Abstract:
For a motor having a plastic bracket, a single-piece ground strap grounds both the motor&#39;s stator and rotor. The ground strap has one end fastened to a bearing retainer of the motor, an opposite end connected to a ground wire, and an intermediate section that is compressed between the stator and the housing. The end connected to the bearing retainer grounds the rotor, and the intermediate section grounds the stator. The ground strap is fastened to the motor using fasteners that already exist in an otherwise identical but ungrounded motor. In some cases, a combination bearing retainer and ground strap is blanked and formed as a unitary piece.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The subject invention generally pertains to electric motors with plastic housings and more specifically to ground strap for such a motor. 
   2. Description of Related Art 
   Motors, particularly for residential or commercial appliances such as vacuum cleaners, carpet cleaners, etc. are preferably grounded to prevent electrical shock to the user should a short circuit occur in the motor. For greatest protection, it is best to ground both the stator and the rotor (armature) of the motor. This is readily accomplished when the stator and rotor are supported by a housing or bracket made of cast aluminum or other electrically conductive material. In such cases, a ground wire is simply attached somewhere to the housing. Cast aluminum housings, unfortunately, are often more expensive and heavier than plastic ones, so some effort has been made to ground motors that have plastic housings. 
   U.S. Pat. Nos. 4,698,534 and 4,621,991, for example, disclose a motor supported by a plastic bracket wherein the rotor is grounded by a disk/clip assembly that is pinched or simply jammed between the plastic bracket and the rotor&#39;s bearing. Although assembling such a connection may be quick and easy, motor vibration and fatigue may eventually diminish electrical continuity between the clip and the bearing. Nonetheless the design offers the advantage of being able to have the clip extend to where it can be crushed or clamped between the stator and the plastic bracket thereby grounding the stator as well. 
   U.S. Pat. No. 5,313,129 discloses a means for grounding a rotor by connecting a ground wire to the rotor&#39;s bearing. Although the rotor appears to be grounded, it is not clear whether the stator is grounded. It appears that grounding the stator would require additional structure. 
   For motors whose housing or bracket is made of plastic, the stator can be grounded by welding a ground lug or wire directly to the side of the stator. Although effective, such a method requires additional equipment and labor. 
   Consequently, there is a need for an easy, inexpensive way to ground both the rotor and the stator of a motor that has a plastic housing and do so with a minimal number of parts. 
   SUMMARY OF THE INVENTION 
   It is an object of the invention to ground both the rotor and the stator of a motor having a plastic housing/bracket. 
   Another object of some embodiments is to fasten a ground strap using existing fasteners of the motor, whereby the fasteners serve multiple purposes. 
   Another object of some embodiments is to render a motor groundable by adding a single part to an otherwise fully functional but ungrounded motor. 
   Another object of some embodiments is to use a stator tie rod to help clamp a ground strap in place. 
   Another object of some embodiments is to use a bearing retainer fastener to help anchor a ground strap. 
   Another object of some embodiments is to configure a ground strap with two generally parallel sections for electrically coupling a rotor bearing to an axial end of a stator, wherein the rotor bearing and the end of the stator are axially offset and parallel to each other. 
   Another object of some embodiments is to minimize the cost and part-count of a grounded motor whose housing is made of plastic. 
   Another object of some embodiments is to provide a very solid connection between a ground strap and a stator and the ground strap and a rotor. 
   Another object of some embodiments is to provide a ground strap with a crushable feature that ensures a positive electrical connection between the ground strap and a stator. 
   Another object of some embodiments is to provide a safe, lightweight air-moving appliance by providing the appliance with a grounded motor whose housing is made of plastic. 
   Another object of some embodiments is to provide a plastic motor housing with a slot for receiving the ground strap so that the slot helps align a hole in the strap with a tie rod that fastens the strap to the housing. 
   Another object of some embodiments is to provide a plastic motor housing with a slot for receiving the ground strap, wherein the slot has a depth that ensures proper compression of the ground strap yet allows the stator to squarely engage the motor housing. 
   Another object of some embodiments is to provide a single-piece combination ground strap and bearing retainer. 
   Another object of some embodiments is to have a motor bracket, an air guide, a bearing retainer, and a ground strap share a common fastener. 
   One or more of these and/or other objects of the invention are provided by a motor that comprises a stator, a rotor, and a plastic housing. To ground the rotor and the stator, a single ground strap has one end fastened to a bearing retainer of the motor, an opposite end made available for connection to a suitable ground wire, and a intermediate section that is compressed between the stator and the housing. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional view of a motor that includes a ground strap according to one embodiment of the invention. 
       FIG. 2  is a top view of the motor of  FIG. 1  but with the stator, rotor and other parts omitted to show a top view of the ground strap. 
       FIG. 3  is a side view of the ground strap used in the embodiment of  FIG. 1 . 
       FIG. 4  is a side view similar to  FIG. 3  but showing a variation of the ground strap. 
       FIG. 5  is a cross-sectional view taken along line  5 — 5  of  FIG. 4 . 
       FIG. 6  is a cross-sectional view of a motor that includes an airguide, wherein portions of the bearing bracket are not illustrated so that the airflow pattern through the motor can be seen more clearly. 
       FIG. 7  is a perspective view of a ground strap that is spot-welded to a bearing retainer. 
       FIG. 8  is a perspective view of a combination bearing retainer and ground strap that is blanked and formed as a unitary piece. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   A motor  10 , shown in  FIGS. 1 and 2 , comprises a stator  12 , a rotor  14 , a plastic housing  16 , and a plastic bracket  18 . Although housing  16  and bracket  18  can be made of a thermosetting polyester-based plastic, the use of other types of plastics is well within the scope of the invention. In some cases, housing  16  and bracket  18  are made of a T40 series compound provided by Rodgers Engineering of Addison, Ill. Two tie rods  20  are each threaded at one end  22  so that the tie rods can screw into housing  16  for clamping stator  12  between housing  16  and bracket  18 . A commutator  24  on rotor  14  is engaged by a pair of brushes (not shown) to deliver electrical current to the rotor. In a currently preferred embodiment, motor  10  is for a portable vacuum cleaner or similar cleaning appliance where rotor  14  drives an impeller  26  that forces air  28  from a fan inlet  30  to a discharge outlet  32  of housing  16 . 
   An inboard bearing  34  in housing  16  and an outboard bearing  36  in bracket  18  support rotor  14  within stator  12 . An annular bearing retainer  38  made of sheet metal is attached to housing  16  by way of a bearing retainer fastener  40 . Bearing retainer fastener  40  represents any fastener for holding bearing retainer  38  to housing  16 . Examples of bearing retainer fastener  40  include, but is not limited to, a rivet, a screw and nut assembly, a self-tapping screw, etc. 
   Only a single piece is added to motor  10  to provide a groundable connection to both stator  12  and rotor  14 . That single piece is a sheet metal ground strap  42  that includes an attachment end  44 , a terminal end  46 , and a central portion  48  therebetween. Attachment end  44  and central portion  48  are generally parallel to each other to accommodate the axial offset between bearing  38  and the lower edge of stator  12 . The dimensions of terminal end  46  are 0.25-inches by 0.032-inches, which is an industry standard that allows a conventional ground wire connector to be attached to terminal end  46 . 
   To ground rotor  14 , fastener  40  connects attachment end  44  to bearing retainer  38 , which in turn is in intimate contact with an outer race of inboard bearing  34 , and bearing  34  is of course in direct contact with rotor  14 . Ground strap  42 , bearing retainer  38 , bearing  34 , and rotor  14  are made of metal (entirely or at least partially) to establish electrical continuity between ground strap  42  and rotor  14 . 
   To ground stator  12 , central portion  48  of ground strap  42  lies along a slot  50  in housing  16  just underneath stator  12 . Tie rod  20  fits through a hole  52  in ground strap  42 , and tightening tie rod  20  pinches or compressively clamps central portion  48  of ground strap  42  between stator  12  and the bottom of slot  50  of housing  16 . Pressing the bottom surface of stator  12  down against central portion  48  establishes electrical continuity between stator  12  and ground strap  42 . 
   Thus, tie rod  20  provides the dual purpose of connecting housing  16 , bracket  18  and stator  12  together plus connecting stator  12  to ground strap  42 . And rivet  40  provides the dual purpose of attaching bearing retainer  38  to housing  16  plus connecting ground strap  42  to bearing retainer  38 . 
   To ensure ground strap  42  is held in compression between stator  12  and housing  16 , central portion  48  includes a crushable curved section  54  as shown in  FIG. 3 . So, even if the material thickness of ground strap  42  is less than the depth of slot  50 , the curved raised section  54  will urge at least some of central portion  48  to protrude above slot  50 , whereby the protruding part will be compressed between stator  12  and the bottom of slot  50 . In some embodiments, the ground strap has a material thickness of 0.032″ and slot  50  has a depth of 0.035″. The actual shape of the curved raised section  54  can vary. In  FIG. 3 , for example, section  54  curves about an axis  56 . And in  FIGS. 4 and 5 , section  54 ′ curves about an axis  58  that lies at a right angle to axis  56 . Numerous other protuberances and shapes will be apparent to those skilled in the art. It is best, however, to have the ground strap be resiliently compressible so that the resilience of strap  42  maintains some upward pressure against the bottom surface of stator  12 . 
   As an alternative to the crushable curved section being on the ground strap, the crushable curved section could be one or more slight protrusions that extend upward from the bottom surface of slot  50 . The protrusion, for example, could be 0.030-inches in diameter and 0.020-inches tall so that the protrusion is readily crushable as the tie rod clamps the stator between the motor&#39;s bracket and housing. The protrusion could be injection molded along with the rest of the motor housing, so the central portion of the ground strap could be flat. 
   Regardless of the crushable curved section&#39;s location, slot  50  also helps align the ground strap&#39;s hole  52  for the insertion of tie rod  20  during assembly. 
   In some cases, motor  10  includes a cooling fan  60  that helps cool motor  10 . In other cases, as shown in  FIG. 6 , a motor  70  is cooled by having an airguide  72  direct a portion of air  28  across the motor&#39;s stator  12  and rotor  74 . Motor  70  also includes a plastic bracket  76 , a plastic housing  78 , a suction bell  80 , impeller  26 , a rotor shaft  82  that couples rotor  74  to impeller  26 , inboard bearing  34 , and tie rods  20  and  22 . Tie rods  20  and  22  extend through bracket  76  and screw into a boss (not shown) of housing  78  to clamp stator  12  between bracket  76  and housing  78 . To more clearly show fastener  40  and other features of motor  70 , a section of the rotor&#39;s shaft  82  has been cutaway. Fastener  40  attaches a bearing retainer  84 , a ground strap  86  and airguide  72  to housing  78 . 
   In this example, fasteners  40  lie along a line  88  that is perpendicular to another line  90  along which tie rods  20  and  22  lie. This can be achieved, as shown in  FIG. 7 , by having an attachment end  44 ′ of ground strap  86  spot-welded or otherwise attached to a point that is midway between holes  92  through which fasteners  40  extend. When assembled, tie rod  20  extends through hole  92  in ground strap  86  to crushably clamp ground strap  86  between stator  12  and the boss borne by housing  78 . 
   Ground strap  86  and bearing retainer  84  as well as ground strap  42  and bearing retainer  38  can be blanked and formed from a single piece of sheet metal.  FIG. 8 , for example, shows a ground strap  94  being an integral extension of a bearing retainer  96 , whereby the combination of ground strap  94  and bearing retainer  96  is a unitary piece  98 . The unitary design ensures electrical continuity between the bearing retainer and the ground strap by eliminating potential problems caused by oxidation, vibration or fatigue. 
   Although the invention is described with reference to a preferred embodiment, it should be appreciated by those skilled in the art that various modifications are well within the scope of the invention. Therefore, the scope of the invention is to be determined by reference to the following claims.