Abstract:
A refrigerator is disclosed. The refrigerator includes a storage compartment, and a shelf assembly provided in the storage compartment to be vertically adjustable, wherein the shelf assembly includes a support unit provided to be movable obliquely, and a shelf unit which is mounted on the support unit to move up and down according to an oblique movement of the support unit. The refrigerator has a simple structure capable of lifting a shelf up and down, thereby facilitating the manufacture and reducing manufacturing costs.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of Korean Patent Application No. 2008-0055157, filed on Jun. 12, 2008 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a refrigerator, and, more particularly, to a vertically adjustable refrigerator. 
     2. Description of the Related Art 
     Generally, a plurality of shelves are installed in a storage compartment of a refrigerator. The shelves, spaced from each other in a vertical direction, divide an inside of the storage compartment in multiple steps to increase space utilization efficiency and allow articles to be easily stored therein. 
     Among conventional shelves for refrigerators, there is a shelf vertically adjustable by a user. An adjustable shelf disclosed in U.S. Pat. No. 5,913,584 includes a mounting assembly fixed to the rear surface of a storage compartment of a refrigerator, a shelf support assembly coupled to the mounting assembly to be vertically movable, a shelf coupled to the shelf support assembly, and a driving unit which lifts the shelf support assembly up and down. 
     The above-mentioned driving unit includes a worm driving gear which is rotated by an operation of the user, a driven gear which is rotated by the driving gear, a gear shaft which is rotated by the driven gear, two spur gears coupled to the opposite ends of the gear shaft, and a gear box which rotatably supports the driving gear and the gear shaft. The gear box is fixed to the shelf support assembly. The driving unit further includes a gear bracket fixed to the mounting assembly and two rack gears installed at the opposite sides of the gear bracket to engage with two spur gears coupled to the opposite ends of the gear shaft. In the driving unit, when the user rotates a rotation knob connected to the driving gear through an operational rod, the two spur gears coupled to the gear shaft are rotated. As a result, the shelf support assembly is lifted up and down. 
     In the conventional adjustable shelf, however, there are problems of reducing productivity and increasing the manufacturing costs because the elevation unit, which lifts the shelf support member up and down, has a complicated configuration. 
     Further, the conventional adjustable shelf is applicable only to a track type shelf fixing structure formed on the rear surface of the refrigerator. Accordingly, it is difficult to apply the conventional adjustable shelf to a refrigerator having no track. 
     The conventional adjustable shelf is mounted to the track provided at the rear surface of the refrigerator. That is, the conventional adjustable shelf is cantilevered at one end thereof. For this reason, the shelf should be manufactured of a material having sufficient stiffness, for example, steel, in order to support load applied to the shelf. 
     Further, since the rear surface of the conventional adjustable shelf is mounted to the track provided at the rear surface of the refrigerator, a shelf frame should be manufactured of a material having sufficient stiffness, for example, steel, in order to support load applied to the shelf. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in order to solve the above problems. It is an aspect of the invention to provide a refrigerator having a simple structure capable of lifting a shelf up and down, thereby facilitating the manufacture and reducing manufacturing costs. 
     Further, it is another aspect of the invention to provide a refrigerator having a shelf driving unit applicable to a refrigerator having no track. 
     Further, it is a further aspect of the invention to provide a refrigerator having a support performance required for a shelf even though the shelf is manufactured using an inexpensive material having insufficient stiffness. 
     Additional aspects and/or advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention. 
     In accordance with an aspect of the invention, there is provided a refrigerator comprising: a storage compartment; and a shelf assembly provided in the storage compartment to be vertically adjustable, wherein the shelf assembly includes a support unit provided to be movable obliquely, and a shelf unit which is mounted on the support unit to move up and down according to an oblique movement of the support unit. 
     Preferably, the shelf assembly further includes a driving unit to drive the support unit. 
     Preferably, the driving unit is a pressing/fixing unit including a plurality of engaging portions provided in the shelf unit, and a stopper provided in the support unit such that the stopper is selectively coupled to and separated from any one of the engaging portions. 
     Preferably, the pressing/fixing unit further includes a pressing portion which moves the stopper forward and backward, and the stopper interlocking with the pressing portion is separated from the engaging portions when a certain external force is applied to the pressing portion. 
     Preferably, the driving unit is a rotation driving unit including a threaded portion, and the rotation driving unit allows the support unit to move forward and backward along inclined portions of guide units by a rotation of the threaded portion. 
     Preferably, the rotation driving unit further includes a knob which is grasped by a user to rotate the threaded portion. 
     Preferably, the refrigerator further includes guide units having inclined portions which guide the oblique movement of the support unit. 
     Preferably, the guide units include a pair of guide units fixed to opposite side surfaces of the storage compartment. 
     Preferably, the shelf unit further includes extended portions, and the guide units further include guide portions which receive the extended portions to guide an elevation of the shelf unit. 
     Preferably, the guide units having the inclined portions which guide the oblique movement of the support unit are formed integrally with opposite sidewalls of the storage compartment. 
     Preferably, the inclined portions are arranged parallel to each other. 
     Preferably, the support unit includes a frame which supports the shelf unit and legs which are extended from the frame and supported by the inclined portions. 
     Preferably, the legs move obliquely along the inclined portions. 
     Preferably, the legs further include rollers to be smoothly movable along the inclined portions. 
     Preferably, the shelf unit is vertically lifted up according to an upward oblique movement of the support unit, and is vertically lifted down according to a downward oblique movement of the support unit. 
     Preferably, the support unit moves forward and backward with respect to the shelf unit. 
     In accordance with another aspect of the invention, there is provided a refrigerator comprising: a storage compartment; and a shelf assembly provided in the storage compartment, wherein the shelf assembly includes guide units provided at opposite sidewalls of the storage compartment and having inclined portions, a support unit moving along the inclined portions, and a shelf unit which is coupled to the support unit to move up and down according to a movement of the support unit. 
     Preferably, the shelf assembly further includes a pressing/fixing unit having a plurality of engaging portions provided in the shelf unit, a stopper provided in the support unit and selectively coupled to and separated from any one of the engaging portions, and a pressing portion which moves the stopper forward and backward. 
     Preferably, the shelf assembly further includes a rotation driving unit which allows the support unit to move forward and backward along the inclined portions of the guide units by a rotation of a threaded portion. 
     In accordance with a further aspect of the invention, there is provided a refrigerator comprising: a storage compartment; and a shelf assembly provided in the storage compartment to be vertically adjustable, wherein the shelf assembly includes a shelf unit, a support unit provided to move forward and backward with respect to the shelf unit and move up and down, a driving unit which drives the support unit, and guide units which guide an oblique movement of the support unit. 
     The refrigerator according to the present invention has a simple structure capable of lifting a shelf up and down, thereby facilitating the manufacture and reducing manufacturing costs. 
     Further, the refrigerator according to the present invention provides a shelf driving unit applicable to a refrigerator having no track. 
     Further, the refrigerator according to the present invention offers a support performance required for a shelf even though the shelf is manufactured using an inexpensive material having insufficient stiffness. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other aspects and advantages of the exemplary embodiments of the invention will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings, of which: 
         FIG. 1  illustrates a perspective view of a refrigerator having a shelf assembly according to a first embodiment of the present invention; 
         FIG. 2  illustrates a perspective view of the shelf assembly of  FIG. 1 ; 
         FIG. 3  illustrates an exploded perspective view of the shelf assembly of  FIG. 2 ; 
         FIG. 4  illustrates a plan view of the guide unit of  FIG. 3  seen in an I direction; 
         FIGS. 5 and 6  illustrate plan views showing a configuration and an operation of the pressing/fixing unit of  FIG. 3 ; 
         FIGS. 7 and 8  illustrate side views showing an operation of the shelf assembly of  FIG. 1 ; 
         FIG. 9  illustrates a perspective view of a shelf assembly according to a second embodiment of the present invention; 
         FIG. 10  illustrates an exploded perspective view of a shelf assembly according to a third embodiment of the present invention; and 
         FIGS. 11 and 12  illustrate side views showing an operation of the shelf assembly of  FIG. 10 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to exemplary embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. The embodiments are described below to explain the present invention by referring to the figures. 
     Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. 
       FIG. 1  illustrates a perspective view of a refrigerator having a shelf assembly according to a first embodiment of the present invention.  FIG. 2  illustrates a perspective view of the shelf assembly of  FIG. 1 . 
     As shown in  FIGS. 1 and 2 , a refrigerator  10  according to the first embodiment of the present invention includes a storage compartment  20  and a shelf assembly  30  disposed in the storage compartment to be vertically adjustable. 
     The storage compartment  20  is divided into a freezing compartment  24  and a cooling compartment  26  by a vertical partition wall  12 . The freezing compartment  24  and the cooling compartment  26  are opened and closed by a freezing compartment door  25  and a cooling compartment door  27 , respectively. The storage compartment  20  is provided with a number of shelves. The shelves partition the inner space of the storage compartment  20  in a vertical direction such that the inner space can be efficiently used. The shelves may include a fixed shelf  29  fixed to an inner wall  28  of the storage compartment  20  and the shelf assembly  30  which is vertically adjustable. 
     The shelf assembly  30  is vertically adjustable according to the size of an article stored in the storage compartment  20 . That is, if an article to be stored under the shelf assembly  30  has a large volume, the shelf assembly  30  can be moved upward. On the other hand, if an article to be stored under the shelf assembly  30  has a small volume, the shelf assembly  30  can be moved downward such that the space above the shelf assembly  30  can be efficiently used. In a case of the fixed shelf  29 , if a stored article is large, the fixed shelf  29  is completely extracted forward and removed such that the inner space of the storage compartment  20  can be efficiently used. On the other hand, in a case of the shelf assembly  30 , the shelf assembly  30  can be variously adjusted in vertical position only with a simple operation without being separated from the supercooling compartment  20 . The shelf assembly  30  is engagedly coupled to protruding parts  21  provided at opposite sides of the inner wall  28  of the storage compartment  20 . The protruding parts  21  are coupled to engaging parts  61  provided at outer side surfaces  63  of guide units  60  of the shelf assembly  30 . The shelf assembly  30  is firmly fixed to the inner wall  28  of the storage compartment  20  as the engaging parts  61  are slidably coupled to the protruding parts  21  provided at four positions, respectively. Hereinafter, the shelf assembly  30  according to the first embodiment of the present invention will be described in detail with reference to  FIGS. 3 and 4 . 
       FIG. 3  illustrates an exploded perspective view of the shelf assembly of  FIG. 2 , and  FIG. 4  illustrates a plan view of the guide unit of  FIG. 3  seen in an I direction. 
     As shown in the drawings, the shelf assembly  30  according to the first embodiment of the present invention includes a support unit  40  disposed to be movable obliquely, a shelf unit  50  which is mounted on the support unit  40  to move up and down according to an oblique movement of the support unit  40 , the guide units  60  which guide the oblique movement of the support unit  40 , guide bars  70  which guide a forward and backward movement of the support unit  40 , and a pressing/fixing unit  80  which fixes the support unit  40  at a desired position in the oblique movement of the support unit  40 . 
     The support unit  40  moves up and down while moving in forward and backward directions along inclined portions  62  provided at the guide units  60 . Accordingly, the support unit  40  moves in oblique directions. That is, when the support unit  40  moves forward, the support unit  40  moves upward along the inclined portions  62  simultaneously with the forward movement. When the support unit  40  moves backward, the support unit  40  moves downward along the inclined portions  62  simultaneously with the backward movement. When the support unit  40  moves up and down, the shelf unit  50  also moves up and down because the shelf unit  50  is placed on the support unit  40 . However, even when the support unit  40  moves forward and backward, the shelf unit  50  does not move forward and backward. This is so because the forward and backward movement of extended portions  52  of the shelf unit  50  is restricted by front and rear wall surfaces  66  and  67  of guide portions  64  disposed on the guide units  60 . The detailed description thereof will be given later. The support unit  40  includes a frame  42  which supports the shelf unit  50 , legs  44  extended from the frame  42 , and rollers  46  disposed at ends of the legs  44 . 
     The frame  42  is a skeleton forming an upper portion of the support unit  40 . The frame  42  includes reinforcement ribs  41  to support the weight of the shelf unit  50  mounted on the frame  42  and stored articles to be placed on the shelf unit  50 . 
     The legs  44  are extended downward at four positions of the frame  42 , respectively. The legs  44  distribute load applied to the frame  42  and transfer the load to the rollers  46  disposed at lower portions of the legs  44 . 
     The rollers  46  are in direct contact with the inclined portions  62 . When the user moves the support unit  40  forward or backward, the support unit  40  smoothly moves forward and backward along the inclined portions  62  because the rollers  46  are provided at the ends of the legs  44  to perform a rolling motion. 
     The shelf unit  50  is coupled to an upper portion of the support unit  40  to allow the stored articles to be placed thereon. The shelf unit  50  has a flat upper surface such that the stored articles can be easily placed thereon. The shelf unit  50  includes a transparent or semitransparent shelf member  54  and a frame  55  that may be formed integrally with or separately from the shelf member  54  to be coupled with the shelf member  54 . The frame  55  may be formed by injection molding a resin material. As described above, the extended portions  52  are also included in the shelf unit  50 . 
     The extended portions  52  are formed to be bent downward from the shelf unit  50 . The extended portions  52  are inserted into the guide portions  64  disposed inside the guide units  60 . Evading parts  53  are formed at positions of the extended portions  52  corresponding to the engaging parts  61  of the guide units  60 . Since the evading parts  53  are provided, the extended portions  52  can be inserted into the guide portions  64  even though the engaging parts  61  are formed. Front and rear surfaces  56  and  57  of the extended portions  52  may be in contact with the front and rear wall surfaces  66  and  67  inside the guide units  60  to restrict the forward and backward movement of the shelf unit  50 . Accordingly in this embodiment, even when the support unit  40  moves forward and backward while moving vertically, the shelf unit  50  moves only vertically. 
     The guide units  60  are coupled to the protruding parts  21  (see  FIG. 2 ) to guide the oblique movement of the support unit  40 . The guide units  60  are provided as a pair of guide units such that two legs  44  are inserted into one of the guide units  60 . The rollers  46  provided at the ends of the legs  44  perform a rolling motion while being in contact with the inclined portions  62  provided inside the guide units  60 . Thus, the guide units  60  include the inclined portions  62  and the guide portions  64  into which the extended portions  52  of the shelf unit  50  are inserted. 
     The two inclined portions  62  are provided in one of the guide units  60 . Flat portions  68  are formed on the bottom surfaces at the ends of the inclined portions  62 , and stop ribs  65  are formed at the upper ends of the inclined portions  62  to prevent separation of the rollers  46 . When the rollers  46  are positioned at the flat portions  68 , the shelf unit  50  is at the lowest state in a downward direction. On the other hand, when the support unit  40  is extracted forward, the rollers  46  can move upward in an oblique direction along the inclined portions  62 . The rollers  46  move to the stop ribs  65  along the inclined portion  62 . At the moment when the rollers  46  reach the stop ribs  65 , the shelf unit  50  is at the highest state in an upward direction. When the shelf unit  50  is at the highest state in an upward direction, an article having a large volume can be stored in a space under the shelf unit  50 . Since the inclined portions  62  generally have an inclined shape enabling movement of the rollers  46 , the inclined shape of the inclined portions  62  may also have a discontinuous shape or a non-uniform gradient. 
     The guide portions  64  are spaces formed in the guide units  60 . Each of the guide portions  64  is divided into three portions by the two engaging parts  61 . As described above, since the evading parts  53  are formed on the extended portions  52  inserted into the guide portions  64 , the extended portions  52  can be inserted into the guide portions  64  without interfering with the engaging parts  61 . The guide portions  64  include a plurality of the front and rear wall surfaces  66  and  67  due to the shape of the engaging parts  61  protruded toward the inside of the guide portions  64 . The front and rear wall surfaces  66  and  67  are in contact with the front and rear surfaces  56  and  57  of the extended portions  52  to restrict the forward and backward movement of the shelf unit  50 . Accordingly, even when the support unit  40  moves forward and backward while moving vertically, the shelf unit  50  moves only vertically. 
     The guide bars  70  allow the support unit  40  to smoothly move forward and backward. The guide bars  70  connect the shelf unit  50  with the support unit  40 . That is, the guide bars  70  pass through guide holes  72  formed on the support unit  40  and, then, the opposite ends of the guide bars  70  are fixed to projection parts  74  formed on the shelf unit  50 . Since the diameter of the guide holes  72  is larger than the diameter of the guide bars  70 , the forward and backward movement of the support unit  40  is not restricted by the guide bars  70 . 
     The pressing/fixing unit  80  is a driving unit which enables the shelf unit  50  to be fixed at a certain vertical position after being moved. Once the user decides an appropriate vertical position of the shelf unit  50  while moving the support unit  40  forward and backward to adjust the vertical position of the shelf unit  50 , it is possible to simply fix the shelf unit  50  at the decided position using the pressing/fixing unit  80 . On the contrary, it is also possible to easily re-move the fixed shelf unit  50  to a desired position. The pressing/fixing unit  80  is arranged between the opposite surfaces of the support unit  40  and the shelf unit  50 . Hereinafter, the configuration of the pressing/fixing unit  80  will be described in detail with reference to  FIGS. 5 and 6 . 
       FIGS. 5 and 6  illustrate plan views showing a configuration and an operation of the pressing/fixing unit of  FIG. 3 . 
     As shown in  FIGS. 5 and 6 , the pressing/fixing unit  80  includes a plurality of engaging portions  82 , a stopper  84  coupled to any one of the engaging portions  82 , and a pressing portion  86  which moves the stopper  84  forward and backward. 
     The engaging portions  82  include ribs formed repeatedly at opposite sides on the lower surface of the shelf unit  50  such that the opposite ribs face each other. A total length L 1  of the engaging portions  82  in the forward and backward directions corresponds to a height L 4  (see  FIG. 8 ) by which the support unit  40  (see  FIG. 3 ) is lifted up while moving in the oblique direction along the inclined portions  62  (see  FIG. 3 ). A length L 2  of each of the engaging portions  82  corresponds to a height by which the shelf unit  50  (see  FIG. 3 ) can be lifted up or down at one time. That is, when the length L 2  of the engaging portions  82  is small, it is possible to lift the shelf unit  50  up and down while finely adjusting the position of the shelf unit  50  (see  FIG. 3 ). 
     The stopper  84  is coupled to or uncoupled from the engaging portions  82 . The ends of the stopper  84  are formed corresponding to the engaging portions  82 . That is, the size of the ends of the stopper  84  corresponds to the length L 2  of the engaging portions  82 . The stopper  84  and the engaging portions  82  are in close contact with each other by an elastic force of elastic springs  83  provided in the stopper  84  until the user applies an external force thereto. One side of the elastic springs  83  are supported by ribs  48  protruded from the support unit  40 , and the other side of the elastic springs  83  are supported by the stopper  84 . Accordingly, when an external force is not exerted, the stopper  84  is maintained in a state of being engaged with the engaging portions  82  by the elastic force of elastic springs  83 . 
     The pressing portion  86  is a portion which is substantially operated by the user to adjust the vertical position of the shelf unit  50 . When the user performs a grasping action after he supports a front end portion of the support unit  40  with the thumb and grabs the pressing portion  86  with the other fingers, the pressing portion  86  is moved in a pressing direction A. When the pressing portion  86  moves in the pressing direction A, the stopper  84  is moved in a separation direction B and separated from the engaging portions  82 . That is, when the pressing portion  86  is moved in the pressing direction A by an external force applied to the pressing portion  86 , projections  85  formed integrally with the pressing portion  86  are also moved in the pressing direction A. The projections  85  are inserted into coupling holes  87  formed in the stopper  84  in the oblique directions. Accordingly, while the pressing portion  86  moves in the pressing direction A, the projections  85  press the inner surfaces of the coupling holes  87  to move the stopper  84  in the separation direction B. 
     The operation of the refrigerator and the shelf assembly having the above configuration according to the first embodiment of the present invention will be described with reference to  FIGS. 5 to 8 . 
       FIGS. 5 and 6  illustrate plan views showing the configuration and the operation of the pressing/fixing unit.  FIGS. 7 and 8  illustrate side views showing an operation of the shelf assembly of  FIG. 1 . For reference, the plan view of  FIG. 5  corresponds to the side view of  FIG. 7  and the plan view of  FIG. 6  corresponds to the side view of  FIG. 8 . 
     The following description will be given in conjunction with the case in which the shelf assembly  30  moves from an initial position corresponding to a minimum height L 3  of the shelf unit  50  to a position corresponding to a maximum height L 4  of the shelf unit  50 . Referring to this description, the movement of the shelf assembly  30  in a direction opposite to that of the above case or from a certain position to another certain position can be easily understood. 
     The shelf assembly  30  may be initially positioned at the minimum height L 3 . When the shelf assembly  30  is positioned at the minimum height L 3 , a space above the shelf assembly  30  is ensured, but a space under the shelf assembly  30  is relatively small. When the user intends to store an article having a large volume in the space under the shelf assembly  30 , he can move the shelf assembly  30  to the maximum height L 4 . 
     In order to move the shelf assembly  30  from the minimum height L 3  to the maximum height L 4 , the user grasps the pressing portion  86  and applies a force thereto in the pressing direction A. As the force is applied to the pressing portion  86 , the pressing portion  86  is moved in the pressing direction A, and at the same time, the stopper  84  is moved in the separation direction B. When the stopper  84  moves in the separation direction B, the stopper  84  is uncoupled from the engaging portions  82 . 
     When the stopper  84  is uncoupled from the engaging portions  82 , the user pulls the support unit  40  forward while grasping the pressing portion  86 . When the support unit  40  is pulled forward, the rollers  46 , which are positioned on the flat portions  68  of the lowest end, perform a rolling motion in an oblique direction C along the inclined portions  62 . Accordingly, even when a relatively heavy article is placed on the shelf unit  50 , the support unit  40  can be moved with only a relatively small force. Several reinforcement ribs  69  are provided under the inclined portions  62  to reinforce strength. 
     The extended portions  52  (see  FIG. 3 ) of the shelf unit  50  are inserted into the guide portions  64  (see  FIG. 4 ) of the guide units  60 . The front and rear surfaces  56  and  57  of the extended portions  52  (see  FIG. 3 ) are in contact with the front and rear wall surfaces  66  and  67  of the guide portions  64  (see  FIG. 4 ) to restrict the forward and backward movement. Accordingly, the shelf unit  50  is influenced only by the vertical movement of the support unit  40  without being influenced by the forward and backward movement of the support unit  40 . The user can lift the shelf unit  50  up to the maximum height L 4  by simply extracting the support unit  40  forward. Since the stop ribs  65  are formed at the ends of the inclined portions  62 , the rollers  46  are prevented from being separated from the inclined portions  62 . Further, since the guide bars  70  are provided between the support unit  40  and the shelf unit  50 , the support unit  40  can be smoothly moved forward and backward. As the support unit  40  is moved up along the inclined portions  62  by the operation of extracting the support unit  40  forward, the shelf unit  50  placed on the support unit  40  is also moved up. 
     When the shelf unit  50  is positioned at the desired maximum height L 4  by the operation of extracting the support unit  40  forward, a grasping force is removed from the pressing portion  86 . When an external force applied to the pressing portion  86  is removed, the pressing portion  86  is moved in the opposite direction to the pressing direction A. When the pressing portion  86  is moved in the opposite direction to the pressing direction A, the stopper  84  is moved in the opposite direction to the separation direction B by the elastic force of the elastic springs  83 . When the stopper  84  is moved in the opposite direction to the separation direction B, the ends of the stopper  84  are engagedly coupled with the engaging portions  82 . When the stopper  84  and the engaging portions  82  are engagedly coupled with each other, the shelf unit  50  does not slide down along the inclined portions  62  even though the external force is removed from the shelf unit  50 . 
       FIG. 9  illustrates a perspective view of a shelf assembly according to a second embodiment of the present invention. 
     A shelf assembly  130  according to the second embodiment of the present invention includes guide units  160  formed on opposite inner walls  128  of a storage compartment  120 . Each guide unit  160  includes an engaging part  161  integrally formed with the corresponding inner wall  128  during the formation of the inner wall  128 . An inclined portion  162  is formed on the engaging part  161 . Stop ribs  165  are formed on upper and lower ends of the inclined portion  162  to prevent a roller  146  from being separated from the inclined portion  162 . Further, a bent portion  163  is formed on the inclined portion  162  in an upward direction to prevent the roller  146  from being laterally separated from the inclined portion  162 . 
     Evading parts  153  are formed on extended portions  152  extended downward from a shelf unit  150  and are fitted around the engaging parts  161 . Since the evading parts  153  are fitted around the engaging parts  161 , the forward and backward movement of the shelf unit  150  is restricted, and the shelf unit  150  only moves up and down. 
       FIG. 10  illustrates an exploded perspective view of a shelf assembly according to a third embodiment of the present invention.  FIGS. 11 and 12  illustrate side views showing an operation of the shelf assembly of  FIG. 10 . 
     A shelf assembly  220  according to the third embodiment of the present invention includes a rotation driving unit  260  which enables a support unit  240  to move obliquely in order to vertically adjust a shelf unit  250 . 
     As shown in  FIG. 10 , the support unit  240  which supports the shelf unit  250  includes frames  241  and  242  which horizontally support the shelf unit  250 , a plurality of legs  243  extended downward from the frames  241  and  242 , and rollers  244  which are rotatably fixed to the ends of the legs  243  and rotate while being supported by rails  232  when the support unit  240  moves along the rails  232 . 
     The frames  241  and  242  connect the upper ends of the legs  243  to each other. The frames  241  and  242  include a pair of lateral frames  241  each connecting a pair of the legs  243  spaced from each other by a specific distance in the forward and backward directions, and a longitudinal frame  242  which connect the pair of lateral frames  241 . 
     In this case, upper surfaces  241   a  of the lateral frames  241  are maintained in contact with the lower surfaces of the shelf unit  250  to support the shelf unit  250 . 
     At the center of the longitudinal frame  242 , there is provided a female threaded portion  242   a  which is threadedly coupled to a threaded portion  262  of the rotation driving unit  260 . At the opposite sides of the longitudinal frame  242 , there are provided guide holes  242   b  into which a pair of guide bars  253  are inserted in order to prevent shaking and allow a smooth sliding motion when the support unit  240  moves forward and backward with respect to the shelf unit  250 . 
     The rotation driving unit  260  is provided to allow the support unit  240  to move obliquely. 
     The rotation driving unit  260  includes a rod  261  rotatably mounted to a central portion of the rear surface of the shelf unit  250  and the threaded portion  262  formed at a specific region of the rod  261 . The rotation driving unit  260  also includes a knob  263  provided at a leading end of the rod  261  to rotate the rod  261 . 
     The threaded portion  262  is inserted into the female threaded portion  242   a  formed in the longitudinal frame  242  of the support unit  240 . Accordingly, the support unit  240  is moved forward and backward in a longitudinal direction of the threaded portion  262  by the rotation of the threaded portion  262 . 
     Thus, when the user rotates the knob  263  clockwise and counterclockwise, the threaded portion  262  is rotated clockwise and counterclockwise, and the support unit  240  is moved forward and backward in the longitudinal direction of the threaded portion  262 . 
     Next, an operation of a shelf assembly according to the embodiment of the present invention will be described. 
       FIG. 11  illustrates the operation of the shelf assembly according to the present invention, in which the shelf unit is lifted down.  FIG. 12  illustrates the operation of the shelf assembly according to the present invention, in which the shelf unit is lifted up. 
       FIG. 11  shows an unlifted state wherein the legs  243  are positioned at horizontal portions  234  and the shelf unit  250  is maintained at a minimum height. When the user rotates the knob  263  in an A direction in the unlifted state to rotate the threaded portion  262  in the A direction, the support unit  240  moves forward in the longitudinal direction of the threaded portion  262  by the rotation of the threaded portion  262 . Accordingly, the support unit  240  moves obliquely in a B direction. 
     The shelf unit  250  loaded on the support unit  240  also moves in the same way according to the oblique movement of the support unit  240 . The shelf unit  250  moves only vertically while the forward movement of the shelf unit  250  is prevented by front portions  238   a  of guide units  230 . In this case, the support unit  240  smoothly moves forward with respect to the shelf unit  250  along the guide bars  253  provided in the shelf unit  250 . Accordingly, the shaking can be prevented when the shelf unit  250  is lifted up and down. 
     Then, when the user removes a force applied to the knob  263 , the rotation of the threaded portion  262  is completed and the legs  243  of the support unit  240  are fixed at specific positions on inclined portions  233 . Accordingly, the support unit  240  does not move downward along the inclined portions  233  and is maintained at a specific position in spite of the weight of the support unit  240 , the shelf unit  250  and food placed on a shelf member  251 . 
     As shown in  FIG. 12 , when the user rotates the knob  263  in a C direction to rotate the threaded portion  262  in the C direction, the support unit  240  moves backward in the longitudinal direction of the threaded portion  262  by the rotation of the threaded portion  262 . Accordingly, the support unit  240  moves obliquely in a D direction. 
     The shelf unit  250  mounted on the support unit  240  also moves in the same way according to the oblique movement of the support unit  240 . The shelf unit  250  can move only vertically and downward while the backward movement of the shelf unit  250  is prevented by rear portions  238   b  of guide portions  238 . In this case, the support unit  240  smoothly moves backward with respect to the shelf unit  250  along the guide bars  253  provided in the shelf unit  250 . Accordingly, the shaking can be prevented when the shelf unit  250  is lifted up and down. 
     As described above, the shelf assembly according to the embodiments of the present invention has a simple configuration by omitting a complicated gear device provided in the conventional elevation unit. Thus, it is possible to simplify a manufacturing process and reducing manufacturing costs. 
     Further, since the shelf unit is supported by a pair of the guide units which are threadedly coupled and fixed to the opposite sidewalls of the storage compartment, the shelf assembly adapted to lift the shelf up and down can be mounted in the storage compartment even when a track is not installed in the storage compartment. Even when the shelf assembly is manufactured using an injection molded product made of a resin material instead of a stiff material such as metal, it is possible to ensure support performance corresponding to the conventional metal shelf. 
     Although a side by side (SBS) refrigerator is explained as an example in the above embodiments, the present invention may be applied to various refrigerators without being limited thereto. 
     Further, although the user directly applies an external force to the driving unit to operate the driving unit in the above embodiments, the driving unit may be automatically operated by an electric device or the like. 
     Although embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in this embodiment without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.