Patent Publication Number: US-2023140447-A1

Title: Display device

Description:
TECHNICAL FIELD 
     The following description relates to a display device. 
     BACKGROUND ART 
     With the development of the information society, there have been growing demands for various types of display devices, and in order to meet these demands, research has been conducted thereon and various display devices have been used recently, including a liquid crystal display (LCD), a plasma display panel (PDP), an electroluminescent display (ELD), a vacuum fluorescent display (VFD), and the like. 
     Among them, a display device using an organic light emitting diode (OLED) has excellent luminance and viewing angle characteristics in comparison with a liquid crystal display device, and requires no backlight unit, such that the OLED display device can be implemented as an ultrathin display device. 
     In addition, a flexible display panel may be bent or rolled on a roller. By using the flexible display panel, a display device rolled or unrolled on the roller may be implemented. Many studies are conducted on the structure of rolling or unrolling the flexible display on the roller. 
     Disclosure of Invention 
     Technical Problem 
     It is an object of the present disclosure to solve the above and other problems. 
     It is another object of the present disclosure to provide a display device having shafts for providing power for opening and closing a door of the display device. 
     It is yet another object of the present disclosure to provide a display device capable of reducing noise and vibration produced during driving of the door of the display device. 
     Solution to Problem 
     According to an aspect of the present disclosure in order to achieve the above objects, there is provided a display device including: a flexible display panel; a roller on which the display panel is rolled or unrolled; a housing having an opening through which the display panel passes, and providing an internal accommodation space, the roller rotatably mounted in the internal accommodation space; and a door assembly mounted in the internal accommodation space at a position adjacent to the opening of the housing, and opening and closing the opening, wherein the door assembly includes: a door for opening and closing the opening; a motor for providing power to the door; a first shaft gear disposed between the motor and the door, and transmitting the power, provided by the motor, to the door; a first shaft having a first end connected to the first shaft gear, and rotating together with the first shaft gear; a joint connected to a second end of the first shaft; a second shaft having a first end connected to the joint, and rotating together with the first shaft; and a second shaft gear connected to a second end of the second shaft, and transmitting power to the door. 
     Advantageous Effects of Invention 
     The display device according to the present disclosure has the following effects. 
     According to at least one of embodiments of the present disclosure, shafts for providing power for opening and closing a door of a display device may be provided. 
     According to at least one of embodiments of the present disclosure, noise and vibration produced during driving of the door of the display device may be reduced. 
     Further scope of applicability of the present disclosure will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the present disclosure, are given by illustration only, since various changes and modifications within the spirit and scope of the present disclosure will become apparent to those skilled in the art from this detailed description. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIGS.  1  to  72    are diagrams illustrating examples of a display device according to embodiments of the present disclosure. 
     
    
    
     MODE FOR THE INVENTION 
     Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings, in which the same or similar elements are designated by the same reference numerals, and a redundant description thereof will be omitted. 
     The terms “module” and “unit” for elements used in the following description are given simply in view of the ease of the description, and do not have a distinguishing meaning or role. 
     In addition, it will be noted that a detailed description of known arts will be omitted if it is determined that the detailed description of the known arts can obscure the embodiments of the present disclosure. Further, the accompanying drawings are used to help easily understand various technical features and it should be understood that the embodiments presented herein are not limited by the accompanying drawings, and the present disclosure should be construed to extend to any alterations, equivalents and substitutes in addition to those which are particularly set out in the accompanying drawings. 
     It will be understood that, although the terms first, second, etc., may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. 
     It will be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present. 
     As used herein, the singular forms are intended to include the plural forms as well, unless the context clearly indicates otherwise. 
     In the following description, even if an embodiment is described with reference to a specific figure, if necessary, reference numeral not shown in the specific figure may be referred to, and reference numeral not shown in the specific figure is used when the reference numeral is shown in the other figures. 
     Referring to  FIG.  1   , a display device  100  may include a display unit  20  and a housing  30 . The housing  30  may have an internal space. At least a portion of the display unit  20  may be located inside the housing  30 . At least a portion of the display unit  20  may be located outside of the housing  30 . The display unit  20  may display an image. 
     A direction parallel to a longitudinal direction of the housing  30  may be referred to as a first direction DR 1 , a positive x-axis direction, a negative x-axis direction, a left direction, or a right direction. A direction in which the display unit  20  displays an image may be referred to as a positive z-axis direction, a forward direction or the front. A direction opposite to the direction in which the display unit  20  displays the image may be referred to as a negative z-axis direction, a rearward direction or the rear. A third direction DR 3  may be parallel to the positive z-axis direction or the negative z-axis direction. A direction parallel to a height direction of the display device  100  may be referred to as a second direction DR 2 , a positive y-axis direction, a negative y-axis direction, an upward direction, or a downward direction. 
     The third direction DR 3  may be a direction perpendicular to the first direction DR 1  and/or the second direction DR 2 . The first direction DR 1  and the second direction DR 2  may be collectively referred to as a horizontal direction. Also, the third direction DR 3  may be referred to as a vertical direction. A left-right direction LR may be parallel to the first direction DR 1 , and an up-down direction UD may be parallel to the second direction DR 2 . 
     Referring to  FIG.  2   , the entire display unit  20  may be located inside the housing  30 . At least a portion of the display unit  20  may be located outside of the housing  30 . A degree of exposure of the display unit  20  to the outside of housing  30  may be adjusted as needed. 
     Referring to  FIG.  3   , the display unit  20  may include a display panel  10  and a plate  15 . The display panel  10  may be flexible. For example, the display panel  10  may be an Organic Light Emitting Display (OLED). 
     The display panel  10  may have a front surface for displaying an image. The display panel  10  may have a rear surface disposed opposite the front surface. The front surface of the display panel  10  may be covered with a light-transmissive material. For example, the light-transmissive material may be a synthetic resin or plastic. 
     The plate  15  may be coupled, fastened, or attached to the rear surface of the display panel  10 . The plate  15  may include a metal material. The plate  15  may be referred to as a module cover  15 , a cover  15 , a display panel cover  15 , a panel cover  15 , and an apron  15 . 
     Referring to  FIG.  4   , the plate  15  may include a plurality of segments  15   c.  A magnet  64  may be disposed in a recess  118  of the segment  15   c.  The recess  118  may be formed in a surface facing the display panel  10  of the segment  15   c.  The recess  118  may be formed in a front surface of each segment  15   c.  The magnet  64  is received in the recess  118 , such that the magnet  64  may be prevented from being exposed to the outside of the segment  15   c.  Even when being in contact with the segments  15   c,  the display panel  10  may be flat without being creased. 
     Referring to  FIG.  5   , a plurality of magnets  64  may be disposed on a link  73 . For example, at least one magnet  64  may be disposed on a first arm  73   a,  and at least one magnet  64  may be disposed on a second arm  73   b.  The plurality of magnets  64  may be spaced apart from each other. 
     Referring to  FIG.  6   , one magnet  64  may be disposed on the first arm  73   a  and the second arm  73   b,  respectively. The magnet  64  may have a shape which is elongated toward a long side of the first arm  73   a  and the second arm  73   b.  As the magnet  64  has a shape which is elongated toward the long side of the first arm  73   a  and the second arm  73   b,  an area of a portion of the link  73  that comes into close contact with the display panel and the module cover may increase. Accordingly, adhesion force between the link  73 , the display panel, and the module cover may be enhanced. 
     Referring to  FIG.  7   , the magnet  64  may be disposed in a recessed portion  321  formed in the link  73 . The recessed portion  321  may have a shape which is recessed inwardly of the link  73 . The magnet  64  may be coupled to the link  73  by at least one screw  187 . 
     A width LHW of the recessed portion  321  which is recessed inwardly of the link  73  may be equal to or greater than a thickness MGW of the magnet  64 . If the thickness MGW of the magnet  64  is greater than the width LHW of the recessed portion  321 , the display panel  10  and the module cover  15  may not come into close contact with the link  73 . In this case, the display panel  10  may be creased or may not be flat. 
     A panel protection part  97  may be disposed on a rear surface of the display panel  10 . The panel protection part  97  may protect the display panel  10  from damage due to friction between the display panel  10  and the module cover  15 . The panel protection part  97  may include a metal material. The panel protection part  97  may be ultra-thin. For example, the panel protection part  97  may have a thickness of about 0.1 mm. 
     As the panel protection part  97  includes a metal material, an attraction force may be generated between the panel protection part  97  and the magnet  64 . Accordingly, even when the module cover  15 , disposed between the panel protection part  97  and the link  73 , does not include a metal material, the module cover  15  may come into close contact with the magnet  64 . 
     Referring to  FIG.  8   , the module cover  15  may come into close contact with the link  73  by an upper bar  75  disposed at an upper side, and a guide bar  234  (see  FIG.  15   ) disposed at a lower side. In the link  73 , a portion between the upper bar  75  and the guide bar  234  may not come into close contact with the module cover  15 . Alternatively, a center portion of the link  73  may not come into close contact with the module cover  15 . The center portion of the link  73  may be in the vicinity of an arm joint  152 . In this case, distances APRD 1  and APRD 2  between the module cover  15  and the link  73  may not be kept constant. In this case, the display panel  10  may be curved or bent. 
     Referring to  FIG.  9   , in the case where the magnet  64  is disposed in the recessed portion  321  of the link  73 , the magnet  64  attracts the panel protection part  97 , such that the module cover  15  may also come into close contact with the magnet  64 . That is, the center portion of the link  73  may come into close contact with the module cover  15 . 
     Referring to  FIG.  10   , a bead  136  may be formed on an upper surface of a segment  15   b.  The bead  136  may have a shape which is recessed inwardly of the segment  15   b.  The bead  136  may have a shape which is recessed in the negative y-axis direction. For example, the bead  136  may be formed by pressing the segment  15   b.  A plurality of beads  136  may be formed on the segment  15   b.  The plurality of beads  136  may be spaced apart from each other. The bead  136  may improve rigidity of the segment  15   b.  The bead  136  may prevent the segment  15   b  from being deformed due to external impact. The segment  15   b  may be fixed to a rear side of the display panel  10  by an adhesive member  70 . The panel protection part  97  (see  FIG.  8   ) may be disposed between the adhesive member  70  and the display panel  10 . For example, the adhesive member  70  may be a double-sided tape. 
     Referring to  FIG.  11   , a source PCB  120  may be disposed over the module cover  15 . During rolling up or rolling down, the source PCB  120  may be changed in position in accordance with the movement of the module cover  15 . An FFC cable  231  may be disposed at the center of the module cover  15  in the first direction. The FFC cable  231  may also be disposed at both ends of the module cover  15  in the first direction. 
     Referring to  FIG.  12   , a segment  15   d  may include a recessed portion  425  which is recessed in the negative z-axis direction. The recessed portion  425  may form 
     a space between the display panel  10  and the module cover  15 . The FFC cable  231  may be accommodated in the space formed by the recessed portion  425 . In addition, the recessed portion  425  may improve rigidity of the segment  15   d.    
     The bead  136  may be disposed on the segment  15   d,  except for a portion where the recessed portion  425  is formed. At the portion where the recessed portion  425  is formed, the thickness of the segment  15   d  decreases in the third direction, such that the bead  136  may not be disposed at the portion. 
     Referring to  FIG.  13   , a segment  15   e  may have a pass-through portion  437  formed at the center thereof in the first direction. The pass-through portion  437  may pass through the center of the segment  15   e  in the second direction. That is, the pass-through portion  437  may be a hole formed in the segment  15   e.  The pass-through portion  437  may be a portion where the FFC cable  231  is disposed. The pass-through portion  43  is formed in the segment  15   e,  such that a thickness of the segment  15   e  may be reduced, compared to the case where the FFC cable  231  is disposed in the recessed portion  425 . 
     The bead  136  may be disposed on the segment  15   e,  except for a portion where the pass-through portion  437  is disposed. At the portion where the pass-through portion  437  is disposed, the thickness of the segment  15   e  decreases in the third direction, such that the bead  136  may not be disposed at the portion. 
     Referring to  FIG.  14   , a top case  167  may cover the source PCB  120  and the upper bar  75 , in addition to the display panel  10  and the module cover  15 . The upper bar  75  has one surface coupled to a rear surface of the module cover  15 , and the other surface coupled to the source PCB  120 . The upper bar  75  may be fixed to the module cover  15  to support the source PCB  120 . 
     A lower end of the FFC cable  231  may be connected to a timing controller board  105  (see  FIG.  15   ) in a panel roller  143  (see  FIG.  15   ). The FFC cable  231  may be wound around or unwound from the panel roller  143  along with the display unit  20 . 
     A portion of the FFC cable  231  may be disposed between the display panel  110  and the module cover  15 . The portion of the FFC cable  231 , which is disposed between the display panel  110  and the module cover  15 , may be referred to as a first portion  231   a.  The first portion  231   a  may be disposed in the recessed portion  425  of the segment  15   d.  Alternatively, the first portion  231   a  may be received in the recessed portion  425  of the plurality of segments  15   d.    
     A portion of the FFC cable  231  may pass through a segment  15   f.  The portion of the FFC cable  231 , which passes through the segment  15   f,  may be referred to as a second portion  231   b.  The segment  15   f  may include a first hole  521   a  formed in a front surface, and a second hole  521   b  formed in a rear surface. The first hole  521   a  and the second hole  521   b  may be connected to each other to form one hole  521 . The hole  521  may pass through the segment  15   f  in the third direction. The second portion  231   b  may pass through the hole  521 . The hole  521  may be referred to as a connection hole  521 . 
     An upper end of the FFC cable  231  may be electrically connected to the source PCB  120 . A portion of the FFC cable  231  may be disposed on the rear surface of the module cover  15 . The portion of the FFC cable  231 , which is disposed on the rear surface of the module cover  15 , may be referred to as a third portion  231   c.  The third portion  231   c  may be electrically connected to the source PCB  120 . 
     The third portion  231   c  may be covered by the top case  167 . Accordingly, the third cover  231   c  may be prevented from being exposed to the outside. 
     Referring to  FIG.  15   , the FFC cable  231  may be connected to the timing controller board  105  mounted in the panel roller  143 . A through-hole  615  may be formed in the panel roller  143 , and the FFC cable  231  may pass through the through-hole  615  to be connected to the timing controller board  105 . 
     The through-hole  615 , formed at a first side of the panel roller  143 , may pass through an outer circumference of the panel roller  143 . The FFC cable  231  may pass through the through-hole  615  to be connected to a first side of the timing controller board  105 . 
     Even when the FFC cable  231  is disposed on the outer circumference of the panel roller  143 , the through-hole  615  may allow the the FFC cable  231  to remain connected to the timing controller board  105 . Accordingly, the FFC cable  231  may be rotated together with the panel roller  143  without being twisted. 
     A portion of the FFC cable  231  may be wound around the panel roller  143 . The portion of the FFC cable  231 , which is wound around the panel roller  143 , may be referred to as a fourth portion  231   d.  The fourth portion  231   d  may come into contact with an outer circumferential surface of the panel roller  143 . 
     A portion of the FFC cable  231  may pass through the through-hole  615 . The portion of the FFC cable  231 , which passes through the through-hole  615 , may be referred to as a fifth portion  231   e.    
     A lower end of the FFC cable  231  may be electrically connected to the timing controller board  105 . A portion of the FFC cable  231  may be disposed inside the panel roller  143 . The portion of the FFC cable  231 , which is disposed inside the panel roller  143 , may be referred to as a sixth portion  231   f.  The sixth portion  231   f  may be electrically connected to the timing controller board  105 . 
     Referring to  FIG.  16   , a lower end of the display panel  10  may be connected to the roller  143 . The display panel  10  may be rolled on or unrolled from the roller  143 . A front surface of the display panel  10  may be coupled to the plurality of source PCBs  120 . The plurality of source PCBs  120  may be spaced apart from each other. 
     A source chip-on film (COF)  123  may connect the display panel  10  and the source PCBs  120 . The source COF  123  may be located on the front surface of the display panel  10 . The roller  143  may include a first part  331  and a second part  337 . The first part  331  and the second part  337  may be fastened by a screw. The timing controller board  105  may be mounted in the roller  143 . 
     The source PCBs  120  may be electrically connected to the timing controller board  105 . The timing controller board  105  may transmit digital video data and a timing control signal to the source PCBs  120 . 
     A cable  117  may electrically connect the source PCBs  120  and the timing controller board  105 . For example, the cable  117  may be a flexible flat cable (FFC). The cable  117  may pass through a hole  331   a.  The hole  331   a  may be formed in a seating part  379  or the first part  331 . The cable  117  may be disposed between the display panel  10  and the second part  337 . 
     The seating part  379  may be formed on an outer circumference of the first part  331 . The seating part  379  may be formed on a stepped portion of the outer circumference of the first part  331 . The seating part  379  may form a space B. When the display unit  20  is rolled on the roller  143 , the source PCBs  120  may be accommodated in the seating part  379 . As the source PCBs  120  are accommodated in the seating part  379 , the source PCBs  120  may not be twisted or bent, and durability may be improved. 
     The cable  117  may electrically connect the timing controller board  105  and the source PCBs  120 . 
     Referring to  FIG.  17   , the roller  143 , around which the display unit  20  is rolled, may be mounted on the first base  31 . The first base  31  may be a base of the housing  30 . The roller  143  may be elongated in the longitudinal direction of the housing  30 . The first base  31  may be connected to a side surface  30   a  of the housing  30 . 
     Referring to  FIGS.  18  and  19   , beams  31   a  may be formed at the first base  31 . The beams  31   a  may improve bending or torsional rigidity of the first base  31 . Many components may be mounted on the first base  31 , such that the first base  31  may be under a heavy load. As the rigidity of the first base  31  is improved, sagging of the first base  31  due to the load may be prevented. For example, the beams  31   a  may be formed by a press process. 
     A second base  32  may be spaced upwardly from the first base  31 . A space  51  may be formed between the first base  31  and the second base  32 . The roller  143 , around which the display unit  20  is rolled, may be accommodated in the space  51 . The roller  143  may be disposed between the first base  31  and the second base  32 . 
     The second base  32  may be connected to the side surface  30   a  of the housing  30 . A bracket  33  may be fastened to an upper surface of the first base  31 . The bracket  33  may be fastened to the side surface  30   a  of the housing  30 . 
     Beams  32   a  may be formed at the second base  32 . The beams  32   a  may improve bending or torsional rigidity of the second base  32 . The beams  32   a  may be formed by a press process. 
     A third part  32   d  may be connected to the first part  32   b  and the second part  32   c.  A fourth part  32   e  may be connected to the first part  32   b  and the second part  32   c.  A space S 2  may be formed between the third part  32   d  and the fourth part  32   e.  Accordingly, bending or torsional rigidity of the second base  32  may be improved. The third part  32   d  may be referred to as a reinforcing rib  32   d  or a rib  32   d.  The fourth part  32   e  may be referred to as a reinforcing rib  32   e  or a rib  32   e.    
     Many components may be mounted on the second base  32 , such that the second base  32  may be under a heavy load. As the rigidity of the second base  32  is improved, sagging of the second base  32  due to the load may be prevented. 
     A first reinforcing plate  34  may be disposed between the first base  31  and the second base  32 . The first reinforcing plate  34  and the second base  32  may be fastened by a screw. The first reinforcing plate  34  may support the second base  32 . The first reinforcing plate  34  may prevent sagging of the second base  32 . The first reinforcing plate  34  may be disposed at a center portion of the first base  31  or at a center portion of the second base  32 . The first reinforcing plate  34  may have a curved portion  34   a.  The curved portion  34   a  may be formed along the roller  143 . The curved portion  34   a  may not come into contact with the roller  143  or the display unit  20  rolled on the roller  143 . The curved portion  34  may be maintained at a predetermined distance from the roller  143 , so as not to interrupt rotation of the roller  143 . 
     A second reinforcing plate  35  may be fastened to the first base  31  and the first reinforcing plate  34 . The second reinforcing plate  35  may support the first reinforcing plate  34 . The second reinforcing plate  35  may be disposed at the rear of the first reinforcing plate  34 . The second reinforcing plate  35  may be disposed at the rear of the first base  31 . The second reinforcing plate  35  may be disposed perpendicular to the first base  31 . The second reinforcing plate  35  may be fastened to the beams  31   a  of the first base  31 . The second base  32  may face the front surface or the rear surface of the housing  30 . 
     Referring to  FIG.  20   , a second base  32   f  may not form a space. If the second base  32   f  is not under a heavy load, beams  32   g  of the second base  32   f  may provide sufficient rigidity to the second base  32   f.  A first base  31 ′ may include beams  31   a′.    
     Referring to  FIGS.  21  and  22   , a motor assembly  810  may be mounted on the second base  32 . The motor assembly  810  may have drive shafts formed on both sides thereof. A right drive shaft and a left drive shaft of the motor assembly  810  may rotate in the same direction. Alternatively, the right drive shaft and the left drive shaft of the motor assembly  810  may rotate in opposite directions. 
     The motor assembly  810  may include a plurality of motors. The plurality of motors may be connected in series to each other. The plurality of motors of the motor assembly  810  are connected in series, thereby outputting high torque. 
     Lead screws  840  may be disposed on each of the left side and the right side of the motor assembly  810 . The motor assembly  810  may be connected to the lead screws  840 . Couplings  811  may connect the lead screws  840  with the drive shafts of the motor assembly  810 . 
     The lead screw  840  may have a screw thread formed in a longitudinal direction. A direction of the screw thread formed on the right lead screw  840  may be opposite to a direction of the screw thread formed on the left lead screw  840 . The direction of the screw thread formed on the right lead screw  840  may be the same as the direction of the screw thread formed on the left lead screw  840 . A pitch of the left lead screw  840  may be the same as a pitch of the right lead screw  850 . 
     Bearings  830   a  and  830   b  may be mounted on the second base  32 . The bearings  830   a  and  830   b  may support both sides of the lead screws  840 . The bearings  830   a  and  830   b  may include inner bearings  830   b  disposed close to the motor assembly  810 , and outer bearings  830   a  disposed away from the motor assembly  810 . The lead screws  840  may be rotated stably by the bearings  830   a  and  830   b.    
     Slides  820  may be engaged with the lead screws  840 . The slides  820  may be movable in a longitudinal direction of the lead screws  840  during rotation of the lead screws  40 . The slide  820  may move between the outer bearing  830   a  and the inner bearing  830   b.  The slides  820  may be disposed at the left lead screw  840  and the right lead screw  840 , respectively. A left slide  820  may be engaged with the left lead screw  840 . A right slide  20  may be engaged with the right lead screw  840 . 
     The left slide  820  and the right slide  820  may be disposed symmetrically with respect to the motor assembly  810 . By driving the motor assembly  810 , the left slide  820  and the right slide  820  may be moved away from or closer to each other by the same distance. 
     Referring to  FIG.  23   , the motor assembly  810  may include a plate  813 . The plate  813  may be referred to as a mount plate  813  or a motor mount plate  813 . Coupling portions  32   h  may be formed on an upper surface of the second base  32 . The plate  813  may be fastened to the coupling portions  32   h  b screws S. The motor assembly  810  may be spaced apart from the upper surface of the second base  32 . Washers  813  may be disposed between the plate  813  and the screws S. The washers  813  may include a rubber material. The washers  813  may reduce vibration occurring in the motor assembly  810 . The washers  813  may improve driving stability of the display device  100 . 
     Referring to  FIG.  24   , a guide rail  860  may be disposed at the second base  32 . The guide rail  860  may be disposed in parallel to the lead screw  840 . The slide  820  may be engaged with the guide rail  860 . A first stopper  861   b  may be disposed on a first side of the guide rail  860 , and a second stopper  861   a  may be disposed on a second side of the guide rail  860 . The slide  820  may move within a limited range between the first stopper  861   b  and the second stopper  861   a.    
     A spring  850  may surround the lead screw  840 . The lead screw  840  may pass through the spring  850 . The spring  850  may be disposed between the inner bearing  830   b  and the slide  820 . A first side of the spring  850  may come into contact with the inner bearing  830   b,  and a second side of the spring  850  may come into contact with the slide  820 . The spring  850  may provide elasticity to the slide  820 . 
     When the slide  820  is stopped by the first stopper  861   b,  the spring  850  may be compressed to the maximum. When the slide  820  is stopped by the first stopper  861   b,  the spring  850  has a minimum length. When the slide  820  is stopped by the first stopper  861   b,  a distance between the slide  820  and the inner bearing  830   b  may be minimum. 
     Referring to  FIG.  25   , when the slide  820  is stopped by a second stopper  861   a,  the spring  850  may be stretched to the maximum. When the slide  820  is stopped by the second stopper  861   b,  the spring  850  may have a maximum length. When the slide  820  is stopped by the second stopper  861   a,  a distance between the slide  820  and the inner bearing  830   b  may be maximum. 
     Referring to  FIG.  26   , the first part  820   a  may be engaged with the guide rail  860 . The first part  820   a  may move along the guide rail  860 . The first part  820   a  may be restricted from moving in a longitudinal direction of the guide rail  860 . The second part  820   b  may be disposed over the first part  820   a.  The first part  820   a  and the second part  820   b  may be fastened to each other by a screw. The second part  820   b  may be spaced apart from the guide rail  860 . The lead screw  840  may pass through the second part  820   b.  For example, the second part  820   b  may include a male thread engaged with a female thread. Accordingly, even when the lead screw  840  is rotated, the slide  820  is stably movable along the guide rail  860  without being rotated. 
     The third part  820   c  may be coupled to a first side of the second part  820   b.  The third part  820   c  may come into contact with the spring  850 . The third part  820   c  may receive elastic force from the spring  850 . 
     Referring to  FIGS.  27  and  28   , a link mount  920  may be mounted on the second base  32 . A first side of a second arm  912  may be pivotally connected to the link mount  920 . A second side of the second arm  912  may be pivotally connected to a joint  913 . The second side of the second arm  912  may be pivotally connected to a second shaft  913   b.  A first side of a rod  870  may be pivotally connected to the slide  820 . A second side of the rod  870  may be pivotally connected to the second arm  912  or a third arm  915 . A first side of the third arm  915  may be pivotally connected to the link mount  920 . A second side of the third arm  915  may be pivotally connected to the second side of the rod  870 . The link mount  920  may include a shaft  921 . The second arm  912  or the third arm  911  may be pivotally connected to the shaft  921 . 
     A link bracket  951  may be referred to as a link cap  951 . The link bracket  951  may be coupled to a top case  950 . The top case  950  may be referred to as a case top  950 , an upper bar  950 , a top  950 , or a bar  950 . The top case  950  may be disposed at an upper end of the display unit  20 . The display unit  20  may be fixed to the top case  950 . 
     A first side of a first arm  911  may be pivotally connected to the joint  913 . The first side of the first arm  911  may be pivotally connected to a first shaft  913   a.  A second side of the first arm  911  may be pivotally connected to the link bracket  951  or the top case  950 . 
     A gear g 1  may be formed on the first side of the first arm  911 . A gear g 2  may be formed on the second side of the second arm  912 . The gear g 1  of the first arm  911  and the gear g 2  of the second arm  912  may be engaged with each other. 
     When the slide  820  moves closer to the outer bearing  830   a,  the second arm  912  or the third arm  915  may be raised. In this case, a direction in which the second arm  912  or the third arm  915  is raised may be referred to as a raised direction DRS. 
     The second arm  912  may include a protrusion  914  protruding in the raised direction DRS. The protrusion  914  may be referred to as a connection part  914 . The third arm  915  may include a protrusion  916  protruding in the raised direction DRS. The protrusion  916  may be referred to as a connection part  916 . The protrusion  914  of the second arm  912  and the protrusion  916  of the third arm  915  may face or come into contact with each other. The second side of the rod  870  may be fastened to the protrusion  914  of the second arm  912  or the protrusion  916  of the third arm  915 . 
     A link  910  may include the first arm  911 , the second arm  912 , the third arm  915 , and/or the joint  913 . 
     Referring to  FIGS.  29  and  30   , an angle formed between the second arm  912  or the third arm  915  and the second base  32  may be referred to as theta S. When the rod  870  is connected to an upper side of the second part  820   b,  an angle formed between the rod  870  and the second base  32  may be referred to as theta A, and a minimum force required for the rod  870  to raise the second arm  912  or the third arm  915  may be referred to as Fa. When the rod  870  is connected to the middle of the second part  820   b,  an angle formed between the rod  870  and the second base  32  may be referred to as theta B, and a minimum force required for the rod  870  to raise the second arm  912  or the third arm  915  may be referred to as Fb. When the rod  870  is connected to a lower side of the second part  820   b,  an angle formed between the rod  870  and the second base  32  may be referred to as theta C, and a minimum force required for the rod  870  to raise the second arm  912  or the third arm  915  may be referred to as Fc. 
     If theta S is the same, a relationship of theta A&lt;theta B&lt;theta C may be formed. Further, if theta S is the same, a relationship of Fc&lt;Fb&lt;Fa may be formed. If the angle formed between the second arm  912  or the third arm  915  and the second base  32  is the same, as an angle between the rod  870  and the second base  32  increases, a force required for raising the second arm  912  or the third arm  915  may be reduced. As the rod  870  is connected to the lower side of the second part  820   b,  load applied to the motor assembly  810  may be reduced. 
     Referring to  FIG.  31   , a rod  870 ′ may not be connected to a protrusion of a second arm  912 ′ or a protrusion of a third arm  915 ′. If an angle formed between the second arm  912 ′ or the third arm  915 ′ and the second base  32  is theta S, an angle formed between the rod  870 ′ and the second base  32  may be referred to as theta  1 , and a minimum force required for the rod  870 ′ to raise the second arm  912 ′ or the third arm  915 ′ may be referred to as F 1 . 
     Referring to  FIG.  32   , the rod  870  may be connected to the protrusion  914  of the second arm  912  or the protrusion  916  of the third arm  915 . If an angle formed between the second arm  912  or the third arm  915  and the second base  32  is theta S, an angle formed between the rod  870  and the second base  32  may be referred to as theta  2 , and a minimum force required for the rod  870  to raise the second arm  912  or the third arm  915  may be referred to as F 2 . 
     Referring to  FIG.  33   , if theta S is the same, theta  2  may be greater than theta  1 . If theta S is the same, F 1  may be greater than F 2 . If the angle formed between the second arms  912  and  912 ′ and the second base  32  is the same, as an angle formed between the rods  870  and  870 ′ and the second base  32  increases, a force required for raising the second arms  912  and  912 ′ may be reduced. As the rod  870  is connected to the protrusions  914  and  916 , a less force is required to raise the second arm  912 , compared to the case where the rod  870 ′ is not connected to the protrusions. As the rod  970  is connected to the protrusions  914  and  916 , load applied to the motor assembly  810  may be reduced. 
     Referring to  FIG.  34   , the second arm  912  or the third arm  915  may have a central axis CR. When the rod  870  is fastened to the second arm  912  at a position spaced apart from the central axis CR by a distance r, an angle formed between the rod  870  and the second base  32  may be referred to as theta  2 , and a minimum force required for the rod  870  to raise the second arm  912  or the third arm  915  may be referred to as F 3 . When the rod  870  is fastened to the second arm  912  at a position spaced apart from the central axis CR by a distance r′, an angle formed between the rod  870  and the second base  32  may be referred to as theta  2 ′, and a minimum force required for the rod  870  to raise the second arm  912  or the third arm  915  may be referred to as F 4 . When the rod  870  is fastened to the second arm  912  at a position spaced apart from the central axis CR by a distance r″, an angle formed between the rod  870  and the second base  32  may be referred to as theta  2 ″, and a minimum force required for the rod  870  to raise the second arm  912  or the third arm  915  may be referred to as F 5 . 
     Referring to  FIG.  35   , if theta S is the same, theta  2 ″ may be greater than theta  2 ′, and theta  2 ′ may be greater than theta  2 . If theta S is the same, F 3  may be greater than F 4 , and F 4  may be greater than F 5 . As the rod  870  is fastened at a position far away from the central axis CR, a force required for raising the second arm  912  may be reduced. As the rod  870  is fastened at a position far away from the central axis CR, load applied to the motor assembly  810  may be reduced. 
     Referring to  FIG.  36   , the first arm  911  and the second arm  912  may come into contact with or may be disposed close to the rear surface of the display unit  20 . As the first arm  911  and the second arm  912  may come into contact with or may be disposed close to the rear surface of the display unit  20 , the display unit  20  may be stably rolled on or unrolled from the roller. The link mount  920  may include a first part  922  and a second part  923 . The first part  922  and the second part  923  may face each other. A space S 4  may be formed between the first part  922  and the second part  923 . The first part  922  may face the display unit  20 . The first part  922  may be disposed closer to the display unit  20  than the second part  923 . The second arm  912  may be pivotally connected to a front surface of the first part  922 . A portion of the third arm  915  may be accommodated in the space S 4 , and may be pivotally connected to the first part  922  or the second part  923 . 
     Referring to  FIG.  37   , the rod  870  may include a first part  871  and a second part  872 . The first part  871  may have a connection part  871   a  formed on a first side thereof. The second part  872  of the slide  820  may have a space S 5  formed therein. The connection part  871   a  may be inserted into the space S 5 . The connection part  871   a  may be pivotally connected to the second part  820   b  (see  FIG.  36   ) of the slide  820 . A second side of the first part  871  may be connected to a first side of the second part  872 . The second side of the second part  872  may be pivotally connected to the second arm  912  or the third arm  915 . The first part  871  may have a space S 3  formed therein. The first part  871  may have a hole  871   b.  The lead screw  840  may be accommodated in the hole  871   b  or the space S 3 . 
     A distance between the second part  872  and the display unit  20  may be D 1 . The second arm  912  may have a thickness of W 1 . A portion of the third arm  915  which is accommodated in the space S 4  may have a thickness of W 3 . The thickness W 3  may be equal to a distance between the first part  922  and the second part  923 . A portion of the third arm  915 , which is not accommodated in the space S 4 , may have a thickness of W 2 . The first part  922  may have a thickness of W 4 . The thickness W 2  may be greater than the thickness W 3 . The thickness W 2  may be equal to a sum of the thickness W 3  and the thickness W 4 . The distance D 1  may be a sum of the thickness W 1  and the thickness W 2 . 
     The second arm  912  may come into contact with or may be disposed close to the rear surface of the display unit  20 . The third arm  915  may be disposed between the second arm  912  and the second part  872 . The third arm  915  allows the second part  872  to stably transmit power for raising the second arm  912 . In order to stably raise the second arm  912  or the third arm  915 , the second part  872  may move forward with respect to a rotation axis of the lead screw  840 , to be connected to the first part  871 . In this manner, a clearance gap between the second arm  912  and the second part  872  may be minimized. 
     Referring to  FIG.  38   , a pusher  930  may be mounted to the link mount  920 . The pusher  930  may be referred to as a lifter  930 . A second part  932  may be fastened to a first part  931 . The second part  932  may come into contact with or may be separated from the link bracket  951 . The second part  932  may be a material having high elasticity. The first part  931  may be a material having lower elasticity than the second part  932 . The first part  931  may be a material having higher rigidity than the second part  932 . The first part  931  and the second part  932  may be collectively referred to as a head  936 . The head  936  may be disposed at the upper side of the link mount  920 . 
     A third part  933  may be connected to the first part  931 . Alternatively, the third part  933  may extend downwardly from the first part  931 . The third part  933  may be referred to as a tail  933 . A fourth part  934  may protrude from the third part  933 . The link mount  920  may have a space S 6 , and the third part  933  may be accommodated in the space S 6 . The space S 6  may be opened upward. The space S 6 , in which the third part  933  is accommodated, may be next to the space S 4  (see  FIG.  37   ) in which the third arm  915  is accommodated. The second part  932  of the link mount  920  may have a hole  924 . The hole  924  may be a long hole which is vertically elongated. The hole  924  may have a length of H 1 . The fourth part  934  may be inserted into the hole  924 . A spring  935  may be accommodated in the space S 6 . The spring  935  may be disposed under the third part  933 . The spring  935  may provide an elastic force to the third part  933  in a vertical direction. 
     The head  936  may be greater than a diameter of the space S 6 . When the head  936  is caught by an upper end of the space S 6 , the head  936  may have a minimum height from the second base  32 . The minimum height of the head  936  may be H 2 . When the head  936  has the minimum height, the fourth part  934  may be caught by a lower end of the space S 6 . When the head  936  has the minimum height, the spring  935  may provide a maximum elastic force. When the head  936  has the minimum height, a height of the top case  950  may be minimum. 
     While being in contact with the link bracket  951 , the pusher  930  may provide an elastic force to the link bracket  951 . In this manner, load applied to the motor assembly  810  to raise the link  910  may be reduced. 
     Referring to  FIG.  39   , when the link  910  is sufficiently raised, the pusher  930  may be separated from the link bracket  951 . When the pusher  930  is separated from the link bracket  951 , the head  936  has a maximum height from the second base  32 . The maximum height of the head  936  may be H 3 . When the head  936  has the maximum height, the fourth part  934  may be stopped at an upper end of the hole  924  (see  FIG.  38   ). When the head  936  has the maximum height, the spring  935  may be stretched to the maximum. When the head  936  has the maximum height, the spring  935  may provide a minimum elastic force. The maximum height H 3  of the head  936  may be substantially equal to a sum of the minimum height H 2  of the head  936  and the length H 1  of the hole. 
     Referring to  FIG.  40   , the display unit  20  may be rolled on the roller  143  to the maximum. The display device  100  may be symmetric with respect to the motor assembly  810 . The top case  950  may have a minimum height. The slide  820  may be at a position closest to the inner bearing  830   b.  The slide  820  may be stopped by the first stopper  861   b.  The spring  850  may be compressed to the maximum. The pusher  930  may come into contact with the link bracket  951 . The pusher  930  may have a minimum height. 
     Referring to  FIG.  41   , about half of the display unit  20  may be rolled on the roller  413 . The display device  100  may be symmetric with respect to the motor assembly  810 . A half of the display device  100  may be unrolled from the roller  143 . The slide  820  may be disposed between the first stopper  861   b  and the second stopper  861   a.  The pusher  930  may be separated from the link bracket  951 . The pusher  930  may have a maximum height. 
     Referring to  FIG.  42   , the display unit  20  may be unrolled from the roller  143  to the maximum. The display device  100  may be symmetric with respect to the motor assembly  810 . The top case  950  may have a maximum height. The slide  820  may be disposed closest to the outer bearing  830   a.  The slide  820  may be stopped by the second stopper  861   a.  The spring  850  may be stretched to the maximum. The pusher  930  may be separated from the link bracket  951 . The pusher  930  may have a maximum height. 
     Referring to  FIGS.  43  to  46   , link mounts  920   a  and  920   b  may be mounted on the base  31 . The link mounts  920   a  and  920   b  may include a right link mount  920   a,  which is spaced rightwardly from a first right bearing  830   a,  and a left link mount  920   b  which is spaced leftwardly from a second left bearing  830   d.    
     Links  910   a  and  910   b  may be connected to the link mounts  920   a  and  920   b.  The links  910   a  and  910   b  may include a right link  910   a  connected to the right link mount  920   a  and a left link  910   b  connected to the left link mount  920   b.    
     The right link  910   a  may be referred to as a first link. The left link  910   b  may be referred to as a second link. The right link mount  920   a  may be referred to as a first link mount  920   a.  The left link mount  920   b  may be referred to as a second link mount  920   b.    
     The links  910   a  and  910   b  may include first arms  911   a  and  911   b,  second arms  912   a  and  912   b,  and arm joints  913   a  and  913   b.  First sides of the second arms  912   a  and  912   b  may be rotatably connected to the link mounts  920   a  and  920   b.  Second sides of the second arms  912   a  and  912   b  may be rotatably connected to the arm joints  913   a  and  913   b.  First sides of the first arms  911   a  and  911   b  may be rotatably connected to the arm joints  913   a  and  913   b.  Second sides of the first arms  911   a  and  911   b  may be rotatably connected to link brackets  951   a  and  951   b.    
     The link brackets  951   a  and  951   b  may include a right link bracket  951   a  connected to the first arm  911   a  of the right link  910   a,  and a left link bracket  951   b  connected to the first arm  911   b  of the left link  910   b.  The link brackets  951   a  and  951   b  may be connected to the upper bar  950 . 
     The upper bar  950  may connect the right link bracket  951   a  and the left link bracket  951   b.    
     Rods  870   a  and  870   b  may connect sliders  860   a  and  860   b  with the links  910   a  and  910   b.  First sides of the rods  870   a  and  870   b  may be rotatably connected to the sliders  860   a  and  860   b.  Second sides of the rods  870   a  and  870   b  may be rotatably connected to the second arms  912   a  and  912   b.  The rods  870   a  and  870   b  may include a right rod  870   a  for connecting the right slider  860   a  and the second arm  912   a  of the right link  910   a,  and a left rod  870   b  for connecting the left slider  860   b  and the second arm  912   b  of the left link  910   b.  The right rod  870   a  may be referred to as a first rod  870   a.  The left rod  870   b  may be referred to as a second rod  870   b.    
     Specifically, a structure formed by a right lead screw  840   a,  the right slider  860   a,  the right rod  870   a,  and the right link  910   a  will be described below. The right slider  860   a  may include a body  861   a  and a rod mount  862   a.  The body  861   a  may have a screw thread SS formed on an outer circumference thereof. The screw thread formed on the body  861   a  may be engaged with a screw thread RS of the right lead screw  840   a.  The right lead screw  840   a  may pass through the body  861   a.    
     The rod mount  862   a  may be formed on the right side of the body  861   a.  The rod mount  862   a  may be rotatably connected to a first side of the right rod  870   a.  The rod mount  862   a  may include a first rod mount  862   a   1  and a second rod mount  862   a   2 . The first rod mount  862   a   1  may be disposed at the front of the right lead screw  840   a.  The second rod mount  862   a   2  may be disposed at the rear of the right lead screw  840   a.  The first rod mount  862   a   1  and the second rod mount  862   a   2  may be spaced apart from each other. The second rod mount  862   a   2  may be spaced apart from the first rod mount  862   a   1  in the negative z-axis direction. The right lead screw  840   a  may be disposed between the first rod mount  862   a   1  and the second rod mount  862   a   2 . 
     The rod mount  862   a  may be rotatably connected to a first side of the rod  870   a  by a connection member C 1 . The connection member C 1  may pass through the rod mount  862   a  and the right rod  870   a.    
     The right rod  870   a  may be rotatably connected to the second arm  912   a  by a connection member C 2 . The connection member C 2  may pass through the second arm  912   a  and the right rod  870   a.    
     The right rod  870   a  may include a transmission unit  871   a  connected to the second arm  912   a  of the right link  910   a,  and a cover  872   a  connected to the rod mount  862   a  of the right slider  860   a.  The transmission unit  871   a  may transmit a force, generated during movement of the right slider  860   a  along the right lead screw  840   a,  to the right link  910   a.    
     The cover  872   a  may include a first plate  873   a  disposed at the front of the right lead screw  840   a.  The first plate  873   a  may be disposed perpendicular to the base  31 . Alternatively, the first plate  873   a  may face the right lead screw  840   a.    
     The cover  872   a  may include a second plate  874   a  disposed at the rear of the right lead screw  840   a.  The second plate  874   a  may be disposed perpendicular to the base  31 . Alternatively, the second plate  874   a  may face the right lead screw  840   a.  Alternatively, the second plate  874   a  may be spaced apart from the first plate  873   a.  The right lead screw  840   a  may be disposed between the first plate  873   a  and the second plate  874   a.    
     The cover  872   a  may include a third plate  875   a  connecting the first plate  873   a  and the second plate  874   a.  The third plate  875   a  may be connected to the transmission unit. The third plate  875   a  may be disposed over the right lead screw  840   a.    
     The cover  872   a  may include a fourth plate  876   a  connecting the first plate  873   a  and the second plate  874   a.  The fourth plate  876   a  may be connected to the third plate  875   a.  The fourth plate  876   a  may be disposed over the right lead screw  840   a.    
     A first side of the first plate  873   a  may be connected to the first rod mount  862   a   1 . The first plate  873   a  may be connected to the first rod mount  862   a   1  by a connection member C 1 ′. A second side of the first plate  873   a  may be connected to the third plate  875   a.    
     A first side of the second plate  874   a  may be connected to the second rod mount  862   a   2 . The second plate  874   a  may be connected to the second rod mount  862   a   2  by a connection member C 1 . A second side of the second plate  874   a  may be connected to the third plate  875   a.    
     When the right slider  860   a  moves closer to the motor assembly  810 , the right lead screw  840   a  and the right rod  870   a  may come into contact with each other. When the right lead screw  840   a  and the right rod  870   a  come into contact with each other, mutual interference may occur, and movement of the right slider  860   a  may be restricted. 
     The cover  872   a  may provide the space Si formed therein. The first plate  873   a,  the second plate  874   a,  the third plate  875   a,  and the fourth plate  876   a  may form the space S 1 . When the right slider  860   a  moves closer to the motor assembly  810 , the right lead screw  840   a  may be accommodated in the space S 1  provided by the cover  872   a,  or may escape. By the space S 1  provided by the cover  872   a,  the right slider  860   a  may move closer to the motor assembly  810 , compared to the case where the cover  872   a  is not provided. That is, as the cover  872   a  provides the space S 1  formed therein, a movable range of the right slider  860   a  may be expanded. In addition, as the right lead screw  840   a  is accommodated in the cover  872   a,  there is an effect of reducing the size of the housing  30  (see  FIG.  2   ). 
     Further, the cover  872   a  may limit a minimum value of an angle theta S formed between the second arm  912   a  and the base  31 . When the angle theta S is sufficiently reduced, the third plate  875   a  of the cover  872   a  may come into contact with the second arm  912   a  and may support the second arm  912   a.  As the third plate  875   a  supports the second arm  912   a,  the minimum value of theta S may be limited, and sagging of the second arm  912   a  may be prevented. That is, the cover  872   a  may serve as a stopper for preventing sagging of the second arm  912   a.  In addition, as the third plate  875   a  limits the minimum value of theta S, an initial load for raising the second arm  912   a  may be reduced. 
     The lead screws  840   a  and  840   b  may be driven by one motor assembly  810 . As the lead screws  840   a  and  840   b  are driven by one motor assembly  810 , the second arms  912   a  and  912   b  may be raised while being symmetric to each other. However, when the lead screws  840   a  and  840   b  are driven by one motor assembly  810 , a load applied to the motor assembly  810  for raising the second arms  912   a  and  912   b  may be extremely increased. In this case, the third plate  875   a  limits the minimum value of theta S, thereby reducing the load applied to the motor assembly  810  for raising the second arms  912   a  and  912   b.    
     A structure formed by the left lead screw  840   b,  the left slider  860   b,  the left rod  870   b,  and the left link  910   b  may be symmetric to the aforementioned structure formed by the right lead screw  840   a,  the right slider  860   a,  the right rod  870   a,  and the right link  910   a.  In this case, a symmetry axis may be a symmetry axis ys of the motor assembly  810 . 
     Referring to  FIG.  47   , guides  850   a,    850   b,    850   c,  and  850   d  may be connected to the bearings  830   a,    830   b,    830   c,  and  830   d.  The guides  850   a,    850   b,    850   c,  and  850   d  may include right guides  850   a  and  850   b  disposed on the right side of the motor assembly  810 , and left guides  850   c  and  850   d  disposed on the left side of the motor assembly  810 . 
     First sides of the right guides  850   a  and  850   b  may be connected to the first right bearing  830   a,  and second sides thereof may be connected to the second right bearing  830   b.  The right guides  850   a  and  850   b  may be disposed parallel to the right lead screw  840   a.  Alternatively, the right guides  850   a  and  850   b  may be spaced apart from the right lead screw  840   a.    
     The right guides  850   a  and  850   b  may include a first right guide  850   a  and a second right guide  850   b.  The first right guide  850   a  and the second right guide  850   b  may be spaced apart from each other. The right lead screw  840   a  may be disposed between the first right guide  850   a  and the second right guide  850   b.    
     The right slider  860   a  may include a protrusion. Alternatively, the display device may include a protrusion formed on the right slider  860   a.  The protrusion may be formed on the body of the slider. The protrusion may include a front protrusion (not shown) protruding from the body  861   a  of the right slider  860   a  in the positive z-axis direction, and a rear protrusion  865   a  protruding from the body of the slider in the negative z-axis direction. 
     The first right guide  850   a  may pass through the rear protrusion  865   a.  Alternatively, a first hole  863   a  formed in the rear protrusion may be included, and the first right guide  850   a  may pass through the first hole  863   a.  The first hole  863   a  may be formed in the x-axis direction. The first hole  863   a  may be referred to as a hole  863   a.    
     The second right guide (not shown) may pass through the front protrusion (not shown). Alternatively, a second hole (not shown) formed in the front protrusion may be included, and the second right guide may pass through the second hole. The second hole may be formed in the x-axis direction. 
     When the right slider  860   a  moves along the right lead screw  840   a,  the right guides  850   a  and  50   b  may guide more stable movement. As the right guides  850   a  and  850   b  stably guide the right slider  860   a,  the right slider  860   a  may move along the right lead screw  840   a  without rotating with respect to the right lead screw  840   a.    
     A structure formed by the left guides  850   c  and  850   d,  the left bearings  830   a,    830   b,    830   c,  and  830   d,  the left slider  860   b,  and the left lead screw  840   b  may be symmetric to the aforementioned structure formed by the right guides  850   a  and  850   b,  the right bearings  830   a,    830   b,    830   c,  and  830   d,  the right slider  860   a,  and the right lead screw  840   a.  In this case, a symmetry axis may be a symmetry axis ys of the motor assembly  810 . 
     Referring to  FIG.  48   , first springs  841   a  and  841   b  may be inserted into the lead screws  840   a  and  840   b.  Alternatively, the lead screws  840   a  and  840   b  may pass through the first springs  841   a  and  841   b.  The first springs  841   a  and  841   b  may include a first right spring  841   a  disposed on the right side of the motor assembly  810 , and a first left spring  841   b  disposed on the left side of the motor assembly  810 . 
     The first right spring  841   a  may be disposed between the right slider  860   a  and the second right bearing  830   b.  A first end of the first right spring  841   a  may come into contact or may be separated from the right slider  860   a.  A second end of the first right spring  841   a  may come into contact or may be separated from the second right bearing  830   b.    
     When the second arm  912   a  fully lies with respect to the base  31 , a distance between the right slider  860   a  and the second right bearing  830   b  may be a distance RD 3 . While not being compressed or stretched, the first right spring  841   a  may have a greater length than the distance RD 3 . Accordingly, when the second arm  912   a  fully lies with respect to the base  31 , the first right spring  841   a  may be compressed between the right slider  860   a  and the second right bearing  830   b.  Further, the first right spring  841   a  may provide a restoring force to the right slider  860   a  in the positive x-axis direction. 
     When the second arm  912   a  changes from a fully lying position to a raised position with respect to the base  31 , the restoring force provided by the first right spring  841   a  may assist in raising the second arm  912   a.  As the first right spring  841   a  assists in raising the second arm  912   a,  load applied to the motor assembly  810  may be reduced. 
     The lead screws  840   a  and  840   b  may be driven by one motor assembly  810 . As the lead screws  840   a  and  840   b  are driven by one motor assembly  810 , the second arms  912   a  and  912   b  may be raised while being symmetric to each other. However, when the lead screws  840   a  and  840   b  are driven by one motor assembly  810 , a load applied to the motor assembly  810  for raising the second arms  912   a  and  912   b  may be extremely increased. In this case, the first right spring  841   a  assists in raising the second arm  912   a,  thereby reducing the load on the motor assembly  810  and the load applied to the motor assembly  810  for raising the second arm  912   a.    
     Alternatively, when the second arm  912   a  changes from a raised position to a fully lying position with respect to the base  31 , the restoring force provided by the first right spring  841   a  may lessen the impact caused when the second arm  912   a  lies with respect to the base  31 . That is, when the second arm  912   a  lies with respect to the base  31 , the first right spring  841   a  may act as a damper. As the first right spring  841   a  acts as a damper, the load on the motor assembly  810  may be reduced. 
     A structure formed by the first left spring  841   b,  the left bearings  830   a,    830   b,    830   c,  and  830   d,  the left slider  860   b,  the left lead screw  840   b,  and the second arm  912   a  may be symmetric to the aforementioned structure formed by the first right spring  841   a,  the right bearings  830   a,    830   b,    830   c,  and  830   d,  the right slider  860   a,  the right lead screw  840   a,  and the second arm  912   a.  In this case, a symmetry axis may be a symmetry axis ys of the motor assembly  810 . 
     Referring to  FIG.  49   , the second springs  851   a  and  851   b  may be inserted into the guides  850   a,    850   b,    850   c,  and  850   d.  Alternatively, the guides  850   a,    850   b,    850   c,  and  850   d  may pass through the second springs  851   a  and  851   b.  The second springs  851   a  and  851   b  may include a second right spring  851   a  disposed on the right side of the motor assembly  810 , and a second left spring  851   b  disposed on the left side of the motor assembly  810 . 
     There may be a plurality of second right springs  851   a.  The second right springs  851   a  may include springs  940   a  and  940   b  inserted into the first right guide  850   a,  and springs  940   a  and  940   b  inserted into the second right guide  850   b.  Alternatively, the second right springs  851   a  may include springs  940   a  and  940   b  through which the first right guide  850   a  passes, and springs  940   a  and  940   b  through which the second right guide  850   b  passes. 
     The guides  850   a,    850   b,    850   c,  and  850   d  may include stopper protrusions  852   a  and  852   b.  The stopper protrusions  852   a  and  952   b  may include a right stopper protrusion  852   a  disposed on the right side of the motor assembly  810 , and a left stopper protrusion  852   b  disposed on the left side of the motor assembly  810 . 
     The right stopper protrusion  852   a  may be disposed between the right slider  860   a  and the second right bearing  830   b.  Further, the second right spring  851   a  may be disposed between the right slider  860   a  and the second right bearing  830   b.  A first end of the second right spring  851   a  may come into contact with or may be separated from the right slider  860   a.  A second end of the second right spring  851   a  may come into contact with or may be separated from the right stopper protrusion  852   a.    
     When the second arm  912   a  fully lies with respect to the base  31 , a distance between the right slider  860   a  and the right stopper protrusion  852   a  may be a distance RD 4 . While not being compressed or stretched, the second right spring  851   a  may have a greater length than the distance RD 4 . Accordingly, when the second arm  912   a  fully lies with respect to the base  31 , the second right spring  851   a  may be compressed between the right slider  860   a  and the second stopper protrusion  852   a.  Further, the second right spring  851   a  may provide a restoring force to the right slider  860   a  in the positive x-axis direction. 
     When the second arm  912   a  changes from a fully lying position to a raised position with respect to the base  31 , the restoring force provided by the second right spring  851   a  may assist in raising the second arm  912   a.  As the second right spring  851   a  assists in raising the second arm  912   a,  load applied to the motor assembly  810  may be reduced. 
     The lead screws  840   a  and  840   b  may be driven by one motor assembly  810 . As the lead screws  840   a  and  840   b  are driven by one motor assembly  810 , the second arms  912   a  and  912   b  may be raised while being symmetric to each other. However, when the lead screws  840   a  and  840   b  are driven by one motor assembly  810 , a load applied to the motor assembly  810  for raising the second arms  912   a  and  912   b  may be extremely increased. In this case, the second right spring  851   a  assists in raising the second arm  912   a,  thereby reducing the load on the motor assembly  810  and the load applied to the motor assembly  810  for raising the second arm  912   a.    
     Alternatively, when the second arm  912   a  changes from a raised position to a fully lying position with respect to the base  31 , the restoring force provided by the second right spring  851   a  may lessen the impact caused when the second arm  912   a  lies with respect to the base  31 . That is, when the second arm  912   a  lies with respect to the base  31 , the second right spring  851   a  may act as a damper. As the second right spring  851   a  acts as a damper, the load of the motor assembly  810  may be reduced. 
     A structure formed by the second left spring  851   b,  the left stopper protrusion  852   b,  the left slider  860   b,  the left guides  850   c  and  850   d,  and the second arm  912   a  may be symmetric to the aforementioned structure formed by the second right spring  851   a,  the right stopper protrusion  852   a,  the right slider  860   a,  the right guides  850   a  and  850   b,  and the second arm  912   a.  In this case, a symmetry axis may be a symmetry axis ys of the motor assembly  810 . 
     Referring to  FIGS.  50  to  52   , the second arm  912   a  may be raised by receiving the restoring force from the first right spring  841   a  and the second right spring  851   a.    
     An angle formed between the second arm  912   a  and the base  31  may be an angle theta S. An angle formed between the right rod  870   a  and the base  31  may be an angle theta T. A force required for the motor assembly  810  to move the right slider  860   a  in the positive x-axis direction may be FA. A force applied by the first right spring  841   a  to the right slider  860   a  may be FB. A force applied by the second right spring  851   a  to the right slider  860   a  may be FC. A force transmitted by the right rod  870   a  to the second arm  912   a  may be FT. 
     When the second arm  912   a  fully lies with respect to the base  31 , the angles theta S and theta T may have minimum values. When the second arm  912   a  changes from a fully lying position to a raised position with respect to the second base  31 , the angles theta S and theta T may gradually increase. 
     When the second arm  912   a  fully lies with respect to the base  31 , the first right spring  841   a  may be compressed. The compressed first right spring  841   a  may provide the restoring force FB to the right slider  860   a.  The restoring force FB may be applied in the positive x-axis direction. When the second arm  912   a  fully lies with respect to the base  31 , compression displacement of the first right spring  841   a  may be maximum, and a magnitude of the restoring force FB may have a maximum value. When the second arm  912   a  changes from a fully lying position to a raised position with respect to the base  31 , the compression displacement of the first right spring  841   a  may gradually decrease, and the magnitude of the restoring force FB may gradually decrease. 
     When the second arm  912   a  fully lies with respect to the base  31 , the second right spring  851   a  may be compressed. The compressed second right spring  851   a  may provide the restoring force FC to the right slider  860   a.  The restoring force FC may be applied in the positive x-axis direction. When the second arm  912   a  fully lies with respect to the base  31 , compression displacement of the second right spring  851   a  may be maximum, and a magnitude of the restoring force FC may have a maximum value. When the second arm  912   a  changes from a fully lying position to a raised position with respect to the base  31 , the compression displacement of the second right spring  851   a  may gradually decrease, and the magnitude of the restoring force FC may gradually decrease. 
     The force FT transmitted by the right rod  870   a  to the second arm  912   a  may be a resultant force composed of the force FA required for the motor assembly  810  to move the right slider  860   a  in the positive x-axis direction, the restoring force FB of the first right spring  841   a,  and the restoring force FC of the second right spring  851   a.    
     When the second arm  912   a  starts to be raised after fully lying with respect to the base  31 , the motor assembly  810  may be under a maximum load. In this case, a magnitude of the restoring force FB provided by the first right spring  841   a  may be maximum. Further, the restoring force FC provided by the second springs  851   a  and  851   b  may be maximum. 
     When the second arm  912   a  changes from a fully lying position to a raised position with respect to the base  31 , the restoring force provided by the first right spring  841   a  and the second right spring  851   a  may assist in raising the second arm  912   a.  As the first right spring  841   a  and the second right spring  851   a  assist in raising the second arm  912   a,  the load on the motor assembly  810  may be reduced. 
     The first right spring  841   a  and the second right spring  851   a  may simultaneously provide the restoring force (a resultant force of the restoring force FB and the restoring force FC) to the right slider  860   a.  The restoring force (the resultant force of the restoring force FB and the restoring force FC) may be provided to the right slider  860   a  until a distance RD 5  between the right slider  860   a  and the right stopper protrusion  852   a  becomes equal to a length of the second right spring  851   a.    
     When the distance RD 5  between the right slider  860   a  and the right stopper protrusion  852   a  becomes equal to the length of the second right spring  851   a,  compression displacement of the second right spring  851   a  may become zero. When the compression displacement of the second right spring  851   a  becomes zero, the restoring force FC provided by the second right spring  851   a  to the right slider  860   a  may become zero. 
     When the distance RD 5  between the right slider  860   a  and the right stopper protrusion  852   a  is greater than the length of the second right spring  851   a,  only the first right spring  841   a  may provide the restoring force FB to the right slider  860   a.  The restoring force FB may be provided to the right slider  860   a  until a distance RD 6  between the right slider  860   a  and the second right bearing  830   b  becomes equal to a length of the first right spring  841   a.    
     When the distance RD 6  between the right slider  860   a  and the right second bearing  830   b  becomes equal to the length of the first right spring  841   a,  compression displacement of the first right spring  841   a  may become zero. When the compression displacement of the first right spring  841   a  becomes zero, the restoring force FB provided by the first right spring  841   a  to the right slider  860   a  may become zero. 
     When the distance RD 6  between the right slider  860   a  and the second right bearing  830   b  is greater than the length of the first right spring  841   a,  the motor assembly  810  may raise the second arm  912   a  without receiving the restoring force from the first right spring  841   a  or the second right spring  851   a.    
     A structure formed by the first left spring  841   b,  the second left spring  851   b,  the left stopper protrusion  852   b,  the left slider  860   b,  the left guides  850   c  and  850   d,  the left lead screw  840   b,  the left rod  870   b,  and the second arm  912   a  may be symmetric to the aforementioned structure formed by the first right spring  841   a,  the second right spring  851   a,  the right stopper protrusion  852   a,  the right slider  860   a,  the right guides  850   a  and  850   b,  the right lead screw  840   a,  the right rod  870   a,  and the second arm  912   a.  In this case, a symmetry axis may be a symmetry axis ys of the motor assembly  810 . 
     Referring to  FIG.  53   , pushers  930   a  and  930   b  may be connected to the link mounts  920   a  and  920   b.  The pushers  930   a  and  930   b  may include a right pusher  930   a  disposed on the right side of the motor assembly  810 , and a left pusher  930   b  disposed on the left side of the motor assembly  810 . 
     The link mounts  920   a  and  920   b  may form an accommodation space A. The accommodation space A may serve to accommodate the springs  940   a  and  940   b  and the pushers  930   a  and  930   b.  The springs  940   a  and  940   b  may include the right spring  940   a  disposed on the right side of the motor assembly  810 , and a left spring  940   b  disposed on the left side of the motor assembly  810 . The accommodation space A may be referred to as an internal space A. 
     The link mounts  920   a  and  920   b  may have a first hole  922   a  for connecting the accommodation space A and an external space (a first hole corresponding to the link mount  920   b  is not shown). The first hole  922   a  may be formed in an upper surface of the link mounts  920   a  and  920   b.  The first hole  922   a  may be referred to as a hole  922   a.    
     The pushers  930   a  and  930   b  may be disposed perpendicular to the base  31 . Alternatively, the pushers  930   a  and  930   b  may be disposed parallel to the y-axis. The springs  940   a  and  940   b  may be disposed perpendicular to the base  31 . Alternatively, the springs  940   a  and  940   b  may be disposed parallel to the y-axis. 
     The pushers  930   a  and  930   b  may include first parts  931   a  and  931   b  and second parts  932   a  and  932   b.  The second parts  932   a  and  932   b  may be connected to lower sides of the first parts  931   a  and  931   b.  Lower ends of the second parts  932   a  and  932   b  may be connected to the springs  940   a  and  940   b.  All or a portion of the second parts  932   a  and  932   b  may be accommodated in the accommodation space A formed by the link mounts  920   a  and  920   b.  The second parts  932   a  and  932   b  may have the same diameter as a diameter of the first hole  922   a,  or may have a smaller diameter than the diameter of the first hole  922   a.  The second parts  932   a  and  932   b  may pass through the first hole  922   a.    
     The first parts  931   a  and  931   b  may be disposed outside of the link mounts  920   a  and  920   b.  Alternatively, the first parts  931   a  and  931   b  may be disposed outside of the accommodation space A of the link mounts  920   a  and  920   b.  The first parts  931   a  and  931   b  may have a greater diameter than the diameter of the first hole  922   a.    
     The first parts  931   a  and  931   b  may come into contact with or may be separated from the link brackets  951   a  and  951   b.  For example, when the second arms  912   a  and  912   b  fully lie with respect to the base  31 , the first parts  931   a  and  931   b  may come into contact with the link brackets  951   a  and  951   b.  Alternatively, when second arms  912   a  and  912   b  are fully raised with respect to the base  31 , the first parts  931   a  and  931   b  may be separated from the link brackets  951   a  and  951   b.    
     When the first parts  931   a  and  931   b  come into contact with the link brackets  951   a  and  951   b,  the pushers  930   a  and  930   b  may receive force from the link brackets  951   a  and  951   b.  The pushers  930   a  and  930   b  may receive a downward force. The pushers  930   a  and  930   b  may receive the force in the negative y-axis direction. Alternatively, the link brackets  951   a  and  951   b  may press the pushers  930   a  and  930   b.  The link brackets  951   a  and  951   b  may downwardly press the pushers  930   a  and  930   b.  Alternatively, the link brackets  951   a  and  951   b  may press the pushers  930   a  and  930   b  in the negative y-axis direction. 
     When the first parts  931   a  and  931   b  receive the force, the springs  940   a  and  940   b  may be compressed. The compressed springs  940   a  and  940   b  may provide a restoring force to the pushers  930   a  and  930   b.  The restoring force may be opposite to a direction of the force applied to the first parts  931   a  and  931   b.  Alternatively, the restoring force may be applied in the positive y-axis direction. 
     The link mounts  920   a  and  920   b  may have a second hole  921   a  (a second hole corresponding to the link mount  920   b  is not shown). The second hole  921   a  may connect the accommodation space A and an external space. All or a portion of the springs  940   a  and  940   b  may be exposed to the outside through the second hole  921   a.  All or a portion of the pushers  930   a  and  930   b  may be exposed to the outside through the second hole  921   a.  During maintenance or repair of the display device, service providers may check an operating state of the pushers  930   a  and  30   b  through the second hole  921   a.  The second hole  921   a  may provide convenience in repair and maintenance for the service providers. 
     Referring to  FIGS.  54  to  56   , the right link  910   a  may be raised by receiving a restoring force from the right pusher  930   a.  The following description will be given based on the right link  910   a.    
     An angle formed between the second arm  912   a  and the base  31  may be an angle theta S. A force transmitted by the right rod  870   a  to the second arm  912   a  may be FT. A force transmitted by the right pusher  930   a  to the right link bracket  951   a  may be FP. 
     Referring to  FIG.  54   , when the second arm  912   a  fully lies with respect to the base  31 , the angle theta S may have a minimum value. The right spring  940   a  connected to the right pusher  930   a  may be compressed to the maximum, and a magnitude of the restoring force FP may have a maximum value. The compressed right spring  940   a  may provide the restoring force FP to the right pusher  930   a.  The right pusher  930   a  may transmit the restoring force FP to the right link bracket  951   a.  The restoring force FP may be applied in the positive y-axis direction. 
     When the second arm  912   a  fully lies with respect to the base  31 , a distance HL from the base  31  to the right pusher  930   a  may have a minimum value. The first part  931   a  of the right pusher  930   a  may protrude to the outside of the right link mount  920   a,  and all of the second part  932   a  of the right pusher  930   a  may be accommodated in an accommodation space  923   a  of the right link mount  920   a.    
     Referring to  FIG.  55   , when the second arm  912   a  changes from a fully lying position to a raised position with respect to the base  31 , the angle theta S may gradually increase. Compression displacement of the right spring  940   a  may gradually decrease, and a magnitude of the restoring force FP may gradually decrease. 
     As the angle theta S gradually increases, at least a portion of the second part  932   a  of the right pusher  930   a  may protrude to the outside of the right link mount  920   a.  A length of the right pusher  930   a  that protrudes to the outside of the right link mount  920   a  may be referred to as a length HP. A distance HL from the base  31  to an upper end of the right pusher  930   a  may increase by HP, compared to the case where the second arm  912   a  fully lies with respect to the base  31 . 
     Referring to  FIG.  56   , while the second arm  912   a  is raised with respect to the base  31 , the right pusher  930   a  and the right link bracket  951   a  may be separated from each other. The compression displacement of the right spring  940   a  may become zero. When the compression displacement of the right spring  940   a  becomes zero, the restoring force FP provided by the right pusher  930   a  to the right link bracket  951   a  may become zero. 
     In addition, the length HP of the second part  932   a  of the right pusher  930   a  that protrudes to the outside of the right link mount  920   a,  may have a maximum value. Further, the distance HL from the base  31  to the upper end of the right pusher  930   a  may have a maximum value. 
     That is, while the right pusher  930   a  and the right link bracket  951   a  are in contact with each other, the right pusher  930   a  applies a restoring force to the right link bracket  951   a,  thereby assisting in raising the second arm  912   a,  and reducing the load on the motor assembly  810 . 
     The lead screws  840   a  and  840   b  may be driven by one motor assembly  810 . As the lead screws  840   a  and  840   b  are driven by one motor assembly  810 , the second arms  912   a  and  912   b  may be raised while being symmetric to each other. However, when the lead screws  840   a  and  840   b  are driven by one motor assembly  810 , a load applied to the motor assembly  810  for raising the second arms  912   a  and  912   b  may be extremely increased. In this case, the right pusher  930   a  applies the restoring force to the right link bracket  951   a,  thereby assisting in raising the second arm  912   a,  and reducing the load on the motor assembly  810 . 
     Alternatively, when the second arm  912   a  changes from a raised position to a fully lying position with respect to the base  31 , the restoring force provided by the right pusher  930   a  to the right link bracket  951   a  may lessen the impact caused when the second arm  912   a  lies with respect to the base  31 . That is, the restoring force provided by the right pusher  930   a  to the right link bracket  951   a  may act as a damper when the second arm  912   a  lies with respect to the base  31 . As the right pusher  930   a  acts as a damper, the load on the motor assembly  810  may be reduced. 
     A structure formed by the left pusher  930   b,  the left spring  940   b,  the left link bracket  951   b,  the left link mount  920   b,  and the left rod  870   b  may be symmetric to the aforementioned structure formed by the right pusher  930   a,  the right spring  940   a,  the right link bracket  951   a,  the right link mount  910   a,  and the right rod  870   a.  In this case, a symmetry axis may be a symmetry axis ys of the motor assembly  810 . 
     Referring to  FIGS.  57  to  59   , the panel roller  143  may be mounted on the base  31 . The panel roller  143  may be mounted in front of the lead screws  840   a  and  840   b.  Alternatively, the panel roller  143  may be disposed parallel to a longitudinal direction of the lead screws  840  and  840   b.  Alternatively, the panel roller  143  may be spaced apart from the lead screws  840   a  and  840   b.    
     The display unit  20  may include the display panel  10  and the module cover  15 . A lower side of the display unit  20  may be connected to the panel roller  143 , and an upper side of the display unit  20  may be connected to the upper bar  75 . The display unit  20  may be rolled on or unrolled from the panel roller  143 . 
     A distance from the symmetry axis ys of the motor assembly  810  to the right slider  860   a  may be a distance RD. A distance from the symmetry axis ys of the motor assembly  810  to the left slider  860   b  may be a distance LD. A distance from the right slider  860   a  to the left slider  860   b  may be a distance SD. The distance SD may be a sum of the distance RD and the distance LD. A distance from the base  31  to the upper end of the display unit  20  may be a distance HD. 
     Referring to  FIG.  57   , when the second arms  912   a  and  912   b  fully lie with respect to the base  31 , the distance SD between the right slider  860   a  and the left slider  860   b  may have a minimum value. The distance RD from the symmetry axis ys of the motor assembly  810  to the right slider  860   a  may be equal to the distance LD from the symmetry axis ys of the motor assembly  810  to the left slider  860   b.    
     When the second arms  912   a  and  912   b  fully lie with respect to the base  31 , the distance HD from the base  31  to the upper end of the display unit  20  may have a minimum value. 
     When the second arms  912   a  and  912   b  fully lie with respect to the base  31 , the first springs  841   a  and  841   b  may come into contact with the sliders  860   a  and  860   b.  Further, the second springs  851   a  and  851   b  may come into contact with the sliders  860   a  and  860   b.  In addition, the pushers  930   a  and  930   b  may come into contact with the link brackets  951   a  and  951   b.    
     When the second arms  912   a  and  912   b  fully lie with respect to the base  31 , an amount of compression of the first springs  841   a  and  841   b  may have a maximum value, and a magnitude of the restoring force provided by the first springs  841   a  and  841   b  to the sliders  860   a  and  860   b  may have a maximum value. 
     When the second arms  912   a  and  912   b  fully lie with respect to the base  31 , an amount of compression of the second springs  851   a  and  851   b  may have a maximum value, and a magnitude of the restoring force provided by the second springs  851   a  and  851   b  to the sliders  860   a  and  860   b  may have a maximum value. 
     When the second arms  912   a  and  912   b  fully lie with respect to the base  31 , an amount of compression of the springs  940   a  and  940   b  may have a maximum value, and a magnitude of the restoring force provided by the springs  940   a  and  940   b  to the pushers  930   a  and  930   b  may have a maximum value. 
     When the second arms  912   a  and  912   b  start to be raised with respect to the base  31 , the second arms  912   a  and  912   b  may be raised by receiving the restoring force from the first springs  841   a  and  841   b,  the second springs  851   a  and  851   b,  and springs  940   a  and  940   b.  In this manner, the load applied to the motor assembly  810  may be reduced. 
     Referring to  FIG.  58   , while the second arms  912   a  and  912   b  are raised with respect to the base  31 , the distance SD between the right slider  860   a  and the left slider  860   b  may gradually increase. Even when the distance SD increases, the distance LD and the distance RD may be equal to each other. That is, the right slider  860   a  and the left slider  860   b  may be disposed symmetrical to each other with respect to the symmetry axis ys of the motor assembly  810 . In addition, a raised degree of the second arms  912   a  and  912   b  of the right link  910   a  with respect to the base  31  may be equal to a raised degree of the second arms  912   a  and  912   b  of the left link  910   b  with respect to the base  31 . 
     While the second arms  912   a  and  912   b  are raised with respect to the base  31 , the distance HD from the base  31  to the upper end of the display unit  20  may gradually increase. The display unit  20  may be unrolled from the panel roller  143 . Alternatively, the display unit  20  may be unfolded from the panel roller  143 . 
     When the second arms  912   a  and  912   b  are sufficiently raised with respect to the base  31 , the first springs  841   a  and  841   b  may be separated from the sliders  860   a  and  860   b.  In addition, when the second arms  912   a  and  912   b  are sufficiently raised with respect to the base  31 , the second springs  851   a  and  851   b  may be separated from the sliders  860   a  and  860   b.  Further, when the second arms  912   a  and  912   b  are sufficiently raised with respect to the base  31 , the pushers  930   a  and  930   b  may be separated from the link brackets  951   a  and  951   b.    
     Separation of the first springs  841   a  and  841   b  from the sliders  860   a  and  860   b,  separation of the second springs  851   a  and  851   b  from the sliders  860   a  and  860   b,  and separation of the pushers  930   a  and  930   b  from the link brackets  951   a  and  951   b  may be performed independently of each other. That is, a sequence of separation of the first springs  841   a  and  841   b  from the sliders  860   a  and  860   b,  separation of the second springs  851   a  and  851   b  from the sliders  860   a  and  860   b,  and separation of the pushers  930   a  and  930   b  from the link brackets  951   a  and  951   b  may vary. 
     An angle formed between an axis xs 1  parallel to the base  31  and the second arm  912   a  may be referred to as theta R. Further, an angle formed between the axis xs 1  parallel to the base  31  and the first arm  911   a  may be referred to as theta R′. The axis xs 1  may be parallel to the x axis. 
     When the second arm  912   a  fully lies with respect to the base  31 , or while the second arm  912   a  is raised with respect to the base  31 , or when raising of the second arm  912   a  with respect to the base  31  is complete, theta R and theta R′ may be maintained equal to each other. 
     An angle formed between an axis xs 2  parallel to the base  31  and the second arm  912   b  may be referred to as theta L. Further, an angle formed between the axis xs 2  parallel to the base  31  and the first arm  911   b  may be referred to as theta L′. The axis xs 2  may be parallel to the x axis. 
     When the second arm  912   b  fully lies with respect to the base  31 , or while the second arm  912   b  is raised with respect to the base  31 , or when raising of the second arm  912   a  with respect to the base  31  is complete, theta L and theta L′ may be maintained equal to each other. 
     The axis xs 1  and the axis xs 2  may be the same axis. 
     Referring to  FIG.  59   , when the second arms  912   a  and  912   b  are fully raised with respect to the base  31 , the distance SD between the right slider  860   a  and the left slider  860   b  may have a maximum value. Even when the distance SD has the maximum value, the distance LD and the distance RD may be equal to each other. 
     When the second arms  912   a  and  912   b  are fully raised with respect to the base  31 , the distance HD from the base  31  to the upper end of the display unit  20  may have a maximum value. 
     Referring to  FIGS.  60  and  61   , a door assembly  40  may be mounted in the housing  40  at a position adjacent to an upper plate of the housing  40 . A door  41  may cover an opening  30 P of the upper plate of the housing  40 . The opening  30 P of the upper plate of the housing  40  may be elongated in a longitudinal direction of the housing  40  on the upper plate of the housing  40 . A door  41  may be an elongated plate. The door  41  may move by sliding in a forward and backward direction of the housing  30 . A holder  42  may be coupled to the door  41 . A first side of the holder  42  may be fixed to a lower surface of the door  41 . In this case, the holder  42  may be disposed adjacent to a right end in a longitudinal direction of the door  41 . The holder  42  may be referred to as a door holder  42 . 
     The holder  42  may be coupled to a slider  43 . A second side of the holder  42  may be fixed to the slider  43 . The slider  43  may move on a rail  44 . The rail  44  may be fixed to a lower side of the upper plate of the housing  30 . The rail  44  may have slots  44 S extending in the forward and backward direction of the housing  30 . The slider  43  is inserted into the slots  44 S and may move in an extended direction of the slots  44 S. A rack gear  43 R may be formed on a lower surface of the slider  43 . 
     A shaft gear  45  may include a first gear  45   a  and a second gear  45   b.  The first gear  45   b  may be engaged with the rack gear  43 R, and the second gear  45   a  may be fixed to the first gear  45   b.  For example, the first gear  45   b  and the second gear  45   a  may be integrally formed with each other. A shaft  50  may be inserted into the first gear  45   a  and the second gear  45   b.  The shaft  50 , the first gear  45   a,  and the second gear  45   b  may rotate together. 
     A motor  49  may provide torque. For example, the motor  49  may provide a torque of 4 kgm. The motor  49  may be mounted in the housing  30 . The motor  49  may be connected to a worm  48 . When the motor  49  rotates, the worm  48  may rotate. A worm gear  47  may be engaged with the worm  48  connected to the motor  49 . The worm gear  47  may be connected to a torque limiter  46 . For example, the torque limiter  46  may limit a torque exceeding 2 kgm. The torque limiter  46  may be connected to the shaft gear  45 . For example, the torque limiter  46  may be engaged with the second gear  45   a  of the shaft gear  45 . 
     When the motor  49  rotates, the worm  48  may provide power to the worm gear  47 . Accordingly, when driving of the door  41  is interrupted, it is possible to prevent damage to the mechanism and injury caused when a user&#39;s hand is caught, and the like. 
     Referring to  FIGS.  61  and  62   , the shaft  50  may be elongated in the longitudinal direction of the housing  30  (see  FIG.  1   ). The shaft gear  45  (see  FIG.  60   ) may be coupled to both ends of the shaft  50 . The shaft  50  may include a shaft body  51  and gear coupling parts  52  and  53 . The gear coupling parts  52  and  53  may include a first part  52  and a second part  53 . The first part  52  may have a smaller diameter than a diameter of the shaft body  51 . The first part  52  may include fixing parts  521  and  522 . The fixing parts  521  and  522  may be formed by cutting out an outer circumferential surface of the first part  52 . The second part  53  may have a smaller diameter than a diameter of the first part  52 . 
     The gear coupling parts  52  and  53  may include first gear coupling parts  52   a  and  52   a  and second gear coupling parts  52   b  and  53   b.  The first gear coupling parts  52   a  and  53   a  may be formed at a first end of the shaft body  51 , and the second gear coupling parts  52   b  and  53   b  may be formed at a second end of the shaft body  51 . For example, the gear coupling parts  52  and  53  may be sequentially processed. As a second end of the shaft  50  is fixed to a shelf jig, the first gear coupling parts  52   a  and  53   a  may be formed at a first end of the shaft  50 . After the first gear coupling parts  52   a  and  53   a  are formed, the first end of the shaft  50  is fixed to the shelf jig, and then the second gear coupling parts  52   b  and  53   b  may be formed at the second end of the shaft  50 . In this case, it may be difficult for the first gear coupling parts  52   a  and  53   a  and the second gear coupling parts  52   b  and  53   b  to have a concentric axis. In this case, vibration and noise may occur during rotation of the shaft  50 . 
     In the case where the shaft  50  is formed as a polygonal shaft having a polygonal cross-section, a concentric axis may be provided when the aforementioned shaft  50  is formed, but there may be a problem in that when the shaft  50  rotates, the shaft  50  may be caught by a holder  60 H (see  FIG.  68   ) holding the shaft  50 . 
     Referring to  FIG.  63   , if the shaft  50  is reduced in length, vibration and noise occurring during rotation of the shaft  50  may be reduced even when the gear coupling parts  52  and  53  do not have a concentric axis. The first shaft  50   a  may be coupled to the second shaft  50   b  by a joint  59 . The joint  59  may include an insertion-fixing portion  591 , into which the first coupling parts  52   a  and  53   a  and the second coupling parts  52   b  and  53   b  are inserted and fixed. Here, when torque applied to the shaft  50  increases, the shafts  50  run idle in the joint  59 , thereby leading to a loss of power. 
     Referring to  FIG.  64   , the shaft  50  may include the shaft body  51 , the gear coupling parts  52  and  53 , and a joint hole  51   h.  The shaft body  51  may be an elongated bar. The gear coupling parts  52  and  53  may be formed at the first end of the shaft body  51 . The gear coupling parts  52  and  53  may include the first part  52  and the second part  53 . The first part  52  may have a smaller diameter than a diameter of the shaft body  51 . The first part  52  may include a fixing part  521 . The fixing part  521  may be formed by cutting out an outer circumferential surface of the first part  52 . The fixing part  521  may include a first fixing part  521   a  formed by cutting out a portion of the outer circumferential surface of the first part  52 , and a second fixing part  521   b  formed opposite to the first fixing prat  521   a  with respect to a central axis of the first part  52 . The second part  53  may have a smaller diameter than a diameter of the first part  52 . 
     The joint hole  51   h  may be formed at a position adjacent to the second end of the shaft body  51 . The joint hole  51   h  may be formed by passing through both sides of the outer circumferential surface of the shaft body  51 . The joint hole  51   h  may be formed in a direction perpendicular to a central axis of the shaft body  51 . 
     Referring to  FIGS.  65  and  66   , a joint  60  may include a body  61  and locking pins  62  and  64 . The body  61  may have an elongated cylindrical shape. An inclined surface  61   a  may be formed between an upper surface and an inner circumferential surface at both ends of the body  61 . Accordingly, the shaft  50  (see  FIG.  64   ) may be easily inserted. 
     The locking pins  62  and  64  may pass through the outer and inner circumferential surfaces of the body  61  in a direction perpendicular to an axial direction of the body  61 . First ends of the locking pins  62  and  64  may protrude to the outer circumferential surface on a first side of the body  61 , and second ends of the locking pints  62  and  64  may protrude to the outer circumferential surface on a second side of the body  61 . The first ends of the locking pins  62  and  64  may be disposed opposite to the second ends of the locking pins  62  and  64  with respect to the body  61 . The locking pins  62  and  64  may have slits  62   s  and  64   s.  The slits  62   s  and  64   s  may be formed by cutting the outer and inner circumferential surfaces of the locking pins  62  and  64  in a longitudinal direction of the locking pins  62  and  64 . There may be a plurality of locking pins  62  and  64 . A first locking pin  62  may be disposed adjacent to a first end of the body  61 , and a second locking pin  64  may be disposed adjacent to a second end of the body  61 . 
     The first shaft  50   a  may be inserted into the joint  60  in a longitudinal direction of the joint  60 . The second shaft  50   b  may be inserted into the joint  60  in the longitudinal direction of the joint  60 . The first shaft  50   a  may be disposed opposite to the second shaft  50   b  with respect to the joint  60 . An end surface of the first shaft  50   a  may face an end surface of the second shaft  50   b.  The joint holes  51   h  (see  FIG.  64   ) of the shafts  50   a  and  50   b  may be aligned at positions of the locking pins  62  and  64 . 
     Referring to  FIG.  67   , press-fit pins f 1  and f 2  may be inserted into the locking pins  62  and  64 . A diameter of the press-fit pins f 1  and f 2  may be greater than an inner diameter of the locking pins  62  and  64 . As the press-fit pins f 1  and f 2  are inserted into the locking pins  62  and  64 , the locking pins  62  and  64  may increase in diameter. Accordingly, the locking pins  62  and  64  may firmly fix the body  61  of the joint  60  and the shafts  50   a  and  50   b.  The joint  60  may be held in the holder  60 H. The joint  60  may be rotated in the holder  60 H. 
     The holder  60 H may include a body  60 H 1  and hangers  60 H 2  and  60 H 3 . 
     The hangers  60 H 2  and  60 H 3  may be formed at the body  60 H 1 . An inner diameter of the hangers  60 H 2  and  60 H 3  may correspond to an outer diameter of the joint  60 . The hangers  60 H 2  and  60 H 3  may hold the joint  60 , and the joint  60  may be rotated at a position fixed by the hangers  60 H 2  and  60 H 3 . A first hanger  60 H 2  may be formed in an arch shape on one surface of the body  60 H 1 . A second hanger  60 H 3  may be formed in an arch shape on the other surface of the body  60 H 1 . The second hanger  60 H 3  may be spaced apart from the first hanger  60 H 2  by a first distance D 1 . The first hanger  60 H 2  may have a first width D 2 , and the second hanger  60 H 3  may have a second width D 3 . A sum of the first width D 2 , the second width D 3 , and the first distance D 1  may be smaller than a gap between the locking pins f 1  and f 2 . 
     Referring to  FIG.  68   , the shaft  50  may be rotated in the holder  60 H. The shaft  50  may be held in the holder  60 H. The holder  60 H may include the body  60 H 1  and the hangers  60 H 2  and  60 H 3 . The hangers  60 H 2  and  60 H 3  may be formed at the body  60 H 1 . An inner diameter of the hangers  60 H 2  and  60 H 3  may correspond to an outer diameter of the shaft  50 . The hangers  60 H 2  and  60 H 3  may hold the shaft  50 , and the shaft  50  may be rotated at a position fixed by the hangers  60 H 2  and  60 H 3 . The first hanger  60 H 2  may be formed in an arch shape on one surface of the body  60 H 1 . The second hanger  60 H 3  may be formed in an arch shape on the other surface of the body  60 H 1 . The second hanger  60 H 3  may be spaced apart from the first hanger  60 H 2  by a first distance D 1 . The first hanger  60 H 2  may have a first width D 2 , and the second hanger  60 H 3  may have a second width D 3 . 
     Referring to  FIGS.  69  and  70   , a first shaft gear  45  may be mounted adjacent to the motor  49  and may receive a driving force from the motor  49 . The first shaft gear  45  may be fixed to the first shaft  50   a  to provide torque to the first shaft  50   a.  The first shaft  50   a  may be elongated in the left-to-right direction of the housing  30  and may be held by the first holder  60 H. 
     A second shaft gear  45 ′ may be mounted on a second side which is opposite to a first side of the housing  30  on which the motor  49  is disposed, and may be fixed to the second shaft  50   b.  The second shaft gear  45 ′ may be rotated by the rotation of the second shaft  50   b.  The first shaft  50   a  may be connected or coupled to the second shaft  50   b  by the joint  60  (see  FIGS.  65  to  67   ).  FIG.  67    may be a diagram illustrating connection of the first shaft  50   a  and the second shaft  50   b  between  FIG.  69    and  FIG.  70   . The shafts  50  may be two or more in number. When the number of the shafts  50  increases to n, the number of the joints  60  may increase accordingly to n−1. 
     The door  41  may be opened and closed in such a manner that the first shaft gear  45  moves the first slider  43  in the forward and backward direction of the housing  30 , and the second shaft gear  45 ′ moves the second slider  43 ′ in the forward and backward direction of the housing  30 . 
     Accordingly, the door  41  may be opened and closed as the shafts  50  rotate without being caught by the holder  60 H or causing noise and/or vibration. 
     Referring to  FIG.  71   , an example is illustrated in which the door  41  mounted on the upper plate of the housing  30  is opened by the rotation of the shaft  50 . Referring to  FIG.  72   , an example is illustrated in which a display unit  20 P moves upward through the opening  30 P of the upper plate which is opened by movement of the door  41 . 
     According to one aspect of the present disclosure, there is provided a display device including: a flexible display panel; a roller around which the display panel is rolled or unrolled; a housing having an opening through which the display panel passes, and providing an internal accommodation space, the roller rotatably mounted in the internal accommodation space; and a door assembly mounted in the internal accommodation space at a position adjacent to the opening of the housing, and opening and closing the opening, wherein the door assembly includes: a door for opening and closing the opening; a motor for providing power to the door; a first shaft gear disposed between the motor and the door, and transmitting the power, provided by the motor, to the door; a first shaft having a first end connected to the first shaft gear, and rotating together with the first shaft gear; a joint connected to a second end of the first shaft; a second shaft having a first end connected to the joint, and rotating together with the first shaft; and a second shaft gear connected to a second end of the second shaft, and transmitting power to the door. 
     In addition, according to another aspect of the present disclosure, the first shaft may include: an elongated first shaft body; a first gear coupling part formed at a first end of the first shaft body, the first gear coupling part having the first shaft gear fixed thereto, and a first part having a smaller diameter than a diameter of the first shaft body, the first part having an outer circumferential surface which is partially cut out; and a first joint hole disposed adjacent to a second end of the first shaft body, and formed by passing through the first shaft body in a diametral direction of the first shaft body. 
     In addition, according to another aspect of the present disclosure, the joint may include: a body which has an elongated cylindrical shape, and into which another side of the first shaft body is inserted; a first locking pin having an elongated cylindrical shape, and passing through the body in a diametral direction of the body at a position corresponding to the first joint hole; and a first press-fit pin inserted into the first locking pin and the first joint hole. 
     In addition, according to another aspect of the present disclosure, the first locking pin may further include slits formed in a longitudinal direction of the first locking pin, wherein a gap between the slits of the first locking pin increases when the first press-fit pin may be inserted. 
     In addition, according to another aspect of the present disclosure, the second shaft may include: an elongated second shaft body; a second gear coupling part formed at a second end of the second shaft body, the second gear coupling part having the second shaft gear fixed thereto, and a second part having a smaller diameter than a diameter of the second shaft body, the second part having an outer circumferential surface which is partially cut out; and a second joint hole disposed adjacent to a first end of the second shaft body, and formed by passing through the second shaft body in a diametral direction of the second shaft body. 
     In addition, according to another aspect of the present disclosure, the joint may include: a second locking pin having an elongated cylindrical shape and passing through the body of the joint in the diametral direction of the body at a position corresponding to the second joint hole of the second shaft inserted into the body of the joint; and a second press-fit pin inserted into the second locking pin and the second joint hole. 
     In addition, according to another aspect of the present disclosure, the second locking pin may further include slits formed in a longitudinal direction of the second locking pin, wherein a gap between the slits of the second locking pin increases when the second press-fit pin is inserted. 
     In addition, according to another aspect of the present disclosure, there is further included a holder which is fixed to an inside of the housing, and in which the joint is held, wherein the holder may include: a body; and hangers surrounding the joint so that the joint may be rotated in the body of the holder. 
     In addition, according to another aspect of the present disclosure, the hangers may be formed between the first locking pin and the second locking pin. 
     In addition, according to another aspect of the present disclosure, the door assembly may further include: a door holder fixed to a lower side of the door; a slider coupled to the door holder and moving in a forward and rearward direction of the housing; and a rack gear formed on the slider in the forward and rearward direction of the housing, wherein the first shaft gear may be engaged with the rack gear. 
     In addition, according to another aspect of the present disclosure, the door assembly may further include: a worm fixed to a rotating shaft of the motor; a worm gear engaged with the worm; and a torque limiter engaged with the first shaft gear and the work gear. 
     In addition, according to another aspect of the present disclosure, when the door opens an opening of the housing, the display panel may be unrolled from the roller by moving out of the housing through the opening, and when the display panel is rolled on the roller to move into the housing, the door may close the opening of the housing. 
     In addition, according to another aspect of the present disclosure, the roller and the housing may be elongated in a longitudinal direction of a bottom side of the display panel, wherein the first shaft and the second shaft may be elongated in the longitudinal direction of the housing and may be disposed in series, and the joint may be disposed between the first shaft and the second shaft. 
     In addition, according to another aspect of the present disclosure, the motor may be disposed adjacent to a left side or a right side of the display panel. 
     Certain embodiments or other embodiments of the invention described above are not mutually exclusive or distinct from each other. Any or all elements of the embodiments of the invention described above may be combined or combined with each other in configuration or function. 
     For example, a configuration “A” described in one embodiment of the invention and the drawings and a configuration “B” described in another embodiment of the invention and the drawings may be combined with each other. Namely, although the combination between the configurations is not directly described, the combination is possible except in the case where it is described that the combination is impossible. 
     Although embodiments have been described with reference to a number of illustrative embodiments thereof, it should be understood that numerous other modifications and embodiments can be devised by those skilled in the art that will fall within the scope of the principles of this disclosure. More particularly, various variations and modifications are possible in the component parts and/or arrangements of the subject combination arrangement within the scope of the disclosure, the drawings and the appended claims. In addition to variations and modifications in the component parts and/or arrangements, alternative uses will also be apparent to those skilled in the art.