thus optional. 
   The oil collector  35  has a body portion  37  and oil collecting portion  38 . The body portion  37  is optionally cylindrical with a through hole for receiving the shaft  16 . The body portion  37  is preferably a press fit onto the shaft  16  and is fixed to the shaft  16  in an oil tight manner to prevent oil migrating along the shaft passing between the shaft  16  and the oil collector  35 . Bearing adhesive may be used to ensure an oil tight fit. It is, however, highly preferable that the press fit is an oil tight press fit which remains oil tight over the full range of operating temperatures, namely, from −40° C. to +180° C. for the automotive application. 
   The collector portion  38  of the oil collector  35  extends from the body portion  37  in an integral, one-piece, monolithic construction. The radial height of the oil collector  35  is restricted as the oil collector is fitted to the shaft  16  before the rotor  14  is fitted to the end cap  13 , which supports the brush gear  20 , thus the oil collector  35  must pass between the brush cages  21  during assembly. In this embodiment, the brush gear  20  comprises two cage brushes  22 , which make sliding contact with the commutator to transfer electrical power thereto. The cages  21  approach the commutator surface to provide stable support for the brushes  22  but this close support limits the radial height of the oil collector  35 , giving the collector a squat appearance. Thus, the oil collector portion  38  has a radial portion  39 , an axial portion  40  and a lip  41 . The radial portion  39  extends radially and from the body portion  37 , preferably, but not necessarily, at an edge of the body portion  37  remote from the bushing  18  to maximize the volume of the oil collector portion  38 . The axial portion  40  extends from the radial portion  39  towards the first bushing  18 . The distal end of the axial portion  40  optionally surrounds at least a portion of the cylindrical portion  25  of the first bushing  18  and the intervening washer  34 , if present. The lip  41  extends radially inward at the distal end of the axial portion  40 . Optionally, the lip  41  is partially reentrant, to positively trap migrating oil. 
   The radial portion  39 , axial portion  40  and lip  41  form an annular cavity or oil collecting space  42 , which retains migrating oil rather than slinging it away from the shaft. Such slinging usually means the oil coats an inner surface of the end cap and could drip on to the commutator. 
   Optionally, the oil collector is adapted to allow the collected oil to return to the bushing  18 . This is achieved through the location of the lip  41  and optionally by the shape of the lip. 
   In the arrangement shown in  FIG. 2 , assuming the motor is mounted horizontally as shown, at rest oil  43  will pool to the bottom of the oil collecting space  42  until it reaches the top of the lip  41  at its lowest point has illustrated in  FIG. 2  and in the cross-section view of  FIG. 3 . 
   As migrating oil is collected, it is retained by the lip  41  and pools at rest in the bottom of the oil collecting space  42  until the oil level rises above the lip  41 , at which time the oil  43  bridges the gap between the lip  41  and the bushing  18  whereby as the bushing cools the oil  43  returns to the bushing  18  under capillary action, thus returning lost oil to the bushing  18 . The oil collector now functions as an oil recirculating system. 
   Alternatively, oil may drip from the oil collector onto the first bushing. This ‘dripping back recirculation’ works for all positions with shaft horizontal within a deviation of −45° to +45° depending on the chosen shape of the oil collecting space  42 . It will not work for recirculation with the shaft vertical and the bearing on top. In this orientation, the only function is to collect the migrating oil and prevent it from reaching the commutator. However, recirculation will occur when the shaft is vertical if the commutator is above the first bushing. In this situation, any oil collected by the collector can drip directly onto the bushing. 
   The dripping recirculation occurs when the collected oil  43  is on the top side of the oil collecting space which may occur when the motor is rotated and stopped. For example the motor may be rotated through 180° or odd multiples there of. The oil may then, under gravity, drip from the lip  41  to the bushing  18  below, especially the shank  25 . Some of the oil  43  may remain in the oil collecting space  42  to be dripped out at a future time following operation of the motor. The remaining oil again pools at the bottom of the oil collecting space. 
     FIG. 4  illustrates an alternative lip arrangement. The lip  41  extends radially inwardly and axially outwardly and thus under centrifugal action during rotation of the rotor, still positively holds or retains oil but when stationery delivers the oil further axially along the cylindrical portion  25  of the first bushing  18 . Here the lip  41  is located radially about the cylindrical portion  25  of the bushing. This assists the return of collected oil to the bushing. 
   The oil collector  35  is of a high temperature material, preferably bronze, brass or steel, so that the coefficient of thermal expansion can be similar to that of the material the shaft  16 . Thus maintaining the oil tight connection between the oil collector and the shaft over a wide temperature range. 
   The outer surface of the oil collector  35  may be treated so as to have or form a oleophobic surface to further prevent oil migration over the outer surface of the oil collector  35 . 
   When the oil collector  35  is made of metal, the body portion  37  may be adapted to make direct contact with the first bushing  18  (as shown in  FIG. 4 ). Thus eliminating the need for the low friction washer (washer  34  of  FIG. 2 ) which normally would be provided between the oil collector and the first bushing. 
   The low friction washer can be eliminated also if the oil collector is made of suitable, low friction engineering plastics, but such material may lose the advantage of the similarity of the coefficient of thermal expansion. 
   Although the invention has been described with reference to a preferred embodiment, it should be appreciated by those in the art that various modifications are possible within the scope of the invention. Therefore, the scope of the invention is to be determined by reference to the claims that follow. 
   For example, while the preferred bushing is shown as a self aligning bushing, a sleeve bushing would be acceptable and it is desirable, although not essential, that the bushing have a shank formed by an axially extending cylindrical portion of diameter generally less than the major diameter of the bushing. Sleeve bushings may be directly press fitted into bearing cavities formed in the end cap or motor housing. 
     FIG. 5  illustrates an example where the first bushing  18  is a sleeve bushing having a body portion  24  and a shank formed by an axially extending cylindrical portion  25 . The body portion  24  is cylindrical and pressed into a bearing recess in the end cap  13 . 
   Although the invention is described with reference to one or more preferred embodiments, it should be appreciated by those skilled in the art that various modifications are possible. Therefore, the scope of the invention is to be determined by reference to the claims that follow. 
   In the description and claims of the present application, each of the verbs “comprise”, “include”, “contain” and “have”, and variations thereof, are used in an inclusive sense, to specify the presence of the stated item but not to exclude the presence of additional items.