BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a clothes dryer, and particularly, to a structure for a roller capable of supporting a drum inside a clothes dryer. 
     2. Background of the Invention 
     Recently, a clothes dryer serves to dry an object to be dried by absorbing moisture inside the object by blowing blast generated by an electric heater or a gas heater into a drum. According to a method for processing humid air generated when drying the object, the clothes dryer is largely classified into an exhaustion-type clothes dryer and a condensation-type clothes dryer. 
       FIG. 1  is a view showing a clothes dryer in accordance with the conventional art. 
     Referring to  FIG. 1 , the conventional clothes dryer comprises a body  10  that forms appearance, a drum  20  rotatably installed in the body  10 , a door  30  through which an object to be dried is introduced into the clothes dryer, etc. Although not shown, the conventional clothes dryer further comprises a circulation duct having both ends connected to the drum thus to form a flow path for air circulation; a heater disposed in the circulation duct for heating air; a blowing fan for forcibly circulating air; etc. The condensation-type clothes dryer comprises a heat exchanger for removing moisture included in air exhausted from the drum  20 . 
     The drum  20  is rotated by receiving a driving force, through a belt (not shown), from a motor (not shown) installed at an inner lower side of the body  10 . The clothes dryer requires a means configured to prevent the drum  20  to be downwardly deformed due to a load of laundry and a load of the drum  20 , and configured to support a lower side of the drum  20  for smooth rotation of the drum  20 . For this, a supporting roller  40  is generally installed below the drum  20 . 
       FIGS. 2 and 3  are perspective and sectional views of the supporting roller. 
     Referring to  FIGS. 2 and 3 , the supporting roller  40  of the drum  20  includes a roller shaft  41 ; a roller frame  42  slidably installed at the roller shaft  41  and rotated; a roller outer circumferential portion  43  formed of rubber having an elastic force to support the drum  20 , and attached to an outer circumference of the roller frame  42 ; a bearing  44  disposed between the roller frame  42  and the roller shaft  41 , and configured to allow the roller frame  42  to be smoothly rotated; and a triangular pin  45  configured to support the roller frame  42  at both sides so as to prevent the roller frame  42  from being separated from the roller shaft  41 . 
     The supporting roller  40  of the conventional clothes dryer is mounted to the roller shaft  41  by using an oil-less bearing formed between the roller frame  42  and the roller shaft  41 . However, when a load supported by the bearing increases, oil included in the oil-less bearing may be discharged out. This may degrade a lubricating characteristic of the supporting roller  40 , and may cause noise occurrence and damage of the supporting roller  40 . Especially, in the case of 24-inch clothes dryer rather than 27-inch clothes dryer, the above problems may become more severe due to a narrow inner space and an overload. 
     Furthermore, in the conventional bearing, the roller outer circumferential portion  43  contacting the drum  20  has a convexed portion at a central part thereof. This may cause only parts of the entire region of the roller outer circumferential portion  43  to contact the bearing. As a result, a stress applied to the supporting roller  40  is concentrated on specific regions, thereby shortening the lifespan of the supporting roller  40 . 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to provide a clothes dryer capable of providing a structure of a roller which supports a drum of the clothes dryer, the roller configured to endure even a large load applied thereto, capable of prolonging the lifespan of the roller by preventing stress concentration by increasing a contact area between the roller and the drum, and capable of enhancing the reliability. 
     To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described herein, there is provided a clothes dryer, comprising: a body; a drum rotatably installed in the body; and a roller disposed below the drum, and configured to support the drum, wherein the roller is mounted to a roller shaft by a ball bearing. 
     When coupling the roller to the roller shaft, may be used a ball bearing, not the conventional oil-less bearing. This may enable the bearing of the roller shaft to have stronger endurance against thermal deformation or abrasion due to heat. 
     A stopping portion for mounting the ball bearing may be formed at the roller shaft. 
     A screw thread for nut mounting may be further formed at one side of the roller shaft, and a screw thread for fixing the roller into the clothes dryer may be formed at another side of the roller shaft. As a nut is mounted to the screw thread after mounting the roller to the roller shaft, the roller may be more stably mounted to the roller shaft. 
     A contact portion of the roller contacting the drum may be formed to be flat. 
     The roller may comprise a roller frame configured to insert the roller shaft; and an outer wheel portion mounted to the roller frame, and configured to encompass an outer circumferential surface of the roller frame. 
     Since the contact portion of the roller may be formed to be flat, a stress applied to the roller may be distributed to prolong the lifespan of the roller. 
     A plurality of concave-convex portions may be formed on an outer circumferential surface of the roller frame, and a plurality of concave-convex portions engaged with the concave-convex portions may be formed on an inner circumferential surface of the outer wheel portion. As the concave-convex portions of the roller frame may be coupled to the concave-convex portions formed on the inner circumferential surface of the outer wheel portion, the roller frame may be stably coupled to the outer wheel portion. The outer wheel portion of the roller may be formed of rubber having an elastic force. 
     Under these configurations, even when a large load may be applied to the roller, the roller may endure the load. This may prolong the lifespan of the roller that supports the drum, and enhance the reliability of the clothes dryer. 
     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. 
       In the drawings: 
         FIG. 1  is a view showing a clothes dryer in accordance with the conventional art; 
         FIG. 2  is a perspective view of a roller of  FIG. 1 ; 
         FIG. 3  is a sectional view of  FIG. 2 ; 
         FIG. 4  is a sectional view of a roller which supports a drum of a clothes dryer according to the present invention; and 
         FIG. 5  is a perspective view of a roller shaft of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Description will now be given in detail of the present invention, with reference to the accompanying drawings. 
     Hereinafter, a clothes dryer according to the present invention will be explained in more detail with reference to the attached drawings. 
       FIGS. 4 and 5  are views respectively showing a roller for supporting a drum of a clothes dryer according to the present invention. 
     The roller for supporting a drum of a clothes dryer according to the present invention is mounted to a roller shaft  51  by a ball bearing  53 . That is, in the present invention, the roller is provided with a ball bearing, not the conventional oil-less bearing. This may enable the bearing of the roller shaft  51  to have a stronger endurance against thermal deformation or abrasion due to heat. 
     The roller shaft  51  for mounting the roller is provided with a bearing mounting surface  51   e  for mounting the ball bearing  53 . For stable mounting of the ball bearing  53 , a stopping portion  51   c  having a diameter a little larger than that of the bearing mounting surface  51   e  is disposed on a side surface of the bearing mounting surface  51   e . On the basis of the bearing mounting surface  51   e  of the roller shaft  51 , a screw portion  51   a  for mounting a nut  52  is formed at an opposite side to the stopping portion  51   c . A stepped portion  51   d  may be formed between the screw portion  51   a  and the bearing mounting surface  51   e , thereby showing the position of the nut coupled to the screw portion  51   a . After inserting the ball bearing  53  into the bearing mounting surface  51   e  of the roller shaft  51 , the nut  52  is coupled to the screw portion  51   a . As the ball bearing  53  is disposed between the stopping portion  51   c  and the nut  52 , the ball bearing  53  is prevented from being separated from the roller shaft  51 . That is, owing to the stopping portion  51   c  and the nut  52  mounted to the screw portion  51   a , the ball bearing  53  can be easily and precisely mounted on the roller shaft  51 , and the ball bearing  53  can be prevented from being separated from the roller shaft  51 . 
     At one side of the roller shaft  51 , the ball bearing  53  is mounted. And, another side of the roller shaft  51  is mounted to a fixing member  61  inside the clothes dryer. For this, a screw thread  51   b  is formed at another side of the roller shaft  51 . The roller shaft  51  is inserted into the fixing member  61  inside the clothes dryer, and a nut  60  are coupled to the screw thread  51   b , thereby fixing the roller shaft  51  to a predetermined position inside the clothes dryer. 
     An outer circumferential surface of the roller, i.e., a contact portion contacting the drum is formed to be flat. This may enable a contact surface of the roller contacting the drum to have a wider area. Therefore, a stress applied to the outer circumferential surface of the roller decreases to prolong the lifespan of the roller. 
     Preferably, the roller includes a roller frame  54  configured to insert the roller shaft  51  therein by using the ball bearing  53 ; and an outer wheel portion  55  mounted to the roller frame  54 , and configured to encompass an outer circumferential surface of the roller frame  54 . In this case, an outer circumferential surface  55   a  of the outer wheel portion  55  directly contacting the drum is formed to be flat thus to have a wider contact area. As the contact portion of the outer circumferential surface  55   a  contacting the drum is formed to be flat, the stress applied to the roller is distributed to prolong the lifespan of the roller. 
     A plurality of concave-convex portions are formed on an outer circumferential surface of the roller frame  54 , and a plurality of concave-convex portions engaged with the concave-convex portions are formed on an inner circumferential surface of the outer wheel portion  55 . As the concave-convex portions of the roller frame  54  are coupled to the concave-convex portions formed on the inner circumferential surface of the outer wheel portion  55 , the roller frame  54  can be stably coupled to the outer wheel portion  55 . The outer wheel portion  55  of the roller is preferably formed of rubber having an elastic force. 
     The foregoing embodiments and advantages are merely exemplary and are not to be construed as limiting the present disclosure. The present teachings can be readily applied to other types of apparatuses. This description is intended to be illustrative, and not to limit the scope of the claims. Many alternatives, modifications, and variations will be apparent to those skilled in the art. The features, structures, methods, and other characteristics of the exemplary embodiments described herein may be combined in various ways to obtain additional and/or alternative exemplary embodiments. 
     As the present features may be embodied in several forms without departing from the characteristics thereof, it should also be understood that the above-described embodiments are not limited by any of the details of the foregoing description, unless otherwise specified, but rather should be construed broadly within its scope as defined in the appended claims, and therefore all changes and modifications that fall within the metes and bounds of the claims, or equivalents of such metes and bounds are therefore intended to be embraced by the appended claims.
Disclosed is a clothes dryer comprising a drum rotatably disposed in a body; and a roller disposed below the drum, and configured to support the drum. The roller is mounted to a roller shaft by a ball bearing, and a contact portion of an outer circumferential surface of the roller contacting the drum is formed to be flat.