Patent Publication Number: US-2015079451-A1

Title: Battery pack

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Korean Patent Application No. 10-2013-0110617, filed on Sep. 13, 2013, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
     BACKGROUND 
     1. Field 
     The described technology generally relates to a battery pack. 
     2. Description of the Related Technology 
     In contrast to primary batteries, secondary batteries are rechargeable. Secondary, or rechargeable, batteries are widely used as energy sources in devices such as mobile electronic devices, electric vehicles, hybrid electric vehicles, electric bicycles, and uninterruptible power supplies. Either single-cell secondary batteries or multi-cell secondary batteries (secondary battery packs) in which a number of cells are connected can be used based on the external devices using the secondary batteries. 
     Small mobile devices such as cellular phones may be operated for a limited period of time using single-cell secondary batteries. However, battery packs having a high-output and high-capacity are more suitable for devices having long operating times and consuming large amount of power such as electric vehicles or hybrid electric vehicles. Such battery packs have coupling housings for holding in place an arrangement of battery cells. 
     SUMMARY OF CERTAIN INVENTIVE ASPECTS 
     One inventive aspect is a battery pack having an improved coupling structure for easily assembling two or more battery cells as a module and substantially guaranteeing sufficient coupling strength. 
     Another aspect is a battery pack including a plurality of battery cells, a case frame having an opened end to receive the battery cells, and a finishing plate configured to be coupled to the opened end of the case frame, wherein the case frame includes a pair of side plates extending in an arrangement direction of the battery cells and an end plate disposed between the side plates to form a closed end opposite to the opened end, wherein the side plates and the end plate are formed in one piece. 
     The side plates and the end plate may be connected without joints therebetween. 
     The side plates and the finishing plate may form a hook coupling structure and the hook coupling structure may include a fastening hole and a catch jaw fastenable to the fastening hole. 
     A protection layer may be formed of a material different from that of the catch jaw may be formed on the catch jaw. 
     The catch jaw may be formed on the finishing plate and the fastening hole may be formed in an end part of the side plates to receive the catch jaw. 
     The finishing plate may include a first coupling tab protruding toward the side plates and the catch jaw may be formed on the first coupling tab. 
     The finishing plate may further include a second coupling tab formed at a position different from that of the first coupling tab and protruding toward the side plates and the first and second coupling tabs may be fitted on opposing surfaces of the side plates. 
     The first coupling tab may be fitted on an inner surface of the side plates and the second coupling tab may be fitted on an outer surface of the side plates. 
     A tapered portion may be formed on an end part of at least one of the first and second coupling tabs for facilitating coupling of the finishing plate to the side plates. 
     The side plates may include a guide tab extending toward the second coupling tab for guiding the assembly of the second coupling tab. 
     The first and second coupling tabs may extend substantially parallel to each other in an assembling direction of the finishing plate. 
     The first and second coupling tabs may be formed at alternating positions along an edge of the finishing plate. 
     A leading end of the second coupling tab may extend further from the finishing plate than a peak position of the catch jaw. 
     A position alignment structure may be formed on the finishing plate and the side plates for aligning the finishing plate and the side plates with each other. 
     The position alignment structure may include an alignment slot and an alignment pin insertable into the alignment slot. 
     The alignment slot may extend in an assembling direction of the finishing plate. 
     The finishing plate may include first and second coupling tabs formed at different positions and protruding toward the side plates so as to be fitted on opposing surfaces of the side plates, the side plates may include a guide tab protruding toward the second coupling tab to guide assembly of the second coupling tab, the alignment slot may be formed in the second coupling tab, and the alignment pin may be formed at a position of the side plates that is aligned with the guide tab in an assembling direction of the guide tab or the finishing plate. 
     A catch jaw may be formed on the first coupling tab for hook coupling between the finishing plate and the side plates. 
     Another aspect is a battery pack including a plurality of battery cells, a case frame housing the battery cells and including a pair of side plates forming an open end in the case frame, wherein each of the side plates has one of i) a hole and ii) a hook configured to be inserted into the hole adjacent to the opened end, and a finishing plate having the other of the hole and the hook and configured to be connected to the side plates. 
     The finishing plate may further include a first coupling tab protruding towards the side plates and wherein the hook is formed on the first coupling tab. Each of the side plates includes opposing surfaces, wherein the finishing plate further includes a second coupling tab formed at a different position from that of the first coupling tab and protruding toward the side plates, and wherein the first and second coupling tabs are configured to be respectively fitted on the opposing surfaces of the side plates. Each of the side plates includes a guide tab extending toward the second coupling tab. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded perspective view illustrating a battery pack according to an embodiment. 
         FIG. 2  is a perspective view illustrating the arrangement of the battery cells depicted in  FIG. 1 . 
         FIGS. 3 and 4  are perspective views illustrating a potion of  FIG. 1  to illustrate the assembly of a finishing plate and a case frame. 
         FIG. 5  is an enlarged perspective view illustrating assembling portions of the finishing plate and the case frame. 
         FIG. 6  is an enlarged perspective view illustrating the finishing plate. 
     
    
    
     DETAILED DESCRIPTION OF CERTAIN INVENTIVE EMBODIMENTS 
     Reference will now be made in detail to embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. In this regard, the present embodiments may have different forms and should not be construed as being limited to the descriptions set forth herein. Accordingly, the embodiments are merely described below, by referring to the figures, to explain aspects of the present description. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. Expressions such as “at least one of,” when preceding a list of elements, modify the entire list of elements and do not modify the individual elements of the list. 
     A battery pack will now be described in detail with reference to the accompanying drawings, in which exemplary embodiments of the described technology are shown. 
       FIG. 1  is an exploded perspective view illustrating a battery pack according to an embodiment. Referring to  FIG. 1 , the battery pack includes a plurality of battery cells  10  arranged in an arrangement direction Z1. The battery pack also includes finish, side, and end plates  120 ,  140 , and  150  surrounding the battery cells  10 . 
       FIG. 2  is a perspective view illustrating the battery cells  10  depicted in  FIG. 1 . Referring to  FIGS. 1 and 2 , the battery cells  10  may be secondary battery cells such as lithium ion battery cells. The battery cells  10  may have any shape such as a cylindrical or prismatic shape. In addition, the battery cells  10  may be any type of battery cells such as polymer battery cells. That is, the battery cells  10  are not limited to any shape or type of battery cell. 
     In some embodiments, each of the battery cells  10  include a case  10   b , an electrode assembly (not shown) disposed in the case  10   b , and electrode terminals  10   a  electrically connected to the electrode assembly and exposed to the exterior of the case  10   b . In these cases, the electrode terminals  10   a  exposed to the exterior of the case  10   b  form portions from the topside of the case  10   b . Although not shown, the electrode assembly may include a positive electrode, a separator, and a negative electrode. The electrode assembly may be a jelly-roll or stack type electrode assembly. The case  10   b  accommodates the electrode assembly and the electrode terminals  10   a  are exposed to the exterior of the case  10   b  for electric connection with an external circuit. 
     According to some embodiments, neighboring battery cells  10  are electrically connected to each other by connecting electrode terminals  10   a  of the neighboring battery cells  10 . In these cases, neighboring battery cells  10  are electrically connected to each other in series, in parallel, or in a combination thereof by connecting electrode terminals  10   a  of the neighboring battery cells  10 . 
     A safety vent  10 ′ may be formed in the case  10   b . The safety vent  10 ′ is relatively weak so that if the pressure inside of the case  10   b  increases to a critical level or higher, the safety vent  10 ′ may be fractured to release gas from the inside of the case  10   b.    
     Spacers  50  may be disposed between neighboring battery cells  10 . The spacers  50  may electrically insulate the neighboring battery cells  10  from each other. For example, the cases  10   b  of the battery cells  10  may be charged and the spacers  50  formed of an electrical insulation material and disposed between the cases  10   b  may prevent electrical interference between neighboring pairs of the battery cells  10 . 
     In addition, the spacers  50  may function as heat-dissipating paths between the battery cells  10 . To this end, heat-dissipating holes  50 ′ may be formed in the spacers  50 . Heat-dissipating holes  140 ′ (refer to  FIG. 1 ) may be formed in the side plates  140  which substantially cover sides of the spacers  50  as described later and the heat-dissipating holes  140 ′ of the side plates  140  may be aligned with the heat-dissipating holes  50 ′ of the spacers  50  to form heat-dissipating paths between the battery cells  10 . 
     The spacers  50  may be disposed between the battery cells  10  and substantially prevent thermal expansion (swelling) of the battery cells  10 . The cases  10   b  of the battery cells  10  are formed of a deformable material such as metal. Thus, the spacers  50  may be formed of a less deformable material such as polymer to substantially suppress swelling the battery cells  10 . 
     The spacers  50  may be disposed on outermost sides of the battery cells  10  in the arrangement direction Z1 as well as between the battery cells  10 . In detail, referring to  FIG. 1 , an end plate  150  and a finishing plate  120  are disposed on both ends of the battery cells  10  in the arrangement direction Z1. A spacer  50  is disposed between the end plate  150  and the corresponding outermost battery cell  10  and another spacer  50  is disposed between the finishing plate  120  and the corresponding outermost battery cell  10 , so as to electrically insulate the end and finishing plates  150  and  120  from the outermost battery cells  10 . 
     Referring to  FIG. 1 , the battery pack further includes a module case ( 120  and  160 ) in which the battery cells  10  are arranged and bound in the arrangement direction Z1. 
     The module case ( 120  and  160 ) includes a case frame  160  having an opened end to receive and house the battery cells  10  and the finishing plate  120  configured to be coupled to the opened end of the case frame  160  for closing the opened end. 
     The case frame  160  substantially covers three sides of the battery cell array ( 10 ,  50 ) disposed in the arrangement direction Z1. The battery cell array ( 10 ,  50 ) may include the battery cells  10  and the spacers  50  arranged in the arrangement direction Z1. In some embodiments, the case frame  160  includes a pair of side plates  140  extending in the arrangement direction Z1 to substantially cover both sides of the battery cell array ( 10 ,  50 ) and the end plate  150  disposed between the side plates  140  to substantially cover an end of the battery cell array ( 10 ,  50 ). The side plates  140  and the end plate  150  may be formed together in one piece. In these cases, the side plates  140  are smoothly connected to the end plate  150  without any joints therebetween. For example, the case frame  160  may be formed by bending a raw material sheet without using any fasteners. Thus, additional fasteners such as bolts and nuts are not required to couple the side plates  140  and the end plate  150 . 
     Alternatively, the side plates  140  and the end plate  150  may be coupled by a method such as welding without using any fasteners. In the current embodiment, the side plates  140  and the end plate  150  are formed of a raw material sheet. That is, the side plates  140  and the end plate  150  are smoothly connected without any welding points or joints therebetween. 
     The finishing plate  120  is coupled to the opened end of the case frame  160 . In some embodiments, the battery cell array ( 10 ,  50 ) is inserted into the case frame  160  through the opened end of the case frame  160 . Then, the opened end of the case frame  160  is closed by coupling the finishing plate  120  to the opened end. Next, the case frame  160  and the finishing plate  120  are fastened to each other by a mechanical fastening method. In some embodiments, the finishing plate  120  is slid into the opened end of the case frame  160  in an assembling direction and locked thereto by a hook coupling structure so that the finishing plate  120  is substantially prevented from being separated from the case frame  160  without a releasing action. 
     The finishing and end plates  120  and  150  may be disposed on opposing ends of the battery cell array ( 10 ,  50 ). The finishing and end plates  120  and  150  may include flanges  128 ,  129 ,  158 , and  159  extending in directions away from the battery cell array ( 10 ,  50 ). The flanges  128 ,  129 ,  158 , and  159  are bent from edge portions of the finishing and end plates  120  and  150  away from the battery cell array ( 10 ,  50 ). The flanges  128 ,  129 ,  158 , and  159  may include upper flanges  128  and  158  and lower flanges  129  and  159  respectively formed on upper and lower sides of the finishing and end plates  120  and  150 . The flanges  128 ,  129 ,  158 , and  159  may function as coupling positions at which neighboring elements are coupled to the finishing plate  120  and the end plate  150 . Fastening holes may be formed in the flanges  128 ,  129 ,  158 , and  159  and screws may be inserted into the fastening holes. In some embodiments, a cover member (not shown) is disposed on top of the battery cell array ( 10 ,  50 ) and coupled to the finishing and end plates  120  and  150  through the fastening holes formed in the flanges  128 ,  129 ,  158 , and  159 . The flanges  128 ,  129 ,  158 , and  159  may enhance the mechanical strength of the finishing and end plates  120  and  150 . 
     The side plates  140  may be disposed on both lateral sides of the battery cells  10 . In other words, the side plates  140  substantially cover both sides of the battery cells  10  arranged in the arrangement direction Z1. The side plates  140  may be provided as a pair of plates on opposing lateral sides of the battery cells  10 . The side plates  140  may extend in the arrangement direction Z1 of the battery cells  10  and may be coupled to the finishing plate  120  disposed on an end of the battery cells  10 . 
     The side plates  140  have a substantially plate shape. The side plates  140  include catch jaws (or hooks)  148  bent toward the battery cells  10  to support portions of the bottom sides of the battery cells  10 . As shown in  FIG. 1 , the pair of side plates  140  are disposed on opposing lateral sides of the battery cells  10  and the catch jaws  148  are bent from the side plates  140  to face each other and support the bottom sides of the battery cells  10 . 
     The catch jaws  148  may extend along the entire length of the side plates  140  in the arrangement direction Z1 of the battery cells  10  and ends of the catch jaws  148  may be coupled to the lower flange  129  of the finishing plate  150  using screws. To this end, coupling holes may be formed in the catch jaws  148  and the lower flanges  129 . For example, the side plates  140  and the finishing plate  120  may be coupled by aligning the coupling holes of the catch jaws  148  and the lower flange  129  and inserting fasteners into the coupling holes and tightening the fasteners. 
     The heat-dissipating holes  140 ′ may be formed in the side plates  140 . For example, the heat-dissipating holes  140 ′ of the side plates  140  may form patterns. Air may flow to the battery cells  10  through the heat-dissipating holes  140 ′, and thus, heat may be rapidly dissipated from the battery cells  10  during operation of the battery cells  10 . 
       FIGS. 3 and 4  are perspective views illustrating a potion of  FIG. 1  to illustrate the assembling of the finishing plate  120  and the case frame  160 .  FIG. 5  is an enlarged perspective view illustrating assembling portions of the finishing plate  120  and the case frame  160 .  FIG. 6  is an enlarged perspective view illustrating the finishing plate  120 . 
     Referring to  FIGS. 3 and 4 , the case frame  160  and the finishing plate  120  may be coupled by a hook coupling structure. In this case, the finishing plate  120  is slid in an assembling direction into an end of the case frame  160  to an assembling position P. Once at the assembling position P, catch jaws  121  of the finishing plate  120  are inserted in fastening holes  141  so that the position of the finishing plate  120  is fixed in the end of the case frame  160 . Thus, the catch jaws  121  are hook coupled to the fastening holes  141 . 
     According to some embodiments, the catch jaws  121  are provided in the side plates  140  and the fastening holes are provided in the finishing plate  120 . In these cases, the finishing plate  120  is slid an assembling direction into an end of the case frame  160  to an assembling position P. Once at the assembling position P, catch jaws  121  of the side plates  140  are inserted in fastening holes  141  so that the position of the finishing plate  120  is fixed in the end of the case frame  160 . Thus, the catch jaws  121  are hook coupled to the fastening holes  141 . 
     Referring to  FIG. 5 , the catch jaws  121  of the finishing plate  120  are formed on first coupling tabs  122  protruding from the finishing plate  120  in the assembling direction. Each of the catch jaws  121  include a first surface  121   a  sloped in a first direction and a second surface  121   b  steeply sloped in a second direction different from the first direction. Thus, the catch jaws  121  may be subject to low resistance when being inserted in the fastening holes  141  in the assembling direction but may be subject to high resistance when being pulled out of the fastening holes  141  in the opposite direction, that is, a separation direction. 
     In other words, the catch jaws  121  are easily inserted into the fastening holes  141  in the assembling direction of the finishing plate  120  so that the catch jaws  121  are hook coupled to the fastening holes  141 . The hook coupling between the catch jaws  121  and the fastening holes  141  is not easily released in the opposite direction, that is, in the separation direction. 
     The finishing plate  120  may further include second coupling tabs  123  formed at different positions and protruding toward the side plates  140 . For example, the first and second coupling tabs  122  and  123  may extend substantially parallel to each other in the assembling direction of the finishing plate  120 . In some embodiments, the first and second coupling tabs  122  and  123  are staggered along sides of the finishing plate  120 . In these cases, the first and second coupling tabs  122  and  123  are alternately arranged along lateral sides of the finishing plate  120 . In some embodiments, the second coupling tabs  123  are formed between the first coupling tabs  122 . For example, the first coupling tabs  122  may be formed at three points approximately equally-spaced on each lateral side of the finishing plate  120  and the second coupling tabs  123  may be formed between the first coupling tabs  122 . 
     The first and second coupling tabs  122  and  123  may substantially prevent free releasing of the hook coupling. The first and second coupling tabs  122  and  123  may protrude from the finishing plate  120  in the assembling direction of the finishing plate  120  and may fit on opposite surfaces of each of the side plates  140 . 
     The first coupling tabs  122  may fit on inner surfaces  140   a  of the side plates  140  and the second coupling tabs  123  may fit on outer surfaces  140   b  of the side plates  140 . Since the first and second coupling tabs  122  and  123  are fitted in a staggered manner on the inner and outer surfaces  140   a  and  140   b  of the side plates  140 , the first and second coupling tabs  122  and  123  may be in tight contact with the inner and outer surfaces  140   a  and  140   b . At this time, the catch jaws  121  of the first coupling tabs  122  are pushed into the fastening holes  141  of the side plates  140 , and thus, the catch jaws  121  are hook coupled to the fastening holes  141  and may not be freely released. The first and second coupling tabs  122  and  123  fitted on the side plates  140  may enhance the hook coupling due to the staggered fit therebetween. 
     In some embodiments, the case frame  160  is formed by bending a sheet of a raw material to form the pair of side plates  140  and the end plate  150 . At this time, the pair of side plates  140  bent from the end plate  150  may be deformed away from each other by a spring-back phenomenon, and as a result, the catch jaws  121  of the finishing plate  120  inserted in the fastening holes  141  of the side plates  140  may be easily separated from the fastening holes  141 . However, the first and second coupling tabs  122  and  123  fitted on the side plates  140  in a staggered manner on the inner and outer surfaces  140   a  and  140   b  may substantially suppress the spring-back phenomenon of the side plates  140 , and thus, the catch jaws  121  formed on the first coupling tabs  122  may be pushed into the fastening holes  141  of the side plates  140 . 
     The first and second coupling tabs  122  and  123  protrude from the finishing plate  120  and fit on the inner and the outer surfaces  140   a  and  140   b  of the side plates  140 . The first coupling tabs  122  are fastened to the fastening holes  141  of the side plates  140  through the catch jaws  121 . The second coupling tabs  123  may be guided to assembling positions of the side plates  140  by guide tabs  143  protruding from the side plates  140 . The guide tabs  143  may protrude from the side plates  140  toward the second coupling tabs  123  to guide the second coupling tabs  123  to their assembled positions. 
     In some embodiments, when the first and second coupling tabs  122  and  123  are assembled, the first coupling tabs  122  move along inner sides  140   a  of the side plates  140  and the second coupling tabs  123  move along outer sides  140   b  of the side plates  140 . Consequently, the first and second coupling tabs  122  and  123  and the guide tabs  143  may be elastically deformed. 
     Since the first and second coupling tabs  122  and  123  are moved inward along opposite surfaces of the side plates  140 , a large frictional force may be applied to the first and second coupling tabs  122  and  123  when the first and second coupling tabs  122  and  123  move along the side plates  140 . For example, since the first and second coupling tabs  122  and  123  tightly contact the inner and outer surfaces  140   a  and  140   b  of the side plates  140 , contact portions thereof may be mechanically damaged during frictional resistance. Tapered portions  122   a  may be formed on end portions of the first coupling tabs  122  to reduce such frictional resistance. Tapered portions  123   a  may also be formed on end portions of the second coupling tabs  123 . In this case, the tapered portions  122   a  and  123   a  of the first and second coupling tabs  122  and  123  have opposite shapes so as to be easily moved inward on the inner and outer surfaces  140   a  and  140   b  of the side plates  140 . In addition, tapered portions  143   a  may also be formed on end portions of the guide tabs  143  facing the second coupling tabs  123  and configured to slide and fit on the second coupling tabs  123 . For example, the tapered portions  143   a  and  123   a  of the guide tabs  143  and the second coupling tabs  123  may have complementary shapes so that the guide tabs  143  and the second coupling tabs  123  may be easily coupled to each other through a sliding motion. 
     As described above, each of the catch jaws  121  include the first surface  121   a  gradually sloped in the first direction and the second surface  121   b  steeply sloped in the second direction. The expression “steeply sloped” means that the catch jaws  121  have surfaces substantially perpendicular to the first coupling tabs  122 . 
     The first and second surfaces  121   a  and  121   b  meet each other at peak positions P1 of the catch jaws  121 . When the finishing plate  120  is assembled in the ends of the side plates  140 , the first surfaces  121   a  of the catch jaws  121  are inserted into the fastening holes  141  in an assembling direction and the peak positions P1 of the catch jaws  121  reach assembled positions P. Then, the second surfaces  121   b  of the catch jaws  121  are caught in the fastening holes  141  to form stable coupling. Before the peak positions P1 of the catch jaws  121  reach assembling positions P, the first coupling tabs  122  may be moved inward on the inner surfaces  140   a  of the side plates  140  and the second coupling tabs  123  may be pulled inward on the outer surfaces  140   b  of the side plates  140  due to the deviations of the first coupling tabs  122 . 
     To substantially prevent the above deviation in the second coupling tabs  123 , the relative positions of the catch jaws  121  and the second coupling tabs  123  may be set in a manner such that end positions P2 of the second coupling tabs  123  protrude further than the peak positions P1 of the catch jaws  121  in the assembling direction of the finishing plate  120 . Then, before the peak positions P1 of the catch jaws  121  reach the assembling position P, the second coupling tabs  123  may reach the outer surfaces  140   b  of the side plates  140 . For example, the end positions P2 of the second coupling tabs  123  may be between start positions P3 and peak positions P1 of the catch jaws  121  in the assembling direction of the finishing plate  120 . 
     While making tight contact with the inner surfaces  140   a  of the side plates  140 , the catch jaws  121  of the first coupling tabs  122  slide past the inner surfaces  140   a  of the side plates  140  until reaching the fastening holes  141 . At this time, the catch jaws  121  may be worn due to frictional interference with the side plates  140 . Protection layers  121   c  (refer to  FIG. 6 ) may be formed on the catch jaws  121  to prevent such friction and wear. For example, the protection layers  121   c  may include a material different from the material of the catch jaws  121 . The protection layers  121   c  may include coating layers formed on the catch jaws  121  or covers detachably attached to the catch jaws  121 . In some embodiments, the protection layers  121   c  may be formed of stainless steal (SUS). 
     As shown in  FIGS. 3 and 4 , a position alignment structure is provided on each of the finishing and side plates  120  and  140  for aligning the finishing and side plates  120  and  140  with each other. Alignment slots  125  are formed in the second coupling tabs  123  and alignment pins  145  insertable into the alignment slots  125  are formed on the side plates  140 . In the present embodiment, the alignment pins  145  are formed to be aligned with the guide tabs  143  in a direction substantially parallel to the assembling direction so as to be inserted into the alignment slots  125 . 
     In some embodiments, when the finishing plate  120  is assembled, the alignment slots  125  of the finishing plate  120  and the alignment pins  145  of the side plates  140  are fitted together. In these cases, the alignment slots  125  are elongated in the assembling direction of the finishing plate  120 . Assembling of the alignment slots  125  and the alignment pins  145  may improve alignment of the case frame  160  and the finishing plate  120 . For example, relative vertical movements of the case frame  160  and the finishing plate  120  may be limited. 
     As described above, according to at least one embodiment, the case frame having an opened end is coupled with the finishing plate using a hook coupling structure to close the opened end and a plurality of battery cells are packaged as a module using the hook coupling between the case frame and the finishing plate. 
     According to at least one embodiment, the first and second coupling tabs protrude from different positions of the finishing plate to enhance the hook coupling structure and are fitted on opposite surfaces of the case frame by an interference fitting method so that the catch jaws engaged in the hook coupling may be securely held in the fastening holes in a tight contact state and may not be freely separated from the fastening holes. 
     Furthermore, according to at least one embodiment, the position alignment structure is provided on the finishing plate and the case frame for aligning the finishing plate and the case frame. Therefore, the finishing plate and the case frame may be easily aligned with each other and thus may be easily fastened to each other. 
     It should be understood that the exemplary embodiments described herein should be considered in a descriptive sense only and not for purposes of limitation. Descriptions of features or aspects within each embodiment should typically be considered as available for other similar features or aspects in other embodiments. 
     While one or more embodiments of the described technology have been described with reference to the figures, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.