Patent Application
20090208261
This application claims priority to Japanese Patent Application No. 2008-036717, which was filed on Feb. 18, 2008, the contents of which are incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to a fixing device for fixing a toner image formed on a recording sheet, onto the recording sheet, as well as to an image forming apparatus having the fixing device.
2. Description of the Related Art
An electrophotographic image forming apparatus for forming images by means of electrophotography, such as a copying machine and a printer, is provided with a fixing device. As the fixing device, as shown in
In order to convey the recording sheet 202, the fixing device 7 has a post-fixing roller 75 and a post-fixing driven roller element 76 which rotates depending on the rotation of the post-fixing roller 75, that are located posteriorly of the heat roller 71 in a direction P in which the recording sheet 202 is conveyed; that is, located downstream of the heat roller 71 in a conveyance direction P. As the post-fixing roller 75 is rotated, with the recording medium 202 caught in the space between the post-fixing roller 75 and the post-fixing driven roller element 76, these rollers convey the recording sheet 202.
As shown in
Upon the re-engagement between the fixing gear 310 and the paper discharge roller-side gear 320, the recording sheet is returned to a normally conveyed state where the tooth jumping has not occurred as yet. However, as shown in
A technique to prevent jumping of gear teeth, although it is not related to fixing device development, is disclosed in Japanese Unexamined Patent Publication JP-A 9-286529 (1997). JP-A 9-286529 discloses a sheet feeding apparatus. In the sheet feeding apparatus disclosed in JP-A 9-286529, when a paper-feeding roller is put under a load which is greater than a predetermined level, the engagement between the paper-feeding roller and a driving shaft is released so that only the driving shaft is rotated, that is, the driving shaft is idled. This makes it possible to reduce the load applied to the driving gear and thereby prevent occurrence of tooth jumping in the driving gear.
If, in the fixing device, the driving shaft is idled to prevent occurrence of tooth jumping in the fixing gear corresponding to the driving gear as in the technique disclosed in JP-A 9-286529, the conveyance of the recording sheet will be brought to a stop. Therefore, just as in the case where the tooth jumping occurs, the post-fixing driven roller element leaves some impressions on the image surface. The technique disclosed in JP-A 9-286529 is thus not applicable to the fixing device.
An object of the invention is to provide a fixing device capable of preventing conveyance of a recording sheet from being impeded in the presence of variation in drive load on a fixing roller, as well as to provide an image forming apparatus having the fixing device.
The invention provides a fixing device for fixing a toner image formed on a recording sheet, onto the recording sheet, comprising:
a fixing roller adapted to be rotatable about a predetermined axis of rotation;
a pressure roller disposed in pressure-contact with the fixing roller so as to be rotatable depending on a rotation of the fixing roller, the pressure roller holding and conveying a recording sheet on which a toner image is formed in cooperation with the fixing roller in a nip region where the pressure roller and the fixing roller abut against each other;
a fixing gear coupled to the fixing roller, for driving the fixing roller; and
a driving section having a driving gear engageable with the fixing gear, for driving the fixing gear by rotating the driving gear,
wherein the fixing roller and the fixing gear are arranged concentrically and coupled to each other by inserting a key formed in one of the fixing roller and the fixing gear into a keyway formed in the other of the fixing roller and the fixing gear, and
the keyway is designed such that the key is allowed to move along a direction in which the fixing roller is rotated.
According to the invention, the driving gear is rotated by the driving section, which leads to rotation of the fixing gear in meshing engagement with the driving gear, thereby causing the fixing roller coupled to the fixing gear to rotate about the axis of rotation of the fixing roller itself. As the fixing roller is rotatably driven, the pressure roller kept in pressure-contact therewith is rotated depending on the rotation of the fixing roller. In the nip region where the fixing roller and the pressure roller abut against each other, the recording sheet on which a toner image is formed, is held and conveyed between the rollers, whereupon the toner image is fixed onto the recording sheet. The fixing roller and the fixing gear are arranged concentrically and coupled to each other by inserting the key formed in one of the fixing roller and the fixing gear into the keyway formed in the other of the fixing roller and the fixing gear. The keyway is so designed that the key can move along the direction in which the fixing roller is rotated, which enables the fixing roller and the fixing gear to be uncoupled from each other by moving the key along the direction in which the fixing roller is rotated. When drive load on the fixing roller is caused to vary due to the interaction between the nip region and the recording sheet, for example, when the fixing roller receives application of a force other than the driving force transmitted thereto from the driving section via the fixing gear, the fixing roller is accelerated while the key moves relative to the keyway along the direction in which the fixing roller is rotated, thereby uncoupling the fixing roller and the fixing gear. This makes it possible to prevent the application of a force from the fixing roller onto the fixing gear, and it is thereby possible to prevent the tooth jumping between the fixing gear and the driving gear. Accordingly, the conveyance of the recording sheet can be prevented from being impeded, which enables formation of images without quality deterioration. For example, in the case where a post-fixing driven roller element is disposed downstream of the nip region in a conveyance direction of the recording sheet, it is possible to protect the recording sheet from impressions made by the post-fixing driven roller element.
Further, in the invention, it is preferable that in a cross section of the keyway perpendicular to the axis of rotation of the fixing roller, a maximum possible angle α of displacement of the key relative to the keyway is equal to or greater than a nip angle θ1 on the nip region side, out of angles made by two line segments which are formed by connecting the axis of rotation of the fixing roller with one and the other of two intersections of the fixing roller and the recording sheet being in the passage through the nip region.
According to the invention, the keyway is designed such that in the cross section perpendicular to the axis of rotation of the fixing roller, the maximum possible angle α of displacement of the key relative to the keyway is equal to or greater than the nip angle θ1. As a result, if the fixing roller receives any force other than the driving force transmitted thereto from the fixing gear upon passing of the recording sheet through the nip region, the key is allowed to move relatively to the keyway along the direction in which the fixing roller is rotated, by the nip angle θ1 or larger angle displacement around the axis of rotation of the fixing roller, and it is therefore possible to more reliably prevent the fixing roller and the fixing gear from coming into contact with each other at a position to which the key has moved. It is thus possible to more reliably prevent the application of a force from the fixing roller onto the fixing gear and therefore possible to more reliably prevent the occurrence of the tooth jumping between the fixing gear and the driving gear.
Further, in the invention, it is preferable that a keyway portion defining the keyway, and the key are formed so as to each have a one-side contact region which is allowed to contact each other when the fixing roller rotates in a direction relative to the key, and the other-side contact region which is allowed to contact each other when the fixing roller rotates in the other direction relative to the key, and
in the cross section of the keyway perpendicular to the axis of rotation of the fixing roller, a keyway width angle β on the side including the keyway, out of angles made by a line segment which is formed by connecting the one-side contact region of the keyway portion with the axis of rotation of the fixing roller and a line segment which is formed by connecting the other-side contact region of the keyway portion with the axis of rotation of the fixing roller, is equal to or greater than a sum angle (θ1+θ2) consisting of a nip angle θ1 on the nip region side, out of the angles made by the two line segments which are formed by connecting the axis of rotation of the fixing roller with one and the other of the two intersections of the fixing roller and the recording sheet being in the passage through the nip region, plus a key width angle θ2 on the side including the key, out of angles made by a line segment which is formed by connecting the one-side contact region of the key with the axis of rotation of the fixing roller and a line segment which is formed by connecting the other-side contact region of the key with the axis of rotation of the fixing roller.
According to the invention, the keyway is designed so that in the cross section perpendicular to the axis of rotation of the fixing roller, the keyway width angle β is equal to or greater than the sum angle (θ1+θ2) consisting of the nip angle θ1 and the key width angle θ2. As a result, if the fixing roller receives any force other than the driving force transmitted thereto from the fixing gear upon passing of the recording sheet through the nip region, the key is allowed to move relatively to the keyway along the direction in which the fixing roller is rotated, by the nip angle θ1 or larger angle displacement around the axis of rotation of the fixing roller, and it is therefore possible to more reliably prevent the fixing roller and the fixing gear from coming into contact with each other at a position to which the key has moved. It is thus possible to more reliably prevent the application of a force from the fixing roller onto the fixing gear and therefore possible to more reliably prevent the occurrence of the tooth jumping between the fixing gear and the driving gear.
Further, in the invention, it is preferable that the key is formed in an inner peripheral surface of the fixing gear, and
the keyway is formed in an one axial end of the fixing roller.
According to the invention, the key connecting the fixing gear with the fixing roller is formed in the inner peripheral surface of the fixing gear, and the keyway into which the key is inserted, is formed in the one axial end of the fixing roller. This makes it easy to form the key and the keyway that allows the key to move along the direction in which the fixing roller is rotated.
Further, in the invention, it is preferable that the keyway is formed by notching the one axial end of the fixing roller so as to extend along the direction in which the fixing roller is rotated.
According to the invention, the keyway is formed by notching the one axial end of the fixing roller so as to extend along the direction in which the fixing roller is rotated. This makes it easier to form the key and the keyway that allows the key to move along the direction in which the fixing roller is rotated.
Further, in the invention, it is preferable that the driving section has a coupling gear, and
the driving gear is coupled to another roller via the coupling gear.
According to the invention, the driving gear is coupled to another roller via the coupling gear. The driving section is therefore capable of rotating both of the fixing roller and the other roller. Now that the driving section for rotating the other roller is capable of functioning also as a driving section for rotating the fixing roller, the structure of the fixing device can be simpler.
Further, in the invention, it is preferable that the recording sheet is heavy paper.
According to the invention, the heavy paper as the recording sheet is held and conveyed in the nip region where the fixing roller and the pressure roller abut against each other. The keyway into which is inserted the key for coupling the fixing roller and the fixing gear, is so designed that the key is allowed to move along the direction in which the fixing roller is rotated, with the result that if the fixing roller receives any force other than the driving force transmitted thereto from the fixing gear upon passing of the heavy paper through the nip region, the key will move relatively to the keyway along the direction in which the fixing roller is rotated, so that the fixing roller and the fixing gear will be uncoupled from each other. This makes it possible to prevent the application of the force from the fixing roller onto the fixing gear, and it is thereby possible to prevent the tooth jumping between the fixing gear and the driving gear. That is why the fixing device is favorable in the case where the recording sheet is the heavy paper.
Further, in the invention, it is preferable that the recording sheet is gloss paper.
According to the invention, the gloss paper as the recording sheet is held and conveyed in the nip region where the fixing roller and the pressure roller abut against each other. The keyway into which is inserted the key for coupling the fixing roller and the fixing gear, is so designed that the key is allowed to move along the direction in which the fixing roller is rotated, with the result that if the fixing roller receives any force other than the driving force transmitted thereto from the fixing gear upon passing of the gloss paper through the nip region, the key will move relatively to the keyway along the direction in which the fixing roller is rotated, so that the fixing roller and the fixing gear will be uncoupled from each other. This makes it possible to prevent the application of the force from the fixing roller onto the fixing gear, and it is thereby possible to prevent the tooth jumping between the fixing gear and the driving gear. That is why the fixing device is favorable in the case where the recording sheet is the gloss paper.
Further, the invention provides an image forming apparatus comprising:
a toner image forming section for forming a toner image on a recording sheet, and
the fixing device mentioned above, for fixing the formed toner image onto the recording sheet.
According to the invention, the image forming apparatus includes the toner image forming section and the fixing device of the invention, and the toner image forming section forms the toner image on the recording image while the fixing device of the invention fixes the toner image onto the recording sheet. In the fixing device of the invention, as described above, the tooth jumping can be prevented between the paper discharge roller-side gear and the driving gear so that the conveyance of the recording sheet is not impeded, and therefore in the case, for example, where the post-fixing driven roller element is disposed downstream of the nip region in the conveyance direction of the recording sheet, it is possible to protect the recording sheet from impressions made by the post-fixing driven roller element. Accordingly, by constituting an image forming apparatus so as to include the fixing device of the invention, it is possible to provide the image forming apparatus which is capable of forming high-quality images without impressions made by rollers such as the post-fixing driven roller element.
Other and further objects, features, and advantages of the invention will be more explicit from the following detailed description taken with reference to the drawings wherein:
Now referring to the drawings, preferred embodiments of the invention are described below.
With the image forming apparatus 100 placed on a horizontal plane, at the top of the apparatus main body 110 is disposed a document platen 92 made of a light-transmitting glass on which a document is placed. On the top side of the document platen 92 is mounted the automatic document processing device 120. The automatic document processing device 120 conveys a document onto the document platen 92 automatically. Moreover, since the automatic document processing device 120 is so designed as to be rotatable in a direction indicated by an arrow M, it follows that a user is able to place a document with his/her hands by leaving the top of the document platen 92 open.
Image data to be processed in the image forming apparatus 100 of this embodiment corresponds to a color image of black (K) color, cyan (C) color, magenta (M) color, and yellow (Y) color. In order to form latent images of the four colors on an individual basis, the developing device 2, the photoreceptor drum 3, the charging device 5, and the cleaner unit 4 are each correspondingly four in number to deal with black, cyan, magenta, and yellow, respectively. In this way, four units of image stations are made up.
The charging device 5 serves as a charging section for uniformly charging the surface of the photoreceptor drum 3 to a predetermined potential. While, in this embodiment, the charging device 5 is realized by using a charging device of non-contact charger type, instead of a charger-type charging device as shown in
The exposure unit 1, which is an optical scanning device, to be more specific, an image writing device, is built as a laser scanning unit (LSU for short) having a laser emitting section, a reflection mirror, and so forth. In the exposure unit 1 are arranged a polygon mirror for scanning laser beams and optical elements such as a lens and a mirror for directing laser light reflected from the polygon mirror to the photoreceptor drum 3. In another embodiment of the invention, the exposure unit 1 may be built as a writing head comprising light-emitting elements e.g. electroluminescence (EL for short) elements or light-emitting diodes (LED for short) arranged in an array.
The exposure unit 1 has the function of exposing the photoreceptor drum 3 in a charged state to light in accordance with inputted image data to thereby form an electrostatic latent image corresponding to the image data on the surface of the photoreceptor drum 3. The developing device 2 turns the electrostatic latent images formed on their respective photoreceptor drums 3 into visual images by means of toners of four colors (YMCK). Moreover, the cleaner unit 4 removes and collects residual toner remaining on the surface of the photoreceptor drum 3 following the completion of development and image transfer process.
The intermediate transfer belt unit 6 located above the photoreceptor drums 3 comprises an intermediate transfer belt 61, an intermediate transfer belt driving roller 62, an intermediate transfer belt driven roller 63, an intermediate transfer roller 64, and an intermediate transfer belt cleaning unit 65. Four pieces of the intermediate transfer rollers 64 are provided to deal with the colors YMCK on an individual basis.
The intermediate transfer belt driving roller 62, the intermediate transfer belt driven roller 63, and the intermediate transfer rollers 64 allow the intermediate transfer belt 61 to be rotatably driven while being suspended in a tensioned state. Moreover, the intermediate transfer rollers 64 impart a transfer bias to transfer a toner image borne on the photoreceptor drum 3 onto the intermediate transfer belt 61.
The intermediate transfer belt 61 is disposed in contact with each of the photoreceptor drums 3. As for the function of the intermediate transfer belt 61, the toner images of four colors formed on the photoreceptor drums 3 are superimposedly transferred one after another onto the intermediate transfer belt 61, whereupon a color toner image (multi-color toner image) is formed on the intermediate transfer belt 61. For example, the intermediate transfer belt 61 is constructed of a film having a thickness of ca. 100 μm to 150 μm in an endless belt shape.
Transfer of toner images from the photoreceptor drum 3 to the intermediate transfer belt 61 is effected by the intermediate transfer roller 64 kept in contact with the reverse side of the intermediate transfer belt 61. The intermediate transfer roller 64 receives application of a high-voltage transfer bias for toner-image transfer, to be more specific, a high voltage of a polarity reverse to the polarity of charge on the toner. For example, if the polarity of charge on the toner is negative (−), a positive (+) high voltage is applied to the intermediate transfer roller 64. The intermediate transfer roller 64 is, for example, a roller constructed of a metal (e.g. stainless)-made shaft of 8 mm to 10 mm in diameter used as a base, the surface of which is covered with a conductive elastic material (such for example as EPDM and urethane foam). By virtue of the conductive elastic material, a high voltage can be uniformly impressed on the intermediate transfer belt 61. While, in this embodiment, a roller-shaped component is used as the transfer electrode, a brush-shaped component may be used instead.
As described above, the toner images obtained as the result of visualization of the electrostatic latent images on the photoreceptor drums 3 in accordance with the different colors are stacked on top of each other on the intermediate transfer belt 61. The thereby stacked, image data-based toner images are moved, as the intermediate transfer belt 61 is rotated, to a location where the recording sheet and the intermediate transfer belt 61 make contact with each other so as to be transferred onto the recording sheet by a transfer roller 10 disposed at the aforementioned contact location.
At this time, the intermediate transfer belt 61 and the transfer roller 10 are brought into pressure-contact with each other in a nip region having a predetermined dimension. Moreover, the transfer roller 10 receives application of a voltage for transferring the toner onto the recording sheet, to be more specific, a high voltage of a polarity reverse to the polarity of charge on the toner. For example, if the polarity of charge on the toner is negative (−), a positive (+) high voltage is applied to the transfer roller 10. Further, in order to obtain the nip region steadily, in constructing the transfer roller 10 and the intermediate transfer belt driving roller 62, one of them is made of a hard material (metal, etc.) and the other is made of a soft material such as an elastic roller element (elastic rubber roller, resin foam roller, etc.).
Furthermore, as described above, the toner that adhered to the intermediate transfer belt 61 upon contact with the photoreceptor drum 3, or the toner that remains on the intermediate transfer belt 61 due to the transfer roller 10 having not effected transfer on the paper sheet, is causative of mixing of toner colors in the subsequent process steps. Therefore, in this construction, the adherent/residual toner is removed and collected by the intermediate transfer belt cleaning unit 65. The intermediate transfer belt cleaning unit 65 is provided with a cleaning blade as a cleaning member which is brought into contact with the intermediate transfer belt 61. The intermediate transfer belt 61 contacted by the cleaning blade is supported, at its back side, by the intermediate transfer belt driven roller 63.
The paper-feeding cassette 81 is a tray on which recording sheets e.g. pieces of recording paper for use in image formation are piled up. In a state where the image forming apparatus 100 is placed on a horizontal plane, the paper-feeding cassette 81 is disposed below the exposure unit 1 of the apparatus main body 110. Moreover, recording sheets for use in image formation can be placed on the manual paper-feeding cassette 82, too. Further, the paper catch tray 91 disposed on the upper part of the apparatus main body 110 is a tray on which printed recording sheets, namely recording sheets having printed images are accumulated in a face-down manner; that is, the recording sheets are accumulated, with their toner image-bearing surfaces pointing downward.
In addition, in the apparatus main body 110 is provided a sheet conveyance path S in substantially vertical form for allowing the recording sheets placed on the paper-feeding cassette 81 as well as on the manual paper-feeding cassette 82 to be fed via the transfer roller 10 and the fixing unit 7 to the paper catch tray 91. In the vicinity of the sheet conveyance path S ranging from the paper-feeding cassette 81 or the manual paper-feeding cassette 82 to the paper catch tray 91, there are arranged pick-up rollers 11a and 11b, a plurality of conveying rollers 12a through 12d, a registration roller 13, the transfer roller 10, the fixing unit 7, and so forth.
The conveying rollers 12a through 12d are a plurality of compact rollers arranged along the sheet conveyance path S, for facilitating and assisting the conveyance of the recording sheets. Among the conveying rollers 12a through 12d, the paper discharge roller 12b is disposed downstream of the fixing unit 7 in a conveyance direction of the recording sheet and near the paper catch tray 91 and discharges thereto the recording sheet fed from the fixing unit 7. Moreover, the pick-up roller 11a is disposed in the vicinity of the end of the paper-feeding cassette 81, for picking up the recording sheets one by one from the paper-feeding cassette 81 and feeds them to the sheet conveyance path S. Likewise, the pick-up roller 11b is disposed in the vicinity of the end of the manual paper-feeding cassette 82, for picking up the recording sheets one by one from the manual paper-feeding cassette 82 and feeds them to the sheet conveyance path S.
Further, the registration roller 13 temporarily holds the recording sheet in the process of being conveyed along the sheet conveyance path S. The registration roller 13 has the capability of conveying the recording sheet to the transfer roller 10 in a timed relationship such that a leading end of the toner image borne on the photoreceptor drum 3 is aligned with a leading end of the recording sheet.
Thus, in the image forming apparatus 100 are disposed the paper-feeding cassette 81 for storing the recording sheets in advance and the manual paper-feeding cassette 82. In order to supply the recording sheets from these paper-feeding cassettes 81 and 82, the pick-up rollers 11a and 11b are arranged to direct the recording sheets one by one to the sheet conveyance path S.
The recording sheet conveyed from each of the paper-feeding cassettes 81 and 82 is conveyed by the conveying roller 12a on the sheet conveyance path S to the registration roller 13, and is then conveyed to the transfer roller 10 in a timed relationship such that the leading end of the recording sheet is aligned with the leading end of the image-data object borne on the intermediate transfer belt 61, whereupon the image data is committed onto the recording sheet. After that, during the passage of the recording sheet through the fixing unit 7, the yet-to-be fixed toner borne on the recording sheet is molten under application of heat and then fixed to the recording sheet, and eventually the recording sheet is discharged, through the paper discharge roller 12b located posteriorly of the fixing unit 7, onto the paper catch tray 91.
The sheet conveyance path is adapted for a print-job request of single-sided printing on the recording sheet. On the other hand, in order to deal with a print-job request of double-sided printing, after the single-sided printing is completed and the tail end of the recording sheet that has passed through the fixing unit 7 is caught hold of by the last paper discharge roller 12b, the paper discharge roller 12b is rotated in a reverse direction so as to direct the recording sheet to the conveying rollers 12c and 12d. Then, after the recording sheet passes through the registration roller 13 and the back side of the recording sheet is subjected to printing process, the recording sheet is discharged onto the paper catch tray 91.
The fixing unit 7 comprises the heat roller 71 acting as a fixing roller and the pressure roller 72. The heat roller 71 and the pressure roller 72 are rotated, with the recording sheet lying therebetween. Moreover, the heat roller 71 is so designed that the temperature of its outer peripheral surface can be adjusted to a predetermined fixing temperature, based on signals from a temperature detector 74, by a control unit that will hereafter be described. The heat roller 71 has a function of pressing the toner onto the recording sheet under application of heat in cooperation with the pressure roller 72, thereby melting, mixing and pressing the toner images of multiple colors transferred onto the recording sheet and eventually causing it to be thermally fixed onto the recording sheet. In addition, an external heating belt 73 is disposed to heat the heat roller 71 from outside.
Being constructed of elastic rollers, the heat roller 71 and the pressure roller 72 undergo elastic deformation at their surfaces when brought into pressure-contact with each other. The recording sheet 202 is retained in the nip region 203 where the heat roller 71 and the pressure roller 72 abut against each other, and is conveyed as the heat roller 71 and the pressure roller 72 are rotated. The recording sheet 202 is fed in such a manner that its toner image-bearing surface, namely image surface 202a onto which the toner image is transferred by the transfer roller 10 can be brought into contact with the heat roller 71.
In order to convey the recording sheet 202 that has passed through the nip region 203, the fixing unit 7 has a post-fixing roller 75 and a post-fixing driven roller element 76 which moves depending on the rotation of the post-fixing roller 75 that are located downstream of the nip region 203 in a conveyance direction P of the recording sheet 202. The post-fixing roller 75 and the post-fixing driven roller element 76 are rotated with the recording sheet 202 caught in the space between them, whereby the recording sheet 202 can be conveyed. In order to simplify an understanding of the invention, in
With several coupling gears, the fixing gear 220 is coupled to a motor which rotates the paper discharge roller 12b shown in
The fixing gear 220 and the heat roller 71 become coupled to each other by inserting the key 222 formed on the fixing gear 220 into the keyway 221 formed in the one-side rotary shaft 200 of the heat roller 71. The roller main body 201 is formed integrally with the one-side rotary shaft 200 and is thus coupled to the fixing gear 220 via the one-side rotary shaft 200. The driving force transmitted to the fixing gear 220 is then transmitted therethrough to the one-side rotary shaft 200 and the rotary main body 201. Under this driving force, the heat roller 71 is driven to rotate about its axis. The pressure roller 72 shown in
The keyway 221 is designed such that the key 222 is allowed to move along directions Q in which the heat roller 71 is rotated. To be more specific, the keyway 221 is designed such that the key 222 can move relatively to the keyway 221 along the directions Q in which the heat roller 71 is rotated. The keyway 221 is formed by notching the free end of the one-side rotary shaft 200 that corresponds to one axial end of the heat roller 71 so as to extend along the directions Q in which the heat roller 71 is rotated. In other words, the keyway 221 is designed such that the heat roller 71 has a circular arch-shaped section on a virtual plane perpendicular to an axial direction thereof, to be specific, so as to constitute a part of a circular ring. More specifically, the keyway 221 is formed so as to extend along the directions Q in which the heat roller 71 is rotated and along the axis of the heat roller 71.
A keyway portion 221a defining the keyway 221, and the key 222 are formed so as to each have a one-side contact region 240a, 240b which is allowed to contact each other when the heat roller 71 rotates in one direction Q1 relative to the key 222, out of the directions Q in which the heat roller 71 is rotated, and the other-side contact region 241a, 241b which is allowed to contact each other when the heat roller 71 rotates in the other direction Q2 relative to the key 222, out of the directions Q in which the heat roller 71 is rotated. In the present embodiment, an entire surface region of keyway portion 221a in the one direction Q1 relative to the key 222, out of the directions in which the heat roller 71 is rotated, constitutes the one-side contact region 240a of the keyway portion 221a, while an entire surface region of keyway portion 221a in the other direction Q2 relative to the key 222, out of the directions in which the heat roller 71 is rotated, constitutes the other-side contact region 241a of the keyway portion 221a. Moreover, an entire side region of the key 222 in the one direction Q1 of the directions in which the heat roller 71 is rotated, constitutes the one-side contact region 240b of the key 222, while an entire side region of the key 222 in the other direction Q2 of the directions in which the heat roller 71 is rotated, constitutes the other-side contact region 241a of the key 222.
The keyway portion 221a and the one-side contact region 240a, 240b of the key 222 constitute, among the contact surfaces between the keyway portion 221a and the key 222, the contact surface located downstream in the one direction Q1 of the direction of rotation caused by the fixing gear 220, that is, the contact surface located upstream in the other direction Q2 of the directions of backward rotation which is opposite to the direction of rotation caused by the fixing gear 220. Moreover, the keyway portion 221a and the other-side contact region 241a, 241b of the key 222 constitutes, among the contact surfaces between the keyway portion 221a and the key 222, the contact surface located upstream in the one direction Q1 of the direction of rotation, that is, the contact surface located downstream in the other direction Q2 of the directions of rotation.
The key 222 is substantially quadrangular prism-shaped, extending along the axis of the heat roller 71. In more detail, the key 222 is designed such that the heat roller 71 has a circular arch-shaped section on a virtual plane perpendicular to an axial direction thereof, to be specific, such that the key 222, jointly with the keyway portion 221a, constitutes a part of a circular ring. In one side of the direction of axis of the heat roller 71 relative to the fixing gear 220, sealing is provided as a gear stopper although not shown in the drawings. This gear stopper allows the fixing gear 220 to be prevented from falling off the rotary shaft 200 of the heat roller 71. Also at the free end (not shown) of rotary shaft extending in the other direction along the axis of the heat roller 71, a sealing is provided. This sealing allows the rotary shaft to be prevented from falling off its bearing.
Returning to
The fixing gear 220 is coupled, via the first coupling gear 230 and the second coupling gear 231, to the post-fixing roller driving gear 232. The first coupling gear 230 engages with the fixing roller driving gear 220, the second coupling gear 231 engages with the first coupling gear 230, and the post-fixing roller driving gear 232 engages with the second coupling gear 231. The post-fixing roller driving gear 232 is coupled to the post-fixing roller 75 shown in
The post-fixing roller 75 is rotatably driven under the driving force transmitted from the fixing gear 220, through the first coupling gear 230 and the second coupling gear 231, to the post-fixing roller driving gear 232. In this embodiment, since the fixing gear 220 is driven to rotate in a counterclockwise direction, it follows that the post-fixing roller driving gear 232 is driven to rotate in a clockwise direction. Thereby, the post-fixing roller 75 is driven to rotate in a clockwise direction. The post-fixing driven roller element 76, shown in
In a case where the recording sheet 202 is thin, such as so-called thin paper having a basis weight as small as 40 g/m2 or below and a thickness size as small as 0.1 mm or below, for example, as shown in
On the other hand, in a case where the recording sheet 202 is of the type that has a certain thickness, e.g. a thickness size of 0.3 mm or more, such as gloss paper or heavy paper, the directions of power transmission indicated by the arrows B and C in
At this time, in the case where the keyway 303 is so formed as to prevent the key 311 from moving along the direction in which the heat roller 300 is rotated as in the case of the above-described related art shown in
The paper discharge roller-side gear 320, which is rotatably driven by a motor, is designed to effect power transmission in a direction reverse to the direction of the arrow E, namely in the power transmission direction of the arrow B shown in
Upon occurring of the tooth jumping, until such time that the normal recording-sheet conveyance condition returns following the completion of speeding up of the heat roller 300 and re-engagement between the paper discharge roller-side gear 320 and the fixing gear 310, as shown in
Hence, in this embodiment, the keyway 221 is so formed as described above that the key 222 is allowed to move along the direction in which the heat roller 71 is rotated; to be specific, that the maximum possible angle α of displacement of the key relative to the keyway 221 is defined as will hereafter be described, whereby it is possible to prevent occurrence of tooth jumping between the fixing gear 220 and the paper discharge roller-side gear 211, and thereby prevent the conveyance of the recording sheet 202 from being impeded.
In the present embodiment, referring to the relation between the heat roller 71 and the recording sheet 202 at the time that the recording sheet 202 passes through the nip region 203 as shown in
In a cross section perpendicular to an axis of rotation of the heat roller 71, an angle formed by two intersections (hereafter referred to as “sheet roller intersections”) A1 and B1 of the heat roller 71 and the recording sheet 202 at the time that the recording sheet 202 is passing through the nip region 203, and a center of rotation O1 of the heat roller 71; namely an angle ∠A1O1B1 is specified as O1, and an angle made by both ends A2 and B2 of the key 222 of the fixing gear 220 along the direction in which the fixing gear 220 is rotated, and a center of rotation O2 of the fixing gear 220; namely an angle ∠A2O2B2 is specified as O2. Out of the two sheet roller intersections A1 and B1, the sheet roller intersection B1 located upstream in the conveyance direction P of the recoding sheet 202 is hereafter referred to as an upstream-side sheet roller intersection while the sheet roller intersection A1 located downstream in the conveyance direction P of the recoding sheet 202 is hereafter referred to as a downstream-side sheet roller intersection.
The angle ∠A1O1B1 represented by θ1 corresponds to a nip angle that is an angle on the nip region side 203 out of the angles made by two line segments L1 and L2 which connect the two sheet roller intersections A1 and B1 respectively with an axis of rotation of the heat roller 71. Since the fixing gear 220 and the heat roller 71 are arranged concentrically, the center of rotation O2 of the fixing gear 220 coincides with the center of rotation O1 of the heat roller 71. In addition, since the heat roller 71 is rotated by the fixing gear 202, the direction in which the heat roller 71 is rotated coincides with the direction in which the fixing gear 220 is rotated. Hence it follows that the angle ∠A2O2B2 represented by O2 corresponds to a key width angle that is an angle on the side including the key 222 out of the angles made by the line segment L3 which connects the one-side contact region 240b of the key 222 with the axis of rotation of the heat roller 71, and the line segment L4 which connects the other-side contact region 241b of the key 222 with the axis of rotation of the heat roller 71.
The width of the keyway 221 is determined as follows. The elastic energy that has accumulated in the heat roller 71 and the pressure roller 72 starts to be released from when the tail end 202b of the recording sheet 202 comes out of the upstream-side sheet roller intersection B1. This causes the heat roller 71 to be subjected to a force F that is oriented in the direction in which the speed of rotation, i.e. the number of revolution, increases; namely along the one direction Q1 of the directions in which the heat roller 71 is rotated by the fixing gear 220. The heat roller 71 and the pressure roller 72 are each deformed to a larger extent at a position closer to a center, namely a center part N between both ends of the nip region 203 in the conveyance direction P of the recoding sheet 202. The force F, which is being generated on the heat roller 71 and oriented in the direction of increasing the number of revolution, is gradually increased toward and peaked at the center N of the nip region 203; to be specific, the part N which includes a line segment L5 which is formed by connecting the center of rotation O1 of the heat roller 71 and a center of rotation O3 of the pressure roller 72.
It is contemplated that the force F being generated in the one direction Q1 of the directions in which the heat roller 71 is rotated, becomes larger than the force R such as a frictional force acting in the other direction Q2 of the directions of rotation opposite to the one direction Q1 of the directions in which the heat roller 71 is rotated by the fixing gear 220, during the period from the time that the tail end 202b of the recording sheet 202 passes through the upstream-side sheet roller intersection B1 to the time that the tail end 202b of the recording sheet 202 passes through the center N of the nip region 203. Further, it is contemplated that the force F acting in the one direction Q1 of the directions in which the heat roller 71 is rotated by the fixing gear 220, becomes smaller than the force R acting in the other direction Q2 of the directions of rotation opposite to the direction Q1 in which the heat roller 71 is rotated by the fixing gear 220, and the heat roller 71 will then have a negative acceleration, during the period from the time that the tail end 202b of the recording sheet 202 passes through the center N of the nip region 203 to the time that the tail end 202b of the recording sheet 202 passes through the downstream-side sheet roller intersection A1.
For easy understanding, it is regarded herein that the number of revolution of the heat roller 71 exceeds the number of revolution of the fixing gear 220 at the time that the tail end 202b of the recording sheet 202 is passing through the upstream-side sheet roller intersection B1, and the number of revolution of the heat roller 71 is equal to the number of revolution of the fixing gear 220 at the time that the tail end 202b of the recording sheet 202 is passing through the downstream-side sheet roller intersection A1.
The keyway is designed to have such a shape that key 222 does not collide even when the heat roller 71 gains speed, by adjusting an angle indicating the width of the keyway 221; namely a keyway width angle β, that is the angle ∠A3O2B3 formed by connecting the both ends A3 and B3 of the keyway 222 along the directions Q in which the heat roller 71 is rotated, and the center of rotation O2 of the heating roller 71. It means that the keyway width angle β is selected from such a range that the keyway portion 221a does not bump into the key 222 when the heat roller 71 has increased its speed. To be specific, the keyway width angle β is selected at a level equal to or greater than a sum angle (θ1+θ2), which is obtained by adding up the nip angle θ1 and the key width angle θ2.
Although the keyway width angle β includes the key width angle θ2, the width of the keyway 221 can be estimated by using the maximum possible angle α of displacement of the key 222 relative to the keyway 221 because the key width angle θ2 is fixed as long as the dimension of the key 222 is fixed. By forming the keyway 221 such that the maximum possible angle α of displacement of the key 222 is equal to or greater than the nip angle θ1, it is possible to prevent tooth jumping between the fixing gear 220 and the paper discharge roller-side gear 211, and thereby prevent the conveyance of the recording sheet 202 from being impeded.
In the state that the recording sheet 202 has not yet passed through the nip region 203 as shown in
As shown in
Accordingly, given that an outer diameter d0 of the roller main body 201 of each of the heat roller 71 and the pressure roller 72 is 50 mm, the width W of the nip region 203 is 0.5 mm, and the thickness dimension T3 of the recording sheet 202 is 0.3 mm, then the nip angle θ1 is, as a concrete figure, 20.5°, which is determined by the following equation (1):
Returning to
As described above, in the present embodiment, the keyway 221 is designed such that the key 222 is allowed to move along the directions Q in which the heat roller 71 is rotated, and as a result, the heat roller 71 and the fixing gear 220 can be uncoupled when the keyway 221 moves in the one direction Q1 of the directions in which the heat roller 71 is rotated by the fixing gear 220; in other words, when the key 222 moves in the other direction Q2 of the directions of rotation opposite to the direction Q1 of rotation caused by the fixing gear 220. When the drive load on the heat roller 71 is caused to vary due to the interaction between the nip region 203 and the recording sheet 202, for example, as shown in
At this time, the key 222 formed in the fixing gear 220 moves along the directions Q in which the heat roller 71 is rotated, with respect to the keyway 221 formed in the heat roller 71; to be more specific, the key 222 moves relatively to the keyway 221 in the other direction Q2 of the directions in which the heat roller 71 is rotated, resulting in uncoupling of the heat roller 71 and the fixing gear 220. This prevents the application of a force from the heat roller 71 onto the fixing gear 220 and thereby prevents the tooth jumping between the fixing gear 220 and the paper discharge roller-side gear 211. To sum up, in the embodiment, the fixing gear 220 can be made so as not to be accelerated when the heat roller 71 is accelerated, whereby it is possible to prevent the tooth jumping between the fixing gear 220 and the paper discharge roller-side gear 211. Accordingly, the conveyance of the recording sheet 202 can be prevented from being impeded, resulting in formation of images without quality deterioration. For example, as in the case of this embodiment, by disposing the post-fixing driven roller element 76 downstream of the nip region 203 in the conveyance direction P of the recording sheet 202, it is possible to protect the recording sheet 202 from impressions made by the post-fixing driven roller element 76.
Especially in a case where the recording medium 202 is heavy paper or gloss paper, having such a large thickness dimension as 0.3 mm, the elastic energy built up in the heat roller 71 and the pressure roller 72 is large, and so is the force F applied in the one direction Q1 of the directions in which the heat roller 71 is rotated by the fixing gear 220. Even in that case, according to the present embodiment, the fixing gear 220 is prevented from receiving a force from the heat roller 71 so that the tooth jumping does not occur between the fixing gear 220 and the paper discharge roller-side gear 211. It is therefore possible to provide the fixing unit 7 that is preferable in the case of using heavy paper or gloss paper as the recording sheet 202.
Further, in this embodiment, the keyway 221 is formed such that the maximum possible angle α of displacement of the key 222 relative to the keyway 221 is equal to or greater than the nip angle θ1. The key 222 is thus allowed to move relatively to the keyway 221 along the directions Q in which the heat roller 71 is rotated, by the nip angle θ1 or larger angle displacement around the axis of rotation of the heat roller 71, in the case where the heat roller 71 receives application of a force other than the driving force transmitted thereto from the fixing gear 220 when the recording sheet 202 is passing through the nip region 203. This makes it possible to prevent the heat roller 71 and the fixing gear 220 from coming into contact with each other at a position to which the key 222 has moved when the speed-up of the heat roller 71 or any other factor caused the key 222 to move relatively to the keyway 211. It is therefore possible to keep the fixing gear 220 more reliably from any force that may be given from the heat roller 71, which thus enables prevention of tooth jumping with higher reliability between the fixing gear 220 and the paper discharge roller-side gear 211.
In other words, the keyway 221 is designed such that the keyway width angle β is equal to or greater than the sum angle (θ1+θ2) consisting of the nip angle θ1 plus the key width angle θ2. The key 222 is thus allowed to move relatively to the keyway 221 along the directions Q in which the heat roller 71 is rotated, by the nip angle θ1 or larger angle displacement around the axis of rotation of the heat roller 71, in the case where the heat roller 71 receives application of a force other than the driving force transmitted thereto from the fixing gear 220 when the recording sheet 202 is passing through the nip region 203. This makes it possible to prevent the heat roller 71 and the fixing gear 220 from coming into contact with each other at a position to which the key 222 has moved when the speed-up of the heat roller 71 or any other factor caused the key 222 to move relatively to the keyway 211. It is therefore possible to keep the fixing gear 220 more reliably from any force that may be given from the heat roller 71, which thus enables prevention of tooth jumping with higher reliability between the fixing gear 220 and the paper discharge roller-side gear 211.
It is contemplated that the elastic energy is released and the heat roller 71 is accelerated from the time the tail end 202b of the recording sheet 202 reaches the upstream-side sheet roller intersection B1 until the tail end 202b passes by the downstream-side sheet roller intersection A1 as shown in
Assuming that the tail end 202b of the recording sheet 202 moves instantly from the upstream-side sheet roller intersection B1 to the downstream-side sheet roller intersection A1, the movement of the recording sheet 202 may accompany an arc motion of the key 222 by the nip angle θ1, at a maximum, around the axis of rotation of the heat roller 71, resulting in that the key 222 moves relatively to the keyway 221. In fact, it takes the tail end 202b of the recording sheet 202 some time to move from the upstream-side sheet roller intersection B1 to the downstream-side sheet roller intersection A1, which means that the key 222 keeps away from the keyway portion 221a at a constant speed; the key 222 is not brought into contact with the keyway portion 221a, while the recording sheet 202 is moving.
Accordingly, as in the case of the embodiment, by forming the keyway 221 such that the maximum possible angle α of displacement of the key 222 is equal to or greater than the nip angle θ1; in other words, such that the keyway width angle β is equal to or greater than the sum angle (θ1+θ2) consisting of the nip angle θ1 plus the key width angle θ2, it is possible to secure margin space between the key 222 and the keyway 221a, to be more specific, between the key 222 and the keyway 221a on the upstream side in the one direction Q1 of the directions in which the heat roller 71 is rotated by the fixing gear 220. This makes it possible to reliably prevent the heat roller 71 and the fixing gear 220 from coming into contact with each other at a position to which the key 222 has moved when the speed-up of the heat roller 71 caused the key 222 to move relatively to the keyway 221. This means that it is possible to prevent undesirable contact of the keyway portion 221a with the key 222 caused by the acceleration of the heat roller 71; to be more specific, possible to prevent so-called inverse contact that the keyway portion 221a and the key 222 come into contact with each other on the upstream side in the one direction Q1 of the directions in which the heat roller 71 is rotated by the fixing gear 220.
The keyway width angle β is not particularly restricted in terms of its upper limit. Nevertheless, depending on the keyway width angle β, it may not be possible to provide sufficient strength for the member in which the keyway 221 is formed, i.e., the rotary shaft 200 of the heat roller 71 in the present embodiment. It is therefore preferable that a material, a shape, etc. of the member in which the keyway 221 is formed, be taken into account in selecting the keyway width angle β at such a degree as to provide sufficient strength for the member in which the keyway 221 is formed. As the maximum possible angle α of displacement of the key 222 relative to the keyway 221 is determined by using the keyway width angle β and the key width angle θ2, the upper limit of the maximum possible angle α of displacement is selected just as in the case of that of the keyway width angle β.
The keyway width angle β, which is selected to be equal to or greater then the sum angle (θ1+θ2) consisting of the nip angle θ1 plus the key width angle θ2, will change according to the nip angle θ1 where the key width angle θ2 is constant. The degree of the nip angle θ1 changes depending on materials constituting the heat roller 71 and the pressure roller 72, the thickness dimension of the recording sheet 202, or the like factor. The keyway width angle β is therefore determined based on the key width angle θ2 and the nip angle θ1 that is obtained, for example, as follows: at a step in manufacturing the fixing unit 7, a recording sheet 202 having the largest thickness dimension is selected out of the recording sheets 202 which the fixing unit 7 being manufactured is expected to dealt with, and fed to the nip region 203 between the heat roller 71 and the pressure roller 72, and when the selected recording sheet 202 is passing through the nip region 203, the nip angle θ1 is measured.
Furthermore, according to the present embodiment, the key 222 coupling the fixing gear 220 to the heat roller 71 is formed in an inner peripheral surface of the fixing gear 220, and the keyway 221 into which the key 222 is inserted, is formed in the free end of the one-side rotary shaft 200 of the heat roller 71. This makes it easy to form the keyway 221 and the key 222 such that the key 222 is allowed to move along the directions Q in which the heat roller 71 is rotated, as compared to the case where the keyway is formed in the fixing gear and the key is formed in the heat roller.
Further, according to the present embodiment, the keyway 221 is formed by notching the free end of the one-side rotary shaft 200 of the heat roller 71 so as to extend along the directions Q in which the heat roller 71 is rotated. This makes it easier to form the keyway 221 that allows the key 222 to move along the directions Q in which the heat roller 71 is rotated.
Further, according to the present embodiment, the paper discharge roller-side gear 211 coupled to the fixing gear 220 is coupled to another roller, specifically the paper discharge roller 12b, via a coupling gear (not shown), and the driving section 210 is therefore capable of rotating both of the heat roller 71 and the paper discharge roller 12b. The driving section 210 for rotating the paper discharge roller 12b is therefore capable of functioning also as a driving section for rotating the heat roller 71, which enables to simplify the structure of the fixing unit 7.
Furthermore, according to the present embodiment, the image forming apparatus 100 has the excellent fixing unit 7 in which the fixing gear 220 and the paper discharge roller-side gear 211 are prevented from the tooth jumping therebetween as described above so that the conveyance of the recording sheet 202 is not impeded. It is therefore possible to provide the excellent image forming apparatus 100 which is capable of forming high-quality images without impressions made by rollers such as the post-fixing driven roller element 76.
Although the key 222 is formed in the fixing gear 220 and the keyway 221 is formed in the heat roller 71 in the above embodiment, it may also be possible to form a keyway in a fixing gear and to form a key in a heat roller.
As illustrated in
In more detail, the key 251 protrudes externally from the one-side rotary shaft 200A of the heat roller 71A in a radial direction thereof and is formed so as to have a shape of bar which extends along an axis of the heat roller 71A. The keyway 250 is formed by notching the inner peripheral surface of the fixing gear 220A so as to extend in a circumferential direction of the fixing gear 220A along the direction in which the heat roller 71 is rotated. The keyway 250 is formed so as to penetrate the fixing gear 220A in its axial direction from one side to the other side. To be specific, the keyway 250 has an axially uniform shape and is formed so as to have a circular arch-shaped section on a virtual plane perpendicular to an axial direction thereof, to be specific, so as to constitute a part of a circular ring.
While the heat roller 71 and the pressure roller 72 are each an elastic roller in the above embodiment, one of the heat roller 71 and the pressure roller 72 may be a rigid roller in another embodiment of the invention. So long as one of the heat roller 71 and the pressure roller 72 is formed of an elastic roller even if the other is formed of a rigid roller, the pressure-contact of these rollers will cause the surface part of the elastic roller to undergo deformation. In that case, the same effect as achieved in the above embodiment will be produced by forming the keyway so as to allow the key to move along the direction in which the heat roller is rotated; to be specific, so that the maximum possible angle α of displacement of the key becomes the nip angle θ1 or larger, which means that the keyway width angle β becomes equal to or greater than the sum angle (θ1+θ2) consisting of the nip angle θ1 plus the key width angle θ2.
The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning and the range of equivalency of the claims are therefore intended to be embraced therein.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2008-036717 | Feb 2008 | JP | national |
