Patent Publication Number: US-2007107194-A1

Title: Method for manufacturing rotary drum for drum washing machine and pressing device therefor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application claims the benefit of Korean Patent Application No. 2005-0105992, filed Nov. 7, 2005, 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 method for manufacturing a rotary drum for a drum washing machine, and more particularly, to a method for manufacturing a rotary drum for a drum washing machine in which the strength at a bonding part of the rotary drum is improved so that the bonding part is not deformed even when the rotary drum is rotated at a high speed, and a pressing device used therefor.  
      2. Description of the Related Art  
      Generally, washing machines, which wash laundry using electric power, are divided into a vertical shaft drum washing machine, in which a washing tub is vertically installed in a main body, and a drum washing machine, in which a washing tub is horizontally installed in a main body. The drum washing machine vertically moves the laundry together with washing water along the inner circumferential surface of a washing tub, i.e., a rotary drum, which is vertically installed, by the alternate rotations of the rotary drum in the clockwise and counterclockwise directions, thereby washing the laundry.  
      Generally, a rotary drum for a drum washing machine includes a cylindrical portion for forming the circumferential surface thereof, and front and rear portions connected to front and rear ends of the cylindrical portion for forming the front and rear surfaces thereof. The cylindrical portion is formed by roll-forming a flat plate member and by bonding both circumferential ends of the rolled plate.  
      In the above process of forming the cylindrical portion, the bonding of both circumferential ends of the rolled plate member is achieved by mechanical pressing using seaming and by welding. The bonding of the ends of the plate member by welding is advantageous in that a connection part maintains high strength. However, considering the rotary drum is used in an environment where it is exposed to water for an extended period of time, a welding part is likely to be damaged and caused to rust and rust stains soak through and damage laundry. Further, a welding process deteriorates efficiency of manufacturing the rotary drum.  
      The bonding of the ends of the plate member by mechanical pressing does not cause the above problems, and is advantageous in that the fatigue strength at the bonding part formed by mechanical pressing is higher than the fatigue strength at the bonding part formed by welding. Further, the bonding by mechanical pressing does not consume a separate material except for the pressing die, thereby reducing production costs of the rotary drum compared to the bonding by welding.  
      When the bonding by mechanical pressing is conventionally performed, both circumference ends of a material are bent and are engaged with each other so as to form a joint part, and the joint part is pressed so that contact areas between the overlapped portions of the ends are increased and the strength at the obtained bonding part is maintained. In the above conventional mechanical pressing method, when the joint part is pressed, sharply bending parts are obtained and fatigue due to plastic deformation is accumulated on the bending parts, thereby reducing the bonding strength of the bonding part.  
      Recently, a rotary drum suitable for use in high-speed dehydration has been researched. In the above rotary drum suitable for use in high-speed dehydration, strong tensile force due to the rotation of the rotary drum at a high speed is applied to a bonding part of the rotary drum. Accordingly, when the bonding part does not maintain sufficient bonding strength, the bonding part is deformed or damaged. Thus, it is difficult to apply the conventional mechanical pressing method, causing low bonding strength, to a rotary drum suitable for use in high-speed dehydration. However, since the mechanical pressing method has many advantages compared to the welding method, if the bonding strength problem of the mechanical pressing method is solved, a rotary drum for use in high-speed dehydration can employ the mechanical pressing method.  
     SUMMARY OF THE INVENTION  
      Therefore, one aspect of the invention is to provide a method for manufacturing a rotary drum for a drum washing machine, which is suitable for high-speed rotation, using a mechanical pressing technique, and a pressing device used therefor.  
      In accordance with one aspect, the present invention provides a method for manufacturing a rotary drum for a drum washing machine, in which a cylindrical portion of the rotary drum is formed by bonding both circumferential ends of a cylindrical processed material, including: bending one circumferential end of the processed material in a U shape to form a first overlapping part and a first bending part, and bending the other circumferential end of the processed material in a U shape to form a second overlapping part and a second bending part; overlapping the first and second overlapping parts with each other to form a connection part; and pressing the connection part so that the original shapes of the first and second bending parts are maintained and the first and second overlapping parts are adhered to each other to form a bonding part. When the original shapes of the first and second bending parts are maintained during the pressing of the connection part as described above, fatigue due to plastic deformation is not accumulated on the first and second bending parts, thereby improving the bonding strength of the rotary drum.  
      The pressing of the connection part may be achieved by a press die including upper and lower dies respectively having a plurality of first and second protrusions disposed in the forward and backward direction of the connection part and having designated spaces between lower ends of the first protrusions and the lower die and/or between upper ends of the second protrusions and the upper die when the upper and lower dies contact each other.  
      The first and second bending parts may have a radius of curvature of 0.5 mm, and the first and second overlapping parts may have a length of 3-6 mm.  
      Further, the first and second protrusions may have a height of 2.5-3 mm, and a thickness of 1.2 mm.  
      The lower ends of the first protrusions may respectively have planes downwardly inclined in the leftward or rightward direction, and the upper ends of the second protrusions may respectively have planes downwardly inclined in the same direction of the inclined plane of the lower ends of the first protrusions.  
      In accordance with another aspect, the present invention provides a method for manufacturing a rotary drum for a drum washing machine, in which a cylindrical portion of the rotary drum is formed by bonding both circumferential ends of a cylindrical processed material, including: bending one circumferential end of the processed material in a U shape to form a first overlapping part and a first bending part, and bending the other circumferential end of the processed material in a U shape to form a second overlapping part and a second bending part; overlapping the first and second overlapping parts with each other to form a connection part; and pressing the connection part into a corrugated shape by upper and lower dies disposed above and below the connection part, wherein the highest points of the connection part in the corrugated shape do not contact the upper die and the lowest points of the connection part in the corrugated shape do not contact the lower die when the connection part is maximally pressed in the pressing of the connection part.  
      In accordance with another aspect, the present invention provides a pressing device having upper and lower dies for pressing a connection part of a cylindrical processed material disposed between the upper and lower dies to form a cylindrical portion of a rotary drum for a drum washing machine, including: a plurality of first protrusions protruded downwardly from the upper die and disposed in the longitudinal direction of the connection part, and a plurality of second protrusions protruded upwardly from the lower die and entering between the first protrusions in the pressing of the connection part for deforming the connection part into a corrugated shape; and designated spaces formed between lower ends of the first protrusions and the lower die and/or between upper ends of the second protrusions and the upper die when the upper and lower dies contact each other.  
      Additional aspects and/or advantages of the invention will be set forth in part in the description which follows and, in part, will be apparent from the description, or may be learned by practice of the invention.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings in which:  
       FIG. 1  is a longitudinal sectional view of a rotary drum obtained by a method for manufacturing a rotary drum for a drum washing machine in accordance with the present invention;  
       FIG. 2  is a flow chart illustrating the method for manufacturing the rotary drum in accordance with the present invention;  
       FIGS. 3 and 4  are views respectively illustrating bending and overlapping steps of a process for manufacturing a cylindrical portion in the method of the present invention;  
      FIGS.  5  to  7  are views illustrating a pressing step of the process for manufacturing the cylindrical portion in the method of the present invention; and  
       FIG. 8  is a table showing test results of a tensile strength of the rotary drum manufactured by the method of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Reference will now be made in detail to the embodiment of the present invention, an example of which is illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. The embodiment is described below to explain the present invention by referring to the annexed drawings.  
       FIG. 1  is a longitudinal sectional view of a rotary drum obtained by a method for manufacturing a rotary drum for a drum washing machine in accordance with the present invention, and  FIG. 2  is a flow chart illustrating the method for manufacturing the rotary drum in accordance with the present invention.  
      As shown in  FIG. 1 , the rotary drum  1  obtained by the method of the present invention includes a cylindrical portion  10 , and front and rear portions  20  and  30  connected to front and rear ends of the cylindrical portion  10 . The cylindrical portion  10 , the front portion  20 , and the rear portion  30  are separately molded, and are then bonded by a seaming step. Reference numerals  2  and  3  of  FIG. 1  represent seaming parts obtained by bonding the front and rear portions  20  and  30  to the cylindrical portion  10 .  
      With reference to  FIG. 2 , a process for manufacturing the front portion  20  and the rear portion  30  will be described, as below. The front portion  20  is molded by a blanking step  21  in which a plate member having a circular shape is punched out of a raw material using a punch and a die, a drawing step  22  in which the plate member is curved, a trimming step  23  in which unnecessary parts are cut off from the plate member, a bending step  24  in which the circumference of the plate member is bent to be connected to the cylindrical portion  10 , a piercing step  25  in which several holes are formed through the plate member, and a curling step  26  in which a part of the front portion  20  corresponding to the circumference of an opening, through which laundry is put into the drum, is curled. Further, the rear portion  30  is molded by a series of steps similar to that of the front portion  20 , i.e., a blanking step  31 , a drawing step  32 , a trimming step  33 , a piercing step  34 , and a bending step  35 .  
       FIGS. 3 and 4  are views respectively illustrating bending and overlapping steps of a process for manufacturing a cylindrical portion in the method of the present invention, and FIGS.  5  to  7  are views illustrating a pressing step of the process for manufacturing the cylindrical portion in the method of the present invention. Hereinafter, with reference to FIGS.  2  to  7 , the process for manufacturing the cylindrical portion  10  will be described.  
      A cylindrical processed material  40 , both circumferential ends of which are not connected to each other, is formed by a blanking step  11  in which a plate member having a rectangular shape is punched out of a raw material, a piercing step  12  in which dehydration holes for dehydrating laundry therethrough are formed through the plate member, and a roll-forming step  13  in which the plate member having the rectangular plate shape is converted into a cylindrical shape by passing through a gap between rollers. Then, the cylindrical portion  10  of a rotary drum  1  is completely manufactured from the above processed material  40  by a bending step  14 , an overlapping step  15 , and a pressing step  16 .  
      As shown in  FIG. 3 , in the bending step  14 , one circumferential end  41  of the processed material  40  is bent in a U shape in one direction so that a first overlapping part  42  and a first bending part  43  are formed, and the other circumferential end  44  of the processed material  40  is bent in a U shape in the opposite direction so that a second overlapping part  45  and a second bending part  46  are formed. The first overlapping part  42  and the second overlapping part  45  may have a length of approximately 3-6 mm. In  FIG. 3 , the circumferential end  41  of the processed material  40  is bent upwardly, and the circumferential end  44  of the processed material  40  is bent downwardly.  
      In order to prevent the accumulation of fatigue onto the first bending portion  43  and the second bending portion  46  due to the sharp bending of the first bending portion  43  and the second bending portion  46 , a U-shaped bending method is employed. The first bending portion  43  and the second bending portion  46  may have a radius of curvature (R) of approximately 0.5 mm. When the radius of curvature (R) of the first and second bending portions  43  and  46  is excessively small, the first and second bending portions  43  and  46  have an approximately V shape and fatigue is accumulated onto the first and second bending portions  43  and  46 . On the other hand, when the radius of curvature (R) of the first and second bending portions  43  and  46  is excessively large, the first and second bending portions  43  and  46  contact a pressing die in the pressing step  16  and are pressed by the pressing die, and thus fatigue caused by plastic deformation is accumulated onto the first and second bending portions  43  and  46 .  
      As shown in  FIG. 4 , after the bending step  14 , the overlapping step  15  in which the first overlapping part  42  and the second overlapping part  45  are overlapped with each other so as to form a connection part  47  is performed. In the overlapping step  15 , both ends of the cylindrical processed material  40  are temporarily overlapped with each other. After the overlapping step  15 , the pressing step  16  in which the connection part  47  is firmly pressed to form a bonding part  48  (with reference to  FIG. 1 ) is performed. Here, the connection part  47  and the bonding part  48  essentially designate the same part, but are distinguished from each other in that the bonding part  48  is obtained after the pressing step  16 .  
      In order to improve the bonding strength when two materials are bonded by a pressing step, it is important to increase dimensions of contact portions of the two materials by applying proper molding pressure to the materials. In the drum washing machine, strong tensile force is generated by the rotation of the rotary drum  1  at a high speed and load is concentrated onto the first bending part  43  and the second bending part  46 , thereby spreading the first and second bending parts  43  and  46  and releasing the bonding between the first and second bending parts  43  and  46 . Thus, it is more important to prevent fatigue from being accumulated onto the first and second bending parts  43  and  46  and to protect the first and second bending parts  43  and  46  so that residual stress due to plastic deformation not occurs in the first and second bending parts  43  and  46 .  
      Accordingly, in the present invention, the pressing step  16  is performed using a press die, which is designed such that fatigue due to press is scarcely accumulated onto the first and second bending portions  43  and  46 .  FIG. 5  is a sectional view of the press die taken along the longitudinal direction of the connection part (in the X-direction of  FIG. 1 ) illustrating a state in which the connection part is pressed between upper and lower dies,  FIG. 6  is a sectional view of the press die illustrating a state in which the pressing of the connection part is completed, and  FIG. 7  is a sectional view of the press die taken along the circumferential direction of the rotary drum illustrating a state in which the connection part is pressed between first protrusions and second protrusions. Although  FIG. 7  illustrated that the first protrusions and the second protrusions are positioned on the same plane, the second protrusions are substantially located in front of or in the rear of the corresponding first protrusions.  
      As shown in  FIGS. 5 and 6 , the press die used in the pressing step  16  of the present invention includes an upper die  50  having a plurality of first protrusions  51  disposed in the longitudinal direction of the connection part  47  and a lower die  60  having a plurality of second protrusions  61  disposed in the longitudinal direction of the connection part  47 . The first protrusions  51  are protruded downwardly from the upper die  50 , and the second protrusions  61  are protruded upwardly from the lower die  60 . The second protrusions  61 , which alternate with the first protrusions  51 , are respectively located between the first protrusions  51  in the pressing step  16 , thereby deforming the connection part  47  of the processed material  40 , located between the first and lower dies  50  and  60 , into a corrugated shape.  
      The press die, which is employed by the present invention, is configured such that designated spaces  70  are formed between lower ends  52  of the first protrusions  51  and the lower die  60  and between upper ends  62  of the second protrusions  61  and the upper die  50  when the upper and lower dies  50  and  60  contact each other under the condition that any processed material is not located between the upper and lower dies  50  and  60 . Through the above configuration of the press die, the highest points (P) and the lowest points (Q) of the connection part  47  do not contact the upper and lower dies  50  and  60  when the first and second overlapping parts  42  and  45  are pressed into the corrugated shape, thereby allowing the first and second bending parts  43  and  46  to maintain their original shapes. That is, these spaces  70  prevent the upper and lower dies  50  and  60  from completely contacting each other in the pressing step, thus preventing the first and second bending parts  43  and  46  from being sharply plastically deformed. Although this embodiment describes the spaces  70  existing between the lower ends  52  of the first protrusions  51  and the lower die  60  and between the upper ends  62  of the second protrusions  61  and the upper die  50 , the spaces  70  may exist only between the lower ends  52  of the first protrusions  51  and the lower die  60  or only between the upper ends  62  of the second protrusions  61  and the upper die  50 .  
      When the spaces  70  exists between the upper and lower dies  50  and  60  as described above, pressure applied to the connection part  47  is reduced. Accordingly, in order to compensate for the reduction in the above pressure applied to the connection part  47 , the first and second protrusions  51  and  61  may have a height (h) of approximately 2.5-3 mm and a thickness (t) of approximately 1.2 mm.  
      As shown in  FIG. 7 , the lower ends  52  of the first protrusions  51  and the upper ends  62  of the second protrusions  61  respectively may have incline planes  52   a  and  62   a , which are downwardly inclined in the rightward direction. The incline planes  52   a  and  62   a  allow the first and second bending portions  43  and  46 , on which load caused by the rotation of the rotary drum is concentrated, to be pressed by pressure higher than pressure applied to other portions of the connection part  47 . Thereby, the first and second bending parts  43  and  46  are protected by the spaces  70  formed between the lower ends  52  of the first protrusions  51  and the lower die  60  and between the upper ends  62  of the second protrusions  61  and the upper die  50  so that fatigue accumulated on the first and second bending parts  43  and  46  is minimized, and proper pressure is applied to the first and second bending parts  43  and  46  so that the strength of the connection part  47  is improved. Here, the incline planes  52   a  and  62   a  may be downwardly inclined in the leftward direction.  
       FIG. 8  is a table showing test results of a tensile strength of the rotary drum manufactured by the method of the present invention. As shown in  FIG. 8 , the rotary drum manufactured by the method of the present invention has a bonding strength of 760 kgf on the average. Considering that the bonding strength of the rotary drum used in a high-speed dehydration mode is 450 kgf, the rotary drum manufactured by the method of the present invention is suitable for use in the high-speed dehydration mode.  
      As apparent from the above description, the present invention provides a method for manufacturing a rotary drum for a drum washing machine, which is suitable for high-speed rotation, using a mechanical press bonding technique, and a pressing device used therefor. That is, since a connection part of the rotary drum is not deformed and damaged when the rotary drum is rotated at a high speed, the method of the present invention improves the reliability of the rotary drum, and has improved productivity compared to a method, in which both ends of a cylindrical portion are bonded by welding.  
      Although an embodiment of the invention has 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.