Patent Publication Number: US-2023136116-A1

Title: Electronic device with hinge assembly

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of International Application No. PCT/KR2022/014059 designating the United States, filed on Sep. 21, 2022, in the Korean Intellectual Property Receiving Office and claiming priority to Korean Patent Application No. 10-2021-0150590, filed on Nov. 4, 2021, in the Korean Intellectual Property Office, the disclosures of which are incorporated by reference herein in their entireties. 
    
    
     BACKGROUND 
     1. Field 
     The disclosure relates to an electronic device including a hinge assembly. 
     2. Description of Related Art 
     Recently, with the development of display-related technologies, electronic devices with flexible displays are being developed. A flexible display may be used in the form of a flat surface, and may also be deformed to be used in a specific shape. For example, an electronic device with a flexible display may be implemented in a foldable form to be folded or unfolded about at least one folding axis. 
     SUMMARY 
     To implement a folding operation or unfolding operation of an electronic device, a hinge assembly may be provided between a first housing and a second housing. The hinge assembly may have a structure for generating a force to maintain a predetermined folding state of the electronic device. To implement such a structure, a cam structure and a spring may be used in the hinge assembly. A separate pin member may be used to rotatably connect some components to a bracket. However, if a separate pin member is used, the cost of the hinge assembly may increase due to an increase in the number of components, and errors may be accumulated, thereby reducing the quality of the hinge module. 
     According to embodiments, an electronic device including a hinge assembly that may rotatably connect some components (e.g., an intermediate member) to a bracket, instead of using a separate pin member may be provided. 
     According to embodiments, an electronic device including a hinge assembly with a decrease in a number of components and a reduced manufacturing cost and with an increased productivity and quality may be provided. 
     According to one embodiment, an electronic device includes a display including a first area, a second area, and a folding area between the first area and the second area, a first housing configured to support the first area, a second housing configured to support the second area, and a hinge assembly configured to connect the first housing and the second housing, and having a pair of hinge axes Ha and Hb. The hinge assembly includes a hinge bracket including an intermediate protrusion formed to protrude in a direction of a middle axis M perpendicular to the pair of hinge axes Ha and Hb, a pair of hinge structures connected to the hinge bracket to be rotatable about the pair of hinge axes Ha and Hb, and an intermediate member including a through-hole into which the intermediate protrusion is inserted, to be rotatable about the middle axis M with respect to the hinge bracket. The intermediate protrusion includes a protrusion base having a first radius R 1 , and a head including a projection which is formed on the protrusion base and which has a second radius R 2  greater than the first radius R 1 . The through-hole has a shape corresponding to a shape of the head. 
     According to various embodiments, the electronic device includes a display including a first area, a second area, and a folding area between the first area and the second area, a first housing configured to support the first area, a second housing configured to support the second area, a hinge assembly configured to connect the first housing and the second housing, and having a pair of hinge axes Ha and Hb. The hinge assembly may include a hinge bracket including an intermediate protrusion formed to protrude in a direction of a middle axis M perpendicular to the pair of hinge axes Ha and Hb, two pairs of hinge structures connected to the hinge bracket to be rotatable about the pair of hinge axes Ha and Hb, a pair of intermediate members connected to the intermediate protrusion and overlapping each other, to be rotatable about the middle axis M with respect to the hinge bracket, respectively, each of the pair of intermediate members and including a through-hole into which the intermediate protrusion is inserted. The intermediate protrusion may include a protrusion base having a first radius R 1 , and a head including a projection which is formed on the protrusion base and which has a second radius R 2  greater than the first radius R 1 . The through-hole or may have a shape corresponding to a shape of the head. 
     According to embodiments, some components (e.g., an intermediate member) may be rotatably connected to a bracket without a need to use a separate pin member. 
     According to embodiments, a number of components of a hinge assembly may be reduced, instead of using a separate pin member, and accordingly, a manufacturing cost and weight of the hinge assembly may be reduced. 
     According to embodiments, accumulated errors between components may be reduced by reducing the number of components of the hinge assembly, and thus it may be possible to increase a quality of the hinge assembly. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features, and advantages of certain embodiments of the present disclosure will be more apparent from the following detailed description, taken in conjunction with the accompanying drawings, in which: 
         FIG.  1    is a block diagram illustrating an electronic device in a network environment according to one embodiment; 
         FIG.  2 A  is a diagram illustrating an unfolded state of an electronic device according to one embodiment; 
         FIG.  2 B  is a diagram illustrating a folded state of an electronic device according to one embodiment; 
         FIG.  2 C  is a perspective view illustrating an example of a fully unfolded state or an intermediate state of the electronic device according to one embodiment; 
         FIG.  3    is a front view illustrating a state in which a hinge assembly is applied to an electronic device according to one embodiment; 
         FIG.  4 A  is a perspective view illustrating an unfolded state of a hinge assembly according to one embodiment; 
         FIG.  4 B  is a front view illustrating an unfolded state of a hinge assembly according to one embodiment; 
         FIG.  4 C  is a rear view illustrating an unfolded state of a hinge assembly according to one embodiment; 
         FIG.  4 D  is an exploded perspective view illustrating a hinge assembly according to one embodiment; 
         FIG.  4 E  is a perspective view illustrating a hinge bracket according to one embodiment; 
         FIG.  4 F  is a front view illustrating an intermediate protrusion according to one embodiment; 
         FIG.  4 G  is a perspective view illustrating a hinge structure according to one embodiment; 
         FIG.  4 H  is a perspective view illustrating an intermediate member according to one embodiment; 
         FIGS.  4 I,  4 J, and  4 K  illustrate a process in which an intermediate member is rotatably connected to a hinge bracket according to one embodiment; 
         FIG.  4 L  is a cross-sectional view taken along line A-A of  FIG.  4 K ; 
         FIG.  4 M  illustrates a force and torque acting on one hinge structure of  FIGS.  4 A through  4 C ; 
         FIG.  4 N  is a perspective view illustrating an intermediate state of a hinge assembly according to one embodiment; 
         FIG.  4 O  is a rear view illustrating an intermediate state of a hinge assembly according to one embodiment; 
         FIG.  4 P  illustrates a force acting on one hinge structure of  FIGS.  4 N and  4 O ; 
         FIG.  4 Q  is a perspective view illustrating a folded state of a hinge assembly according to one embodiment; 
         FIG.  4 R  is a rear view illustrating a folded state of a hinge assembly according to one embodiment; 
         FIG.  4 S  illustrates a force and torque acting on one hinge structure of  FIGS.  4 Q and  4 R ; 
         FIGS.  4 T,  4 U, and  4 V  are rear views schematically illustrating a hinge assembly according to one embodiment, and illustrate a process in which a balance between both sides of the hinge assembly is achieved in a situation in which one rotation member starts to rotate first; 
         FIGS.  4 W,  4 X, and  4 Y  are rear views schematically illustrating a hinge assembly according to one embodiment, and illustrate a process in which a balance between both sides of the hinge assembly is achieved in a situation in which one rotation member starts to rotate first; 
         FIG.  5    is a rear view schematically illustrating a hinge assembly according to one embodiment; 
         FIG.  6 A  is a perspective view illustrating an intermediate protrusion according to one embodiment; 
         FIG.  6 B  is a cross-sectional view taken along line B-B of  FIG.  6 A ; 
         FIG.  7 A  is a perspective view illustrating an unfolded state of a hinge assembly according to one embodiment; 
         FIG.  7 B  is a front view illustrating an unfolded state of a hinge assembly according to one embodiment; 
         FIG.  7 C  is a perspective view illustrating a folded state of a hinge assembly according to one embodiment; 
         FIG.  7 D  is an exploded perspective view illustrating a hinge assembly according to one embodiment; 
         FIG.  7 E  is a perspective view illustrating a hinge bracket according to one embodiment; 
         FIG.  7 F  is an exploded perspective view illustrating a hinge structure according to one embodiment; 
         FIGS.  7 G,  7 H, and  7 I  illustrate a state in which a rotation plate and a fixing plate are in surface contact with each other according to one embodiment; 
         FIG.  8    is a rear view schematically illustrating a hinge assembly according to one embodiment; 
         FIG.  9 A  is a front view illustrating a hinge assembly according to one embodiment; 
         FIG.  9 B  is a perspective view illustrating a process of connecting a first intermediate member to a hinge bracket according to one embodiment; 
         FIG.  9 C  is a perspective view illustrating a process of connecting a second intermediate member to a hinge bracket according to one embodiment; 
         FIG.  9 D  is a perspective view illustrating a state in which a first intermediate member and a second intermediate member are connected to a hinge bracket according to one embodiment; 
         FIG.  9 E  is a cross-sectional view taken along line C-C of  FIG.  9 A ; and 
         FIG.  9 F  is a cross-sectional view taken along line D-D of  FIG.  9 A . 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, embodiments will be described in detail with reference to the accompanying drawings. When describing the embodiments with reference to the accompanying drawings, like reference numerals refer to like elements and a repeated description related thereto will be omitted. 
       FIG.  1    is a block diagram illustrating an electronic device  101  in a network environment  100  according to one embodiment. 
     Referring to  FIG.  1   , the electronic device  101  in the network environment  100  may communicate with an electronic device  102  via a first network  198  (e.g., a short-range wireless communication network), or communicate with an electronic device  104  or a server  108  via a second network  199  (e.g., a long-range wireless communication network). According to one embodiment, the electronic device  101  may communicate with the electronic device  104  via the server  108 . According to one embodiment, the electronic device  101  may include a processor  120 , a memory  130 , an input module  150 , a sound output module  155 , a display module  160 , an audio module  170 , and a sensor module  176 , an interface  177 , a connecting terminal  178 , a haptic module  179 , a camera module  180 , a power management module  188 , a battery  189 , a communication module  190 , a subscriber identification module (SIM)  196 , or an antenna module  197 . In some embodiments, at least one (e.g., the connecting terminal  178 ) of the above components may be omitted from the electronic device  101 , or one or more other components may be added to the electronic device  101 . In some embodiments, some (e.g., the sensor module  176 , the camera module  180 , or the antenna module  197 ) of the components may be integrated as a single component (e.g., the display module  160 ). 
     The processor  120  may execute, for example, software (e.g., a program  140 ) to control at least one other component (e.g., a hardware or software component) of the electronic device  101  connected to the processor  120 , and may perform various data processing or computation. According to one embodiment, as at least a part of data processing or computation, the processor  120  may store a command or data received from another component (e.g., the sensor module  176  or the communication module  190 ) in a volatile memory  132 , process the command or the data stored in the volatile memory  132 , and store resulting data in a non-volatile memory  134 . According to one embodiment, the processor  120  may include a main processor  121  (e.g., a central processing unit (CPU) or an application processor (AP)) or an auxiliary processor  123  (e.g., a graphics processing unit (GPU), a neural processing unit (NPU), an image signal processor (ISP), a sensor hub processor, or a communication processor (CP)) that is operable independently of, or in conjunction with the main processor  121 . For example, when the electronic device  101  includes the main processor  121  and the auxiliary processor  123 , the auxiliary processor  123  may be adapted to consume less power than the main processor  121  or to be specific to a specified function. The auxiliary processor  123  may be implemented separately from the main processor  121  or as a part of the main processor  121 . 
     The auxiliary processor  123  may control at least some of functions or states related to at least one (e.g., the display module  160 , the sensor module  176 , or the communication module  190 ) of the components of the electronic device  101 , instead of the main processor  121  while the main processor  121  is in an inactive (e.g., sleep) state or along with the main processor  121  while the main processor  121  is an active state (e.g., executing an application). According to one embodiment, the auxiliary processor  123  (e.g., an ISP or a CP) may be implemented as a portion of another component (e.g., the camera module  180  or the communication module  190 ) that is functionally related to the auxiliary processor  123 . According to one embodiment, the auxiliary processor  123  (e.g., an NPU) may include a hardware structure specified for artificial intelligence (AI) model processing. An artificial intelligence model may be generated by machine learning. Such learning may be performed by, for example, the electronic device  101  in which artificial intelligence is performed, or performed via a separate server (e.g., the server  108 ). Learning algorithms may include, but are not limited to, for example, supervised learning, unsupervised learning, semi-supervised learning, or reinforcement learning. The AI model may include a plurality of artificial neural network layers. An artificial neural network may include, for example, a deep neural network (DNN), a convolutional neural network (CNN), a recurrent neural network (RNN), a restricted Boltzmann machine (RBM), a deep belief network (DBN), and a bidirectional recurrent deep neural network (BRDNN), a deep Q-network, or a combination of two or more thereof, but is not limited thereto. The AI model may additionally or alternatively include a software structure other than the hardware structure. 
     The memory  130  may store various pieces of data used by at least one component (e.g., the processor  120  or the sensor module  176 ) of the electronic device  101 . The various pieces of data may include, for example, software (e.g., the program  140 ) and input data or output data for a command related thereto. The memory  130  may include the volatile memory  132  or the non-volatile memory  134 . 
     The program  140  may be stored as software in the memory  130 , and may include, for example, an operating system (OS)  142 , middleware  144 , or an application  146 . 
     The input module  150  may receive a command or data to be used by another component (e.g., the processor  120 ) of the electronic device  101 , from the outside (e.g., a user) of the electronic device  101 . The input module  150  may include, for example, a microphone, a mouse, a keyboard, a key (e.g., a button), or a digital pen (e.g., a stylus pen). The sound output module  155  may output a sound signal to the outside of the electronic device  101 . The sound output module  155  may include, for example, a speaker or a receiver. The speaker may be used for general purposes, such as playing multimedia or playing record. The receiver may be used to receive an incoming call. According to one embodiment, the receiver may be implemented separately from the speaker or as a part of the speaker. 
     The display module  160  may visually provide information to the outside (e.g., a user) of the electronic device  101 . The display module  160  may include, for example, a control circuit for controlling a display, a hologram device, or a projector and control circuitry to control a corresponding one of the display, the hologram device, and the projector. According to one embodiment, the display module  160  may include a touch sensor adapted to sense a touch, or a pressure sensor adapted to measure an intensity of a force incurred by the touch. 
     The audio module  170  may convert a sound into an electric signal or vice versa. According to one embodiment, the audio module  170  may obtain the sound via the input module  150  or output the sound via the sound output module  155  or an external electronic device (e.g., an electronic device  102  such as a speaker or headphones) directly or wirelessly connected to the electronic device  101 . 
     The sensor module  176  may detect an operational state (e.g., power or temperature) of the electronic device  101  or an environmental state (e.g., a state of a user) external to the electronic device  101 , and generate an electric signal or data value corresponding to the detected state. According to one embodiment, the sensor module  176  may include, for example, a gesture sensor, a gyro sensor, an atmospheric pressure sensor, a magnetic sensor, an acceleration sensor, a grip sensor, a proximity sensor, a color sensor, an infrared (IR) sensor, a biometric sensor, a temperature sensor, a humidity sensor, or an illuminance sensor. 
     The interface  177  may support one or more specified protocols to be used for the electronic device  101  to be coupled with the external electronic device (e.g., the electronic device  102 ) directly (e.g., by wire) or wirelessly. According to one embodiment, the interface  177  may include, for example, a high-definition multimedia interface (HDMI), a universal serial bus (USB) interface, a secure digital (SD) card interface, or an audio interface. 
     The connecting terminal  178  may include a connector via which the electronic device  101  may be physically connected to an external electronic device (e.g., the electronic device  102 ). According to one embodiment, the connecting terminal  178  may include, for example, an HDMI connector, a USB connector, an SD card connector, or an audio connector (e.g., a headphone connector). 
     The haptic module  179  may convert an electric signal into a mechanical stimulus (e.g., a vibration or a movement) or an electrical stimulus which may be recognized by a user via his or her tactile sensation or kinesthetic sensation. According to one embodiment, the haptic module  179  may include, for example, a motor, a piezoelectric element, or an electric stimulator. 
     The camera module  180  may capture a still image and moving images. According to one embodiment, the camera module  180  may include one or more lenses, image sensors, ISPs, or flashes. 
     The power management module  188  may manage power supplied to the electronic device  101 . According to one embodiment, the power management module  188  may be implemented as, for example, at least a part of a power management integrated circuit (PMIC). 
     The battery  189  may supply power to at least one component of the electronic device  101 . According to one embodiment, the battery  189  may include, for example, a primary cell which is not rechargeable, a secondary cell which is rechargeable, or a fuel cell. 
     The communication module  190  may support establishing a direct (e.g., wired) communication channel or a wireless communication channel between the electronic device  101  and the external electronic device (e.g., the electronic device  102 , the electronic device  104 , or the server  108 ) and performing communication via the established communication channel. The communication module  190  may include one or more communication processors that are operable independently of the processor  120  (e.g., an AP) and that support a direct (e.g., wired) communication or a wireless communication. According to one embodiment, the communication module  190  may include a wireless communication module  192  (e.g., a cellular communication module, a short-range wireless communication module, or a global navigation satellite system (GNSS) communication module) or a wired communication module  194  (e.g., a local area network (LAN) communication module, or a power line communication (PLC) module). A corresponding one of these communication modules may communicate with the external electronic device  104  via the first network  198  (e.g., a short-range communication network, such as Bluetooth™, wireless-fidelity (Wi-Fi) direct, or infrared data association (IrDA)) or the second network  199  (e.g., a long-range communication network, such as a legacy cellular network, a 5G network, a next-generation communication network, the Internet, or a computer network (e.g., a LAN or a wide area network (WAN))). These various types of communication modules may be implemented as a single component (e.g., a single chip), or may be implemented as multi components (e.g., multi chips) separate from each other. The wireless communication module  192  may identify and authenticate the electronic device  101  in a communication network, such as the first network  198  or the second network  199 , using subscriber information (e.g., international mobile subscriber identity (IMSI)) stored in the SIM  196 . 
     The wireless communication module  192  may support a 5G network after a 4G network, and next-generation communication technology, e.g., new radio (NR) access technology. The NR access technology may support enhanced mobile broadband (eMBB), massive machine type communications (mMTC), or ultra-reliable and low-latency communications (URLLC). The wireless communication module  192  may support a high-frequency band (e.g., a mmWave band) to achieve, e.g., a high data transmission rate. The wireless communication module  192  may support various technologies for securing performance on a high-frequency band, such as, e.g., beamforming, massive multiple-input and multiple-output (MIMO), full dimensional MIMO (FD-MIMO), an array antenna, analog beam-forming, or a large scale antenna. The wireless communication module  192  may support various requirements specified in the electronic device  101 , an external electronic device (e.g., the electronic device  104 ), or a network system (e.g., the second network  199 ). According to one embodiment, the wireless communication module  192  may support a peak data rate (e.g., 20 Gbps or more) for implementing eMBB, loss coverage (e.g., 164 dB or less) for implementing mMTC, or U-plane latency (e.g., 0.5 ms or less for each of downlink (DL) and uplink (UL), or a round trip of 1 ms or less) for implementing URLLC. 
     The antenna module  197  may transmit or receive a signal or power to or from the outside (e.g., the external electronic device) of the electronic device  101 . According to one embodiment, the antenna module  197  may include an antenna including a radiating element including a conductive material or a conductive pattern formed in or on a substrate (e.g., a printed circuit board (PCB)). According to one embodiment, the antenna module  197  may include a plurality of antennas (e.g., array antennas). In such a case, at least one antenna appropriate for a communication scheme used in a communication network, such as the first network  198  or the second network  199 , may be selected by, for example, the communication module  190  from the plurality of antennas. The signal or the power may be transmitted or received between the communication module  190  and the external electronic device via the at least one selected antenna. According to one embodiment, another component (e.g., a radio frequency integrated circuit (RFIC)) other than the radiating element may be additionally formed as a part of the antenna module  197 . 
     According to one embodiment, the antenna module  197  may form a mmWave antenna module. According to one embodiment, the mmWave antenna module may include a PCB, an RFIC disposed on a first surface (e.g., a bottom surface) of the PCB or adjacent to the first surface and capable of supporting a designated a high-frequency band (e.g., the mmWave band), and a plurality of antennas (e.g., array antennas) disposed on a second surface (e.g., a top or a side surface) of the PCB, or adjacent to the second surface and capable of transmitting or receiving signals in the designated high-frequency band. 
     At least some of the above-described components may be coupled mutually and communicate signals (e.g., commands or data) therebetween via an inter-peripheral communication scheme (e.g., a bus, general purpose input and output (GPIO), serial peripheral interface (SPI), or mobile industry processor interface (MIPI)). 
     According to one embodiment, commands or data may be transmitted or received between the electronic device  101  and the external electronic device  104  via the server  108  coupled with the second network  199 . Each of the external electronic devices  102  and  104  may be a device of the same type as or a different type from the electronic device  101 . According to one embodiment, all or some of operations to be executed by the electronic device  101  may be executed at one or more external electronic devices (e.g., the external electronic devices  102  and  104 , and the server  108 ). For example, if the electronic device  101  needs to perform a function or a service automatically, or in response to a request from a user or another device, the electronic device  101 , instead of, or in addition to, executing the function or the service, may request one or more external electronic devices to perform at least part of the function or the service. The one or more external electronic devices receiving the request may perform the at least part of the function or the service requested, or an additional function or an additional service related to the request, and may transfer an outcome of the performing to the electronic device  101 . The electronic device  101  may provide the outcome, with or without further processing of the outcome, as at least part of a reply to the request. To that end, a cloud computing, distributed computing, mobile edge computing (MEC), or client-server computing technology may be used, for example. The electronic device  101  may provide ultra low-latency services using, e.g., distributed computing or mobile edge computing. In another embodiment, the external electronic device  104  may include an Internet-of-things (IoT) device. The server  108  may be an intelligent server using machine learning and/or a neural network. According to one embodiment, the external electronic device  104  or the server  108  may be included in the second network  199 . The electronic device  101  may be applied to intelligent services (e.g., smart home, smart city, smart car, or healthcare) based on 5G communication technology or IoT-related technology. 
       FIG.  2 A  is a diagram illustrating an unfolded state of an electronic device  200  according to one embodiment.  FIG.  2 B  is a diagram illustrating a folded state of the electronic device  200  according to one embodiment.  FIG.  2 C  is a perspective view illustrating an example of a fully unfolded state or an intermediate state of the electronic device  200  according to one embodiment. 
     The electronic device  200  of  FIGS.  2 A through  2 C  is an example of the electronic device  101  of  FIG.  1   , and may be a foldable or bendable electronic device. 
     In  FIG.  2 C  and other following drawings, illustrated is a spatial coordinate system defined by an X-axis, a Y-axis, and a Z-axis that are orthogonal to each other. Here, the X-axis may represent a width direction of an electronic device, the Y-axis may represent a length direction of the electronic device, and the Z-axis may represent a height (or thickness) direction of the electronic device. In the following description, a “first direction” may refer to a direction parallel to the Z-axis. 
     Referring to  FIGS.  2 A and  2 B , in one embodiment, the electronic device  200  may include a foldable housing  201 , and a flexible or foldable display  250  (hereinafter, the “display”  250  in short) (e.g., the display module  160  of  FIG.  1   ) disposed in a space formed by the foldable housing  201 . A surface on which the display  250  is disposed (or a surface on which the display  250  is viewed from outside of the electronic device  200 ) may be defined as a front surface of the electronic device  200 . In addition, a surface opposite to the front surface may be defined as a rear surface of the electronic device  200 . In addition, a surface surrounding a space between the front surface and the rear surface may be defined as a side surface of the electronic device  200 . 
     According to one embodiment, the foldable housing  201  may include a first housing structure  210 , a second housing structure  220  including a sensor area  222 , a first rear cover  215 , a second rear cover  225 , and a hinge structure  230 . Here, the hinge structure  230  may include a hinge cover that covers a foldable portion of the foldable housing  201 . The foldable housing  201  of the electronic device  200  is not limited to the shape and combination shown in  FIGS.  2 A and  2 B , and may be implemented in a different shape or a different combination of components. For example, in one embodiment, the first housing structure  210  and the first rear cover  215  may be integrally formed, and the second housing structure  220  and the second rear cover  225  may be integrally formed. 
     According to one embodiment, the first housing structure  210  may be connected to the hinge structure  230 , and may include a first surface facing a first direction, and a second surface facing a second direction opposite to the first direction. The second housing structure  220  may be connected to the hinge structure  230 , and may include a third surface facing a third direction and a fourth surface facing a fourth direction opposite to the third direction. The second housing structure  220  may rotate with respect to the first housing structure  210  about the hinge structure  230 . A state of the electronic device  200  may be changed to a folded state or an unfolded state. 
     According to one embodiment, the first surface may face the third surface in a state in which the electronic device  200  is fully folded, and the third direction may be identical to the first direction in a state in which the electronic device  200  is fully unfolded. 
     According to one embodiment, the first housing structure  210  and the second housing structure  220  may be disposed on both sides with respect to a folding axis A and generally may be symmetrical with respect to the folding axis A. As to be described hereinafter, an angle or distance between the first housing structure  210  and the second housing structure  220  may vary depending on whether the state of the electronic device  200  is the unfolded state, the folded state, or an intermediate state (e.g., a partially folded state or a partially unfolded state). According to one embodiment, unlike the first housing structure  210 , the second housing structure  220  may additionally include the sensor area  222 , in which various sensors are arranged. However, the first housing structure  210  and the second housing structure  220  may have mutually symmetrical shapes in areas other than the sensor area  222 . In one embodiment, the sensor area  222  may be additionally disposed in or replaced with at least a partial area of the second housing structure  220 . The sensor area  222  may include, for example, a camera hole area, a sensor hole area, an under-display camera (UDC) area, and/or an under-display sensor (UDS) area. 
     According to one embodiment, as shown in  FIG.  2 A , the first housing structure  210  and the second housing structure  220  may together form a recess for accommodating the display  250 . In one embodiment, due to the sensor area  222 , the recess may have at least two different widths in a direction perpendicular to the folding axis A. For example, the recess may have a first width w 1  between a first portion  210   a  of the first housing structure  210  parallel to the folding axis A and a first portion  220   a  of the second housing structure  220  formed on a periphery of the sensor area  222 , and a second width w 2  formed by a second portion  210   b  of the first housing structure  210  and a second portion  220   b  of the second housing structure  220  not corresponding to the sensor area  222  and being parallel to the folding axis A. In this case, the second width w 2  may be greater than the first width w 1 . In one embodiment, the first portion  220   a  and the second portion  220   b  of the second housing structure  220  may be at different distances from the folding axis A. The widths of the recess are not limited to the shown example. In one embodiment, the recess may have a plurality of widths due to the shape of the sensor area  222  or asymmetrical portions of the first housing structure  210  and the second housing structure  220 . According to one embodiment, the sensor area  222  may be formed to have a predetermined area adjacent to one corner of the second housing structure  220 . However, the arrangement, shape, and size of the sensor area  222  are not limited to the shown example. For example, in one embodiment, the sensor area  222  may be provided at another corner of the second housing structure  220  or in a predetermined area between an upper corner and a lower corner. In one embodiment, components embedded in the electronic device  200  to perform various functions may be exposed to the front surface of the electronic device  200  through the sensor area  222  or through one or more openings provided in the sensor area  222 . In one embodiment, the components may include various types of sensors. The sensors may include, for example, at least one of a front camera, a receiver, or a proximity sensor. According to one embodiment, the sensor area  222  may not be included in the second housing structure  220  or may be formed at a position different from that shown in the drawings. 
     According to one embodiment, at least a portion of the first housing structure  210  and the second housing structure  220  may be formed of a metal material or a non-metal material having a selected magnitude of rigidity to support the display  250 . At least a portion of the first housing structure  210  and the second housing structure  220  formed of the metal material may provide a ground plane for the electronic device  200 , and may be electrically connected to a ground line formed on a PCB disposed in the foldable housing  201 . 
     According to one embodiment, the first rear cover  215  may be disposed on one side of the folding axis A on the rear surface of the electronic device  200 , and may have, for example, a substantially rectangular periphery that may be enclosed by the first housing structure  210 . Similarly, the second rear cover  225  may be disposed on another side of the folding axis A on the rear surface of the electronic device  200 , and may have a periphery that may be enclosed by the second housing structure  220 . 
     According to one embodiment, the first rear cover  215  and the second rear cover  225  may be substantially symmetrical with respect to the folding axis A. However, the first rear cover  215  and the second rear cover  225  are not necessarily mutually symmetrical. For example, the first rear cover  215  and the second rear cover  225  in the electronic device  200  may have various shapes. In one embodiment, the first rear cover  215  may be formed integrally with the first housing structure  210 , and the second rear cover  225  may be formed integrally with the second housing structure  220 . 
     According to one embodiment, the first rear cover  215 , the second rear cover  225 , the first housing structure  210 , and the second housing structure  220  may form a space in which various components (e.g., a PCB, or a battery) of the electronic device  200  are to be arranged. In one embodiment, one or more components may be disposed or visually exposed on the rear surface of the electronic device  200 . For example, at least a portion of a sub-display may be visually exposed through a first rear area  216  of the first rear cover  215 . In one embodiment, one or more components or sensors may be visually exposed through a second rear area  226  of the second rear cover  225 . In one embodiment, the sensors may include a proximity sensor and/or a rear camera. 
     According to one embodiment, a front camera exposed to the front surface of the electronic device  200  through one or more openings provided in the sensor area  222 , or a rear camera exposed through the second rear area  226  of the second rear cover  225  may include one or more lenses, an image sensor, and/or an ISP. A flash may include, for example, a light emitting diode (LED) or a xenon lamp. In some embodiments, two or more lenses (e.g., infrared camera, wide-angle, and telephoto lenses) and image sensors may be arranged on one surface of the electronic device  200 . 
     Referring to  FIG.  2 B , the hinge cover may be disposed between the first housing structure  210  and the second housing structure  220  to cover internal components (e.g., the hinge structure  230 ). According to one embodiment, the hinge structure  230  may be covered by a portion of the first housing structure  210  and a portion of the second housing structure  220 , or may be exposed to the outside, depending on the state (e.g., the unfolded state, the intermediate state, or the folded state) of the electronic device  200 . 
     In an example, when the electronic device  200  is in the unfolded state (e.g., a fully unfolded state) as illustrated in  FIG.  2 A , the hinge structure  230  may be covered by the first housing structure  210  and the second housing structure  220  not to be exposed. In another example, when the electronic device  200  is in the folded state (e.g., a fully folded state), as shown in  FIG.  2 B , the hinge structure  230  may be exposed to the outside between the first housing structure  210  and the second housing structure  220 . In another example, when the first housing structure  210  and the second housing structure  220  are in an intermediate state of being folded with a predetermined angle, at least a portion of the hinge cover  230  may be exposed to the outside between the first housing structure  210  and the second housing structure  220 . However, the area exposed in this example may be smaller than that in the fully folded state. In one embodiment, the hinge cover  230  may have a curved surface. 
     According to one embodiment, the display  250  may be disposed in a space formed by the foldable housing  201 . For example, the display  250  may be seated on the recess formed by the foldable housing  201  and may be viewed from the outside through the front surface of the electronic device  200 . For example, the display  250  may constitute most of the front surface of the electronic device  200 . Accordingly, the front surface of the electronic device  200  may include the display  250 , and a partial area of the first housing structure  210  and a partial area of the second housing structure  220 , which are adjacent to the display  250 . In addition, the rear surface of the electronic device  200  may include the first rear cover  215 , a partial area of the first housing structure  210  adjacent to the first rear cover  215 , the second rear cover  225 , and a partial area of the second housing structure  220  adjacent to the second rear cover  225 . 
     According to one embodiment, the display  250  may refer to a display having at least a partial area that is deformable into a flat surface or a curved surface. In one embodiment, the display  250  may include a folding area  253 , a first area  251  disposed on one side of the folding area  253  (e.g., on the left side of the folding area  253  shown in  FIG.  2 A ), and a second area  252  disposed on the other side of the folding area  253  (e.g., on the right side of the folding area  253  shown in  FIG.  2 A ). 
     However, such an area division of the display  250  shown in  FIG.  2 A  is merely an example, and the display  250  may be divided into a plurality of areas (e.g., four or more areas, or two areas) depending on a structure or functions thereof. In an example, as shown in  FIG.  2 A , the display  250  may be divided into areas based on the folding area  203  extending in parallel to the folding axis A. In another example, the display  250  may be divided into areas based on another folding axis (e.g., a folding axis parallel to a width direction of an electronic device). 
     According to one embodiment, the display  250  may be coupled to or disposed adjacent to a touch panel including a touch sensing circuit and a pressure sensor for measuring a strength (a pressure) of a touch. For example, the display  250  may be coupled to or disposed adjacent to a touch panel for detecting a stylus pen of an electromagnetic resonance (EMR) type, as an example of the touch panel. 
     According to one embodiment, the first area  251  and the second area  252  may have globally symmetrical shapes around the folding area  253 . However, unlike the first area  251 , the second area  252  may include a notch that is cut depending on a presence of the sensor area  222 , but may have a shape symmetrical to the first area  251  in the other areas. For example, the first area  251  and the second area  252  may include portions having mutually symmetrical shapes and portions having mutually asymmetrical shapes. 
     According to one embodiment, an edge thickness of each of the first area  251  and the second area  252  may be different from an edge thickness of the folding area  253 . The edge thickness of the folding area  253  may be less than those of the first area  251  and the second area  252 . For example, the first area  251  and the second area  252  may be asymmetrical in terms of thickness when viewed in a cross section thereof. For example, an edge of the first area  251  may be formed to have a first radius of curvature, and an edge of the second area  252  may be formed to have a second radius of curvature different from the first radius of curvature. In another example, the first area  251  and the second area  252  may be symmetrical in terms of thickness when viewed in the cross section thereof. 
     Hereinafter, each area of the display  250 , and operations of the first housing structure  210  and the second housing structure  220  depending on the state (e.g., the folded state, the unfolded state, or the intermediate state) of the electronic device  200  will be described. 
     According to one embodiment, when the electronic device  200  is in the unfolded state (e.g.,  FIG.  2 A ), the first housing structure  210  and the second housing structure  220  may be arranged to face the same direction while forming an angle of 180 degrees. The surface of the first area  251  of the display  250  and the surface of the second area  252  thereof may face the same direction (e.g., a front direction of an electronic device) while forming 180 degrees. The folding area  253  may form the same plane in conjunction with the first area  251  and the second area  252 . 
     According to embodiments, when the electronic device  200  is in the folded state (e.g.,  FIG.  2 B ), the first housing structure  210  and the second housing structure  220  may be arranged to face each other. The surface of the first area  251  and the surface of the second area  252  of the display  250  may face each other, forming a narrow angle (e.g., between 0 degrees to 10 degrees). At least a portion of the folding area  253  may form a curved surface having a predetermined curvature. 
     According to one embodiment, when the electronic device  200  is in the intermediate state, the first housing structure  210  and the second housing structure  220  may be arranged to form a predetermined angle therebetween. The surface of the first area  251  and the surface of the second area  252  of the display  250  may form an angle greater than that in the folded state and smaller than that in the unfolded state. At least a portion of the folding area  253  may include a curved surface having a predetermined curvature, and the curvature may be smaller than that in the folded state. 
     An upper part of  FIG.  2 C  illustrates a state in which the electronic device  200  is fully unfolded, and a lower part of  FIG.  2 C  illustrates an intermediate state in which the electronic device  200  is partially unfolded. As described above, the state of the electronic device  200  may be changed to the folded state or the unfolded state. According to one embodiment, when viewed in a direction of a folding axis (e.g., the folding axis A of  FIG.  2 A ), the electronic device  200  may be folded in two types, i.e., an “in-folding” type in which the front surface of the electronic device  200  is folded to form an acute angle, and an “out-folding” type in which the front surface of the electronic device  200  is folded to form an obtuse angle. In an example, in the state in which the electronic device  200  is folded in the in-folding type, the first surface of the first housing structure  210  may face the third surface of the second housing structure  220 . In the fully unfolded state, the first surface of the first housing structure  210  and the third surface of the second housing structure  220  may face the same direction (e.g., a direction parallel to the z-axis). 
     In another example, when the electronic device  200  is folded in the out-folding type, the second surface of the first housing structure  210  may face the fourth surface of the second housing structure  220 . 
     In addition, although not shown in the drawings, the electronic device  200  may include a plurality of hinge axes (e.g., two parallel hinge axes including the folding axis A of  FIG.  2 A  and another axis parallel to the folding axis A). In this example, the electronic device  200  may also be folded in a “multi-folding” type in which the in-folding type and the out-folding type are combined. Also, although not shown in the drawings, a hinge axis may be formed in a vertical direction or a horizontal direction when the electronic device  200  is viewed from above. In an example, all the plurality of hinge axes may be arranged in the same direction. In another example, some of the plurality of hinge axes may be arranged in different directions and folded. 
     The in-folding type may refer to a state in which the display  250  is not exposed to the outside in the fully folded state. The out-folding type may refer to a state in which the display  250  is exposed to the outside in the fully folded state. The lower part of  FIG.  2 C  shows the intermediate state in which the electronic device  200  is partially unfolded in an in-folding process. 
     Although the state in which the electronic device  200  is folded in the in-folding type will be described below for convenience&#39;s sake, it should be noted that the description may be similarly applied in the state in which the electronic device  200  is folded in the out-folding type. 
     The electronic device according to one embodiment may be one of various types of electronic devices. The electronic device may include, for example, a portable communication device (e.g., a smartphone), a computer device, a portable multimedia device, a portable medical device, a camera, a wearable device, or a home appliance device. According to one embodiment of the disclosure, the electronic device is not limited to those described above. 
     It should be appreciated that various embodiments of the present disclosure and the terms used therein are not intended to limit the technological features set forth herein to particular embodiments and include various changes, equivalents, or replacements for a corresponding embodiment. In connection with the description of the drawings, like reference numerals may be used for similar or related components. It is to be understood that a singular form of a noun corresponding to an item may include one or more of the things, unless the relevant context clearly indicates otherwise. As used herein, “A or B,” “at least one of A and B,” “at least one of A or B,” “A, B or C,” “at least one of A, B and C,” and “at least one of A, B, or C,” each of which may include any one of the items listed together in the corresponding one of the phrases, or all possible combinations thereof. Terms such as “1st” and “2nd,” or “first” and “second” may be used to simply distinguish a corresponding component from other components, and do not limit the components in other aspects (e.g., importance or order). It is to be understood that if an element (e.g., a first element) is referred to, with or without the term “operatively” or “communicatively”, as “coupled with”, “coupled to”, “connected with”, or “connected to” another element (e.g., a second element), it means that the element may be coupled with the other element directly (e.g., wiredly), wirelessly, or via a third element. 
     As used in connection with various embodiments of the disclosure, the term “module” may include a unit implemented in hardware, software, or firmware, and may interchangeably be used with other terms, for example, “logic,” “logic block,” “part,” or “circuitry.” A module may be a single integral component, or a minimum unit or part thereof, adapted to perform one or more functions. For example, according to one embodiment, the module may be implemented in a form of an application-specific integrated circuit (ASIC). 
     Various embodiments of the present disclosure as set forth herein may be implemented as software (e.g., the program  140 ) including one or more instructions that are stored in a storage medium (e.g., an internal memory  136  or an external memory  138 ) that is readable by a machine (e.g., the electronic device  101 ). For example, a processor (e.g., the processor  120 ) of the machine (e.g., the electronic device  101 ) may invoke at least one of the one or more instructions stored in the storage medium, and execute it. This allows the machine to be operated to perform at least one function according to the at least one instruction invoked. The one or more instructions may include a code generated by a compiler or a code executable by an interpreter. The machine-readable storage medium may be provided in the form of a non-transitory storage medium. Here, the term “non-transitory” simply means that the storage medium is a tangible device, and does not include a signal (e.g., an electromagnetic wave), but this term does not differentiate between where data is semi-permanently stored in the storage medium and where the data is temporarily stored in the storage medium. 
     According to one embodiment, a method according to one embodiment of the disclosure may be included and provided in a computer program product. The computer program product may be traded as a product between a seller and a buyer. The computer program product may be distributed in the form of a machine-readable storage medium (e.g., a compact disc read-only memory (CD-ROM)), or be distributed (e.g., downloaded or uploaded) online via an application store (e.g., PlayStore™), or between two user devices (e.g., smartphones) directly. If distributed online, at least part of the computer program product may be temporarily generated or at least temporarily stored in the machine-readable storage medium, such as memory of the manufacturer&#39;s server, a server of the application store, or a relay server. 
     According to various embodiments, each component (e.g., a module or a program) of the above-described components may include a single entity or multiple entities, and some of the multiple entities may be separately disposed in different components. According to various embodiments, one or more of the above-described components may be omitted, or one or more other components may be added. Alternatively or additionally, a plurality of components (e.g., modules or programs) may be integrated into a single component. In such a case, the integrated component may still perform one or more functions of each of the plurality of components in the same or similar manner as they are performed by a corresponding one of the plurality of components before the integration. According to embodiments, operations performed by the module, the program, or another component may be carried out sequentially, in parallel, repeatedly, or heuristically, or one or more of the operations may be executed in a different order or omitted, or one or more other operations may be added. 
       FIG.  3    is a front view illustrating a state in which a hinge assembly is applied to an electronic device according to one embodiment. 
     Referring to  FIG.  3   , an electronic device  300  (e.g., the electronic device  101  of  FIG.  1    or the electronic device  200  of  FIGS.  2 A through  2 C ) according to one embodiment may be a foldable electronic device. For example, the electronic device  300  may be folded or unfolded about a folding axis A. However, this is merely an example, and a size, shape, structure, and folding axis of the electronic device  300  are not limited thereto. For example, the electronic device  300  of  FIG.  3    may include the folding axis A in a Y-axis direction, which is a long side direction, however, an electronic device according to one embodiment may also include a folding axis in an X-axis direction, which is a short side direction. 
     The electronic device  300  according to one embodiment may include a housing  310  (e.g., the foldable housing  201  of  FIGS.  2 A through  2 C ), a display (not shown) (e.g., the display module  160  of  FIG.  1   , or the display  250  of  FIGS.  2 A through  2 C ), a hinge assembly  400 , and a synchronization, or sync, assembly  320 . 
     In one embodiment, the housing  310  may form at least a portion of an exterior of the electronic device  300 . The housing  310  may include a first housing  311  (e.g., the first housing structure  210  of  FIGS.  2 A through  2 C ), a second housing  312  (e.g., the second housing structure  220  of  FIGS.  2 A through  2 C ), and a hinge housing  313 . 
     In one embodiment, the first housing  311  and the second housing  312  may be foldably connected to each other by the hinge assembly  400 . An angle or distance between the first housing  311  and the second housing  312  may vary depending on whether the electronic device  300  is in a flat state or unfolded state, a folded state, or an intermediate state. The hinge housing  313  may be disposed between the first housing  311  and the second housing  312  to provide a space for mounting internal components (e.g., the hinge assembly  400  and/or the sync assembly  320 ). For example, the hinge housing  313  may be configured to cover the hinge assembly  400  and/or the sync assembly  320  so that the hinge assembly  400  and/or the sync assembly  320  may not be exposed to the outside. 
     In one embodiment, the first housing  311  and the second housing  312  may provide a space in which the display  250  is disposed. The display  250  may be, for example, a foldable flexible display. For example, the display  250  may include a first area (e.g., the first area  251  of  FIG.  2 C ), a second area (e.g., the second area  252  of  FIG.  2 C ), and a folding area (e.g., the folding area  253  of  FIG.  2 C ) between the first area and the second area. The first housing  311  may be disposed at a position corresponding to the first area  251  of the display  250  to support the first area  251  of the display  250 . The second housing  312  may be disposed at a position corresponding to the second area  252  of the display  250  to support the second area  252  of the display  250 . 
     In one embodiment, the hinge assembly  400  may be disposed between the first housing  311  and the second housing  312  to connect the first housing  311  and the second housing  312 . For example, the hinge structure  230  of  FIG.  2 B  may include a plurality of hinge assemblies  400 . The plurality of hinge assemblies  400  may be spaced apart along the folding axis A. For example, as shown in  FIG.  3   , four hinge assemblies  400  may be spaced apart along the folding axis A. However, this is merely an example, and a number of hinge assemblies  400  is not limited thereto. The hinge assembly  400  may implement folding or unfolding operations of the electronic device  300 . The hinge assembly  400  may operate between a folded state in which the first area  251  and the second area  252  face each other and an unfolded state in which the first area  251  and the second area  252  do not face each other. The hinge assembly  400  may generate a force to maintain a predetermined folded state of the electronic device  300 . For example, when the electronic device  300  is in the folded state, the hinge assembly  400  may generate a force to maintain the folded state of the electronic device  300 . When the electronic device  300  is in the unfolded state, the hinge assembly  400  may generate a force to maintain the unfolded state of the electronic device  300 . When the electronic device  300  is in the intermediate state, the hinge assembly  400  may generate a force to maintain the intermediate state of the electronic device  300 . The hinge assembly  400  will be further described below. 
     In one embodiment, the sync assembly  320  may be disposed between the first housing  311  and the second housing  312  and may synchronize folding angles between the first housing  311  and the second housing  312 . For example, the sync assembly  320  may include a bracket  321 , a slider  322 , and a pair of rotators  323   a  and  323   b . The bracket  321  may be disposed between the first housing  311  and the second housing  312 . For example, the bracket  321  may be fixedly connected to the hinge housing  313 . The slider  322  may be connected to the bracket  321  to be movable in the direction of the folding axis A with respect to the bracket  321 . One side of each of the pair of rotators  323   a  and  323   b  may be connected to the first housing  311  or the second housing  312 , and the other side may be connected to the slider  322 . When the first housing  311  or the second housing  312  is folded about the folding axis A, the folding angles between the first housing  311  and the second housing  312  may be synchronized by a helical rotation of the pair of rotators  323   a  and  323   b  and a movement of the slider  322  in the direction of the folding axis A. However, this is merely an example, and the structure of the sync assembly  320  is not limited thereto. For example, the sync assembly  320  may also synchronize the folding angles between the first housing  311  and the second housing  312  through a gear structure. 
       FIG.  4 A  is a perspective view illustrating an unfolded state of a hinge assembly according to one embodiment.  FIG.  4 B  is a front view illustrating the unfolded state of the hinge assembly according to one embodiment.  FIG.  4 C  is a rear view illustrating the unfolded state of the hinge assembly according to one embodiment.  FIG.  4 D  is an exploded perspective view illustrating the hinge assembly according to one embodiment.  FIG.  4 E  is a perspective view illustrating a hinge bracket according to one embodiment.  FIG.  4 F  is a front view illustrating an intermediate protrusion according to one embodiment.  FIG.  4 G  is a perspective view illustrating a hinge structure according to one embodiment.  FIG.  4 H  is a perspective view illustrating an intermediate member according to one embodiment. 
     Referring to  FIGS.  4 A through  4 H , a hinge assembly  400  according to one embodiment may include a hinge bracket  410 , a pair of hinge structures  420   a  and  420   b , an intermediate member  430 , and a pair of elastic members  450   a  and  450   b.    
     The hinge bracket  410  according to one embodiment may be fixedly connected to a housing (e.g., the housing  310  of  FIG.  3   ). For example, the hinge bracket  410  may be fixedly connected to a hinge housing (e.g., the hinge housing  313  of  FIG.  3   ). At least a portion of a lower surface (e.g., a surface facing the −z-axis direction) of the hinge bracket  410  may have a curved surface. For example, the lower surface of the hinge bracket  410  may be formed to correspond to the shape of the inside of the hinge housing  313 . At least a portion of an upper surface (e.g., a surface facing the +z-axis direction) of the hinge bracket  410  may have a flat surface. 
     In one embodiment, the hinge bracket  410  may include a pair of first rail structures  411   a  and  411   b , an intermediate member arrangement space  412 , a pair of open spaces  413   a  and  413   b , a pair of bracket fixing holes  414   a  and  414   b , and an intermediate protrusion  415 . 
     In one embodiment, the hinge bracket  410  may include the pair of first rail structures  411   a  and  411   b  such that the pair of hinge structures  420   a  and  420   b  may be rotatably coupled. At least a portion of a cross section of the pair of first rail structures  411   a  and  411   b  may have an arc shape in a direction from the upper surface (e.g., the surface facing the +z-axis direction) to the lower surface (e.g., the surface facing the −z-axis direction). The pair of first rail structures  411   a  and  411   b  may be formed to protrude in an arc shape with a predetermined angle. For example, portions at both sides (e.g., a +y side and a −y side) of the first rail structure  411   a  or  411   b  may be relatively recessed such that a cross section of the first rail structure  411   a  or  411   b  facing an x-z plane may protrude in an arc shape. The hinge structure  420   a  or  420   b  may be connected to the hinge bracket  410  in a direction (e.g., the +x-axis direction or the −x-axis direction) perpendicular to a folding axis (e.g., the folding axis A of  FIG.  3   ). For example, a second rail structure  423  formed in the hinge structure  420   a  or  420   b  may be inserted into the first rail structure  411   a  or  411   b  in the +x-axis direction or the −x-axis direction, so that the hinge structure  420   a  or  420   b  may be connected to the hinge bracket  410 . The second rail structure  423  of each of the hinge structures  420   a  and  420   b  may be inserted into the pair of first rail structures  411   a  and  411   b  to interoperate with the pair of first rail structures  411   a  and  411   b . The pair of first rail structures  411   a  and  411   b  may be formed to diagonally face each other. For example, the pair of first rail structures  411   a  and  411   b  may be formed to be point-symmetric with respect to the center of the hinge bracket  410 , when the hinge bracket  410  is viewed from the front side. For example, one first rail structure  411   a  may be formed at a position relatively biased in the −x-axis direction and/or the +y-axis direction in comparison to the other first rail structure  411   b , and the other first rail structure  411   b  may be formed at a position relatively biased in the +x-axis direction and/or the −y-axis direction in comparison to the first rail structure  411   a . The arc shapes of the pair of first rail structures  411   a  and  411   b  may define a pair of hinge axes Ha and Hb. For example, centers of the arc shapes of the pair of first rail structures  411   a  and  411   b  may be defined as the pair of hinge axes Ha and Hb. The pair of hinge axes Ha and Hb may be parallel to the folding axis A. The pair of hinge axes Ha and Hb may be spaced apart from each other by a designated interval. 
     In one embodiment, the intermediate member arrangement space  412  may be formed near the center of the hinge bracket  410 . In one embodiment, the intermediate member arrangement space  412  may be formed at the center of the hinge bracket  410 , when the hinge bracket  410  is viewed from the front side. For example, the intermediate member arrangement space  412  may be recessed with a shape corresponding to an outer shape of a central portion  431  of the intermediate member  430 . The intermediate member  430  that will be described below may be rotatably disposed in the intermediate member arrangement space  412 . 
     In one embodiment, the pair of open spaces  413   a  and  413   b  may be formed on both sides (e.g., a −x side and a +x side based on  FIG.  4 E ) of the hinge bracket  410 . For example, when a direction parallel to the hinge axis Ha or Hb corresponds to a y-axis when the hinge bracket  410  is viewed from the front side, the pair of open spaces  413   a  and  413   b  may be formed on both sides (e.g., the −x side and +x side based on  FIG.  4 E ) of the hinge bracket  410 . The open space  413   a ,  413   b  may be a space for an arrangement of at least the elastic member  450   a  or  450   b  and/or the extension  432   a  or  432   b  of the intermediate member  430 . For example, at least a portion of the open space  413   a ,  413   b  may be formed to have a longitudinal direction in the y-axis direction. The pair of open spaces  413   a  and  413   b  may be formed to diagonally face each other. For example, the pair of open spaces  413   a  and  413   b  may be formed to be point-symmetric with respect to the center of the hinge bracket  410  when the hinge bracket  410  is viewed from the front side. The pair of open spaces  413   a  and  413   b  may substantially communicate with the intermediate member arrangement space  412 . For example, one open space  413   a  may substantially communicate with the intermediate member arrangement space  412  in the −x-axis direction and/or the +y-axis direction, and the other open space  413   b  may substantially communicate with the intermediate member arrangement space  412  in the +x-axis direction and/or the −y-axis direction. 
     In one embodiment, a first connection projection  4131   a  or  4131   b  may protrude in a direction parallel to the hinge axis Ha or Hb in the open space  413   a  or  413   b . For example, one first connection projection  4131   a  may protrude in the +y-axis direction in one open space  413   a , and the other first connection projection  4131   b  may protrude in the −y-axis direction in the other open space  413   b . The first connection projection  4131   a  or  4131   b  may be inserted into and connected to one end portion (e.g., an end portion facing the −y-axis direction, or an end portion facing the +y-axis direction) of the elastic member  450   a  or  450   b.    
     In one embodiment, the hinge bracket  410  may include the pair of bracket fixing holes  414   a  and  414   b  to fix the hinge bracket  410  to the hinge housing  313 . For example, a fastening member (e.g., a screw, a bolt, a pin, and/or a combination fastening structure) may be inserted into each of the pair of bracket fixing holes  414   a  and  414   b . The pair of bracket fixing holes  414   a  and  414   b  may be formed to diagonally face each other. For example, the pair of bracket fixing holes  414   a  and  414   b  may be formed to be point-symmetric with respect to the center of the hinge bracket  410  when the hinge bracket  410  is viewed from the front side. For example, one bracket fixing hole  414   a  may be formed at a position relatively biased in the +x-axis direction and/or +y-axis direction, in comparison to the other bracket fixing hole  414   b , and the other bracket fixing hole  414   b  may be formed at a position relatively biased in the −x-axis direction and/or the −y-axis direction, in comparison to the one bracket fixing hole  414   a . For example, the pair of bracket fixing holes  414   a  and  414   b  may be disposed in a diagonal direction that crosses a direction in which the pair of first rail structures  411   a  and  411   b  are disposed. 
     In one embodiment, the intermediate protrusion  415  may protrude from the intermediate member arrangement space  412  in a direction perpendicular to the hinge axis Ha or Hb. A direction in which the intermediate protrusion  415  protrudes may be defined as a middle axis M. The middle axis M may be positioned between the pair of hinge axes Ha and Hb and may be perpendicular to the pair of hinge axes Ha and Hb. For example, the middle axis M may be oriented in the z-axis direction. The intermediate member  430  that will be described below may be rotatably connected to the intermediate protrusion  415 . 
     In one embodiment, the intermediate protrusion  415  may include a protrusion base  4151  and a head  4152 . 
     In one embodiment, the protrusion base  4151  may be formed to protrude from the intermediate member arrangement space  412  of the hinge bracket  410  in a direction of the middle axis M. For example, the protrusion base  4151  may protrude in the +z-axis direction. The protrusion base  4151  may substantially have a cylindrical shape. For example, the protrusion base  4151  may be formed with a first radius R 1  about the middle axis M. However, this is merely an example, and a shape of the protrusion base  4151  is not limited thereto. For example, at least a portion of an outer surface of the protrusion base  4151  may not have a cylindrical shape. For example, a cut face or a cut groove may be formed on at least a portion (e.g., a lower portion (e.g., a −z side portion) of a projection  41521  that will be described below) of the outer surface of the protrusion base  4151  for reasons of the manufacturing process. 
     In one embodiment, the head  4152  may be formed on the protrusion base  4151 . For example, the head  4152  may be formed on an upper end portion of the protrusion base  4151 . At least a portion of the head  4152  may be formed to have a radius greater than that of the protrusion base  4151 . For example, the head  4152  may include the projection  41521  having a second radius R 2  greater than the first radius R 1 . For example, at least a point of the projection  41521  may have the second radius R 2  from the middle axis M. In one embodiment, for ease of assembling between the intermediate protrusion  415  and the intermediate member  430 , a chamfer may also be formed at an upper edge side (e.g., a side facing the +z direction) of the head  4152 . 
     In one embodiment, a single projection  41521 , or a plurality of projections  41521  may be formed. For example, a pair of projections  41521  may be formed. A pair of projections  41521   a  and  41521   b  may protrude in directions opposite to each other. For example, the pair of projections  41521   a  and  41521   b  may protrude in a direction in which the intermediate member arrangement space  412  substantially communicates with the pair of open spaces  413   a  and  413   b . For example, one projection  41521   a  may substantially protrude in the −x-axis direction and the +y-axis direction, and the other projection  41521   b  may substantially protrude in the +x-axis direction and the −y-axis direction. The pair of projections  41521   a  and  41521   b  may protrude in a direction inclined by a designated angle AG 1  with respect to the x-axis. The angle AG 1  at which the direction in which the pair of projections  41521   a  and  41521   b  protrudes is inclined with respect to the x-axis may be greater than an angle (e.g., an angle AG 2  of  FIGS.  4 C and/or  4 R ) at which the intermediate member  430  is rotated about the middle axis M when the hinge assembly  400  is operating. For example, the direction in which the pair of projections  41521   a  and  41521   b  protrudes may be a direction substantially inclined at 45 degrees with respect to the x-axis. However, this is merely an example, and a number of projections  41521  and a direction of the projection  41521  are not limited thereto. 
     In one embodiment, the pair of hinge structures  420   a  and  420   b  may be rotatably connected to the hinge bracket  410 . For example, the pair of hinge structures  420   a  and  420   b  may be rotatably connected to the pair of first rail structures  411   a  and  411   b , respectively. The pair of hinge structures  420   a  and  420   b  may be arranged to diagonally face each other. For example, the pair of hinge structures  420   a  and  420   b  may be arranged to be point-symmetric with respect to the center of the hinge assembly  400 , when the hinge assembly  400  is viewed from the front side. 
     In one embodiment, the hinge structure  420   a  or  420   b  may include a first body  421 , a second body  422 , the second rail structure  423 , and a first cam structure  424 . 
     In one embodiment, the first body  421  may be formed in a plate shape. The first body  421  may be fixedly connected to a first housing (e.g., the first housing  311  of  FIG.  3   ) or a second housing (e.g., the second housing  312  of  FIG.  3   ). The first body  421  may be parallel to a front surface (e.g., a surface facing the +z direction based on the state of  FIG.  3   ) of the first housing  311  or the second housing  312 . At least one housing fixing hole  4211  for fixing the hinge structure  420   a  or  420   b  to the first housing  311  or the second housing  312  may be formed in the first body  421 . For example, the housing fixing hole  4211  may be formed to penetrate the first body  421  in the z-axis direction. For example, a fastening member (e.g., a screw, a bolt, a pin, and/or a combination fastening structure) may be inserted into the housing fixing hole  4211 . Although three housing fixing holes  4211  are formed as shown in  FIG.  4 G , this is merely an example, and a number of housing fixing holes  4211  is not limited thereto. 
     In one embodiment, the second body  422  may be formed to extend from at least a portion of one end of the first body  421 . For example, referring to  FIG.  4 G , the second body  422  may be formed to extend in the +x-axis direction from at least a portion of an end portion of the first body  421  facing the +x-axis direction. The second body  422  may be formed integrally with the first body  421 . At least a portion of a cross section of the second body  422  may have an arc shape in the direction from the upper surface (e.g., the surface facing the +z-axis direction) to the lower surface (e.g., the surface facing the −z-axis direction). For example, at least a portion of a cross section of the second body  422  facing the x-z plane may have an arc shape. 
     In one embodiment, the second rail structure  423  may be formed on a lower side (e.g., a side facing the −z-axis direction) of the second body  422 . The second rail structure  423  may be recessed in the second body  422 . The second rail structure  423  may be recessed from a lower end portion (e.g., an end portion facing the −z-axis direction) of the second body  422  in an upward direction (e.g., the +z-axis direction) such that at least a portion of the lower end portion (e.g., the end portion facing the −z-axis direction) of the second body  422  may be opened. At least a portion of a cross section of the second rail structure  423  may have an arc shape in the direction from the upper surface (e.g., the surface facing the +z-axis direction) to the lower surface (e.g., the surface facing the −z-axis direction). For example, at least a portion of a cross section of the second rail structure  423  facing the x-z plane may have an arc shape. The arc shape of the second rail structure  423  may correspond to the arc shape of the first rail structure  411   a  or  411   b . The hinge structure  420   a  or  420   b  may be connected to the hinge bracket  410  such that the first rail structure  411   a  or  411   b  may be inserted into the second rail structure  423 . The second rail structure  423  may rotate about the hinge axis Ha or Hb within a designated angle range along the first rail structure  411   a  or  411   b . Based on the above structure, in a state in which the first rail structure  411   a  or  411   b  is inserted into the second rail structure  423 , the hinge structure  420   a  or  420   b  may rotate about the hinge axis Ha or Hb within a designated angle range with respect to the hinge bracket  410 . For example, the hinge structure  420   a  or  420   b  may rotate in the x-z plane about the hinge axis Ha or Hb formed by the first rail structure  411   a  or  411   b  and the second rail structure  423 . The first rail structure  411   a  or  411   b  and the second rail structure  423  may allow only a rotational motion of the hinge structure  420   a  or  420   b  on the x-z plane and may restrict a translational motion and/or a rotational motion of the hinge structure  420   a  or  420   b  in another direction. For example, an inner portion  4111   a  or  4111   b  of the first rail structure  411   a  or  411   b  in the radial direction may be formed to be stepped with a relatively large width (e.g., a width in the y-axis direction), in comparison to an outer portion  4112   a  or  4112   b  of the first rail structure  411   a  or  411   b , and an inner portion  4231  of the second rail structure  423  in the radial direction may be formed to be stepped with a relatively large width (e.g., a width in the y-axis direction), in comparison to an outer portion  4232  of the second rail structure  423 . The pair of first rail structures  411   a  and  411   b  protrude and the second rail structure  423  is recessed, as illustrated and described above, however, this is merely an example. For example, the pair of first rail structures  411   a  and  411   b  may be recessed, and the second rail structure  423  may protrude. 
     In one embodiment, the first cam structure  424  may be formed on one surface of the second body  422 . For example, based on  FIG.  4 G , the first cam structure  424  may be formed on a surface of the second body  422  facing the −y-axis direction. The first cam structure  424  may be formed along an arc having the hinge axis Ha or Hb as a center. For example, the first cam structure  424  may be formed along an arc shape of the lower side (e.g., the side facing the −z direction) of the second body  422 . The first cam structure  424  may include at least one crest and/or one trough structure. For example, the first cam structure  424  may protrude to include a first inclined surface  4241 , a first flat surface  4242 , and a second inclined surface  4243 . 
     In one embodiment, the intermediate member  430  may be disposed between the pair of hinge structures  420   a  and  420   b . The intermediate member  430  may be disposed in the intermediate member arrangement space  412  and connected to the hinge bracket  410  through the intermediate protrusion  415 . The intermediate member  430  may be rotatable with respect to the hinge bracket  410  about the middle axis M perpendicular to the pair of hinge axes Ha and Hb. A pair of second cam structures  434   a  and  434   b  interoperating with the first cam structure  424  may be formed on both end portions (e.g., an end portion facing the −x-axis direction and an end portion facing the +x-axis direction) of the intermediate member  430 . 
     In one embodiment, the intermediate member  430  may include the central portion  431 , a pair of extensions  432   a  and  432   b , a pair of second connection projections  433   a  and  433   b , and the pair of second cam structures  434   a  and  434   b.    
     In one embodiment, the central portion  431  may be a portion positioned at the center of the intermediate member  430 . An outer circumferential surface of the central portion  431  may be formed in a substantially cylindrical shape. A through-hole  4311  may be formed in the center of the central portion  431 . For example, the through-hole  4311  may penetrate the central portion  431  in the z-axis direction. The central portion  431  may be disposed in the intermediate member arrangement space  412  such that the intermediate protrusion  415  of the hinge bracket  410  may be inserted into the through-hole  4311 . The central portion  431  may be disposed in the intermediate member arrangement space  412  to be rotatable with respect to the hinge bracket  410  about the middle axis M perpendicular to the pair of hinge axes Ha and Hb. The central portion  431  may be formed to have a height (e.g., a height in the z-axis direction) substantially corresponding to the protrusion base  4151  of the intermediate protrusion  415 . Based on the above configuration, when the intermediate member  430  is inserted into the intermediate protrusion  415 , the head  4152  of the intermediate protrusion  415  may pass through the through-hole  4311  to be exposed to an upper side (e.g., a side facing the +z direction) of the intermediate member  430 . 
     In an embodiment, the intermediate protrusion  415  may be inserted into the through-hole  4311 . The through-hole  4311  may have a shape substantially corresponding to a shape of the head  4152  of the intermediate protrusion  415 . In one embodiment, for ease of assembling between the intermediate protrusion  415  and the intermediate member  430 , a chamfer may also be formed at a lower edge side (e.g., a side facing the −z direction) of the through-hole  4311 . 
     In one embodiment, the through-hole  4311  may include a main hole  43111  and a recessed portion  43112 . 
     In one embodiment, the main hole  43111  may be a substantially cylindrical hole. For example, the main hole  43111  may be formed with a first radius R 1 . A radius of the main hole  43111  may substantially correspond to a radius of the intermediate protrusion  415 . The recessed portion  43112  may be recessed radially from the main hole  43111  to have a radius greater than that of the main hole  43111 . For example, the recessed portion  43112  may be formed such that at least a point of the recessed portion  43112  may have the second radius R 2 . 
     In one embodiment, a number of recessed portions  43112  and/or a shape of the recessed portion  43112  may correspond to a number of protrusions  41521  of the intermediate protrusion  415  and/or a shape of the projection  41521 . A single or a plurality of recessed portions  43112  may be formed. For example, a pair of recessed portions  43112  may be formed. A pair of recessed portions  43112   a  and  43112   b  may be recessed in directions opposite to each other. For example, the pair of recessed portions  43112   a  and  43112   b  may be formed in a direction corresponding to a pair of projections  41521   a  and  41521   b.    
     In one embodiment, the pair of extensions  432   a  and  432   b  may be portions extending to both sides (e.g., a side facing the −x direction and a side facing the +x direction) of the central portion  431 . For example, one extension  432   a  may extend from the central portion  431  in the −x direction, and the other extension  432   b  may extend from the central portion  431  in the +x direction. The pair of extensions  432   a  and  432   b  may be formed to be point-symmetric with respect to the center, when the intermediate member  430  is viewed from the front side. For example, the one extension  432   a  may be formed at a position relatively biased in the −x-axis direction and/or the +y-axis direction, in comparison to the other extension  432   b , and the other extension  432   b  may be formed at a position relatively biased in the +x-axis direction and/or the −y-axis direction, in comparison to the one extension  432   a . In a state in which the central portion  431  is disposed in the intermediate member arrangement space  412 , the pair of extensions  432   a  and  432   b  may be disposed in the pair of open spaces  413   a  and  413   b , respectively. 
     In one embodiment, the pair of extensions  432   a  and  432   b  may be formed with a height (e.g., a height in the z-axis direction) greater than that of the central portion  431 . For example, an upper surface  4312  (e.g., a surface facing the +z direction) of the central portion  431  as shown in  FIG.  4 L  may be formed to be stepped below (e.g., in the −z direction) an upper surface  4321   a ,  4321   b  (e.g., a surface facing the +z direction) of the extension  432   a ,  432   b . Based on the above configuration, when the intermediate member  430  is inserted into the intermediate protrusion  415 , a space  4313  in which the head  4152  passing through the through-hole  4311  may be located may be formed. 
     In one embodiment, the second cam structure  434   a  or  434   b  may be formed on one surface (e.g., a surface facing the +y-axis or −y-axis direction) of the extension  432   a  or  432   b , and the second connection projection  433   a  or  433   b  may be formed on another surface (e.g., a surface facing the −y-axis or +y-axis direction). 
     In one embodiment, the pair of second connection projections  433   a  and  433   b  may be formed to protrude from one surface of the pair of extensions  432   a  and  432   b . The pair of second connection projections  433   a  and  433   b  may be formed to be point-symmetric with respect to the center, when the intermediate member  430  is viewed from the front side. For example, one second connection projection  433   a  may protrude from one surface (e.g., the surface facing the −y-axis direction) of one extension  432   a , and the other second connection projection  433   b  may protrude from one surface (e.g., the surface facing the +y-axis direction) of the other extension  432   b . The second connection projection  433   a  or  433   b  may be inserted into and connected to the other end portion (e.g., an end portion facing the +y-axis direction, or an end portion facing the −y-axis direction) of the elastic member  450   a  or  450   b.    
     In one embodiment, the pair of second cam structures  434   a  and  434   b  may be formed to protrude from the other surface of the pair of extensions  432   a  and  432   b . The pair of second connection projections  433   a  and  433   b  may be formed to be point-symmetric with respect to the center, when the intermediate member  430  is viewed from the front side. The second cam structure  434   a  or  434   b  may be formed on a surface opposite to the second connection projection  433   a  or  433   b . For example, one second cam structure  434   a  may be formed on another surface (e.g., the surface facing the +y-axis direction) of the one extension  432   a , and the other second cam structure  434   b  may be formed on another surface (e.g., the surface facing the −y-axis direction) of the other extension  432   b . The second cam structure  434   a  or  434   b  may be formed along an arc having the hinge axis Ha or Hb as a center. The second cam structure  434   a  or  434   b  may include at least one crest and/or trough structure. For example, the second cam structure  434   a  or  434   b  may protrude to include a third inclined surface  4341 , a second flat surface  4342 , and a fourth inclined surface  4343 . 
     In one embodiment, the elastic member  450   a  or  450   b  may generate an elastic force. For example, the elastic member  450   a  or  450   b  may generate an elastic force in the longitudinal direction. The elastic member  450   a  or  450   b  may be disposed in a direction parallel to the hinge axis Ha or Hb to generate an elastic force in the longitudinal direction. For example, the elastic member  450   a  or  450   b  may have a shape of a spring with an empty central space. The elastic member  450   a  or  450   b  may be disposed in the open space  413   a  or  413   b  such that one end portion (e.g., the end portion facing the −y-axis direction or +y-axis direction) may be connected to the first connection projection  4131   a  or  4131   b , and that another end portion (e.g., the end portion facing the +y-axis direction or −y-axis direction) may be connected to the second connection projection  433   a  or  433   b . For example, the one end portion (e.g., the end portion facing the −y-axis direction or +y-axis direction) of the elastic member  450   a  or  450   b  may be supported by the hinge bracket  410 , and the other end portion (e.g., the end portion facing the +y-axis direction or −y-axis direction) may be supported by the intermediate member  430 . The elastic member  450   a  or  450   b  may provide an elastic force to the extension  432   a  or  432   b  in a direction in which the second cam structure  434   a  or  434   b  is pressed toward the first cam structure  424   a  or  424   b . For example, one elastic member  450   a  may press the one extension  432   a  in the +y-axis direction, and the other elastic member  450   b  may press the other extension  432   b  in the −y-axis direction. As a result, the elastic force of the elastic member  450   a  or  450   b  may generate torque for rotating the intermediate member  430  in the direction in which the second cam structure  434   a  or  434   b  is pressed toward the first cam structure  424   a  or  424   b . Based on the above structure, the second cam structure  434   a  or  434   b  and the first cam structure  424   a  or  424   b  may be in close contact with each other. In one embodiment, the elastic member may also be formed of a torsion spring for generating an elastic force in a rotation direction. For example, the elastic member may be inserted into the intermediate protrusion  415  to generate an elastic force to rotate the intermediate member  430  in one direction (e.g., a clockwise direction). 
       FIGS.  4 I through  4 K  illustrate a process in which an intermediate member is rotatably connected to a hinge bracket according to one embodiment.  FIG.  4 L  is a cross-sectional view taken along line A-A of  FIG.  4 K . 
     Referring to  FIGS.  4 I through  4 L , in one embodiment, the intermediate member  430  may be rotatably connected to the intermediate protrusion  415  of the hinge bracket  410 . The intermediate member  430  may be inserted into the intermediate protrusion  415  in a state in which the through-hole  4311  and the head  4152  of the intermediate protrusion  415  are aligned such that the shape of the through-hole  4311  and the shape of the head  4152  correspond to each other, as shown in  FIGS.  4 I and  4 J . The state in which the through-hole  4311  and the head  4152  are aligned such that the shape of the through-hole  4311  and the shape of the head  4152  of the intermediate protrusion  415  correspond to each other may indicate a state in which a position of the projection  41521  of the head  4152  and a position of the recessed portion  43112  of the through-hole  4311  are aligned with each other. In the state in which the position of the projection  41521  of the head  4152  and the position of the recessed portion  43112  of the through-hole  4311  are aligned with each other, the projection  41521  may pass through the recessed portion  43112 , and accordingly the intermediate member  430  may be inserted into the intermediate protrusion  415 . When the intermediate member  430  is inserted into the intermediate protrusion  415 , the head  4152  of the intermediate protrusion  415  may pass through the through-hole  4311  and may be exposed to the upper side (e.g., a side facing the +z direction) of the intermediate member  430 . 
     In one embodiment, referring to  FIGS.  4 K and  4 L , in a state in which the intermediate member  430  is inserted into the intermediate protrusion  415 , the intermediate member  430  may be rotated with respect to the intermediate protrusion  415  such that the head  4152  and the through-hole  4311  may be out of alignment with each other. A state in which the head  4152  and the through-hole  4311  are out of alignment with each other may indicate a state in which the position of the projection  41521  of the head  4152  and the position of the recessed portion  43112  of the through-hole  4311  are out of alignment with each other. For example, the intermediate member  430  may be rotated in a counterclockwise direction by a designated angle with respect to the intermediate protrusion  415 . When the head  4152  and the through-hole  4311  are out of alignment with each other in the state in which the intermediate member  430  is inserted into the intermediate protrusion  415 , it may be difficult for the projection  41521  of the head  4152  to pass through the through-hole  4311 , to prevent the intermediate member  430  from being separated from the intermediate protrusion  415  in the direction (e.g., the +z-axis direction) of the middle axis M. For example, the head  4152  of the intermediate protrusion  415  may support the upper surface  4312  (e.g., the surface facing the +z direction) of the central portion  431  of the intermediate member  430  on the upper side, to prevent the intermediate member  430  from deviating in the +z-axis direction. In addition, an angle (e.g., the angle AG 1  of  FIG.  4 F ) at which the projection  41521  of the intermediate protrusion  415  is inclined with respect to the x-axis may be formed to be greater than an angle (e.g., the angle AG 2  of  FIGS.  4 C and/or  4 R ) at which the intermediate member  430  is rotated about the middle axis M with respect to the x-axis during an operation of a hinge assembly (e.g., the hinge assembly  400  of  FIG.  4 A ). Thus, it may be possible to prevent the intermediate member  430  from being separated from the intermediate protrusion  415  even during an operation of the hinge assembly  400 . 
     Based on the above structure, it may be possible to prevent the intermediate member  430  from being separated while rotatably connecting the intermediate member  430  to the hinge bracket  410  without using a separate pin member. Therefore, since there is no need to use a separate pin member, a number of components may be reduced, thereby reducing a manufacturing cost and a weight. Also, if the number of components is reduced, an accumulated error between components may be reduced, and thus a quality of the hinge assembly (e.g., the hinge assembly  400  of  FIG.  4 A ) may be enhanced. In addition, a process of welding a pin member to a bracket may not be required, in comparison to a case in which a separate pin member is used, and thus a deformation and/or thermal damage to components that may occur during welding may not occur. As a result, a reliability and/or durability of the hinge assembly  400  may be enhanced. 
       FIG.  4 M  illustrates a force and torque acting on one hinge structure of  FIGS.  4 A through  4 C . 
     Hereinafter, the force and torque acting on one hinge structure  420   a  in the unfolded state of the hinge assembly  400  according to one embodiment will be described with reference to  FIGS.  4 A through  4 C and  4 M . However, this is for convenience of description, and it will be obvious that the other hinge structure  420   b  may also operate in a manner corresponding to that of the one hinge structure  420   a . The unfolded state of the hinge assembly  400  may refer to a state in which the pair of hinge structures  420   a  and  420   b  are fully unfolded with respect to the hinge bracket  410 . 
     In one embodiment, in the state in which the hinge assembly  400  is unfolded, the first cam structure  424  and the second cam structure  434   a  may be arranged to be alternately engaged with each other. For example, a crest portion of the second cam structure  434   a  may be inserted into a trough portion of the first cam structure  424 . For example, the first inclined surface  4241  of the first cam structure  424  and the third inclined surface  4341  of the second cam structure  434   a  may contact each other. The elastic member  450   a  may provide an elastic force Fs 1  to the extension  432   a  of the intermediate member  430  in a direction in which the second cam structure  434   a  is pressed toward the first cam structure  424 . For example, the elastic force Fs 1  may be provided in the +y-axis direction by the elastic member  450   a . If the elastic member  450   a  applies the elastic force Fs 1  in the +y-axis direction in a state in which the crest portion of the second cam structure  434   a  is inserted into the trough portion of the first cam structure  424 , the intermediate member  430  may be in a state of being rotated by a predetermined angle (e.g., the angle AG 2 ) about the middle axis M in a direction in which the extension  432   a  approaches the hinge structure  420   a . For example, when the hinge assembly  400  is viewed from the front side, the intermediate member  430  may be in a state of being rotated in the clockwise direction by a predetermined angle (e.g., the angle AG 2 ) about the middle axis M. 
     In one embodiment, the elastic force Fs 1  of the elastic member  450   a  may be applied to press the second cam structure  434   a  to the first cam structure  424  in the +y-axis direction. The first cam structure  424  and the second cam structure  434   a  may apply a reaction force to each other in a direction perpendicular to inclined surfaces (e.g., the first inclined surface  4241  and the third inclined surface  4341 ) that are in contact with each other. For example, a reaction force Fc 1  may be applied to the first cam structure  424  by the second cam structure  434   a  in a direction perpendicular to the first inclined surface  4241  and the third inclined surface  4341 . For example, the reaction force Fc 1  may be applied to the first cam structure  424  by the second cam structure  434   a  in a direction between the +x-axis direction and the +y-axis direction.  FIG.  4 M  illustrates an x-axis direction component Fc 1 _ x  of the reaction force Fc 1  applied to the first inclined surface  4241  of the first cam structure  424 . Referring to  FIG.  4 M , the x-axis direction component Fc 1 _ x  of the reaction force Fc 1  may generate counterclockwise torque T 1  about the hinge axis Ha. The counterclockwise torque T 1  may be torque in a direction to allow the hinge structure  420   a  to be further unfolded with respect to the hinge bracket  410 . Based on the above structure, in the state in which the hinge structure  420   a  is unfolded, the elastic force Fs 1  of the elastic member  450   a  may act as torque to further unfold the hinge structure  420   a . For example, in the state in which the hinge structure  420   a  is unfolded, the elastic force Fs 1  of the elastic member  450   a  may act as a kind of open detent force to allow the hinge structure  420   a  to remain unfolded. Therefore, the hinge structure  420   a  may start to be folded with respect to the hinge bracket  410  only when a force greater than the open detent force is applied. If the force greater than the open detent force is not applied, the unfolded state of the hinge structure  420   a  with respect to the bracket  410  may be maintained. 
       FIG.  4 N  is a perspective view illustrating an intermediate state of the hinge assembly according to one embodiment.  FIG.  4 O  is a rear view illustrating the intermediate state of the hinge assembly according to one embodiment.  FIG.  4 P  illustrates a force acting on one hinge structure of  FIGS.  4 N and  4 O . 
     Hereinafter, a force acting on one hinge structure  420   a  in an intermediate state of the hinge assembly  400  according to one embodiment will be described with reference to  FIGS.  4 N through  4 P . However, this is merely for convenience of description, and it will be obvious that the other hinge structure  420   b  may also operate in a manner corresponding to that of the one hinge structure  420   a . The intermediate state of the hinge assembly  400 , which is a state between the unfolded state and the folded state, may refer to a state in which the pair of hinge structures  420   a  and  420   b  are rotated by a designated angle range about the hinge axes Ha and Hb with respect to the hinge bracket  410   
     In one embodiment, in the intermediate state of the hinge assembly  400 , the first cam structure  424  and the second cam structure  434   a  may be disposed such that a flat surface of the first cam structure  424  and a flat surface of the second cam structure  434   a  may contact each other. For example, the crest portion of the second cam structure  434   a  and a crest portion of the first cam structure  424  may contact each other. For example, the first flat surface  4242  of the first cam structure  424  and the second flat surface  4342  of the second cam structure  434   a  may contact each other. The elastic member  450   a  may provide an elastic force Fs 2  to the extension  432   a  of the intermediate member  430  in the direction in which the second cam structure  434   a  is pressed toward the first cam structure  424 . For example, the elastic force Fs 2  may be provided in the +y-axis direction by the elastic member  450   a . In a process in which one hinge structure  420   a  unfolded with respect to the hinge bracket  410  is rotated to be in the intermediate state, the first cam structure  424  and the second cam structure  434   a  that are alternately arranged may face each other such that the flat surface of the first cam structure  424  and the flat surface of the second cam structure  434   a  may contact each other. Accordingly, the intermediate member  430  may be rotated by a predetermined angle about the middle axis M in a direction in which the extension  432   a  moves away from the hinge structure  420   a . For example, the extension  432   a  or  432   b  of the intermediate member  430  may be disposed parallel to the x-axis direction, when the hinge assembly  400  is viewed from the front side. 
     In one embodiment, the elastic force Fs 2  of the elastic member  450   a  may be applied to press the second cam structure  434   a  to the first cam structure  424  in the +y-axis direction. The first cam structure  424  and the second cam structure  434   a  may apply a reaction force to each other in a direction perpendicular to flat surfaces (e.g., the first flat surface  4242  and the second flat surface  4342 ) that are in contact with each other. For example, a reaction force Fc 2  may be applied to the first cam structure  424  by the second cam structure  434   a  in a direction perpendicular to the first flat surface  4242  and the second flat surface  4342 . For example, the reaction force Fc 2  may be applied to the first cam structure  424  by the second cam structure  434   a  in the +y-axis direction.  FIG.  4 P  illustrates the reaction force Fc 2  applied to the first flat surface  4242  of the first cam structure  424 . Referring to  FIG.  4 P , since the reaction force Fc 2  does not include an x-axis direction component or z-axis direction component, any torque about the hinge axis Ha may not be generated. Instead, the reaction force Fc 2  may act as a normal force that generates a friction force between the first flat surface  4242  and the second flat surface  4342 . Accordingly, due to the reaction force Fc 2  acting perpendicular to the first flat surface  4242  and the second flat surface  4342 , a large friction force may be generated between the first flat surface  4242  and the second flat surface  4342 . Based on the above structure, in the intermediate state of the hinge structure  420   a , the elastic force Fs 2  of the elastic member  450   a  may generate a friction force to prevent the hinge structure  420   a  from being unfolded or folded. For example, in the intermediate state of the hinge structure  420   a , the elastic force Fs 2  of the elastic member  450   a  may act as an intermediate state stopping force to maintain the hinge structure  420   a  in the intermediate state. Therefore, the hinge structure  420   a  may start to be folded or unfolded with respect to the hinge bracket  410  only when a force greater than the intermediate state stopping force is applied. If the force greater than the intermediate state stopping force is not applied, the intermediate state of the hinge structure  420   a  with respect to the hinge bracket  410  may be maintained. 
       FIG.  4 Q  is a perspective view illustrating a folded state of the hinge assembly according to one embodiment.  FIG.  4 R  is a rear view illustrating the folded state of the hinge assembly according to one embodiment.  FIG.  4 S  illustrates a force and torque acting on one hinge structure of  FIGS.  4 Q and  4 R . 
     Hereinafter, a force and torque acting on the hinge structure  420   a  in the folded state of the hinge assembly  400  according to one embodiment will be described with reference to  FIGS.  4 Q through  4 S . However, this is merely for convenience of description, and it will be obvious that the other hinge structure  420   b  may also operate in a manner corresponding to that of the one hinge structure  420   a . The folded state of the hinge assembly  400  may refer to a state in which the pair of hinge structures  420   a  and  420   b  are fully folded with respect to the hinge bracket  410 . 
     In one embodiment, in the state in which the hinge assembly  400  is folded, the first cam structure  424  and the second cam structure  434   a  may be alternately engaged with each other. For example, the crest portion of the second cam structure  434   a  may be inserted into the trough portion of the first cam structure  424 . For example, the second inclined surface  4243  of the first cam structure  424  and the fourth inclined surface  4343  of the second cam structure  434   a  may contact each other. The elastic member  450   a  may provide an elastic force Fs 3  to the extension  432   a  of the intermediate member  430  in the direction in which the second cam structure  434   a  is pressed toward the first cam structure  424 . For example, the elastic force Fs 3  may be provided in the +y-axis direction by the elastic member  450   a . In a process in which the one hinge structure  420   a  in the intermediate state is rotated to be folded with respect to the hinge bracket  410 , the first cam structure  424  and the second cam structure  434   a  disposed such that the flat surfaces contact with each other may be alternately arranged. Here, if the elastic member  450   a  applies the elastic force Fs 3  in the +y-axis direction, the intermediate member  430  may be rotated by a predetermined angle (e.g., the angle AG 2 ) about the middle axis M in the direction in which the extension  432   a  approaches the hinge structure  420   a . For example, when the hinge assembly  400  is viewed from the front side, the intermediate member  430  may be in a state of being rotated in the clockwise direction by a predetermined angle (e.g., the angle AG 2 ) about the middle axis M. 
     In one embodiment, the elastic force Fs 3  of the elastic member  450   a  may be applied to press the second cam structure  434   a  to the first cam structure  424  in the +y-axis direction. The first cam structure  424  and the second cam structure  434   a  may apply a reaction force to each other in a direction perpendicular to inclined surfaces (e.g., the second inclined surface  4243  and the fourth inclined surface  4343 ) that are in contact with each other. For example, the reaction force Fc 3  may be applied to the first cam structure  424  by the second cam structure  434   a  in a direction perpendicular to the second inclined surface  4243  and the fourth inclined surface  4343 . For example, the reaction force Fc 3  may be applied to the first cam structure  424  by the second cam structure  434   a  in a direction between the −x-axis direction and the +y-axis direction.  FIG.  4 S  illustrates an x-axis direction component Fc 3 _ x  of the reaction force Fc 3  applied to the second inclined surface  4243  of the first cam structure  424 . Referring to  FIG.  4 S , the x-axis direction component Fc 3 _ x  of the reaction force Fc 3  may generate clockwise torque T 3  about the hinge axis Ha. The clockwise torque T 3  may be torque in a direction to allow the hinge structure  420   a  to be further folded with respect to the hinge bracket  410 . Based on the above structure, in the state in which the hinge structure  420   a  is folded, the elastic force Fs 3  of the elastic member  450   a  may act as torque to allow the hinge structure  420   a  to be further folded. For example, in the state in which the hinge structure  420   a  is folded, the elastic force Fs 3  of the elastic member  450   a  may act as a kind of close detent force to allow the hinge structure  420   a  to remain folded. Accordingly, the hinge structure  420   a  may start to be unfolded with respect to the hinge bracket  410  only when a force greater than the close detent force is applied. If the force greater than the close detent force is not applied, the folded state of the hinge structure  420   a  with respect to the bracket  410  may be maintained. 
       FIGS.  4 T through  4 V  are rear views schematically illustrating the hinge assembly according to one embodiment, and illustrate a process in which a balance between both sides of the hinge assembly is achieved in a situation in which one rotation member starts to rotate first. 
       FIG.  4 T  illustrates a state in which the pair of hinge structures  420   a  and  420   b  are rotated by the same angle. In the state of  FIG.  4 T , a reaction force Fca 1  acting between the first cam structure  424   a  and the second cam structure  434   a  and a reaction force Fcb 1  acting between the first cam structure  424   b  and the second cam structure  434   b  may be equal to each other, to achieve the balance between both sides.  FIG.  4 U  illustrates a state in which one hinge structure  420   b  first starts to rotate about the hinge axis Hb. A rotation angle of the one hinge structure  420   b  may be synchronized with a rotation angle of the other hinge structure  420   a  by a sync assembly (e.g., the sync assembly  320  of  FIG.  3   ). However, a gap may be formed between components of the sync assembly  320  in a manufacturing process. Due to the gap, the rotation angle of the one hinge structure  420   b  and the rotation angle of the other hinge structure  420   a  may not be temporarily the same. In such a state, as shown in  FIG.  4 U , the first cam structure  424   b  of the one hinge structure  420   b  that starts to rotate first may start to push the second cam structure  434   b , so that the intermediate member  430  may be rotated about the middle axis M in one direction (e.g., a clockwise direction). If the intermediate member  430  is rotated about the middle axis M in one direction (e.g., a clockwise direction), a distance between the first cam structure  424   a  and the second cam structure  434   a  of the other hinge structure  420   a  may relatively increase. Therefore, a reaction force Fca 2  between the first cam structure  424   a  and the second cam structure  434   a  in the other hinge structure  420   a  may be relatively reduced in comparison to a reaction force Fcb 2  between the first cam structure  424   b  and the second cam structure  434   b  in the one hinge structure  420   b  that starts to rotate first. For example, the reaction force Fca 2  between the first cam structure  424   a  and the second cam structure  434   a  and the reaction force Fcb 2  between the first cam structure  424   b  and the second cam structure  434   b  may be temporarily imbalanced.  FIG.  4 V  illustrates a state in which the other hinge structure  420   a  is rotated about the hinge axis Ha, to solve an imbalance between reaction forces. If the reaction force Fca 2  between the first cam structure  424   a  and the second cam structure  434   a  and the reaction force Fcb 2  between the first cam structure  424   b  and the second cam structure  434   b  are imbalanced, the other hinge structure  420   a  may be rotated about the hinge axis Ha until reaction forces Fca 3  and Fcb 3  on both sides are balanced, because a rotation resistance of the other hinge structure  420   a  having a relatively small reaction force Fca 2  is less than that of the one hinge structure  420   b  having a relatively large reaction force Fcb 2 . As a result, the other hinge structure  420   a  may be rotated about the hinge axis Ha until the same angle as the rotation angle of the one hinge structure  420   b  is formed. In such a state, the reaction forces Fca 3  and Fcb 3  may be balanced with each other as shown in  FIG.  4 V . As a result, since the pair of hinge structures  420   a  and  420   b  interoperate with each other through the intermediate member  430  in the hinge assembly  400  according to one embodiment, the other hinge structure  420   a  may be rotated in compensation via the intermediate member  430  even though the one hinge structure  420   b  starts to rotate first. Therefore, the hinge assembly  400  according to one embodiment may complement the sync assembly  320  by reducing a rotation angle imbalance of the pair of hinge structures  420   a  and  420   b  by itself. As a result, the pair of hinge structures  420   a  and  420   b  may be smoothly rotated. 
       FIGS.  4 W through  4 Y  are rear views schematically illustrating the hinge assembly according to one embodiment, and illustrate a process in which a balance between both sides of the hinge assembly is achieved in a situation in which one rotation member starts to rotate first. 
       FIG.  4 W  illustrates a state in which the pair of hinge structures  420   a  and  420   b  are rotated by the same angle. In the state of  FIG.  4 W , a reaction force Fca 1  acting between the first cam structure  424   a  and the second cam structure  434   a , and a reaction force Fcb 1  acting between the first cam structure  424   b  and the second cam structure  434   b  on both sides may be equal to each other, to achieve the balance between both sides.  FIG.  4 X  illustrates a state in which one hinge structure  420   b  first starts to rotate about the hinge axis Hb. In a process in which the first cam structure  424   b  of the one hinge structure  420   b  that starts to rotate first pushes the second cam structure  434   b , the intermediate member  430  may be pushed in one direction (e.g., the +x direction) by a gap between a through-hole (e.g., the through-hole  4311  of  FIG.  4 H ) of the intermediate member  430  and an intermediate protrusion (e.g., the intermediate protrusion  415  of  FIG.  4 E ) of a hinge bracket (e.g., the hinge bracket  410  of  FIG.  4 E ). In such a state, since the first cam structure  424   a  and the second cam structure  434   a  in the other hinge structure  420   a  interfere with each other, the reaction force Fca 2  between the first cam structure  424   a  and the second cam structure  434   a  in the other hinge structure  420   a  may relatively increase in comparison to the reaction force Fcb 2  between the first cam structure  424   b  and the second cam structure  434   b  in the one hinge structure  420   b . For example, the reaction force Fca 2  between the first cam structure  424   a  and the second cam structure  434   a  and the reaction force Fcb 2  between the first cam structure  424   b  and the second cam structure  434   b  on both sides may be temporarily imbalanced.  FIG.  4 Y  illustrates a state in which the intermediate member  430  is rotated in one direction (e.g., a clockwise direction) about a new middle axis M′, to solve an imbalance between reaction forces. To solve the interference between the first cam structure  424   a  and the second cam structure  434   a  in the other hinge structure  420   a , the intermediate member  430  may be rotated about the new middle axis M′ in the clockwise direction until the reaction force Fca 3  between the first cam structure  424   a  and the second cam structure  434   a  and the reaction force Fcb 3  between the first cam structure  424   b  and the second cam structure  434   b  on both sides are equal. In such a state, the reaction forces Fca 3  and Fcb 3  may be balanced with each other as shown in  FIG.  4 Y . As a result, since the pair of hinge structures  420   a  and  420   b  interoperate with each other through the intermediate member  430  in the hinge assembly  400  according to one embodiment, the reaction forces Fca 3  and Fcb 3  on both sides may be balanced even though the one hinge structure  420   b  starts to rotate first. Therefore, the hinge assembly  400  according to one embodiment may prevent a reaction force imbalance of the pair of hinge structures  420   a  and  420   b . As a result, the pair of hinge structures  420   a  and  420   b  may be smoothly rotated. 
     In one embodiment, as described above with reference to  FIGS.  4 A through  4 Y , the intermediate member  430  may be rotated about the middle axis M between the pair of hinge structures  420   a  and  420   b , and accordingly rotation operations of the hinge structure  420   a  and  420   b  may interoperate with each other through the intermediate member  430 . Thus, the hinge assembly  400  may be smoothly folded or unfolded. In addition, by preventing tilting caused by a gap between components, the thickness of the hinge assembly  400  and the number of components of the hinge assembly  400  may be reduced, and a sufficient elastic force of the elastic member  450   a  or  450   b  may be secured. 
       FIG.  5    is a rear view schematically illustrating a hinge assembly according to one embodiment. 
     Referring to  FIG.  5   , in one embodiment, a pair of second cam structures  534   a  and  534   b  may be formed as components separate from an intermediate member  530 . For example, at least a central portion  531  and a pair of extensions  532   a  and  532   b  of the intermediate member  530  may be integrally formed, and the pair of second cam structures  534   a  and  534   b  may be separate components in contact with the pair of extensions  532   a  and  532   b . An elastic member  550   a  or  550   b  may press the extension  532   a  or  532   b  toward the second cam structure  534   a  or  534   b  such that the extension  532   a  or  532   b  may remain in contact with the second cam structure  534   a  or  534   b . In addition, for a longitudinal alignment of the elastic member  550   a  or  550   b , the extension  532   a  or  532   b , and the second cam structure  534   a  or  534   b , a guide member  560   a  or  560   b  penetrating the elastic member  550   a  or  550   b , the extension  532   a  or  532   b , and the second cam structure  534   a  or  534   b  in a longitudinal direction (e.g., a y-axis direction) may also be separately provided. A hinge assembly  500  of  FIG.  5    may operate in substantially the same manner as the hinge assembly  400  described above with reference to  FIGS.  4 A through  4 Y . 
       FIG.  6 A  is a perspective view illustrating an intermediate protrusion according to one embodiment.  FIG.  6 B  is a cross-sectional view taken along line B-B of  FIG.  6 A . 
     Referring to  FIGS.  6 A and  6 B , an intermediate protrusion  615  according to one embodiment may include a protrusion base  6151 , a head  6152  having a protrusion  61521 , and a groove  6153 . 
     In one embodiment, the groove  6153  may be recessed in at least a portion of an outer surface of the protrusion base  6151 . For example, the groove  6153  may be formed along a circumference of the protrusion base  6151 . For example, a single groove  6153 , or a plurality of grooves  6153  may be formed. The plurality of grooves  6153  may be formed to be spaced apart from each other in the direction of the middle axis M in the protrusion base  6151 . Although two grooves  6153  are illustrated in  FIGS.  6 A and  6 B , this is merely an example. A number of grooves  6153  and/or a shape of the groove  6153  is not limited thereto. 
     In one embodiment, when a lubricant is applied around the protrusion base  6151 , the lubricant may be introduced into a space formed by the groove  6153  to fill the space. The lubricant may include, for example, grease. The lubricant introduced into the groove  6153  may reduce a rotational frictional resistance of an intermediate member (e.g., the intermediate member  430  of  FIG.  4 D ) with respect to the intermediate protrusion  615 . 
       FIG.  7 A  is a perspective view illustrating an unfolded state of a hinge assembly according to one embodiment.  FIG.  7 B  is a front view illustrating the unfolded state of the hinge assembly according to one embodiment.  FIG.  7 C  is a perspective view illustrating a folded state of the hinge assembly according to one embodiment.  FIG.  7 D  is an exploded perspective view illustrating the hinge assembly according to one embodiment.  FIG.  7 E  is a perspective view illustrating a hinge bracket according to one embodiment.  FIG.  7 F  is an exploded perspective view illustrating a hinge structure according to one embodiment.  FIGS.  7 G through  7 I  illustrate a state in which a rotation plate and a fixing plate are in surface contact with each other according to one embodiment. 
     Referring to  7 A through  7 I, a hinge assembly  700  according to one embodiment may include a hinge bracket  710 , a pair of hinge structures  720   a  and  720   b , an intermediate member  730 , a pair of elastic members  750   a  and  750   b , and a fixing plate  760 . The description of the configuration of the hinge assembly  400  provided with reference to  FIGS.  4 A through  4 Y  may be similarly applied to a configuration of the hinge assembly  700  of  FIGS.  7 A through  7 I  which is substantially the same as that of the hinge assembly  400  of  FIGS.  4 A through  4 Y , unless otherwise described. 
     In one embodiment, the hinge bracket  710  may include a pair of first rail structures  711   a  and  711   b , an intermediate member arrangement space  712 , a pair of open spaces  713   a  and  713   b , a pair of bracket fixing holes  714   a  and  714   b , and an intermediate protrusion  715 . 
     In one embodiment, the hinge bracket  710  may include a pair of first rail structures  711   a  and  711   b  so that the pair of hinge structures  720   a  and  720   b  may be rotatably coupled. At least a portion of a cross section of the pair of first rail structures  711   a  and  711   b  may have an arc shape in a direction from an upper surface (e.g., a surface facing the +z-axis direction) of the hinge bracket  710  to a lower surface (e.g., a surface facing the −z-axis direction) of the hinge bracket  710 . The pair of first rail structures  711   a  and  711   b  may be recessed in an arc shape with a predetermined angle. For example, the first rail structure  711   a  or  711   b  may be recessed from an upper side (e.g., a +z side) to a lower side (e.g., a −z side) such that a cross section of the first rail structure  711   a  or  711   b  with respect to an x-z plane may have an arc shape. At least a portion of the hinge structure  720   a  or  720   b  may be inserted into the first rail structure  711   a  or  711   b  in a direction (e.g., an +x-axis direction or a −x-axis direction) perpendicular to a folding axis (e.g., the folding axis A of  FIG.  3   ). For example, a second rail structure  7213  of each of the pair of hinge structures  720   a  and  720   b  may be inserted into the pair of first rail structures  711   a  and  711   b  to interoperate with the pair of first rail structures  711   a  and  711   b . The pair of first rail structures  711   a  and  711   b  may be formed to diagonally face each other. For example, the pair of first rail structures  711   a  and  711   b  may be formed to be point-symmetric with respect to a center of the hinge bracket  710 , when the hinge bracket  710  is viewed from the front side. For example, one first rail structure  711   a  may be formed at a position relatively biased in the −x-axis direction and/or the −y-axis direction, in comparison to the other first rail structure  711   b , and the other first rail structure  711   b  may be formed at a position relatively biased in the +x-axis direction and/or the +y-axis direction, in comparison to the one first rail structure  711   a . The arc shapes of the pair of first rail structures  711   a  and  711   b  may define the pair of hinge axes Ha and Hb. For example, centers of the arc shapes of the pair of first rail structures  711   a  and  711   b  may be defined as a pair of hinge axes Ha and Hb. The pair of hinge axes Ha and Hb may be parallel to the folding axis A. The pair of hinge axes Ha and Hb may be spaced apart from each other by a designated interval. 
     In one embodiment, the intermediate member arrangement space  712  and the intermediate protrusion  715  may be formed near the center of the hinge bracket  710 . The description of the intermediate member arrangement space  412  and the intermediate protrusion  415  of the hinge assembly  400  provided with reference to  FIGS.  4 A through  4 Y  may be similarly applied to the intermediate member arrangement space  712  and the intermediate protrusion  715 , unless otherwise described. 
     In one embodiment, the pair of open spaces  713   a  and  713   b  may be formed on both sides (e.g., the −x side and the +x side) of the hinge bracket  710 . The pair of open spaces  713   a  and  713   b  may be formed on the sides (e.g., the −x side and the +x side) of the hinge bracket  710 , respectively, when the hinge bracket  710  is viewed from the front side. The open space  713   a ,  713   b  may be a space for an arrangement of at least the elastic member  750   a  or  750   b , an extension (e.g., the extension  432   a  or  432   b  of  FIG.  4 H ) of the intermediate member  730 , and the fixing plate  760 . For example, at least a portion of the open space  713   a ,  713   b  may be formed to have a longitudinal direction in the y-axis direction. The pair of open spaces  713   a  and  713   b  may be formed to diagonally face each other. For example, the pair of open spaces  713   a  and  713   b  may be formed to be point-symmetric with respect to the center of the hinge bracket  710 , when the hinge bracket  710  is viewed from the front side. 
     In one embodiment, a first connection projection  7131   a  or  7131   b  may protrude in a direction parallel to the hinge axis Ha or Hb in the open space  713   a  or  713   b . For example, one first connection projection  7131   a  may protrude in the +y-axis direction in one open space  713   a , and the other first connection projection  7131   b  may protrude in the −y-axis direction in the other open space  713   b . The first connection projection  7131   a  or  7131   b  may be inserted into and connected to one end portion (e.g., an end portion facing the −y-axis direction, or an end portion facing the +y-axis direction) of the elastic member  750   a  or  750   b.    
     In one embodiment, a protruding pin  7132   a  or  7132   b  may protrude in a direction parallel to the hinge axis Ha or Hb in the open space  713   a  or  713   b . The protruding pin  7132   a  or  7132   b  may be formed in a longitudinal direction (e.g., the y-axis direction) parallel to the hinge axis Ha or Hb. The protruding pin  7132   a  or  7132   b  may protrude to face the first connection projection  7131   a  or  7131   b . For example, one protruding pin  7132   a  may protrude in the +y-axis direction in one open space  713   a , and the other protruding pin  7132   b  may protrude in the −y-axis direction in the other open space  713   b . The protruding pins  7132   a  or  7132   b  may have an arc-shaped cross section. For example, a cross section of the protruding pin  7132   a  or  7132   b  facing the x-z plane may have an arc shape. The arc shape of the protruding pin  7132   a  or  7132   b  may be an arc having the hinge axis Ha or Hb as a center. However, this is merely an example, and the shape of the protruding pin  7132   a  or  7132   b  is not limited thereto. For example, the protruding pin  7132   a  or  7132   b  may also have a shape of a circular rod. At least one fixing plate  760  and at least one rotation plate  723  may be inserted into the protruding pin  7132   a  or  7132   b.    
     In one embodiment, the hinge bracket  710  may include a pair of bracket fixing holes  714   a  and  714   b  to fix the hinge bracket  710  to a hinge housing (e.g., the hinge housing  313  of  FIG.  3   ). For example, a fastening member (e.g., a screw, a bolt, a pin, and/or a combination fastening structure) may be inserted into each of the pair of bracket fixing holes  714   a  and  714   b . The pair of bracket fixing holes  714   a  and  714   b  may be formed to diagonally face each other. For example, the pair of bracket fixing holes  714   a  and  714   b  may be formed to be point-symmetric with respect to the center of the hinge bracket  710  when the hinge bracket  710  is viewed from the front side. For example, one bracket fixing hole  714   a  may be formed at a position relatively biased in the +x-axis direction and/or −y-axis direction, in comparison to the other bracket fixing hole  714   b , and the other bracket fixing hole  714   b  may be formed at a position relatively biased in the −x-axis direction and/or the +y-axis direction, in comparison to the one bracket fixing hole  714   a . For example, a direction in which the pair of bracket fixing holes  714   a  and  714   b  is arranged may cross a direction in which the pair of first rail structures  711   a  and  711   b  is arranged. 
     In one embodiment, the pair of hinge structures  720   a  and  720   b  may be rotatably connected to the hinge bracket  710 . For example, the pair of hinge structures  720   a  and  720   b  may be rotatably connected to the pair of first rail structures  711   a  and  711   b . The pair of hinge structures  720   a  and  720   b  may be arranged to diagonally face each other. For example, the pair of hinge structures  720   a  and  720   b  may be disposed to be point-symmetric with respect to the center of the hinge assembly  700 , when the hinge assembly  700  is viewed from the front side. 
     In one embodiment, each of the pair of hinge structures  720   a  and  720   b  may include a first part  721 , a second part  722 , a rotation plate  723 , a spacer  724 , and a second alignment pin  725 . 
     In one embodiment, the first part  721  may be rotatably connected to the first rail structure  711   a  or  711   b  of the hinge bracket  710 . The first part  721  may be fastened to a first housing (e.g., the first housing  311  of  FIG.  3   ) or a second housing (e.g., the second housing  312  of  FIG.  3   ). 
     In one embodiment, the first part  721  may include a first body  7211 , a second body  7212 , a second rail structure  7213 , and an alignment pin  7214 . 
     In one embodiment, the first body  7211  may be formed in a plate shape. The first body  7211  may be fixedly connected to the first housing (e.g., the first housing  311  of  FIG.  3   ) or the second housing (e.g., the second housing  312  of  FIG.  3   ). The first body  7211  may be disposed parallel to the front surface (e.g., the surface facing the +z-axis direction based on the state of  FIG.  3   ) of the first housing  311  or the second housing  312 . At least one housing fixing hole  72111  for fixing the hinge structure  720   a  or  720   b  to the first housing  311  or the second housing  312  may be formed in the first body  7211 . For example, the housing fixing hole  72111  may be formed to penetrate the first body  7211  in the z-axis direction. For example, a fastening member (e.g., a screw, a bolt, a pin, and/or a combination fastening structure) may be inserted into the housing fixing hole  72111 . Although two housing fixing holes  72111  are formed as shown in  FIG.  7 F , this is merely an example, and a number of housing fixing holes  72111  is not limited thereto. 
     In one embodiment, the second body  7212  may be formed to extend from at least a portion of one end of the first body  7211 . For example, in  FIG.  7 F , the second body  7212  may be formed to extend in the −x-axis direction from at least a portion of an end of the first body  7211  facing the −x-axis direction. The second body  7212  may be formed integrally with the first body  7211 . At least a portion of a cross section of the second body  7212  may have an arc shape in the direction from the upper surface (e.g., the surface facing the +z-axis direction) to the lower surface (e.g., the surface facing the −z-axis direction). For example, at least a portion of a cross section of the second body  7212  facing the x-z plane may have an arc shape. 
     In one embodiment, the second rail structure  7213  may be formed on a lower side (e.g., a side facing the −z-axis direction) of the second body  7212 . The second rail structure  7213  may be formed to protrude from the second body  7212 . At least a portion of a cross section of the second rail structure  7213  may have an arc shape in the direction from the upper surface (e.g., the surface facing the +z-axis direction) to the lower surface (e.g., the surface facing the −z-axis direction). For example, at least a portion of a cross section of the second rail structure  7213  facing the x-z plane may have an arc shape. The arc shape of the second rail structure  7213  may correspond to the arc shape of the first rail structure  711   a  or  711   b . The hinge structure  720   a  or  720   b  may be connected to the hinge bracket  710  such that the second rail structure  7213  may be inserted into the first rail structure  711   a  or  711   b . The second rail structure  7213  may rotate about the hinge axis Ha or Hb within a designated angle range along the first rail structure  711   a  or  711   b . Based on the above structure, in a state in which the second rail structure  7213  is inserted into the first rail structure  711   a  or  711   b , the hinge structure  720   a  or  720   b  may rotate within a designated angle range with respect to the hinge bracket  710 . For example, the hinge structure  720   a  or  720   b  may rotate in the x-z plane about the hinge axis Ha or Hb formed by the first rail structure  711   a  or  711   b  and the second rail structure  7213 . The first rail structure  711   a  or  711   b  and the second rail structure  7213  may allow only a rotational motion of the hinge structure  720   a  or  720   b  on the x-z plane and may restrict a translational movement and/or a rotational motion of the hinge structure  720   a  or  720   b  in another direction. For example, an outer portion of the first rail structure  711   a  or  711   b  in the radial direction may be formed to be stepped with a relatively large width (e.g., a width in the y-axis direction), in comparison to an inner portion thereof, and an outer portion of the second rail structure  7213  in the radial direction may be formed to be stepped with a relatively large width (e.g., a width in the y-axis direction), in comparison to an inner portion thereof. The pair of first rail structures  711   a  and  711   b  protrude and the second rail structure  7213  is recessed, as illustrated and described above, however, this is merely an example. For example, the pair of first rail structures  711   a  and  711   b  may be recessed, and the second rail structure  7213  may protrude. 
     In one embodiment, a first alignment pin  7214  and a first alignment hole  7215  may be formed in the first body  7211 . The first alignment pin  7214  and the first alignment hole  7215  may be configured to align and connect the second part  722 , the rotation plate  723 , and the spacer  724  to the first part  721 . For example, the first alignment pin  7214  and the first alignment hole  7215  may be formed in an end portion of the first body  7211  facing the −y-axis direction. The first alignment pin  7214  may be formed to protrude from one end portion (e.g., an end portion facing the −y-axis direction) of the first body  7211  in a longitudinal direction (e.g., the −y-axis direction). The first alignment pin  7214  may extend in a direction parallel to the hinge axis Ha or Hb. The first alignment pin  7214  may be inserted into the second part  722 , the rotation plate  723 , and the spacer  724 . The first alignment hole  7215  may be recessed from the one end portion (e.g., the end portion facing the −y-axis direction) of the first body  7211  in an inward direction (e.g., a +y-axis direction). The first alignment hole  7215  may be recessed in a direction parallel to the hinge axis Ha or Hb. The second alignment pin  725  may be inserted into the first alignment hole  7215 . The first alignment pin  7214  and the first alignment hole  7215  may be spaced apart from each other by a designated interval. 
     In one embodiment, the second part  722  may be a component formed separately from the first part  721 . The second part  722  may be connected to one end portion of the first part  721  by the first alignment pin  7214  and the second alignment pin  725 . The second part  722  may be connected to the first body  7211  of the first part  721  to be disposed on an opposite side of the second body  7212 . For example, referring to  FIG.  7 F , the second part  722  may be connected to an end portion of the first part  721  facing the −y-axis direction to be disposed on the opposite side of the second body  7212 . 
     In one embodiment, the second part  722  may include a third body  7221 , a fourth body  7222 , a first cam structure  7223 , a rotation guide groove  7224 , and a protrusion guide  7225 . 
     In one embodiment, the third body  7221  may be connected to the first part  721 . A second alignment hole  72211  may be formed in the third body  7221 . The second alignment hole  72211  may be formed to penetrate the third body  7221 . For example, the second alignment hole  72211  may penetrate the third body  7221  in the y-axis direction. At least one second alignment hole  72211  may be formed. For example, a pair of second alignment holes  72211  may be formed, and the first alignment pin  7214  and the second alignment pin  725  may be inserted into the second alignment holes  72211 , respectively. 
     In one embodiment, the fourth body  7222  may be formed to extend from one end portion of the third body  7221 . For example, the fourth body  7222  may be formed to extend in the −x-axis direction from an end portion of the third body  7221  facing the −x-axis direction, as shown in  FIG.  7 F . The fourth body  7222  may be formed in an arc shape of a predetermined angle. In a state in which the second part  722  is connected to the first part  721 , the fourth body  7222  may be disposed on the opposite side of the second body  7212 . 
     In one embodiment, the first cam structure  7223  may be formed on one surface of the fourth body  7222 . For example, in the state in which the second part  722  is connected to the first part  721 , the first cam structure  7223  may be formed on a surface of the fourth body  7222  facing the second body  7212 . For example, referring to  FIG.  7 D , the first cam structure  7223  may be formed on a surface of the fourth body  7222  facing the −y-axis direction. The first cam structure  7223  may be formed along an arc having the hinge axis Ha or Hb as a center. The first cam structure  7223  may include at least one crest and/or one trough structure. For example, the first cam structure  7223  may protrude to include a first inclined surface, a first flat surface, and a second inclined surface. 
     In one embodiment, the rotation guide groove  7224  may be formed on another surface of the fourth body  7222 . The rotation guide groove  7224  may be formed on a surface opposite to the surface on which the first cam structure  7223  is formed. For example, in the state in which the second part  722  is connected to the first part  721 , the rotation guide groove  7224  may be formed on a surface opposite to a surface of the fourth body  7222  facing the second body  7212 . For example, in  FIG.  7 F , the rotation guide groove  7224  may be formed on a surface of the fourth body  7222  facing the −y-axis direction. The rotation guide groove  7224  may be recessed along an arc having the hinge axis Ha or Hb as a center. An end portion of the protruding pin  7132   a  or  7132   b  of the hinge bracket  710  may be inserted into the rotation guide groove  7224 . When the hinge structure  720   a  or  720   b  is rotated about the hinge axis Ha or Hb, an end portion of the protruding pin  7132   a  or  7132   b  may move along the rotation guide groove  7224 , so that a path of rotation of the second part  722  relative to the protruding pin  7132   a  or  7132   b  may be guided. 
     In one embodiment, the protrusion guide  7225  may be formed to protrude from another surface of the fourth body  7222 . The protrusion guide  7225  may be formed on the same side as a side on which the rotation guide groove  7224  is formed. For example, in  FIG.  7 F , the protrusion guide  7225  may be formed to protrude in the −y-axis direction from a side of the fourth body  7222  facing the −y-axis direction. At least a portion of a cross section of the protrusion guide  7225  may have an arc shape in the direction from the upper surface (e.g., the surface facing the +z-axis direction) to the lower surface (e.g., the surface facing the −z-axis direction). For example, a lower surface (e.g., a surface facing the −z-axis direction) of the protrusion guide  7225  may be formed in a shape of an arc having the hinge axis Ha or Hb as a center. The lower surface (e.g., the surface facing the −z-axis direction) of the protrusion guide  7225  may be seated on a guide seating portion  72322  of the rotation plate  723 , and relative positions of the second part  722  and the rotation plate  723  may be aligned. In addition, the lower surface (e.g., the surface facing the −z-axis direction) of the protrusion guide  7225  may rotate along an arc-shaped upper surface (e.g., a surface facing the +z-axis direction) of the fixing plate  760  while being in contact with the arc-shaped upper surface, and a path of rotation of the second part  722  relative to the fixing plate  760  may be guided. 
     In one embodiment, the rotation plate  723  may be formed in a plate shape. At least one rotation plate  723  may be provided. The rotation plate  723  may be connected to one end portion of the second part  722  by the first alignment pin  7214  and the second alignment pin  725 . For example, the rotation plate  723  may be connected to an end portion of the second part  722  facing the −y-axis direction, as shown in  FIG.  7 F . 
     In one embodiment, the rotation plate  723  may include a first portion  7231  and a second portion  7232 . 
     In one embodiment, the first portion  7231  may correspond to the third body  7221  of the second part  722 . A third alignment hole  72311  may be formed in the first portion  7231 . The third alignment hole  72311  may be formed to penetrate the first portion  7231 . For example, the third alignment hole  72311  may penetrate the first portion  7231  in the y-axis direction. At least one third alignment hole  72311  may be formed. For example, a pair of third alignment holes  72311  may be formed, and the first alignment pin  7214  and the second alignment pin  725  may be inserted into the third alignment holes  72311 , respectively. 
     In one embodiment, the second portion  7232  may correspond to the fourth body  7222  of the second part  722 . The second portion  7232  may be formed to extend from one end portion of the first portion  7231 . For example, the second portion  7232  may be formed to extend in the −x-axis direction from an end portion of the first portion  7231  facing the −x-axis direction, as shown in  FIG.  7 F . The second portion  7232  may be formed in an arc shape of a predetermined angle. 
     In one embodiment, a rotation guide hole  72321  may be formed in the second portion  7232 . The rotation guide hole  72321  may be formed to penetrate the second portion  7232 . For example, the rotation guide hole  72321  may penetrate the second portion  7232  in the y-axis direction. The rotation guide hole  72321  may be formed along an arc having the hinge axis Ha or Hb as a center. The protruding pin  7132   a  or  7132   b  of the hinge bracket  710  may be inserted into the rotation guide hole  72321 . When the hinge structure  720   a  or  720   b  is rotated about the hinge axis Ha or Hb, the protruding pin  7132   a  or  7132   b  may move along the rotation guide hole  72321 , so that a path of rotation of the rotation plate  723  relative to the protruding pin  7132   a  or  7132   b  may be guided. 
     In one embodiment, a guide seating portion  72322  may be formed in the second portion  7232 . The guide seating portion  72322  may be formed by recessing an upper surface (e.g., a surface facing the +z-axis direction) of the second portion  7232 . The guide seating portion  72322  may be formed in a shape of an arc having the hinge axis Ha or Hb as a center. The guide seating portion  72322  may be formed in a shape corresponding to that of the lower surface (e.g., the surface facing the −z-axis direction) of the protruding guide  7225 . The protrusion guide  7225  of the second part  722  may be seated on the guide seating portion  72322 . The lower surface (e.g., the surface facing the −z-axis direction) of the protrusion guide  7225  may be seated on the guide seating portion  7232  of the rotation plate  723 , and relative positions of the second part  722  and the rotation plate  723  may be aligned. 
     In one embodiment, the spacer  724  may correspond to the first portion  7231  of the rotation plate  723 . The spacer  724  may be formed in a plate shape. A fourth alignment hole  7241  may be formed in the spacer  724 . The fourth alignment hole  7241  may be formed to penetrate the spacer  724 . For example, the fourth alignment hole  7241  may penetrate the spacer  724  in the y-axis direction. At least one fourth alignment hole  7241  may be formed. For example, a pair of fourth alignment holes  7241  may be formed, and the first alignment pin  7214  and the second alignment pin  725  may be inserted into the fourth alignment holes  7241 , respectively. 
     In one embodiment, at least one spacer  724  may be provided. The rotation plate  723  and the spacer  724  may be alternately connected to one side of the second part  722 . For example, the rotation plate  723  and the spacer  724  may be alternately disposed. For example, as shown in  FIG.  7 F , three rotation plates  723  and two spacers  724  may be alternately disposed. However, this is merely an example, and a number of rotation plates  723  and a number of spacers  724  are not limited thereto. Since an area of a rotation plate  723  is greater than an area of a spacer  724 , a gap with a predetermined width may be formed between a plurality of rotation plates  723  when the rotation plates  723  and the spacers  724  are alternately disposed. 
     In one embodiment, the second alignment pin  725  may align positions of the first part  721 , the second part  722 , the rotation plate  723 , and the spacer  724  and connect the first part  721 , the second part  722 , the rotation plate  723 , and the spacer  724  to each other. The second alignment pin  725  may pass through the first alignment hole  7215  of the first part  721 , the second alignment hole  72211  of the second part  722 , the third alignment hole  72311  of the rotation plate  723 , and the fourth alignment hole  7241  of the spacer  724 . For example, an end portion of the second alignment pin  725  may be fixedly connected to the first alignment hole  7215  of the first part  721 . For example, a screw thread may be formed in the end portion of the second alignment pin  725 , and a screw thread corresponding to the formed screw thread may be formed in the first alignment hole  7215  of the first part  721 . Based on the above structure, the first part  721 , the second part  722 , the rotation plate  723 , and the spacer  724  may be fixedly connected to each other by the second alignment pin  725 . 
     In one embodiment, the first alignment pin  7214  and the second alignment pin  725  may align positions of the first part  721 , the second part  722 , the rotation plate  723 , and the spacer  724  and connect the first part  721 , the second part  722 , the rotation plate  723 , and the spacer  724  to each other. The first alignment pin  7214  and the second alignment pin  725  may have a longitudinal direction (e.g., a y-axis direction) parallel to the hinge axis Ha or Hb. The first alignment pin  7214  and the second alignment pin  725  may be spaced apart from each other, to limit translational movements in the x-axis direction and the z-axis direction between the first part  721 , the second part  722 , the rotation plate  723 , and the spacer  724 , and limit rotational movements in all directions. The first alignment pin  7214  is formed integrally with the first part  721 , as illustrated and described above, however, this is merely an example. For example, the first alignment pin  7214  may also be provided as a separate component such as the second alignment pin  725 . 
     In one embodiment, the fixing plate  760  may be formed in a plate shape. At least one fixing plate  760  may be provided. A fixing hole  761  may be formed in the fixing plate  760 . For example, the fixing hole  761  may be formed to penetrate the fixing plate  760 . For example, the fixing hole  761  may penetrate the fixing plate  760  in the y-axis direction. The fixing hole  761  may have an arc shape corresponding to the protruding pin  7213   a  or  7213   b  of the hinge bracket  710 . Since the protruding pin  7213   a  or  7213   b  is inserted into the fixing hole  761  of the fixing plate  760 , the fixing plate  760  may be connected to the protruding pin  7213   a  or  7213   b . An insertion structure of the fixing hole  761  and the protruding pin  7213   a  or  7213   b  may limit translational movements of the fixing plate  760  in the x-axis direction and the z-axis direction with respect to the hinge bracket  710 , and rotational movements in all directions. At least a portion of an upper surface (e.g., a surface facing the +z-axis direction) of the fixing plate  760  may be formed in an arc shape. For example, the upper surface (e.g., the surface facing the +z-axis direction) of the fixing plate  760  may be formed in a shape of an arc having the hinge axis Ha or Hb as a center. The upper surface (e.g., the surface facing the +z-axis direction) of the fixing plate  760  may have a shape corresponding to that of the lower surface (e.g., the surface facing the −z-axis direction) of the protrusion guide  7225 . The protrusion guide  7225  may be seated on the upper surface (e.g., the surface facing the +z-axis direction) of the fixing plate  760 . When the second part  722  is rotated about the hinge axis Ha or Hb, the lower surface (e.g., the surface facing the −z-axis direction) of the protrusion guide  7225  may rotate along the arc-shaped upper surface (e.g., the surface facing the +z-axis direction) of the fixing plate  760  while being in contact with the upper surface of the fixing plate  760 , and a path of rotation of the second part  722  relative to the fixing plate  760  may be guided. 
     In one embodiment, the fixing plate  760  may be alternately disposed with the rotation plate  723 . For example, the fixing plate  760  and the rotation plate  723  may be alternately disposed in the y-axis direction. A thickness of the fixing plate  760  may substantially correspond to a thickness of the spacer  724 . For example, as shown in  FIG.  7 A , three fixing plates  760  and three rotation plates  723  may be alternately disposed in the y-axis direction. However, this is merely an example, and a number of fixing plates  760  and a number of rotation plates  723  are not limited thereto. The fixing plate  760  and the rotation plate  723  that are alternately disposed may be in surface contact with each other in at least some areas. An elastic force provided by the elastic member  750   a  or  750   b  may be perpendicular to a surface of each of the fixing plate  760  and the rotation plate  723 . Accordingly, the elastic force of the elastic member  750   a  or  750   b  may act as a normal force to generate a friction force between the fixing plate  760  and the rotation plate  723 . A direction in which the fixing plate  760  and the rotation plate  723  are inserted into the protruding pin  7132   a  or  7132   b  may coincide with a direction of the elastic force of the elastic member  750   a  or  750   b . For example, the fixing plate  760  and the rotation plate  723  may be inserted in the y-axis direction into the protruding pin  7132   a  or  7132   b  protruding in the y-axis direction, and the elastic force of the elastic member  750   a  or  750   b  may be directed in the y-axis direction. Based on the above structure, the fixing plate  760  and the rotation plate  723  may be in close contact with each other in the y-axis direction by the elastic force of the elastic member  750   a  or  750   b . Accordingly, a large friction force may be generated between the fixing plate  760  and the rotation plate  723 . For example, as shown in  FIGS.  7 G through  7 I , when the hinge structure  720   a  or  720   b  is rotated about the hinge axis Ha or Hb, the rotation plate  723  may be relatively rotated while being in surface contact with the fixing plate  760 , and a large friction force may be generated between the rotation plate  723  and the fixing plate  760 . As a result, the friction force acting between the rotation plate  723  and the fixing plate  760  may further increase an open detent force, an intermediate state stopping force, and/or a close detent force. 
       FIG.  8    is a rear view schematically illustrating a hinge assembly according to one embodiment. 
     Referring to  FIG.  8   , in one embodiment, hinge structures  820   a  or  820   b  may be formed to be symmetrical with each other about the x-axis. For example, the hinge structure  820   a  or  820   b  may include a first body  821   a  or  821   b , a second body  822   a  or  822   b  extending from one side (e.g., a side facing the −y-axis direction or +y-axis direction) of the first body  821   a  or  821   b , and a third body  825   a  or  825   b  extending from another side (e.g., a side facing the +y-axis direction or −y-axis direction) of the first body  821   a  or  821   b . A first cam structure  824   a  or  824   b  may be formed in the second body  822   a  or  822   b , and a third cam structure  826   a  or  826   b  may be formed in the third body  825   a  or  825   b.    
     In one embodiment, a pair of intermediate members  830  may be provided. For example, the intermediate members  830  may include a first intermediate member  830   a  and a second intermediate member  830   b . The first intermediate member  830   a  and the second intermediate member  830   b  may be disposed to cross each other. For example, the first intermediate member  830   a  and the second intermediate member  830   b  may share the same middle axis M, but may be respectively disposed in diagonal directions crossing each other. For example, the first cam structure  824   a  of one hinge structure  820   a  may interoperate with one second cam structure  834   a _ a  of the first intermediate member  830   a , and the third cam structure  826   a  of the one hinge structure  820   a  may interoperate with one second cam structure  834   a _ b  of the second intermediate member  830   b . For example, the first cam structure  824   b  of the other hinge structure  820   b  may interoperate with another second cam structure  834   b _ a  of the first intermediate member  830   a , and the third cam structure  826   b  of the other hinge structure  820   b  may interoperate with another second cam structure  834   b _ b  of the second intermediate member  830   b . A hinge assembly  800  of  FIG.  8    may operate in substantially the same manner as the hinge assembly  400  described with reference to  FIGS.  4 A through  4 Y . The hinge assembly  800  of  FIG.  8    may generate a detent force greater than that of the hinge assembly  400  of  FIGS.  4 A through  4 Y . 
       FIG.  9 A  is a front view illustrating a hinge assembly according to one embodiment.  FIG.  9 B  is a perspective view illustrating a process of connecting a first intermediate member to a hinge bracket according to one embodiment.  FIG.  9 C  is a perspective view illustrating a process of connecting a second intermediate member to a hinge bracket according to one embodiment.  FIG.  9 D  is a perspective view illustrating a state in which a first intermediate member and a second intermediate member are connected to a hinge bracket according to one embodiment.  FIG.  9 E  is a cross-sectional view taken along line C-C of  FIG.  9 A .  FIG.  9 F  is a cross-sectional view taken along line D-D of  FIG.  9 A . 
     Referring to  FIGS.  9 A through  9 F , a hinge assembly  900  according to one embodiment may include a hinge bracket  910 , two pairs of hinge structures  920   a ,  920   b ,  920   c , and  920   d , a pair of intermediate members  930   a  and  930   b , and a pair of elastic members  950   a  and  950   b.    
     In one embodiment, the hinge bracket  910  may include two pairs of protruding pins  9132   a ,  9132   b ,  9132   c , and  9132   d . The two pairs of hinge structures  920   a ,  920   b ,  920   c , and  920   d  may be rotatably connected to the two pairs of protruding pins  9132   a ,  9132   b ,  9132   c , and  9132   d , respectively. For example, two hinge structures  920   a  and  920   c  may be rotatable about a first hinge axis Ha, and the other two hinge structures  920   b  and  920   d  may be rotatable about a second hinge axis Hb. However, this is merely an example, and the hinge bracket  910  may include two pairs of rail structures (e.g., the first rail structures  711   a  and  711   b  of  FIG.  7 E ), and the two pairs of hinge structures  920   a ,  920   b ,  920   c , and  920   d  may also be rotatably connected to the two pairs of rail structures. 
     In one embodiment, an intermediate protrusion  915  may be formed near a center of the hinge bracket  910 . The description of the intermediate protrusion  415  of the hinge assembly  400  provided with reference to  FIGS.  4 A through  4 Y  may be similarly applied to the intermediate member protrusion  915 , unless otherwise described. 
     In one embodiment, the pair of intermediate members  930   a  and  930   b  may include central portions  931   a  and  931   b  and extensions  932   a  and  932   b , respectively. Through-holes  9311   a  and  9311   b  may be formed in the central portions  931   a  and  931   b , respectively. The description of the through-hole  4311  of the hinge assembly  400  provided with reference to  FIGS.  4 A through  4 Y  may be similarly applied to the through-holes  9311   a  and  9311   b , unless otherwise described. 
     In one embodiment, the pair of intermediate members  930   a  and  930   b  may be rotatably connected to the intermediate protrusion  915 . For example, the pair of intermediate members  930   a  and  930   b  may be connected to the intermediate protrusion  915  and overlap each other. For example, the first intermediate member  930   a  may be disposed below (e.g., in the −z direction) the second intermediate member  930   b , and the second intermediate member  930   b  may be disposed above (e.g., in the +z direction) the first intermediate member  930   a . For example, the first intermediate member  930   a  may first be connected to the intermediate protrusion  915 , and the second intermediate member  930   b  may be connected to the intermediate protrusion  915  above (e.g., in the +z direction) the first intermediate member  930   a . For example, the pair of intermediate members  930   a  and  930   b  may be arranged such that longitudinal directions thereof may cross each other with respect to the middle axis M. For example, the pair of intermediate members  930   a  and  930   b  may cross to form an X-shape about the middle axis M. 
     In one embodiment, the first intermediate member  930   a  may be inserted into the intermediate protrusion  915  in a state in which the through-hole  9311   a  is aligned with a head  9152 . For example, when the first intermediate member  930   a  is inserted into the intermediate protrusion  915 , the first intermediate member  930   a  may be rotated in one direction (e.g., a counterclockwise direction) with respect to the intermediate protrusion  915  such that the through-hole  9311   a  may be out of alignment with the head  9152 . The second intermediate member  930   b  may be inserted into the intermediate protrusion  915  in a state in which the through-hole  9311   b  is aligned with the head  9152 . For example, when the second intermediate member  930   b  is inserted into the intermediate protrusion  915 , the second intermediate member  930   b  may be rotated in one direction (e.g., a counterclockwise direction) with respect to the intermediate protrusion  915  such that the through-hole  9311   b  may be out of alignment with the head  9152 . Based on the above configuration, the pair of intermediate members  930   a  and  930   b  may be prevented from being separated from the intermediate protrusion  915 . However, this is merely an example, and a direction in which the pair of intermediate members  930   a  and  930   b  are rotated to prevent the pair of intermediate members  930   a  and  930   b  from being separated from the intermediate protrusion  915  is not limited thereto. 
     In one embodiment, in the first intermediate member  930   a , an upper surface  9312   a  (e.g., a surface facing the +z direction) of the central portion  931   a  may be formed to be stepped below (e.g., in the −z direction) an upper surface  9321   a  (e.g., a surface facing the +z direction) of the extension  932   a . In the second intermediate member  930   b , a lower surface  9314   b  (e.g., a surface facing the −z direction) of the central portion  931   b  may be formed to be stepped above (e.g., in the +z direction) a lower surface  9322   b  (e.g., a surface facing the −z direction) of the extension  932   b . For example, a sum of a height (e.g., a height in the z direction) of the central portion  931   a  of the first intermediate member  930   a  and a height (e.g., a height in the z direction) of the central portion  931   b  of the second intermediate member  930   b  may substantially correspond to a height (e.g., a height in the z direction to a bottom surface of a projection  91521  of a head  9152 ) of a protrusion base  9151 . For example, each of the height (e.g., the height in the z direction) of the central portion  931   a  of the first intermediate member  930   a  and the height (e.g., the height in the z direction) of the central portion  931   b  of the second intermediate member  930   b  may be substantially half the height of the protrusion base  9151 . Based on the above configuration, when the pair of intermediate members  930   a  and  930   b  are inserted into the intermediate protrusion  915 , the head  9152  of the intermediate protrusion  915  may be exposed to an upper side (e.g., a side facing the +z direction) of the pair of intermediate members  930   a  and  930   b  by passing through the through-holes  9311   a  and  9311   b . In addition, based on the above configuration, the extension  932   a  of the first intermediate member  930   a  and the extension  932   b  of the second intermediate member  930   b  may be positioned at substantially the same height (e.g., the height in the z direction), and accordingly the elastic member  950   a  or  950   b  may be disposed between the pair of intermediate members  930   a  and  930   b.    
     In one embodiment, the pair of elastic members  950   a  and  950   b  may be arranged between both end portions of the pair of intermediate members  930   a  and  930   b . For example, the first elastic member  950   a  may be positioned between one end portion of the first intermediate members  930   a  and one end portion of the second intermediate members  930   b , and the second elastic member  950   b  may be positioned between the other end portion of the first intermediate members  930   a  and the other end portion of the second intermediate members  930   b . For example, the pair of elastic members  950   a  and  950   b  may generate an elastic force to rotate the first intermediate member  930   a  in one direction (e.g., a counterclockwise direction), and generate an elastic force to rotate the second intermediate member  930   b  in another direction (e.g., a clockwise direction). 
     The hinge assembly  900  of  FIGS.  9 A through  9 F  may operate in substantially the same manner as the hinge assembly  800  described above with reference to  FIG.  8   . 
     According to one embodiment, an electronic device  300  may include a display  250  including a first area  251 , a second area  252 , and a folding area  253  between the first area  251  and the second area  252 , a first housing  311  configured to support the first area  251 , a second housing  312  configured to support the second area  252 , and a hinge assembly  400  configured to connect the first housing  311  and the second housing  312 , and having a pair of hinge axes Ha and Hb. The hinge assembly  400  may include a hinge bracket  410  including an intermediate protrusion  415  formed to protrude in a direction of a middle axis M perpendicular to the pair of hinge axes Ha and Hb, a pair of hinge structures  420   a  and  420   b  connected to the hinge bracket  410  to be rotatable about the pair of hinge axes Ha and Hb, and an intermediate member  430  including a through-hole  4311  into which the intermediate protrusion  415  is inserted, to be rotatable about the middle axis M with respect to the hinge bracket  410 . The intermediate protrusion  415  may include a protrusion base  4151  having a first radius R 1 , and a head  4152  including a projection  41521  which is formed on the protrusion base  4151  and which has a second radius R 2  greater than the first radius R 1 . The through-hole  4311  may have a shape corresponding to a shape of the head  4152 . 
     In one embodiment, the through-hole  4311  may include a main hole  43111  having the first radius R 1 , and a recessed portion  43112  recessed radially from the main hole  43111  to have the second radius R 2 . 
     In one embodiment, the intermediate member  430  may be inserted into the intermediate protrusion  415  in a state in which the head  4152  and the through-hole  4311  are aligned such that the shape of the head  4152  and the shape of the through-hole  4311  correspond to each other. 
     In one embodiment, when the intermediate member  430  is inserted into the intermediate protrusion  415 , the intermediate member  430  may be rotated with respect to the intermediate protrusion  415  such that the head  4152  and the through-hole  4311  may be out of alignment with each other. 
     In one embodiment, the head  4152  and the through-hole  4311  may be out of alignment with each other in a state in which the intermediate member  430  is inserted into the intermediate protrusion  415 , to prevent the intermediate member  430  from being separated from the intermediate protrusion  415  in a direction of the middle axis M. 
     In one embodiment, a number of projections  41521  and a number of recessed portions  43112  may be equal. 
     In one embodiment, a pair of projections  41521  may be formed to protrude in directions opposite to each other, and a pair of recessed portions  43112  may be formed to protrude in directions opposite to each other. 
     In one embodiment, the intermediate member  430  may include a central portion  431  in which the through-hole  4311  is formed and which is rotatably connected to the hinge bracket  410 , and a pair of extensions  432   a  and  432   b  which extend from the central portion  431  to both sides. 
     In one embodiment, an upper surface of the central portion  431  may be formed to be stepped below an upper surface of the extension  432   a ,  432   b.    
     In one embodiment, the intermediate protrusion  415  may further include a groove  4153  recessed in at least a portion of an outer surface of the protrusion base  4151 . 
     In one embodiment, the groove  4153  may be formed along a circumference of the protrusion base  4151 . 
     In one embodiment, a plurality of grooves  4153  may be formed. The plurality of grooves  4153  may be formed to be spaced apart from each other in the direction of the middle axis M in the protrusion base  4151 . 
     In one embodiment, a lubricant may be disposed in the groove  4153  to reduce a rotational frictional resistance of the intermediate member  430  with respect to the intermediate protrusion  415 . 
     In one embodiment, a first cam structure  424  may be formed on each of the pair of hinge structures  420   a  and  420   b , and a second cam structure  434   a ,  434   b  interoperating with the first cam structure  424  may be formed in each of both end portions of the intermediate member  430 . 
     In one embodiment, the intermediate member  430  may further include an elastic member  450   a ,  450   b  configured to provide an elastic force to the intermediate member  430  in a direction in which the intermediate member  430  is rotated in one direction about the middle axis M. 
     According to various embodiments, the electronic device  300  may include a display  250  including a first area  251 , a second area  252 , and a folding area  253  between the first area  251  and the second area  252 , a first housing  311  configured to support the first area  251 , a second housing  312  configured to support the second area  252 , a hinge assembly  900  configured to connect the first housing  311  and the second housing  312 , and having a pair of hinge axes Ha and Hb. The hinge assembly  900  may include a hinge bracket  910  including an intermediate protrusion  915  formed to protrude in a direction of a middle axis M perpendicular to the pair of hinge axes Ha and Hb, two pairs of hinge structures  920   a ,  920   b ,  920   c , and  920   d  connected to the hinge bracket  910  to be rotatable about the pair of hinge axes Ha and Hb, a pair of intermediate members  930   a  and  930   b  connected to the intermediate protrusion  915  and overlapping each other, to be rotatable about the middle axis M with respect to the hinge bracket  910 , respectively, each of the pair of intermediate members  930   a  and  930   b  including a through-hole  9311   a  or  9311   b  into which the intermediate protrusion  915  is inserted. The intermediate protrusion  915  may include a protrusion base  9151  having a first radius R 1 , and a head  9152  including a projection  91521  which is formed on the protrusion base  9151  and which has a second radius R 2  greater than the first radius R 1 . The through-hole  9311   a  or  9311   b  may have a shape corresponding to a shape of the head  9152 . 
     In one embodiment, each of the pair of intermediate members  930   a  and  930   b  may include a central portion  931   a  or  931   b  in which the through-hole  9311   a  or  9311   b  is formed and which is rotatably connected to the hinge bracket  910 , and a pair of extensions  932   a  or  932   b  which extend from the central portion  931   a  or  931   b  to both sides of each of the intermediate members  930   a  and  930   b.    
     In one embodiment, the pair of intermediate members  930   a  and  930   b  may be arranged such that longitudinal directions of the intermediate members  930   a  and  930   b  cross each other with respect to the middle axis M. 
     In one embodiment, the first intermediate member  930   a  disposed on a relatively lower side of the pair of intermediate members  930   a  and  930   b  may be formed such that an upper surface of the central portion  931   a  of the first intermediate member  930   a  is stepped below an upper surface of the extension  932   a  of the first intermediate member  930   a , and the second intermediate member  930   b  disposed on a relatively upper side of the pair of intermediate members  930   a  and  930   b  may be formed such that a lower surface of the central portion  931   b  of the second intermediate member  930   b  is stepped above a lower surface of the extension  932   b  of the second intermediate member  930   b.    
     In one embodiment, a pair of elastic members  950   a  and  950   b  disposed between both ends of the pair of intermediate members  930   a  and  930   b  may be further included.