Patent Publication Number: US-11022921-B2

Title: Image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is based on and claims priority pursuant to 35 U.S.C. § 119(a) to Japanese Patent Application Nos. 2019-074624, filed on Apr. 10, 2019, and 2019-222336, filed on Dec. 9, 2019, in the Japan Patent Office, the entire disclosure of each of which is hereby incorporated by reference herein. 
     BACKGROUND 
     Technical Field 
     This disclosure relates to an image forming apparatus. 
     Background Art 
     Various types of image forming apparatuses include a driven unit, a drive source to drive the driven unit, and a drive transmitter having a drive gear and a driven gear to transmit the driving force to the driven unit. The drive gear transmits a driving force from the drive source. The driven gear is meshed with the drive gear. One of the drive gear and the driven gear is a crowned gear by the process of gear crowning. 
     SUMMARY 
     At least one aspect of this disclosure provides an image forming apparatus including a drive unit and a drive transmitter. The drive transmitter includes a drive source, a drive gear, and a driven gear. The drive source is configured to drive the driven unit. The drive gear is configured to receive a driving force from the drive source. The driven gear is meshed with the drive gear. The drive transmitter is configured to transmit the driving force from the drive source to the driven unit. The drive gear or the driven gear is a crowned gear crowning-processed, and the crowned gear has a crowning amount less than 50 μm. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Exemplary embodiments of this disclosure will be described in detail based on the following figured, wherein: 
         FIG. 1  is a diagram illustrating an overall schematic configuration of an image forming apparatus according to the present embodiment of this disclosure; 
         FIG. 2  is a diagram illustrating a schematic configuration of a drive device included in the image forming apparatus of  FIG. 1 ; 
         FIG. 3  is a diagram illustrating a variation of the drive device of  FIG. 2 ; 
         FIG. 4A  is a diagram illustrating a schematic configuration of the drive device of  FIG. 2 , including a support mechanism of a motor shaft; 
         FIG. 4B  is an enlarged view illustrating an area “a” encircled by a broken line in  FIG. 4A ; 
         FIGS. 5A, 5B, and 5C  are views of meshing of a drive gear and a driven gear on the occurrence of misalignment; 
         FIG. 6  is a graph of vibration data in a case in which the driven gear has the crowning amount of 0 μm; 
         FIG. 7  is a graph of vibration data in a case in which the driven gear is a crowned gear having the crowning amount of 20 μm  FIG. 8  is a graph of vibration data in a case in which the driven gear is a crowned gear having the crowning amount of 50 μm; 
         FIG. 9  is a diagram for explaining specifications of a drive gear and a driven gear in Verification Test 2; 
         FIG. 10  is a diagram illustrating a helical gear; 
         FIG. 11  is a graph of the results of Verification Test 2; 
         FIGS. 12A and 12B  are graphs of the results of tests on the crowning amounts and the face widths of crowned gears; 
         FIG. 13  is a graph of the results of tests conducted under a condition in which the drive gear and the driven gear have various crowning amounts; 
         FIG. 14  is a diagram illustrating an example of a schematic configuration of a sheet conveying device; and 
         FIG. 15  is a diagram illustrating the sheet conveying device, in a state of a multi-sheet feeding in which a plurality of sheets is conveyed in layers. 
     
    
    
     The accompanying drawings are intended to depict embodiments of the present disclosure and should not be interpreted to limit the scope thereof. The accompanying drawings are not to be considered as drawn to scale unless explicitly noted. 
     DETAILED DESCRIPTION 
     It will be understood that if an element or layer is referred to as being “on,” “against,” “connected to” or “coupled to” another element or layer, then it can be directly on, against, connected or coupled to the other element or layer, or intervening elements or layers may be present. In contrast, if an element is referred to as being “directly on,” “directly connected to” or “directly coupled to” another element or layer, then there are no intervening elements or layers present. Like numbers referred to like elements throughout. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     Spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements describes as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, term such as “below” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors herein interpreted accordingly. 
     The terminology used herein is for describing particular embodiments and examples and is not intended to be limiting of exemplary embodiments of this disclosure. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “includes” and/or “including,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     Referring now to the drawings, embodiments of the present disclosure are described below. In the drawings for explaining the following embodiments, the same reference codes are allocated to elements (members or components) having the same function or shape and redundant descriptions thereof are omitted below. 
     Descriptions are given below of an electrophotographic color copier that is an image forming apparatus according to an embodiment of the present disclosure with reference to the drawings. 
     First, a description is given of the detailed configuration of an image forming apparatus  500  according to the present embodiment of this disclosure. 
       FIG. 1  is a diagram illustrating an overall schematic configuration of the image forming apparatus  500  according to the present embodiment of this disclosure. 
     The image forming apparatus  500  according to the present embodiment is a tandem-type image forming apparatus and employs a dry two-component developing method using dry two-component developer. As illustrated in  FIG. 1 , the image forming apparatus  500  includes a housing  100 , a sheet feeding table  200 , a scanner  300 , and an automatic document feeder  400 . The housing  100  is installed on the sheet feeding table  200 . The scanner  300  is attached to the housing  100 . The automatic document feeder  400  is attached to the scanner  300 . 
     The image forming apparatus  500  performs image formation by receiving image data that is image data read by the scanner  300  or by receiving print data sent from an external device such as a personal computer. 
     As illustrated in  FIG. 1 , the housing  100  contains four photoconductor drums  1 Y,  1 M,  1 C, and  1 K that are rotary bodies functioning as four cylindrical latent image bearers for each color of yellow (Y), magenta (M), cyan (C), and black (K). Hereinafter, the photoconductor drums  1 Y,  1 M,  1 C, and  1 K are occasionally referred to in a singular form without suffixes as the “photoconductor drum  1 .” Note that other devices and units, which have the structures basically identical to each other and provide different colors of toners to an image in a printing process, are also referred to in a singular form without suffixes. The photoconductor drum  1 Y,  1 M,  1 C, and  1 K are aligned in contact with an intermediate transfer belt  5  along a belt moving direction in which the intermediate transfer belt  5  moves. The intermediate transfer belt  5  is an endless belt supported by a plurality of rollers including a drive roller. 
     Electrophotographic process members or devices such as charging device  2  (i.e., charging devices  2 Y,  2 M,  2 C, and  2 K), a developing device  9  (i.e., developing devices  9 Y,  9 M,  9 C, and  9 K) for each color, a cleaning device  4  (i.e., cleaning devices  4 Y,  4 M,  4 C, and  4 K), and an electric discharging device  3  (i.e., electric discharging devices  3 Y,  3 M,  3 C, and  3 K) are disposed around the photosensitive drum  1  (i.e., the photoconductor drums  1 Y,  1 M,  1 C, and  1 K) in the order of image formation. An optical writing device  17  is disposed above the photoconductor drums  1 Y,  1 M,  1 C, and  1 K. Primary transfer rollers  6 Y,  6 M,  6 C, and  6 K, which are primary transfer units, are disposed at respective positions facing the photoconductor drums  1 Y,  1 M,  1 C, and  1 K, respectively, via the intermediate transfer belt  5 . The primary transfer rollers  6 Y,  6 M,  6 C, and  6 K subsequently transfer respective single-color toner images formed on the surfaces of the photoconductor drums  1 Y,  1 M,  1 C, and  1 K onto the surface of the intermediate transfer belt  5 , to form a composite toner image. 
     The intermediate transfer belt  5  is wound around stretching rollers  11 ,  12 , and  13  and a tension roller  14 . The stretching roller  12  functions as a drive roller that is rotated by the driving of a drive source. The intermediate transfer belt  5  rotates along with rotation of the stretching roller  12 , together with the stretching rollers  11  and  13  and the tension roller  14 . A belt cleaning device  19  is disposed at a position facing the stretching roller  13  via the intermediate transfer belt  5 . The belt cleaning device  19  cleans the intermediate transfer belt  5  by removing residual toner remaining on the surface of the intermediate transfer belt  5  after secondary transfer in which the composite toner image formed on the surface of the intermediate transfer belt  5  is transferred onto a recording medium such as a sheet. The stretching roller  11  is a secondary transfer opposing roller that is disposed facing the secondary transfer roller  7  that functions as a secondary transfer unit. The stretching roller  11  (i.e., secondary transfer opposing roller) and the secondary transfer roller  7  form a secondary transfer nip region via the intermediate transfer belt  5 . 
     A sheet conveying belt  15  is disposed downstream from the secondary transfer nip region in the sheet conveyance direction. The sheet conveying belt  15  is stretched by a pair of stretching rollers  16  and conveys the sheet having the toner image after secondary transfer, to a fixing device  18 . The fixing device  18  includes a pair of fixing rollers  8  that forms a fixing nip region. In the fixing device  18 , the image that is formed on but yet unfixed to the sheet is fixed to the sheet by application of heat and pressure in the fixing nip region by the pair of fixing rollers  8 . 
     Next, a description is given of a copying operation performed by the image forming apparatus  500  according to the present embodiment. 
     In a case in which the image forming apparatus  500  according to the present embodiment of this disclosure form a full-color image, an original document is set on a document table  401  of the automatic document feeder  400 . Note that the automatic document feeder  400  is hereinafter referred to as the ADF  400 . Alternatively, the ADF  400  is opened to set the original document on an exposure glass  301  of the scanner  300 , and then is closed to press the original document against the exposure glass  301 . Thereafter, as a start button is pressed by a user, when the original document is set on the document table  401  of the ADF  400 , the original document is conveyed to the exposure glass  301  of the scanner  300 . Then, the scanner  300  is driven so that a first moving body  302  and a second moving body  303  start travelling. Consequently, light emitted from the first moving body  302  reflects on the original document placed on the exposure glass  301 , and then reflects on a mirror (or mirrors) of the second moving body  303 . Then, the light is guided to a reading sensor  305  through an image forming lens  304 . Accordingly, the scanner  300  reads image data of the original document. 
     As a user presses the start button of the image forming apparatus  500 , a motor is driven to rotate the stretching roller  12  that functions as a drive roller, thereby rotating the intermediate transfer belt  5 . At the same time, the photoconductor drum  1  (i.e., the photoconductor drums  1 Y,  1 M,  1 C, and  1 K) is uniformly charged by the charging device  2  (i.e., the charging devices  2 Y,  2 M,  2 C, and  2 K) while a photoconductor drive device drives to rotate the photoconductor drum  1  (i.e., the photoconductor drums  1 Y,  1 M,  1 C, and  1 K) in a direction indicated by arrow in  FIG. 1 . A detailed description of the photoconductor drive device is given below. Thereafter, the optical writing device  17  emits a light beam L (i.e., light beams L Y , L M , L C , and L K ) to form a single-color electrostatic latent image on the surface of the photoconductor drum  1 . The single-color electrostatic latent image is developed by the developing device  9  (i.e., the developing devices  9 Y,  9 M,  9 C, and  9 K) with toner of the corresponding color in the developer. In a developing process, a given amount of developing bias is applied to between a developing roller  91  and the photoconductor drum  1 , so that the toner supplied on the developing roller  91  is electrostatically attracted to the electrostatic latent image formed on the surface of the photoconductor drum  1  in a given clearance (i.e., development gap) formed between the developing roller  91  and the photoconductor drum  1 . 
     The toner image thus developed is conveyed to the primary transfer position at which the photoconductor drum  1  and the intermediate transfer belt  5  contact along with rotations of the photoconductor drum  1 . The primary transfer roller  6  applies a given bias voltage to a back face of the intermediate transfer belt  5  at this primary transfer position. Then, the toner image formed on the photoconductor drum  1  is drawn toward the intermediate transfer belt  5  by the primary transfer electric field generated by application of the given bias voltage, so that the toner image is transferred onto the intermediate transfer belt  5  as primary transfer. In the similar manner as described above, the respective single-color toner images, which are yellow, magenta, cyan, and black toner images, are sequentially transferred in layers onto the surface of the intermediate transfer belt  5  as primary transfer. It is to be noted that, after secondary transfer, residual toner remaining on the surface of the intermediate transfer belt  5  is removed by the belt cleaning device  19 . 
     Further, as the start button is pressed by a user, a sheet feeding roller  202  of the sheet feeding table  200  according to the size of a sheet selected by the user starts rotating to feed a sheet from a selected one of sheet feed trays  201 . When a plurality of sheets is fed from the selected sheet feed tray  201 , the plurality of sheets is separated one by one by each pair of sheet separation rollers  203 . After being conveyed to a sheet conveyance passage  204 , the separated sheet is conveyed by pairs of sheet conveying rollers  205  to a sheet conveyance passage  101  in the housing  100  of the image forming apparatus  500 . The sheet thus conveyed is stopped when the sheet comes to contact with a pair of registration rollers  102 . It is to be noted that, when feeding a sheet that is not set on any of the sheet feed trays  201  but is set on a bypass sheet tray  105 , the sheet is fed by a sheet feeding roller  104 , separated one by one by a sheet separating roller  108 , and conveyed to the housing  100  through a bypass sheet conveyance passage  103 . Then, the sheet from the bypass sheet tray  105  is stopped when the sheet comes to contact with the pair of registration rollers  102 . 
     After four single-color toner images are transferred and overlaid onto the surface of the intermediate transfer belt  5  to form a composite toner image, the composite toner image is conveyed along with rotations of the intermediate transfer belt  5 , to the secondary transfer position at which the intermediate transfer belt  5  comes to face the secondary transfer roller  7 . Further, the pair of registration rollers  102  starts rotating to convey the sheet to the secondary transfer position, in synchronization with timing at which the composite toner image formed as described above on the intermediate transfer belt  5  is conveyed to the secondary transfer position. At the secondary transfer position, the secondary transfer roller  7  applies a given bias voltage to the back face of the sheet. With the bias voltage generated in the secondary transfer electric field by the application of the bias voltage and the contact pressure at the secondary transfer position, the composite toner image formed on the intermediate transfer belt  5  is collectively transferred onto the sheet as secondary transfer. Thereafter, the sheet having the composite toner image after secondary transfer is conveyed to the fixing device  18  along with movement of the sheet conveying belt  15 , so that the pair of fixing rollers  8  provided in the fixing device  18  performs a fixing operation to the sheet. Then, the sheet to which the composite toner image has been fixed during the fixing operation is conveyed by a pair of sheet ejecting rollers  106  onto a sheet ejection tray  107  provided outside the housing  100  of the image forming apparatus  500 . The ejected sheet is stacked on the sheet ejection tray  107 . Alternatively, the direction of conveyance of the sheet is switched by a switching claw  109  so that the sheet enters a sheet reversing device  110 . In the sheet reversing device  110 , the sheet is reversed and conveyed to the transfer position again. After a toner image is formed on the back face of the sheet at the transfer position, the sheet having toner images on both faces is ejected by the pair of sheet ejecting rollers  106  onto the sheet ejection tray  107 . 
     In the present embodiment, the photoconductor drum  1  and the image forming units, such as the developing device  9 , disposed around the photoconductor drum  1  are composed in a process cartridge of each color. The process cartridge is detachably attached to the housing  100  of the image forming apparatus  500 . Specifically, the process cartridge of each color integrally supports the photoconductor drum  1 , the charging device  2 , the developing device  9 , the cleaning device  4 , and the electric discharging device  3 . Note that the process cartridge may support at least the photoconductor drum  1  and the developing device  9 . 
     Next, a description is given of an example of a drive device included in the image forming apparatus  500 . 
       FIG. 2  is a diagram illustrating a schematic configuration of a drive device  30  included in the image forming apparatus  500  of  FIG. 1 . 
     The drive device  30  drives the pair of fixing rollers  8  of the fixing device  18  as a driven unit. The drive device  30  has a drive motor  31  as a drive source. The drive motor  31  has a motor shaft  31   a  (as a drive shaft) made of metal. Directly on the motor shaft  31   a , the teeth of a drive gear  32  made of metal are formed. A driven gear  33  made of resin meshes with the drive gear  32  and is mounted on the edge of a roller shaft of a fixing roller (drive roller)  8   a  of the pair of fixing rollers  8 . The pair of fixing rollers  8  includes the fixing roller (drive roller)  8   a  and a pressure roller (driven roller)  8   b . The drive device  30  includes a gear train including the drive gear  32  and a driven gear  33  and functions as a drive transmitter. 
     The drive gear  32  is a normal gear with the crowning amount of 0 μm and having the tooth trace parallel to the axial direction of the drive gear  32 . On the other hand, the driven gear  33  is a crowned gear crowning-processed and has the crowning amount less than 50 μm. To be more specific, the crowned gear in the embodiments of this disclosure is a gear with crowned teeth having surfaces outwardly curved in a convex shape in the lengthwise direction of the teeth of the gear. In the present embodiment, the driven gear  33  is a crowned gear. However, the drive gear  32  may be a crowned gear having the crowning amount less than 50 μm. 
       FIG. 3  is a diagram illustrating a variation of the drive device  30 . 
     The drive device  30  of the variation illustrated in  FIG. 3  has a connecting joint  34  so that the fixing device  18  is detachably attached with respect to the housing  100  of the image forming apparatus  500 . 
     The fixing device  18  applies heat and pressure to the sheet passing between the rollers of the pair of fixing rollers  8  to fix the four-color toner image that is transferred onto the surface of the sheet. As described above, in order to apply a given pressure to the sheet, one roller of the pair of fixing rollers  8  is pressed against the other roller of the pair of fixing rollers  8  with pressing force that is greater than the rollers of the other pairs of sheet conveying rollers. With this configuration, the fixing device  18  has a heavy torque load among the units in the image forming apparatus  500 . Therefore, the load torque applied to the meshing portion of the drive gear  32  and the driven gear  33  is high, and the vibration at the time of gear meshing increases. As a result, the noise of the fixing device  18  may increase. 
     Further, in the drive device  30  illustrated in  FIG. 3 , a driven side coupling  34   b  is provided in the fixing device  18  and a drive side coupling  34   a  is mounted on the edge of a gear shaft  33   b  of the driven gear  33 . In a case in which the fixing device  18  is detachably attached to the housing  100  as illustrated in  FIG. 3 , when coupling the driven side coupling  34   b  to the drive side coupling  34   a , the gear shaft  33   b  tilts to easily cause misalignment between the drive gear  32  and the driven gear  33 . Due to occurrence of such misalignment, vibration at the time of gear meshing increases, and therefore the noise of the fixing device  18  is likely to increase. 
     Note that misalignment occurs between the drive gear  32  and the driven gear  33  even in the configuration illustrated in  FIG. 2 , due to manufacturing error, assembly error, or both. For example, an assembly error of the drive motor  31  to the motor mounting face of the housing  100  causes the motor shaft  31   a  to tilt with respect to the motor mounting face of the housing  100 , which is referred to as the tilt of the shaft. Due to the tilt of the motor shaft  31   a , misalignment occurs between the drive gear  32  and the driven gear  33 . 
       FIGS. 4A and 4B  are diagrams for explaining a support of the motor shaft  31   a .  FIG. 4A  is a diagram illustrating a schematic configuration of the drive device  30  of  FIG. 2 , including a support mechanism of the motor shaft  31   a .  FIG. 4B  is an enlarged view illustrating an area “a” encircled by a broken line in  FIG. 4A . 
     The drive motor  31  is a brushless motor, in which two ball bearings  131  and  132  are provided to receive the motor shaft  31   a . As described above, the motor shaft  31   a  has one end supported by the two ball bearings  131  and  132  and the opposed end having the drive gear  32 . The opposed end functions as a free end of the motor shaft  31   a . Therefore, the motor shaft  31   a  is easily warped by the force applied to the tooth surface of the drive gear  32 , and therefore the tilt of motor shaft  31   a  occurs easily. 
     Further, as illustrated in  FIG. 4B , the brushless motor may have backlash between an inner ring  132   b  of the ball bearing  132  (for example, the ball bearing  132  as illustrated in  FIG. 4B ) and the motor shaft  31   a , between an outer ring  132   a  of the ball bearing  132  and the housing  100 , between the outer ring  132   a  of the ball bearing  132  and a ball  132   c  of the ball bearing  132 , and between the inner ring  132   b  of the ball bearing  132  and the ball  132   c  of the ball bearing  132 . Among the above-described backlash, the backlash between the inner ring  132   b  of the ball bearing  132  and the motor shaft  31   a  and the backlash between the outer ring  132   a  of the ball bearing  132  and the housing  100  are eliminated by pressing the ball bearing  132  between the motor shaft  31   a  and the housing  100 . However, a radial clearance, which is an inner clearance or the backlash between the outer ring  132   a  of the ball bearing  132  and the ball  132   c  of the ball bearing  132  or the backlash between the inner ring  132   b  of the ball bearing  132  and the ball  132   c  of the ball bearing  132 , is not eliminated and has the backlash of 5 μm to 10 μm. Due to the above-described backlash, the tilt of the motor shaft  31   a  increases. 
     As described above, it is likely that the positional deviation of the motor shaft  31   a  is ±0.35 mm at the maximum and the tilt angle of the motor shaft  31   a  is ±0.7 degrees at the maximum due to accumulation of the tilt of the motor shaft  31   a  caused by the drive motor  31  alone, the tilt of the motor shaft  31   a  caused by the assembly error when attaching the drive motor  31  to the motor mounting face of the housing  100 , and the tilt of the motor shaft  31   a  caused by force applied to the tooth face of the drive gear  32  at the start of driving after assembly. 
     Note that the above-described positional deviation is obtained by the following equation:
 
Positional Deviation=Length (Face Width) of Drive Gear 32*tan(Tilt Angle of Motor Shaft 31 a ).
 
     As described above, it is likely that, even in the configuration illustrated in  FIG. 2 , misalignment occurs between the drive gear  32  and the driven gear  33  due to the tilt of the motor shaft  31   a  described above, so that vibration at the time of gear meshing of the drive gear  32  and the driven gear  33 , and therefore the noise of the image forming apparatus  500  increases. 
     It is preferable that the diameter of the drive gear  32  is relatively small so as to reduce the size of the image forming apparatus  500  and the drive gear  32  obtains a large reduction ratio. Further, the drive gear  32  is made of metal from the point of view of the reduction in durability of the drive gear  32  caused by the reduction of the size. Further, it is preferable that the drive gear  32  is formed directly on the motor shaft  31   a . Accordingly, while reducing the size of the image forming apparatus  500 , the metallic drive gear  32  preferably obtains a large reduction ratio and achieves high durability. However, the metal gear is harder than the resin gear. Therefore, unlike the resin gear, the metal gear is not capable of sufficiently absorbing the load with elastic deformation. As a result, the vibration of the gears at the meshing increases, and therefore the noise may increase. 
     Further, in a case in which the vibration at the gear meshing between the drive gear  32  and the driven gear  33  is transmitted to, for example, the photoconductor drum  1  to vibrate the photoconductor drum  1  vibrates in a rotational direction in which the photoconductor drum  1  rotates, an abnormal image such as an image with banding may be generated. 
     As an example, a known image forming apparatus includes a driven gear to be an asymmetric crowned gear that is a crowned gear with asymmetric crowned teeth, in which the position of the maximum tooth thickness is shifted from the center in a tooth trace direction. The crowning amount of the asymmetric crowned gear of the known image forming apparatus is 70 μm. 
     However, the noise of the device is increased. 
     In order to restrain such vibration between the drive gear  32  and the driven gear  33  at the gear meshing, the precision of gears has been enhanced and resin gears have been employed. However, in recent years, demands for lower noise of gears at the gear meshing and higher quality of gears have risen. In order to achieve the above-described demands, a crowned gear is employed as the driven gear  33  of the present embodiment. A crowned gear is employed as the driven gear  33  to restrain vibration of gears at the gear meshing that is likely to occur at occurrence of misalignment. In addition, by performing an appropriate crowning to set the crowning amount less than 50 μm to the driven gear  33 , an increase in noise at the gear mesh frequency is restrained, and therefore the noise of the gear meshing is reduced. 
       FIGS. 5A, 5B, and 5C  are views of gear meshing of the drive gear  32  and the driven gear  33  on the occurrence of misalignment. To be more specific,  FIG. 5A  is a diagram illustrating a case in which the drive gear  32  and the driven gear  33  are normal gears having the crowning amount of 0 μm,  FIG. 5B  is a diagram illustrating a case in which the driven gear  33  with an appropriate crowning amount, and  FIG. 5C  is a diagram illustrating a case in which the driven gear  33  with an excessive crowning amount. 
     There are cases that misalignment occurs since the gear shaft of the driven gear  33  is tilted or the driven gear  33  is tilted with respect to the gear shaft of the driven gear  33  due to the backlash between the driven gear  33  and the gear shaft of the driven gear  33 . In such cases, when the drive gear  32  and the driven gear  33  are normal gears, as illustrated in  FIG. 5A , the drive gear  32  and the driven gear  33  are not meshed in the whole face width but the lower part (in  FIG. 5A ) of a tooth  32   a  of the normal drive gear  32  and the lower part (in  FIG. 5A ) of a tooth  33   a  of the normal driven gear  33  are only meshed. This state is a partial contact state in which the driving force of a gear is received in a part of the face width of another gear. When the driving force is transmitted in such a partial contact state, the drive transmission is unstable to result in an increase in vibration and rotational unevenness. As a result, noise at the gear meshing increases and the image quality deteriorates. 
     On the other hand, in a case in which the driven gear  33  has an excessive crowning amount S 2  of 50 μm or greater, as illustrated in  FIG. 5C , even when the misalignment occurs, the position of tooth contact is located in the substantially center in the tooth trace direction. This configuration restrains twist of a tooth or teeth, twist of a gear in the rotational direction, or both caused by application of the load at one end side of a tooth or teeth. As a result, vibration of the whole gear is reduced. However, the face width at which the tooth  32   a  of the drive gear  32  and the tooth  33   a  of the driven gear  33  mesh with each other is significantly narrow, and the load concentrates on a significantly small area between the tooth  32   a  of the drive gear  32  and the tooth  33   a  of the driven gear  33 . Due to the above-described load concentration, noise increases at the gear mesh frequency increases. Details of the increase in noise due to load concentration are described below. 
     By contrast, as illustrated in  FIG. 5B , the driven gear  33  has the appropriate crowning amount S 1 , which is, for example, the crowning amount S 1  less than 50 μm, the gear meshing portion (the tooth contact portion) between the tooth  32   a  of the drive gear  32  and the tooth  33   a  of the driven gear  33  is located closer to the center in the face width when compared with the gear meshing portion between the tooth of the normal drive gear and the tooth of the normal driven gear with the crowning amount of 0 μm. Further, the greater face width in which the tooth  32   a  of the drive gear  32  and the tooth  33   a  of the driven gear  33  mesh with each other is achieved when compared with the configuration illustrated in  FIG. 5C , with the excessive crowning amount S 2  of the driven gear  33 . As a result, vibration generated at the gear meshing of the gears and rotation unevenness of the gears are restrained, and therefore an increase in noise and deterioration in image quality are restrained. 
     Further, among the plurality of gears in the image forming apparatus  500 , it is preferable that a gear mounted on a motor shaft or a gear meshing with the gear on the motor shaft is a crowned gear. When the load is applied, the load is removed toward an upstream side in a drive transmission direction in which the driving force is transmitted, due to backlash, on the downstream side, from the gear meshing portion of the gear on the motor shaft, in the drive transmission direction. However, the gear mounted on the motor shaft is a gear that directly receives the driving force from the drive motor, that is, the highest load is applied to the gear meshing portion of the gear mounted on the motor shaft and the gear meshing with the gear mounted on the motor shaft. Therefore, by employing a crowned gear as a gear mounted on the motor shaft or a gear meshing with the gear mounted on the motor shaft and by providing the crowning amount less than 50 μm to the crowned gear, noise of the gear meshing is effectively restrained when a misalignment of gears occurs. 
     Verification Test 1. 
     A gear meshing verification test, Verification Test 1, was conducted with the drive device  30  illustrated in  FIG. 3  to evaluate the gear meshing on cases in which the driven gear  33  is a normal gear (with the crowning amount=0 μm), the driven gear  33  is a crowned gear with the crowning amount of 20 μm, and the driven gear  33  is a crowned gear with the crowning amount of 50 μm. 
     The driving conditions for the evaluation of the gear meshing are as follows:
         Load of Driven Unit (Fixing Device): 2.0 [N·m];   Rotation Speed of Drive Motor: 2000 [rpm];   Deceleration of Driving: 15;   Drive Gear  32 : Metal Gear (Normal Gear);   Driven Gear  33 : Resin Gear (Crowned Gear); and   Gear Meshing Frequency of Drive Gear  32  and Driven Gear  33 : 600 Hz to 700 Hz.       

     Under the above-described driving conditions, the drive device  30  was driven to measure vibration of the drive gear  32  and the driven gear  33 . 
       FIGS. 6 to 8  are graphs rendering the results of the tests. Specifically,  FIG. 6  is a graph of vibration data in a case in which the driven gear  33  is a normal gear (having the crowning amount of 0 μm).  FIG. 7  is a graph of vibration data in a case in which the driven gear  33  is a crowned gear having the crowning amount of 20 μm.  FIG. 8  is a graph of vibration data in a case in which the driven gear  33  is a crowned gear having the crowning amount of 50 μm. In  FIGS. 6 to 8 , an X axis (horizontal axis) represents frequency and a Y axis (vertical axis) represents acceleration (vibration). 
     In a case in which the driven gear  33  is a normal gear (having the crowning amount of 0 μm as illustrated in  FIG. 6 , the load applied to the tooth concentrates on the end portion of the driven gear  33  since the driven gear  33  meshes with the drive gear  32  at the end portion, as illustrated in  FIG. 5A . Therefore, vibrations at various frequencies were observed due to vibrations, such as the twist of the tooth (teeth) in the rotational direction of the driven gear  33  and the twist of the driven gear  33  in the rotational direction of the driven gear  33 . 
     Further, in a case in which the driven gear  33  is a crowned gear having the crowning amount of 50 μm, the tooth contact position of the driven gear  33  with the drive gear  32  is located in the substantially center in the face width direction. Therefore, neither tooth nor gear is twisted in the rotational direction of the driven gear  33 . Accordingly, as illustrated in  FIG. 8 , vibrations of frequencies other than the gear meshing frequency (in a range of 600 Hz to 700 Hz) are sufficiently restrained. However, as illustrated in  FIG. 5C , since the tooth contact width of the driven gear  33  is relatively narrow and the driving force is transmitted locally, the load concentrates on the center of the tooth, and therefore the vibration caused by the gear meshing frequency increased. 
     By contrast, in a case in which the driven gear  33  is a crowned gear having the crowning amount of 20 μm, the tooth contact position of the driven gear  33  is located in the substantially center in the face width direction, as illustrated in  FIG. 5B . Therefore, the degree of twist of tooth and gear is restrained in the rotational direction of the driven gear  33  and, as illustrated in  FIG. 7 , vibrations of frequencies other than the gear meshing frequency (in the range of 600 Hz to 700 Hz) are sufficiently restrained. Further, as illustrated in  FIG. 5B , with the appropriate tooth contact width, vibration of the gear mesh frequency was also restrained. 
     The sound pressure level when the driven gear  33  is a normal gear (having the crowning amount of 0 μm) is 60 [dB], which is the same as the sound pressure level when the driven gear  33  is a crowned gear having the crowning amount of 50 μm. On the other hand, the sound pressure level when the driven gear  33  is a crowned gear having the crowning amount of 20 μm is reduced to 59 [dB]. When compared with the sound pressure level of 60 [dB], the crowned gear having the crowning amount of 20 μm has reduced the sound energy amount by 30%. Accordingly, by employing the crowned gear having the crowning amount of 20 μm as the driven gear  33 , the noise of the image forming apparatus  500  is greatly reduced. 
     Actually, in addition to the above-described tests with the driven gear  33 , various crowned gears having different crowning amounts were evaluated. Through the tests, it has been proved that, if the driven gear  33  has at least a small crowning amount, in other words, if the driven gear  33  is a crowned gear, vibration of the driven gear  33  is reduced when compared with the driven gear  33  being a normal gear (with the crowning amount of 0 μm), thereby reducing noise of the image forming apparatus  500  or adverse effect on the image. From the above-described results, the driven gear  33  is a crowned gear having the crowning amount less than 50 μm. By so doing, when compared with the driven gear  33  being a normal gear (with the crowning amount of 0 μm), the driven gear  33  having the crowning amount less than 50 μm reduces the noise. 
     Verification Test 2. 
     In Verification Test 2, the tilt angle of the motor shaft  31   a  is changed to check the relation of the crowning amount of the driven roller  33  and the rotational unevenness of the driven roller  33 . 
     As illustrated in  FIG. 9 , Verification Test 2 was conducted with a normal gear (with the crowning amount of 0 μm) as the drive gear  32  and six (6) different crowned gears having different crowning amounts C as the driven gear  33 . The drive gear  32  and the driven gear  33  have the face width W of 10 mm. The drive gear  32  and the driven gear  33  are helical gears having a helix angle of 12 degrees. Note that the helix angle α is an angle of inclination of the helical tooth with respect to the axial direction of the gears, as illustrated in  FIG. 10 . The gear meshing frequency between the drive gear  32  and the driven gear  33  is 600 Hz to 700 Hz. 
     Further, as illustrated in  FIG. 9 , the drive motor  31  is tilted to adjust a tilt angle θ of the motor shaft  31   a . The gear meshing position of the helical tooth changes from one end side to the opposed end side in the axial direction of a gear. A shaft tilt direction in which the shaft is tilted to cause a partial contact of gears on a first meshing side of a helical tooth is indicated as a positive (+) shaft tilt direction and a shaft tilt direction in which the shaft is tilted to cause a partial contact on a last meshing side of the helical tooth is indicated as a negative (−) shaft tilt direction. In Verification Test 2, as illustrated in  FIG. 4A , the positive shaft tilt direction indicates the tilt of the motor shaft  31   a  in which the leading end of the motor shaft  31   a  is tilted in a direction to move away from the driven gear  33 . On the other hand, the negative shaft tilt direction indicates the tilt of the motor shaft  31   a  in which the leading end of the motor shaft  31   a  is tilted in a direction to approach the driven gear  33 . In Verification Test 2, the rotational unevenness of the driven gear  33  was measured the angle of every 0.5 degree in a range from −1.0 degree to +1.0 degree. As illustrated in  FIG. 4A , an encoder  35  was mounted on the gear shaft  33   b  of the driven gear  33  so that the encoder  35  measured the rotational unevenness of the driven gear  33 . The graph of  FIG. 11  represents the results of the measurement. 
     As illustrated in the graph of  FIG. 11 , the crowned gear having the crowning amount in the range of 10 μm to 30 μm restrained the rotational unevenness, compared with the normal gear (with the crowning amount of 0 μm), in the range of the maximum tilt angle of the motor shaft  31   a  (−0.7 degrees to +0.7 degrees) due to the variation of parts and the accumulation of assembly errors (Accumulation Range). On the other hand, when the tilt angle of the driven gear  33  was −0.5 degrees, the crowned gear having the crowning amount of 40 μm was worse than the normal gear (with the crowning amount of 0 μm) in the rotational unevenness. By contrast, however, when the tilt angle of the driven gear  33  was +0.5 degrees or +1.0 degree, the crowned gear having the crowning amount of 40 μm had the least rotational unevenness and the average value of the rotational unevenness of the crowned gear was sufficiently lower than the normal gear (with the crowning amount of 0 μm). From the above-described results of Verification Test 2, the crowned gear having the crowning amount of 40 μm was also expected to enhance the rotational unevenness of the driven gear  33  sufficiently. 
       FIGS. 12A and 12B  are graphs of the results of the test checking the crowning amount C and the face width W. 
     Specifically,  FIG. 12A  is a graph of the results of the test conducted under the condition that the helical tooth has the helix angle α of 12 degrees and  FIG. 12B  is a graph of the results of the test conducted under the condition that the helical tooth has the helix angle α of 20 degrees. 
     Note that, in  FIGS. 12A and 12B , the motor shaft  31   a  was tilted by +0.5 degrees and the rotational unevenness was measured with the encoder  35  illustrated in  FIG. 4A . The gear meshing frequency between the drive gear  32  and the driven gear  33  is 600 Hz to 700 Hz. 
     As can be seen from  FIGS. 12A and 12B , when a crowned gear is used, the face width of the crowned gear is preferably set to 8 mm or greater, which preferably reduces the rotational unevenness equal to or lower than the rotational unevenness of the normal gear (having the crowning amount of 0 μm). Since the crowned gear meshes with another gear in the center of the tooth surface, the contact ratio of the crowned gear is reduced easily when compared with the contact ratio of the normal gear. Further, as the face width W decreases, the curvature (curvature) of the tooth surface with respect to the crowning amount C increases, and therefore the contact ratio tends to decrease easily. Generally, it is known that, when the contact ratio is below 1.2, the gears do not rotate smoothly, which results in an increase in the rotational unevenness and noise. Therefore, when the face width of the crowned gear is below 8 mm, the contact ratio goes below 1.2, which is an insufficient contact ratio to exert a rotational unevenness restraining effect by a crowned gear on the occurrence of misalignment (in other words, an effect to restrain a rotational unevenness of the gear by setting the tooth contact position in the center area in the face width direction). Accordingly, the effect to worsen is greater than the rotational unevenness restraining effect. As a result, the crowned gear is considered to worsen in the rotational unevenness than the normal gear (with the crowning amount of 0 μm). Therefore, when employing a crowned gear, the face width W is set to 8 mm or greater. To be more specific, the face width of the crowned gear as the drive gear  32  or the driven gear  33  is preferably set to be 8 mm or greater. By so doing, the contact ratio remains at 1.2 or greater and the rotational unevenness caused by a decrease in the contact ratio is restrained. 
     As illustrated in  FIG. 12A , when the face width of the tooth of a crowned gear is beyond 30 mm, the rotational unevenness restraining effect by the crowned gear decreases. As the face width W of a crowing gear increases, the curved portion (curvature) of the tooth surface with respect to the crowning amount C decreases. As a result, it is considered that the rotational unevenness restraining effect decreases since the tooth contact position on the occurrence of misalignment is one end side in the face width. Therefore, in order to sufficiently obtain the rotational unevenness restraining effect by the crowned gear, the face width of the crowned gear is preferably set to be 30 mm or smaller. To be more specific, the face width of the crowned gear as the drive gear  32  or the driven gear  33  is preferably set to be 30 mm or smaller. 
     Note that, as illustrated in  FIG. 12B , in a case in which the helix angle α is 20 degrees, when the face width W of the tooth of the crowned gear is greater than 22 mm, the rotational unevenness of the normal gear (with the crowning amount of 0 μm) and the rotational unevenness of the crowned gear increase excessively. Therefore, both the normal gear (with the crowning amount of 0 μm) and the crowned gear cannot be used as the driven gear  33 . However, a crowned gear in at least an acceptable range (for example, 8 mm to 22 mm) of the face width restrains the rotational unevenness more effectively than the normal gear (with the crowning amount of 0 μm). 
       FIG. 13  is a graph of the results of tests conducted in a condition in which the drive gear  32  and the driven gear  33  have various crowning amounts. 
     Note that the graph of  FIG. 13  renders the results of the tests conducted under the conditions that the motor shaft  31   a  was tilted by +0.5 degrees and the encoder  35  illustrated in  FIG. 4A  was used to measure the rotational unevenness. The gear meshing frequency between the drive gear  32  and the driven gear  33  is 600 Hz to 700 Hz. 
     As illustrated in the graph of  FIG. 13 , when the crowning amount of the drive gear  32  and the sum of the crowning amounts of the drive gear  32  and the driven gear  33  are identical (in other words, the crowning amount of the drive gear  32  is the same as the total crowning amounts of the drive gear  32  and the driven gear  33 ), the possible rotational unevenness of the drive gear  32  and the possible rotational unevenness of the driven gear  33  are substantially the same. Therefore, when the total crowning amount of the drive gear  32  and the driven gear  33  are in a range of 10 μm to 40 μm, the rotational unevenness of the drive gear  32  and the rotational unevenness of the driven gear  33  are restrained preferably. In other words, the sum of the crowning amount of the drive gear  32  and the crowning amount of the drive gear  32  and the driven gear  33  is 10 μm or greater and 40 μm or smaller. Note that, considering the processing cost, it is preferable that either the drive gear  32  or the driven gear  33  is a crowned gear. 
     As described above, a description has been given of the drive device  30  of the fixing device  18  having a heavier load in the image forming apparatus  500 , as one embodiment to which this disclosure is applied. However, this disclosure may also be applied to a sheet conveying device in which a transfer sheet is conveyed. By applying this disclosure to the sheet conveying device, noise impact of the image forming apparatus  500  is effectively restrained. 
       FIG. 14  is a diagram illustrating an example of a schematic configuration of a sheet conveying device  600 . 
     The sheet conveying device  600  includes a pair of sheet conveying rollers  111 , a pair of sheet conveying rollers  112 , an upper conveyance guide plate  113   a , and a lower conveyance guide plate  113   b . The pair of sheet conveying rollers  112  is disposed downstream from the pair of sheet conveying rollers  111  in the sheet conveyance direction. The upper conveyance guide plate  113   a  and the lower conveyance guide plate  113   b  guide the sheet P conveyed between the pair of sheet conveying rollers  111  and the pair of sheet conveying rollers  112 . 
     As illustrated in  FIG. 14 , the sheet conveying device  600  includes a drive device  40 A configured to drive a pair of sheet conveying rollers  111  and a drive device  40 B configured to drive a pair of sheet conveying rollers  112 . The drive device  40 A and the drive device  40 B transmit respective driving forces generated by one drive motor or respective drive motors to the pair of sheet conveying rollers  111  and the pair of sheet conveying rollers  112 , respectively, via a plurality of gears. As illustrated in  FIG. 14 , the sheet conveying device  600  includes a plurality of drive devices, each driving at least a pair of sheet conveying rollers. According to this configuration, the drive devices generate vibration and noise. Since the load on each pair of sheet conveying rollers is relatively light, noise generated in each drive device is relatively small. However, since the image forming apparatus includes a plurality of drive devices, the total amount of noise of the plurality of drive devices contributes to an increase in noise of the whole image forming apparatus. 
     Therefore, among the plurality of gears of each drive device, a gear that meshes with a metallic drive gear directly mounted on the motor shaft of the drive motor is a crowned gear having the crowning amount less than 50 μm. Accordingly, noise impact of each driving device is restrained, and therefore noise impact of the image forming apparatus is effectively reduced. Further, by setting the total crowning amount of the crowning amount of the drive gear and the crowning amount of the drive gear and the driven gear meshing with the drive gear, to a value in the range of 10 μm to 40 μm, the rotational unevenness of any sheet conveying rollers of the plurality of drive devices in the image forming apparatus is restrained, and therefore the sheet is conveyed stably at a specified speed. In other words, by setting the sum of the crowning amount of the drive gear  32  and the crowning amount of the drive gear  32  and the driven gear  33  to 10 μm or greater and 40 μm or smaller, the sheet is conveyed stably at the specified speed. Accordingly, density unevenness in an image due to a change in the sheet conveying speed is restrained. 
       FIG. 15  is a diagram illustrating the sheet conveying device  600 , in a state of a multi-sheet feeding in which a plurality of sheets is conveyed at a time while being overlapped. 
     As illustrated in  FIG. 15 , at the time of a multi-sheet feeding, a load is applied abruptly on the pair of sheet conveying rollers (in  FIG. 15 , the pair of sheet conveying rollers  111 ). In this case, there is a risk that the pair of sheet conveying rollers locks to damage or break the gear train. As illustrated in  FIGS. 5A and 5C , in a case in which the tooth contact width is small (narrow), the load concentrates on a significantly small area, and therefore the risk of damaging or breaking the gear is relatively high. By contrast, when a crowned gear having the crowning amount less than 50 μm is employed, the tooth contact width is increased, and therefore the risk of damaging or breaking the gear is reduced. 
     Further, this disclosure is also applicable to a gear train such as a gear train that transmits the driving force to the photoconductor drum  1 , a gear train that transmits the driving force to each roller of the developing device  9 , a gear train that transmits the driving force to the intermediate transfer belt  5 , and a gear train that transmits the driving force to the secondary transfer roller  7 . By applying this disclosure to the above-described gear trains, noise is restrained and deterioration in image quality due to vibration and rotational unevenness is restrained. 
     The configurations according to the above-descried embodiments are not limited thereto. This disclosure can achieve the following aspects effectively. 
     Aspect 1. 
     In Aspect 1, an image forming apparatus (for example, the image forming apparatus  500 ) includes a driven unit (for example, the fixing device  18  and the sheet conveying device  600 ), and a drive transmitter (for example, the drive device  30  including the gear train) including a drive source (for example, the drive motor  31 ) configured to drive the driven unit, a drive gear (for example, the drive gear  32 ) configured to receive a driving force from the drive source, and a driven gear (for example, the driven gear  33 ) meshed with the drive gear. The drive transmitter is configured to transmit the driving force from the drive source to the driven unit. The drive gear or the driven gear is a crowned gear crowning-processed. The crowned gear has a crowning amount less than 50 μm. 
     According to this configuration, as described in verification tests (which are Verification Test 1 and Verification Test 2), by setting the drive gear or the driven gear as a crowned gear having the crowning amount less than 50 μm, vibration is more restrained when compared with a configuration in which normal gears having no crowning amount (that is, normal gears with the crowning amount of 0 μm) are meshed with each other, and therefore noise is more reduced. 
     Aspect 2. 
     In Aspect 1, the drive gear (for example, the drive gear  32 ), the driven gear (for example, the driven gear  33 ), or both is the crowned gear, and a sum of a crowning amount of the drive gear and a crowning amount of the driven gear is 10 μm or greater and 40 μm or smaller. 
     According to this configuration, as described with reference to  FIGS. 11 and 13 , the rotational unevenness of the gear or gears is restrained. 
     Aspect 3. 
     In Aspect 1 or Aspect 2, a face width of the drive gear (for example, the drive gear  32 ) or the driven gear (for example, the driven gear  33 ), that is the crowned gear, is 8 mm or greater. 
     According to this configuration, as described with reference to  FIG. 12 , the rotational unevenness of the gear or gears is restrained. 
     Aspect 4. 
     In any one of Aspects 1 to 3, a face width of the drive gear (for example, the drive gear  32 ) or the driven gear (for example, the driven gear  33 ), that is, the crowned gear, is 30 mm or smaller. 
     According to this configuration, as described with reference to  FIG. 12 , the crowned gear provides the rotational unevenness restraining effect of the gear or gears sufficiently. 
     Aspect 5. 
     In any of Aspects 1 to 4, the drive gear (for example, the drive gear  32 ) is mounted on a drive shaft (for example, the motor shaft  31   a ) of the drive source (for example, the drive motor  31 ). 
     According to this configuration, as described in the embodiments above, the gear mounted on the drive shaft directly receives the driving force of the drive source. Therefore, unlike other gears, when a load is applied, the gear cannot reduce the load. Therefore, the greatest load is applied to the meshing portion at which the gear mounted on the drive shaft and the driven gear mesh with each other. Therefore, by setting the gear mounted on the drive shaft or the gear meshed with the gear mounted on the drive shaft to be a crowned gear having the crowning amount less than 50 μm, vibration and noise in misalignment are effectively restrained. 
     Aspect 6. 
     In any one of Aspects 1 to 5, the drive gear (for example, the drive gear  32 ) is made of metal and the driven gear (for example, the driven gear  33 ) is made of resin. 
     According to this configuration, as described in the embodiments above, different from a resin gear, a hard metal drive gear does not deform elastically and therefore has a low effect of attenuating vibration. For this reason, vibration and noise are likely to increase at the meshing portion at which the gear meshes with the metal gear. Therefore, by employing a metal gear or a resin gear that meshes with the metal gear as a crowned gear having the crowning amount less than 50 μm, vibration and noise in misalignment are effectively restrained. 
     Aspect 7. 
     In any one of Aspects 1 to 6, the driven gear (for example, the driven gear  33 ) is the crowned gear. 
     According to this configuration, vibration and noise at the meshing portion of the drive gear (for example, the drive gear  32 ) and the driven gear are restrained. 
     Aspect 8. 
     In any one of Aspects 1 to 7, the drive gear (for example, the drive gear  32 ) is the crowned gear. 
     According to this configuration, vibration and noise at the meshing portion of the drive gear and the driven gear (for example, the driven gear  33 ) are restrained. 
     Aspect 9. 
     In any one of Aspects 1 to 8, wherein the driven unit (for example, the fixing device  18  and the sheet conveying device  600 ) is a fixing device (for example, the fixing device  18 ). 
     According to this configuration, as described in the embodiments above, the gear of the drive transmitter to transmit the driving force of the drive source (for example, the drive motor  31 ) to the fixing device having a heavier load in the image forming apparatus is a crowned gear having the crowning amount less than 50 μm. By doing so, the noise of the image forming apparatus is restrained effectively. 
     Aspect 10. 
     In any of the first to ninth aspects, the driven unit (for example, the fixing device  18 , the sheet conveying device  600 ) is a sheet conveying device (for example, the sheet conveying device  600 ). 
     According to this configuration, as described in the embodiments above, the sheet conveying device includes a plurality of drive devices, and therefore noise is generated in each of the plurality of driving devices. Therefore, the total amount of noise of the plurality of drive devices contributes to an increase in noise of the whole image forming apparatus. Therefore, the gear of the drive transmitter that conveys the driving force of the drive source to each pair of sheet conveying rollers in the sheet conveying device is a crowned gear having the crowning amount of less than 50 μm. By so doing, noise of each driving device is restrained, and therefore noise of the image forming apparatus is effectively reduced. Further, when a sudden change in load occurs due to the occurrence of multi-sheet feeding, this configuration prevents the gear or gears from damage or breakage. 
     In the above-described embodiments, the sheet P for image formation is employed as a recording medium on which an image is formed. However, the sheet P is not limited to the recording medium but also includes thick paper, postcard, envelope, plain paper, thin paper, coated paper, art paper, tracing paper, and the like. The sheet P further includes a non-paper material such as OHP sheet, OHP film, resin film, and any other sheet-shaped material on which an image may be formed. 
     The effects described in the embodiments of this disclosure are listed as the examples of preferable effects derived from this disclosure, and therefore are not intended to limit to the embodiments of this disclosure. 
     The embodiments described above are presented as examples to implement this disclosure and are not intended to limit the scope of this disclosure. These novel embodiments can be implemented in various other forms, and various omissions, replacements, or changes can be made without departing from the gist of this disclosure. These embodiments and their variations are included in the scope and gist of this disclosure, and are included in the scope of this disclosure recited in the claims and its equivalent. 
     Any one of the above-described operations may be performed in various other ways, for example, in an order different from the one described above.