Patent Publication Number: US-2023163664-A1

Title: Outer-rotor brushless motor for a power tool

Description:
FIELD 
     This disclosure relates to a brushless motor assembly for a rotary tool, and particularly to a compact outer-rotor motor assembly for a cordless power tool such as a ratchet wrench. 
     BACKGROUND 
     A brushless direct-current (BLDC) motor typically includes a stator that is electronically commuted through various phases and a permanent magnet rotor that is rotatably driven relative to the stator as the phases of the stator are sequentially energized. The stator is commonly provided as a cylindrical core with a hollow center that receives the rotor therein. The rotor is mounted on a rotor shaft. 
     In some power tool applications, an outer-rotor BLDC motor is provided. Outer-rotor BLDC motors are typically capable of building more inertia in the rotor shaft due to the greater mass of the rotor and are more suitable for certain power tool applications. U.S. Publication No. 2019/0058373, which is incorporated herein by reference, provides an example of a nailer that is provided with an outer-rotor BLDC motor, where a flywheel is integrally mounted on the outer surface of the rotor. 
     What is needed is a compact outer rotor motor having a high power density suitable for portable power tool applications. U.S. Pat. Application No. 17/125,031 filed Dec. 17, 2020, which is incorporated herein by reference in its entirety, describes examples of compact outer rotor motors. This disclosure provides additional improvements on a similar compact outer-rotor motor. 
     SUMMARY 
     According to an embodiment, a power tool is provided including a housing, a brushless direct-current (BLDC) motor disposed within the housing, and a battery receptacle configured to receive a removable battery pack. The BLDC motor includes a rotor shaft on which a rear motor bearing and a front motor bearing are mounted; a motor can through which the rotor shaft extends and including a substantially cylindrical body having an open end and a radial wall distanced from the open end, the radial wall forming a first bearing pocket arranged to receive the front motor bearing therein; a stator assembly including a stator core having an aperture extending therethrough, stator teeth radially extending outwardly from the stator core and defining slots therebetween, and stator windings wound around the stator teeth; a stator mount including an axial member on which the stator assembly is mounted, and a radial member coupled to the open end of the motor can, the radial member forming a second bearing pocket arranged to support the rear motor bearing; an outer rotor comprising a cylindrical rotor core supporting at least one permanent magnet around an outer surface of the stator core; and a fan mounted on the rotor shaft inside the motor can to generate an airflow through the motor can. In an embodiment, the motor can includes exhaust openings formed around the fan to allow the airflow to be expelled radially away from the fan, and intake openings formed at a distance from the exhaust openings along a radial plane between a rear end of the stator assembly and the stator mount to allow an airstream to be received radially into the motor can for generating the airflow through the motor can. 
     In an embodiment, the motor includes a rotor mount configured to secure the outer rotor to the rotor shaft. The rotor mount includes an outer rim arranged to couple to the outer rotor, at least one radial element extending inwardly from the outer rim, and an inner body mounted on the rotor shaft. 
     In an embodiment, the radial element of the rotor mount is made up of a series of spaced apart radial elements forming the fan. 
     In an embodiment, the radial member of the stator mount includes openings that allow a secondary airstream to be received axially into the motor can, the airstream and the secondary airstream cooperatively generating the airflow through the motor can. 
     In an embodiment, the housing includes exhaust openings aligned with the exhaust openings of the motor can to allow the airflow to be expelled radially away from the housing. 
     In an embodiment, the housing further includes intake openings aligned with the intake openings of the motor can to receive the airstream to be received radially into the housing. 
     In an embodiment, the power tool does not include any additional fans for cooling the motor. 
     In an embodiment, the fan includes a maximum diameter that is smaller than or equal to an outer diameter of the outer rotor. 
     In an embodiment, the outer diameter of the outer rotor is smaller than approximately 35 mm. 
     In an embodiment, the outer diameter of the outer rotor is smaller than approximately 32 mm. 
     According to another aspect of the invention, in an embodiment, a brushless direct-current motor is provided including: a rotor shaft on which a rear motor bearing and a front motor bearing are mounted; a motor housing through which the rotor shaft extends; a stator assembly including a stator core having an aperture extending therethrough, stator teeth radially extending outwardly from the stator core and defining a slots therebetween, and stator windings wound around the stator teeth; a stator mount including an axial member on which the stator assembly is mounted, and a radial member coupled to the motor housing, the radial member forming a bearing pocket arranged to support the rear motor bearing; and an outer rotor comprising a cylindrical rotor core supporting at least one permanent magnet around an outer surface of the stator core. In an embodiment, the stator core and the radial member of the stator mount include corresponding alignment features to radially affix the stator core on the axial member. 
     In an embodiment, the stator core includes a stack of steel laminations, and the alignment feature of the stator core is formed integrally in the stack of steel laminations. 
     In an embodiment, the alignment feature of the stator core extends through at least half an axial length of the stator core. 
     In an embodiment, the alignment features include a tongue and groove structure. 
     In an embodiment, the alignment feature of the radial member of the stator mount is a groove formed through an outer circumferential surface of the radial member. 
     In an embodiment, the radial member of the stator mount includes an elongated cylindrical member projecting axially from the radial member into the aperture of the stator core. 
     In an embodiment, the motor housing includes a substantially cylindrical body having an open end and a radial wall distanced from the open end, the radial wall forming a second bearing pocket arranged to receive the front motor bearing therein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings described herein are for illustration purposes only and are not intended to limit the scope of this disclosure in any way. 
         FIG.  1    depicts a perspective view of an electric power tool, according to an embodiment; 
         FIG.  2    depicts a side view of the electric power tool with a housing half removed to expose an outer-rotor brushless motor therein, according to an embodiment; 
         FIG.  3    depicts a side perspective view of the motor, according to an embodiment; 
         FIG.  4    depicts a side exploded view of the motor, according to an embodiment; 
         FIG.  5    depicts a side cross-sectional view of the motor, according to an embodiment; 
         FIG.  6    depicts a partial cross-sectional view of a front portion of the motor can coupled to a ratchet head, according to an embodiment; 
         FIG.  7    depicts a partial exploded view of the stator assembly and the stator mount, according to an embodiment; 
         FIG.  8    depicts a partial side cross-sectional view of the stator assembly and the stator mount, according to an embodiment; and 
         FIG.  9    depicts an axial view of the stator assembly, according to an embodiment. 
     
    
    
     Throughout this specification and figures like reference numbers identify like elements. 
     DETAILED DESCRIPTION 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are intended to provide an explanation of various embodiments of the present teachings. 
     Referring to  FIGS.  1 - 2   , an electric power tool  10  is briefly described herein, according to an embodiment. In an embodiment, power tool  10 , which in this exemplary embodiment is an electric ratchet wrench for applying torque to a fastener, includes a housing  12  formed by two clam shells. The housing  12  is elongated along a longitudinal axis and includes a motor case  13  within which an electric brushless motor  100  is disposed, a handle portion  14  extending rearwardly from the motor case  13  within which a control and/or power module  20  are supported, and a battery receiving portion  15  disposed at a rear end of the handle portion  14 . 
     The control and/or power module  20  is thus disposed between the battery receiving portion  15  and the motor  100 . In an embodiment, the control and/or power module  20  includes control and switching components, for example an inverter switch circuit controlled by a programable controller, that controls flow of electric current to the motor  100 . In an embodiment, a trigger assembly  19  is mounted on the handle portion  14  of the housing  12  that electrically communicates with the control and/or switch module  20 . In an embodiment, the trigger assembly  19  includes a trigger switch  21  engageable by a user. Actuation of the trigger switch  21  sends a signal to the controller to begin operating the motor  100 . 
     In an embodiment, the battery receiving portion  15  is configured to receive and lock in a sliding battery pack  16 , such as a 20V Max power tool battery pack. In an embodiment, the battery receiving portion  15  allows the battery pack  16  to be received along a sliding axis that is substantially parallel to the longitudinal axis of the housing  12 . This ensures that the battery pack  16  is contained within approximately an envelope of the housing  12 . In an embodiment, when viewed from a side, the total width D1 of the battery receiving portion  15  plus the battery pack  16  is between approximately 25% to 40% greater than a width D2 of the motor case formed around the motor  100 , and between approximately 15% to 25% greater than a width D3 formed by the trigger switch  21  and the handle portion  14 . 
     In an embodiment, a ratchet head  18  is mounted on a front end of the housing  12  forward of the motor case  13 . In an embodiment, motor  100  is orientated along the longitudinal axis of the housing  12  to provide a rotary output to the ratchet head  18  to drive an output member  22 . In an embodiment, as described later in detail, a nut  24  is mounted at the end of the housing  12  to secure the ratchet head  18  to the motor  100 . 
       FIG.  3    depicts a side perspective view of the motor  100 , according to an embodiment.  FIG.  4    depicts a side exploded view of the motor  100 , according to an embodiment.  FIG.  5    depicts a side cross-sectional view of the motor  100 , according to an embodiment. 
     As shown in these figures, motor  100  is an outer-rotor brushless (BLDC) motor  100  contained in a motor can (or motor housing)  102 . In an embodiment, motor  100  includes an inner stator assembly  110  disposed within an outer rotor assembly  140 , according to an embodiment. 
     In an embodiment, stator assembly  110  includes a stator lamination stack  112  formed by a series of steel laminations. The stator lamination stack  112  is mounted on a stator mount  114  and supports a series of stator windings  116 . In an exemplary embodiment, the stator windings  116  are wound in three phases, which, when respectively energized by the control and/or power module  20 , cause rotation of the rotor assembly  140 . Further, in an embodiment, a set of power wires  130  are received through the stator mount  114  and coupled to the stator windings  116 . 
     In an embodiment, the stator mount  114  includes an elongated cylindrical portion  122  sized to be received securely within a central aperture of the stator lamination stack  112 . In an embodiment, the stator lamination stack  112  may be press-fitted over the cylindrical portion  122  of the stator mount  114 . In an embodiment, stator mount  114  further includes a radial body  120  at an end of the cylindrical portion  122  outside the body of the stator lamination stack  112 . The radial body  120  forms a center bearing support pocket  124 , which as described below, securely receives a rear bearing  162  of the rotor assembly  140 . 
     In an embodiment, a positional sensor board  126  is mounted on an end of the stator lamination stack  112 , between the stator lamination stack  112  and the stator mount  114 . In an embodiment, the positional sensor board  126  includes a series of Hall sensors positioned for sensing a rotary position of the rotor assembly  140 . A set of signal wires  128  are secured to the positional sensor board  126  to carry signals from the Hall sensors to the control and/or power module  20 . 
     In an embodiment, rotor assembly  140  includes a cylindrical rotor core  142  formed around the stator assembly  110 , and a series of permanent magnets  144  surface-mounted on the inner surface of the rotor core  142  facing the stator assembly  110  with a small airgap therebetween. As the stator windings  116  are energized in a controlled pattern, they magnetically interact with permanent magnets  144 , thus causing the rotation of the rotor. In an embodiment, the rotor assembly  140  mounted securely on a rotor shaft  160  via a rotor mount  146 . Rotation of the rotor assembly  140  causes rotation of the rotor shaft  160 . A pinion  166  is mounted on a front end of the rotor shaft  160  for coupling the rotor shaft  160  to gear components (not shown) of the ratchet head  18 . 
     In an embodiment, rotor mount  146  includes an inner body  148  that is substantially cylindrical and is mounted on the rotor shaft  160  via a bushing  150 . The rotor mount  146  further includes a radial body  152  extending from the inner body  148  and an outer ring  154  that is securely coupled to the end of the rotor core  142  via a lip  158  shaped to be form-fittingly received through the end of the rotor core  143 . A fan  156  is formed by a series of spaced-apart fan blades extending between the radial body  152  and the outer ring  154 . As the rotor assembly  140  is rotated, the fan  156  generates an airflow through the stator assembly  110  and the rotor assembly  140 . 
     US Pat. Application No. 17/125,031 filed Dec. 17, 2020, which is incorporated herein by reference in its entirety, provides further detail on the features described above and is referenced for further detail. 
     In an embodiment, motor can  102  includes a generally cylindrical body having two open ends. The stator assembly  110  and rotor assembly  140  are received within the motor can  102 , with an air gap maintained between the rotor core  142  and the inner surface of the motor can  102 . 
     In an embodiment, the stator mount  114  is secured to one end of the motor can  102  via a set of fasteners  118 . Since the cylindrical portion  122  of the stator mount  114  supports the stator assembly  110 , the stator mount  114  provides structural support for the stator assembly  110  relative to the motor can  102 . 
     In an embodiment, the rotor mount  146  is received within the motor can  102  along with the rotor assembly  140 . The motor can  102  includes a radial wall  103  that projects inwardly and forms a bearing support pocket  104  adjacent the rotor mount  146 . The bearing support pocket  104  receives a front bearing  164  of the rotor shaft  160 . Further, since the bearing support pocket  124  of the stator mount  114  supports the rear bearing  162 , the stator mount  114  and the motor can  102  cooperatively provides structural support for the rotor assembly  140  to be freely rotatably within the motor can  102 . In an embodiment, the rotor shaft  160  extends through the bearing support pocket  104 . The pinion  166 , which is coupled to the end of the rotor shaft  160 , is provided within the front portion  108  of the motor can  102 . 
       FIG.  6    depicts a partial cross-sectional view of the front portion  108  of the motor can  102  coupled to the ratchet head  18 . In an embodiment, the ratchet head  18  includes an open rear end  170  that includes approximately the same outer diameter D4 as the front portion  108  of the motor can  102 . The outer circumferences of both the open rear end  170  of the ratchet head  18  and the front portion  108  of the motor can  102  are threaded, providing a uniform outer surface that allows the nut  24  to be fastened and securely support the ratchet head  18  to the motor can  102 . In an embodiment, the nut  24  has an outer diameter that is smaller than or equal to the diameter of the motor case  13 . 
       FIG.  7    depicts a partial exploded view of the stator assembly  110  and the stator mount  114 , according to an embodiment.  FIG.  8    depicts a partial side cross-sectional view of the stator assembly  110  and the stator mount  114 , according to an embodiment.  FIG.  9    depicts an axial view of the stator assembly  110 , according to an embodiment. 
     As shown in these figures, according to an embodiment, the stator lamination stack  112  of the stator assembly  110  includes a stator core  180  mounted having an annular body sized to be fittingly mounted on the elongated cylindrical portion  122  of the stator mount  114 . The lamination stack  112  further includes on a series of radially-outwardly projecting teeth on which the stator windings  116  are wound. The elongated cylindrical portion  122  of the stator mount  114  extends approximately through the entire length of the stator lamination stack  112 . 
     To properly align the stator mount  114  and the stator assembly  110  during the assembly process, in an embodiment, a tongue and groove structure is provided. In an embodiment, the stator core  180  is provided with a tongue  182  that projects radially from its inner surface. The tongue  182  may be approximately 0.5 to 1 millimeter in lateral width, and in an embodiment, extends through approximately half to ¾ of the length of stator lamination stack  112 . In an embodiment, tongue  182  may be formed by forming at least a subset of the steel laminations using a die that integrally incorporates the tongue  182  into the lamination stack  112 . In an embodiment, the elongated cylindrical portion  122  of the stator mount  114  is provided with a corresponding groove  184  sized to slidingly receive the tongue  182  therein as the stator assembly  110  is mounted. 
     It should be understood that in an exemplary embodiment, the cylindrical portion  122  of the stator mount  114  may be provided with a tongue and the stator lamination stack  112  of the stator assembly  110  may be provided with a corresponding groove. It should further be understood that instead of a tongue and groove structure, other alignment and/or poka-yoke structures may be utilized to radially align the stator mount  114  and the stator assembly  110 . 
     Referring to  FIGS.  1 - 4 ,  7  and  8   , in an embodiment, the motor can  102  is provided with a series of exhaust openings  190  formed radially around the fan  156 . In an embodiment, four exhaust openings  190  are provided around the fan  156 . The motor case  13  portion of the housing  12  similarly includes a series of exhaust openings  200  radially aligned with the exhaust openings  190  of the motor can  102 . As the fan  156  rotates, the airflow generated through the motor  100  is radially expelled through the exhaust openings  190  of the motor can  102  and exhaust openings  200  of the tool housing  12 . In an embodiment, the radial wall  103  of the motor can  102  and/or the stator assembly  110  cooperatively or individually form a baffle for the fan  156  to expel the airflow radially. 
     In an embodiment, the motor can  102  further includes a series of radial intake openings  192  provided near a rear end of the motor can  102  adjacent the stator mount  114 . In an embodiment, the radial intake openings  192  are provided along a radial plane between the positional sensor board  126  and the stator mount  114 . In an embodiment, four intake openings  192  are provided in alignment with the exhaust openings  190 . The housing  12  similarly includes a series of radial intake openings  202  radially aligned with the intake openings  192  of the motor can  102 . In an embodiment, the radial intake openings  192  of the motor can  102  and the radial intake openings  202  of the housing  12  cooperate to allow entry of a first airstream of ambient air in the radial direction into the motor can  102 , bypassing the handle portion  14  of the housing  12 . Further, in an embodiment, the housing  12  includes a second set of intake openings  204  provided at or adjacent the battery receiving portion  15 , which allow entry of a second airstream of ambient air through at least the handle portion  14  of the housing  12 . In an embodiment, the stator mount  114  includes a series of openings  194  formed in the radial body  120 . The openings  194  allow entry of the second airstream from the handle portion  12  along approximately the axial direction into the motor can  102 . Accordingly, the total airflow in the motor can  102  includes the first airstream received radially through the radial intake openings  192  and the second airstream received through the openings  194  of the radial body  120 . 
     In an embodiment, the fan  156  is fully contained within the motor can  102  and includes an outer diameter that is approximately smaller than or equal to the outer diameter of the rotor core  142 . In an embodiment, the maximum diameter of the rotor core  142  is smaller than approximately 35 mm, preferably smaller than approximately 32 mm, preferably smaller than approximately 30.5 mm. This arrangement allows the motor case  13  of the tool housing  12  to include a small diameter, particularly in a front portion of the housing  12  where the ratchet head  18  is mounted. In an embodiment, motor case  13  includes an outer diameter that is smaller than approximately 45 mm, at least in a lateral direction, and at least at the front portion of the housing  12 . 
     In an embodiment, the fan  156  rotates at a top no-load speed of approximately greater than or equal to 22,000 rpm, preferably greater than 27,000 rpm. In an embodiment, the ratchet head  18  provides significant gear reduction, which enables the fan  156  to rotate at a significantly higher speed than the output member  22 . The rotational speed of the fan  156 , combined with the arrangement of the air intake and exhaust openings described above, provide sufficient cooling airflow for the motor  100  and other power tool  10  components without a need for a secondary fan and despite the small diameter of the fan  156 . This is a significant improvement over similar compact motor applications that typically require a fan having a larger diameter than the rotor core to efficiently cool the motor. 
     In an embodiment, at least one of the openings  194  of the radial body  120  extends to the outer periphery of the radial body  120  forming a cutout area  196 . The cutout area  196  allows the power wires  130  and/or the signal wires  128  to be radially slid into the corresponding opening  194  after the stator assembly  110  is mounted on the stator mount  114 . This eliminates the additional step of axially inserting the power wires  130  and/or the signal wires  128  into the opening  194  prior to mounting the stator assembly  110  on the stator mount  114 . 
     Example embodiments have been provided so that this disclosure will be thorough, and to fully convey the scope to those who are skilled in the art. Numerous specific details are set forth such as examples of specific components, devices, and methods, to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to those skilled in the art that specific details need not be employed, that example embodiments may be embodied in many different forms and that neither should be construed to limit the scope of the disclosure. In some example embodiments, well-known processes, well-known device structures, and well-known technologies are not described in detail. 
     The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting. As used herein, the singular forms “a,” “an,” and “the” may be intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms “comprises,” “comprising,” “including,” and “having,” are inclusive and therefore specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. The method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed. 
     When an element or layer is referred to as being “on,” “engaged to,” “connected to,” or “coupled to” another element or layer, it may be directly on, engaged, connected or coupled to the other element or layer, or intervening elements or layers may be present. In contrast, when an element is referred to as being “directly on,” “directly engaged to,” “directly connected to,” or “directly coupled to” another element or layer, there may be no intervening elements or layers present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., “between” versus “directly between,” “adjacent” versus “directly adjacent,” etc.). As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     Although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms may be only used to distinguish one element, component, region, layer or section from another region, layer or section. Terms such as “first,” “second,” and other numerical terms when used herein do not imply a sequence or order unless clearly indicated by the context. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the example embodiments. 
     Terms of degree such as “generally,” “substantially,” “approximately,” and “about” may be used herein when describing the relative positions, sizes, dimensions, or values of various elements, components, regions, layers and/or sections. These terms mean that such relative positions, sizes, dimensions, or values are within the defined range or comparison (e.g., equal or close to equal) with sufficient precision as would be understood by one of ordinary skill in the art in the context of the various elements, components, regions, layers and/or sections being described. 
     The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.