Abstract:
An image forming apparatus has a high-speed mode and a low-speed mode and includes a speed switch unit configured to select the high-speed mode or the low-speed mode by switching a rotation direction of a drive source. The speed switch unit includes a drive gear attached to a rotating shaft of the drive source; a first drive gear series transmitting a rotating power of the drive source upon rotation in a first direction to an image carrier; and a second drive gear series transmitting a rotating power of the drive source upon rotation in a second direction to the image carrier, the second drive gear series having a larger reduction ratio than the first drive gear series. The speed switch unit causes the drive gear to be selectively connected to the first drive gear series or the second drive gear series depending on the rotating direction of the drive source.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to image forming apparatuses, such as copy machines, printers, facsimile machines, plotters, and multifunction peripherals (MFP) incorporating multiple image forming functions, such as copying and printing functions. More particularly, the present invention relates to an image forming apparatus having multiple image formation speed modes. 
     2. Description of the Related Art 
     An image forming apparatus is known in which a low-speed mode or a high-speed mode can be selected by a user. In the low-speed mode, image quality may be given priority, while in the high-speed mode, speed (productivity) may be given priority. In this type of an image forming apparatus, a drive source, such as a motor, may be connected to an image carrier, such as a photosensitive drum, via a series of drive gears. When the gear ratio of the series of drive gears is fixed, the high-speed mode and the low-speed mode may be switched by varying the number of rotations of the drive source. 
     In this type of an image forming apparatus, noise may increase in the high-speed mode. The noise during an image formation operation is known to be largely due to the noise level of the gear meshing frequency of drive source gears. The gear meshing frequency is the number of times two gears mesh with each other per second. For example, the gear meshing frequency of a drive source is the number of times a motor gear and a transmission gear mesh with each other per second. Thus, the gear meshing frequency, and hence the noise level, can be reduced by decreasing the number of rotations of the motor in the drive source. Desirably, the gear meshing frequency should be lowered below 100 Hz because the sound of such frequencies is difficult for humans to hear. 
     The drive source in this type of image forming apparatus may include a so-called FG (frequency-generating) output motor equipped with a frequency generator. Typically, the FG output motor has a pattern of frequency-generating pulse shapes (“FG pattern”) disposed opposite a magnet of a rotating part of the motor. As the motor rotates, electromagnetic induction is caused between the magnet and the FG pattern, thereby producing a pulse current. Based on the pulse current, a feedback control is performed so that the rotating speed of the motor can be controlled (see Japanese Laid-Open Patent Application No. 09-46995, for example). The FG output motors are frequently used as a drive source for image forming apparatuses because of their inexpensive rotation control mechanism. 
     As mentioned above, the high-speed mode and the low-speed mode may be switched by changing the number of rotations of the drive source when the gear ratio the series of drive gears is fixed. In this case, when the rotation speed of the drive source in the high-speed mode is lowered in order to reduce the noise level of the gear meshing frequency of the drive source gears, the number of rotations for the low-speed mode also decreases because of the fixed gear ratio. As a result, the frequency generator may not be able to produce a sufficient level of pulse signal for the feedback control of the rotation speed of the motor. 
     Japanese Laid-Open Patent Application No. 2002-089638 discusses a drive apparatus including various motors, a simple planetary gear mechanism as an intermediate speed-reduction mechanism, and various speed-reduction units. In this drive apparatus, the motors and the speed-reduction units can be selectively engaged with the simple planetary gear mechanism on an input and an output end, respectively, in order to reduce vibration and noise. 
     Japanese Laid-Open Patent Application No. 2007-212806 discusses a rotating drive apparatus including a drive source, a series of gears, and a driven member. The gears are coupled via planetary gears for increasing accuracy of rotation of an output shaft and reducing the size in the shaft axial direction, while allowing the detachment of the driven member from the rotating drive apparatus. 
     SUMMARY OF THE INVENTION 
     In one aspect of the present invention, a swing gear mechanism includes a frame having a first and a second arch-shaped guide opening having a first end and a second end; a first swing gear supported by the frame with a shaft of the first swing gear being guided in the first arch-shaped guide-opening; a second swing gear supported by the frame with a shaft of the second swing gear being guided in the second arch-shaped guide opening; and a drive gear meshed with the first and the second swing gears and configured to rotate in a first or a second direction. The first swing gear and the second swing gear are displaced to the first end of the corresponding arch-shaped guide openings upon rotation of the drive gear in the first direction, or to the second end of the corresponding arch-shaped guide openings upon rotation of the drive gear in the second direction. 
     In another aspect of the present invention, an image forming apparatus includes the swing gear mechanism. 
     In yet another aspect of the present invention, an image forming apparatus has a high-speed mode and a low-speed mode and includes a drive source configured to be rotated in a first direction or a second direction; an image carrier configured to be rotated by the drive source; an optical scanning unit configured to scan the image carrier with a beam of light in order to form an electrostatic latent image on the image carrier; a developing unit configured to develop the electrostatic latent image on the image carrier into a visible image; a transfer unit configured to transfer the visible image onto a recording medium directly or indirectly; and a speed switch unit configured to select the high-speed mode or the low-speed mode by switching a rotation direction of the drive source. The speed switch unit includes a drive gear attached to a rotating shaft of the drive source; a first drive gear series configured to transmit a rotating power of the drive source upon rotation in the first direction to the image carrier; and a second drive gear series configured to transmit a rotating power of the drive source upon rotation in the second direction to the image carrier, the second drive gear series having a larger reduction ratio than the first drive gear series. The speed switch unit is configured to cause the drive gear to be selectively connected to the first drive gear series or the second drive gear series depending on the rotating direction of the drive source. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a laser color printer as an image forming apparatus according to an embodiment of the present invention; 
         FIG. 2  illustrates a drive mechanism of the laser color printer in a high-speed mode; 
         FIG. 3A  illustrates a swing-gear mechanism in the drive mechanism of the laser color printer; 
         FIG. 3B  illustrates an assembly of a motor (drive source) and the swing-gear mechanism; 
         FIG. 4  illustrates the drive mechanism of the laser color printer according to the present embodiment in a low-speed mode; 
         FIG. 5  is a graph indicating torque sound pressure level with respect to the number of rotations of the drive source; and 
         FIG. 6  illustrates a drive mechanism according to a conventional technology. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  illustrates a laser color printer  100  as an image forming apparatus according to an embodiment of the present invention. In the laser color printer  100 , photosensitive drums (image carriers)  20 Y (yellow),  20 M (magenta),  20 C (cyan), and  20 K (black) are disposed side by side along an extended surface of an intermediate transfer belt  50  which is supported by support rollers  102   a ,  102   b , and  102   c . The laser color printer  100  further includes an optical scan unit  105  (exposure unit); charging units (not shown), developing units  106 Y,  106 M,  106 C and  106 K; a primary transfer roller (not shown) disposed inside the intermediate transfer belt  50 ; a cleaning unit (not shown); and a neutralizing unit (not shown). 
     The optical scan unit  105  is configured to emit laser beams L 1 , L 2 , L 3 , and L 4  in accordance with image information signals for the various colors. The laser beams L 1 , L 2 , L 3 , and L 4  hit the photosensitive drums  20 Y,  20 M,  20 C, and  20 K, thereby forming electrostatic latent images of the various color components on the photosensitive drums  20 Y,  20 M,  20 C, and  20 K. The latent images are thereafter rendered into visible toner images by the developing units  106 Y,  106 M,  106 C, and  106 K, as well known in the art. 
     The toner images of the various colors are successively transferred onto the intermediate transfer belt  50 , forming an overlaid color image. The overlaid image is then transferred onto a transfer sheet  120  (recording medium) by the secondary transfer roller  102   d . The transfer sheet  120  is fed from the sheet-feeding cassette  111  at a predetermined timing. Thereafter, the intermediate transfer belt  50  is cleaned by the cleaning unit. The transfer sheet  120  with the color image transferred thereon is transported to the fusing unit  114  where the color image is fused onto the transfer sheet  120  using heat and pressure. The fused transfer sheet is then ejected onto an ejected sheet tray  110 . 
       FIG. 2  illustrates a drive mechanism  1  for the image forming apparatus  100 . In  FIG. 2 , the intermediate transfer belt  50  (indicated by broken lines) is supported across belt gears  36  and  15 , which are integrally formed with the support rollers  102   a  and  102   b , respectively. The drive mechanism  1  includes a drive gear  3  for driving the photosensitive drum  20 K and a drive gear  5  for driving the color photosensitive drums  20 Y,  20 M, and  20 C. The drive gears  3  and  5  are fixed to rotating shafts  6   a  of FG-output-type motors  6  (drive source), as will be described below. 
     The drive gear  3  is meshed with a speed-reduction gear  7 . The speed-reduction gear  7  is meshed with a drum gear  9  that is integral with the photosensitive drum  20 K. The speed-reduction gear  7  is also meshed with a speed-reduction gear  11 . The speed-reduction gear  11  is coupled with a belt gear  15  via an idler gear  13 . The belt gear  15  is integral with the support roller  102   b . Rotation of the motor  6  for the drive gear  3  in counter-clockwise direction (“second direction”) causes the drum gear  9  to rotate in a direction indicated by the corresponding arrow (counter-clockwise direction) via the speed-reduction gear  7 . At the same time, the belt gear  15  is caused to rotate in a direction indicated by the corresponding arrow (clockwise direction). 
     The drive gear  5  for driving the color photosensitive drums  20 Y,  20 M, and  20 C is meshed with swing gears  17  and  19 . The swing gear  17  is engageable with a speed-reduction gear  21 . The other swing gear  19  is engageable with a speed-reduction gear  22  meshed with the speed-reduction gear  21 . The speed-reduction gear  21  is also meshed with a drum gear  23  that is integral with the photosensitive drum  20 M. Idler gears  25  and  27  are meshed with the speed-reduction gear  21  on an input end. The idler gear  25  is further engaged with a drum gear  31  via a speed-reduction gear  29 . The drum gear  31  is integral with the photosensitive drum  20 Y. The idler gear  27  is also engaged with a drum gear  35  via a speed-reduction gear  33 . The drum gear  35  is integral with the photosensitive drum  20 C. 
     The belt gear  36  is integral with the support roller  102   a  ( FIG. 1 ). Toner supply units  38 Y,  38 M,  38 C, and  38 K are configured to supply the various colors of toner to the developing unit  106 Y,  106 M,  106 C, and  106 K. The speed-reduction gear  22  is disposed above a center line of the photosensitive drum  20 M (magenta); namely, the drum gear  23 . In this way, the space between the photosensitive drums  20 M and  20 C and additionally defined by the toner supply unit  38 C, for example, can be effectively utilized for a structure (including the swing gears  17  and  19  and guide openings  43  and  45 ) for enabling the switching between the high-speed mode and the low-speed mode, as will be described later. 
       FIG. 3A  illustrates a swing-gear mechanism, and  FIG. 3B  illustrates an assembly of the FG-output-type motor  6 , the drive gear  5 , and the swing gears  17  and  19 . The FG-output-type motor  6  to which the drive gear  5  is fixed may include a frequency generator for detecting a rotation speed by an electromagnetic pattern generating method. The electromagnetic pattern generating method may involve generating a pulse signal using an electromagnetic pattern (rotation speed detecting unit) disposed between a rotating part and a fixed part (which are not illustrated) of the motor  6  when the motor  6  rotates by a predetermined angle. The time interval of generation of such pulse signals may be detected as a speed and supplied for a feedback control. 
     Referring to  FIG. 3B , the motor  6  is supported on a motor circuit board  37  (drive source fixing unit) and a frame  39 . On the motor circuit board  37 , there may be formed the FG pattern as a part of the aforementioned electromagnetic pattern. The swing gears  17  and  19  are supported between the frame  39  and another frame  41  having the guide openings  43  and  45  in them. The swing gears  17  and  19  are movable in the guide openings  43  and  45 . The swing gears  17  and  19  are pressurized in a thrust direction by thrust springs  47  and  49 . The gears  17  and  19  are integral with shafts that are movable in the guide openings  43  and  45 . The guide openings  43  and  45  have a smooth arc shape so that the shafts of the gears  17  and  19  can smoothly move therein. The ends of the guide openings  43  and  45  have a shape conforming to the circumferential surface of the shafts of the swing gears  17  and  19 . 
     When the motor  6  rotates in one direction or the other, the swing gears  17  and  19  are displaced in the guide openings  43  and  45  by a pressing force provided by the rotation of the motor  6 , so that the swing gears  17  and  19  rotate with their shafts abutted against one or the other end of the guide openings  43  and  45 .  FIG. 3A  illustrates the case where the swing gear  17  is displaced to the right while the swing gear  19  is displaced to the left with reference to the drawing in a swinging motion when the motor  6  rotates in counter-clockwise direction (“second direction”) in the low-speed mode. 
     On the other hand, in the high-speed mode, the motor  6  rotates in clockwise direction (“first direction”) with reference to  FIGS. 2 and 3 , for example. In this case, the swing gear  17  is displaced to the left and meshed with the speed-reduction gear  21  as illustrated in  FIG. 2 , so that the color photosensitive drums  20 M,  20 Y, and  20 C are rotated at high speed. In this case, the swing gear  17 , the speed-reduction gear  21 , and the drum gear  23  constitute a first drive gear series for the high-speed mode, the swing gear  17  being the most upstream gear. The swing gear  19 , the speed-reduction gear  22 , the speed-reduction gear  21  and the drum gear  23  constitute a second drive gear series (for the low-speed mode), with the swing gear  19  being the most upstream gear. 
     When the motor  6  rotates in the first (clockwise) direction with reference to  FIG. 2 , for example, the swing gear  19  is disengaged from the speed-reduction gear  22 , so that the second drive gear series is rendered incapable of transmitting drive power. Referring to  FIG. 4 , when the motor  6  rotates in the second (counter-clockwise) direction for the low-speed mode, the swing gear  19  is meshed with the speed-reduction gear  22 , so that the color photosensitive drums  20 M,  20 Y,  20 C are rotated at a low speed. In the low-speed mode, the swing gear  17  is disengaged from the speed-reduction gear  21 , thus rendering the first drive gear series incapable of transmitting drive power. The structure including the drive gear  5 , the first drive gear series, the second drive gear series, and the swing-gear mechanism may be hereafter referred to as a “speed switch unit”. 
     Table 1 below illustrates a specification of the drive mechanism  1  according to an embodiment of the present invention. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
             
             
               
                   
                 Torque of photosensitive drum and roller 
                 0.5 N · m 
               
               
                   
                 102b 
               
               
                   
                 Number of photosensitive drums driven by 
                 3 
               
               
                   
                 drive gear 5 
               
               
                   
                 Gear transmission efficiency 
                 0.95 
               
               
                   
                 Rotation speed (rpm) of photosensitive 
                 94.03 
               
               
                   
                 drum (high-speed mode) 
               
               
                   
                 Rotation speed (rpm) of photosensitive 
                 47.02 
               
               
                   
                 drum (low-speed mode) 
               
               
                   
                 Rotation speed (rpm) of support roller 
                 117.00 
               
               
                   
                 102b (high-speed mode) 
               
               
                   
                 Rotation speed (rpm) of support roller 
                 58.50 
               
               
                   
                 102b (low-speed mode) 
               
               
                   
                 Number of teeth of drive gear 5 
                 8 
               
               
                   
                   
               
               
                   
                 *The number of teeth of drive gear 5 may be selected depending on the cost of bar material prior to formation of teeth in it. 
               
             
          
         
       
     
     In accordance with the present embodiment, the number of rotations of the motor  6  in the high-speed mode may be set at 700 rpm, as illustrated in Table 2. 700 rpm is a relatively low speed that can be controlled by a FG-output-type motor and that satisfies the condition that the gear meshing frequency be below 100 Hz, which corresponds to the low-frequency sound that is hard for humans to hear. In this case, the gear meshing frequency is 93.3 Hz, indicating a sufficient decrease in noise. 
     In accordance with the present embodiment, in order to switch to the low-speed mode, the motor  6  is rotated in the second direction so that the motor  6  is engaged with the speed-reduction gear  21  via the swing gear  19  and the speed-reduction gear  22 . Thus, a lower rotation speed is achieved by increasing the reduction ratio compared to the case where the motor  6  is rotated in the first direction. 
     Thus, the difference in the number of rotations of the photosensitive drums between the high-speed mode and the low-speed mode is provided by varying the reduction ratio of the drive gear series while the number of rotations of the motor  6  is set at a constant value of 700 rpm, for example. In this way, two or more speed modes can be realized without changing the rotation speed of the motor  6 , so that the rotation speed of the motor  6  can be set to a low speed at all times that contributes to a decrease in noise. Thus, the gear meshing frequency of the drive gear  5  can be made lower than the low-frequency sound of 100 Hz in any of the multiple speed modes. 
     
       
         
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Drive gear 
                 5 (for color drums) 
                 3 (for (K) drum) 
                 3 (for belt 50) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Gear 
                 5→17→ 
                 5→19→22→ 
                 3→7→9 
                 3→7→9 
                 3→11→ 
                 3→11→ 
               
               
                 sequence 
                 21→23 
                 21→23 
                   
                   
                 13→15 
                 13→15 
               
               
                 Speed mode 
                 High 
                 Low 
                 High 
                 Low 
                 High 
                 Low 
               
               
                 Rpm of 
                 700.0 
                 700.0 
                 1400.0 
                 700.0 
                 1400.0 
                 700.0 
               
               
                 drive 
                   
                   
                   
                   
                   
                   
               
               
                 source 
                   
                   
                   
                   
                   
                   
               
               
                 Gear ratio 
                 7.4 
                 14.9 
                 14.9 
                 14.9 
                 12.0 
                 12.0 
               
               
                 Output (W) 
                 18 
                 9 
                 13 
                 6 
                 13 
                 6 
               
               
                 Shaft 
                 0.126 
                 0.252 
                 0.089 
                 0.089 
                 0.089 
                 0.089 
               
               
                 torque 
                   
                   
                   
                   
                   
                   
               
               
                 (N · m) 
                   
                   
                   
                   
                   
                   
               
               
                 Sound 
                 50.0 
                 49.0 
                 53.0 
                 49.0 
                 53.0 
                 49.0 
               
               
                 pressure 
                   
                   
                   
                   
                   
                   
               
               
                 level 
                   
                   
                   
                   
                   
                   
               
               
                 (dBA) 
                   
                   
                   
                   
                   
                   
               
               
                 Meshing 
                 93.3 
                 93.3 
                 186.7 
                 93.3 
                 186.7 
                 93.3 
               
               
                 frequency 
                   
                   
                   
                   
                   
                   
               
               
                 (Hz) 
               
               
                   
               
               
                 *Reduction ratio is the ratio of the numbers of rotation of the drive source to the photosensitive drum or the support roller. 
               
             
          
         
       
     
     Table 2 corresponds to a case where the aforementioned speed switch unit (including the drive gear, the first and the second drive gear series, and the swing-gear mechanism) is not applied to the drive gear  3  for the photosensitive drum  20 K (for black). However, in another embodiment of the present invention, the speed switch unit may be applied to the drive gear  3  for the photosensitive drum  20 K in the same way as for the color photosensitive drums  20 Y,  20 M, and  20 C for enhanced noise reduction purposes. 
       FIG. 5  is a graph indicating torque and sound pressure level with respect to the number of rotations (rpm). The initial rpm of “700” is the number of rotations in the high-speed mode. The second rpm of “700” is the number of rotations in the low-speed mode. In the low-speed mode, torque increases due to the increased reduction ratio. The corresponding values are shown in Table 3. 
     
       
         
               
               
               
             
           
               
                   
               
               
                   
                   
                 Sound pressure level 
               
               
                 rpm 
                 Torque (N · m) 
                 (dBA) 
               
               
                   
               
             
             
               
                 700.0 (High- 
                 0.126 
                 49.0 
               
               
                 speed mode) 
               
               
                 700.0 (Low- 
                 0.252 
                 50.0 
               
               
                 speed mode) 
               
               
                 1400.0 
                 0.126 
                 53.0 
               
               
                   
               
             
          
         
       
     
       FIG. 6  illustrates a conventional drive mechanism in which the speed switch unit according to the foregoing embodiment of the present invention is not used. As illustrated, the drive gear  5  is directly meshed with the speed-reduction gear  21 . Thus, drive power from the drive source is transmitted by a series of drive gears including the drive gear  5 , the speed-reduction gear  21 , and the drum gear  23  in a fixed manner, so that the rotation direction of the motor  6  is fixed to the second direction (counter-clockwise direction). 
     In this conventional example, the number of rotations of the motor  6  in the low-speed mode may be fixed at 700 rpm while the high-speed mode may be provided by doubling the rotation speed of the motor  6  to 1400 rpm. In this case, in the high-speed mode, the gear meshing frequency of the drive gear  5  is 186.7 Hz as illustrated in Table 4 below, which is far above the low-frequency sound threshold of 100 Hz, resulting in a large noise level. If the rotation speed in the high-speed mode is lowered in order to reduce the noise, the decrease in rotation speed is directly reflected in the low-speed mode because of the fixed reduction ratio of the drive gear series. As a result, the rotation speed in the low-speed mode greatly decreases, making it impossible to control the FG-output-type motor  6 . 
     
       
         
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
               
                 Drive gear 
                 5 (for color drums) 
                 3 (for (K) drum) 
                 3 (for belt 50) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Gear 
                 5→21→23 
                 5→21→23 
                 3→7→9 
                 3→7→9 
                 3→11→ 
                 3→11→ 
               
               
                 sequence 
                   
                   
                   
                   
                 13→15 
                 13→15 
               
               
                 Speed mode 
                 High 
                 Low 
                 High 
                 Low 
                 High 
                 Low 
               
               
                 Rpm of 
                 1400.0 
                 700.0 
                 1400.0 
                 700.0 
                 1400.0 
                 700.0 
               
               
                 drive 
                   
                   
                   
                   
                   
                   
               
               
                 source 
                   
                   
                   
                   
                   
                   
               
               
                 Gear ratio 
                 14.9 
                 14.9 
                 14.9 
                 14.9 
                 12.0 
                 12.0 
               
               
                 Output (W) 
                 18 
                 9 
                 13 
                 6 
                 13 
                 6 
               
               
                 Shaft 
                 0.126 
                 0.126 
                 0.089 
                 0.089 
                 0.089 
                 0.089 
               
               
                 torque 
                   
                   
                   
                   
                   
                   
               
               
                 (N · m) 
                   
                   
                   
                   
                   
                   
               
               
                 Sound 
                 53.0 
                 49.0 
                 53.0 
                 49.0 
                 53.0 
                 49.0 
               
               
                 pressure 
                   
                   
                   
                   
                   
                   
               
               
                 level 
                   
                   
                   
                   
                   
                   
               
               
                 (dBA) 
                   
                   
                   
                   
                   
                   
               
               
                 Meshing 
                 186.7 
                 93.3 
                 186.7 
                 93.3 
                 186.7 
                 93.3 
               
               
                 frequency 
                   
                   
                   
                   
                   
                   
               
               
                 (Hz) 
               
               
                   
               
               
                 *Reduction ration values may be in integers so that an image position error due to motor vibration can be cancelled. 
               
             
          
         
       
     
     Although this invention has been described in detail with reference to certain embodiments, variations and modifications exist within the scope and spirit of the invention as described and defined in the following claims. 
     The present application is based on Japanese Priority Application No. 2009-198660 filed Aug. 28, 2009, the entire contents of which are hereby incorporated by reference.