JP7745828B2 - Belt driving device, heating device, fixing device and image forming apparatus - Google Patents

Description

The present invention relates to a belt drive device, a heating device, a fixing device, and an image forming apparatus.

One example of a belt drive device installed in an image forming device such as a copier or printer is a fixing device equipped with an endless belt.

In such fixing devices, when the belt rotates, it slides against a sliding member such as a heater located inside the belt, generating sliding resistance (friction) between the belt and the sliding member. The sliding resistance that occurs between the belt and the sliding member accelerates belt wear. Therefore, a common measure to reduce sliding resistance is to apply a lubricant such as grease or oil to the inner surface of the belt.

However, the amount of lubricant between the belt and the sliding member tends to decrease over time, and as the amount of lubricant decreases, it becomes difficult to achieve a satisfactory sliding resistance reduction effect. In response to this problem, Patent Document 1 (JP 2018-194696 A) proposes a configuration in which a lubricant reservoir is formed between a plate-shaped member provided on the inside of the belt and the inner surface of the belt to store lubricant, thereby suppressing belt wear caused by a lack of lubricant over the long term. Patent Document 1 also proposes a method of rotating the belt in the opposite direction to its normal rotation direction (during the fixing process) in order to return lubricant that accumulates under the belt due to the influence of gravity to the lubricant reservoir.

It has been discovered that the following phenomenon occurs when a belt is rotated in the opposite direction from its normal rotation direction. As shown in Figure 23, the surface of belt 600 typically has numerous microprotrusions 601 that constitute the belt's 600 surface roughness. Therefore, when belt 600 rotates in one direction, as indicated by the arrow in Figure 23, the microprotrusions 601 wear away as they slide against sliding member 700, forming tips of the microprotrusions 601 that point in the opposite direction to the belt's 600 rotation direction (the direction of movement of the belt surface). Then, as shown in Figure 24, when belt 600 rotates in the opposite direction, the tips of the microprotrusions 601 on the belt surface bend in the opposite direction due to the sliding resistance with sliding member 700. This phenomenon becomes more pronounced the longer the belt rotates in the opposite direction. Therefore, if the belt rotates longer in the opposite direction, the tips of the microprotrusions may bend in the opposite direction and chip, potentially accelerating belt wear.

Patent Document 1 above proposes a method of rotating the belt in the opposite direction to return the accumulated lubricant to the lubricant reservoir, but does not consider any of the adverse effects of rotating the belt in the opposite direction.

In order to solve the above-mentioned problems, the present invention provides a belt drive device comprising: a flexible endless belt; a sliding member arranged to slide on the inner peripheral surface of the belt; an opposing member that contacts the sliding member via the belt and forms a nip portion between the belt and the opposing member; and a lubricant interposed between the inner peripheral surface of the belt and the sliding member, wherein the belt is rotatable in a direction opposite to the rotation direction when a sheet is passed through the nip portion, and the sliding member is configured to move in the direction of belt surface movement in the nip portion as the belt rotates in the opposite direction, and is characterized in that when the temperature of the belt is 100°C or less, or after one minute or more has elapsed since heating of the belt was stopped in an unheated state, the belt rotates in the opposite direction within a range of five rotations or more that is equal to or greater than the movement distance of the sliding member associated with the rotation in the opposite direction.

This invention effectively prevents accelerated belt wear.

1 is a schematic diagram illustrating the configuration of an image forming apparatus according to an embodiment of the present invention. FIG. 2 is a schematic diagram illustrating the configuration of a fixing device according to the present embodiment. FIG. 2 is a cross-sectional view of the fixing belt according to the embodiment. FIG. 2 is a plan view of the heater according to the embodiment. FIG. 10 is a diagram illustrating a state in which the fixing belt is rotated in a normal rotation direction. FIG. 10 is a diagram showing a state in which the fixing belt is rotated in the opposite direction. 10 is a diagram showing the results of examining the relationship between the reverse rotation speed and rotation torque when the fixing belt is rotated in the reverse direction. 10A and 10B are diagrams illustrating an example in which the reverse rotation operation of the fixing belt is controlled based on the rotation torque. 10A and 10B are diagrams illustrating an example in which the reverse rotation operation of the fixing belt is controlled based on any one of the number of sheets of paper passed, the rotation distance, and the elapsed time. FIG. 2 is a cross-sectional view of a fixing belt that does not have an elastic layer. FIG. 10 is a diagram showing an example in which resistance heating elements are arranged continuously in the longitudinal direction of the heater. FIG. 10 is a diagram showing a configuration of a fixing device different from that of the above embodiment. FIG. 10 is a diagram showing a configuration of a fixing device different from that of the above embodiment. FIG. 10 is a diagram showing a configuration of a fixing device different from that of the above embodiment. FIG. 10 is a diagram showing a configuration of a fixing device different from that of the above embodiment. FIG. 10 is a diagram showing a configuration of an image forming apparatus different from that of the above embodiment. FIG. 17 is a diagram showing the configuration of the fixing device shown in FIG. 16 . FIG. 18 is a plan view of the heater shown in FIG. 17. FIG. 18 is a perspective view of the heater and heater holder shown in FIG. 17. 18A and 18B are diagrams showing a method of attaching a connector to the heater shown in FIG. 17. 17 is a diagram showing the arrangement of a temperature sensor and a thermostat provided in the fixing device shown in FIG. 16. FIG. 21 shows a groove portion of the flange shown in FIG. 20. FIG. 10A and 10B are diagrams showing the state of minute protrusions on the belt surface when the belt is rotated in the normal rotation direction. FIG. 10 is a diagram showing the state of minute protrusions on the belt surface when the belt is rotated in the opposite direction.

The present invention will now be described with reference to the accompanying drawings. In each of the drawings used to explain the present invention, components such as parts and components having the same function or shape will be assigned the same reference numerals as far as possible to distinguish them, and once they have been described, their description will be omitted.

Figure 1 is a schematic diagram of an image forming apparatus according to one embodiment of the present invention. In this specification, "image forming apparatus" includes printers, copiers, facsimiles, printing machines, and multifunction machines that combine two or more of these. Furthermore, "image formation" as used in the following description refers not only to the formation of meaningful images such as characters and figures, but also to the formation of meaningless images such as patterns. First, the overall configuration and operation of the image forming apparatus according to this embodiment will be described with reference to Figure 1.

As shown in FIG. 1, the image forming apparatus 100 according to this embodiment includes an image forming unit 200 that forms an image on a sheet-like recording medium such as paper, a fixing unit 300 that fixes the image on the recording medium, a recording medium supply unit 400 that supplies the recording medium to the image forming unit 200, and a recording medium discharge unit 500 that discharges the recording medium outside the apparatus.

The image forming unit 200 is equipped with four process units 1Y, 1M, 1C, and 1Bk as imaging units, an exposure device 6 that forms an electrostatic latent image on the photoreceptor 2 provided in each process unit 1Y, 1M, 1C, and 1Bk, and a transfer device 8 that transfers the image to a recording medium.

Each process unit 1Y, 1M, 1C, and 1Bk has basically the same configuration, except that it contains toner (developer) of a different color: yellow, magenta, cyan, or black, which corresponds to the color separation components of a color image. Specifically, each process unit 1Y, 1M, 1C, and 1Bk includes a photoconductor 2 as an image carrier that carries an image on its surface, a charging member 3 that charges the surface of the photoconductor 2, a developing device 4 that supplies toner as developer to the surface of the photoconductor 2 to form a toner image, and a cleaning member 5 that cleans the surface of the photoconductor 2.

The transfer device 8 includes an intermediate transfer belt 11, a primary transfer roller 12, and a secondary transfer roller 13. The intermediate transfer belt 11 is an endless belt member stretched over multiple support rollers. Four primary transfer rollers 12 are provided inside the intermediate transfer belt 11. Each primary transfer roller 12 contacts each photoconductor 2 via the intermediate transfer belt 11, forming a primary transfer nip between the intermediate transfer belt 11 and each photoconductor 2. The secondary transfer roller 13 contacts the outer peripheral surface of the intermediate transfer belt 11, forming a secondary transfer nip.

The fixing section 300 is provided with a fixing device 20. The fixing device 20 includes a fixing belt 21, which is an endless belt, and a pressure roller 22, which serves as an opposing member facing the fixing belt 21. The fixing belt 21 and the pressure roller 22 come into contact with each other on their outer circumferential surfaces, forming a nip portion (fixing nip).

The recording medium supply unit 400 is provided with a paper feed cassette 14 that stores paper P as a recording medium, and a paper feed roller 15 that feeds paper P from the paper feed cassette 14. Hereinafter, "recording medium" will be described as "paper," but "recording medium" is not limited to paper (paper). "Recording medium" includes not only paper (paper), but also overhead projector sheets or fabric, metal sheets, plastic films, or prepreg sheets made by pre-impregnating carbon fibers with resin. Furthermore, "paper" includes not only plain paper, but also cardboard, postcards, envelopes, thin paper, coated paper (coated paper, art paper, etc.), tracing paper, etc.

The recording medium ejection section 500 is provided with a pair of ejection rollers 17 that eject paper P outside the image forming device, and an ejection tray 18 on which paper P ejected by the ejection rollers 17 is placed.

Next, the printing operation of the image forming apparatus 100 according to this embodiment will be described with reference to Figure 1.

When the printing operation begins in the image forming device 100, the photosensitive elements 2 of each process unit 1Y, 1M, 1C, and 1Bk and the intermediate transfer belt 11 of the transfer device 8 begin to rotate. The paper feed roller 15 also begins to rotate, and paper P is fed out of the paper feed cassette 14. The fed paper P comes to a standstill by contacting a pair of timing rollers 16, and transport of the paper P is temporarily halted until the image to be transferred onto the paper P is formed.

In each process unit 1Y, 1M, 1C, and 1Bk, the surface of the photoconductor 2 is first charged to a uniform high potential by the charging member 3. Next, the exposure device 6 exposes the surface (charged surface) of each photoconductor 2 based on the image information of the original document read by the document reader or the print image information instructed to be printed from a terminal. This reduces the potential of the exposed area, forming an electrostatic latent image on the surface of each photoconductor 2. The development device 4 then supplies toner to this electrostatic latent image, forming a toner image on each photoconductor 2. As the toner images formed on each photoconductor 2 reach the primary transfer nip (position of the primary transfer roller 12) as each photoconductor 2 rotates, they are transferred sequentially onto the rotating intermediate transfer belt 11 so that they overlap one another. In this way, a full-color toner image is formed on the intermediate transfer belt 11. In addition, in the image forming apparatus 100, it is possible to form a monochrome image using any one of the process units 1Y, 1M, 1C, and 1Bk, or to form a two- or three-color image using any two or three of the process units. After the toner image is transferred from the photoreceptor 2 to the intermediate transfer belt 11, residual toner and the like on each photoreceptor 2 is removed by a cleaning member 5.

The toner image transferred onto the intermediate transfer belt 11 is transported to the secondary transfer nip (the position of the secondary transfer roller 13) as the intermediate transfer belt 11 rotates, and is transferred onto the transported paper P by the timing roller 16. The paper P is then transported to the fixing device 20, where the toner image on the paper P is heated and pressed by the fixing belt 21 and pressure roller 22, thereby fixing the toner image to the paper P. The paper P is then transported to the recording medium ejection section 500, and is ejected onto the paper ejection tray 18 by the paper ejection roller 17. This completes the printing process.

Next, the configuration of the fixing device according to this embodiment will be described in detail with reference to Figure 2.

As shown in FIG. 2, the fixing device 20 according to this embodiment includes a fixing belt 21, a pressure roller 22, a heater 23, a heater holder 24, a stay 25, a guide member 26, a temperature sensor 27, and the like.

The fixing belt 21 is a rotating body (fixing member) that comes into contact with the unfixed toner-carrying surface of the paper P to fix the unfixed toner (unfixed image) to the paper P, and is composed of a flexible, endless belt. The diameter of the fixing belt 21 is set to, for example, 15 to 120 mm. In this embodiment, the inner diameter of the fixing belt 21 is set to 25 mm.

As shown in FIG. 3, the fixing belt 21 is, for example, layered from the inner circumferential surface to the outer circumferential surface, with a substrate 210, an elastic layer 211, and a release layer 212, with a total thickness of 1 mm or less. The substrate 210 is 30 to 50 μm thick and is made of a metal material such as nickel or stainless steel, or a resin material such as polyimide. The elastic layer 211 is 100 to 300 μm thick and is made of a rubber material such as silicone rubber, foamed silicone rubber, or fluororubber. The fixing belt 21 includes the elastic layer 211, which prevents minute irregularities from forming on the surface of the fixing belt 21 at the nip, facilitating uniform heat transfer to the toner image on the paper P. The release layer 212 is 10 to 50 μm thick and is made of a material such as PFA (tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer), PTFE (polytetrafluoroethylene), polyimide, polyetherimide, or PES (polyether sulfide). The fixing belt 21 has a release layer 212, which ensures release (peelability) of the toner (toner image).

As shown in FIG. 2, the pressure roller 22 is an opposing member disposed opposite the outer peripheral surface of the fixing belt 21. The pressure roller 22 contacts the heater 23 via the fixing belt 21, forming a nip N between the pressure roller 22 and the fixing belt 21.

The pressure roller 22 is a roller with an outer diameter of, for example, 25 mm, and includes a hollow iron core 220, an elastic layer 221 provided on the outer surface of the core 220, and a release layer 222 provided on the outer surface of the elastic layer 221. The elastic layer 221 is, for example, 3.5 mm thick and made of silicone rubber or the like. The release layer 222 is, for example, about 40 μm thick and made of fluororesin or the like.

The heater 23 is a planar or plate-shaped heat source that heats the fixing belt 21 from the inside. The heater 23 extends longitudinally across the length of the fixing belt 21 (the paper width direction, which intersects with the paper transport direction) and is positioned so that it comes into contact with the inner surface of the fixing belt 21.

As shown in FIG. 4 , the heater 23 according to this embodiment is composed of a plate-shaped substrate 55, multiple resistance heating elements 56 provided on the substrate 55, and an insulating layer 57 covering each resistance heating element 56. The multiple resistance heating elements 56 are spaced apart along the longitudinal direction of the substrate 55 (heater 63). The gap between adjacent resistance heating elements 56 is preferably 0.2 mm or more, and more preferably 0.4 mm or more, from the viewpoint of ensuring insulation between the resistance heating elements 56. Furthermore, if the gap between adjacent resistance heating elements 56 is too large, a temperature drop is likely to occur in the gap. Therefore, from the viewpoint of suppressing temperature unevenness along the longitudinal direction, the gap is preferably 5 mm or less, and more preferably 1 mm or less. Note that multiple resistance heating elements 56 may be arranged along the short side (direction perpendicular to the longitudinal direction) of the surface of the substrate 55 on which the resistance heating elements 56 are arranged.

As shown in FIG. 4, multiple electrode portions 58 and multiple power supply lines 59 are provided on the surface of the substrate 55 on which each resistance heating element 56 is provided. Each resistance heating element 56 is connected in parallel via the power supply lines 59 to each electrode portion 58 provided at both longitudinal ends of the substrate 55. Each resistance heating element 56 and each power supply line 59 is covered with an insulating layer 57 to ensure insulation. Meanwhile, each electrode portion 58 is exposed and not covered by the insulating layer 57 so that a connector serving as a power supply terminal can be connected. Connecting a connector to each electrode portion 58 enables power to be supplied from the power source to each resistance heating element 56. In this state, power is supplied to each resistance heating element 56, causing each resistance heating element 56 to generate heat.

The substrate 55 is made of a material with excellent heat resistance and insulation, such as ceramics such as alumina or aluminum nitride, glass, mica, or polyimide. Alternatively, the substrate 55 may be made of a metal (conductive material) such as stainless steel (SUS), iron, or aluminum, with an insulating layer formed on top. In particular, if the substrate 55 is made of a highly thermally conductive material such as aluminum, copper, silver, graphite, or graphene, the heater 23 can achieve improved thermal uniformity and image quality. The insulating layer 57 is made of a material with excellent heat resistance and insulation, such as ceramics such as alumina or aluminum nitride, glass, mica, or polyimide. The resistive heating element 56 is formed by applying a paste containing silver palladium (AgPd) and glass powder to the surface of the substrate 55 by screen printing or the like, and then firing the substrate 55. Alternatively, each resistive heating element 56 can be made of a resistive material such as silver alloy (AgPt) or ruthenium oxide (RuO ₂ ). The electrode portion 58 and the power supply line 59 are formed by screen printing silver (Ag) or silver palladium (AgPd).

As shown in FIG. 2, in this embodiment, each resistance heating element 56 is provided on the surface of the substrate 55 facing the pressure roller 22 (the nip portion N side), but each resistance heating element 56 may also be provided on the opposite surface. In this case, since the heat from each resistance heating element 56 is transferred to the fixing belt 21 via the substrate 55, it is preferable that the substrate 55 be made of a material with high thermal conductivity, such as aluminum nitride.

The heater holder 24 is disposed inside the fixing belt 21 and is a holding member that holds the heater 23. Because the heater holder 24 is prone to becoming hot due to the heat from the heater 23, it is preferable that it be made of a heat-resistant material. For example, if the heater holder 24 is made of a low-thermal-conductivity, heat-resistant resin such as LCP or PEEK, the heat resistance of the heater holder 24 is ensured while heat transfer from the heater 23 to the heater holder 24 is suppressed, allowing the fixing belt 21 to be heated efficiently.

The stay 25 is a support member that supports the heater holder 24. The stay 25 supports the surface of the heater holder 24 opposite the surface facing the pressure roller 22 along the length of the fixing belt 21, thereby preventing the heater holder 24 from bending due to the pressure of the pressure roller 22 and forming a nip N of uniform width between the fixing belt 21 and the pressure roller 22. To ensure its rigidity, the stay 25 is preferably made of an iron-based metal material such as SUS or SECC.

The guide member 26 is a member that guides the fixing belt 21 from the inside. The guide member 26 has an arc-shaped cross section that follows the inner circumferential surface of the fixing belt 21, and is disposed upstream and downstream of the heater 23 in the rotation direction of the fixing belt 21 (the direction of the arrow in Figure 2). In this embodiment, the guide member 26 is configured integrally with the heater holder 24, but it may also be configured separately.

The temperature sensor 27 is a temperature detection element that detects the temperature of the heater 23. Known temperature sensors such as a thermopile, thermostat, thermistor, or NC sensor can be used as the temperature sensor 27. In this embodiment, a contact-type temperature sensor is used that detects the temperature by contacting the surface of the heater 63 opposite the pressure roller 22 side. Furthermore, the temperature sensor 27 is not limited to a contact-type temperature sensor; it may also be a non-contact-type temperature sensor that is positioned in a non-contact manner with the heater 63 and detects the ambient temperature near the heater 63.

The fixing device 20 according to this embodiment operates as follows.

As shown in FIG. 2, when the pressure roller 22 is driven to rotate, the driving force is transmitted to the fixing belt 21, causing the fixing belt 21 to rotate. The fixing belt 21 is then heated by the heater 23. At this time, the temperature of the heater 23 is detected by the temperature sensor 27, and the amount of heat generated by the heater 23 is controlled based on the detected temperature, heating the fixing belt 21 until it reaches a temperature at which an image can be fixed (fixing temperature), and maintaining that temperature. When paper P carrying an unfixed image is transported between the fixing belt 21 and the pressure roller 22 (nip N), the paper P is heated and pressurized by the fixing belt 21 and the pressure roller 22, and the unfixed image is fixed to the paper P.

In a configuration in which the heater 23 contacts the inner circumferential surface of the fixing belt 21, as in this embodiment, the fixing belt 21 slides against the heater 23 as the fixing belt 21 rotates. For this reason, in this embodiment, a lubricant such as grease or oil is interposed between the inner circumferential surface of the fixing belt 21 and the surface of the heater 23 that contacts it (the surface on the pressure roller 22 side), thereby reducing the sliding resistance that occurs between the fixing belt 21 and the heater 23.

However, lubricant generally evaporates gradually and flows out from between the fixing belt and heater as the fixing belt rotates, so the amount of lubricant between the fixing belt and heater tends to decrease over time. Furthermore, when the amount of lubricant decreases, it is no longer possible to achieve a satisfactory sliding resistance reduction effect, which raises concerns that the rotational torque of the fixing belt may increase and the fixing belt may become more susceptible to wear.

To address this issue, one method for maintaining the effectiveness of the lubricant over a long period of time is to provide a plate-shaped member to form a lubricant reservoir on the inner surface of the belt, as proposed in Patent Document 1 above. However, this method requires the addition of additional components within the fixing belt, which increases costs and hinders device miniaturization. Another method would be to add a mechanism to replenish the lubricant, but this would complicate the mechanism and increase costs. Therefore, it would be preferable to be able to maintain the effectiveness of the lubricant without adding new mechanisms or components.

Therefore, when a fixing device that had reached the end of its life was examined, it was found that there was unused lubricant remaining between the fixing belt and the heater. Specifically, as shown in FIG. 5, it was found that unused lubricant 50 remained upstream and downstream of the area A where the heater 23 and the fixing belt 21 contact (hereinafter referred to as the "inner nip") in the sheet passage direction C (the sheet passage direction, or the belt surface movement direction at the nip) in which the paper passes through the nip N. That is, in this embodiment, the width Wa of the inner nip in the sheet passage direction C is smaller than the width Wb of the entire plane of the heater 23 on the pressure roller 22 side in the sheet passage direction C. Therefore, a gap S is formed between the heater 23 and the fixing belt 21 upstream and downstream of the inner nip A, and the lubricant 50 that leaks out from the inner nip A accumulates in the gap S. Furthermore, as shown in FIG. 5, in a configuration in which the heater 23 is housed in a recess 24a of the heater holder 24, and the upstream and downstream edges 241 of the recess 24a in the paper feed direction C protrude further toward the pressure roller 22 than the surface of the heater 23 facing the pressure roller 22 in order to prevent the fixing belt 21 from contacting the edge of the heater 23, the gap S described above is particularly likely to occur due to the step between the surface of the heater 23 and the heater holder 24.

In this way, unused lubricant remains in the gap between the fixing belt and the heater, and if this remaining lubricant can be effectively utilized, the effectiveness of the lubricant can be sustained.

It is known that the trajectory of the fixing belt when it rotates differs between the upstream side (hereinafter referred to as the "nip entrance side") and downstream side (hereinafter referred to as the "nip exit side") of the nip portion in the paper feed direction. Specifically, as shown in FIG. 5 , when the fixing belt 21 rotates, a force acts on the fixing belt 21 at the nip entrance side E1 (upstream side in the paper feed direction C) to draw the fixing belt 21 into the nip portion N, causing the fixing belt 21 to rotate along a trajectory that brings it close to or into contact with the surfaces of the heater 23 and the heater holder 24. On the other hand, at the nip exit side E2 (downstream side in the paper feed direction C), the fixing belt 21 rotates along a trajectory that moves away from the surfaces of the heater 23 and the heater holder 24 and bulges outward. Therefore, the lubricant 50 that accumulates in each gap S is formed following this trajectory of the fixing belt 21 as it rotates. As long as the fixing belt 21 rotates along a similar trajectory, the lubricant 50 will not generally adhere to the fixing belt 21.

However, if the trajectory of the fixing belt 21 could be changed, it would be possible to cause the accumulating lubricant 50 to adhere to the fixing belt 21. For this reason, in an embodiment of the present invention, the fixing belt 21 is rotated in the opposite direction to its normal rotation direction. That is, as shown in FIG. 6, the fixing belt 21 is rotated in the opposite direction D2 to the normal rotation direction (rotation direction D1 shown in FIG. 5) when passing paper through the nip N. When the fixing belt 21 is rotated in the opposite direction D2 in this way, contrary to the case shown in FIG. 5 above, the fixing belt 21 bulges outward on the nip entrance side E1, and on the nip exit side E2, the fixing belt 21 approaches or comes into contact with the surfaces of the heater 23 and the heater holder 24.

Therefore, as shown in Figure 6, when the fixing belt 21 is rotated in the opposite direction D2, the fixing belt 21 approaches the surface of the heater 23, particularly on the nip exit side E2, so that the inner surface of the fixing belt 21 comes into contact with the lubricant 50 remaining on the nip exit side E2.

Also, as shown in FIG. 6, in this embodiment, when the fixing belt 21 rotates in the opposite direction D2, friction between the heater 23 and the fixing belt 21 causes the heater 23 to move toward the nip entrance side E1 (the direction of belt surface movement in the nip N) within the recess 24a of the heater holder 24. That is, in this embodiment, to avoid interference with the heater holder 24 due to thermal expansion of the heater 23, as shown in FIG. 6, the width W1 of the recess 24a in the paper passage direction C is made larger than the width W2 of the heater 23 in the paper passage direction C, and there is a gap (backlash) in the paper passage direction C between the heater 23 and the recess 24a.

For this reason, during normal rotation as shown in FIG. 5, the heater 23 moves toward the nip exit side E2 as the fixing belt 21 rotates, and abuts against the side surface 24b of the recess 24a on the nip exit side E2. In contrast, as shown in FIG. 6, when the fixing belt 21 rotates in the opposite direction D2, the heater 23 moves toward the nip entrance side E1 as the fixing belt 21 rotates in the opposite direction D2, and abuts against the side surface 24c of the recess 24a on the nip entrance side E1. At this time, the lubricant 50 remaining on the surface of the heater 23 moves as the heater 23 moves, and the lubricant 50 on the nip exit side E2 in particular moves to a position where it is more likely to come into contact with the inner circumferential surface of the fixing belt 21.

In this manner, in this embodiment, by rotating the fixing belt 21 in the direction D2 opposite to its normal rotation direction, the rotational trajectory of the fixing belt 21 can be changed to one that makes it easier for the fixing belt 21 to come into contact with the lubricant 50 on the heater 23, and the position of the heater 23 can be changed to a position where the lubricant 50 can easily come into contact with the fixing belt 21. This allows the lubricant 50 that accumulates on the nip exit side E2 above the heater 23 to adhere to the inner surface of the fixing belt 21. The lubricant 50 that has adhered to the inner surface of the fixing belt 21 can then be supplied to the inner nip A between the fixing belt 21 and the heater 23 as the fixing belt 21 moves in the opposite direction D2.

As described above, in this embodiment, unused and accumulated lubricant 50 can be supplied to the inner nip A between the fixing belt 21 and heater 23 and put to effective use, thereby making it possible to suppress an increase in rotational torque and wear on the fixing belt that accompanies a decrease in lubricant over the long term. Furthermore, in this embodiment, lubricant can be supplied to the inner nip A without the need to add a new mechanism or part, making it possible to suppress an increase in rotational torque and wear on the fixing belt at low cost.

In this embodiment, to avoid the adverse effects of rotating the fixing belt in the reverse direction, the reverse rotation of the fixing belt is controlled as follows. That is, if the rotation distance of the fixing belt in the reverse direction becomes long, as shown in Figure 24, the tips of the minute protrusions 601 on the belt surface will bend in the opposite direction to normal rotation, accelerating belt wear and increasing rotational torque as wear progresses. For this reason, in this embodiment, the rotation distance of the fixing belt in the reverse direction is limited within a certain range.

First, based on Figure 7, we will explain the relationship between the reverse rotation speed when the fixing belt rotates in the opposite direction to its normal rotation direction and the rotational torque generated according to the reverse rotation speed. The graphs R0, R1, R5, and R10 in Figure 7 show the rotational torque of the fixing belt when the reverse rotation speed of the fixing belt is 0, 1, 5, and 10 times.

As shown in Figure 7, when the number of reverse rotations of the fixing belt was zero (graph R0), the rotational torque increased most significantly as the number of sheets passed through the nip increased. In this case, because the fixing belt did not reverse, the lubricant supply to the inner nip due to the reverse rotation described above was not performed. As a result, the amount of lubricant decreased over time as the number of sheets passed increased, resulting in an increase in rotational torque. In contrast, when the number of reverse rotations was one (graph R1) and five (graph R5), the supply of lubricant to the inner nip due to the reverse rotation of the fixing belt was suppressed, resulting in a reduction in the increase in rotational torque. In particular, when the number of reverse rotations was one, the increase in rotational torque was most effectively suppressed. In contrast, when the number of reverse rotations was ten (graph R10), the results were nearly identical to when the fixing belt did not reverse (graph R0), despite the fact that it rotated in the opposite direction. This is thought to be due to the fact that the longer the distance the fixing belt rotated in the opposite direction, the belt surface wore out, resulting in an increase in rotational torque.

Based on the above results, in this embodiment, the number of reverse rotations of the fixing belt is limited to five rotations or less (five rotation distances or less), which is a good way to suppress increases in rotational torque. In this way, by limiting the number of reverse rotations of the fixing belt to five or less, it is possible to suppress the accelerated wear of the fixing belt and the increase in rotational torque that accompanies an increase in the number of reverse rotations. As a result, the occurrence of wear scratches on the heater is also suppressed, making it possible to avoid a decrease in image quality due to such wear scratches. Furthermore, if the number of reverse rotations of the fixing belt is limited to one rotation or less (one rotation distance or less), it is possible to more effectively suppress accelerated wear of the fixing belt and the increase in rotational torque, and further improvements in the durability and reliability of the fixing belt can be expected.

As such, in this embodiment, the upper limit of the number of reverse rotations of the fixing belt is set to five, preferably one or less, thereby effectively suppressing accelerated wear of the fixing belt and an increase in rotational torque. However, the rotation distance of the fixing belt during reverse rotation must be equal to or greater than the heater's travel distance during reverse rotation. That is, as shown in FIG. 6 , rotating the fixing belt 21 in the opposite direction D2 moves the heater 23 toward the nip entrance side E1, moving the lubricant 50 on the nip exit side E2 to a position where it is more likely to come into contact with the fixing belt 21. Therefore, the rotation distance of the fixing belt during reverse rotation is set to be equal to or greater than the heater's travel distance during this time. Therefore, the rotation distance of the fixing belt during reverse rotation is set to be equal to or greater than the heater's travel distance during the reverse rotation of the fixing belt 21, but equal to or less than five rotations. Note that the "distance traveled by the heater as the fixing belt rotates in the reverse direction" refers to the distance corresponding to the gap L between the heater 23 and the opposite side surface (the side surface on the nip entrance side E1) when the heater 23 is in contact with the side surface 24b on the nip exit side E2 of the recess 24a (the state shown in Figure 5).

As described above, in this embodiment, by rotating the fixing belt in reverse for a distance equal to or greater than the heater travel distance associated with the reverse rotation, but no more than five rotations, it is possible to supply accumulating lubricant between the heater and the fixing belt while suppressing the adverse effects associated with reverse rotation. As a result, in this embodiment, it is possible to suppress an increase in rotational torque and wear on the fixing belt over the long term, thereby improving the durability and reliability of the fixing belt.

It is preferable that the rotation speed of the fixing belt during reverse rotation be slower than the rotation speed during normal operation (when paper is passed through the nip N). By slowing the rotation speed during reverse rotation, the rotational torque is reduced and the load on the heater is also reduced, making it possible to more effectively suppress wear due to reverse rotation.

Furthermore, since the rotational torque of the fixing belt increases as the amount of lubricant between the heater and fixing belt decreases, a torque sensor 31 may be used to detect the rotational torque of the pressure roller 22 or fixing belt 21, as shown in FIG. 8. In this case, information on the rotational torque detected by the torque sensor 31 is sent to a control unit 30 provided in the image forming apparatus main body, etc., which determines whether the rotational torque exceeds a predetermined reference value. If the rotational torque exceeds the reference value, the control unit 30 reverses the rotation of the pressure roller 22, thereby rotating the fixing belt 21 in the opposite direction. As a result, the lubricant remaining on the heater 23 is supplied between the heater 23 and the fixing belt 21.

Furthermore, because the amount of lubricant between the heater and the fixing belt tends to decrease over time as the fixing device is used, the reverse rotation of the fixing belt may be controlled based on the number of sheets of paper that have passed through the nip, the rotation distance (cumulative distance) of the fixing belt, or the elapsed time from a preset timing. For example, as shown in FIG. 9 , the control unit 30 provided in the image forming apparatus main body includes at least one of a paper count recording unit 32 that records the number of sheets of paper that have passed through the nip, a rotation distance recording unit 33 that stores the rotation distance of the fixing belt, and an elapsed time measuring unit 34 that measures the elapsed time from a preset timing. Then, when the number of sheets of paper that have passed recorded by the paper count recording unit 32, the rotation distance of the fixing belt that has been recorded by the rotation distance recording unit 33, or the elapsed time that has been measured by the elapsed time measuring unit 34 exceeds a preset reference value, the fixing belt 21 is reversed. This allows lubricant to be supplied when the amount of lubricant between the heater and the fixing belt decreases over time. Additionally, the timing of reverse rotation of the fixing belt may be controlled using information on the number of sheets passed, the rotation distance, or the elapsed time in combination with the detection results of the rotation torque.

The reverse rotation of the fixing belt may also be performed after the fixing belt has been left unheated. Specifically, the reverse rotation of the fixing belt is performed when the temperature of the fixing belt is below 100°C, or after one minute or more has passed since heating of the fixing belt was stopped. In this case, the temperature of the fixing belt is particularly low and the fixing belt has become non-circular. Therefore, by rotating the fixing belt in reverse in this state, the lubricant is more likely to adhere to the inner surface of the fixing belt.

The fixing belt may also be reversed immediately after the fixing belt has completed the fixing process. Specifically, after 30 or more sheets of paper have been continuously passed through the fixing device and the fixing process has been completed (after the paper has been passed through the nip), the fixing belt is reversed within one minute. In this case, the temperature of the accumulated lubricant is high and the viscosity of the lubricant is reduced, so by reversing the rotation of the fixing belt, it becomes easier to supply the lubricant between the heater and the fixing belt.

The lubricant placed between the heater and the fixing belt can contain fluorine grease or silicone oil. Such lubricants become viscous and gel-like at low temperatures, making them prone to adhering to the fixing belt when the belt is rotated in reverse.

Furthermore, to further reduce wear of the fixing belt, as shown in the example of FIG. 10, the fixing belt 21 may be a belt made of a substrate 210 and a surface layer (release layer) 212 provided on the outer periphery of the substrate 210. In this case, since no elastic layer such as a rubber layer is provided between the surface layer (release layer) 212 and the substrate 210, the fixing belt 21 has lower insulation properties and better thermal conductivity from the heater to the fixing belt surface (outer periphery) than fixing belts with elastic layers. Therefore, with a fixing belt such as that shown in FIG. 10, the heater's heat output can be set relatively low. Generally, fixing belts lose strength and become more susceptible to wear as the temperature increases. Therefore, setting the heater's heat output low can suppress temperature increases in the fixing belt and reduce wear of the fixing belt. Furthermore, heat-induced degradation of the lubricant can be suppressed, thereby maintaining the lubricating function over a long period of time and extending the life of the fixing belt.

Furthermore, from the perspective of suppressing wear on the fixing belt, it is preferable that the heater that heats the fixing belt be configured as shown in Figure 4 above, in which multiple resistance heating elements 56 are arranged along the longitudinal direction of the heater 23 (the paper width direction, which intersects with the paper transport direction). In this case, since the heat output of each resistance heating element 56 can be controlled independently of one another, the heat output of each resistance heating element 56 can be individually controlled according to the width of the paper passing through the nip portion, thereby suppressing excessive temperature rise in non-paper passing areas where paper does not pass, and suppressing a decrease in the durability of the fixing belt and deterioration of the lubricant that accompanies temperature rise.

The heater is not limited to the configuration shown in Figure 4, but may also be configured as shown in Figure 11, in which the resistance heating elements 56 are arranged continuously in the longitudinal direction of the heater 23.

The resistive heating element may also have a PTC (positive temperature coefficient of resistance) characteristic. PTC characteristics are a characteristic in which, as the temperature of the resistive heating element increases, the resistance value of the resistive heating element increases and the heater output decreases.

This characteristic suppresses the heater's temperature rise in non-paper-passing areas. That is, when a sheet of paper narrower than the entire longitudinal width of the resistance heating elements is passed through, the temperature of the resistance heating elements rises in the non-paper-passing areas because the paper does not absorb heat. However, because the voltage applied to the resistance heating elements remains constant, the temperature of the resistance heating elements in the non-paper-passing areas rises, causing their resistance values to increase. As a result, the output (heat generation) of the resistance heating elements in the non-paper-passing areas decreases relatively, suppressing excessive temperature rise in the fixing belt. Therefore, by using a heater whose resistance heating elements have PTC characteristics, temperature rise in the fixing belt in non-paper-passing areas can be effectively suppressed, improving the durability of the fixing belt. Furthermore, as shown in Figure 4, when each resistance heating element 56 is electrically connected in parallel, it is possible to suppress temperature rise in the non-paper-passing areas while maintaining printing speed.

Although various embodiments of the present invention have been described above, the present invention is not limited to the above embodiments, and various modifications can of course be made without departing from the spirit of the present invention.

For example, the present invention can also be applied to fixing devices configured as shown in Figures 12 to 15. The configuration of each fixing device shown in Figures 12 to 15 will be described below.

12 differs from the fixing device 20 shown in FIG. 2 above in the location of the temperature sensor 27 that detects the temperature of the heater 23. The rest of the configuration is the same. In the fixing device 20 shown in FIG. 12, the temperature sensor 27 is located upstream of the center M of the nip N in the paper feed direction (the nip entrance side). In contrast, in the fixing device 20 shown in FIG. 4, the temperature sensor 27 is located at the center M of the nip N. When the temperature sensor 27 is located upstream of the center M of the nip N in the paper feed direction, as shown in FIG. 12, the temperature sensor 27 can accurately detect the temperature at the nip entrance side. The nip entrance side is an area where heat from the fixing belt 21 is particularly likely to be lost by the paper P entering the nip N. Therefore, accurately detecting the temperature at the nip entrance side with the temperature sensor 27 ensures image fixability and effectively prevents fixing offset (a condition in which the toner image is not sufficiently heated).

Next, in the embodiment shown in FIG. 13, a heating nip N1, where the heater 23 heats the fixing belt 21, and a fixing nip N2, through which the paper P passes, are formed in separate locations. Specifically, in this embodiment, a nip forming member 68 is disposed inside the fixing belt 21 in addition to the heater 23, and pressure rollers 69 and 70 are pressed against the heater 23 and nip forming member 68, respectively, via the fixing belt 21, thereby forming the heating nip N1 and the fixing nip N2. In this case, the fixing belt 21 is heated in the heating nip N1, and the heat of the fixing belt 21 is applied to the paper P in the fixing nip N2, thereby fixing the unfixed image to the paper P.

Next, the fixing device 20 shown in Figure 14 is an example of the fixing device shown in Figure 13 above, in which the pressure roller 69 on the heater 23 side is omitted, and the heater 23 is formed in an arc shape to match the curvature of the fixing belt 21. Other than that, it is the same configuration as shown in Figure 13. In this case, because the heater 23 is formed in an arc shape, the contact length between the fixing belt 21 and the heater 23 in the belt rotation direction is ensured, and the fixing belt 21 can be heated efficiently.

Next, the fixing device 20 shown in Figure 15 is an example in which a roller 73 is disposed between a pair of belts 71, 72. In this example, the heater 23 disposed within the belt 71 on the left side in Figure 15 contacts the roller 73 via the belt 71, and the nip forming member 74 disposed within the belt 72 on the right side contacts the roller 73 via the belt 72, thereby forming a heating nip N1 and a fixing nip N2.

In Figures 13, 14, and 15, nip forming members 68 and 74 are provided in addition to the heater 23 as members that slide relative to the belt. The present invention is also applicable to configurations in which lubricant is supplied between such nip forming members 68 and 74 and the belt. Furthermore, the present invention is not limited to configurations in which a heater is disposed in a nip portion through which paper passes (fixing nip portion N2), but is also applicable to configurations in which a heater is disposed in a nip portion through which paper does not pass (heating nip portion N1), as shown in Figures 13, 14, and 15.

Furthermore, the image forming apparatus according to the present invention is not limited to the color image forming apparatus shown in FIG. 1, but may also be a monochrome image forming apparatus, a copier, a printer, a facsimile, or a combination machine of these.

For example, the present invention can also be applied to an image forming apparatus configured as shown in Figure 16. The image forming apparatus 100 shown in Figure 16 includes an image forming means 80 consisting of a photosensitive drum or the like, a paper transport unit consisting of a pair of timing rollers 81 or the like, a paper feeder 82, a fixing device 83, a paper discharge device 84, and a reading unit 85. The paper feeder 82 includes multiple paper feed trays, each of which can accommodate paper of a different size.

The reading unit 85 reads the image of the document Q. The reading unit 85 generates image data from the read image. The paper feeder 82 stores multiple sheets of paper P and sends the sheets P to the transport path. The timing rollers 81 transport the sheets P on the transport path to the image forming means 80.

The image forming unit 80 forms a toner image on the paper P. Specifically, the image forming unit 80 includes a photosensitive drum, a charging roller, an exposure device, a developing device, a replenishment device, a transfer roller, a cleaning device, and a static eliminator. The fixing unit 83 applies heat and pressure to the toner image to fix it to the paper P. The paper P with the fixed toner image is transported to the paper discharge unit 84 by a transport roller or the like. The paper discharge unit 84 discharges the paper P outside the image forming apparatus 100.

Next, the fixing device 83 according to this embodiment will be described with reference to Figure 17. Note that in the configuration shown in Figure 27, components that are common to the fixing device 20 of the above embodiment shown in Figure 2 will be assigned the same reference numerals and their description will be omitted.

As shown in FIG. 17, the fixing device 83 includes a fixing belt 21, a pressure roller 22, a heater 23, a heater holder 24, a stay 25, a temperature sensor 27, and the like.

A nip N is formed between the fixing belt 21 and the pressure roller 22. The nip width of the nip N is 10 mm, and the linear speed of the fixing device 83 is 240 mm/s.

The fixing belt 21 has a polyimide base and a release layer, and does not have an elastic layer. The release layer is made of a heat-resistant film material, such as fluororesin. The outer diameter of the fixing belt 21 is approximately 24 mm.

The pressure roller 22 includes a core metal, an elastic layer, and a release layer. The outer diameter of the pressure roller 22 is 24 to 30 mm, and the thickness of the elastic layer is 3 to 4 mm.

The heater 23 includes a base material, a heat insulating layer, a conductive layer containing a resistance heating element, and an insulating layer, and has an overall thickness of 1 mm. The width of the heater 63 in the paper transport direction is 13 mm.

As shown in FIG. 18, the conductor layer of the heater 23 includes multiple resistance heating elements 56, power supply lines 59, and electrode portions 58A-58C. The multiple resistance heating elements 56 are spaced apart in the longitudinal direction (arrow X direction) of the heater 23. Here, if the portions between each pair of resistance heating elements 56 are referred to as "division regions," then, as shown in the enlarged view of FIG. 18, division regions B are formed between each pair of resistance heating elements 56. (Although FIG. 18 illustrates division regions B only within the enlarged view, in reality, division regions B are provided between all pairs of resistance heating elements 56.) Furthermore, in FIG. 18, the direction of arrow Y is a direction that intersects or is perpendicular to the longitudinal direction X of the heater 23 (longitudinal intersecting direction), and is different from the thickness direction of the substrate 55. The direction of arrow Y is also the direction intersecting the arrangement direction of the multiple resistance heating elements 56 (arrangement crossing direction), or the direction along the surface of the substrate 55 on which the resistance heating elements 56 are provided, which is the widthwise direction of the heater 23, or the same direction as the transport direction of paper passing through the fixing device.

The multiple resistance heating elements 56 form a central heating section 35B and heating sections 35A and 35C on both ends that can generate heat independently. For example, of the three electrode sections 58A-58C, when electricity is applied to the leftmost electrode section 58A in Figure 18 and the central electrode section 58B, both end heating sections 35A and 35C generate heat. Furthermore, when electricity is applied to the end electrode sections 58A and 58C, the central heating section 35B generates heat. For example, when fixing small-size paper, only the central heating section 35B generates heat, and when fixing large-size paper, all heating sections 35A-35C generate heat, allowing heating according to the size of the paper.

As shown in FIG. 19, the heater holder 24 according to this embodiment has a recess 24a that accommodates and holds the heater 23. The recess 24a is formed on the heater 23 side of the heater holder 24. The recess 24a is composed of a rectangular (oblong) surface (bottom) 24f of approximately the same size as the heater 23, and four walls (sides) 24b, 24c, 24d, and 24e that intersect with the surface 24f along the four sides that form the outline of the surface 24f. Note that the right-hand wall 24e is not shown in FIG. 19. Alternatively, one of the pair of walls 24d and 24e (left and right) that intersect with the longitudinal direction X of the heater 23 (the direction in which the resistance heating elements 56 are arranged) may be omitted, and the recess 24a may be configured to open at one end of the heater 23 in the longitudinal direction.

As shown in Figure 20, the heater 23 and heater holder 24 according to this embodiment are held by a connector 86. The connector 86 includes a housing made of resin (e.g., LCP) and multiple contact terminals provided within the housing.

The connector 86 is attached to the heater 23 and heater holder 24 in a direction intersecting the longitudinal direction X of the heater 23 (the arrangement direction of the resistance heating elements 56) (see the direction of the arrow from the connector 86 in Figure 20). The connector 86 is attached to the heater 23 and heater holder 24 at one end side of the longitudinal direction X of the heater 23 (the arrangement direction of the resistance heating elements 56), opposite the side where the drive motor of the pressure roller 22 is provided. When the connector 86 is attached to the heater holder 24, a convex portion on one of the connector 86 and the heater holder 24 may engage with a concave portion on the other, allowing the convex portion to move relatively within the concave portion.

When the connector 86 is attached, the heater 23 and heater holder 24 are held in place by being sandwiched between them on both the front and back sides by the connector 86. In this state, each contact terminal comes into contact (pressure welded) with each electrode portion of the heater 23, electrically connecting each resistance heating element 56 to the power supply provided in the image forming device via the connector 86. This allows power to be supplied from the power supply to each resistance heating element 56.

Flanges 87 shown in Figure 20 are belt holding members that are provided at both longitudinal ends of the fixing belt 21 and hold both ends of the fixing belt 21 from the inside. The flanges 87 are inserted into both ends of the stay 25 and fixed to a pair of side plates that are frame members of the fixing device.

Figure 21 shows the arrangement of the temperature sensor 27 and the thermostat 88, which is the current interrupter, in this embodiment.

As shown in FIG. 21, the temperature sensors 27 according to this embodiment are arranged to face the inner circumferential surfaces of the fixing belt 21 on the center Xm side and the end side in the longitudinal direction X. Furthermore, one of these temperature sensors 27 is arranged at a position corresponding to the divided region B (see FIG. 18) between the resistive heating elements of the heater 23.

In addition, thermostats 88 serving as current-cutting members are arranged on the center Xm side and end sides of the fixing belt 21 so as to face the inner circumferential surface of the fixing belt 21. Each thermostat 88 detects the temperature of the inner circumferential surface of the fixing belt 21 or the ambient temperature near the inner circumferential surface. If the temperature detected by the thermostat 88 exceeds a preset threshold, current to the heater 23 is cut off.

Also, as shown in Figures 21 and 22, flanges 87 that hold both ends of the fixing belt 21 are provided with slide grooves 87a. The slide grooves 87a extend in the direction in which the fixing belt 21 moves toward and away from the pressure roller 22. An engagement portion of the fixing device housing engages with the slide grooves 87a. This engagement portion moves relative to the pressure roller 22 within the slide grooves 87a, allowing the fixing belt 21 to move toward and away from the pressure roller 22.

The above describes the configurations of other fixing devices and image forming apparatuses to which the present invention can be applied. However, by applying the present invention to fixing devices and image forming apparatuses with such configurations, the same effects as those of the above-described embodiment can be obtained. In other words, by applying the present invention, it is possible to supply accumulated lubricant between the heater and fixing belt without adding any new mechanisms or parts, thereby improving the durability of the fixing belt through resistance stress.

Furthermore, the present invention is not limited to fixing devices, which are an example of a belt-driven device or a heating device using a belt, but can also be applied to other belt-driven devices and heating devices. For example, the present invention can be applied to heating devices such as heating devices (drying devices) that heat paper to dry liquids such as ink applied to the paper, laminators that thermocompress a film as a covering member onto the surface of a sheet such as paper, and heat sealers that thermocompress the seal portion of packaging material. The present invention can also be applied to belt-driven devices that do not have a heating source such as a heater.

20 Fixing device (heating device, belt driving device)
21 Fixing belt (belt)
22 Pressure roller (opposing member)
23 Heater (heat source, sliding member)
24 heater holder (holding member)
24a: recess 50: lubricant 56: resistance heating element (heating element)
100 Image forming apparatus C Paper passing direction (sheet passing direction)
N Nip P Paper (sheet)

Japanese Patent Application Laid-Open No. 2018-194696

Claims (13)

a flexible endless belt;
a sliding member disposed so as to slide on an inner circumferential surface of the belt;
an opposing member that contacts the sliding member via the belt and forms a nip between the opposing member and the belt;
A belt driving device including a lubricant interposed between an inner circumferential surface of the belt and the sliding member,
the belt is rotatable in a direction opposite to a direction in which the belt rotates when the sheet passes through the nip portion,
the sliding member is configured to be movable in a direction in which the belt surface moves in the nip portion in association with the rotation of the belt in the opposite direction,
a belt driving device that rotates the belt in the opposite direction within a range of not more than five rotations but not more than a moving distance of the sliding member accompanying the rotation in the opposite direction when the temperature of the belt is 100°C or less, or after one minute or more has elapsed in an unheated state since heating of the belt was stopped. a flexible endless belt;
a sliding member disposed so as to slide on an inner circumferential surface of the belt;
an opposing member that contacts the sliding member via the belt and forms a nip between the opposing member and the belt;
a holding member having a recess in which the sliding member is accommodated;
a lubricant interposed between the inner circumferential surface of the belt and the sliding member;
the sliding member is accommodated in a belt driving device so as to be movable in a belt surface movement direction in the nip portion in accordance with the rotation of the belt in the opposite direction,
the belt is rotatable in a direction opposite to a direction in which the belt rotates when the sheet passes through the nip portion,
the sliding member is configured to be movable in a direction in which the belt surface moves in the nip portion in association with the rotation of the belt in the opposite direction,
A belt driving device , characterized in that the rotation of the belt in the opposite direction is performed within a range of not less than a movement distance of the sliding member accompanying the rotation in the opposite direction but not more than five rotations . 3. The belt driving device according to claim 1, wherein the rotation of the belt in the opposite direction is performed within a range of one rotation or less that is equal to or greater than the movement distance of the sliding member accompanying the rotation in the opposite direction . 4. The belt driving device according to claim 1, wherein the rotation speed of the belt in the opposite direction is lower than the rotation speed when the sheet is passed through the nip portion . 5. The belt drive device according to claim 1, wherein the rotation of the belt in the opposite direction is performed based on at least one of the number of sheets that have passed through the nip portion, the rotation distance of the belt, the elapsed time from a preset timing, and the rotational torque of the belt or the opposing member . 3. The belt driving device according to claim 2 , wherein upstream and downstream edges of the recess in the belt surface movement direction protrude toward the opposing member beyond the opposing member-side surface of the sliding member. A belt drive device according to any one of claims 1 to 6, wherein the width of the portion of the sliding member where the opposing member-side surface of the opposing member contacts the inner circumferential surface of the belt in the direction of belt surface movement is smaller than the width of the entire opposing member-side surface of the sliding member in the direction of belt surface movement. A belt drive device according to any one of claims 1 to 7, wherein the lubricant includes fluorine grease or silicone oil. The belt drive device described in any one of claims 1 to 8, wherein the belt has a base material and a surface layer disposed on the outer periphery of the base material, and no elastic layer is provided between the surface layer and the base material. The belt drive device according to any one of claims 1 to 9;
A heating device comprising: a heat source for heating the belt provided in the belt driving device. The heating device described in claim 10, wherein the heat source has multiple heating elements arranged along the longitudinal direction of the heat source, each of which has heat generation controllable independently of the other. A fixing device that fixes an unfixed image to a sheet using the heating device described in claim 10 or 11. An image forming apparatus comprising at least one of the belt drive device described in any one of claims 1 to 9, the heating device described in claim 10 or 11, and the fixing device described in claim 12.