Fixing device

Abstract

To provide a fixing device that performs fixing processing on a toner image formed on a recording material at a fixing nip part formed between a cylindrical film and a roller, and allows stable rotation of the film.SOLUTION: A fixing device has a rotatably supported cylindrical fixing film, a guide 9 that guides rotation of the fixing film, a metal stay 10 that reinforces the guide 9, and a pressure roller that forms a fixing nip with the guide 9 with the fixing film therebetween. The guide 9 has a projection part S at a center part in a Y-axis direction and at a center part in an X-axis direction. The stay 10 has an opening H for supporting the guide 9. The projection part S of the guide 9 is inserted into the opening H of the stay 10. The guide 9 is provided rotatably with respect to the stay 10 with a Z-axis as the fulcrum. The fixing film slides to apply a force to the guide 9 and the guide 9 is rotated by the force received from the fixing film, thereby correcting the alignment with the stay 10.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to a fixing device mounted in an image forming apparatus such as an electrophotographic copying machine or an electrophotographic printer. [Background technology]

[0002] Among fixing devices mounted on electrophotographic copying machines and printers, there is one that fixes a toner image formed on a recording material in a fixing nip portion formed between a film and a roller in a cylindrical housing (Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2014-26267 A Summary of the Invention [Problem to be solved by the invention]

[0004] If the film alignment is misaligned due to the dimensional accuracy of parts or the accuracy of assembly, the stable rotation of the film will be impaired. If the stability of the film rotation decreases, the conveyance of the recording material will deteriorate, causing wrinkles on the recording material and image defects such as reduced fixation of the toner image.

[0005] SUMMARY OF THE PRESENT EMBODIMENT An object of the present invention is to provide a fixing device in which the film rotates stably. [Means for solving the problem]

[0006] In order to solve the above-mentioned problems, the present invention provides a fixing device comprising a cylindrical flexible rotating body, a guide member arranged in the internal space of the rotating body along the longitudinal direction of the rotating body and contacting the inner surface of the rotating body to guide the rotation of the rotating body, a holding member arranged in the internal space of the rotating body along the longitudinal direction of the rotating body and holding the guide member along the longitudinal direction, and a roller forming a fixing nip portion together with the guide member via the rotating body, the fixing device fixing a toner image formed on the recording material to the recording material while sandwiching and transporting the recording material in the fixing nip portion, the guide member is rotatably provided relative to the holding member with an axis that is approximately perpendicular to the fixing nip portion as a fulcrum, and the guide member rotates due to the force received from the rotating body as the rotating body slides, and alignment relative to the holding member is corrected. Effect of the Invention

[0007] According to the present invention, it is possible to provide a fixing device in which the film rotates stably. [Brief description of the drawings]

[0008] [Figure 1] 1 is a cross-sectional view of a fixing device according to a first embodiment of the present invention; [Diagram 2] FIG. 1 is a front view of a fixing device according to a first embodiment of the present invention; [Diagram 3] Guide and stay diagram [Figure 4] Diagram explaining the heat generation principle [Diagram 5] Diagram explaining how the guide works [Figure 6] 11 is a cross-sectional view of a guide of a fixing device according to a second embodiment of the present invention; [Figure 7] Diagram explaining the difference in pressure distribution DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] Example 1 (Configuration of Fixing Device A) The fixing device A of the first embodiment will be described with reference to Figures 1 and 2. The fixing device A is mounted in an image forming apparatus (not shown) such as an electrophotographic copying machine or printer, and is a device that heats and fixes a toner image T formed on a recording material P to the recording material P. Figure 1 is a cross-sectional view of the fixing device A, and Figure 2 is a front view of the fixing device A as viewed from the upstream side in the conveying direction X of the recording material P.

[0010] The fixing device A has a cylindrical fixing film (flexible rotating body) 1 that is rotatably supported, and a guide (guide member) 9 that is made of heat-resistant resin and guides the rotation of the film. It also has a metal stay (holding member) 10 that reinforces the guide 9, and a pressure roller 7 that forms a fixing nip portion N together with the guide 9 via the fixing film 1. The fixing device A fixes the toner image T formed on the recording material P to the recording material P while nipping and transporting the recording material P in the fixing nip portion N.

[0011] The guide 9 is made of PPS (polyphenylene sulfide) and is provided in the internal space of the fixing film 1 along the longitudinal direction (Y-axis direction) of the fixing film 1. A stay 10 is also provided in the internal space of the fixing film 1 along the longitudinal direction of the fixing film 1, reinforcing the guide 9 along the longitudinal direction of the fixing film 1. A magnetic core 2 around which an exciting coil 3 is wound is provided in the internal space of the fixing film 1. The magnetic core 2 has approximately the same length as the film 1 in the Y-axis direction and is of an end-shaped form. The fixing device A is an induction heating type device in which the fixing film 1 generates heat by electromagnetic induction.

[0012] The pressure roller (roller) 7 has a core 7a, a rubber layer 7b, and a release layer 7c. The pressure roller 7 has a diameter of 30 mm. The rubber layer 7b is preferably made of a material with excellent heat resistance, such as silicone rubber or fluororubber. The release layer 7c is preferably made of a material with excellent releasability and heat resistance, such as PFA (tetrafluoroethylene-perfluoroalkylvinylether copolymer) or PTFE (polytetrafluoroethylene). Both ends of the core 7a in the direction (Y-axis direction) perpendicular to the conveying direction (X-axis direction) of the recording material P, i.e., in the axial direction of the pressure roller 7, are rotatably held by the left and right side plate frames (not shown) of the fixing device A via bearings.

[0013] The fixing film 1 has a heat generating layer (base layer) 1a with a diameter of 10 to 50 mm, a rubber layer 1b, and a release layer 1c. The heat generating layer 1a is preferably made of a metal with a low volume resistivity, and in this example, SUS (stainless steel) with a thickness of 20 μm to 100 μm is used. As will be described later, the fixing device A is configured to form a magnetic path in such a shape that most (90% or more) of the magnetic flux that goes out of the fixing film 1 from one end of the magnetic core 2 in the Y-axis direction passes outside the fixing film 1 and returns to the other end of the magnetic core 2. Therefore, since the fixing device A is not a type of device that induces magnetic flux into the layer (meaning the thickness area) of the heat generating layer 1a to heat the heat generating layer, magnetic metals or non-magnetic metals can be used as the material of the heat generating layer 1a. The material of the rubber layer 1b is silicone rubber with a hardness of 20 degrees (JIS (a), 1 kg load) and a thickness of 0.1 mm to 0.3 mm. The release layer 1c is a fluororesin tube having a thickness of 10 μm to 50 μm.

[0014] As shown in FIG. 2, flanges 13a and 13b made of heat-resistant resin are provided at both ends of the stay 10 in the Y-axis direction (longitudinal direction of the fixing device A). Each flange 13a and 13b holds the inner circumferential surface (inner surface) of the end of the film 1 with a holding portion (not shown) inserted into the hollow portion of the film 1. Each flange 13a and 13b has a regulating surface 13a1 and 13b1 that receives the end surface of the film 1 when the film 1 moves toward the Y-axis direction while rotating. The regulating surfaces 13a1 and 13b1 regulate the shifting of the film 1 in the Y-axis direction. A predetermined gap is provided between the guide 9 and the flange 13 and between the guide 9 and the stay 10 in the X and Y directions so that the guide 9 can rotate around a fulcrum, as described later.

[0015] Pressure springs (compression springs) 11a and 11b are provided between both ends of the stay 10 in the Y-axis direction and spring bearing members 12a and 12b of the left and right side plate frames (not shown) of the fixing device A, respectively, to apply a downward force to the stay 10. In the fixing device A of this embodiment, a total pressure of about 100N to 450N (about 10kgf to about 45kgf) is applied to the stay 10. This pressure pressure presses the guide 9 toward the pressure roller via the film 1, forming a fixing nip N. The pressure roller 7 is driven in the direction of arrow K (see FIG. 1) by the power of the motor M. The film 1 rotates following the rotation of the pressure roller 7 while its inner surface slides against the surface of the guide 9. Reference numeral 4 shown in FIG. 2 denotes a temperature detection element that detects the temperature of the film 1.

[0016] 3(a) and 3(b) are diagrams for explaining the configuration of the stay 10 and the guide 9. FIG. 3(a) is a perspective view showing the relationship between the guide 9 and the stay 10, and FIG. 3(b) is a schematic view of these members viewed in the Z-axis direction. As shown in FIG. 3, the guide 9 has a protrusion S at the center in the Y-axis direction and at the center in the X-axis direction. The stay 10 has an opening H for supporting the guide 9 at a position facing the protrusion of the guide 9. The protrusion S of the guide 9 is inserted into the opening H of the stay 10 so as to penetrate through the opening H. In addition, the lower surface of the stay 10 presses the surface of the guide 9 facing the stay 10 by the force of the pressure springs 11a and 11b, so that the guide 9 is pressed toward the pressure roller 7 side. As shown in FIG. 3(b), when the force of the pressure springs 11a and 11b is applied, the stay 10 is positioned and fixed to the side plate frame of the fixing device A by the regulating member 14. On the other hand, the guide 9 is held by the stay 10 so as to be rotatable around the protrusion S in the direction of the arrow.

[0017] 4 is a perspective view of the magnetic core 2 and the excitation coil 3. The magnetic core 2 has a cylindrical shape and is disposed at approximately the center of the internal space of the fixing film 1 in the radial direction of the fixing film 1 by a fixing means (not shown). In the internal space of the fixing film 1, magnetic lines of force (magnetic flux) generated by flowing a high-frequency current through the excitation coil 3 penetrate the inside of the magnetic core 2 along the Y-axis direction. A magnetic field is formed in such a shape that the magnetic lines of force coming out of one end of the magnetic core 2 in the Y-axis pass outside the fixing film 1 and enter the other end of the magnetic core 2. A current flowing in the circumferential direction of the fixing film 1 is induced in the heat generating layer 1a of the fixing film 1 to form a magnetic field that cancels this magnetic field.

[0018] The material of the magnetic core 2 is preferably a material with small hysteresis loss and high relative magnetic permeability, and is preferably a ferromagnetic material made of oxides or alloys with high magnetic permeability, such as sintered ferrite, ferrite resin, amorphous alloy, or permalloy. In addition, it is preferable to make the cross-sectional area as large as possible within the range that can be accommodated in the internal space of the fixing film 1, and the diameter is set to 5 mm to 40 mm. The shape of the magnetic core 2 is not limited to a cylindrical shape, and a rectangular column shape or the like can be selected. In this example, the magnetic core 2 is arranged only in the internal space of the fixing film 1 to form an open magnetic path, but a magnetic core may also be arranged outside the fixing film 1 to form a closed magnetic path. The exciting coil 3 is formed of a single conductor of copper wire with a diameter of 1 to 2 mm coated with heat-resistant polyamideimide. The copper wire is wound around the magnetic core 2 about 10 to 30 times to form a spiral exciting coil 3. In this example, the number of turns is 18.

[0019] When the image forming operation of the image forming apparatus is started, the fixing device A heats the fixing film 1 by electromagnetic induction at a predetermined timing and starts driving the pressure roller 7 by the motor M. The fixing film 1 is rotated by the pressure roller 7 due to the frictional force at the fixing nip N. The high-frequency converter 5 supplies high-frequency current to the exciting coil 3 via the power supply contacts 3a and 3b. The control circuit 6 controls the high-frequency converter 5 based on the temperature detected by the temperature detection element 4 that detects the surface temperature of the fixing film 1. This causes the fixing film 1 to heat up by electromagnetic induction, and the surface temperature of the fixing film 1 is maintained and adjusted to a predetermined target temperature (approximately 150°C to 200°C).

[0020] (Guide 9 alignment correction operation) 5(a) and 5(b) are schematic diagrams showing the alignment correction operation of the guide 9. Fig. 5(a) shows a state in which the central axis Y9 of the guide 9 and the central axis Y10 of the stay 10 are misaligned with respect to the axis Y7 of the core metal 7a of the pressure roller 7. Fig. 5(b) shows a state in which the misalignment between the axis Y7 and the central axis Y9 of the guide 9 is corrected by the rotation of the guide 9 about the protrusion S.

[0021] As shown in FIG. 5(a), consider a state where the central axis Y10 of the stay 10 and the central axis Y9 of the guide 9 are misaligned with respect to the axis Y7 of the pressure roller 7 within the tolerance range due to the assembly accuracy during assembly of the fixing device A. In a state where the central axes Y10 and Y9 are misaligned with respect to the axis Y7 of the pressure roller 7, when the fixing film 1 is rotated by the driving of the pressure roller 7, a force is applied to the fixing film 1 in the Y-axis direction in addition to a force in the X-axis direction, which is the rotation direction. If the rotation continues in this state, the fixing film 1 shifts, and the end face of the fixing film 1 rubs strongly against the flange 13a or the flange 13b, which may reduce the durability of the fixing film 1. In addition, the rotation of the fixing film 1 may not be stable, and wrinkles may occur in the recording material P.

[0022] However, since the fixing device A is configured such that the guide 9 can rotate around the protrusion S as a fulcrum, the guide 9 rotates due to the sliding frictional force of the fixing film 1 as the fixing film 1 rotates. This causes the misalignment between the axis Y7 of the pressure roller 7 and the central axis Y9 of the guide 9 to be corrected to eliminate the misalignment, as shown in Fig. 5(b). As a result, the fixing film 1 rotates stably, and the force acting on the fixing film 1 in the Y-axis direction is also reduced.

[0023] Table 1 shows the results of evaluating the conveying state of the recording material P and the images when an misaligned fixing device was used to confirm the effect of this example, and images were formed on 100 sheets of recording material P in succession, with different types of recording material P.

[0024] As Comparative Example 1, a case where the guide 9 is fixed to the stay 10 so that it cannot be rotated like the device of this example is also shown. The types of recording material P are as follows. Paper type A (XEROX: Vitality, basis weight 75g) was used as plain paper, paper type B (HP: HP EcoFFICIENT Paper, basis weight 60g) was used as thin paper, and paper type C (HP(b)rochure Paper 150g Glossy, basis weight 150g) was used as glossy paper. In the image evaluation shown in the table, "◯" indicates a good image, and "×" indicates the occurrence of image defects such as streaks and uneven gloss. Regarding transportability, "◯" indicates good, and "×" indicates the occurrence of defects such as paper wrinkles.

[0025] [Table 1]

[0026] As shown in Table 1, in Comparative Example 1, when paper type A, which is plain paper, was used, 100 sheets were fed continuously and good images were obtained without any particular problems. In contrast, when 100 sheets of paper type B, such as thin paper, were fed, not only did wrinkles occur in the recording material P, but image defects also occurred in which streaks appeared in the image. Furthermore, when paper type C, such as glossy paper, was used, uneven gloss occurred on the left and right sides of the image.

[0027] On the other hand, in the fixing device A of this example, good images were obtained with all paper types. Since paper type B is thin paper, the paper has low rigidity and is easily affected by deformation of the fixing film 1 when the recording material P is transported. In comparative example 1, the rotation of the fixing film 1 was unstable due to the above-mentioned alignment deviation, and this caused differences in transportability between the left and right sides, resulting in wrinkles in the paper. It is believed that this caused the image to rub against the transport section in the image forming device, resulting in streaky marks on the image. Also, in comparative example 1, there was a difference in pressure distribution at both ends of the fixing nip portion N in the Y-axis direction, resulting in differences in fixability between the left and right sides of the image. As a result, it is believed that uneven gloss occurred with glossy paper with a noticeable gloss, such as paper type C.

[0028] As described above, in the fixing device A, the guide 9 is provided so as to be rotatable with respect to the stay 10, with the axis substantially perpendicular to the fixing nip portion N as a fulcrum. Then, as the fixing film 1 slides, the guide 9 rotates due to the force received from the fixing film 1, and the alignment with respect to the stay 10 is corrected. This makes it possible to provide a fixing device A in which the fixing film 1 rotates stably.

[0029] In the above description, the surface shapes of the guide 9 and the stay 10 are not particularly specified. However, for example, the material, surface shape, surface roughness, contact area, etc. of the guide 9 and the stay 10 may be adjusted so that the frictional force (friction coefficient) between the guide 9 and the stay 10 is smaller than the frictional force (friction coefficient) between the fixing film 1 and the guide 9. In addition to optimizing the shape and material, a lubricant (grease or oil) may be applied between the guide 9 and the stay 10 to adjust the frictional force. With this configuration, it becomes possible to more easily rotate the guide 9 with the protrusion S as a fulcrum by the force that the guide 9 receives from the fixing film 1 due to the rotation of the fixing film 1. This makes it possible to appropriately correct the alignment deviation.

[0030] In addition, the configuration that reduces the contact area between the guide 9 and the stay 10 can also suppress the transfer of heat from the guide 9 to the stay 10, thereby reducing the transfer of heat to components other than the recording material P, and making it possible to further reduce power consumption.

[0031] In this example, the guide 9 is provided with a protrusion S that fits into the opening of the stay 10, but the stay 10 may be provided with a protrusion S and the guide 9 with a recess. In short, it is sufficient that a protrusion serving as a fulcrum and an opening or recess for receiving the protrusion are provided on one side of the guide 9 and the other side of the stay 10. Also, the guide 9 may be supported rotatably by combining with another member. Furthermore, the guide 9 may be supported movably in a direction that corrects the alignment deviation by a movement other than rotation.

[0032] Example 2 In this example, the shape of the guide is different from that of Example 1. Specifically, the shape of the surface forming the fixing nip portion N is not flat, but is curved with a portion PP protruding toward the pressure roller 7 so that the pressure distribution of the fixing nip portion N in the recording material conveyance direction (X-axis direction) is greater on the downstream side than on the upstream side with respect to the center of the fixing nip portion N. By using such a shape, it is possible to improve the fixing performance of the toner, and also to enable fixing at a lower temperature, thereby reducing power consumption.

[0033] FIG. 6(a) shows the cross-sectional shape of the guide 19 of the second embodiment, which is a substitute for the guide 9 of the first embodiment. FIG. 6(b) shows the pressure distribution of the fixing nip N of the second embodiment. For comparison, the cross-sectional shape and pressure distribution of the guide 9 used in the first embodiment are also shown. Point O in the figure is the center in the nip width direction. The surface of the guide 19 of the second embodiment, on which the fixing film 1 slides, has a curved surface with a curvature radius of Rs. As shown in FIG. 6(a), when the pressure roller 7 is pressed against the guide 19 of the second embodiment via the (fixing film 1), the pressure roller 7 takes a shape as shown by the dashed line, and the fixing nip N is formed. If the radius of the pressure roller 7 is Rp, the relationship Rs≧Rp holds.

[0034] When the guide 9 of the first embodiment is used, the shape of the surface forming the fixing nip N is flat, so that the pressure distribution in the fixing nip N is symmetrical between the upstream side and the downstream side with the peak at point O. On the other hand, when the guide 19 of the second embodiment is used, the pressure peak is located downstream of point O.

[0035] For the purpose of explanation, Figure 7(a) shows the pressure distribution of the fixing nip portion N in the "longitudinal center," "longitudinal left," and "longitudinal right" portions in the Y-axis direction (longitudinal direction of the fixing device A) when the guide 19 is fixed with the alignment shifted.

[0036] 7(a), in the "longitudinal left" portion, the pressure peak portion on the downstream side in the conveying direction of the recording material P is smaller than the peak portion in the "longitudinal center" portion, and conversely, in the "longitudinal right" portion, the pressure peak portion on the downstream side is larger than the peak portion in the "longitudinal center" portion. This is because, due to misalignment, in the "longitudinal right" portion, part PP bites into the nip center in the conveying direction, and in the "longitudinal left" portion, part PP falls outside the area of ​​the fixing nip N.

[0037] On the other hand, when the guide surface is flat like the guide 9 and there is a pressure peak in the center in the transport direction, the part corresponding to the peak is unlikely to fall out of the fixing nip N even if the alignment is misaligned. Therefore, the difference in pressure distribution at both ends in the Y-axis direction is small, and the impact of alignment misalignment is small. In this way, in a configuration in which the peak value of the pressure distribution is shifted from the center of the fixing nip N in the transport direction, the peak may fall out of the fixing nip N due to alignment misalignment. This tends to result in a configuration in which the pressure distribution differs between the left and right in the longitudinal direction. If the pressure distribution differs between the left and right in the longitudinal direction, uneven fixing of the toner image occurs.

[0038] In contrast, the guide 19 of the second embodiment is provided to be rotatable about the protrusion S relative to the stay 10, so that the alignment is corrected and the pressure distribution can be made uniform over the longitudinal direction of the fixing device A. This makes it possible to obtain a good image with uniform fixability.

[0039] In addition, by making the surface of the guide 19 connected to the portion PP in the conveying direction (X direction) curved as shown in Fig. 6(a) and providing an alignment correction mechanism as in this embodiment, the alignment correction operation can be easily performed by simply applying pressure from the pressure springs 11a and 11b. This is because the curved shape of the guide 19 can easily follow the axis of the pressure roller 7.

[0040] In this embodiment, the protrusion S of the guide 19 is disposed at the center of the fixing nip N in the X-axis direction, but the position of the protrusion S may be configured to be located upstream of the center of the fixing nip N in the X-axis direction. In this case, the frictional force against the guide 19 caused by the rotation of the fixing film 1 acts toward the downstream side in the conveying direction of the recording material P. Therefore, the guide 19 and the fixing film 1 tend to follow the rotation direction, and the rotation behavior becomes more stable. As a result, even if there is a deviation in alignment, correction is reliably performed. This is also true for the guide 9 shown in the first embodiment.

[0041] Also, by configuring the position of the portion PP of the guide 19 forming the pressure peak and the position of the protrusion S serving as the fulcrum to be different in the X-axis direction, there is an effect that the rotation of the guide 19 for alignment correction becomes smoother. The reason for this is that, due to the relationship between the force point, fulcrum, and point of action, a high frictional force due to sliding resistance acts at the portion PP away from the fulcrum, making it easier for the guide 19 to rotate. The position of the fulcrum may be configured to be provided downstream in the transport direction from the position of the portion PP of the guide 19, or may be configured to be provided at a position of the guide 19 corresponding to a position outside the fixing nip N in the transport direction.

[0042] Also, a pressure distribution peak can be formed on the downstream side within the fixing nip N by configuring the surface of the guide that forms the fixing nip N to be flat and offsetting the center of this guide in the X-axis direction upstream in the transport direction with respect to the core metal 7a of the pressure roller 7 or by configuring the guide to be inclined. Even in this case, alignment can be corrected by configuring the guide to be rotatable.

[0043] In the above-mentioned first and second embodiments, the fixing device A is described as being configured to heat the fixing film 1 by electromagnetic induction. However, the alignment correction mechanism as in the first and second embodiments may be applied to a fixing device in which a plate-shaped heater is brought into contact with the inner surface of the fixing film, and the fixing nip portion N is formed via the fixing film by this heater and a pressure roller. The heater holder (film guide) that holds the heater may be configured to rotate as in the first and second embodiments. Also, the alignment correction mechanism as in the first and second embodiments may be applied to a fixing device in which a halogen heater is disposed in the internal space of the fixing film, and the fixing film is heated by the radiant heat of the halogen heater. [Explanation of symbols]

[0044] 1 Fixing film 7 Pressure roller 9 Guide 10 Stay

Claims

1. A cylindrical flexible rotor; a guide member disposed in an internal space of the rotor along a longitudinal direction of the rotor, the guide member contacting an inner surface of the rotor to guide the rotation of the rotor; a holding member disposed in an internal space of the rotating body along a longitudinal direction of the rotating body and configured to hold the guide member along the longitudinal direction; a roller that forms a fixing nip portion together with the guide member via the rotating body; A fixing device that fixes a toner image formed on a recording material to the recording material while nipping and conveying the recording material at the fixing nip portion, The fixing device is characterized in that the guide member is rotatably mounted relative to the holding member, with an axis that is approximately perpendicular to the fixing nip portion as a fulcrum, and the guide member rotates due to the force received from the rotating body as the rotating body slides, thereby correcting the alignment with the holding member.

2. 2. The fixing device according to claim 1, wherein the protrusion serving as the fulcrum and an opening or recess for receiving the protrusion are provided on one of the guide member and the holding member and the other of the guide member and the holding member.

3. 3. The fixing device according to claim 1, wherein the holding member is a metal stay for reinforcing the guide member.

4. 4. The fixing device according to claim 1, wherein the surface of the guide member that forms the fixing nip portion is curved having a portion that protrudes toward the roller so that the pressure distribution of the fixing nip portion in the recording material transport direction is greater downstream than upstream with respect to the center of the fixing nip portion.

5. 5. The fixing device according to claim 4, wherein the radius of curvature of the curved surface of the guide member is Rs and the radius of the roller is Rp, and the relationship Rs≧Rp is satisfied.

Images