JP2011099896A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent damage and jam of a recording sheet passing through a fixing nip without making a noise and without spoiling economical efficiency. <P>SOLUTION: A toner image, formed on an intermediate transfer belt which is moved to circulate by a main motor in an image forming part, is transferred to the recording sheet while the recording sheet passes through a transfer nip in a transfer part. In a fixing part, a pressure roller is brought into press-contact with a heating roller to form a fixing nip, and the toner image is fixed while the recording sheet passes through the fixing nip. The heating roller is rotated by motive power transmitted from the main motor through a power transmission mechanism 40. The power transmission mechanism 40 changes rotational speed transmitted to the heating roller 51 to two steps by turning on and off a variable speed motor 46. The variable speed motor 46 is switched to an on-state and an off-state based on a loop amount of the recording sheet conveyed from the transfer nip to the fixing nip. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an image forming apparatus that fixes a toner image transferred onto a recording sheet onto the recording sheet in a fixing unit.

  In an electrophotographic image forming apparatus such as a printer or a copying machine, an image forming unit that forms a toner image corresponding to image data, and a toner image formed by the image forming unit is usually recorded on recording paper, an OHP sheet, or the like. A transfer section for transferring to a sheet and a fixing section for fixing the toner image transferred to the recording sheet at the transfer section to the recording sheet are provided, so that a recording sheet on which the toner image is fixed can be obtained. ing.

  In the image forming unit, a toner image is formed on the outer peripheral surface of the rotating photosensitive drum or the outer peripheral surface of an image carrier such as an intermediate transfer belt that moves around. In an image forming apparatus such as a color printer that forms a color image, an intermediate transfer belt is usually used as an image carrier. A color image is formed on the outer peripheral surface of the intermediate transfer belt, and the intermediate transfer belt is moved around. Thus, the toner image is conveyed to the transfer unit.

  The transfer unit is provided with a transfer roller that is pressed against the outer peripheral surface of the intermediate transfer belt, and a transfer nip is formed by the intermediate transfer belt and the transfer roller being pressed against each other. The recording sheet is conveyed to the transfer nip in synchronization with the conveyance of the toner image by the intermediate transfer belt, and the toner image on the intermediate transfer belt is transferred to the recording sheet while the recording sheet passes through the transfer nip. Transcribed above. The recording sheet to which the toner image is transferred is conveyed to the fixing unit.

  In the fixing unit, for example, a heating roller provided with a heat source such as a heater lamp and a pressure roller pressed against the heating roller are provided, and the heating roller and the pressure roller are pressed against each other. As a result, a fixing nip is formed. The recording sheet transported from the transfer unit to the fixing unit is fixed while the toner image on the recording sheet is heated and pressed against the recording sheet while passing through the fixing nip.

  In recent years, the distance from the transfer unit to the fixing unit has been shortened due to a demand for downsizing of the apparatus. For this reason, the recording sheet conveyed from the transfer unit to the fixing unit is usually transferred at the rear end of the recording sheet. Enter the fixing nip before passing through the nip. Therefore, when the leading end portion of the recording sheet enters the fixing nip, the trailing end portion is sandwiched by the transfer nip. For this reason, if the recording sheet conveyance speed in the fixing nip is higher than the recording sheet conveyance speed in the transfer nip, the recording sheet is pulled in the fixing nip, which may cause transfer deviation, damage to the recording sheet, and the like. There is.

  Conversely, if the recording sheet conveyance speed in the fixing nip is slower than the recording sheet conveyance speed in the transfer nip, the pressing force from the subsequent portion of the recording sheet when the leading edge of the recording sheet enters the fixing nip. Therefore, wrinkles may occur when entering the fixing nip, or sheet jamming (JAM) may occur in the fixing nip.

For this purpose, conventionally, together with driving of the photosensitive drum in the image forming unit, the rotation of the main motor that rotates the intermediate transfer belt is transmitted to the heating roller of the fixing unit via the power transmission mechanism, and recording in the transfer nip is performed. The heating roller is rotated so that the sheet conveyance speed is equal to the recording sheet conveyance speed in the fixing nip.
With such a configuration, it is possible to equalize the recording sheet conveyance speed by the intermediate transfer belt in the transfer nip and the recording sheet conveyance speed by the heating roller in the fixing nip. It is possible to prevent the recording sheet from clogging.

However, the heating roller of the fixing unit has a constant conveyance speed of the recording sheet passing through the fixing nip due to variation in shape at the time of manufacture, deformation due to heating by a heat source such as an internal heater lamp, and temporal deformation. Without change.
In the configuration in which the rotation of the main motor is transmitted to the heating roller and the recording sheet conveyance speed varies due to the heating roller, the rotation speed of the heating roller is adjusted to convey the recording sheet at a predetermined speed. There is a problem that it is not easy to do. In this case, the difference in the conveyance speed of the recording sheet between the fixing nip and the transfer nip cannot be eliminated, and damage to the recording sheet, generation of wrinkles, clogging of the recording sheet in the fixing nip, and the like cannot be prevented.

  In order to solve such a problem, a configuration is proposed in which a fixing motor for rotating the heating roller in the fixing unit is provided separately from the main motor for driving the intermediate transfer belt. In this case, even if the conveyance speed of the recording sheet in the fixing nip changes due to deformation of the heating roller or the like, the rotation speed of the heating roller can be adjusted by controlling the rotation driving speed of the fixing motor. The recording sheet can be set to a predetermined conveyance speed.

  However, since the heating roller of the fixing unit rotates with the pressure roller being in pressure contact, the rotational load is large. Therefore, a large motor having a large driving force is required as a fixing motor that rotationally drives the heating roller. In addition, when the recording sheet conveyance speed changes minutely due to deformation of the heating roller, etc., it is necessary to change the rotation speed of the fixing motor minutely. For this reason, a pulse motor that can adjust the rotation speed with high accuracy. A high-performance motor such as the above is also required. However, since large and high-performance motors are expensive, using such a motor as a fixing motor impairs the economy.

  Usually, many motors are used in the image forming apparatus. In particular, in a tandem type color printer provided with four color imaging cartridges, the number of motors used is very large, such as a motor for driving each color imaging cartridge. In such a printer, if a large and high-performance fixing motor is newly provided, the cost will be further increased, and the economy will be impaired.

  Patent Document 1 discloses a configuration in which the rotation speed of the fixing roller is switched by the variable speed driving mechanism in the image forming apparatus that transmits the rotation of the main motor to the fixing roller (heating roller) via the variable speed driving mechanism. Yes. The variable speed drive mechanism is configured to switch the rotation speed of the fixing roller by an electromagnetic clutch, and the electromagnetic clutch is controlled to be turned on and off based on the loop amount of the recording sheet conveyed to the fixing nip. It has become.

  In other words, when the loop amount of the recording sheet increases, the electromagnetic clutch is turned off, and the rotation speed of the fixing roller by the variable speed drive mechanism is increased. When the loop amount of the recording sheet decreases, the electromagnetic clutch is turned on and enabled. The rotation speed of the fixing roller by the speed change drive mechanism is reduced. In this way, by controlling the rotation speed of the fixing roller so that the loop amount of the recording sheet is within a predetermined range, the recording sheet is prevented from being damaged, wrinkles are generated, and the recording sheet is blocked at the fixing nip. Can do.

  Further, in Patent Document 2, in order to prevent the rubber layer of the fixing roller (heating roller) in the fixing unit from deteriorating over time, a pressure roller that is pressed against the fixing roller is used as the fixing roller. On the other hand, the structure which provides the mechanism which makes a pressure reduction state and a separation state is disclosed. With such a configuration, the pressure roller can be separated from the fixing roller during a standby time other than during image fixing, and the fixing roller can be prevented from aging due to the pressure roller. Accordingly, it is possible to suppress a change in the conveyance speed of the recording sheet passing through the fixing nip due to the deformation of the fixing roller.

JP 2003-263065 A JP 2006-119425 A

  In the configuration disclosed in Patent Document 1, the electromagnetic clutch is normally in an on state for reducing the recording sheet conveyance speed and increasing the loop amount while one recording sheet passes through the fixing nip; The recording sheet can be switched to an off state for increasing the recording sheet conveyance speed and reducing the loop amount over about 6 to 8 times. Since an electromagnetic noise is generated when an electromagnetic clutch is switched on and off, an operational noise (impact noise) is generated by switching the electromagnetic clutch six to eight times while one recording sheet passes through the fixing nip. However, there is a problem in that the user feels uncomfortable.

  For this reason, a configuration has been proposed in which the rotation speed of the variable speed drive mechanism is switched using a solenoid instead of an electromagnetic clutch, and the flapper operating noise of the solenoid is reduced by a cushioning material. However, since the solenoid has a short life compared to the electromagnetic clutch, there is a problem that the variable speed drive mechanism cannot be used stably over a long period of time. As a result, the configuration using such a solenoid can be applied only to an image forming apparatus having a relatively short life.

  With the configuration disclosed in Patent Document 2, since stress applied to the fixing roller during standby can be reduced, it is possible to suppress deterioration over time of the rubber layer and the like of the fixing roller. However, there is a problem that it cannot cope with the case where the conveyance speed of the recording sheet that passes through the fixing nip changes due to the fixing roller being heated. In addition, since a special mechanism is provided for alleviating stress applied to the fixing roller during standby, the apparatus may be increased in size and the economy may be impaired.

  SUMMARY OF THE INVENTION The present invention has been made in view of the above-described problems, and an object of the present invention is to prevent damage to the recording sheet passing through the fixing nip, sheet jamming, and the like without causing noise and impairing economic efficiency. An object of the present invention is to provide an image forming apparatus that can prevent the above problem.

  In order to achieve the above object, an image forming apparatus according to the present invention includes a transfer unit that transfers a toner image to a recording sheet, and a front end side of the recording sheet enters while the recording sheet passes through the transfer unit. A fixing unit that fixes the toner image on the recording sheet; and a power transmission mechanism that transmits a rotational force from an input shaft connected to the main motor to an output shaft connected to the driving shaft of the fixing unit. Detecting the loop amount of the recording sheet that has entered the fixing unit while passing through the transfer unit, and controlling the conveyance speed of the recording sheet in the fixing unit so that the loop amount falls within a predetermined range. The power transmission mechanism includes a speed change gear in the middle of a rotational force transmission path between the input shaft and the output shaft, and the rotation of the speed change gear is used for speed change. Depending on the motor By changing, the rotation speed transmitted to the output shaft is changed to the first rotation speed and the second rotation speed, and the loop amount of the recording sheet is controlled by the control of the speed change motor. Is adjusted.

  In the present invention, the output shaft of the power transmission mechanism that transmits the rotation of the main motor to the fixing unit is used for the first rotation speed and the second rotation by rotating and stopping the transmission gear by controlling the rotation and stop of the transmission motor. Since the recording sheet conveyed from the transfer unit to the fixing unit is adjusted so as to have a loop amount within a predetermined range, an impact sound is generated when switching between rotation and stop of the speed change motor. There is no fear and there is no possibility of giving the user a discomfort.

  Further, the speed change motor is configured to change the output shaft of the power transmission mechanism in a state where the rotation of the main motor is transmitted to the fixing unit by the rotation drive and stop control to the first rotation speed and the second rotation speed. Because of the switching configuration, there is no need to use a high-performance and large motor, and therefore a low-cost motor can be used. Thereby, economic efficiency can be improved.

Preferably, the power transmission mechanism is attached to an input gear to which rotation of the input shaft is transmitted, a plurality of planetary gears each rotating by the rotation of the input gear, and the output shaft. And an output gear that revolves when the planetary gear rotates. The plurality of planetary gears revolve when the transmission gear rotates.
Preferably, the second rotation speed is lower than the first rotation speed.

Preferably, the speed change motor is stopped when the loop amount of the recording sheet is larger than the predetermined range, the output shaft is set to the first rotation speed, and is smaller than the predetermined range. And the output shaft is driven to the second rotation speed.
Preferably, the fixing unit includes a heating roller and a pressure roller pressed against each other, and the heating roller is rotated by the output shaft of the power transmission mechanism.

  Preferably, the speed changing motor is rotated for a predetermined time every predetermined time when the main motor is stopped.

1 is a schematic diagram illustrating a configuration of a color laser printer of a tandem type intermediate transfer system that is an embodiment of an image forming apparatus of the present invention. FIG. 2 is a schematic diagram illustrating a schematic configuration of a sheet conveyance path and its peripheral portion provided from a transfer unit to a fixing unit in the printer of the present embodiment. It is a schematic diagram which shows schematic structure of the drive system of the main motor provided in the printer of this embodiment. FIG. 3 is a perspective view illustrating a configuration of a power transmission mechanism provided in a fixing unit transmission unit in the printer of the present embodiment. It is a longitudinal cross-sectional view of the power transmission mechanism. It is a side view of the power transmission mechanism. It is a perspective view which decomposes | disassembles and shows the power transmission mechanism. It is a block diagram which shows the structure of the control system of the speed-change motor in the printer of this embodiment. It is a flowchart for demonstrating the control which the control part of the control system performs.

<Schematic configuration of image forming apparatus>
FIG. 1 is a schematic diagram showing the configuration of a tandem intermediate transfer type color laser printer (hereinafter simply referred to as “printer”) as an embodiment of the image forming apparatus of the present invention. When the printer receives an instruction to execute a print job, the printer forms a full-color or monochrome image on a recording sheet such as a recording sheet or an OHP sheet by a known electrophotographic method based on the instruction.

  The printer is formed by an image forming unit A capable of forming a full color image with toners of yellow (Y), magenta (M), cyan (C), and black (K), and the image forming unit A. A transfer unit C that transfers a toner image to a recording sheet, a paper feeding unit B that supplies the recording sheet to the transfer unit C, and a fixing unit D that fixes the toner image transferred to the recording sheet by the transfer unit C are provided. ing.

The image forming unit A includes an intermediate transfer belt 21 that is wound around a pair of rollers 23A and 23B in a horizontal state at a substantially central portion of the printer and can be moved around. The intermediate transfer belt 21 is moved in a direction indicated by an arrow X by a main motor 25 schematically shown in FIG.
Below the intermediate transfer belt 21, an image forming unit for sequentially forming toner images on the intermediate transfer belt 21 with yellow (Y), magenta (M), cyan (C), and black (K) toners. (Imaging cartridges) 10Y, 10M, 10C, and 10K are arranged in that order from the upstream side in the circumferential movement direction of the intermediate transfer belt 21.

The image forming unit 10Y that forms a Y-color toner image includes a photosensitive drum 11Y disposed opposite to the intermediate transfer belt 21, a charging device 12Y that charges the surface of the photosensitive drum 11Y, and a charging device 12Y. And a developing unit 13Y that develops the electrostatic latent image formed on the surface of the photosensitive drum 11Y after charging with a Y-color toner.
The electrostatic latent image on the surface of the photoconductor drum 11Y is formed by being exposed and scanned by the laser light emitted from the exposure device 22 after being charged by the charging device 12Y. The Y toner image on the surface of the photosensitive drum 11Y developed by the developing unit 13Y is transferred onto the intermediate transfer belt 21 by the primary transfer roller 14Y.

  Each of the image forming units 10M, 10C, and 10K for the toners of M, C, and K has the same configuration as the image forming unit 10Y for the Y toner except that only the color of the toner used is different. The photosensitive drums 11M, 11C, and 11K, the charging devices 12M, 12C, and 12K, and the developing devices 13M, 13C, and 13K are provided, and the photosensitive drums 11M, 11C, and 11K are provided. An electrostatic latent image is formed by irradiating laser light from the exposure device 22.

The respective toner images formed on the respective photosensitive drums 11M, 11C, and 11K are the same areas as the areas where the Y-color toner images are transferred onto the intermediate transfer belt 21 by the primary transfer rollers 14M, 14C, and 14K. Each is transcribed above.
The color toner image forming units 10Y, 10M, and 10C other than the K color toner image forming unit 10K have color unit motors (not shown) for driving the respective photosensitive drums 11Y, 11M, and 11C. Is provided.

  The toner image transferred onto the intermediate transfer belt 21 is conveyed to the transfer portion C by the circumferential movement of the intermediate transfer belt 21. The transfer section C is provided on the side of the roller 23A located on the downstream side in the toner image transport direction, and has a secondary transfer roller 24 disposed to face the intermediate transfer belt 21 via the sheet transport path 33. is doing. The secondary transfer roller 24 is in pressure contact with the intermediate transfer belt 21, and a transfer nip is formed in the pressure contact portion.

  The sheet feeding unit B that conveys the recording sheet to the transfer unit C includes a recording sheet storage unit 31 provided below the intermediate transfer belt 21. The recording sheet storage unit 31 includes recording paper, an OHP sheet, and the like. Recording sheets are housed. The recording sheets stored in the recording sheet storage unit 31 are fed out one by one by a feeding roller 32 and are conveyed to a sheet conveyance path 33 that extends upward through the transfer nip of the transfer unit C.

  A timing roller pair 35 that conveys the recording sheet conveyed in the sheet conveyance path 33 to the transfer section C at a predetermined timing is provided upstream of the transfer section C in the sheet conveyance path 33 in the recording sheet conveyance direction. ing. The timing roller pair 35 conveys the recording sheet to the transfer nip in synchronization with the timing at which the toner image formed on the intermediate transfer belt 21 is conveyed to a position facing the secondary transfer roller 24.

The toner image formed on the intermediate transfer belt 21 is pressed against the recording sheet passing through the transfer nip, and is collectively transferred onto the recording sheet by applying a bias voltage.
The feeding roller 32, the timing roller pair 35 and the like in the sheet feeding unit B are rotated by the rotation of the main motor 25 being transmitted.

A fixing unit D is provided above the transfer unit C, and the recording sheet on which the toner image is transferred at the transfer nip is conveyed to the fixing unit D through the sheet conveyance path 33.
A recording sheet sensor 38 that detects a recording sheet conveyed to the timing roller pair 35 is provided upstream of the timing roller pair 35 in the sheet conveyance path 33 in the recording sheet conveyance direction. The sheet conveyance path 33 is provided with a loop sensor 60 that detects the loop amount when the recording sheet conveyed from the transfer unit C to the fixing unit D is bent in a loop shape. A specific configuration of the loop sensor 60 will be described later.

  The fixing unit D includes a heating roller 51 and a pressure roller 52 arranged to face each other with the sheet conveyance path 33 interposed therebetween, and the fixing nip is formed when the heating roller 51 and the pressure roller 52 are pressed against each other. Is formed. A heater lamp 53 is disposed at the axial center of the heating roller 51, and the heating roller 51 is heated by the heater lamp 53.

  The heating roller 51 is configured to be rotated by being transmitted while the rotation of the main motor 25 is shifted by a power transmission mechanism 40 (see FIG. 4) described later. The pressure roller 52 is rotated by the heating roller 51. Rotate following the rotation. The toner image on the recording sheet conveyed through the sheet conveying path 33 is heated and pressurized while passing through the fixing nip, and is fixed on the recording sheet.

  The sheet conveying path 33 passes through the fixing nip of the fixing unit D and extends above the fixing unit D, and a pair of discharge rollers 36 is provided at the end of the sheet conveying path 33 located above the fixing unit D. Has been placed. The recording sheet on which the toner image has been fixed in the fixing unit D is discharged onto a paper discharge tray 37 provided at the top of the printer with a pair of paper discharge rollers 36 with the surface on which the toner image is formed facing downward. Is done.

  FIG. 2 is a schematic diagram illustrating a schematic configuration of the sheet conveyance path 33 provided from the transfer unit C to the fixing unit D and its peripheral portion. When the recording sheet that has passed the detection region of the recording sheet sensor 38 passes through the transfer nip of the transfer portion C, it passes through the detection region of the loop sensor 60 and then enters the fixing nip of the fixing portion D. When the leading end portion of the recording sheet enters the fixing nip, the subsequent portion is sandwiched by the transfer nip.

  The recording sheet conveyed while being sandwiched between the fixing nip and the transfer nip is transferred to the fixing nip and the transfer nip when the conveyance speed of the leading end portion in the fixing nip is slower than the conveyance speed of the subsequent portion in the transfer nip. It will be bent in a loop between the nip. The loop sensor 60 is switched between an on state and an off state according to the loop amount of the recording sheet.

The loop sensor 60 includes a sensor arm 61 that is rotatably attached to the side of the sheet conveyance path 33 by a support shaft 63, and a photo interrupter 62 that is switched between an on state and an off state by the rotation of the sensor arm 61. ing.
The sensor arm 61 includes a sheet detection unit 61a that protrudes from the support shaft 63 toward the sheet conveyance path 33, and a light shielding unit 61b that protrudes from the support shaft 63 in the opposite direction to the detection unit 61a.

The photo interrupter 62 has a light emitting part and a light receiving part that face each other, and is turned on when the light receiving part receives light emitted from the light emitting part, and the light shielding part 61b of the sensor arm 61 is turned on by the light emitting part. When the light emitted from the light is blocked and the light receiving unit does not receive light, the light is turned off.
As indicated by a two-dot chain line in FIG. 2, when the recording sheet conveyed through the sheet conveyance path 33 is bent in a loop shape, the sheet detection unit 61 a of the sensor arm 61 is pressed by the bent portion and is supported by the support shaft 63. Rotate around the center. As shown by a one-dot chain line in FIG. 2, the light shielding portion 61 b of the sensor arm 61 is provided when the loop amount (deflection amount) of the recording sheet conveyed to the fixing portion D is equal to or less than a predetermined amount. The rotation of the sheet detection unit 61a is small, and the light emitted from the light emitting unit in the photo interrupter 62 is not blocked. As a result, the photo interrupter 62 is turned on. On the contrary, when the loop amount of the recording sheet becomes larger than the predetermined amount, the rotation amount of the sheet detection unit 61a is increased, and the light shielding unit 61b shields the light emitted from the light emitting unit. As a result, the photo interrupter 62 is turned off.

  FIG. 3 is a schematic diagram showing a schematic configuration of a drive system of the main motor 25 provided in the printer of the present embodiment. The main motor 25 (see FIG. 1) is a driving source for each of the intermediate transfer belt 21, the image forming unit 10K, the paper feeding unit B, and the fixing unit D. As described above, the image forming units 10Y, 10M, and 10C are provided with color unit motors that drive the respective photosensitive drums 11Y, 11M, and 11C.

  The rotation of the output shaft 25a of the main motor 25 is transmitted to the sheet feeding unit B through the sheet feeding unit transmission unit 71, and the image is transmitted through the image forming unit transmission unit 72 and the intermediate transfer belt transmission unit 73 in the image forming unit A. It is transmitted to the forming unit A and transmitted to the fixing unit D via the fixing unit transmission unit 74. Each of the transmission units 71 to 74 is configured by a plurality of gears. Accordingly, when the output shaft 25a of the main motor 25 is rotated at a constant speed, the image forming unit A, the paper feeding unit B, and the fixing unit D are each driven at a predetermined rotational speed.

  FIG. 4 is a perspective view showing the configuration of the power transmission mechanism 40 provided in the fixing unit transmission unit 74. The power transmission mechanism 40 is provided in the fixing unit transmission unit 74, and the rotation speed of the input shaft 56 that is rotated by transmission of rotation from the main motor 25 is set to the first rotation speed and the second rotation speed. It is transmitted to the output shaft 48 while being shifted to the speed. A power output gear 55 is attached to the output shaft 48, and the rotation of the power output gear 55 is transmitted to the heating roller 51 (see FIG. 2) of the fixing unit D to rotate the heating roller 51.

  FIG. 5 is a longitudinal sectional view for explaining the configuration of the power transmission mechanism 40, FIG. 6 is a side view of the power transmission mechanism 40, and FIG. 7 is an exploded perspective view showing the power transmission mechanism 40. As shown in FIGS. 4 to 7, the power transmission mechanism 40 includes a power input gear 54 attached to the input shaft 56, a speed change input gear 41 meshed with the power input gear 54, and the speed change input gear 41. A shift output gear 43 that is in a power transmission state by one planetary gear 45 (see FIG. 6) and a shift gear 42 disposed between the shift input gear 41 and the shift output gear 43 are provided.

The transmission output gear 43 is fixedly attached to the output shaft 48 that passes through the shaft center portion, and therefore the output shaft 48 rotates integrally with the transmission output gear 43. The output shaft 48 passes through the respective shaft centers of the transmission input gear 41 and the transmission gear 42 in a coaxial state, and each of the transmission input gear 41 and the transmission gear 42 is supported by the output shaft 48 so as to be freely rotatable. ing.
The transmission output gear 43 is an internal gear having an inner tooth 43a on the inner peripheral surface, and each of the three planetary gears 45 meshes with the inner tooth 43a. Therefore, as each planetary gear 45 rotates, the transmission output gear 43 rotates in a predetermined direction.

  The transmission gear 42 that is rotatable with respect to the output shaft 48 is a normal spur gear having outer teeth 42b formed on the outer peripheral surface thereof, and is fixed to the output shaft 46a of the transmission motor 46 on the outer teeth 42b. The transmission gear 47 is engaged. The transmission gear 42 is provided with three through holes 42a parallel to the output shaft 48 at equal intervals in the circumferential direction around the output shaft 48, and three planets in each through hole 42a. Each of the gears 45 passes through the respective through holes 42a in a coaxial state.

  Each planetary gear 45 is rotatably supported by a planetary gear support 44 integrally formed with the transmission gear 42. The planetary gear support 44 includes a disc-shaped support plate 44 a disposed between the transmission gear 42 and the transmission output gear 43 and three through holes 42 a provided in the transmission gear 42. And three support shafts 44b that pass coaxially, and the planetary gear 45 is rotatably supported on each support shaft 44b. The support plate 44a is disposed at a predetermined interval with respect to the transmission gear 42 while being coaxially fitted to the output shaft 48, and is fitted within the transmission output gear 43 with a predetermined interval. Has been.

  The speed change input gear 41 arranged coaxially with respect to the output shaft 48 has a cylindrical shape, and has outer teeth 41a on the outer peripheral surface and inner teeth 41b on the inner peripheral surface. A power input gear 54 meshes with the outer teeth 41a, and three planetary gears 45 mesh with the inner teeth 41b. The transmission input gear 41 is rotated by the rotation of the power input gear 54 meshed with the external teeth 41a, and the planetary gears 45 respectively meshed with the internal teeth 41b are rotated by the rotation of the transmission input gear 41. The transmission output gear 43 rotates in the same direction as the transmission input gear 41 by the rotation of each planetary gear 45.

  The number of teeth of the internal teeth 41b of the transmission input gear 41 and the internal teeth 43a of the transmission output gear 43 are different. For example, the number of teeth of the internal gear 41b of the transmission input gear 41 is different from the number of teeth of the transmission output gear 43. 43a is increased by only three. Thus, when the transmission gear 42 is stopped, the rotation speed of the transmission output gear 43 to which the rotation of the transmission input gear 41 is transmitted by each planetary gear 45 is different from the rotation speed of the transmission input gear 41. It becomes the rotation speed.

  When the speed change motor 46 is turned on and is driven to rotate at a constant speed, the transmission gear 42 is rotated at a constant speed, whereby the transmission gear 47 is rotated at a constant speed. It is rotated. As a result, each planetary gear 45 rotates in a direction opposite to the rotation direction with respect to the transmission output gear 43 rotated by each planetary gear 45. As a result, the rotational speed of the transmission output gear 43 becomes a second rotational speed that is lower than when the transmission gear 42 is not rotating.

  As described above, the power transmission mechanism 40 has the first shift state in which the rotation of the power input gear 54 is shifted to the first rotation speed and transmitted to the power output gear 55 by the speed change gear 42 not being rotated, When the gear 42 is rotated, the speed is changed to a second rotational speed that is lower than the first rotational speed and the rotation of the power input gear 54 is transmitted to the power output gear 55. Switch to stage.

  Note that, at the first rotation speed, the recording sheet conveyance speed by the heating roller 51 that is rotated by the rotation of the power output gear 55 being transmitted is the recording sheet conveyance speed at the transfer nip (reference conveyance speed of the image forming apparatus). The recording sheet conveyance speed by the heating roller 51 is lower than the recording sheet conveyance speed in the transfer nip at the second rotation speed.

  FIG. 8 is a block diagram showing the configuration of the control system of the speed change motor 46 in the printer of this embodiment. As shown in FIG. 8, the output of the loop sensor 60 and the output of the recording sheet sensor 38 are given to the control unit 49. The control unit 49 is configured to switch the speed change motor 46 between an on state and an off state based on the output of the loop sensor 60.

  In the printer having such a configuration, the main motor 25 is driven by receiving a print execution instruction, and the image forming unit A, the paper feeding unit B, the transfer unit C, and the fixing unit D are driven. In this state, when a toner image is formed on the intermediate transfer belt 21 by the image forming unit A, the toner image is transferred onto the recording sheet by the secondary transfer roller 24 of the transfer unit C, and the toner image is transferred. The recorded sheet is conveyed to the fixing unit D via the sheet conveyance path 33.

FIG. 9 is a flowchart for explaining a control process executed by the control unit 49 when the recording sheet conveyed to the transfer unit C has the toner image transferred by the transfer unit C and conveyed to the fixing unit. is there. Hereinafter, control executed by the control unit 49 will be described based on this flowchart.
Note that when the leading edge of the recording sheet passes through the transfer nip, the speed-changing motor 46 is in an OFF state, and accordingly, the rotation of the main motor 25 is transmitted to rotate the rotating power input gear 54. The power transmission mechanism 40 transmits the power to the power output gear 55 at the first rotational speed. Accordingly, the heating roller 51 in the fixing unit D is slightly faster than the recording sheet conveyance speed in the transfer nip.

  In this state, when the recording sheet sensor 38 detects the recording sheet conveyed to the timing roller pair 35 and is turned on (see step S11 in FIG. 9, the same applies hereinafter), the control unit 49 determines that the predetermined time has elapsed. It will be in a standby state until it passes (step S12). This standby time is set to a time required for the leading end of the recording sheet to reach a position before the heating roller 51 by a predetermined distance of about several millimeters after passing through the transfer nip.

  When the predetermined time has elapsed, the control unit 49 turns on the speed change motor 46 (step S13). As a result, the transmission gear 47 is rotated, and the transmission gear 42 is rotated in the direction opposite to the rotation direction of the transmission input gear 41. As a result, the transmission output gear 43 is rotated at a second rotation speed that is lower than the first rotation speed. Accordingly, the rotational speed of the power output gear 55 is decreased, and the rotational speed of the heating roller 51 in the fixing unit D is decreased.

  As described above, the recording sheet enters the fixing nip of the fixing portion D in a state where the rotation speed of the heating roller 51 is reduced. In this case, the subsequent portion of the recording sheet is sandwiched in the transfer nip, and the recording sheet conveyance speed in the fixing nip is lower than the recording sheet conveyance speed in the transfer nip. In the sheet conveyance path 33 before the fixing nip, the recording sheet bends in a loop shape. As the recording sheet continues to be conveyed in each of the fixing nip and the transfer nip, the loop amount of the recording sheet gradually increases.

  When the loop amount of the recording sheet increases, the rotation amount of the sensor arm 61 in the loop sensor 60 increases. When the loop amount of the recording sheet exceeds a predetermined amount, the light intercepting portion 61b of the sensor arm 61 puts the light receiving portion of the photo interrupter 62 in a light shielding state, and the photo interrupter 62 (loop sensor 60) is turned off (step). “YES” in S14).

In such a state, the control unit 49 turns off the speed change motor 46 (step S15). When the transmission motor 46 is turned off, the transmission gear 47 stops rotating, so that the transmission gear 42 stops rotating, and the transmission output gear 43 is released from the rotation state at the second rotational speed. The first rotation speed is higher.
Accordingly, the heating roller 51 is rotated at the first rotation speed, and the recording sheet conveyance speed in the fixing nip becomes faster than the recording sheet conveyance speed in the transfer nip. Accordingly, the loop amount of the recording sheet is gradually reduced. Then, it is confirmed whether or not the recording sheet sensor 38 is in an off state (“NO” in step S16). If the recording sheet sensor 38 is in an off state, the recording sheet has passed through the transfer nip. It is confirmed that the determined predetermined time has not elapsed ("NO" in step S17), and such a state is continued until the recording sheet becomes equal to or less than the predetermined loop amount (step S14).

  Thereafter, when the loop sensor 60 is turned on ("NO" in step S14), the speed change motor 46 is turned on (step S13), the speed change output gear 43 becomes the second rotational speed, and the heating roller 51 is turned on. A low speed state corresponding to the second rotation speed is obtained. As a result, the loop amount of the recording sheet gradually increases, and then the photo interrupter 62 (loop sensor 60) is turned off (“YES” in step S14). Thereby, the speed change motor 46 is turned off (step S15), the speed change output gear 43 becomes the first rotation speed, and the rotation speed of the heating roller 51 becomes the high speed state corresponding to the first rotation speed.

  Thereafter, such control is repeated, and the recording sheet passes through the fixing nip of the fixing portion D while repeatedly increasing and decreasing the loop amount. Then, when the recording sheet sensor 38 is turned off (“YES” in step S16) and then a predetermined time has elapsed (“YES” in step S17), the control unit 49 performs on / off control of the transmission motor 46. Ends.

  As described above, since the recording sheet conveyed to the fixing nip of the fixing unit maintains a loop-formed state when passing through the fixing nip, a large tensile force and pressing force are applied to the recording sheet. There is no fear. Therefore, there is no risk of transfer displacement, damage to the recording sheet, no wrinkling of the recording sheet, and no possibility of clogging the recording sheet in the fixing nip.

In the printer of this embodiment, the shift motor 46 is set at predetermined time intervals (for example, every 5 hours) at the time of standby when the printing operation is not executed and the driving of the main motor 25 is stopped. Is rotated for a predetermined time to change the position where the heating roller 51 and the pressure roller 52 are pressed against each other.
That is, in the standby state, the drive of the main motor 25 is stopped, so that the rotation of the power input gear 54 is stopped and the speed change input gear 41 is stopped. Therefore, the heating roller 51 is stopped. ing.

  In this state, when the speed change motor 46 is driven for a predetermined time set in advance, the speed change gear 42 is rotated over a predetermined rotation angle. Due to the rotation of the transmission gear 42, the planetary gear support 44 integrated with the transmission gear 42 rotates, and the three planetary gears 45 supported by the planetary gear support 44 revolve around the output shaft 48. As a result, the internal gear 43a of the transmission output gear 43 with which the planetary gears 45 mesh with each other causes the transmission output gear 43 to rotate as the planetary gear 45 revolves around the output shaft 48.

  When the speed change output gear 43 is thus rotated over a predetermined rotation angle, the output shaft 48 rotates together with the speed change output gear 43, and the power output gear 55 attached to the output shaft 48 is also integrated. Rotate. Thereby, the heating roller 51 is rotated over a predetermined rotation angle. Then, when the heating roller 51 is rotated over a predetermined rotation angle, the pressure roller 52 also follows and rotates.

Thereby, in the standby state, the position where the heating roller 51 and the pressure roller 52 are pressed against each other is changed, so that the same portion of the heating roller 51 is pressed against the pressure roller 52 for a long time. Is prevented. Therefore, it is possible to prevent the heating roller 51 from creeping due to the pressure contact with the pressure roller 51 during standby.
The power transmission mechanism 40 in the present embodiment shifts the rotation speed of the heating roller 51 that is rotated by transmission of the rotation of the main motor in two stages by the rotation and stop of the transmission gear 42 by the transmission motor 46. The speed change motor 46 is not substantially involved in the power transmission of the heating roller 51. Therefore, since the speed change motor 46 does not require power for rotating the heating roller 51, a small motor can be used. Conventionally, such a speed change motor 46 only needs to have a driving force of about 1/5 of the driving force of the fixing motor provided in the fixing unit for directly driving the heating roller.

Further, since the speed change motor 46 can change the rotational speed of the heating roller 51 in two stages by controlling the rotation and stop of the speed change gear 42 by switching between the on state and the off state, the speed change motor 46 is The performance of a pulse motor or the like that can adjust the rotation speed is not necessary, and a low-cost motor such as a DC motor that can rotate at a constant speed can be used.
In addition, the speed change motor 46 does not generate an impact sound that is generated when the electromagnetic clutch is switched during the rotation and stop control, and thus there is no possibility of giving the user unpleasant feeling.

  Furthermore, since the heating roller 51 can be rotated over a predetermined angle by the rotational drive of the speed change motor 46 during standby, the pressure contact position between the heating roller 51 and the pressure roller 52 during standby can be changed. Thereby, creep deformation of the heating roller 51 can be prevented. Therefore, the heating roller 51 can be used stably over a long period of time, and it is not necessary to provide a special mechanism for preventing the heating roller 51 from being deformed by creep.

  In the above-described embodiment, the output shaft 48 is rotated at the first rotational speed while the transmission motor 46 is stopped from rotating, and the transmission motor 46 enters the constant speed rotation state. However, the present invention is not limited to such a configuration. For example, when the speed change motor 46 is rotated at a low speed, the output shaft 48 is rotated at the first speed to change the speed. The output shaft 48 may be rotated at the second rotation speed when the motor 46 is rotated at a high speed.

The image forming apparatus according to the present invention is not limited to a tandem color digital printer, and may be a printer that forms a monochrome image. Furthermore, the present invention can be applied to a copying machine, a FAX, an MFP (Multiple Function Peripheral) that can form a color or monochrome image.
Furthermore, the toner image formed on the intermediate transfer belt in the transfer unit is not limited to be transferred to the recording sheet, but the toner image formed on the image carrier such as a photosensitive drum is transferred to the recording sheet. I just need it. In this case, the photosensitive drum as an image carrier is driven by the power of the main motor being transmitted.

  Furthermore, in the above embodiment, the fixing roller is pressed against the fixing roller to form the fixing nip. However, a fixing belt may be used instead of the fixing roller. Further, not only the pressure roller but also a pressure belt may be used, or a pressure member provided in a fixed manner may be used.

  The present invention relates to an image forming apparatus that fixes a toner image formed on a recording sheet onto a recording sheet by a fixing unit, and the recording sheet passing through the fixing nip is damaged without causing noise. It can be prevented without impairing economic efficiency.

A Image forming unit B Paper feeding unit C Transfer unit D Fixing unit 10Y, 10M, 10C, 10K Image forming unit 12 First slider 13 Second slider 21 Intermediate transfer belt 22 Exposure device 24 Secondary transfer roller 25 Main motor 33 Sheet conveyance Road 38 Recording sheet sensor 40 Power transmission mechanism 41 Transmission input gear 41a External teeth 41b Internal teeth 42 Transmission gear 43 Transmission output gear 43a Internal teeth 45 Planetary gear 46 Transmission motor 47 Transmission gear 48 Rotating shaft 51 Heating roller 52 Pressure roller 53 Heater lamp 54 Power input gear 56 Input shaft 55 Power output gear 60 Loop sensor 61 Sensor arm 62 Photo interrupter 71 Paper feed section transmission section 72 Image forming unit transmission section 73 Intermediate transfer belt transmission section 74 Fixing section transmission section

Claims (6)

A transfer portion for transferring a toner image to a recording sheet;
A fixing unit that is arranged so that the leading end side of the recording sheet enters while the recording sheet passes through the transfer unit, and fixes the toner image on the recording sheet;
A power transmission mechanism that transmits a rotational force from an input shaft coupled to a main motor to an output shaft coupled to a drive shaft of the fixing unit;
Detecting a loop amount of the recording sheet that has entered the fixing unit while passing through the transfer unit, and controlling the conveyance speed of the recording sheet in the fixing unit so that the loop amount falls within a predetermined range. An image forming apparatus configured,
The power transmission mechanism includes a transmission gear in the middle of a rotational force transmission path between the input shaft and the output shaft, and the output shaft is changed by changing the rotation of the transmission gear by a transmission motor. The rotational speed transmitted to the first rotational speed and the second rotational speed are changed,
An image forming apparatus, wherein a loop amount of the recording sheet is adjusted by control of the speed change motor. The power transmission mechanism includes an input gear to which rotation of the input shaft is transmitted,
A plurality of planetary gears each rotating by rotation of the input gear;
An output gear attached to the output shaft and revolving by rotation of the plurality of planetary gears,
The image forming apparatus according to claim 1, wherein the plurality of planetary gears revolve by rotation of the transmission gear.   The image forming apparatus according to claim 2, wherein the second rotation speed is lower than the first rotation speed.   The speed-changing motor is stopped when the loop amount of the recording sheet is larger than the predetermined range, and the output shaft is set to the first rotational speed and is smaller than the predetermined range. 4. The image forming apparatus according to claim 3, wherein the image forming apparatus is driven to set the output shaft to the second rotation speed. 5.   The fixing unit includes a heating roller and a pressure roller that are pressed against each other, and the heating roller is rotated by the output shaft of the power transmission mechanism. The image forming apparatus described in the item.   The image forming apparatus according to claim 5, wherein the speed change motor is rotated for a predetermined time every predetermined time when the main motor is stopped.

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