Patent Publication Number: US-2023152738-A1

Title: Transmission mechanism, rotational force transmission device, and image forming apparatus

Description:
CROSS REFERENCE OF RELATED APPLICATION 
     Japanese Patent Application No. 2021-186747, filed on Nov. 17, 2021, and Japanese Patent Application No. 2021-186748, filed on Nov. 17, 2021 are herein incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The disclosure relates to a transmission mechanism, a rotational force transmission device, and an image forming apparatus, and, in particular, relates to a transmission mechanism that is disposed between, for example, a first gear and a second gear disposed side by side on the same rotary shaft and transmits the toque of the first gear to the second gear, a rotational force transmission device including the transmission mechanism, the first gear, and the second gear and transmits toque to a detachable unit, and an image forming apparatus including the transmission mechanism or the rotational force transmission device. 
     Description of the Background Art 
     There is a known a rotational force transmission device that transmits driving force from a drive source to a rotary member of a seat conveyance device. The rotational force transmission device includes a first gear (first ratchet gear) provided with a first interlocker, a second gear (second ratchet gear) provided with a second interlocker that engages with the first interlocker, and an urging member that causes the first gear and the second gear to he urged against each other. The first gear and the second gear rotate in a first direction to transmit driving force from the drive source to the rotary member. The first interlocker has a first face where the first gear rotating in the first direction transmits the driving force to the second gear while the first and second interlockers are engaged. The first face tilts in a direction that allows the first interlocker to rotate in the first direction while the first or second interlocker retracts in a direction against the urging force of the urging member to disengage from the other interlocker. The first and second interlockers disengage before the torque on the first and second interlockers due to the driving force from the drive source exceeding the allowable torque that deforms or breaks the first or second gear. 
     In the conventional technique, ratchet gears are used as torque limiters to prevent damage to gears and other components. When the rotational force transmission device disclosed in the conventional technique is disposed on a part that transmits rotational force to a detachable unit such as a fusing unit detachable from the device body, at the time of installment of the detachable unit to the device body, there is a risk of damaging the gears due to an excessive load caused by the impact of the gear of the detachable unit and the gear of the rotational force transmission device meshing with each other. Relatively large force is required to incorporate the urging member into the body of the rotational force transmission device. In the conventional technique, the urging member, the gears, etc., are individually mounted, so that the gears must be mounted while the urging member is held down with large force. This makes assembly difficult. 
     An object of the disclosure is, therefore, to provide a novel transmission mechanism, a novel rotational force transmission device, and a novel image forming apparatus. 
     Another object of the disclosure is to provide a transmission mechanism, a rotational force transmission device, and an image forming apparatus that can be readily assembled and properly prevent damage to the gears when the detachable unit is installed. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the disclosure, a rotational force transmission device includes: a first gear and a second gear disposed side by side on a same rotary shaft, the first gear and the second gear rotating about the rotary shaft in a first direction by driving force from a drive source to transmit rotational force to a detachable unit; a transmission mechanism disposed on a side of the first gear adjacent to the second gear to rotate together with the first gear, the transmission mechanism comprising a first ratchet disposed at an end adjacent to the second gear; and a ratchet member disposed on a side of the second gear adjacent to the first gear to rotate together with the second gear, the ratchet member comprising a second ratchet at an end adjacent to the first gear, the second ratchet engaging with the first ratchet, wherein, the transmission mechanism comprises: a first sleeve disposed adjacent to the first gear and comprising a first interlocker; a second sleeve disposed adjacent to the second gear in a manner movable in an axial direction of the rotary shaft, the second sleeve comprising the first ratchet and a second interlocker, the second interlocker engaging with the first interlocker with a rotation clearance allowing movement in a circumferential direction of the rotary shaft by a predetermined distance; and a coil spring disposed between the first sleeve and the second sleeve, the coil spring urging the second sleeve toward the ratchet member by restoring force against compression, the coil spring urging the first sleeve and the second sleeve in a direction causing disengagement of the first interlocker and the second interlocker by restoring force against twisting, when a rotational load equal to or greater than a predetermined value is applied between the first ratchet and the second ratchet, the first ratchet and the second ratchet disengage as a result of the second sleeve moving away from the ratchet member against urging force of the coil spring, and when the drive source is stopped, the second sleeve is able to idly rotate in a second direction opposite to the first direction relative to the first sleeve by a circumferential distance corresponding to the rotation clearance formed between the first interlocker and the second interlocker by the urging force of the coil spring. 
     According to the first aspect, the ratchet member, the second sleeve, and the coil spring constitute a torque limiter mechanism, and the first sleeve, the second sleeve, and the coil spring constitute a buffer mechanism. Thus, damage to the gears when the detachable unit is locked can be appropriately prevented, and damage to the gears when the detachable unit is installed can be appropriately prevented. 
     According to the first aspect, the coil spring functions as a compression spring in the torque limiter mechanism and as a torsion spring in the buffer mechanism, that is, one coil spring is shared by both mechanisms. The second sleeve is shared by the torque limiter mechanism and the buffer mechanism. Therefore, it is possible to reduce parts, cost, and size. 
     According to a second aspect of the disclosure, which is dependent on the first aspect, one of the first interlocker and the second interlocker is a protrusion extending along the axial direction of the rotary shaft and having a length in the circumferential direction of the rotary shaft equal to a first width, the other one of the first interlocker and the second interlocker is a groove extending along the axial direction of the rotary shaft and having a length in the circumferential direction of the rotary shaft equal to a second width, the second width being larger than the first width, and the coil spring urges the first sleeve and the second sleeve to cause the protrusion to abut on one of two sides of the groove. 
     According to a third aspect of the disclosure, which is dependent on the first aspect, the first ratchet has a plurality of first tilt faces and a plurality of second tilt faces, the first tilt faces being disposed at predetermined intervals along the circumferential direction of the rotary shaft and tilting by a first tilt angle to the axial direction of the rotary shaft, the second tilt faces being disposed between the first tilt faces and tilting by a second tilt angle to the axial direction of the rotary shaft, the second tilt angle being larger than the first tilt angle, and the second ratchet has a plurality of third tilt faces and a plurality of fourth tilt faces, the third tilt faces tilting by the first tilt angle to the axial direction of the rotary shaft and engaging with the first tilt faces, respectively, the fourth tilt faces tilting by the second tilt angle to the axial direction of the rotary shaft and engaging with the second tilt faces, respectively. 
     According to a fourth aspect of the disclosure, which is dependent on the third aspect, the first tilt angle is within a range of 2 to 15 degrees, both inclusive. 
     According to a fifth aspect of the disclosure, which is dependent on the first aspect, the first sleeve comprises a first restrictor abutting on a first end of a wire rod constituting the coil spring in the circumferential direction of the rotary shaft to restrict a circumferential position of the first end, and the second sleeve comprises a second restrictor abutting on a second end of the wire rod constituting the coil spring to restrict a circumferential position of the second end. 
     According to a sixth aspect of the disclosure, which is dependent on the first aspect, the rotation clearance between the first interlocker and the second interlocker is equivalent to at least one pitch and no more than three pitches of the second gear. 
     According to a seventh aspect of the disclosure, an image forming apparatus includes: the rotational force transmission device according to the first aspect disposed on a body of the image forming apparatus, and a fusing unit removably disposed in the body and receiving rotational force from the rotational force transmission device. 
     According to an eighth aspect of the disclosure, a transmission mechanism is disposed between a first gear and a second gear and transmits rotational force of the first gear to the second gear, the first gear and the second gear being disposed side by side on a same rotary shaft, the transmission mechanism including: a first sleeve rotatable around the rotary shaft and comprising a first interlocker; a second sleeve disposed adjacent to the first sleeve in a manner rotatable around the rotary shaft and comprising a second interlocker engaging with a first interlocker with a rotation clearance allowing movement in a circumferential direction of the rotary shaft by a predetermined distance; and an urging member disposed between the first sleeve and the second sleeve and urging the first sleeve and the second sleeve in a direction causing the first interlocker and the second interlocker to disengage, wherein, the first sleeve comprises a first retainer, and the second sleeve comprises a second retainer engaging with the first retainer in an axial direction of the rotary shaft, and the first sleeve, the second sleeve, and the urging member are unitized by the first retainer and the second retainer engaging with each other. 
     In the eighth aspect, the first interlocker and the second interlocker are engaged with a clearance in the circumferential direction, and the first sleeve and the second sleeve are urged by the urging member in a direction that causes the first interlocker and the second interlocker to disengage. This allows the second sleeve to idly rotate relative to the first sleeve by the circumferential distance corresponding to the clearance. Therefore, the transmission mechanism functions as a buffer mechanism to alleviate the impact of, for example, meshing another gear with the second gear. 
     According to the eighth aspect, the transmission mechanism is unitized by disposing the urging member between the first sleeve and the second sleeve. Thus, the transmission mechanism can be readily assembled into the apparatus body. 
     According to a ninth aspect of the disclosure, which is dependent on the eighth aspect, the urging member is a coil spring, the urging member urges the first sleeve and the second sleeve in a direction causing the first interlocker and the second interlocker to disengage by restoring force against twisting, and the urging member urges the first sleeve and the second sleeve in a direction causing the first sleeve and the second sleeve to move away from each other in the axial direction of the rotary shaft by restoring force against compression. 
     According to a tenth aspect of the disclosure, which is dependent on the eighth aspect, one of the first retainer and the second retainer is an engagement claw extending in the axial direction of the rotary shaft, and the other one of the first retainer and the second retainer is a claw-receiving hole to which the engagement claw is fitted. 
     According to an eleventh aspect of the disclosure, which is dependent on the tenth aspect, a length of the claw-receiving hole in the circumferential direction of the rotary shaft is larger than a length of the engagement claw in the circumferential direction of the rotary shaft. 
     According to a twelfth aspect of the disclosure, which is dependent on the eighth aspect, one of the first interlocker and the second interlocker is a protrusion extending along the axial direction of the rotary shaft and having a length in the circumferential direction of the rotary shaft equal to a first width, the other one of the first interlocker and the second interlocker is a groove extending along the axial direction of the rotary shaft and having a length in the circumferential direction of the rotary shaft equal to a second width, the second width being larger than the first width, and the urging member is a coil spring and urges the first sleeve and the second sleeve to cause the protrusion to abut on one of two sides of the groove. 
     According to a thirteenth aspect of the disclosure, an image forming apparatus includes the transmission mechanism according to the eighth aspect, wherein the transmission mechanism transmits rotational force to a fusing unit. 
     According to the disclosure, since the rotational force transmission device functions as a torque limiter mechanism and a buffer mechanism, gear breakage can be properly prevented when the detachable unit is locked, and gear breakage can be properly prevented when the detachable unit is installed. According to the disclosure, since the coil spring and the second sleeve are shared as components of the torque limiter mechanism and the buffer mechanism, it is possible to reduce parts, cost, and size. The transmission mechanism is unitized ley disposing the urging member between the first sleeve and the second sleeve. Thus, the transmission mechanism can be readily assembled into the body of the rotational force transmission device. 
     The above-described objects, other objects, features, and advantages of the present disclosure will become more apparent from the detailed description of the following embodiment given with reference to the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic cross-sectional view of the internal structure of an image forming apparatus including a rotational force transmission device including a transmission mechanism, according to a first embodiment of the disclosure. 
         FIG.  2    is a perspective view of the image forming apparatus with the fusing unit removed. 
         FIG.  3    is a schematic cross-sectional view of the fusing unit. 
         FIG.  4    is a cross-sectional view of a fusing unit drive device. 
         FIG.  5    is a perspective view of the rotational force transmission device. 
         FIG.  6    is an exploded perspective diagram illustrating the configuration of the rotational force transmission device. 
         FIG.  7    is a perspective vie of a ratchet member of the rotational force transmission device. 
         FIG.  8    is a front view of the ratchet member. 
         FIG.  9    is an exploded perspective diagram illustrating the configuration of the transmission mechanism of the rotational force transmission device. 
         FIG.  10    is a cross-sectional view of the transmission mechanism. 
         FIG.  11    is a perspective view of a first sleeve of the transmission mechanism. 
         FIG.  12    is a first perspective view of a second sleeve of the transmission mechanism. 
         FIG.  13    is a second perspective view of the second sleeve. 
         FIG.  14    is an illustration of the internal structure of the rotational force transmission device with a first ratchet and a second ratchet engaged. 
         FIG.  15    is an illustration of the internal structure of the rotational force transmission device with a first ratchet and a second ratchet disengaged. 
         FIG.  16    is an illustration of the buffering operation of the rotational force transmission device. 
         FIG.  17    is an exploded perspective diagram illustrating the configuration of the transmission mechanism of the rotational force transmission device according to a second embodiment of the disclosure. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     Referring to  FIGS.  1  and  2   , an image forming apparatus  10 , which is one embodiment of the disclosure, is an apparatus that forms a multicolored or monochromatic image on a sheet of paper (a recording medium) by an electrophotographic method. As described in detail below, the image forming apparatus  10  includes a rotational force transmission device  104  disposed in the apparatus body  12 . The rotational force transmission device  104  includes an input gear  122 , an output gear  124 , and a transmission mechanism  126  (see  FIG.  6   ), and transmits ciriving force (rotational force) from a motor  102  to a fusing unit  46 , which is an example of a detachable unit. 
     The basic configuration of the image forming apparatus  10  will now be briefly described. Note that in this specification, the front-back direction (depth direction) of the image forming apparatus  10  and its components are defined such that the surface facing the user&#39;s standing position, that is, the surface on the side to which an operation unit  26  is disposed is the forward surface (front surface). The left-right direction (transverse directions) of the image forming apparatus  10  and its components are defined with reference to the state in which the image forming apparatus  10  is viewed from a user. 
     As illustrated in  FIGS.  1    and.  2 , the image forming apparatus  10  according to the present embodiment is a multifunction peripheral (MFP) having a copier function, a printer function, a scanner function, a facsimile function, and the like. The image forming apparatus  10  includes an apparatus body  12  including an image former  30 , etc., and an image reading device  14  disposed above the apparatus body  12 . 
     The image reacting device  14  includes a document table  16  that is made of transparent material. A document pressing cover  18  is attached in a freely openable/closable manner on the upper portion of the document table  16  via a hinge or the like. The document pressing cover  18  is provided with an automatic document feeder (ADF)  24  that automatically feeds, one sheet at a time, documents placed on a document loading tray  20  to an image reacting position  22 . On the front surface side of the document table  16 , the operation unit  26  is provided to accept the user&#39;s input operation, such as a print instruction. The operation unit  26  is appropriately provided with a display such as a touch screen, various operation buttons, etc. 
     An image reader  28  that includes a light source, multiple mirrors, an imaging lens, a line sensor, and the like is installed in the image reading device  14 . The image reader  28  exposes a document surface to the light source, and leads reflected light, which is reflected from the document surface, to the imaging lens by using the mirrors. Then, the reflected light is focused on a light receiving element of the line sensor by the imaging lens, for imaging. The line sensor detects the luminance and chromaticity of the reflected light focused on the light receiving element for imaging, and generates image data based on an image on the surface of the document. As the line sensor, a charge-coupled device (CCD) or a contact image sensor (CIS), etc., is used. 
     The apparatus body  12  incorporates a controller (not illustrated) including a CPU and memory, an image former  30 , etc. In response to the input operation to the operation unit  26  by the user, the controller sends a control signal to each component in the image forming apparatus  10  and causes the image forming apparatus  10  to perform various types of operation. 
     The image former  30  includes an exposure unit  32 , a developing device  34 , the photoreceptor drum  36 , a cleaner unit  38 , a charger  40 , an intermediate transfer belt unit  42 , a secondary transfer roller  44 , a fusing unit  46 , and the like, forms an image on paper that is advanced from a sheet feed tray  48  or a manual sheet feed tray  50 , and discharges the image-formed paper into an output tray  52 . As the image data used to form the image on the paper, image data read by the image reader  28 , image data sent from an external computer, or the like is used. 
     The image data handled by the image forming apparatus  10  correspond to a color image in four colors including black (K), cyan (C), magenta (M), and yellow (Y). Accordingly, four each of the developing devices  34 , the photoreceptor drums  36 , the cleaner units  38 , and the chargers  40  are provided to form four types of latent images corresponding to the four colors, and these constitute four image stations. 
     The photoreceptor drum  36  is an image carrier in which a photosensitive layer is formed on a surface of a conductive cylindrical base body, and the charger  40  is a member that charges a surface of this photoreceptor drum  36  to a predetermined potential. The exposure unit  32 , which is a laser scanning unit (LSU) that includes a laser emitter and a reflection mirror, forms an electrostatic latent image corresponding to the image data on the surface of the photoreceptor drum  36  by exposing the surface of the charged photoreceptor drum  36 . The developing device  34  visualizes the electrostatic latent image, which is formed on the surface of the photoreceptor drum  36 , by using toners in four colors (Y, M, C, and K). The cleaner unit  38  removes a residual toner remaining on the surface of the photoreceptor drum  36  after the development and the image transfer. 
     The intermediate transfer belt unit  42  includes an intermediate transfer belt  54 , a drive roller  56 , a driven roller  58 , and four intermediate transfer rollers  60 , and is disposed above the photoreceptor drum  36 . The intermediate transfer belt  54  is a flexible endless belt that is stretched across multiple rollers such as the drive roller  56  and the driven roller  58 , and is disposed such that its surface (outer circumferential surface) comes into contact with the surface of the photoreceptor drum  36 . The intermediate transfer belt  54  rotates (rotationally moves) in a predetermined direction with a rotary drive of the drive roller  56 . The intermediate transfer rollers  60  are arranged at positions in which the intermediate transfer rollers  60  face the photoreceptor drums  36  across the intermediate transfer belt  54 , respectively. At the time of image formation, the toner images of the respective colors formed on the respective photoreceptor drums  36  are sequentially superimposed and transferred onto the intermediate transfer belt  54  by using the intermediate transfer rollers  60 , to form a multi-color toner image on the intermediate transfer belt  54 . 
     The secondary transfer roller  44  is disposed so as to face the drive roller  56  across the intermediate transfer belt  54 . When the sheet passes through a secondary transfer nip between the secondary transfer roller  44  and the intermediate transfer belt  54 , the toner image formed on the intermediate transfer belt  54  is transferred to the sheet. 
     The fusing unit  46  (fusing device) includes a fusing belt  62  and a pressure roller  64 , and is disposed above the secondary transfer roller  44  (on the downstream side in the sheet conveyance direction). The fusing unit  46  is detachable from the apparatus body  12 . A fusing pad  76  and a heat source  82  are disposed on the inner side of the fusing belt  62  (see  FIG.  3   ). The fusing belt  62  is heated to a predetermined fusing temperature by the heat source  82 . The pressure roller ( 34  presses the fusing belt  62  between the pressure roller  64  and the fusing pad  76 . When the sheet passes through a fusing nip N between the pressure roller  64  and the fusing belt  62 , the toner image transferred to the sheet is melted, mixed, and pressed, and the toner image thermally fuses to the sheet. 
     A fusing unit drive device  100  is fixed on the back side of the fusing unit  46  of the apparatus body  12  (right rear portion of the apparatus body  12 ). When the fusing unit  46  is installed on the apparatus body  12 , the fusing unit  46  and the fusing unit drive device  100  are connected, and the pressure roller  64  is rotationally driven by receiving rotational driving force from the fusing unit drive device  100 . Specific configurations of the fusing unit  46  and the fusing unit drive device  100  will be described below. 
     In such an apparatus body  12 , a first sheet conveyance path L 1  is formed to advance the paper from the sheet feed tray  48  or the manual sheet feed tray  50  to the output tray  52  through a registration roller  68 , the secondary transfer roller  44 , and the fusing unit  46 . A second sheet conveyance path L 2  is also formed to return the sheet, which has passed through the fusing unit  46  after single-sided printing, to an upstream side of the secondary transfer roller  44  in a sheet conveyance direction in the first sheet conveyance path L 1  when double-sided printing is performed on the sheet. Multiple conveyance rollers  66  are appropriately provided in the first sheet conveyance path L 1  and the second sheet conveyance path L 2  to apply auxiliary propulsion force to the sheet. 
     The configuration of the fusing unit  46  will now be described with reference to  FIG.  3   . The fusing unit  46  includes a fusing belt  62  and a pressure roller  64  as an example of a pressure rotary body, and fuses the toner image on the sheet by passing the sheet through the fusing nip N formed between the fusing bell  62  and the pressure roller  64 . 
     Specifically, the fusing unit  46  includes a heater unit  70  including the fusing belt  62 , etc., and the pressure roller  64 , as illustrated in  FIG.  3   . The components of the heater unit  70  and the pressure roller  64  are integrally held by a fusing frame (not illustrated) in a predetermined mode. 
     The heater unit  70  includes the fusing belt  62  formed in a substantially cylindrical shape and extending in the front-back direction. (the width direction of the sheet). The fusing belt  62  is, for example, formed. by disposing a release layer on the surface of a belt-like base material composed of a synthetic resin, such as polyimide, or a metal, such as nickel. Such a fusing belt  62  is rotatable around its axis, and its inner diameter is, for example, 30 mm. The fusing pad  76 , a support member  78 , a reflector  80 , and the heat source  82  are disposed on the inner side of the fusing belt  62 . 
     The fusing pad  76  is a fixed member fixed in sliding contact with the inner circumferential surface of the fusing belt  62 , and is formed in a long plate shape extending along the axial direction of the fusing belt  62 . The fusing pad  76  has a sliding contact sheet  76   a  on its outer circumferential surface (at least a sliding contact surface with the fusing belt  62 ), and sliding oil for reducing the frictional force with the fusing belt  62  is applied to the sliding contact sheet  76   a . The length of the fusing pad  76  is the same as the length (width) of the fusing belt  62  in the axial direction. 
     The support member  78  supports the fusing pad  76  while pressing it against the inner circumferential surface of the fusing belt  62 , and the two ends of the support member  78  are fixed to the fusing frame. In the present embodiment, the support member  78 , which has a substantially L-shaped cross-section, includes a long plate-shaped fixing part  78   a  to which the fusing pad  76  is fixed, and a long plate-shaped erected part  78   b  that is erected from the width direction end of the fixing part  78   a . The reflector  80  having a plate-like shape is attached to the support member  78  so as to cover the surface adjacent to the heat source  82 . 
     The heat source  82  is a member for heating the fusing belt  62 , and extends along the axial direction of the fusing belt  62 . The heat source  82  is, for example, a lamp heater such as a halogen lamp. In the present embodiment, the heat source  82  includes a first lamp heater  82   a  that heats a central portion of the fusing belt  62  in the axial direction, and a second lamp heater  82   b  that heats the two ends of the fusing belt  62  in the axial direction. 
     The pressure roller  64  opposes the fusing pad  76  across the fusing belt  62 . The pressure roller  64  is disposed so as to extend parallel to the axial direction of the fusing belt  62 , and presses the fusing belt  62  against the fusing pad  76  to form the fusing nip N between the pressure roller  64  and the fusing belt  62 . 
     A gear (not illustrated) is provided at the rear end of the roller shaft (not illustrated) of the pressure roller  64 . The output gear  124  of the rotational force transmission device  104  described below is connected to this gear to connect the motor  102  (see  FIG.  4   ) to the roller shaft of the pressure roller  64 , and the pressure roller  64  is rotationally driven by the driving force from the motor  102 . The fusing belt  62  is driven to rotate in a direction opposite to the rotation direction of the pressure roller  64  in conjunction with the rotational driving of the pressure roller  64 . That is, the pressure roller  64  is brought into contact with the outer circumferential surface of the fusing belt  62  to form the fusing nip N with respect to the fusing belt  62 , and the rotational driving force is transmitted to the fusing belt  62  via the fusing nip N. This causes the fusing belt  62  to be driven and rotated. 
     The fusing unit  46  includes a temperature sensor  94 , such as a thermopile, that detects the surface temperature of the fusing belt  62 . A release plate  96  is disposed downstream of the fusing nip N in the sheet conveyance direction and prevents the sheet from being wound around the fusing belt  62 . 
     The configuration of the fusing unit drive device  100  will now be described. As illustrated in  FIG.  4   , the fusing unit drive device  100  is a device for providing rotational driving force to the fusing unit  46 , and includes a motor  102 , which is an example of a drive source, and the rotational force transmission device  104 . The rotational force transmission device  104  is connected to the motor  102  via a pinion gear  106  and an intermediate gear  108 . The motor  102 , the rotational force transmission device  104 , the pinion gear  106 , the intermediate gear  108 , etc., are held in a predetermined mode by a frame  110 . The driving force from the motor  102  is transmitted to the pressure roller  64  of the fusing unit  46  through the pinion gear  106 , the intermediate gear  108 , and the rotational force transmission device  104 . 
     As illustrated in  FIGS.  5  and  6   , the rotational force transmission device  104  includes a synthetic resin input gear  122  (first gear) and a synthetic resin output gear  124  (second gear) disposed side by side on the same rotary shaft  120  (spindle). When the fusing unit  46  is installed on the apparatus body  12 , the gear disposed at the rear end of the roller shaft of the pressure roller  64  is meshed with the output gear  124  of the rotational force transmission device  104 . The rotational force transmission device  104  transmits the rotational force to the pressure roller  64  of the fusing unit  46  when the input gear  122  and the output gear  124  rotate in a first direction X (positive rotation direction) around the rotary shaft  120  by the driving force from the motor  102 . 
     Here, it is preferable to provide a torque limiter mechanism in order to prevent gear breakage due to a high load on the fusing unit drive device  100  when the fusing unit  46  is locked due to breakage of the fusing belt  62 . It is preferable to provide buffering mechanism that alleviates the impact in order to prevent gear breakage due to an impact of the engagement of the gear of the pressure roller  64  and the output gear  124  of the rotational force transmission device  104  when the fusing unit  46  is installed on the apparatus body  12 . 
     Thus, in the present embodiment by adopting the following configuration for the rotational force transmission device  104 , the rotational force transmission device  104  is made to functionas a torque limiter mechanism and a buffer mechanism while saving components and reducing costs by sharing components. By unitizing the urging members with other parts, the ease of assembly to the apparatus body  12  is improved. The configuration of the rotational force transmission device  104  will now be described in detail. 
     As illustrated in  FIGS.  5  and  6   , a transmission mechanism  126  and a ratchet member  128  are disposed on the same rotary shaft  120  between the input gear  122  and the output gear  124 . The transmission mechanism  126  is mounted on the front face (output gear  124  side) of the input gear  122 . The transmission mechanism  126  is composed of three members: a first sleeve  130 , a second sleeve  132 , and a coil spring  134 , which is an example of an urging member. These three members are combined with each other to form a unit. At the rear end of the transmission mechanism  126  (specifically, the rear end of the first sleeve  130 ), a fitting projection  160  is formed to be fitted into a fitting hole  122   a  formed in the input gear  122 . The transmission mechanism  126  is connected and fixed to the input gear  122  by fitting the fitting hole  122   a  and the fitting projection  160 , and rotates about the rotary shaft  120  together with the input gear  122 . The front end of the transmission mechanism  126  (the end adjacent to the input gear  122 , specifically the front end of the second sleeve  132 ) has a first ratchet  190 . 
     Referring to  FIGS.  7  and  8   , together with  FIG.  6   , the ratchet member  128  is made of synthetic resin and is mounted on the rear face of the output gear  124  (the side adjacent to the input gear  122 ). The ratchet member  128  includes an annular base  140  having at the center an insertion hole  140   a  through which the rotary shaft  120  is passed. On the front face of the base  140 , four fitting projections  142  having a substantially trapezoidal cross-section are formed to be fitted into fitting holes  124  formed in the output gear  124 . The ratchet member  128  is connected and fixed to the output gear  124  by fitting the fitting holes  124   a  and the fitting projections  142 , and rotates about the rotary shaft  120  together with the output gear  124 . A short cylinder  144  is formed on the rear face of the base  140  and projects rearward from the periphery thereof, and a second ratchet  146  engaging with the first ratchet  190  is formed on the rear end surface of the short cylinder  144  (that is, the end of the ratchet member  128  adjacent to the input gear  122 ). 
     The second ratchet  146  is composed of teeth  146   a  in the shape of saw teeth arranged in the circumferential direction. Each of the teeth  146   a  has a third tilt face  146   b  and a fourth tilt face  146   c  that respectively engage with a first tilt face  190   b  and a second tilt face  190   c  of the first ratchet  190 , as described below. That is, the second ratchet  146  of the present embodiment has multiple third tilt faces  146   b  that tilt by a first tilt angle θ 1  to the axial direction of the rotary shaft  120  and engaged with the respective first tilt faces  190   b , and multiple fourth tilt faces  146   c  that tilt by a second tilt angle θ 2  to the axial direction of the rotary shaft  120  and engaged with the respective second tilt faces  190   c.    
     As illustrated in  FIGS.  9  and  10   , the transmission mechanism  126  consists of three members: a first sleeve  130  disposed adjacent to the input gear  122 , a second sleeve  132  disposed adjacent to the output gear  124 , and a coil spring  134  disposed between the first sleeve  130  and the second sleeve  132 . These three members are unitized prior to installation in the apparatus body  12 . By forming a unit by disposing the coil spring  134  between the first sleeve  130  and the second sleeve  132 , the transmission mechanism  126  can be readily installed in (assembled to) the apparatus body  12 . 
     Referring to  FIG.  11    together with  FIGS.  9  and  10   , the first sleeve  130  is composed of synthetic resin and has a short cylindrical base  150  provided with an insertion hole  150   a  through which the rotary shaft  120  passes in the center. At the front end of the base  150 , two engagement claws  152 , which are an example of a first retainer, are formed. Each of the engagement claws  152  protrudes forward from the front end of the base  150  and extends in the axial direction of the rotary shaft  120 . The tip of the engagement claw  152  is provided with a catch  152   a  protruding outward. The first sleeve  130  has a tubular cylinder  154  surrounding the base  150  and a tubular first holder  156  surrounding the cylinder  154 . The base  150 , the cylinder  154 , and the first holder  156  are connected at their rear ends by an annular plate-like connector  158 . On the rear face of the connector  158 , two fitting projections  160  having a substantially trapezoidal cross-section are formed to be fitted into fitting holes  122   a  formed in the input gear  122 . 
     A first interlocker  162  is formed on the outer circumferential surface of the cylinder  154 . In the present embodiment, the first interlocker  162  has multiple substantially rectangular plate-like protrusions that are arranged at predetermined intervals in the circumferential direction of the rotary shaft  120  and extend in the axial direction of the rotary shaft  120 . A flange-shaped first catch  164  projecting farther outward than the first holder  156  is formed on the periphery of the connector  158 , and a short cylindrical fold  166  projecting forward is formed on the periphery of the first catch  164 . A first restrictor  168  having a rectangular cross-section and protruding forward is formed on the front face of the first catch  164 . 
     Referring to  FIGS.  12  and  13    together with  FIGS.  9  and  10   , the second sleeve  132  is composed of synthetic resin and has a short cylindrical base  170  provided with an insertion hole  170   a  through which the rotary shaft  120  passes in the center. The second sleeve  132  includes a tubular cylinder  172  surrounding the base  170 , and the rear end of the base  170  and the center of the cylinder  172  are connected by an annular plate-shaped connector  174 . 
     The base  170  and the connector  174  are each provided with two claw-receiving holes  176 , which are each an example of a second retainer that engages with the first retainer (the engagement claw  152 ) of the first sleeve  130  in the axial direction of the rotary shaft  120 . The claw-receiving holes  176  are holes each having a rectangular cross-section formed by cutting the rear portion of the base  170  and the inner periphery of the connector  158  in the thickness direction of the base  170 . In the present embodiment, the length W 2  of the claw-receiving hole  176  in the circumferential direction of the rotary shaft  120  is larger than the length W 1  of the engagement claw  152  in the circumferential direction of the rotary shaft  120 . That is a rotation clearance equal to the difference between W 1  and W 2  is provided between the engagement claw  152  and the claw-receiving hole  176  to allow movement in the circumferential direction of the rotary shaft  120 . That is, the engagement claw  152  is attached to the claw-receiving hole  176  with a rotation clearance equal to the difference between W 1  and W 2 , i.e., a clearance that allows movement in the circumferential direction of the rotary shaft  120 . 
     By fitting the engagement claw  152  into the claw-receiving hole  176  and locking the catch  152   a  of the engagement claw  152  to the inner periphery of the connector  174 , the second sleeve  132  is integrated (unitized) with the first sleeve  130  so as to he retained by in the first sleeve  130  in the axial direction (front) of the rotary shaft  120 . By making the length of the claw-receiving hole  176  in the axial direction of the rotary shaft  120  is larger than the length of the catch  152   a  of the engagement claw  152 , the second sleeve  132  is integrated with the first sleeve  130  so as to he movable in the axial direction of the rotary shaft  120 . 
     A second interlocker  178  to be engaged with the first interlocker  162  of the first sleeve  130  is formed on the inner circumferential surface at the rear of the cylinder  172 . In the present embodiment, the second interlocker  178  has multiple grooves that are arranged at predetermined intervals in the circumferential direction of the rotary shaft  120  and extend in the axial direction of the rotary shaft  120 . Each of the grooves is formed by two projections  180 . The engagement of the first interlocker  162  with the second interlocker  178  causes the rotational force of the first sleeve  130  to be transmitted to the second sleeve  132 . 
     In the present embodiment, the length W 4  (second width) of the second interlocker  178  in the circumferential direction of the rotary shaft  120  is larger than the length W 3  (first width) of the first interlocker  162  in the circumferential direction of the rotary shaft  120 . That is, the first interlocker  162  and the second interlocker  178  are engaged with a rotation clearance equal to the difference between W 3  and W 4  to allow movement in the circumferential direction of the rotary shaft  120 . This rotation clearance a (see  FIG.  16   ) is equivalent to at least one pitch and no more than three pitches in terms of the pitch of the output gear  124 . That is, the second interlocker  178  engages with the first interlocker  162  with a rotation clearance that allows the second interlocker  178  to move by a predetermined distance in the circumferential direction of the rotary shaft  120 . The clearance between the engagement claw  152  and the claw-receiving hole  176  in the circumferential direction is equal to or slightly larger than α. Thus, it can be said that the rotation clearance α is defined by the difference between the lengths of the first interlocker  162  and the second interlocker  178  in the circumferential direction. 
     A short cylindrical second holder  182  is disposed on the outer circumferential surface of the front end of the cylinder  172 . A flange-shaped second catch  184  is formed on the front end of the second holder  182 , and a short cylindrical fold  186  projecting rearward is formed on the periphery of the second catch  184 . A second restrictor  188  having a rectangular cross-section and protruding rearward is formed on the rear face of the second catch  184 . 
     The first ratchet  190  is then formed at the front end of the cylinder  172 . The first ratchet  190  is composed of teeth  190   a  in the shape of saw teeth arranged in the circumferential direction. Each of the teeth  190   a  has a first tilt face  190   b  formed on the face of the side pushing the second ratchet  146  when the transmission mechanism  126  rotates in the first direction X to transmit the driving force from the motor  102  to the ratchet member  128 , and a second tilt face  190   c  formed on the opposite side. The second tilt angle θ 2  of the second tilt face  190   c  relative to the axial direction of the rotary shaft  120  larger than the first tilt angle θ 1  of the first tilt face  190   b . That is, the first ratchet  190  of the present embodiment has multiple first tilt faces  190   b  that tilt by the first tilt angle θ 1  to the axial direction of the rotary shaft  120 , and multiple second tilt faces  190   c  that are disposed between the first tilt faces  190   b  and tilt by the second tilt angle θ 2 , which is larger than the first tilt angle θ 1 , to the axial direction of the rotary shaft  120 . It is preferred that the first tilt angle θ 1  be, for example, within the range of 2 to 15 degrees, both inclusive, and the second tilt angle θ 2  be, for example, within the range of 60 and 80 degrees, both inclusive. 
     Returning to  FIGS.  9  and  10   , the coil spring  134 , which is an example of an urging member, is a general-purpose compression coil spring in which a metal wire is spirally wound at a predetermined pitch in the axial direction and is disposed to be sandwiched between the first sleeve  130  and the second sleeve  132 . Specifically, the rear end of the coil spring  134  is held by the first holder  156  of the first sleeve  130  by being externally fitted to the first holder  156 , and the front end of the coil spring  134  is held by the second holder  182  of the second sleeve  132  by being externally fitted to the second holder  182 . At this time, the rear end  134   a  of the coil spring  134  abuts on the first catch  164  of the first sleeve  130 , and the front end  134   b  of the coil spring  134  abuts on the second catch  184  of the second sleeve  132 . The first sleeve  130  and the second sleeve  132  are then urged away from each other in the axial direction of the rotary shaft  120  by the restoring force against compression of the coil spring  134 . That is, the coil spring  134  urges the second sleeve  132  toward the ratchet member  128 . 
     The coil spring  134  is slightly twisted in the radial direction when it is attached to the first sleeve  130  and the second sleeve  132 . In this state, one end  134   c  (one end face) of the wire rod forming the coil spring  134  and the first restrictor  168  of the first sleeve  130  abut on each other in the circumferential direction of the rotary shaft  120 , to restrict the circumferential position of the one end  134   c . The other end  134   d  (the other end face) of the wire rod forming the coil spring  134  and the second restrictor  188  of the second sleeve  132  abut on each other in the circumferential direction of the rotary shaft  120 , to restrict the circumferential position of the other end  134   d . This causes the coil spring  134  to urge the first sleeve  130  and the second sleeve  132  in the direction in which the first interlocker  162  and the second interlocker  178  disengage by the restoring force against torsion, that is, in the direction in which the first interlocker  162  (protrusion) abuts on one side face  178   a  of the two side faces of the second interlocker  178  (groove) (see  FIG.  16   ). The side face  178   a  is the side face remote from the side face  178   b  abutting on the first interlocker  162  when the first sleeve  130  rotates in the first direction X. Thus, it can also be said that the coil spring  134  urges the first sleeve  130  and the second sleeve  132  so that the rotation clearance with the second interlocker is provided farther than the first interlocker  162  in the first direction X by the restoring force against torsion. 
     As illustrated in  FIG.  14   , when the rotational force transmission device  104  described above drives the fusing unit  46 , the input gear  122  receives the driving force from the motor  102 , thereby rotating the input gear  122  in the first direction X. With this, the transmission mechanism  126  rotates in the first direction X, and the first tilt face  190   b  of the first ratchet  190  pushes the third tilt face  146   b  of the second ratchet  146 , to rotate the ratchet member  128  and the output gear  124  in the first direction X. This causes the driving force of the motor  102  to be transmitted to the pressure roller  64  of the fusing unit  46 , and the pressure roller  64  rotates in the forward rotation direction (the conveying direction of a sheet toward the output tray  52 ). 
     As illustrated in  FIG.  15   , when the fusing unit  46  is locked while the motor  102  is driven and a rotational load equal to or greater than a predetermined value is applied between the first ratchet  190  and the second ratchet  146 , the first ratchet  190  and the second ratchet  146  disengage as a result of the second sleeve  132  moving away from the ratchet member  128  (rearward) against the urging force of the coil spring  134 . The second sleeve  132  moves away from the ratchet member  128  when a rotational load equal to or greater than a predetermined value is applied between the first ratchet  190  and the second ratchet  146  because the first ratchet  190  and the second ratchet  146  have the first tilt faces  190   b  and the third tilt faces  146   b . That is, in the present embodiment, the torque limiter mechanism is composed of three parts: the ratchet member  128 , the second sleeve  132 , and the coil spring  134 . 
     In the present embodiment, as illustrated in  FIG.  16   , the first interlocker  162  and the second interlocker  178  are engaged with a rotation clearance in the circumferential direction of the rotary shaft  120 , and the first sleeve  130  and the second sleeve  132  are urged by the coil spring  134  in the direction of the disengagement of the first interlocker  162  and the second interlocker  178 , to cause the first interlocker  162  abuts on one side face  178   a  of the second interlocker  178 . This forms a non-transmission section in which no rotational force is transmitted by a circumferential. distance equal to the rotation clearance α in a free state in which the motor  102  is stopped, and enables the second sleeve  132  to idle in the second direction Y relative to the first sleeve  130 . Thus, when the fusing unit  46  is installed on the apparatus body  12 , that is, when the gear of the pressure roller  64  is engaged with the output gear  124  of the rotational force transmission device  104  in a free state in which the motor  102  is stopped, the second sleeve  132 , the ratchet member  128 , and the output gear  124  can idle relative to the first sleeve  130 , so that the impact during this engagement can be mitigated. That is, in the present embodiment, the buffer mechanism is composed of three parts: the first sleeve  130 , the second sleeve  132 , and the coil spring  134 . In other words, the buffer mechanism is composed of three parts: the first sleeve  130 , the second sleeve  132 , and the coil spring  134 , which constitute the transmission mechanism  126 . Even if the fusing unit  46  is locked (the pressure roller  64  does not rotate) for some reason, the fusing unit  46  can be readily removed from the apparatus body  12  because of the rotation clearance. 
     As described above, according to the present embodiment, since the rotational force transmission device  104  functions as a torque limiter mechanism and a buffer mechanism, gear breakage can be properly prevented when the fusing unit  46  is locked, and gear breakage can be properly prevented when the fusing unit  46  is installed. 
     According to the present embodiment, since the coil spring  134  is disposed between the first sleeve  130  and the second sleeve  132  and unitized, the transmission mechanism  126  can be readily assembled on to the apparatus body  12 . In other words, it excels in assembly performance. Since the transmission mechanism  126  also functions as a buffer mechanism, gear damage can be appropriately prevented when the fusing unit  46  is installed. 
     According to the present embodiment, the coil spring  134  functions as a compression spring in the torque limiter mechanism (and ratchet mechanism) and as a torsion spring in the buffer mechanism, that is, one coil spring  134  is shared by both mechanisms. The second sleeve  132  is shared by the torque limiter mechanism and the buffer mechanism. Therefore, it is possible to reduce parts, cost, and size. 
     Second Embodiment 
     Rotational force transmission device  104  according to the second embodiment of the disclosure will now be described with reference to  FIG.  17   . In the second embodiment, the configuration of the coil spring  134  of the transmission mechanism  126  of the rotational force transmission device  104  differs from that of the first embodiment. Since the other components are the same, the components that are common to those according to the above-described first embodiment are denoted by same reference numbers, and redundant descriptions are omitted or simplified. 
     As illustrated in  FIG.  17   , in the present embodiment, the two ends of the wire rod constituting the coil spring  134  have a first stretched section  134   e  and a second stretched section  134   f  that are stretched in the axial direction. The first catch  164  of the first sleeve  130  has an axially extending fitting hole  164   a  (another example of the first restrictor) having a circular cross-section, and the second catch  184  of the second sleeve  132  has an axially extending fitting hole  184   a  (another example of the second restrictor) having a circular cross-section. The circumferential position of the first stretched section  134   e  is restricted as a result of fitting and locking the first stretched section  134   e  into the fitting hole  164   a . The circumferential position of the second stretched section  134   f  is restricted as a result of fitting and locking the second stretched section  134   f  into the fitting hole 
     The second embodiment achieves the same effect as that of the first embodiment described above, and gear damage can be appropriately prevented when the fusing unit  46  is installed. 
     In the above-described embodiment, the output gear and the second ratchet member are formed as separate parts, and the second ratchet member is attached to the output gear. However, the output gear and the second ratchet member may be molded into a single unit (one-piece molding) in advance. Similarly, the input gear and the first sleeve can be molded into a single unit in advance. 
     In the above-described embodiment, the first interlocker formed on the first sleeve is a protrusion, and the second interlocker formed on the second sleeve is a groove. Alternatively, the first interlocker may be a groove, and the second interlocker may be a protrusion. The first retainer formed on the first sleeve is an engagement claw, and the second retainer formed on the second sleeve is a claw-receiving hole. Alternatively, the first retainer may be the claw-receiving hole, and the second retainer may be an engagement claw. 
     In the above-described embodiment, a multifunction peripheral combining a copier, a facsimile machine, a printer, etc., exemplifies an image forming apparatus. Alternatively, the image forming apparatus may be a multifunction peripheral any one of the copier, the facsimile machine, the printer, etc., or a combination of at least two of the copier, the facsimile machine, the printer, etc. The image forming apparatus may be a monochrome machine. 
     In each of the above-described embodiments, the rotational force transmission device is applied to a fusing unit drive device. Alternatively, the rotational force transmission device may be applied to other rotational force transmission parts of the image forming apparatus as well as to rotational force transmission parts included in any type of device other than the image forming apparatus. 
     The specific numerical values, materials, and the like described above are mere examples, and can appropriately be changed according to need such as a product specification.