Abstract:
A permanent magnet electric motor has a stator and a rotor. The stator includes a stator housing having opposed axial ends and magnets affixed to an inner surface of the stator housing, and overmold material overmolded around the plurality of magnets to securely hold the plurality of magnets to the inner surface of the stator housing. The thickness of the overmold material, as measured from the inner surface of the stator housing to an inner surface of the overmold material, is greater at edges of the magnets than at a center of the magnet.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. application Ser. No. 12/443,196 filed Mar. 27, 2009, which is the national phase of PCT Application No. PCT/US07/21797 filed Oct. 12, 2007, which claims the benefit of U.S. Provisional Application No. 60/851,813 filed Oct. 13, 2006. Contents of all the above-cited applications are incorporated herein by their entireties. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present disclosure relates to power tools and electric motors therefore including permanent magnet DC motors in which a stator has a stator housing assembly having a housing to which permanent magnets are affixed to an inner surface thereof and overmolded with plastic. It also relates to power tools and electric motors therefore in which composite magnetic material is molded on the inner surfaces of the stator housing to form magnets. 
       BACKGROUND 
       [0003]    In U.S. Pat. Nos. 6,522,042, 6,983,529 and 7,088,024, it is described that anchors for stator housings are formed in the housing or a flux ring, magnets are placed in the housing or flux ring such as between the anchors, and a plastic material is overmolded that fills around the anchors to secure the magnets to the flux ring or housing. It is also described that, alternatively, a magnet composite material is molded in the flux ring or housing and fills around the anchors to form molded magnets that are held in place in by the anchors. The entire disclosures of U.S. Pat. Nos. 6,522,042, 6,983,529 and 7,088,024 are incorporated by reference herein. 
       SUMMARY 
       [0004]    In accordance with an aspect of the present disclosure, a permanent magnet electric motor has a stator and a rotor. The stator has a stator housing with opposed axial ends and features skived in the stator housing to extend radially inwardly from an inner surface of the stator housing proximate to at least one of the axial ends of the stator housing. An overmolding of material is molded around the features. In an aspect, the overmolding of material is a magnetic composite material and is molded to form magnets. In an aspect, magnets are placed on the inner surface of the stator housing and the overmolding of material is a plastic that is over molded around the magnets and the features. 
         [0005]    In an aspect, the features hold the magnets in place during the molding of the overmolding around the magnets. 
         [0006]    In an aspect, the magnets have essentially the same inner radius and outer radius and the overmolding of material is thicker at edges of each magnet than at the center of each magnet. 
         [0007]    In an aspect, the magnets are flat magnets and the overmolding of material is thicker at edges of each magnet than at the center of each magnet. 
         [0008]    In an aspect a power tool has such a permanent magnet DC motor. 
         [0009]    In an aspect, a power too has a housing with a permanent magnet electric motor in the housing, with an member coupled to the electric motor. The electric motor has a rotor and a stator but not an end plate. The stator has a stator housing having opposed axial ends and a plurality of magnets affixed to an inner surface of the stator housing and an overmolding of material molded around the magnets. The overmolding of material includes a pilot feature that mates with a pilot feature of a bearing support of the power tool. 
         [0010]    Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
     
    
     
       DRAWINGS 
         [0011]    The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
           [0012]      FIG. 1  is a side perspective view of a prior art power tool; 
           [0013]      FIG. 2  is a side end view of a stator housing having skived anchors in accordance with an aspect of the present disclosure; 
           [0014]      FIG. 3  is a section taken along the line  3 - 3  of  FIG. 2 ; 
           [0015]      FIG. 4  is a perspective view showing an empty stator housing with skived anchors in the ID of the stator housing around the peripheries of both axial ends of the stator housing; 
           [0016]      FIG. 5  is a perspective view showing the stator housing of  FIG. 4  with magnets placed on the inner surface of the stator housing between the skived anchors; 
           [0017]      FIG. 6  is a perspective view showing the stator housing of  FIG. 4  with an overmolding around the magnets and skived anchors; and 
           [0018]      FIG. 7  is a perspective view showing a power tool with a bearing support combined with a ring gear housing piloted by the overmolding in accordance with an aspect of the present disclosure; and 
           [0019]      FIG. 8  is a perspective view of a stator housing having flat magnets on an inner surface of the stator housing with an overmolding of material therearound in accordance with an aspect of the present disclosure. 
       
    
    
     DESCRIPTION 
       [0020]    Referring now to  FIG. 1 , a prior art power tool  10  is shown. The power tool  10  includes a housing  12  which surrounds a motor  14 . An activation member  16  is coupled with the motor and a power source  18 . The power source  18  includes either a power cord (AC current) or includes a battery pack  19  (DC current). The motor  14  is coupled with an output member  20  that includes a transmission  22  and a chuck  24 . The chuck  24  is operable to retain a tool (not shown). 
         [0021]    The motor includes a stator assembly  30 . The stator assembly  30  includes a stator housing  32 , a flux ring  34  and magnets  36 . The flux ring  34  is an expandable or split flux ring. An armature  40  includes a shaft  42 , a rotor  44  and a commutator  50  coupled with the shaft  42 . The rotor  44  includes laminations  46  and windings  48 . The motor  14  also includes end plates  52  and  54 . End plate  52  includes a front bearing  56  which supports one end of a shaft  42 . The shaft  42  is coupled with a pinion  60  that is part of the output member  20 . Brushes  62  and  64  are associated with the commutator  50 . A rear bearing  70  is also coupled with the end plate  54  to balance rotation of the shaft  42 . 
         [0022]    While motor  14  is illustratively shown as a permanent magnet DC (“PMDC”) motor in which magnets  36  are affixed to an inner surface of flux ring  34 , it should be understood that motor  14  could be other types of motors that utilize permanent magnets, such as a brushless motor in which the rotor has permanent magnets and the stator has electronically commutated windings. Referring now to  FIG. 1 , a prior art power tool  10  is shown in which a motor in accordance with aspects of the present disclosure can be used. The power tool  10  is illustrated as a drill, however, any type of power tool may be used in accordance with the present invention. The power tool  10  includes a housing  12  which surrounds a motor  14 . An activation member  16  is coupled with the motor and a power source  18 . The power source  18  includes either a power cord (AC current) or includes a battery (DC current) (not shown). The motor  14  is coupled with an output member  20  that includes a transmission  22  and a chuck  24 . The chuck  24  is operable to retain a tool (not shown). 
         [0023]    The motor includes a stator assembly  30 . The stator assembly  30  includes a stator housing  32 , a flux ring  34  and magnets  36 . The flux ring  34  is an expandable or split flux ring. An armature  40  includes a shaft  42 , a rotor  44  and a commutator  50  coupled with the shaft  42 . The rotor  44  includes laminations  46  and windings  48 . The motor  14  also includes end plates  52  and  54 . End plate  52  includes a front bearing  56  which supports one end of a shaft  42 . The shaft  42  is coupled with a pinion  60  that is part of the output member  20 . Brushes  62  and  64  are associated with the commutator  50 . A rear bearing  70  is also coupled with the end plate  54  to balance rotation of the shaft  42 . 
         [0024]    Referring  FIG. 2 , in accordance with an aspect of the present disclosure, a stator assembly  200  includes a stator housing  202  have magnets affixed to inner surface  204  of stator housing  202 . The magnets can be flat magnets, designated with reference number  206  or arcuate magnets, designated with reference number  208  For illustrative purposes, stator assembly  200  is shown as having both flat and arcuate magnets, but it should be understood that stator assembly  200  would typically have either all flat magnets or all arcuate magnets. 
         [0025]    The magnets may illustratively be formed placing stator housing  202  in a mold and molding a magnet composite material on inner surface  204  of stator housing  202 . The magnets may alternatively be preformed, placed on inner surface  204  of stator housing  202  and affixed thereto. 
         [0026]    Material of the stator housing  202  is skived at  210  to create features  212  ( FIG. 3 ) therein in which a molding  302  of either a magnet composite or an overmolding, such as of plastic, molds around. The features may illustratively be raised features  304  and may also include recesses  306 . With several of these features  212  on the stator housing  202 , the molded part, such overmolding  600  ( FIG. 6 ) or molded magnets is well retained within the stator housing  202  axially and angularly. Additionally, these features  212  can be created using a die set and appropriate tooling so that their angular spacing is precisely controlled. Thus they may be used as the angular locators of the magnets during the molding process in which the overmolding is molded. For example, as shown in  FIG. 5 , arcuate magnets  208  are retained between features  212  in stator housing  202  prior to being overmolded. 
         [0027]    Axially outer ends of the features  212  can be parallel (shown at  308 ) with the ends  310  of the stator housing  202 ). The axially outer ends of features  212  may alternatively angled slightly (shown at  312 ) to better key the plastic of the overmolding radially to inner surface  204  of the stator housing  202 . The axial outer ends of the features  212  may also be chamfered (as shown at  800  in  FIG. 8 ). Axially inner ends  314  of features  212  may be raised above the inner surface  204  of the stator housing  202  to retain overmolding  600  ( FIG. 6 ) axially within the stator housing. 
         [0028]    In an aspect, with reference to  FIG. 4 , stator housing  202  has skived features  212  formed around the peripheries of both axial ends  310  (only one of which is shown in  FIG. 4 ) of stator housing  402 . At least one of the axial ends  310  includes a notch  400  therein. The skived features  212  act as anchors and prevent overmolding  600  ( FIG. 6 ) from rotating in stator housing  202 . Notch  400  in one or both axial ends  310  of stator housing  202  and flats  404  on outer surface  406  of stator housing  202  cooperate to prevent stator housing  202  from rotating in the power tool housing, such as housing  12  of power tool  10 . Also, flats  404  may illustratively be used to key the stator housing  202  in housing  12  of power tool  10 . 
         [0029]    In such a process, the magnets could be partially or fully magnetized so that they are self-retained against inner surface  204  of the stator housing  202 . Locating pins in the molding tool can additionally be used to position the magnets axially within the stator housing  202 . Thus after molding, the magnets are in the proper position and well-secured in the stator housing  202 . In such a case, the magnet arcs could be arcuate in shape, or they could be flat magnets as described in the patent application titled “Motor Can and Magnet Manufacturing Design,” (attorney docket no. 0275K-001245) filed concurrently herewith, the entire disclosure of which is incorporated herein by reference. And multiple flat magnets could be placed between the skived anchors, as shown in  FIG. 2 . 
         [0030]    In a variation, the magnets may be un-magnetized and features in the mold tooling may be used to properly locate and retain the magnets during the molding process. Or, the magnets may be glued to the stator housing  202  to locate and secure them to the stator housing  202  for molding. Or, the magnets could be adhered to the stator housing  202  by means of a double sided adhesive. 
         [0031]    The stator housing could be made using the drawn over mandrel (DOM) process, or it could be made from stamped and rolled housings. For the magnets, they can be pre-formed discrete magnets, or they could be a composite blend of magnet and polymer material that is molded directly into the stator housing  202 . In the case of discrete magnets, they could be of various compositions, including but not limited to ferrite, sintered NdFeB, compression bonded NdFeB. 
         [0032]    During the overmolding process, if the magnets are designed having the “same OR and IR”, or are flat magnets, as described in the above referenced patent application titled “Motor Can and Magnet Manufacturing Design,” this provides the additional benefit of the overmolding having thicker molded walls at the edges of the magnets. This benefit can be used in either of two ways. First, the thicker molding at the edges of the magnets provides increased strength for magnet retention. Secondly, the wall thickness of the overmolding at the center of the magnets can be minimized, or made to essentially zero, while still having sufficient wall thickness at the edges of the magnet for sufficient magnet retention and a feasible molding process.  FIG. 8  shows a stator assembly  800  having a stator housing  802  with a plurality of flat magnets  804  (only one of which is shown in  FIG. 8 ) affixed to an inner surface of the stator housing  802  by an overmolding  806  of material. Overmolding  806  is thicker at edges  808  of magnets  804  than at center  810  of magnets  804 . In an aspect, overmolding  806  is at least twenty percent thicker at the edges  808  of magnets  804  than at the center  810  of magnets  804 . It should be understood that magnets  804  can also be arcuate magnets having the same OR and IR. 
         [0033]    FIG. 38A of U.S. Pat. No. 7,088,024 describes the motor end plate piloted by the overmolding. With reference to  FIG. 7 , in accordance with an aspect of the present disclosure, functional parts of power tool  700 , such as gear case/ring gear  702 , are piloted by the overmolding where there is no separate motor end plate. That is, the end plate is functionally combined into other parts of the power tool—such as shown at  704  in  FIG. 7  showing a bearing support combined with a ring gear housing. (Note the overmolding is not shown in  FIG. 7 .) But in the case of overmolding, the armature bearing support, hence alignment of the armature within the overmolding, is improved with less tolerance stackups.  FIG. 6  shows pilot features  602 , such as holes, in overmolding  600  that pilots bearing support/ring gear housing  704 . 
         [0034]    The above provides the advantages of a robust means of holding the magnets to a stator housing. Also, formed pilot features in the overmolding can be used to align the front bearing &amp; armature shaft to the inner surface of the overmolding for reduced chances of the armature stack contacting the overmolding. 
         [0035]    Overmolding also provides the advantage of improving corrosion resistance of magnets, especially for NdFeB magnets, which are prone to corrosion. Overmolding also allows the use of alternative magnet grades or coatings that are less expensive. Overmolding also provides a method of discrete magnet retention that lessens the dependency on the quality of the magnet gluing process or the quality of the magnet coating process.