Image forming apparatus

Abstract

To prevent a force with which a positioning part of a fixing device is pressed against a positioning surface of a fixing housing from being reduced when a nip pressure is reduced SOLUTION: A fixing device 80 includes a first fixing member (heating roller 81), a second fixing member (pressure roller 82), an arm 84, a spring 85, a cam 86, an input gear, and a positioning part. The arm 84 can press the second fixing member toward the first fixing member. The spring 85 biases the arm. The cam 86 applies the first nip pressure at the first phase and applies the second nip pressure at the second phase. The positioning portion positions the fixing housing with respect to the main housing. When rotating the input gear, the drive gear presses the input gear so that the positioning portion is pressed against the positioning surface. The friction mechanism MM applies a greater frictional resistance to the heating roller 81 when the arm 84 is in the second position than when the arm 84 is in the first position.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present disclosure relates to an image forming apparatus having a fuser with variable nip pressure. [Background technology]

[0002] Conventionally, an image forming apparatus having a fixing unit capable of changing nip pressure is known (see Patent Document 1). The fixing unit is detachable from the main body of the image forming apparatus and has a heating roller, a pressure roller that forms a nip with the heating roller, and a pressure lever that holds the pressure roller. In this fixing unit, the nip pressure between the heating roller and the pressure roller can be changed by rotating a cam that abuts against the pressure lever. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-025571 Summary of the Invention [Problem to be solved by the invention]

[0004] The fixing unit has an input gear to which a rotational driving force is input. The input gear meshes with a drive gear of the image forming apparatus main body and receives the rotational driving force. In this case, a positioning portion of the fixing unit relative to the main body housing may be configured to be pressed against a positioning surface of the main body housing. In this case, if the fixing unit can be changed between a first nip pressure and a second nip pressure, the rotational resistance of the input gear is smaller when the nip pressure is the second nip pressure, which is lower than when the nip pressure is the first nip pressure, and therefore the pressing force that the input gear receives from the drive gear is smaller. As a result, there is a problem in that when the nip pressure is the second nip pressure, the force with which the positioning portion of the fixing unit is pressed against the positioning surface of the main body housing is smaller.

[0005] Therefore, an object of the present disclosure is to prevent a decrease in the force with which the positioning portion of the fixing unit is pressed against the positioning surface of the main body housing when the nip pressure is reduced. [Means for solving the problem]

[0006] In order to achieve the above object, the image forming apparatus of the present application includes a main body housing, a drive gear supported by the main body housing, and a fixing unit. The fixing unit is detachably attached to the main body housing and includes a fixing housing, a first fixing member, a second fixing member, an arm, a spring, a cam, an input gear, and a positioning portion. The first fixing member is supported by the fixing housing and has a roller that rotates about a first axis extending in a first direction. The second fixing member forms a nip portion that nips a sheet between the first fixing member and the second fixing member. The arm can press the second fixing member toward the first fixing member. The spring has one end engaged with the fixing housing and the other end engaged with the arm. The spring biases the arm to press the second fixing member toward the first fixing member. The cam can rotate between a first phase and a second phase. In the first phase, the cam moves the arm to a first position to apply a first nip pressure to the nip portion, and in the second phase, moves the arm to a second position to apply a second nip pressure to the nip portion that is lower than the first nip pressure. The input gear meshes with the drive gear and applies a rotational driving force to the first fixing member. The positioning portion contacts a positioning surface of the main housing to position the fixing housing relative to the main housing. The drive gear meshes with the input gear, and when rotating the input gear, presses the input gear so that the positioning portion is pressed against the positioning surface. The fixing unit has a friction mechanism that applies frictional resistance to the roller, and the friction mechanism applies a greater frictional force when the arm is in the second position than when the arm is in the first position.

[0007] The drive gear meshes with the input gear and presses the input gear so that the positioning portion is pressed against the positioning surface when the input gear is rotated. When the nip pressure is the second nip pressure, which is lower, the rotational resistance of the input gear is smaller than when the nip pressure is the first nip pressure, so the pressing force received by the input gear is smaller. However, because the friction mechanism applies a greater rotational resistance when the fixing unit is at the second nip pressure than when it is at the first nip pressure, it is possible to prevent the pressing force on the input gear from becoming smaller.As a result, even when the nip pressure is at the second nip pressure, which is smaller, it is possible to prevent the force with which the positioning part is pressed against the positioning surface of the main body housing from becoming smaller.

[0008] The friction mechanism may also have a friction pad that comes into sliding contact with the surface of the roller.

[0009] Since the friction mechanism has a friction pad that is in sliding contact with the surface of the roller, it is possible to apply stable rotational resistance to the input gear.

[0010] The fixing device may also have a pad base that holds the friction pad. The pad base may extend in the circumferential direction of the roller, with one end thereof being fastened to the fixing housing and the other end being fastened to the arm.

[0011] One end of the pad base is engaged with the arm, so that the sliding state between the friction pad and the roller can be changed by moving the arm.

[0012] The pad base may also be engaged with the arm via a tension spring.

[0013] The pad base is engaged with the arm via a tension spring, so that the friction force can be gradually changed when the position of the arm changes.

[0014] The pad base may also be engaged with the fixing housing via a tension spring.

[0015] The pad base is engaged with the fixing housing via a tension spring, so that the friction force can be gradually changed when the position of the arm is changed.

[0016] The arm may be rotatable about a second axis extending in the first direction, and the roller may be located between the friction pad and the second axis as viewed from the first direction.

[0017] The input gear may be disposed at one end of the roller in the first direction, and the friction pad may be disposed at one end of the roller in the first direction.

[0018] Since the input gear and the friction pad are both disposed at one end of the roller in the first direction, it is possible to prevent the roller from being twisted by the friction force between the friction pad and the roller. [Effects of the Invention]

[0019] According to the present disclosure, it is possible to prevent a decrease in the force with which the positioning portion of the fixing unit is pressed against the positioning surface of the main body housing when the nip pressure is reduced. [Brief explanation of the drawings]

[0020] [Figure 1] 1 is a cross-sectional view of an image forming apparatus according to an embodiment. [Figure 2] 1A is a diagram showing the state before the fixing unit is attached to the main body housing, and FIG. 1B is a diagram showing the state when the fixing unit is positioned at the attachment position. [Figure 3] FIG. 10 is a diagram showing the positions of the cam, arm, and friction pad in the state of the first nip pressure. [Figure 4] FIG. 10 is a diagram showing the positions of the cam, arm, and friction pad in the state of the second nip pressure. [Figure 5] FIG. 10 is a diagram showing the positions of the cam, arm, and friction pad in the state of the third nip pressure. [Figure 6] FIG. 10 is a view of the heating roller and the friction pad as seen from a third direction. [Figure 7] 10 is a diagram showing the direction of a force pressing against the input gear when the drive gear rotates the input gear; FIG. [Figure 8] 10A and 10B are diagrams showing another embodiment in which the pad base is engaged with the fixing housing via a tension spring. [Figure 9] FIG. 10 is a diagram showing another embodiment, which shows a friction mechanism that does not use a tension spring. DETAILED DESCRIPTION OF THE INVENTION

[0021] Next, an embodiment of the present invention will be described in detail with reference to the accompanying drawings. As shown in FIG. 1, the image forming apparatus 1 includes a main body housing 10, a sheet supply unit 20, a process unit 30, a fixing unit 80, and a sheet discharge unit 9.

[0022] The main body housing 10 has a front cover 11, a discharge tray 12, and a rear cover 13. The main body housing 10 has a front opening 10A and a rear opening 10B. The front cover 11 opens and closes the front opening 10A of the main body housing 10. The rear cover 13 opens and closes the rear opening 10B of the main body housing 10.

[0023] The sheet supply unit 20 includes a sheet tray 21 and a supply mechanism 22. The sheet tray 21 stores sheets S such as paper. The supply mechanism 22 supplies the sheets S in the sheet tray 21 to the process unit 30.

[0024] The process section 30 forms a developer image on the sheet S. The process section 30 includes an exposure device SC, a process unit PU, and a transfer unit .

[0025] The exposure device SC is located above the process unit PU. The exposure device SC emits a light beam to expose the surface of the photosensitive drum 51.

[0026] The process unit PU is located between the sheet tray 21 and the exposure device SC. The process unit PU is detachable from the main body housing 10 through an opening 10A in the main body housing 10, which is opened by opening the front cover 11. The process unit PU includes a drum cartridge 50 and a plurality of toner cartridges 60.

[0027] The drum cartridge 50 includes a plurality of photosensitive drums 51, a plurality of chargers 52 corresponding to the plurality of photosensitive drums 51, and a drum frame 53.

[0028] The drum frame 53 supports the photosensitive drum 51 and the charger 52. The drum frame 53 is movably supported by the main body casing 10. A toner cartridge 60 is detachably attached to the drum frame 53.

[0029] The plurality of toner cartridges 60 contain toner of different colors. Each toner cartridge 60 includes a developing roller 61, a supply roller 62, a layer thickness regulating blade 63, a toner containing section 64 that contains toner, and an agitator 65.

[0030] The agitator 65 agitates the toner in the toner storage unit 64. The agitator 65 supplies the toner to the supply roller 62. The supply roller 62 supplies the toner to the development roller 61. The layer thickness regulating blade 63 regulates the toner on the development roller 61 to a constant thickness.

[0031] The transfer unit 70 is located between the sheet tray 21 and the process unit PU. The transfer unit 70 includes a drive roller 71, a driven roller 72, a conveyor belt 73, and a plurality of transfer rollers 74. The conveyor belt 73 is an endless belt for conveying the sheet S. The drive roller 71 and the driven roller 72 rotate the conveyor belt 73. The transfer roller 74 is located inside the conveyor belt 73. The transfer roller 74 sandwiches the conveyor belt 73 between itself and the photosensitive drum 51.

[0032] The fixing unit 80 includes a heating roller 81 as an example of a first fixing member, a pressure roller 82 as an example of a second fixing member, and a fixing housing 83.

[0033] Heating roller 81 is supported by fixing housing 83. Heating roller 81 rotates around a first axis 1X extending in a first direction. Heating roller 81 receives a driving force from the main body of image forming apparatus 1 to rotate. Heating roller 81 has heater 81A and roller 81B. Heater 81A is disposed inside roller 81B and heats roller 81B.

[0034] The pressure roller 82 forms a nip portion NP that nips the sheet S between itself and the heating roller 81. The pressure roller 82 rotates in accordance with the rotation of the heating roller 81.

[0035] The fixing housing 83 supports the heating roller 81 and the pressure roller 82 .

[0036] The charger 52 charges the surface of the photosensitive drum 51. The exposure device SC exposes the surface of the photosensitive drum 51. As a result, an electrostatic latent image is formed on the photosensitive drum 51. The development roller 61 supplies toner to the photosensitive drum 51. As a result, a toner image is formed on the photosensitive drum 51.

[0037] The sheet supply unit 20 transports the sheet S between the photosensitive drum 51 and the transfer roller 74. The toner image on the photosensitive drum 51 is transferred to the sheet S. The sheet S onto which the toner image has been transferred is transported between the heating roller 81 and the pressure roller 82. As a result, the fixing unit 80 thermally fixes the developer image formed on the sheet S.

[0038] The sheet discharge section 9 has a conveying roller 9 A and a discharge roller 9 B. The conveying roller 9 A conveys the sheet S to the discharge roller 9 B. The discharge roller 9 B discharges the sheet S onto the discharge tray 12.

[0039] The fixing unit 80 is detachable from the main body housing 10. More specifically, the fixing unit 80 is detachable from the main body housing 10 through an opening 10B of the main body housing 10 that is opened by opening the rear cover 13. In other words, the main body housing 10 detachably supports the fixing unit 80. The fixing unit 80 is attachable to an attachment position of the main body housing 10.

[0040] The second direction is a direction along the mounting direction in which the fixing unit 80 moves from the outside of the main body housing 10 toward the inside of the main body housing 10. In this embodiment, the second direction is a direction perpendicular to the first direction and along the front-to-rear direction of the image forming apparatus 1, with one side of the second direction being a direction facing the rear side of the image forming apparatus 1. In the following description, the upstream side of the mounting direction of the fixing unit 80 is referred to as one side of the second direction, and the downstream side is referred to as the other side of the second direction. Arrows indicating each direction in the drawings will refer to one side of the respective direction. The third direction is a direction perpendicular to the first and second directions. In this embodiment, the third direction is a direction along the up-down direction of the image forming apparatus 1. In the following description, the downward direction is referred to as one side of the third direction, and the upward direction is referred to as the other side of the third direction.

[0041] As shown in FIG. 2(a), the fixing unit 80 is removed from the mounting position to one side in the second direction. In the following description, the direction in which the fixing unit 80 is removed is also referred to as the removal direction. Then, as shown in FIG. 2(b), the fixing unit 80 is mounted from one side to the other side in the second direction. In the following description, the direction in which the fixing unit 80 is mounted is also referred to as the mounting direction.

[0042] 2(a), the fixing unit 80 has a second shaft 120 and a first shaft 130. Note that only the other side of the second shaft 120 and the first shaft 130 in the first direction is shown. The second shaft 120 and the first shaft 130 extend from the fixing housing 83 in the first direction.

[0043] The second shaft 120 is an axis having a cylindrical surface on its outer surface. The second shaft 120 is located on one side surface and the other side surface of the fixing housing 83 in the first direction. The two second shafts 120 protrude toward one side and the other side in the first direction, respectively. The second shaft 120 is located at an end of the fixing housing 83 on one side in the third direction. The second shaft 120 is located at an end of the fixing housing 83 on the other side in the second direction. In other words, the second shaft 120 is located at the downstream end in the mounting direction. The second shaft 120 is an example of a positioning portion for the fixing unit 80.

[0044] The first shaft 130 is a shaft having a cylindrical outer surface. The first shaft 130 is located on one side surface and the other side surface of the fixing housing 83 in the first direction. The two first shafts 130 protrude toward one side and the other side in the first direction, respectively. The first shaft 130 is located at one end of the fixing housing 83 in the third direction. The first shaft 130 is located upstream of the second shaft 120 in the mounting direction. The first shaft 130 is also located on one side of the second shaft 120 in the third direction. The first shaft 130 is located upstream of the center of the fixing housing 83 in the mounting direction. As a result, the downstream side of the first shaft 130 is heavier than the upstream side of the first shaft 130 in the mounting direction. In the following description, an axis that passes through the center of the first shaft 130 and extends in the first direction is referred to as a sixth axis 6X.

[0045] The main body housing 10 has a first recess G3, a second recess G4, and a mounting spring SP.

[0046] The first recess G3 is a groove recessed toward one side in the third direction. The first recess G3 has a second reference surface G31 and a stopper surface G32. The second reference surface G31 is perpendicular to the third direction. The stopper surface G32 extends upward from the downstream end of the second reference surface G31 in the mounting direction. The second reference surface G31 of the first recess G3 is an example of a positioning surface for the main housing 10.

[0047] The second recess G4 extends in the second direction and opens to the upstream side in the mounting direction.

[0048] The mounting spring SP is a spring for mounting the fixing unit 80 to the main body housing 10. In this embodiment, the mounting spring SP is a torsion spring. The mounting spring SP biases the fixing unit 80 against the main body housing 10.

[0049] 2(b), when the fixing unit 80 is attached to the main body housing 10, the first shaft 130 fits into the second recess G4. The second recess G4 stops the first shaft 130 from rotating around the second shaft 120.

[0050] The second shaft 120 fits into the first recess G3 when the fixing unit 80 is attached to the main body housing 10. When the second shaft 120 fits into the first recess G3, the second shaft 120 comes into contact with the second reference surface G31.

[0051] The second reference surface G31 comes into contact with the second shaft 120 when the fixing unit 80 is attached, thereby preventing the second shaft 120 from moving in the third direction. In this manner, the second shaft 120 comes into contact with the second reference surface G31 to position the fixing housing 83 relative to the main housing 10.

[0052] One end of the mounting spring SP contacts the second shaft 120. The other end of the mounting spring SP contacts the main body housing 10. The mounting spring SP biases the second shaft 120 toward the second reference surface G31.

[0053] As shown in FIGS. 3 to 7, the fixing device 80 further includes a nip pressure change mechanism NM and a friction mechanism MM.

[0054] The nip pressure change mechanism NM is a mechanism that changes the nip pressure between the heating roller 81 and the pressure roller 82. The nip pressure change mechanism NM can change the nip pressure at the nip portion NP by moving at least one of the heating roller 81 and the pressure roller 82. In this embodiment, the nip pressure change mechanism NM changes the nip pressure by moving the pressure roller 82 relative to the heating roller 81. As shown in FIG. 3 , the nip pressure change mechanism NM has an arm 84, a spring 85, and a cam 86.

[0055] The arm 84 is rotatably supported by the fixing housing 83. The arm 84 is rotatable about a second axis 2X extending in the first direction. By rotating about the second axis 2X, the arm 84 is movable between a first position shown in FIG. 3 and a second position shown in FIG. 4. The arm 84 is also movable between the second position shown in FIG. 4 and a third position shown in FIG. 5. The arm 84 is capable of pressing the pressure roller 82 toward the heat roller 81. The arm 84 presses the pressure roller 82 toward the heat roller 81 in all of the first position, the second position, and the third position.

[0056] The arm 84 has one end 84A, another end 84B, a first portion 84C, and a second portion 84D.

[0057] One end 84A is rotatably supported by the fixing housing 83. More specifically, one end 84A has a notch that fits into a fixing housing shaft 83F of the fixing housing 83, and is supported rotatably around the fixing housing shaft 83F.

[0058] The other end 84B has a cam follower 84E. The cam follower 84E is capable of contacting a cam 86.

[0059] The first portion 84C and the second portion 84D are located between the one end 84A and the other end 84B. The first portion 84C supports the shaft 82A of the pressure roller 82. The second portion 84D is a portion to which the spring 85 is connected. The second portion 84D has a first hole 1H to which the spring 85 is connected. The distance from the one end 84A to the first portion 84C is shorter than the distance from the one end 84A to the second portion 84D.

[0060] The spring 85 is a tension spring. One end of the spring 85 is attached to the fixing housing 83, and the other end is attached to the arm 84. The fixing housing 83 has a first hook portion 83A. One end of the spring 85 is hooked onto the first hook portion 83A. The other end of the spring 85 is hooked onto the first hole 1H of the second portion 84D of the arm 84.

[0061] The spring 85 biases the arm 84 so as to press the pressure roller 82 toward the heating roller 81. The spring 85 presses the pressure roller 82 against the heating roller 81 via the arm 84.

[0062] The cam 86 is rotatably supported by the fixing housing 83. The cam 86 is rotatable about the third axis 3X. When the cam 86 rotates, it moves at least one of the heating roller 81 and the pressure roller 82, thereby changing the nip pressure that nips the sheet S.

[0063] Specifically, the cam 86 is rotatable between a first phase shown in Fig. 3 and a second phase shown in Fig. 4. The cam 86 is also rotatable between the second phase shown in Fig. 4 and a third phase shown in Fig. 5. The cam 86 moves the arm 84 by rotating.

[0064] As shown in FIG. 3, when the cam 86 is in the first phase, the arm 84 is positioned at the first position, and the nip portion NP is applied with a first nip pressure P1. As shown in FIG. 4, when the cam 86 is in the second phase, the arm 84 is positioned at the second position to apply a second nip pressure P2 to the nip portion NP, which is lower than the first nip pressure P1 (P1>P2). As shown in FIG. 5, when the cam 86 is in the third phase, the arm 84 is positioned at the third position to apply a third nip pressure P3 to the nip portion NP, which is lower than the second nip pressure P2 (P2>P3).

[0065] The friction mechanism MM is a mechanism that applies rotational resistance due to friction to the heating roller 81. The friction mechanism MM has a friction pad 91, a pad base 92, a tension spring 93, and a hook 94.

[0066] When viewed from the first direction, friction pad 91 is located on the opposite side of second axis 2X across heating roller 81. In other words, when viewed from the first direction, heating roller 81 is located between friction pad 91 and second axis 2X.

[0067] 6, the friction pad 91 is disposed at one end in the first direction of the heat roller 81. The friction pad 91 is disposed outside the passing range SE of the sheet S.

[0068] As shown in Figure 3, friction pad 91 is made of a material that generates friction between itself and heating roller 81, such as resin, rubber, cloth, or nonwoven fabric. Friction pad 91 has a flat plate shape. Friction pad 91 comes into sliding contact with the surface of heating roller 81. By coming into sliding contact with the surface of heating roller 81, friction pad 91 generates rotational resistance in heating roller 81.

[0069] The pad base 92 is made of a flexible material such as resin, rubber, cloth, nonwoven fabric, etc. The pad base 92 has a flat plate shape and extends in the circumferential direction of the heating roller 81. The pad base 92 holds the friction pad 91.

[0070] One end of the pad base 92 is engaged with the fixing housing 83, and the other end is engaged with the arm 84. Here, the fixing housing 83 has a second hook portion 83B. One end of the pad base 92 is engaged with the second hook portion 83B by a hook 94. The other end of the pad base 92 is engaged with the second hole 2H of the arm 84 by a pull spring 93. In this way, the pad base 92 engages with the arm 84 via the pull spring 93.

[0071] When the arm 84 is in the first position, the friction pad 91 is in contact with the heating roller 81. Therefore, when the heating roller 81 rotates, the friction pad 91 applies rotational resistance to the heating roller 81 due to friction.

[0072] 4, when the arm 84 moves from the first position to the second position, the arm 84 pulls the pad base 92 to one side in the second direction and one side in the third direction via the pull spring 93. Therefore, the pad base 92 is pulled more strongly by the arm 84 when the arm 84 is in the second position than when the arm 84 is in the first position. As a result, the friction pad 91 is pressed more strongly against the heating roller 81 when the arm 84 is in the second position than when the arm 84 is in the first position. In this way, the friction mechanism MM applies a greater friction force to the heating roller 81 when the arm 84 is in the second position than when the arm 84 is in the first position.

[0073] 5, when the arm 84 moves from the second position to the third position, the arm 84 pulls the pad base 92 to one side in the second direction and one side in the third direction via the pull spring 93. Therefore, the pad base 92 is pulled more strongly by the arm 84 when the arm 84 is at the third position than when the arm 84 is at the second position. As a result, the friction pad 91 is pressed more strongly against the heating roller 81 when the arm 84 is at the third position than when the arm 84 is at the second position. In this way, the friction mechanism MM applies a greater friction force to the heating roller 81 when the arm 84 is at the third position than when the arm 84 is at the second position.

[0074] 7, the image forming apparatus 1 further includes a first drive gear DG1 and a second drive gear DG2 as examples of drive gears, and the fixing device 80 includes a first input gear NG1 and a second input gear NG2 as examples of input gears.

[0075] The first drive gear DG1 is supported by the main body housing 10. The first drive gear DG1 is rotatable about a fourth axis 4X that extends in the first direction.

[0076] The second drive gear DG2 is supported by the main body housing 10 and is rotatable about a fifth axis 5X extending in the first direction.

[0077] As shown in Fig. 6, the first input gear NG1 is fixed to one end of the heating roller 81 in the first direction, and is rotatable about the same first axis 1X as the heating roller 81. When the fixing unit 80 is in the mounting position, the first input gear NG1 meshes with the first drive gear DG1, causing the heating roller 81 to rotate. As shown in Fig. 7, when the first drive gear DG1 rotates counterclockwise, the first input gear NG1 rotates clockwise, applying a rotational drive force to the heating roller 81, causing the heating roller 81 to rotate clockwise.

[0078] As shown in FIG. 7, the first axis 1X is located between the fourth axis 4X and the first shaft 130 in the third direction.

[0079] The second input gear NG2 is rotatable about the same third axis 3X as the cam 86. The second input gear NG2 meshes with the second drive gear DG2 to rotate the cam 86. In Figure 7, if it is desired to rotate the cam 86 clockwise, the second drive gear DG2 is rotated counterclockwise, and if it is desired to rotate the cam 86 counterclockwise, the second drive gear DG2 is rotated clockwise.

[0080] In the third direction, the third axis 3X is located between the fifth axis 5X and the first shaft 130.

[0081] When the fixing unit 80 is attached to the attachment position, the first input gear NG1 is located downstream of the second input gear NG2 in the attachment direction in which the fixing unit 80 is attached to the main body housing 10.

[0082] When the fixing unit 80 is attached to the attachment position, the first shaft 130 is located on a straight line L1 that passes through the first axis 1X and the fourth axis 4X when viewed from the first direction. When the fixing unit 80 is attached to the attachment position, the first shaft 130 is located on a straight line L2 that passes through the third axis 3X and the fifth axis 5X when viewed from the first direction. The first shaft 130 is located below the first axis 1X and the third axis 3X.

[0083] As described above, the following effects can be obtained in this embodiment. As shown in FIG. 7, when the heating roller 81 is rotated, the first drive gear DG1 is rotated counterclockwise in FIG. 7. This causes the first input gear NG1, which is meshed with the first drive gear DG1, to rotate clockwise. At this time, the heating roller 81 is in contact with and pressed by the pressure roller 82, creating resistance to rotation. While the first drive gear DG1 rotates the first input gear NG1, it applies a force F1 to the first input gear NG1 in a direction that forms a pressure angle α with respect to the common tangent L3 of the pitch circles of the first drive gear DG1 and the first input gear NG1. As a result, while the first drive gear DG1 rotates the first input gear NG1, the second shaft 120, which serves as a positioning unit, is pressed against the second reference surface G31, which serves as a positioning surface, by a force F2. The pressure angle α is, for example, 20°, but may be a different value, such as 14.5°. The force F2 is a component perpendicular to the second reference plane G31 of the force with which the second shaft 120 presses the first recess G3.

[0084] Here, when the nip pressure at the nip portion NP is the second nip pressure P2, which is smaller than the first nip pressure P1, the rotational resistance of the heating roller 81 is smaller than when the nip pressure is the first nip pressure P1, and therefore the pressing force received by the first input gear NG1 is smaller. However, in the present embodiment, since the fixing device 80 has the friction mechanism MM that applies a larger rotational resistance when the nip pressure is second nip pressure P2 than when the nip pressure is first nip pressure P1, it is possible to prevent the pressing force received by the first input gear NG1 from decreasing. As a result, even when the nip pressure is second nip pressure P2, which is small, it is possible to prevent the force with which the second shaft 120, which is the positioning portion, is pressed against the second reference surface G31, which is the positioning surface of the main body housing 10, from decreasing.

[0085] Furthermore, since the friction mechanism MM has the friction pad 91 that comes into sliding contact with the surface of the heating roller 81, rotational resistance can be applied stably to the first input gear NG1.

[0086] Furthermore, one end of the pad base 92 is engaged with the arm 84, so that the sliding state between the friction pad 91 and the heating roller 81 can be changed by the movement of the arm 84.

[0087] Furthermore, since the pad base 92 is engaged with the arm 84 via the tension spring 93, the frictional force can be gradually changed when the position of the arm 84 changes.

[0088] In addition, since the first input gear NG1 and the friction pad 91 are both positioned at one end of the heating roller 81 in the first direction, the frictional force between the friction pad 91 and the heating roller 81 can be prevented from twisting the heating roller 81.

[0089] Although the embodiment has been described above, the present invention is not limited to the above embodiment and can be practiced by appropriately modifying it as exemplified below. In the following description, the same components as those in the embodiment described above are given the same reference numerals and their description will be omitted.

[0090] In the above-described embodiment, the pad base is engaged with the arm via the tension spring, but the pad base may be engaged with the fixing housing via the tension spring. 8 includes a friction pad 91, a pad base 92, a pull spring 93A, and a hook 94A. One end of the pad base 92 is hooked onto the arm 84 by the hook 94A. The other end of the pad base 92 is hooked onto the fixing housing 83 by the pull spring 93A. In this way, the pad base 92 is engaged with the fixing housing 83 via the pull spring 93A. In the configuration of FIG. 8, the pad base 92 also has the tension spring 93A, so that the frictional force can be gradually changed when the position of the arm changes.

[0091] In the above-described embodiment, the friction mechanism is engaged with the arm via the tension spring, but it is not necessary to use the tension spring. For example, the friction mechanism MM3 shown in Figures 9(a) and (b) has a friction pad 191 and a pad base 192. The pad base 192 has a plate shape with a bent portion. The base end of the pad base 192 is fixed to the arm 84. The pad base 192 has the friction pad 191 fixed to the tip end. As shown in FIG. 9(a), when the arm 84 is in the first position, the friction pad 191 does not contact the heating roller 81. As shown in FIG. 9(b), when the arm 84 is located at the second position, the friction pad 191 comes into contact with the heating roller 81, and the friction pad 191 can apply a frictional force to the heating roller 81. The configuration shown in FIGS. 9(a) and 9(b) also makes it possible to prevent the pressing force received by the first input gear NG1 from becoming smaller when the second nip pressure is P2.

[0092] 6, the friction pad 91 is disposed at one end of the heating roller 81 in the first direction, but the friction pad 91 may also be disposed at the other end of the heating roller 81 in the first direction. Furthermore, the friction pad 91 may also be disposed at both ends of the heating roller 81 in the first direction.

[0093] In the above-described embodiment, the second shaft 120 is given as an example of a positioning portion of the fixing unit 80, and the second reference surface G31 of the first recess G3 is given as an example of a positioning surface of the main body housing 10, but the positioning portion and the positioning surface can be set arbitrarily.

[0094] In the above-described embodiment, a heating roller is used as an example of the first fixing member. However, the first fixing member is not limited to a heating roller and may be, for example, an endless belt whose inner circumferential surface is heated by a heater. Alternatively, an external heating method in which a heater is disposed outside the rotating body and heats the outer circumferential surface of the rotating body, or an induction heating method may be used. Alternatively, a heater may be disposed inside the endless belt and the rotating body in contact with the outer circumferential surface of the endless belt may be indirectly heated. Alternatively, the rotating body and the endless belt may each have a built-in heater.

[0095] In the above-described embodiment, a pressure roller was given as an example of the second fixing member, but the second fixing member is not limited to a heat roller and may be, for example, an endless belt with a pressing member disposed inside.

[0096] In the above-described embodiment, the first input gear NG1 meshes with the first drive gear DG1 when the fixing unit 80 is located in the mounting position, thereby rotating the heating roller 81. However, the first input gear NG1 may be configured to rotate the pressure roller 82. In this case, the heating roller 81 rotates in accordance with the rotation of the pressure roller 82.

[0097] Furthermore, the image forming apparatus is not limited to a color printer, but may be any of a monochrome printer, a multifunction machine, a copier, etc. Furthermore, although an electrophotographic image forming apparatus is exemplified in the above embodiment, the present invention is not limited to this, and may be, for example, an inkjet image forming apparatus.

[0098] Furthermore, the elements described in the above-described embodiment and modifications can be implemented in appropriate combinations. [Explanation of symbols]

[0099] 1. Image forming device 10 Main unit housing 80 Fixing unit 81 Heating roller 82 Pressure roller 83 Fixing housing 84 Arm 85 spring 86 Cam 91 Friction Pad 92 Pad Base 93 Pull spring MM friction mechanism NP nip section

Claims

1. A main body housing; a drive gear supported by the main body housing; a fixing unit detachable from the main body housing, A fixing housing; a first fixing member supported by the fixing housing and having a roller that rotates around a first axis extending in a first direction; a second fixing member that forms a nip portion that nips a sheet between itself and the first fixing member; an arm capable of pressing the second fixing member toward the first fixing member; a spring having one end engaged with the fixing housing and the other end engaged with the arm, the spring biasing the arm so as to press the second fixing member toward the first fixing member; a cam that is rotatable between a first phase and a second phase, the cam moving the arm so that, in the first phase, the arm is positioned at a first position to apply a first nip pressure to the nip portion, and in the second phase, the arm is positioned at a second position to apply a second nip pressure to the nip portion, the second nip pressure being a nip pressure smaller than the first nip pressure; an input gear that meshes with the drive gear and applies a rotational driving force to the first fixing member; a positioning portion that contacts a positioning surface of the main body housing to position the fixing housing relative to the main body housing; the drive gear meshes with the input gear, and when rotating the input gear, presses the input gear so that the positioning portion is pressed against the positioning surface; The image forming apparatus is characterized in that the fixing unit has a friction mechanism that applies frictional rotational resistance to the roller, and that applies a greater frictional force when the arm is in the second position than when the arm is in the first position.

2. 2. The image forming apparatus according to claim 1, wherein the friction mechanism has a friction pad that comes into sliding contact with the surface of the roller.

3. the fixing device has a pad base that holds the friction pad; 3. The image forming apparatus according to claim 2, wherein the pad base extends in a circumferential direction of the roller, one end of the pad base is engaged with the fixing housing, and the other end of the pad base is engaged with the arm.

4. 4. The image forming apparatus according to claim 3, wherein the pad base is engaged with the arm via a tension spring.

5. 4. The image forming apparatus according to claim 3, wherein the pad base is engaged with the fixing housing via a tension spring.

6. the arm is rotatable about a second axis extending in the first direction; 3. The image forming apparatus according to claim 2, wherein the roller is located between the friction pad and the second axis when viewed from the first direction.

7. the input gear is disposed at one end of the roller in the first direction, 7. The image forming apparatus according to claim 2, wherein the friction pad is disposed at one end of the roller in the first direction.

Images