Patent Application
20080080910
The above aspects, other advantages and further features of the present invention will become more apparent by describing in detail illustrative, non-limiting embodiments thereof with reference to the accompanying drawings, in which:
At the outset, a brief description will be given of a general setup of a laser printer as an example of an image-forming apparatus according to a first embodiment of the present invention. In the drawings, to which reference will be made,
As shown in
The feeder unit 4 includes a sheet feed tray 6 and a sheet pressure plate 7. The sheet feed tray 6 is removably installed in a bottom space provided in the body casing 2. The sheet pressure plate 7 is provided in the sheet feed tray 6. The feeder unit 4 also includes a sheet feed roller 8, a sheet feed pad 9, and a paper powder remover rollers 10, 11. The sheet feed roller 8 and the sheet feed pad 9 are provided above an edge of one side of the sheet feed tray 6. The paper powder remover rollers 10, 11 are provided along a route of conveyance of the sheet 3 downstream relative to the sheet feed roller 8 in a direction of the conveyance of the sheet 3. The feeder unit 4 further includes a resist roller 12 located ‘downstream’ relative to the paper powder remover rollers 10, 11. Hereupon, and in the following description as well, the term ‘downstream’ or ‘upstream’ is and will be used alone to represent a relative position along the route of conveyance of the sheet 3, downstream or upstream with respect to the direction of conveyance of the sheet 3.
The feeder unit 4 constructed as described above is configured to bring one sides of sheets 3 in the sheet feed tray 6 close to the sheet feed roller 8 by means of the sheet pressure plate 7, feed the sheets 3 one after another by means of the sheet feed roller 8 and the sheet feed pad 9 to pass each sheet 3 through rollers 10, 11 and 12 to the image-forming unit 5 on a one-by-one basis.
The image-forming unit 5 includes a scanner unit 16, a process cartridge 17, a fixing device 18, and other components.
The scanner unit 16 is disposed in an upper space provided in the body casing 2. The scanner unit 16 includes a laser beam emitter (not shown), a polygon mirror 19 configured to be driven to spin, lenses 20, 21, reflecting mirrors 22, 23, 24, and other components. A laser beam formed in accordance with image data and emitted from the laser beam emitter is transmitted or reflected by the polygon mirror 19, lens 20, reflecting mirrors 22, 23, lens 21, and reflecting mirror 24 in this sequence as indicated by a chain line, so as to scan a surface of the photoconductor drum 27 in the process cartridge 17 at high speed.
The process cartridge 17 is disposed below the scanner unit 16, and detachably installed in the body casing 2. A hollow housing 51 making up the outer frame of the process cartridge 17 accommodates a developer cartridge 28, a photoconductor drum 27, a scorotron charger 29, a transfer roller 30, and other components.
The developer cartridge 28 is detachably attached to the housing 51, and includes a development roller 31, a doctor blade 32, a supply roller 33 and a toner hopper 34. Toner in the toner hopper 34 is supplied to the development roller 31 by the action of the supply roller 33 rotating in a direction indicated by arrow (counterclockwise), and at the same time becomes positively charged by friction between the supply roller 33 and the development roller 31. The toner supplied onto the development roller 31 goes between the doctor blade 32 and the development roller 31 as the development roller 31 rotates in a direction indicated by arrow (counterclockwise), to form a thin film in a predetermined thickness, so that the film of toner is retained on the development roller 31.
The photoconductor drum 27 is supported by the housing 51 in such a manner that the photoconductor drum 27 is rotatable in a direction indicated by arrow (clockwise). The photoconductor drum 27 has its drum body grounded, while a positively charged photoconductive layer forms a cylindrical surface of the drum body.
The scorotron charger 29 is disposed over the photoconductor drum 27 and opposed to the photoconductive surface of the photoconductor drum 27 with a gap left between the photoconductor drum 27 and the scorotron charger 29 so as to keep the scorotron charger 29 from contact with the photoconductor drum 27. The scorotron charger 29 may be a known charger of scorotron type having a charging wire made of tungsten or the like for generating corona discharge and configured to positively charge the surface of the photoconductor drum 27 uniformly.
The transfer roller 30 is disposed under the photoconductor drum 27 and opposed to the photoconductive surface of the photoconductor drum 27, so as to have contact with the photoconductive surface of the photoconductor drum 27. The transfer roller 30 is supported by the housing 51 in such a manner that the transfer roller 30 is rotatable counterclockwise. The transfer roller 30 has a metal roller shaft covered with a conductive rubber material. In the transfer process, a transfer bias is applied to the transfer roller 30.
In operation, the photoconductive surface of the photoconductor drum 27 is positively charged uniformly by the scorotron charger 29, and then exposed to a rapidly scanning laser beam from the scanner unit 16. This exposure process lowers the potential of an exposed area(s) on the photoconductive surface, thus forming an electrostatic latent image based upon the image data. Hereupon, “electrostatic latent image” is an invisible image produced on the uniformly positively charged surface of the photoconductor drum 27 with the exposed areas made lower in potential by exposure to the laser beam. Next, as the development roller 31 rotates, toner particles carried on the development roller 31 come in contact with the opposed photoconductor drum 27; then the toner particles are supplied onto the surface of the photoconductor drum 27, and transferred to the areas corresponding to the electrostatic latent image formed thereon. The toner particles are retained selectively, i.e., solely in the areas corresponding to the electrostatic latent image, and thus visualize the latent image, to form a toner image. The process described above is called reversal process.
Thereafter, as the photoconductor drum 27 and the transfer roller 30 rotate so that the sheet 3 is held and fed forward between the rollers 27 and 30, the toner image formed on the surface of the photoconductor drum 27 is transferred to the sheet 3 while the sheet 3 is conveyed between the photoconductor drum 27 and the transfer roller 30.
The fixing device 18, which is disposed downstream relative to the process cartridge 17, includes a heating roller 41, a pressure roller 42 configured to be pressed against the heating roller 41, and a pair of conveyor rollers 43 disposed downstream relative to the heating roller 41 and the pressure roller 42. In the fixing device 18 constructed as described above, the toner image transferred onto the sheet 3 is fixed by heating and fusing the toner while the sheet 3 goes between the heating roller 41 and the pressure roller 42. Thereafter, the sheet 3 is conveyed by the conveyor rollers 43 to a sheet output path 44. The sheet 3 forwarded to the sheet output path 44 is then discharged by sheet output rollers 45 onto a sheet output tray 46.
A detailed structure of the fixing device according to exemplary embodiments of the present invention will be described hereafter. In the drawings to which reference will be made,
As shown in
The heating roller 41 is a cylindrical member having a hollow in which a halogen heater HH is installed, so that the heating roller 41 can be heated by the halogen heater HH. The heating roller 41 is rotatably supported at each end thereof by a bearing unit (not shown) fixed to the frame 62. The halogen heater HH has two ends fixed to the frame 62. The heating roller 41 may be of 25 mm in diameter as shown in
The pressure roller 42 is a cylindrical member having a rotation shaft 42a of which two end portions projecting outwardly along its rotation axis from both ends of the pressure roller 42, respectively, are supported by bearing units (not shown) fixed to the pressure arm 63; thus the pressure roller 42 is rotatable on its rotation axis (rotation shaft 42a) relative to the pressure arm 63. The driving force input element 66 is fixed to a tip end of one of the end portions of the rotation shaft 42a passing through and projecting from the pressure arm 63.
The pressure roller 42 may be of 25 mm in diameter, for example, as shown in
A diameter Dp of the pressure roller 42 and a nip width (contact width) N between the heating roller 41 and the pressure roller 42 in circumferential directions thereof, as shown in
0.24<N/Dp<0.6 (1)
where the nip width N is a length of a curve formed on a plane perpendicular to the rotation axis of the pressure roller 42 along the surface of a recessed portion of the pressure roller 42 which is produced when the pressure roller 42 is pressed against the heating roller 41 (or a length of an arc formed on a plane perpendicular to the rotation axis of the heating roller 41 along a surface in contact with the recessed portion of the pressure roller 42).
The nip width N may be measured, for example, by a method of utilizing a sheet of paper one side of which is solidly painted in black all over its surface (such a sheet will hereinafter referred to as “black sheet”), as follows. Specifically, in this method, a black sheet is held between the heating roller 41 and the pressure roller 42 for a predetermined period of time, and then removed therefrom for observation. An area (strip) of the black sheet held by the two rollers 41, 42 (extending in a direction perpendicular to the direction of conveyance) should become glossier than the other areas. The width of the glossier strip (length from end to end in the direction of conveyance of the sheet) is measured by vernier calipers, or the like, to thereby measure the nip width N.
The diameter Dp of the pressure roller 42 and a diameter Dh of the heating roller 41 are configured to have a relationship represented by the following expression (2):
Dp/2<Dh<2Dp (2)
The pressure arm 63 is comprised of a pair of oblong members blanked out from sheet metal, which are disposed symmetrically at the opposite ends of the pressure roller 42, each extending in the direction perpendicular to the rotation axis of the pressure roller 42, as shown in
One end of the extension spring S is attached to the pressure arm 63, and the other end of the extension spring S is attached to the frame 62. The positions of attachment of the extension spring S in the pressure arm 63 and the frame 62 are appropriately determined so that the extension spring S biases the pressure arm 63 toward the heating roller 41, and the pressure roller 42 attached to the pressure arm 63 thus biased is also biased and pressed against the heating roller 41.
The input gear 64 is, as shown in
The intermediate gear 65 is in mesh with the input gear 64 and the driving force input element 66, and rotatably supported by the support shaft 63a provided in the pressure arm 63. Accordingly, the intermediate gear 65 is swingable around the support shaft 62a according as the pressure arm 63 swings, and is moved around the input gear 64.
The driving force input element 66 is located at an end of the pressure roller 42 and fixed coaxially to the pressure roller 42. Thus, the driving force input element 66 is rotatable together with the pressure roller 42 on an axis coincident with the second axis of the pressure roller 42 (rotation shaft 42a) on which the pressure roller 42 is rotatable. Therefore, the driving force input element 66 is swingable around the support shaft 62a according as the pressure arm 63 swings. The driving force input element 66 is connected with the input gear 64 by the intermediate gear 65, and thus a driving force from the output gear 67 in the body casing 2 is transmitted through the input gear 64 and the intermediate gear 65 to the driving force input element 66.
The next discussion will focus on the relative positions of the gears 67, 64-66 during transmission of the driving force from the output gear 67 to the driving force input element 66, to explain how the swinging motion of the pressure arm 63 affects the meshing conditions between the adjacent gears, with reference to
As shown in
It is to be noted that the pressure roller 42 is formed with soft rubber provided at its outermost layer and thus with precision in its external-diameter dimension (i.e., its roundness in cross section) lower than that of the heating roller 41. Therefore, in the normal operation as mentioned above, the pressure arm 63 may shift (swing) downwardly as shown in
Furthermore, the relative positions of the input gear 64, intermediate gear 65 and driving force input element 66 would not change relative to the pressure arm 63, and thus the input gear 64, intermediate gear 65 and driving force input element 66 would never come out of mesh. Consequently, even when the pressure arm 63 swings downwardly, the driving force from the output gear 67 is transmitted through the input gear 64, intermediate gear 65 and driving force input element 66, to the pressure roller 42 without fail.
Assuming that the pressure roller 42 were rotatably supported by the frame 62 so that the rotation axis of the pressure roller 42 is fixed relative to the frame 62 (i.e., relative to the rotation axis of the heating roller 41), the possible variations in external diameter of the pressure roller 42 due to its low dimensional precision would not be able to be accommodated, and cause the contact area (nip width) between the heating roller 41 and the pressure roller 42 to vary. Resultantly, a feeding force applied to the sheet between the heating roller 41 and the pressure roller 42 would vary, which would possibly form wrinkles in the sheet.
In contrast, according to the present embodiment, the pressure arm 63 swingable up and down is provided, so that the pressure roller 42 is moved toward and away from the heating roller 41, and thus the variations of the feeding force as mentioned above can be suppressed. As a result, wrinkles which would be formed in the sheet can be prevented.
With this embodiment, the following advantageous effects can be exerted.
Since N/Dp is greater than 0.24, the nip width N can be made so great as could be achieved to the limit placed by the size (diameter) of the pressure roller 42. Accordingly, the time and area of contact between the sheet 3 and the heating roller 41 when the sheet 3 passes between the two rollers 41 and 42 can be increased, so that the heating roller 41 can be rendered operable at lower temperatures and/or at higher speeds. Moreover, since N/Dp<0.6, the excessive increase in the nip width which would increase the driving torque of the pressure roller 42 can be suppressed, and thus the overload on the fixing device can be avoided. This feature of N/Dp<0.6 in this embodiment also serve to suppress excessive load on the sheet 3 passing between the pressure roller 42 and the heating roller 41.
Incidentally, the nip width N which satisfies the expression (1), if applied to a fixing device of ‘heating roller driving’ type (in which rotation of a heating roller causes a pressure roller to rotate), would cause the heating roller to slip on the sheet being conveyed by the pressure roller. Therefore, the nip width N which satisfies the expression (1) may be deemed effective particularly in the ‘pressure roller driving’ arrangement as in the present embodiment. In other words, the present embodiment can realize a fixing device with a heating roller operable at lower temperatures and/or at higher speeds which would be impossible due to a likelihood of a slip in the ‘heating roller driving’ scheme.
Since the diameter Dp of the pressure roller 42 and the diameter Dh of the heating roller 41 have a relationship represented by the expression (2): Dp/2<Dh<2Dp, the nip width N between the rollers 41 and 42 can be made as great as possible. Moreover, the dimensions of the rollers 41 and 42 are well balanced, and thus the space around the rollers 41, 42 can be utilized effectively.
Since the heating roller 41 is configured not to reciprocate, the entry point of the sheet 3 can be determined in a fixed position, and thus the image quality can be improved. Since the heating roller 41 is configured not to reciprocate, the load which would otherwise be imposed on the halogen heater HH can be suppressed, and thus damage to electric system (electric terminal HI) fixed to the frame of the halogen heater HH can be prevented.
Regardless of the swinging motion of the pressure arm 63, the relative positions of the output gear 67 and the input gear 64 (distance between the axes thereof), and the relative positions of the input gear 64, the intermediate gear 65 and the driving force input element 66 (distances between the axes of adjacent gears) can be maintained constant at all times. Therefore, the gears 67, 64-66 are kept in constant mesh, and thus good transmission of the driving force to the pressure roller 42 can be ensured.
The present invention is not limited to the first embodiment described above, but can rather be implemented in various alternative forms, as will be demonstrated below.
Although only one intermediate gear 65 is provided in the first embodiment, the number of the intermediate gears applicable to the present invention is not limited to one; rather, more than one intermediate gear may be provided as the case may be. In an alternative embodiment, the input gear 64 and the driving force input element 66 may be adapted to mesh together directly.
In the first embodiment, an extension spring S configured to bias the pressure arm 63 toward the heating roller 41 so that the pressure roller 42 is biased and pressed against the heating roller 41 is adopted as a means for biasing the pressure roller 42 toward the heating roller 41, but the present invention is not limited thereto; for example, a compression spring or a torsion spring may be used, instead, or in combination.
Next, a second embodiment of the present invention will be described in detail with reference made to the drawings where appropriate. This embodiment has some commonalities with the first embodiment, and can be considered to provide a modification of the fixing device 18 of the first embodiment as described above. Therefore, the same components as in the first embodiment will be designated by the same reference numerals, and a duplicate description thereof will be omitted. In the drawings to which reference will be made,
Besides components equivalent to those provided in the first embodiment, such as pressure arm 63, driving force input element 66, etc., a fixing device 18′ according to the second embodiment includes, as shown in
The input gear 68 is rotatably supported by a support shaft 62b provided in a position different from an axis (first axis) on which the pressure arm 63 is swingable. The input gear 68 is, as shown in
The swingable arm 69 is comprised of a pair of oblong members, which are disposed symmetrically at the opposite ends of the pressure roller 42, each extending in the direction perpendicular to the rotation axis of the pressure roller 42. Each of the oblong members of the swingable arm 69 has a first point located near one end thereof, and a second point located near the other end thereof. The two oblong members of the swingable arm 69 are fixed relative to each other by the support shaft 62b mentioned above which supports the firsts points of the oblong members, and by a support shaft 69a which supports the second points of the oblong members. The swingable arm 69 is swingably supported at the first points by the support shaft 62b provided in the frame 62 in a manner that permits the swingable arm 69 to swing on its rotation axis (coincident with the support shaft 62b; thus coaxial with the rotation axis of the input gear 68) in its entirety. An extension spring S′ is attached to an appropriate position between first and second points of each oblong member of the swingable arm 69. While one end of the extension spring S′ is attached to the swingable arm 69, the other end thereof is attached to the frame 62.
The planet gear 70 is rotatably supported by the support shaft 69a of the swingable arm 69. The planet gear 70 is configured to mesh with the input gear 68 once the illustrated components (at the least, input gear 68, swingable arm 69, support shafts 62b, 69a, and planet gear 70) are assembled together into the fixing device 18′. The extension spring S′ is configured to bias the second point of the swingable arm 69 toward the pressure roller 42, to thereby cause the planet gear 70 provided at the second point of the swingable arm 69 to be pressed against the driving force input element 66, so that the planet gear 70 is brought into mesh with the driving force input element 66. Furthermore, the pressing force exerted by the planet gear 70 on the driving force input element 66 causes the pressure arm 63 to swing and causes the pressure roller 42 to be pressed against the heating roller 41.
The next topic brought up for discussion with reference to
As shown in
It is to be noted that the pressure roller 42 is formed with soft rubber provided at its outermost layer and thus with precision in its external-diameter dimension (i.e., its roundness in cross section) lower than that of the heating roller 41. Therefore, in the normal operation as mentioned above, the pressure arm 63 may shift (swing) downwardly as shown in
Furthermore, when the pressure arm 63 swings downwardly and causes the driving force input element 66 to move downward, the planet gear 70 thus pressed downward is moved around the input gear 68 while keeping in mesh with the input gear 68 and the driving force input element 66. Accordingly, distances between axes of adjacent ones of gears 67, 68, 70 and 66 are kept constant, and thus the adjacent ones of gears 67, 68, 70 and 66 would never come out of mesh. Consequently, even when the pressure arm 63 swings downwardly, the driving force from the output gear 67 is transmitted through the input gear 68, planet gear 70 and driving force input element 66, to the pressure roller 42 without fail.
Assuming that the pressure roller 42 were rotatably supported by the frame 62 so that the rotation axis of the pressure roller 42 is fixed relative to the frame 62 (i.e., relative to the rotation axis of the heating roller 41), the possible variations in external diameter of the pressure roller 42 due to its low dimensional precision would not be able to be accommodated, and cause the contact area (nip width) between the heating roller 41 and the pressure roller 42 to vary. Resultantly, a feeding force applied to the sheet between the heating roller 41 and the pressure roller 42 would vary, which would possibly form wrinkles in the sheet.
In contrast, according to the present embodiment, the pressure arm 63 swingable up and down is provided, so that the pressure roller 42 is moved toward and away from the heating roller 41, and thus the variations of the feeding force as mentioned above can be suppressed. As a result, wrinkles which would be formed in the sheet can be prevented.
With the second embodiment, the following advantageous effects can be exerted.
Regardless of the swinging motion of the pressure arm 63, the distances between axes of adjacent ones of gears 67, 68, 70 and 66 can be maintained constant at all times. Therefore, the gears 67, 68, 70 and 66 are kept in constant mesh, and thus good transmission of the driving force to the pressure roller 42 can be ensured.
The present invention is not limited to the second embodiment described above, but can rather be implemented in various alternative forms, as will be demonstrated below.
Although the second embodiment adopts the extension spring S′ as a gear biasing mechanism by way of example, the present invention is not limited thereto; for example, a compression spring or a torsion spring may be used, instead or in combination.
Moreover, in the second embodiment, the planet gear 70 is directly in mesh (connected) with the driving force input element 66, but alternatively the planet gear 70 may be connected indirectly, i.e., through one or more of gears rotatably supported by the pressure arm 63, with the driving force input element 66.
Moreover, in the second embodiment the planet gear 70 is directly in mesh (connected) with the input gear 68, but alternatively the planet gear 70 may be connected indirectly, i.e., through one or more gears rotatably supported by the swingable arm 69, with the input gear 68.
Next, a third embodiment of the present invention will be described in detail with reference made to the drawings where appropriate. In the drawings, to which reference will be made,
As shown in
The heating roller 41 is a cylindrical member having a hollow in which a halogen heater HH is installed, so that the heating roller 41 can be heated by the halogen heater HH. The heating roller 41 is rotatably supported at each end thereof by the heating roller bearing unit 80. The heating roller 41 may be of 25 mm in diameter as shown in
The heating roller bearing unit 80 is a cylindrical member made of plastic, and on its outer cylindrical surface is formed a channel 80a having a predetermined width in which the leaf spring BS can be fitted. The heating roller bearing unit 80 is pressed by the leaf spring BS toward the pressure roller 42, and is held between the leaf spring BS and the pressure roller 42. The heating roller bearing unit 80 may be of a conductive or nonconductive (insulating) plastic, and may include ball bearings. The heating roller bearing unit 80 is provided on an annular zone (extending in an axially inside position) at each end of the outer cylindrical surface of the heating roller 41.
The leaf spring BS in this embodiment is made of a single oblong plate bent at two sections toward the same side at the same obtuse angle, symmetrically with respect to a plane perpendicular to the lengthwise direction of the plate. To be more specific, the leaf spring BS includes a base wall portion B1 and a pair of arm portions B2 extending at an angle from both ends of the base wall portion B1 such that the arm portions B2 gradually becomes farther apart from each other toward the ends. The base wall portion B1 of the leaf spring BS is disposed in a spring support portion 82a of the frame 82, which will be described later, and the arm portions B2 extend each along a corresponding tangent to the cylindrical surface of the heating roller 41. The leaf spring BS configured to hold the heating roller bearing unit 80 between the arm portions B2. Specifically, each arm portion B2 of the leaf spring BS supports the bottom of the channel 80a of the heating roller bearing unit 80.
The pair of arm portions B2 is configured to be slightly unfolded (but still not unfolded to its maximum) when the leaf spring BS is installed together with the heating roller 41 and other components in the frame 82 (as shown in
The halogen heater HH is located within the heating roller 41 and fixed at both ends thereof to the frame 8. To be more specific, an electric terminal Hi which projects outwards from each end of the halogen heater HH is welded to a metal sheet M1, and the metal sheet M1 is in turn fixed, by a screw SC made of metal, to a housing-side metal sheet M2 which is formed integrally with a frame 82 made of plastic, so that the halogen heater HH is fixed relative to the frame 82. Accordingly, even when the heating roller 41 is shifted as shown in
The pressure roller 42 includes a body 421 having a cylindrical shape, and a rotation shaft 422 of which a middle portion is disposed inside and coaxially with the cylindrical body 421 and two end portions protrude from two ends of the cylindrical body 421, respectively. Each end portion of the rotation shaft 422 includes a middle-diameter portion 423 having a smaller diameter than the middle portion of the rotation shaft 422, and a small-diameter portion 424 having a smaller diameter than the middle-diameter portion 423, which are arranged in such a manner that the rotation shaft 422 has its diameters reduced stepwise toward each end thereof. The middle-diameter portion 423 is rotatably supported by the pressure roller bearing unit 81. The driving force input element G is fixed to the small-diameter portion 424. Accordingly, upon transmission of a driving force from a driving device (not shown) to the driving force input element G, the pressure roller 42 is caused to rotate.
The rotation shaft 422 is designed to have a lengthwise dimension such that each of the end portions thereof, i.e., middle-diameter portion 423 and small-diameter portion 424, is in an axially outside position of a cylindrical body of the heating roller 41. Therefore, a bearing support portion 82b of the frame 82 for supporting the pressure roller bearing unit 81 can be designed in appropriate dimensions and provided in an appropriate position, and the driving force input element G can be designed in greater dimensions.
The pressure roller 42 may be of 25 mm in diameter, for example, as shown in
A diameter Dp of the pressure roller 42 and a nip width (contact width) N between the heating roller 41 and the pressure roller 42 in circumferential directions thereof are, as in the first embodiment (see
0.24<N/Dp<0.6 (1)
where the nip width N is a length of a curve formed on a plane perpendicular to the rotation axis of the pressure roller 42 along the surface of a recessed portion of the pressure roller 42 which is produced when the pressure roller 42 is pressed against the heating roller 41 (or a length of an arc formed on a plane perpendicular to the rotation axis of the heating roller 41 along a surface in contact with the recessed portion of the pressure roller 42).
The nip width N may be measured, for example, by a method of utilizing a sheet of paper one side of which is solidly painted in black all over its surface (such a sheet will hereinafter referred to as “black sheet”), as follows. Specifically, in this method, a black sheet is held between the heating roller 41 and the pressure roller 42 for a predetermined period of time, and then removed therefrom for observation. An area (strip) of the black sheet held by the two rollers 41, 42 should become glossier than the other areas. The width of the glossier strip is measured by vernier calipers, or the like, to thereby measure the nip width N.
The diameter Dp of the pressure roller 42 and a diameter Dh of the heating roller 41 are configured to have a relationship represented by the following expression (2):
Dp/2<Dh<2Dp (2)
The frame 82 is formed in a container-like shape having an opening which opens toward downward, and mainly contains the heating roller 41 and the pressure roller 42. The spring support portion 82a described above is provided in pair on an upper wall 82c of the frame 82, such that each spring support portion 82a projects downward. The spring support portions 82a are located a predetermined distance (corresponding to the width of the base wall portion B1) apart from each other in the axial directions of the heating roller 41 and opposite each other. With this structure, the base wall portion B1 of the leaf spring BS is held from the axial directions of the heating roller 41 by the spring support portions 82a so that the movement of the leaf spring BS in the axial directions is restricted.
The width of the leaf spring BS is designed to be substantially equal to the width of a channel 80a of the heating roller bearing unit 80, and thus the movement of the heating roller bearing unit 80 in the axial directions is restricted by the leaf spring BS. Since the heating roller 41 and the heating roller bearing unit 80 are normally engaged with each other with a relative movement in the axial directions restricted, the restriction placed on the movement of the heating roller bearing unit 80 by the leaf spring BS results in restriction on the movement of the heating roller 41.
With the third embodiment, the following advantageous effects can be exerted.
Since N/Dp is greater than 0.24, the nip width N can be made so great as could be achieved to the limit placed by the size (diameter) of the pressure roller 42. Accordingly, the time and area of contact between the sheet 3 and the heating roller 41 when the sheet 3 passes between the two rollers 41 and 42 can be increased, so that the heating roller 41 can be rendered operable at lower temperatures and/or at higher speeds. Moreover, since N/Dp<0.6, the excessive increase in the nip width which would increase the driving torque of the pressure roller 42 can be suppressed, and thus the overload on the fixing device can be avoided. This feature of N/Dp<0.6 in this embodiment also serve to suppress excessive load on the sheet 3 passing between the pressure roller 42 and the heating roller 41.
Incidentally, the nip width N which satisfies the expression (1), if applied to a fixing device of ‘heating roller driving’ type, would cause the heating roller to slip on the sheet being conveyed by the pressure roller. Therefore, the present embodiment can realize a fixing device with a heating roller operable at lower temperatures and/or at higher speeds which would be impossible due to a likelihood of a slip in the ‘heating roller driving’ scheme.
Furthermore, the feature of N/Dp being less than 0.6 serves to prevent excessive increase in the nip width, thus reducing the torque applied to the pressure roller 42 when the pressure roller is caused to rotate, to thereby improve the durability of the fixing device 18″. Prevention of excessive increase in the nip width also serves to reduce the load placed on the sheet 3 passing through the heating roller 41 and the pressure roller 42.
Since the diameter Dp of the pressure roller 42 and the diameter Dh of the heating roller 41 have a relationship represented by the expression (2): Dp/2<Dh<2Dp, the nip width N between the rollers 41 and 42 can be made as great as possible. Moreover, the dimensions of the rollers 41 and 42 are well balanced, and thus the space around the rollers 41, 42 can be utilized effectively.
Since the halogen heater HH is fixed relative to the frame 82 while the heating roller 41 is configured to reciprocate independently of the halogen heater HH, damage to the electric system of the halogen heater HH (e.g., construction around electric terminal Hi) can be prevented.
The heating roller 41 allowed to reciprocate mainly by the leaf spring BS alone can be achieved in a smaller number of parts, and thus at a lower cost in comparison, for example, with a configuration in which a pressure roller is pressed against a heating roller by a spring-biased arm.
The pair of arm portions B2 of the leaf spring BS configured to hold the heating roller bearing unit 80 serves to restrict the movement of the heating roller 41 in the direction of conveyance of the sheet 3.
Since the driving force input element G is disposed in an axially outside position of the cylindrical body of the heating roller 41, the driving force input element G may be rendered greater in diameter, so that the torque applied to the driving force input element G can be reduced.
The present invention is not limited to the third embodiment described above, but can rather be implemented in various alternative forms, as will be demonstrated below.
Although the third embodiment adopts the leaf spring BS as a pressing device with an elastic member by way of example, the present invention is not limited thereto; for example, an arm member configured to support a heating roller in a manner that permits the heating roller to rotate and rotatably supported by a frame, in combination with a spring member configured to bias the arm member to press the heating roller against the pressure roller, may be used, instead. Moreover, the elastic member for use in the pressing device may, for example, be a leaf spring curved in an arcuate shape, a coil spring, a torsion spring, etc., instead of the bent leaf spring BS.
It is contemplated that various modifications and changes may be made to the exemplary embodiments of the invention without departing from the spirit and scope of the embodiments of the present invention as defined in the following claims.
In the exemplary embodiments described above, the present invention is applied to a laser printer 1, but the present invention is not limited thereto, but may be applied, for example, to a copier, an all-in-one printer, and other image-forming apparatuses.
In the exemplary embodiments described above, the transfer roller 30 is employed as an example of a transfer unit configured to come in contact with a photoconductor drum, but the present invention is not limited thereto; for example, a non-contact type transfer unit may be employed, instead, which is configured to transfer a developed image of toner from the photoconductor drum to a sheet.
The sheet 3 of paper, such as thick paper (a cardboard), a postcard, thin paper (a flimsy), etc. is used as an example of a recording sheet in the above-exemplified embodiments, but the present invention is not limited thereto; for example, an OHP sheet, etc. may be used, instead.
The halogen heater HH is employed as an example of a heat source in the above-exemplified embodiments, but the present invention is not limited thereto; for example, an induction heating (IH) heater, resistance heater, etc. may be employed, instead.
It is to be understood that the term ‘toner’ used in describing the exemplary embodiments above encompasses any kinds of developer without any limitation on their material or components. Similarly, the developer cartridge 28 as an example of a developer unit configured to supply toner onto the photoconductor, the scanner unit 16 as an example of an exposure apparatus configured to receive a signal of the image and cause a laser beam to scan in accordance with the signal of the image, the photoconductor drum 27 as an example of a photoconductor configured to be scanned by the laser beam from the exposure apparatus to form an electrostatic latent image thereon, are all described in the above embodiments for illustration purposes only, and the present invention is not limited their specific constructions.
In the first and second embodiments, the first point (first axis) of the pressure arm 63 which is supported by the frame 62 in a manner that permits the pressure arm 63 to swing on the first axis is located near one end of the pressure arm 63, and the second point (second axis) of the pressure arm 63 at which the pressure arm 63 is configured to support the pressure roller 42 in a manner that permits the pressure roller 42 to rotate on the second axis is located at an appropriated position that is closer to the other end of the pressure arm 63 than the first point and separate from the first point at a predetermined distance. Thus, the present invention is not limited to the particular embodiments as illustrated in
The nip width N may be any value as long as the expression (1) 0.24<N/Dp<0.6 is satisfied. Preferably but not necessarily, the nip width N may be 6 mm or greater, in that this specific range has been found out practically conformable to actual circumstances (i.e., in terms of the diameter of the pressure roller 42 and/or the size of the sheet 3). In consideration of the relationship between the rollers 41 and the 42 as shown in
Furthermore, the diameter of the pressure roller 42 may preferably but not necessarily be as large as illustrated in
Moreover, the dimension in the radial direction of the rubber layer provided in the pressure roller 42 may preferably but not necessarily be as thick as illustrated in
In order to obtain a desired nip width N by pressing and partially collapsing the pressure roller 42 to a predetermined extent, various parameters, such as a tension of the spring, a hardness of the pressure roller 42, etc., may be adjusted, and it has turned out to be desirable that Asker C hardness of the pressure roller be 37 degrees or less. This makes it possible to increase the nip width N without the need for increasing the spring tension so much, so that the torque for driving the pressure roller 42 can be made smaller.
Some examples of our fixing device implemented according to the above-illustrated embodiments for evaluation will be described below. To be more specific, two experimental results will be shown in which EXAMPLE 1 represents the observations on the relationship between the nip width N and a slip, and EXAMPLE 2 represents the observations on the nip width N and a reflection density decrease rate (toner loss rate).
Conditions of the experiment in EXAMPLE 1 were as follows:
The nip width N was changed stepwise under the above conditions, and an experiment was carried out for each nip width N to investigate whether or not a slip occurs between two rollers which hold a sheet. In the experiment, a predetermined driving force is supplied to the heating roller alone in the fixing device of HL5250 laser printer to provide the ‘heating roller driving’ type fixing device. On the other hand, a predetermined driving force is supplied, contrastively, to the pressure roller alone in the fixing device of the same-model (HL-5250) laser printer to provide the ‘pressure roller driving’ type fixing device.
These experiments have brought about the results as shown in TABLE 1. In TABLE 1, ‘O’ denotes a normal (successful) state in which a slip did not occur, while ‘X’ denotes an abnormal (poor) state in which a slip occurred. In other words, ‘O’ indicates that one roller successfully followed the other roller supplied with a driving force, while ‘X’ indicates that one roller failed to follow the other roller supplied with a driving force. ‘-’ indicates that no test printing operation was carried out.
To be more specific, ‘O’ indicates that 1,000 test printing operations were all completely successful without a slip, while ‘X’ denotes that a slip was observed during 1,000 test printing operations. Determination as to whether a slip had occurred was made by a skilled engineer observing an image printed on a sheet; it was thus determined that a slip had occurred if a disturbance in the printed image was observed.
The results of EXAMPLE 1, as shown in TABLE 1, demonstrate that the ‘heating roller driving’ scheme would cause a slip to occur when N/Dp is 0.24 and thus cannot increase the nip width N any more.
Conditions of the experiment in EXAMPLE 2 were as follows:
100×(RD1−RD2)/RD1
The nip width N was changed stepwise under the above conditions, and an experiment was carried out for each nip width N to investigate how the reflection density decrease rate, which indicates how much the toner has been lost, is affected by the nip width N. Hereupon, the reflection density decrease rate refers to a numerical value which is measured after the toner on the sheet is dried and which indicates how much the toner has been lost. The lower value in reflection density decrease rate exhibits the superiority in fixability performance of the fixing device. In this experiment, the data for 5.0 mm of the nip width N were obtained by means of a ‘heating roller driving’ type fixing device, and the other data were obtained by means of a ‘pressure roller driving’ type fixing device. It is however to be understood that the same results would be obtained even if all the data were obtained by means of a ‘pressure roller driving’ type fixing device in this experiment.
These experiments have brought about the results as shown in TABLE 2, see below, and
It has been shown in EXAMPLE 2 that N/Dp>0.24 is preferable. Consequently, the experimental observations given in EXAMPLE 1 and EXAMPLE 2 have established that N/Dp>0.24 is preferable.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-264301 | Sep 2006 | JP | national |
| 2006-264402 | Sep 2006 | JP | national |
| 2006-264537 | Sep 2006 | JP | national |
