Abstract:
A belt driving apparatus is constructed so that the edge portion of a belt will not run over a stopper or be damaged. A belt is entrained about a plurality of rollers. A drive source is coupled to at least one of the plurality of rollers. The drive source drives the at least one of the plurality of rollers in rotation. A restriction member is provided in the vicinity of at least one longitudinal end portion of at least one of the plurality of rollers. The restriction member prevents the belt from moving outwardly in a longitudinal direction of the at least one of the plurality of rollers and preventing the belt from moving outwardly in a radial direction of the at least one of the plurality of rollers.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a belt driving apparatus, fixing apparatus and image forming apparatus.  
         [0003]     2. Description of the Related Art  
         [0004]     Among the conventional electrophotographic image forming apparatuses are printers, copying machines, and facsimile machines. For printers, a charging unit charges the entire surface of a photoconductive drum to a uniform potential. Then, an exposing unit illuminates the charged surface to form an electrostatic latent image. A developing unit develops the electrostatic latent image into a toner image. The toner image is then transferred onto a recording paper. The recording paper having the toner image thereon is then advanced to a fixing unit where the toner image is fixed into a permanent image.  
         [0005]     The fixing unit includes a fixing roller and a pressure roller in pressure contact with the fixing roller. When the recording paper is pulled in between the fixing roller and the pressure roller, the fixing roller heats the toner image and the pressure roller presses the toner image against the recording paper. In order to increase printing speed, the amount of heat supplied to the toner image needs to be increased.  
         [0006]     For this purpose, a belt type fixing unit has been proposed which includes a heat roller in addition to a fixing roller and a pressure roller. An endless belt is sandwiched between the fixing roller and heater roller unit. A relatively large nip is formed between the belt and pressure roller, and the heat roller and pressure roller are heated to heat the belt.  
         [0007]     If the belt runs crooked due to dimension errors in various members, imbalance of tension applied to the belt, and non-uniform temperature distribution across the length of the fixing roller and heat roller, the edge portion of the belt will be damaged. In order to solve this problem, a flange is provided at the longitudinal ends of the heat roller and serves as a stopper that prevents the belt from shifting toward one ends of the fixing roller and heat roller.  
         [0008]      FIG. 11  illustrates a conventional belt.  FIG. 12  and  FIG. 13  illustrate how the belt shifts to one side.  
         [0009]     Referring to  FIGS. 11-13 , a flange  13  is disposed on one end of a heat roller  11 . The heat roller  11  has flanges  13  at its longitudinal ends and the flanges  13  are rotatably supported by bearings  14 . The bearing  14  is supported by a supporting frame  15 . The heat roller  11  rotates in a direction shown by arrow A. A belt  12  is entrained about the heat roller  11  and runs in a direction shown by arrow B.  
         [0010]     Referring to  FIG. 12 , when the belt  12  shifts toward one longitudinal end of the heat roller  11  to touch the flange  13 , a frictional force is exerted on the belt  12  to cause the edge portion of the belt  12  to deform in a direction shown by arrow C at a point P 1  where the belt  12  contacts the flange  13 . Prolonged application of such a frictional force will eventually cause the edge portion of the belt  12  deform greatly in a radial direction of the flange as shown in  FIG. 13 , so that the belt runs over the flange  13  or becomes damaged.  
       SUMMARY OF THE INVENTION  
       [0011]     The present invention was made in view of the aforementioned problems with the conventional printers.  
         [0012]     An object of the invention is to provide a belt driving apparatus, a fixing apparatus, and an image forming apparatus where the edge portion of a belt will not run over a stopper or be damaged.  
         [0013]     A belt driving apparatus is constructed so that the edge portion of a belt will not run over a stopper or be damaged. A belt is entrained about a plurality of rollers. A drive source is coupled to at least one of the plurality of rollers. The drive source drives the at least one of the plurality of rollers in rotation. A restriction member is provided in the vicinity of at least one longitudinal end portion of at least one of the plurality of rollers. The restriction member prevents the belt from moving outwardly in a longitudinal direction of the at least one of the plurality of rollers and preventing the belt from moving outwardly in a radial direction of the at least one of the plurality of rollers.  
         [0014]     The restriction member may include a flange and a rotating member. The flange is provided at the at least one end portion of the at least one of the plurality of rollers and projects in a radial direction from the at least one of the plurality of rollers. The rotating member is supported to roll on a peripheral edge portion of the belt in the vicinity of the flange and presses the peripheral end portion of the belt against the at least one of the plurality of rollers.  
         [0015]     The rotating member may include a shaft, a resin layer, and a heat-resistant resilient layer applied over the resin layer. The roller rolls on the peripheral edge portion of the belt. The resin layer covers the shaft. The heat-resistant resilient layer covers the resin layer.  
         [0016]     The at least one end portion of the at least one of the plurality of rollers is rotatably received in a space formed in a stationary supporting member in such a way that the at least one end portion is movable within the space relative to the rotating member as the belt moves in the longitudinal direction.  
         [0017]     The rotating member is formed with a tapered circumferential surface conical with respect to a rotational axis of the rotating member, the tapered circumferential surface being tapered toward a longitudinally middle portion of the at least one of the plurality of rollers.  
         [0018]     The tapered circumferential surface  46   c  is such that the difference between a large diameter and a small diameter of the tapered circumferential surface is larger than a thickness of the belt.  
         [0019]     The belt has a three-layer structure consisting of a base layer, a resilient layer, and a mold releasing layer, which are layered in this order from an inner layer of the belt to an outer layer.  
         [0020]     Another belt driving apparatus is constructed so that the edge portion of a belt will not run over a stopper or be damaged. A belt is entrained about a plurality of rollers. A drive source is coupled to at least one of said plurality of rollers, said drive source driving the at least one of said plurality of rollers in rotation. A rotating member is rotatably supported on a stationary supporting member and rolls on an outer circumferential surface of the at least one of the plurality of rollers. The rotating member has a tapered circumferential surface that is conical with respect to a rotational axis of the rotating member. The tapered circumferential surface is tapered toward a longitudinally middle portion of the at least one of the plurality of rollers. An urging member urges the at least one of the plurality of rollers toward the rotating member. The at least one end portion of the at least one of the plurality of rollers is rotatably received in a space formed in the stationary supporting member in such a way that the at least one end portion is movable within the space relative to the rotating member as the belt moves in the longitudinal direction.  
         [0021]     The tapered circumferential surface forms an angle in the range of 30 to 45 degrees with a rotational axis of the at least one of the plurality of rollers.  
         [0022]     The belt has a thickness in the rage of 0.3 to 1.0 mm.  
         [0023]     The belt has a thickness in the rage of 0.5 to 1.0 mm.  
         [0024]     The rotating member includes a flange that projects radially from the rotating member and rotates in contact with an end surface of the at least one of the plurality of rollers.  
         [0025]     A fixing apparatus incorporates the aforementioned belt driving apparatus.  
         [0026]     An image forming apparatus incorporates the aforementioned belt driving apparatus.  
         [0027]     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limiting the present invention, and wherein:  
         [0029]      FIG. 1  is a schematic diagram of a printer;  
         [0030]      FIG. 2  is a side view illustrating a fixing unit according to the first embodiment;  
         [0031]      FIG. 3  illustrates a heat roller and a guide roller according to a first embodiment;  
         [0032]      FIG. 4  is a perspective view of a belt, fixing roller, and heat roller when the belt runs normally;  
         [0033]      FIG. 5  is a perspective view of the belt, the fixing roller, and a heat roller when the belt shifts to one side;  
         [0034]      FIG. 6  and  FIG. 7  are cross-sectional views of a guide roller according to a third embodiment;  
         [0035]      FIG. 8  illustrates a direction in which a force acts so that the heat roller applies tension to a belt;  
         [0036]      FIG. 9  illustrates a heat roller and a guide roller according to a fourth embodiment;  
         [0037]      FIG. 10  illustrates a heat roller and a guide roller according to a fifth embodiment;  
         [0038]      FIG. 11  illustrates a conventional belt; and  
         [0039]      FIG. 12  and  FIG. 13  illustrate how a belt shifts to one side.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0040]     Embodiments of the present invention will be described with reference to the accompanying drawings. By way of example, an image forming apparatus will be described in terms of a color printer.  
       First Embodiment  
       [0041]      FIG. 1  is a schematic diagram of a printer.  FIG. 2  is a side view illustrating a fixing unit according to the first embodiment.  
         [0042]     Referring to  FIG. 1 , image forming sections  20 Y- 20 BK are aligned in a direction in which recording paper is advanced. The image forming sections  20 Y- 20 BK are provided with developing units  22 Y- 22 BK, photoconductive drums  25 Y- 25 Bk, exposing units  21 Y- 21 BK, and transfer rollers  23 Y- 23 BK, respectively. The exposing units  21 Y- 21 BK form electrostatic latent images of corresponding colors on the photoconductive drums  25 Y- 25 BK. The developing units  22 Y- 22 BK develop the electrostatic latent images with toners of corresponding colors into color toner images.  
         [0043]     The transfer rollers  23 Y- 23 BK are under the photoconductive drums  25 Y- 25 BK with a belt  26  sandwiched between the transfer rollers  23 - 23 BK and the photoconductive drums  25 Y- 25 BK. The belt  26  is entrained about a drive roller  27  and a driven roller  28  and runs in a direction shown by arrow E. The paper is fed onto the belt from a paper-feeding unit, not shown. The belt  26  transports the paper through the image forming sections  20 Y- 20 BK, so that the transfer rollers transfer the toner images of the corresponding colors onto the paper to form a full color toner image on the paper.  
         [0044]     When the drive roller  27  is driven in rotation by a motor, not shown, the belt runs and the driven roller  28  also rotates.  
         [0045]     The paper is advanced to a belt type fixing unit  24 , which in turn fixes the color toner image into a permanent full color image. A fixing roller  31  is rotatable in a direction shown by arrow G. A pressure roller  32  is rotatable in a direction shown by arrow H. A heat roller  33  is rotatalbe in a direction shown by arrow I. An endless belt  34  is entrained about the fixing roller  31  and heat roller  33 , and runs in a direction shown by arrow J. A motor M ( FIG. 1 ) is provided for driving the belt  34 . The motor M is coupled to at least one of the fixing roller  31 , heat roller  33  and pressure roller  32 . In the first embodiment, the motor M is coupled to the fixing roller  31 . When the motor M rotates, the belt  34  runs and the pressure roller  32  and heat roller  33  rotate accordingly.  
         [0046]     The pressure roller  32  and heat roller  33  have heat sources  32   a  and  33   a,  respectively. The heat sources  32   a  and  33   a  heat the pressure roller  32  and heat roller  33 , which in turn heat the belt  34 . The pressure roller  32  presses the fixing roller  31  via the belt  34  in a direction shown by arrow K. The heat roller  33  presses the belt  34  in a direction shown by arrow N.  
         [0047]     The fixing roller  31  has an outer diameter of 30 mm. A resilient heat-insulating layer  31   b  in the form of silicone rubber is formed around a solid metal shaft  31   a.  The pressure roller  32  has an outer diameter of 30 mm. A resilient layer  32   c  in the form of silicone rubber is formed around the outer cylindrical surface of an aluminum or iron pipe  32   b.  A mold-releasing layer  32   d  is a fluorinated layer formed on the resilient heat-insulating layer  31   b  for improving the ability of the fixing roller  31  to release the toner. For heat roller  33  has an outer diameter of 24 mm and has an iron or aluminum hollow pipe  33   b.    
         [0048]     The belt  34  has an outer diameter of 60 mm when it is in the shape of a complete ring. The belt  34  has a three-layer of base layer, resilient layer, and mold releasing layer. The base layer is made of a metal such as stainless steal or nickel and has a thickness of about 0.04 mm. Alternatively, the base layer may be made of polyimide resin having a thickness of about 0.1 mm. The resilient layer is made of silicone rubber and has a thickness in the range of 0.15 to 0.3 mm. The mold releasing layer is a fluorinated layer so that the melted toner can be released easily from the mold layer.  
         [0049]     In order to increase the fixing speed to improve printing speed, the nip formed between the belt  34  and the pressure roller  32  needs to be made larger. For this purpose, the resilient heat insulating layer  31   b  is formed on the fixing roller  31  and the resilient layer  32   c  is formed on the pressure roller  32 . Because a full color toner image contains toner images of the respective colors and therefore the surface of the full color toner image is not smooth and flat but irregular. Thus, for uniformly pressing the color toner image, the belt  34  is provided with the aforementioned resilient layer.  
         [0050]     If the belt runs crooked due to dimensional errors in various members, imbalance of tension applied to the belt, and non-uniform temperature distribution across the length of the fixing roller and heat roller, the edge portion of the belt  34  is damaged.  
         [0051]      FIG. 3  illustrates a heat roller and a guide roller according to a first embodiment. Referring to  FIG. 3 , for preventing the belt  34  from shifting in an axis direction of the rollers, an annular sleeve  35  having a flange  35   a  is provided to at least one longitudinal end of the heat roller  33 . In the first embodiment, the sleeve  35  is provided at both longitudinal ends of the heat roller  33  and extends radially. The sleeve  35  may have a sector shaped flange in place of the sleeve  35 . The sleeve  35  has a radius larger than the sum of the radius of the heat roller  33  and the thickness of the belt  34 , so that the sleeve  35  projects further than the belt  34  in the radial direction.  
         [0052]     There is provided a guide roller  36  on at least one longitudinal end side of the heat roller  33 . The guide roller  36  is rotatably supported on the supporting frame  38  and rotates in contact with the surface of the belt  34  and the sleeve  35 . The heat roller  33  is supported by a bearing  37  which in turn is supported by the supporting frame  38 .  
         [0053]     Referring back to  FIG. 2 , the guide roller  36  is in contact with a flange  35   a  of the sleeve  35  and the belt  34  a position at which the belt  34  has run into wrapping contact with the heat roller  33 . The guide roller  36  rotates as the belt  34  runs.  
         [0054]     The guide roller  36  has a heat-resistant resin layer  36   b  formed on a shaft  36   a  so that the resin layer  36   b  will not damage the side surface of the flange  35  and the outer surface of the belt  34 . A heat-resistant resilient layer  36   c  formed of, for example, silicone rubber is formed on the resin layer  36   b.  Thus the guide roller  36  can effectively resiliently press the outer surface of the belt  34 . Alternatively, an additional resin layer may be formed on the heat-resistant layer  36   c.    
         [0055]     The operation of the printer of the aforementioned configuration will be described.  
         [0056]     When a printing operation is initiated, the belt  34  runs in the J direction and may run crooked due to dimensional errors in various members that form the fixing unit  24 , imbalance of tension applied to the belt  34 , and non-uniform temperature distribution across the length of the fixing roller  31  and heat roller  33 . At this moment, the edge portion of the belt  34  abuts the side surface of the sleeve  35 , which in turn prevents the belt  34  from shifting any further.  
         [0057]     The edge portion of the belt  34  moves into contact engagement with the sleeve  35  before the belt  34  wraps around the heat roller  33 . As the sleeve  35  rotates, the frictional force between the sleeve  35  and the belt  34  creates a force that acts on the belt  34  in a radially outward direction.  
         [0058]     Because the guide roller  36  contacts the outer surface o the belt  34  at a position where the belt  34  has wrapped around the heat roller by a small amount, and presses the edge portion of the belt  34  against the heat roller  33 , the edge portion of the belt  34  is prevented from deforming radially outwardly of the heat roller  33 . Thus, the belt  34  will be free from warping but wrap sufficiently around the heat roller  33  for reliable running.  
         [0059]     Thus, the edge portion of the belt  34  will not run over the sleeve  35  or be damaged.  
       Second Embodiment  
       [0060]     In order for a belt to run reliably, the force that causes the belt to shift one side needs to be minimized. A second embodiment is directed to minimizing the force that causes the belt to shift one side.  
         [0061]      FIG. 4  is a perspective view of a belt  34 , the fixing roller  31 , and a heat roller  33  when a belt  34  runs normally.  FIG. 5  is a perspective view of the belt  34 , the fixing roller  31 , and a heat roller  33  when the belt  34  shifts to one side.  
         [0062]     Referring to  FIGS. 4 and 5 , the fixing unit  24  unit  24  includes the fixing roller  31 , pressure roller  32  ( FIG. 1 ), heat roller  33 , and an endless belt  34  entrained about the fixing roller  31  and heat roller  33  and running in a direction shown by arrow J.  
         [0063]     When one longitudinal end of the heat roller  33  (left in  FIG. 5 ) is pivoted upward about another longitudinal end in a direction shown by arrow K, a portion of the belt  34  running in the J direction reaches the heat roller  33  and further runs along the outer circumference of the heat roller  33 . At this moment, the belt runs about the rotational axis of the heat roller  33  in a direction shown by arrow L, thereby wrapping around the heat roller  33 . Therefore, the belt  34  tends to shift in a direction shown by arrow M. Contrary, if the another longitudinal end of the heat roller  33  (right in  FIG. 5 ) is pivoted upward about the one longitudinal end, a portion of the belt  34  running in the J direction reaches the heat roller  33  and further runs along the outer circumference of the heat roller  33 . Therefore, the belt  34  tends to shift in a direction shown by arrow M.  
         [0064]     In this manner, causing the one longitudinal end of the heat roller  33  to pivot slightly about the other longitudinal end or vice versa, the belt  34  can be moved toward the one longitudinal end or the other, thereby minimizing the force that causes the belt to shift one side. This configuration allows the belt  34  to run reliably and increases reliability of the belt  34 .  
       Third Embodiment  
       [0065]     Elements similar to those in the first embodiment have been given the same reference numerals and the description is omitted.  
         [0066]      FIG. 6  and  FIG. 7  are cross-sectional views of a guide roller according to a third embodiment.  FIG. 8  illustrates a direction in which a force acts so that the heat roller applies tension to a belt  34 .  
         [0067]     A bearing  37  is provided to at least one longitudinal end of the heat roller  33 , and is disposed such that the heat roller  33  is somewhat movable within an opening  38   a  formed in the supporting frame  38  toward and away from the shaft  46   a.  When the bearing  37  is at an upper end of the opening  38   a,  there is a gap below the bearing  37  between the bearing  37  and the supporting frame  38  as shown in  FIGS. 6 and 8 . When the bearing  37  is at a lower end of the opening  38   a,  there is a gap above the bearing  37  between the bearing  37  and the supporting frame  38  as shown in  FIG. 7 .  
         [0068]     There are provided guide rollers  46  near the both longitudinal ends of the heat roller  33  such that the guide rollers  46  extend from the supporting frame  38  inwardly to oppose each other. The guide rollers  46  rotate in contact with the longitudinal end portion of the heat roller  33  and the sleeve  35 . The width of the belt  34  is shorter than the length of the heat roller  33  such that the edge portion of the belt  34  is not in contact with neither the sleeve  35  nor the guide rollers  46 . The guide rollers  46  have a conical or tapered circumferential surface  46   c  such that the difference W between a large diameter and a small diameter of the tapered circumferential surface is larger than the thickness of the belt  34 .  
         [0069]     The heat roller  33  is urged by a spring  24   a  in a direction away from the fixing roller  31  ( FIG. 1 ) , i.e., in a direction shown by arrow Q in  FIG. 8  so that the belt  34  has a predetermined tension therein. In this case, the spring applies a force that urges the bearing  37  upward.  
         [0070]     The edge portion of the belt  34  will not contact the sleeve  35  so that the bearing  37  is positioned at an upper position with the outer circumferential surface of the guide roller  46  in contact with the circumferential surface of the heat roller  33 .  
         [0071]     The operation of the printer of the aforementioned configuration will be described.  
         [0072]     When the edge portion of the belt  34  shifts toward a longitudinal end of the heat roller  33  as the belt  34  runs crooked, the edge portion moves into contact engagement with the tapered surface  46   c.  If the belt  34  further shifts, the edge portion of the belt  34  will be caught between the guide roller  46  and the circumferential surface of the heat roller  33 . At this moment, the bearing  37  moves in a direction away from the guide roller  46 . When the belt  34  is completely sandwiched between the guide roller  46  and the heat roller  33 , the bearing  37  is at its lowest position as shown in  FIG. 7 .  
         [0073]     As the bearing  37  moves downward, the heat roller  33  tilt in such a way that one longitudinal end of the heat roller  33  is lower than the other longitudinal end. Therefore, the belt  34  will shift to the opposite direction until the belt  34  reaches an equilibrium point at which the forces causing the belt  34  to shift are equal in magnitude and opposite in direction. In this manner, the edge portions of the belt  34  are not damaged.  
       Fourth Embodiment  
       [0074]     Elements similar to those in the third embodiment have been given the same reference numerals and the description thereof is omitted.  
         [0075]      FIG. 9  illustrates a heat roller and a guide roller according to a fourth embodiment.  
         [0076]     It is to be noted that there is not provided a flange such as the sleeve  35  in the third embodiment. This will be described later. The thickness of a belt  34  is selected to be larger than 0.3 mm and preferably in the range of 0.5 to 1.0 mm. The thickness of the belt  34  according to the fourth embodiment is larger than that of the belt  34  according to the third embodiment.  
         [0077]     When the belt  34  is caught between a heat roller  33  and a tapered surface  46   c  of a guide roller  46 , a bearing  37  at one end of the heat roller  33  moves away from the guide roller  46  so that the heat roller  33  will tilt slightly. As a result, a force is developed which causes the belt  34  to shift the belt  34  in the opposite direction. The larger the inclination of the heat roller  33  become, larger the shifting of the belt  34  is. Thus, the belt  34  will shift to the opposite direction until the belt  34  reaches an equilibrium point at which the forces causing the belt  34  to shift are equal in magnitude and opposite in direction. In this manner, the edge portions of the belt  34  are not damaged.  
         [0078]     In the fourth embodiment, the belt  34  will reach an equilibrium point at which the forces causing the belt  34  to shift are equal in magnitude and opposite in direction, before the edge portion of the belt  34  has been completely caught between the guide roller  46  and the heat roller  33 . For this purpose, the inclination angle θ of a tapered surface  46   c  with respect to the rotational axis of the guide roller  46  is selected to be in the rage of 30 to 45 degrees so as to depress the heat roller  33  when the heat roller  33  moves in its longitudinal direction.  
         [0079]     Because the belt  34  will reach the equilibrium point before the edge portion of the belt  34  has been completely caught between the guide roller  46  and the heat roller  33 , a flange such as the sleeve  35  in the third embodiment need not be provided at longitudinal ends of the heat roller  33 .  
       Fifth Embodiment  
       [0080]     Elements similar to those in the fourth embodiment have been given the same reference numerals and the description thereof is omitted.  
         [0081]      FIG. 10  illustrates a heat roller  33  and a guide roller  46  according to a fifth embodiment.  
         [0082]     The guide rollers  46  are provided on longitudinal end portions of the heat roller  33  in such a way that the guide rollers  46  extend inwardly to oppose each other. The guide roller  46  is formed in one piece with a flange  46   d  having a larger diameter than the guide roller  46 . As shown in  FIG. 10 , the guide roller  46  is disposed such that the flat peripheral surface of the flange  46   d  rotates in contact with the longitudinal end of the heat roller  33  and the guide roller  46  rotates in contact with the circumferential surface of the belt  34 . Even when the belt  34  shifts further after the belt  34  has been completely caught between the guide roller  46  and heat roller  33 , the edge portion of the belt  34  abuts the flange  46   d,  which in turn prevents the belt  34  from shifting any further.  
         [0083]     While the aforementioned embodiments have been described in terms of the belt  34  in the fixing unit  24 , the invention may also be applied to the belt  26  that transports the recording paper.  
         [0084]     While the aforementioned embodiments have been described with respect to the belt  34  entrained about two rollers, the invention may also be applied to a belt entrained about more than two rollers.  
         [0085]     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art intended to be included within the scope of the following claims.