Abstract:
A fixing device having: a heating roller and a presser that form a fixing nip portion therebetween; a pair of brackets having holes for supporting both end portions of the heating roller; a biasing member for pressing the presser against the heating roller to form the fixing nip portion; and a pair of semicircular arc slide bearings that are fitted in the holes of the respective brackets so as to be located between the respective brackets and the heating roller, on an opposite side of the heating roller to a side thereof pressed by the presser.

Description:
This application is based on Japanese Patent Application No. 2010-063395, filed on Mar. 19, 2010, the content of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a fixing device, and particularly to a fixing device to be employed in an electrophotographic image forming apparatus, such as a copier and a printer, as a device for fixing a toner image on a sheet by heat. 
     2. Description of Related Art 
     Generally, in a fixing device of this kind, a heating roller and a pressing roller are juxtaposed to be pressed against each other with a specified pressing force, and while a recording sheet with a toner image thereon is passing between the rollers, the toner image is fixed on the recording sheet. The heating roller is driven to rotate at a predetermined rate, and the heating roller is held by a bearing member that permits the heating roller to freely rotate in the radial direction while regulating the position of the heating roller in the thrust direction. Conventionally, there has been normally used a slide bearing or a C-ring for positional regulation in the thrust direction, in combination with a radial bearing. 
     However, combining two or more components for a bearing member is not good because it requires a larger number of components as well as a larger number of assembly steps. Further, in order to mount the heating roller to such a conventional bearing member, the heating roller is required to be moved in the thrust direction, which is a complicated operation. Moreover, the radial bearing hardly expands/contracts with a temperature change because of the material and the structure thereof, while the heating roller, which is made of aluminum, expands/contracts to a relatively large degree with a temperature change. Thus, when there is a large difference in thermal expansion coefficient between the bearing member and the heating roller, an unexpected backlash or bite occurs with a temperature change. Also, a process of fitting the C-ring, which is a component for regulating the position of the heating roller in the thrust direction, to the heating roller is troublesome, and this process may cause damage on the heating roller. Further, in order to permit the C-ring fitting process, it is necessary to make a slit in the heating roller, which necessitates an elongation of the heating roller, that is, prevents downsizing of the heating roller. 
     Japanese Patent Laid-Open Publication No. 2007-171387 describes that a sleeve-like slide bearing composed of two fired bodies that are made of a material mainly containing tetrafluoroethylene resin into shapes with semicircular arc cross sections in the axial directions is used as a bearing of the heating roller. Although this slide bearing has a favorable characteristic of having a low rotational torque, the slide bearing complicates the structure of the bearing member. Specifically, this bearing member requires another component for regulating the position of the heating roller in the thrust direction, and in mounting the heating roller in the bearing member, it is necessary to move the heating roller in the axial direction. Thus, the above-described problems are left unsolved. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a fixing device that can be assembled easily and that can be assembled into a small size. 
     A fixing device according to a first aspect of the present invention comprises: a heating roller and a presser that form a fixing nip portion therebetween; a pair of brackets having holes for supporting both end portions of the heating roller; a biasing member for pressing the presser against the heating roller to form the fixing nip portion; and a pair of semicircular arc slide bearings that are fitted in the holes of the respective brackets so as to be located between the respective brackets and the heating roller, on an opposite side of the heating roller to a side thereof pressed by the presser. 
     A fixing device according to a second aspect of the present invention comprises: a heating roller and a presser that form a fixing nip portion therebetween; a pair of brackets having holes for supporting both end portions of the heating roller; a biasing member for pressing the presser against the heating roller to form the fixing nip portion; and a pair of semicircular arc slide bearings that are fitted in the holes of the respective brackets so as to be located between the respective brackets and the heating roller, on an opposite side of the heating roller to a side thereof pressed by the presser; wherein each of the slide bearings has a wall protruding outward to a periphery of the hole of the bracket so as to prevent the slide bearing itself from coming off from the bracket; wherein the heating roller is a hollow pipe having a predetermined radius; wherein the semicircular arc slide bearings have a radius almost equal to the radius of the heating roller and are in contact with a peripheral surface of the heating roller; and wherein the slide bearings are made of a resin material having heat resistance and abrasion resistance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       This and other objects and features of the present invention will be apparent from the following description with reference to the accompanying drawings, in which: 
         FIG. 1  is an exploded perspective view of a main part of a fixing device according to an embodiment of the present invention; 
         FIG. 2  is an exploded perspective view of a slide bearing and a bracket of the fixing device; 
         FIG. 3  is a perspective view showing a state where the slide bearing is fitted in the bracket; 
         FIG. 4  is a sectional view of a bearing structure for a heating roller; and 
         FIG. 5  is a sectional view of a modified slide bearing. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, an embodiment of the fixing device according to the present invention is described with reference to the accompanying drawings. It is to be noted that in the drawings, common components and portions are provided with the same reference numerals, and descriptions thereof will not be repeated. 
     As shown in  FIG. 1 , a fixing device according to an embodiment of the present invention comprises a heating roller  10  and a pressing roller  20  juxtaposed to each other and pressed against each other by a predetermined pressing force. Both of the rollers  10  and  20  are fixed to a frame  30  in a structure described hereinafter, and are housed in a cover member (not shown). 
     The heating roller  10  comprises an aluminum cylindrical body, a rubber layer coated on the aluminum cylindrical body and a fluorine resin layer coated on the rubber layer as a mold-release layer. In order to facilitate holding of the heating roller  10 , the rubber layer and the fluorine resin layer are not coated in the end portions of the roller  10 . The pressing roller  20  comprises an aluminum cylindrical body, a silicone rubber layer coated on the aluminum cylindrical body and a fluorine resin tube covered around the silicone rubber layer as a mold-release layer. Further, a heater (not shown) is incorporated in the heating roller  10 . 
     One end portion (first end portion) of the heating roller  10  is held by a bracket  31  via a slide bearing  15 , and the other end portion (second end portion) is held by a bracket  35  via a slide bearing  17 . As shown in  FIG. 2 , the slide bearing  15 , which is shaped into a semicircular arc, is fitted in a hole  32  of the bracket  31  in a direction of arrow A. The slide bearing  15  has a first wall  15   a  protruding outward, that is, in a direction away from the center of the semicircular arc and a second wall  15   b  protruding inward, that is, in a direction toward the center of the semicircular arc. When the slide bearing  15  is fitted in the hole  32 , the first wall  15   a  leans against the bracket  31  in the periphery of the hole  32  from the direction of arrow A, and thereby serves to prevent the slide bearing  15  itself from coming off from the bracket  31 . Then, when the first end portion of the heating roller  10  is fitted to the slide bearing  15 , as shown in  FIG. 4 , the second wall portion  15   b  serves to regulate the position of the heating roller  10  in the direction of arrow A (thrust direction). 
     Further, as shown in  FIG. 2 , the slide bearing  15  has a projected portion  15   c , and the hole  32  of the bracket  31  has a depressed portion  32   a  to come into engagement with the projected portion  15   c . Engagement of the projected portion  15   c  with the depressed portion  32   a  prevents the slide bearing  15  from moving along the circumference of the hole  32 . 
     The first end portion of the heating roller  10  is mounted to the slide bearing  15  from a direction of an arrow B (direction orthogonal to the thrust direction) shown in  FIG. 3 , and then pressed by the pressing roller  20  from the direction of arrow B, and thereby, the heating roller  10  can be held at a predetermined position. The slide bearing  15  supports the first end portion of the heating roller  10  such that the heating roller  10  can rotate freely in the radial direction while preventing the heating roller from moving in the thrust direction with the second wall  15   b.    
     As shown in  FIG. 1 , the other end portion (second end portion) of the heating roller  10  is supported by the slide bearing  17  such that the heating roller  10  can rotate freely in the radial direction. The slide bearing  17  is shaped into a semicircular arc having an L-shaped cross section. This slide bearing  17  is fitted in a hole  36  of the bracket  35  as shown in  FIG. 4 , and the second end portion of the heating roller  10  is mounted to the slide bearing  17  from the direction of arrow B in the same way of mounting the first end portion of the heating roller  10  to the slide bearing  15 . Thereafter, the second end portion of the heating roller  10  is pressed by the pressing roller  20  from the direction of arrow B. 
     As shown in  FIG. 4 , a gear  41  for transmission of rotating force is provided at the second end of the heating roller  10 . A pin  42  standing on an inner circumferential surface of the gear  41  engages with a depressed portion  11  made in the second end surface of the heating roller  10 . Thereby, the heating roller  10  rotates integrally with the gear  41 . Further, the gear  41  engages with another gear  45 , and a flange  46  provided on an outer side of this gear  45  comes into contact with an outer side of the gear  41 , thereby preventing the heating roller  10  from moving in the opposite direction to arrow A. 
     Meanwhile, as shown in  FIG. 1 , the pressing roller  20  is supported by members  26  that are elastically biased by coil springs  25 . More specifically, pins  21  provided on both of the end surfaces of the pressing roller  20  are supported by the members  26  such that the pressing roller  20  can freely rotate. The members  26  are held at fixed positions by engagement with depressed portions  33  and  37  of the brackets  31  and  35 , respectively. 
     In the present embodiment, the first end portion of the heating roller  10  is supported by the slide bearing  15  such that the heating roller  10  can rotate freely in the radial direction and cannot move in the thrust direction (direction of the arrow A). Thus, one component, namely, the slide bearing  15  has two functions. Further, since the slide bearing  15  is shaped into a semicircular arc, the heating roller  10  can be mounted to the slide bearing  15  from the direction orthogonal to the thrust direction (direction of the arrow B). This eliminates the need to move the heating roller  10  in the thrust direction in mounting the heating roller  10  to the slide bearing  15 , thereby facilitating the assembly. Moreover, since no C-rings are used, the assembly is easy, and there is no possibility that the heating roller  10  may be damaged by a C-ring. Also, there is no need to elongate the heating roller  10 . 
     Further, it is preferred that a resin material having abrasion resistance and heat resistance is used for the slide bearings  15  and  17 . For example, PPS resin (polyphenylene sulfide resin) is suited to be used for the slide bearings  15  and  17 . This kind of resin is comparable to the material of the heating roller  10  (aluminum) and to the metal or resin material of the brackets  31  and  35  in coefficient of thermal expansion. Therefore, with temperature changes, this kind of resin expands/contracts to substantially the same degree as the heating roller  10 , the brackets  31  and  35 . Therefore, even when the heating roller  10  expands due to heat during operation of the fixing device, there is no possibility that an unexpected backlash or bite may occur between these members. 
       FIG. 5  shows a modified slide bearing  15 ′. The slide bearing  15 ′ has the same structure as the slide bearing  15  except that the slide bearing  15 ′ has a plurality of (preferably two) small projections  15   d  on the outer circumferential surface at predetermined intervals. The small projections  15   d  of the slide bearing  15 ′ come into contact with the edge surface of the hole  32 , thereby allowing absorption of a difference between the outer shape of the slide bearing  15 ′ and the shape of the hole  32 . That is, when the slide bearing  15  and the edge surface of the hole  32  make surface contact with each other, abrasion may occur locally at an unexpected portion due to errors in roundness of the slide bearing  15  and the hole  32 , which results in a degradation in function as a bearing. However, when the slide bearing  15  and the peripheral surface of the hole  32  make point contact with each other at the small projections  15 ′, only the small projections  15   d  will be abraded, which never leads to a degradation in function as a bearing. 
     Other Embodiments 
     The slide bearing for supporting the second end portion of the heating roller may be designed arbitrarily. Also, although in the above-described embodiment, the flanges of the gears  45  and  46  regulate the position of the second end portion of the heating roller in the thrust direction, it is possible to use any other structure to regulate the position of the second end portion of the heating roller in the thrust direction. Further, various structures are possible to permit the pressing roller to be driven to rotate by rotation of the heating roller while staying in a fixed position. 
     According to the embodiments, the fixing device can be assembled easily and can be downsized. 
     Although the present invention has been described in connection with the preferred embodiments above, it is to be noted that various changes and modifications are possible to those who are skilled in the art. Such changes and modifications are to be understood as being within the scope of the present invention.