Patent Publication Number: US-RE49143-E

Title: Image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an image forming apparatus such as a copying machine or a printer equipped with a function of forming an image on a recording material such as a sheet. 
     Description of the Related Art 
     Japanese Patent Laid-Open No. 9-230657 discloses a configuration in which an annular rib is disposed between a central portion and a tooth surface of a gear and the tooth surface and the annular rib are disposed at an interval so as not to come in contact with each other. According to such a configuration, since the tooth surface and the annular rib do not contact with each other, phenomena are suppressed in which a portion of the tooth surface coming in contact with the annular rib is deformed by shrinkage during molding and thus accuracy of the tooth surface deteriorates. 
     By this configuration, it is considered that variation in a position occurs due to a rotation fluctuation or vibration of an image preparing portion caused by a rotation fluctuation or vibration occurring at a gear engagement cycle and thus a periodic band-like uneven density called a banding image is prevented. 
     However, the invention disclosed in Japanese Patent Laid-Open No. 9-230657 does not cope with reduction in size of a gear and modules for the purpose of miniaturization of an apparatus body in recent years. It is difficult to make the modules smaller in the case of reducing the size of the gear. This reason is that stress applied to a tooth root of the gear rises when the size of the module becomes smaller. 
     Under these circumstance, the inventors paid attention to the fact that a portion of an arm formed between the tooth surface and a rotation support portion is disposed at the center of a tooth width direction in the configuration illustrated in FIG. 3 of Japanese Patent Laid-Open No. 9-230657. The inventors found that it is possible to reduce the size of the module and to lower the stress applied to the tooth root of the gear by changing the arrangement of the arm. 
     SUMMARY OF THE INVENTION 
     The invention is to provide an image forming apparatus capable of suppressing stress concentration on a tooth root of a gear. 
     An image forming apparatus that forms an image on a recording material includes: a first helical gear and a second helical gear that are engaged with each other; and a driving portion that applies a driving force to the first helical gear, wherein at least one of the first helical gear and the second helical gear is a helical gear in which torsional rigidity in a tooth width direction of one side end in a width direction of the gear is larger than torsional rigidity in a tooth width direction of the other side end, and wherein a twist direction of helical teeth and a rotational direction of the first helical gear due to the driving portion are set such that the other side end is engaged earlier than the one side end. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional view illustrating schematically an image forming apparatus according to the invention. 
         FIG. 2A  is a schematic diagram illustrating a state where motors are connected to a photosensitive drum and an intermediate belt unit, respectively, and  FIG. 2B  is a schematic diagram of a driving configuration of a developing device. 
         FIG. 3  is a schematic diagram of a gear arrangement in the driving configuration of the developing device illustrated in  FIG. 2B . 
         FIGS. 4A and 4B  are perspective views illustrating a developing motor gear and a developing reduction gear in detail. 
         FIG. 5  is a cross-sectional view of the developing motor gear and the developing reduction gear. 
         FIGS. 6A to 6F  are perspective views illustrating calculation results of a contact state of teeth, respectively. 
         FIGS. 7A and 7B  are numerical value-attached perspective views illustrating maximum stress in a developing reduction gear and calculation results of occurrence points of the maximum stress. 
         FIG. 8  is a cross-sectional view of a developing motor gear and a developing reduction gear according to a second embodiment. 
         FIG. 9  is a cross-sectional view of a developing motor gear and a developing reduction gear according to a third embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, with reference to the drawings, embodiments of the invention will be exemplarily described in detail. However, dimensions, materials, shapes, and relative positions of components described in the embodiments are appropriately changed depending on structures and various conditions of apparatuses to which the invention is applied and therefore the scope of the invention is not intended to be limited thereto unless otherwise particularly specified. In each of the drawings, components denoted by the same reference numerals have the same structure or operation, and the duplication description thereof will not be appropriately presented. 
     [First Embodiment] 
       FIG. 1  is a schematic cross-sectional view illustrating schematically an image forming apparatus  50  according to the invention. In the following description, each of the stations denoted by reference numerals with Y, M, C, and K means member for yellow, magenta, cyan, and black, and these members will be described below by reference numerals without signs of Y, M, C, and K. The image forming apparatus  50  illustrated in  FIG. 1  is an example of a full-color image forming apparatus (complex machine having all of copying machine, printer function, and FAX function). In  FIG. 1 , the image forming apparatus  50  has a plurality of image forming stations (four image forming stations in this embodiment) which are transversely juxtaposed with each other in an image forming apparatus body (hereinafter, referred to as an “apparatus body  50 A”). 
     Each of the stations includes a drum-like electrophotographic photosensitive drum (referred to as a “photosensitive drum  10 ” in this embodiment) as an “image bearing member”. In this embodiment, the photosensitive drums  10  sequentially bear color images of a yellow (Y) component, a magenta (M) component, a cyan (C) component, and a black (K) component, respectively. These photosensitive drums  10  are rotatably driven at a predetermined process speed in an arrow direction “A” (counterclockwise direction) by a drum motor which is not illustrated in the drawing. 
     For example, a charging device  11 , a scanner unit  12 , a developing device  13 , an intermediate belt unit  14 , and a cleaning device  15  are sequentially disposed around each of the photosensitive drums  10  according to a rotational direction of the photosensitive drum  10 . The charging device  11  (charging portion) is configured to uniformly charge the surface of the photosensitive drum  10 . The scanner unit  12  (exposure portion) is configured to irradiate the photosensitive drum  10  with a laser beam based on image information and form an electrostatic image on the photosensitive drum  10 . 
     The developing device  13  as a “developing portion” is configured to develop the electrostatic image formed on the surface of the photosensitive drum  10  with a toner and generate a developer image (toner image). The intermediate belt unit  14  (electrostatic transfer portion) is configured to transfer the toner image on the photosensitive drum  10  onto a sheet. The cleaning device  15  (cleaning portion) is configured to remove a transfer residual toner remaining on the surface of the photosensitive drum  10  after the transfer. 
     Hereinafter, the image forming station for yellow (Y) out of four colors will be described as an example. A photosensitive drum  10 Y is uniformly subjected to a charging treatment by a charging device  11 Y during a rotation process so as to have predetermined polarity and potential. Then, the photosensitive drum  10 Y is exposed to light by a laser scanner  12 Y, whereby an electrostatic image of image information is formed on the photosensitive drum  10 Y. 
     Next, the electrostatic image formed on the photosensitive drum  10 Y is visualized by a developing device  15 Y 13Y and thus a toner image is formed on the photosensitive drum  10 Y. Subsequently, the toner image formed on the photosensitive drum  10  is transferred onto the intermediate belt unit  14  by a primary transfer roller  16 Y. Thereafter, the toner image on the intermediate belt unit  14  is transferred onto a sheet or other output objects by a secondary transfer roller  17 . Similar processes are performed on the image forming stations for other three colors (magenta (M), cyan (C), and black (K)). 
     [Driving device] 
     A driving device of an image preparing portion which drives the photosensitive drum drums  10 , the intermediate belt unit  14 , and the developing device devices  13  equipped with a driving transmission device (driving force transfer unit), which is a feature of the invention, will be described below. 
       FIG. 2A  is a schematic diagram illustrating a state where motors  100  and  101  are connected to the photosensitive drum drums  10  and the intermediate belt unit  14 , respectively. As illustrated in  FIG. 2A , the photosensitive drums  10 Y,  10 M, and  10 C are driven by the motor  100 , and the photosensitive drum  10 K and the intermediate belt unit  14  are driven by the motor  101 . 
       FIG. 2B  is a schematic diagram illustrating a state where a motor  102  is connected to the developing device devices  13 . As illustrated in  FIG. 2B , the developing devices  13 Y,  13 M,  13 C, and  13 K are driven by the motor  102 . 
       FIG. 3  is a schematic diagram of a gear arrangement in a driving configuration of the developing device devices  13  illustrated in  FIG. 2B . As illustrated in  FIG. 3 , the developing device  13  is devices 13 are driven by a developing drive gear gears  103  provided coaxially with a drive input position. A DC brushless motor is often used as the motor  102 , which generally has a rotation speed from about 2000 to 3000 rpm in terms of efficiency. 
     The rotation speed of the developing device devices  13  to be often used is about 100 to 500 rpm, thereby being reduced by a gear ratio between a developing reduction gear  104 , a developing motor gear  105 , and the developing drive gear gears  103 . In the embodiment shown in FIG. 3, a smaller gear 104A rotates coaxially with the developing reduction gear 104 and is configured to engage with the developing drive gear 103M and the developing drive gear 103C. The diameter of the smaller gear 104A is smaller than the diameter of the developing reduction gear 104. As in this configuration, in a case where a plurality of rotating objects is rotated by one motor, a large load is concentrated on the developing reduction gear  104  compared with a configuration in which one rotation object is rotated by one motor. 
       FIG. 4A  is a perspective view illustrating the developing motor gear  105  and the developing reduction gear  104  in detail.  FIG. 4B  is a view as seen from the back in  FIG. 4A . Referring to  FIGS. 4A and 4B , the shapes of the developing motor gear  105  and the developing reduction gear  104  corresponding to the driving gear of this embodiment will be described below in detail. 
     The motor  102  is provided as a “driving portion” which drives the developing motor gear  105  of the developing device devices  13 . The driving force of the motor  102  is transmitted to the developing device devices  13  through a driving transmission portion. The developing motor gear  105  as a “first helical gear” and the developing reduction gear  104  as a “second helical gear” are disposed to come in contact with each other, and the driving force is transmitted to the developing reduction gear  104  from the developing motor gear  105 . 
     In the case of being viewed from the above in  FIG. 4A , since the developing motor gear  105  has teeth which are cut in a direction from a lower left toward an upper right, it is formed by right-twisted helical teeth. In addition, since the developing reduction gear  104  has teeth which are cut in a direction from a lower right toward an upper left, it is formed by left-twisted helical teeth. In this way, the helical gears coming in contact with each other can be obtained by a combination of the right-twisted teeth and the left-twisted teeth in a reverse direction. 
     As illustrated in  FIG. 4A , the developing motor gear  105  is formed in such a manner of being directly subjected to gear cutting together with a metallic driving shaft  102 X of the motor  102  as a “motor” which generates a driving force. Therefore, the developing motor gear  105  is totally formed of a metal. 
     As illustrated in  FIG. 4B , the developing reduction gear  104  is engaged with the developing motor gear  105 . The developing reduction gear  104  includes a rim  104 c, which is formed of a resin and has an outer circumference formed with teeth, a boss  104 d, which is the center of rotation of the rim  104 c (simultaneously, forming the center of rotation of the gear), and a web  104 e through which the rim  104 c and the boss  104 d are connected to each other. The developing reduction gear 104 may be formed of a resin. 
     In addition, a rib ribs  104 f and a rib  104 g protrude from the face of the web  104 e (projecting from the web 104e in a tooth width direction M, discussed further below). As can be seen in FIG. 4, a projecting dimension of the boss 104d from the web 104e is larger than a projecting dimension of each of the ribs 104f from the web 104e. The rib ribs  104 f radially extends extend in a radial fashion (in a radial ray fashion) (in a direction separated away from the boss  104 d) from the boss  104 d for the purpose of reinforcement of the developing reduction gear  104 . The rib  104 g is concentrically disposed with respect to the boss  104 d. In a direction of the diameter of the developing reduction gear 104, a distance between the boss 104d and the rib 104g is larger than a distance between the rib 104g and the rim 104c. The rib  104 f is ribs 104f are disposed with a predetermined distance from the rim  104 c so as to prevent tooth-face accuracy from deteriorating due to shrinkage during molding and is are formed not to come in contact with the rim  104 c. 
       FIG. 5  is a cross-sectional view of the developing motor gear  105  and the developing reduction gear  104 . The web  104 e is provided on a front side  104 a of the developing reduction gear  104 . For this reason, a tooth width direction M of the web  104 e is positioned at a left end deviated from a center M 1  in the a tooth width direction M. Herein, the tooth width direction M refers to a thickness direction of the gear. 
     Therefore, a gradient of torsional rigidity in the tooth width direction M is formed to be large at the front side  104 a of the developing reduction gear  104  and to be small at a rear side  104 b thereof. That is, the torsional rigidity of developing reduction gear  104  in the tooth width direction M becomes gradually smaller from the front side  104 a (one side) toward the rear side  104 b (the other side) in the tooth width direction M. For this reason, the developing reduction gear  104  refers to a helical gear in which the torsional rigidity in the tooth width direction M of the front side  104 a (one side end) in the tooth width direction M of the developing reduction gear  104  is larger than the torsional rigidity in the tooth width direction of the rear side  104 b (the other side end). 
     In other words, the torsional rigidity in the tooth width direction M becomes gradually smaller from a side where closer to the web  104 e is closer to in the tooth width direction M toward a side where farther from the web  104 e is not closer to in the tooth width direction M. For this reason, it is said that torsional rigidity at the side where farther from the web  104 e is not closer to in the tooth width direction M is smaller than the torsional rigidity at the side where closer to the web  104 e is closer to in the tooth width direction M. At least one of the developing motor gear  105  and the developing reduction gear  104  may be configured in this manner. 
     Returning back to  FIGS. 4A and 4B , the description will be made below. The developing motor gear  105  rotates in a direction indicated by an arrow A, and the developing reduction gear  104  engaged with the developing motor gear  105  rotates in a direction indicated by an arrow B. The A helical gear has a property that comes in contact with the other gear to be engaged from an advancing side in the rotational direction. 
     That is, the a helical gear sequentially comes in contact with the other gear to be engaged from an advancing helical tooth in the advancing direction of each rotating helical teeth. That is, since the developing motor gear  105  is right-twisted and thus rotates in the direction indicated by the arrow A, a rear end  105 X 2  of helical teeth  105 X rotates engages with a helical tooth 104X of the developing reduction gear 104 earlier than a front end  105 X 1  thereof in the direction indicated by the arrow A. In addition, since the developing reduction gear  104  is left-twisted and thus rotates in the direction indicated by the arrow B, a rear end  104 X 2  of helical teeth  104 X rotates engages with a helical tooth 105X of the developing motor gear 105 earlier than a front end  104 X 1  thereof in the direction indicated by the arrow B. Accordingly, the developing motor gear  105  and the developing reduction gear  104  come in contact with the other gear to be engaged from the rear ends  105 X 2  and  104 X 2  advancing in the advancing direction, respectively. 
     In the configuration of this embodiment, the direction of the helical teeth is set such that the contact occurs from the rear side  104 b having the small torsional rigidity. That is, a twist direction of the helical teeth and the rotational direction of the developing motor gear  105  due to the motor  102  are set such that the developing motor gear  105  and the developing reduction gear  104  are engaged with each other in such a manner that the teeth come in contact with each other at the side (the other side in the tooth width direction) (the other side end) having the small torsional rigidity earlier than the side (one side in the tooth width direction) (one side end) having the large torsional rigidity. 
     A simulation experiment is performed to observe the contact state of the teeth of this embodiment configured as described above and to calculate the maximum value of tooth root stress. The simulation experiment is performed using Abaqus which is versatile software for non-linear structure analysis. The developing motor gear  105  is a rigid body and the developing reduction gear  104  is an elastic body having a Young&#39;s modulus of 2200 MPa. A module of the gear is 0.4, a twist angle is 20°, a pressure angle is 20°, the number of teeth of the developing motor gear  105  is 11, the number of teeth of the developing reduction gear  104  is 86, and a driving load is 0.8 N·m. In this way, the number of teeth of the developing motor gear  105  is set to be smaller than the number of teeth of the developing reduction gear  104 . 
       FIGS. 6A to 6F  are perspective views illustrating calculation results of the contact state of the teeth when the developing reduction gear  104  rotates in the direction indicated by an arrow B and an arrow C. In order to make it easy to see the contact state, only the tooth surface of the developing motor gear  105  is illustrated. In addition, the developing reduction gear  104  illustrated in  FIGS. 6A to 6F  looks like a spur gear in external appearance, but has the left-twisted helical teeth which are left-twisted with respect to the axial line as described above in fact.  FIGS. 6A to 6F  are enlarged diagrams of the actual helical teeth, respectively. 
     In  FIGS. 6A to 6F , a contact portion is indicated by a black-painted portion K. The developing reduction gear  104  rotates in the direction indicated by the arrow B in this order of  FIGS. 6A to 6C . As the helical teeth  105 X of the developing motor gear  105  rotate, the developing motor gear  105  continues to come in sequential contact with the developing reduction gear  104  from the rear end  105 X 2  side which is the advancing side of the helical teeth. Then, the contact area between the developing motor gear  105  and the developing reduction gear  104  shifts from the rear end  104 X 2  of the developing reduction gear  104  to the front end  104 X 1 . In addition, it is understood that three helical teeth  105 X of the developing motor gear  105  always come in contact with the developing reduction gear  104  during the rotation. 
       FIGS. 6D to 6F  are illustrated in comparison with this embodiment and are perspective views illustrating calculation results of a contact state of teeth when the developing reduction gear  104  rotates in the direction indicated by an arrow C which is a direction reverse to the direction illustrated in  FIGS. 6A to 6C , respectively. Since the helical teeth  105 X of the developing motor gear  105  rotate in a reverse direction, the advancing direction of the helical teeth is also reverse and thus the developing motor gear  105  continues to come in sequential contact with the developing reduction gear  104  from the front end  105 X 1  side. Then, the number of the helical teeth  105 X of the developing motor gear  105 , which always come in contact with the developing reduction gear  104  during the rotation, is reduced to two. 
     Comparing these two examples with each other, in the rotational direction of this embodiment, since the developing motor gear  105  comes in contact with the developing reduction gear  104  from the rear side  104 b having the small torsional rigidity, and the developing motor gear  105  comes in contact with a deformable portion of the developing reduction gear  104 , the number of the teeth of the developing motor gear  105  coming in contact with the developing reduction gear  104  at all times increases. Meanwhile, in the rotation in the reverse direction, since the developing motor gear  105  comes in contact with the developing reduction gear  104  from the front side  104 a having the large torsional rigidity, and the developing motor gear  105  comes in contact with a hardly deformable portion of the developing reduction gear  104 , the number of the teeth of the developing motor gear  105  coming in contact with the developing reduction gear  104  at all times reduces. 
       FIGS. 7A and 7B  are numerical value-attached perspective views illustrating maximum stress and calculation results of occurrence points of the maximum stress in the developing reduction gear  104 , respectively. It indicates that stress becomes gradually higher in the order from a gray-painted portion K 2  to a black-painted portion K 1 .  FIG. 7A  illustrates the calculation results when the developing reduction gear  104  rotates in the rotational direction (direction indicated by the arrow B) of this embodiment corresponding to  FIGS. 6A to 6C .  FIG. 7B  illustrates the calculation results when the developing reduction gear  104  rotates in the rotational direction (direction indicated by the arrow C) corresponding to  FIGS. 6D to 6F , which is reverse to the rotational direction of this embodiment. 
     A stress value is expressed by the maximum principal stress. Even in any case, the maximum stress occurs in a tooth root in the vicinity of the front side  104 a having the large torsional rigidity. When the maximum stress value in this embodiment (see  FIG. 7A ) is 1, the The maximum stress value during the rotation in the reverse direction (see  FIG. 7B ) becomes is 2.3 times larger than the maximum stress value of this embodiment (see FIG. 7A) during the rotation in the direction indicated by the arrow B. 
     In this embodiment (see  FIG. 7A ), since the contact number of the teeth, that is, the contact area to which the load is applied becomes larger when the developing reduction gear  104  rotates in the direction indicated by the arrow B, the stress value becomes smaller so as to relatively reduce the amount of deformation of each tooth and the maximum stress also becomes smaller, which is 84.5 MPa. Whereas, in the comparative example (see  FIG. 7B ), since the contact number of the teeth, that is, the contact area to which the load is applied becomes smaller when the developing reduction gear  104  rotates in the direction indicated by the arrow C, the stress value becomes larger so as to relatively increase the amount of deformation of each tooth and the maximum stress also becomes larger, which is 194 MPa. 
     According to this embodiment, when high loads are transmitted with a small module, since the rigidity increases to ensure strength and thus deterioration in accuracy of the tooth surface is not caused by the shrinkage during molding, it is possible to provide a driving configuration in which a high-quality image not having a banding image can be output. 
     [Second Embodiment] 
       FIG. 8  is a cross-sectional view of a developing motor gear  105  and a developing reduction gear  104  according to a second embodiment. A configuration of the second embodiment is the same as similar to that of the first embodiment except that a gradient of torsional rigidity in a tooth width direction M of the developing reduction gear  104  is provided, and the description of features of the second embodiment that have the same configuration as the first embodiment will not be presented. 
     The thickness of a rim  104 c of the developing reduction gear  104  becomes gradually thinner from a front side  104 a (one side) toward a rear side  104 b (the other side) in the tooth width direction M. For this reason, the gradient of the torsional rigidity in the tooth width direction M is formed to be large at the front side  104 a and to be small at the rear side  104 b. Thus, the torsional rigidity in the tooth width direction M of the developing reduction gear  104  becomes gradually smaller from the front side  104 a (one side) toward the rear side  104 b (the other side) in the tooth width direction M. In other words, the torsional rigidity in the tooth width direction M becomes gradually smaller from a thick side of the rim  104 c toward a thin side of the rim  104 c. For this reason, it is also considered that the torsional rigidity at the thin side of the rim  104 c is smaller than the torsional rigidity at the thick side of the rim  104 c. 
     [Third Embodiment] 
       FIG. 9  is a cross-sectional view illustrating a developing motor gear  105  and a developing reduction gear  104  according to a third embodiment in detail. A configuration of this embodiment is the same as similar to that of the first embodiment except that a gradient of torsional rigidity in a tooth width direction M of the developing reduction gear  104  is provided, and the description of features of the third embodiment that have the same configuration as the first embodiment will not be presented. 
     In this embodiment, the developing reduction gear  104  is formed with a web  104 e between a boss  104 d and a rim  104 c. The web  104 e is disposed substantially at the center in the tooth width direction M of the developing reduction gear  104 . The web  104 e is formed in a disk-like plate shape around the boss  106  104d. 
     On the premise of this configuration, a rib  104 f radially extends from the boss  104 d (this is the same as the configuration in  FIG. 4B ) and protrudes toward a front side  104 a from the web  104 e (this is different from the configuration in  FIG. 4B ) at the same time. In this way, the rib  104 f is disposed at only the front side  104 a (one side) in the tooth width direction M. For this reason, a gradient of torsional rigidity in the tooth width direction M is formed to be large at the front side  104 a and to be small at a rear side  104 b. 
     Thus, the torsional rigidity in the tooth width direction M of the developing reduction gear  104  becomes gradually smaller from the front side  104 a (one side) toward the rear side  104 b (the other side) in the tooth width direction M. In other words, the torsional rigidity in the tooth width direction M becomes gradually smaller from a side disposed with the rib  104 f toward a side not disposed with the rib  104 f. For this reason, it is also considered that the torsional rigidity in the tooth width direction M at the side not disposed with the rib  104 f is smaller than the torsional rigidity in the tooth width direction M at the side disposed with the rib  104 f. 
     According to any one configuration of the first to third embodiments, it is possible to suppress stress concentration on a gear tooth root even when the module is reduced compared to the related art. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2014-221347, filed Oct. 30, 2014, which is hereby incorporated by reference herein in its entirety.