JP7581760B2 - Charging device, image forming unit and image forming apparatus - Google Patents

Description

The present invention relates to a charging device, an image forming unit, and an image forming apparatus, and is suitable for use in, for example, electrophotographic printers.

Conventionally, image forming devices using electrophotographic processes, such as printers, copiers, facsimile machines, and multifunction devices, often use a contact charging system in which a charging roller, acting as a charging member, contacts the surface of a photosensitive drum to uniformly charge the surface of the photosensitive drum.

In contact charging type image forming devices, the charging roller is in direct contact with the photosensitive drum, so the charging roller picks up dirt such as external additives that adhere to the surface of the photosensitive drum. For this reason, such image forming devices are provided with a cleaning member to clean the surface of the charging roller, and a cleaning roller with a foam surface is often used as the cleaning member (see, for example, Patent Document 1).

JP 2017-83643 A

However, in order to clean the surface of the charging member that applies voltage to the photosensitive drum, the cleaning member may be biased by an elastic member, but there are cases in which the cleaning member does not contact the charging member with an appropriate pressure.

The present invention was made in consideration of the above points, and aims to propose a charging device, an image forming unit, and an image forming device that can bring a cleaning member into contact with a charging member with an appropriate pressure.

In order to solve this problem, the charging device of the present invention includes a charging member that is rotatable and contacts the surface of a charged member to charge it, a cleaning member that is rotatable and cleans the surface of the charged member, an elastic member that is stretchable in the longitudinal direction and brings the cleaning member into contact with the charging member, and a holding member that holds one end side and the other end side of the elastic member in a bent state and brings the outer periphery of the elastic member into contact with the cleaning member, and where the natural length of the elastic member is L1 and the inner length of the elastic member in the bent state is L4, L1 = L4.

The image forming unit of the present invention is also equipped with the charging device described above.

Furthermore, the image forming device of the present invention is provided with the image forming unit described above.

In the present invention, when the cleaning member momentarily separates from the charged member, the entire elastic member stretches, momentarily increasing the urging force, and the elastic member immediately urges the cleaning member toward the charged member, thereby preventing a decrease in cleaning performance. On the other hand, when the cleaning member momentarily comes into strong contact with the charged member, the entire elastic member cannot contract any further, and the urging force drops sharply, preventing dirt adhering to the cleaning member from adhering to the charged member and preventing the occurrence of printing defects.

According to the present invention, the cleaning member can be brought into contact with the charged member with an appropriate pressure.

FIG. 2 is a left side view illustrating a configuration of the image forming apparatus. FIG. 2 is a front view showing the configuration of the charging device according to the first embodiment. FIG. 2 is a cross-sectional view showing a configuration of a charging device. FIG. 4 is a left side view showing the configuration of a bearing portion. FIG. 2 is a front view showing the configuration of the cleaning roller. 6 is a cross-sectional view taken along line BB of FIG. 5, showing the configuration of the cleaning roller according to the first embodiment. FIG. 2 is a diagram showing the configuration of a tension spring. FIG. FIG. FIG. 2 is a block diagram showing a control configuration of the image forming apparatus. 1 is a table showing a list of tension springs. 13 is a table showing evaluation results. 4 is a graph showing the relationship between the center distance and the bearing load. 1 is a graph showing a relationship (1) between a nip amount and a bearing load. 13 is a graph showing the relationship (2) between the nip amount and the bearing load. FIG. 11 is a front view showing a configuration of a charging device according to a second embodiment. FIG. 11 is a cross-sectional view showing a configuration of a cleaning roller according to a second embodiment.

Below, the form for implementing the invention (hereinafter referred to as embodiment) will be explained with reference to the drawings.

1. First embodiment
[1-1. Configuration of Image Forming Apparatus]
As shown in Fig. 1, the image forming device 1 is, for example, a printer using an electrophotographic method, and forms a black-and-white image or a color image on a medium P such as paper or film by performing an image forming operation using a developer such as toner. In the following, a position close to a cassette 7 as viewed from an arbitrary position on the transport path along which the medium P is transported, or a direction toward the cassette 7, is referred to as upstream. Also, a position close to a stacker into which the medium P is discharged and loaded, or a direction toward the stacker, as viewed from an arbitrary position on the transport path, is referred to as downstream. Furthermore, the direction from upstream to downstream is referred to as the transport direction. The image forming device 1 has, from upstream to downstream, a medium supply unit 2, a transport unit 3, an image forming unit 4, a transfer unit 5, and a fixing unit 6 inside a box-shaped housing.

[1-2. Configuration of medium supply unit]
The medium supply unit 2 has a cassette 7 and a hopping roller 8. The cassette 7 is detachably attached to the lower part of the housing of the image forming apparatus 1, and stores a stack of media P. The hopping roller 8 separates and takes out the media P stored in the cassette 7 one by one from the top, and pays them out toward a pair of transport rollers 10 located downstream.

[1-3. Configuration of the conveying unit]
The transport unit 3 has transport roller pairs 10 and 11 arranged in order from upstream to downstream. The transport roller pairs 10 and 11 sandwich and transport the medium P fed from the hopping roller 8, and at the same time, correct the skew of the medium P and transport it along the transport direction toward the transfer belt 26.

[1-4. Configuration of image forming unit]
The image forming unit 4 has, for example, four developing units 12 (12K, 12C, 12M, and 12Y). The developing units 12K, 12C, 12M, and 12Y are arranged in order along the transport direction from the upstream side to the downstream side, and form developer images (toner images) on the medium P using toners (developers) of different colors. Specifically, the developing units 12K, 12C, 12M, and 12Y form black, cyan, magenta, and yellow toner images using black, cyan, magenta, and yellow toners, respectively.

Each of these toner colors contains, for example, a specific colorant, release agent, charge control agent, and treatment agent, and is manufactured by appropriately mixing these components or by surface treatment. Of these, the colorant, release agent, and charge control agent each function as an internal additive. For example, silica or titanium oxide is used as an external additive, and polyester resin is used as a binder resin. In addition, dyes and pigments can be used alone or in combination as colorants. For example, a toner with a circularity of about 0.94 to 0.98 and an average particle size of about 7 μm is used. For example, an external additive with a diameter of about 50 to 200 nm is used.

[1-4-1. Configuration of the developing unit]
The developing units 12K, 12C, 12M, and 12Y have the same configuration except that they form toner images using toners of different colors. Hereinafter, the developing units 12K, 12C, 12M, and 12Y are collectively referred to as the developing unit 12. Each developing unit 12 includes, for example, a photosensitive drum 14, a charging device 16, an exposure device 18, a developing roller 20, a toner supply unit 22, and a cleaning blade 24.

[1-4-1-1. Configuration of the photosensitive drum]
The photosensitive drum 14 is a member that carries an electrostatic latent image on its surface (surface portion), and is constructed using a photoconductor (e.g., an organic photoconductor). Specifically, the photosensitive drum 14 has a conductive support and a photoconductive layer that covers its outer periphery (surface). The conductive support is constructed of a metal pipe made of aluminum or stainless steel, for example. The photoconductive layer has a structure in which, for example, a charge generation layer and a charge transport layer are laminated in order. The photosensitive drum 14 rotates at a predetermined peripheral speed.

The charge generation layer is mainly composed of a charge generation material and a binder resin. Various organic pigments and organic dyes can be used as the charge generation material. Among them, metal-free phthalocyanine, copper indium chloride, gallium chloride, tin, oxytitanium, zinc, vanadium, and other metals or their oxides and chlorides are preferred, as well as phthalocyanines coordinated with monoazo, hisazo, trisazo, or polyazo azo pigments. The charge generation layer is used as a dispersion layer in which fine particles of these charge generation materials are bound with various binder resins, such as polyester resin, polyvinyl acetate, polyacrylic acid ester, polymethacrylic acid ester, polyester, polycarbonate, polyvinyl acetoacetal, polyvinyl propional, polyvinyl butyral, phenoxy resin, epoxy resin, urethane resin, cellulose ester, or cellulose ether.

The charge transport layer is mainly composed of a charge transport material and a binder resin. Examples of the charge transport material include heterocyclic compounds such as carbazole, indole, imidazole, oxazole, pyrazole, oxadiazole, pyrazoline, and thiadiazole, aniline derivatives, hydrazone compounds, aromatic amine derivatives, stilbene derivatives, and electron donating substances such as polymers having groups consisting of these compounds in the main chain or side chain. Examples of the binder resin in the charge transport layer include polycarbonate, polymethyl methacrylate, polystyrene, vinyl polymers such as polyvinyl chloride, polyester, polyester carbonate, polysulfone, polyimide, phenoxy, epoxy, and silicone resins, or polymers or partially crosslinked cured products thereof, and polycarbonate is particularly preferred. The binder resin in the charge transport layer may contain various additives such as antioxidants and sensitizers as necessary.

[1-4-1-2. Configuration of charging device]
As shown in Figures 2, 3 and 4, the charging device 16 is a device that charges the surface of the photosensitive drum 14, and is mainly composed of a charging roller 32 and a cleaning roller 34. The charging roller 32 is arranged with its rotation axis facing in the left-right direction so as to be in contact with the surface of the photosensitive drum 14. The charging roller 32 has a charging roller core metal 32a rotatably supported by outer charging roller bearings 36 provided near both left and right ends. The outer charging roller bearings 36 are biased toward the photosensitive drum 14 by compression springs 38. Therefore, the charging roller 32 is biased toward the photosensitive drum 14. In addition, inner charging roller bearings 40 that rotatably support the charging roller core metal 32a are provided on the left and right inner sides of each of the left and right outer charging roller bearings 36. The inner charging roller bearings 40 have linear bearing arms 40a that are approximately cylindrical and protrude from slightly above the center in the vertical direction on both front and rear sides toward the front and rear upper directions, respectively.

The cleaning roller 34 is arranged with its rotation axis facing the left and right direction so as to be in sliding contact with the charging roller 32. The cleaning roller 34 has a cleaning roller core 34a rotatably supported by cleaning roller bearings 42 provided near both left and right ends. The tension spring arm 44a of the tension spring 44 is engaged with the bearing arm 40a of the inner charging roller bearing 40. The tension spring 44 is bent so as to have a shape like a capital letter "U" rotated 180 degrees as a whole, and the inside of the bent shape of the tension spring coil part 44b in the bent state is wound around the outer circumferential surface of the cleaning roller bearing 42 on the opposite side to the charging roller 32. In this way, the charging device 16 bends the tension spring 44 engaged with the inner charging roller bearing 40, and the inside of the bent shape abuts against the cleaning roller bearing 42, so that the tension spring 44 pulls the bearing arm 40a of the inner charging roller bearing 40, which is coaxial with the charging roller 32. Therefore, the charging device 16 can urge the cleaning roller 34 toward the charging roller 32 so as to contact the charging roller 32 by urging the cleaning roller bearing 42 toward the inner charging roller bearing 40 by the tension spring 44. In the present embodiment, the outer charging roller bearing 36 and the inner charging roller bearing 40 are separate and individual components, but the outer charging roller bearing 36 and the inner charging roller bearing 40 may be integrated.

The bearing section consisting of the inner charging roller bearing 40, cleaning roller bearing 42, and tension spring 44 allows the charging device 16 to nip the cleaning roller 34 against the charging roller 32 with a load, and can respond to wear of the cleaning roller 34 and changes and fluctuations in the thickness of the cleaning roller elastic layer 34b. The cleaning roller 34 in the first embodiment is driven by the rotation of the charging roller 32 due to the frictional force between the surfaces of the charging roller 32 and cleaning roller 34.

[1-4-1-2-1. Configuration of charging roller]
The charging roller 32 has a charging roller core 32a made of a conductive material, and a conductive charging roller elastic layer 32b is formed on the outer circumferential surface of the charging roller core 32a. For the charging roller core 32a, a metal shaft such as electroless nickel-plated SUM or SUS is often used. For the charging roller elastic layer 32b, rubber, thermoplastic elastomer, resin, etc. are often used so that it can be nipped with the photosensitive drum 14 to obtain appropriate discharge with the photosensitive drum 14. The charging roller elastic layer 32b is not limited to a single layer, but has a multi-layer structure of two or more layers as necessary.

As the material for the charging roller elastic layer 32b, for example, a rubber composition containing one or a mixture of two or more of epichlorohydrin rubber (CO, ECO, GECO), ethylene propylene rubber (EPM, EPDM), acrylonitrile-butadiene rubber (NBR), hydrogenated acrylonitrile-butadiene rubber (H-NBR), styrene-butadiene rubber (SBR), butadiene rubber (BR), isoprene rubber (IR), chloroprene rubber (CR), urethane rubber, silicone rubber, etc., as the main component can be used. Among them, rubber containing epichlorohydrin rubber (ECO) as the main component or rubber containing a mixture of epichlorohydrin rubber (ECO) and acrylonitrile-butadiene rubber (NBR) as the main component is often used. In this embodiment, rubber containing epichlorohydrin rubber (ECO) as the main component was used.

Generally, if the resistance of the conductive characteristic of the elastic layer 32b of the charging roller is too large, uneven charging of the surface of the photosensitive drum 14 or poor image quality due to poor charging occurs. Conversely, if the resistance is too small, leakage occurs due to scratches on the surface of the photosensitive drum 14, resulting in poor image quality. Therefore, there is an appropriate resistance region for the conductive characteristic of the elastic layer 32b of the charging roller. In order to obtain an appropriate resistance region, the elastic layer 32b of the charging roller is given a predetermined conductivity by using an ion conductive material, an ion conductive agent, carbon black, metallic oxide, etc. for the elastic layer 32b of the charging roller. The elastic layer 32b of the charging roller can be either electronically conductive or ionically conductive, but since partial resistance unevenness is likely to affect the uneven charging of the photosensitive drum 14, ionically conductive materials are often used rather than electronically conductive materials in order to suppress uneven resistance.

The elastic layer 32b of the charging roller is preferably a resistive layer having a volume resistivity of 10 ⁶ to 10 ⁹ Ω. In the case of ion conductivity, the resistance value of the charging roller 32 varies depending on the temperature, humidity, and measurement voltage, but here, the value measured in an environment of a temperature of 20° C. and a humidity of 50% RH is shown.

The hardness of the elastic layer 32b of the charging roller must be such that a small gap is formed between the surface of the charging roller 32 and the surface of the photosensitive drum 14, and an area that contributes to discharge based on Paschen's law is secured. In order to obtain an appropriate nip, the hardness of the elastic layer 32b of the charging roller is adjusted. At this time, a micro rubber hardness tester MD-1capa (Type A) (manufactured by Kobunshi Keiki Co., Ltd.) is used to measure the hardness of the elastic layer 32b of the charging roller, and peak measurements are performed. At this time, it is preferable that the measured value is in the range of 35 degrees to 80 degrees, but this hardness range also includes the purpose of absorbing cylindrical runout and shape variations of the charging roller 32 and the photosensitive drum 14, so there is no need to stick to this value as long as an appropriate nip is obtained between the charging roller 32 and the photosensitive drum 14.

The outer surface shape of the charging roller elastic layer 32b is formed with a predetermined polishing mark and surface roughness by cutting, polishing process or molding. At this time, the surface roughness of the charging roller 32 varies somewhat depending on the applied voltage and the usage environment, but according to Paschen's law, the maximum height Ry (based on JIS B 0601:1994) is preferably in the range of about 1 μm to 40 μm.

The outer surface of the elastic layer 32b of the charging roller may be subjected to a surface treatment or coating. The surface treatment or coating of the charging roller 32 may prevent the substances contained in the elastic layer 32b of the charging roller from contaminating the photosensitive drum 14, adjust the surface resistance of the elastic layer 32b of the charging roller, or provide an appropriate roughness so that the toner or toner additives adhering to the photosensitive drum 14 are less likely to adhere to the surface of the charging roller 32. The outer surface of the elastic layer 32b of the charging roller may be subjected to a surface treatment such as ultraviolet irradiation or electron beam irradiation.

Coating methods include dipping, spraying, and coating with a coater. Materials to be applied include, for example, acrylic resin, urethane resin, fluororesin, polyamide resin, polycarbonate resin, polyester resin, isocyanate resin, etc., and a combination of one or more of these. In addition to the above, conductive agents may be added to the materials to be applied by coating, if necessary. Particles may also be mixed into these coating materials. Examples of particles to be mixed include, for example, acrylic resin, urethane resin, fluororesin, polyamide resin, polycarbonate resin, polyester resin, isocyanate resin, etc., and a combination of one or more of these.

In this embodiment, the charging roller 32 uses a metal shaft made of free-cutting steel (SUM) with electroless nickel plating as the charging roller core 32a, and rubber whose main component is a mixture of epichlorohydrin rubber (ECO) and acrylonitrile-butadiene rubber (NBR) is used for the charging roller elastic layer 32b. The diameter of the charging roller core 32a is φ6 [mm], and the outer diameter of the charging roller elastic layer 32b is 9.5 [mm].

The surface of the charging roller elastic layer 32b is formed by molding. A water/alcohol mixed solvent is used as the solvent for coating the charging roller elastic layer 32b. A solution containing a polyamide (nylon) polymer and urethane resin particles is applied and the solvent is evaporated, causing the solution to harden. At this time, urethane resin particles with diameters of 20 μm and 10 μm are included.

[1-4-1-2-2. Configuration of cleaning roller]
As shown in FIG. 5, the cleaning roller 34 has a cleaning roller core 34a and a cleaning roller elastic layer 34b on the outer circumferential surface of the cleaning roller core 34a. For example, the cleaning roller core 34a may be made of a metal shaft such as electroless nickel-plated SUM or SUS, or a resin such as polyacetal (POM). The cleaning roller elastic layer 34b may be a one-layer or two or more layer laminate. The cleaning roller elastic layer 34b may be made of a foam or may be a two-layer structure of a solid layer and a foam layer. The cleaning roller elastic layer 34b is arranged so as to spirally wind the entire surface of the cleaning roller core 34a except for both ends. As shown in FIG. 6, in this embodiment, the cleaning roller elastic layer 34b is wound around the surface of the cleaning roller core 34a with a narrow gap in the longitudinal direction of the cleaning roller 34.

The material constituting the cleaning roller elastic layer 34b may be a foamable resin such as polyurethane, polyethylene, polyamide or polypropylene, or a material made by mixing one or more of the following rubber materials: silicone rubber, fluororubber, urethane rubber, ethylene propylene rubber (EPM, EPDM), ethylene propylene rubber (EPM, EPDM), acrylonitrile-butadiene rubber (NBR), hydrogenated acrylonitrile-butadiene rubber (H-NBR), styrene-butadiene rubber (SBR), butadiene rubber (BR), isoprene rubber (IR) or chloroprene rubber (CR). If necessary, auxiliary agents such as foaming assistants, foam stabilizers, catalysts, hardeners, plasticizers and vulcanization accelerators may be added to these.

The cleaning roller elastic layer 34b and the cleaning roller core 34a are bonded by an adhesive 34c. For the adhesive 34c, an adhesive made of an adhesive tape such as a double-sided tape is used. For the adhesive tape, an acrylic adhesive, a rubber-based adhesive, a silicone-based adhesive, or a combination of one or more of these adhesives can be used. In addition, the adhesive tape can be used regardless of the presence or absence of a base material. The cleaning roller 34 used in this embodiment uses a shaft body made of free-cutting steel electrolessly plated with nickel as the material for the cleaning roller core 34a. For the cleaning roller elastic layer 34b, a polyurethane foam, specifically Moltoprene SM-55 manufactured by Inoac Corporation, is used. For the adhesive, an acrylic adhesive, specifically Double-sided Tape 5000NS manufactured by Nitto Denko Corporation, is used. The cleaning roller elastic layer 34b was formed by attaching double-sided tape to the back side of a urethane foam with a width of 8 mm, and winding it spirally around the cleaning roller core 34a with a gap of 0.5 mm or less. The cleaning roller core 34a had a diameter of φ4 mm, and the cleaning roller elastic layer 34b had a diameter of 6.0 mm.

[1-4-1-2-3. Configuration of tension spring]
As shown in Fig. 7, the tension spring 44 is composed of a tension spring coil portion 44b and a tension spring arm portion 44a. The tension spring coil portion 44b is provided in the longitudinal center portion of the tension spring 44, occupies most of the longitudinal direction, and is coil-shaped. The tension spring arm portions 44a are provided on both longitudinal outer sides of the tension spring coil portion 44b, and are hook-shaped. As shown in Fig. 4, the tension spring arm portions 44a of the tension spring 44 are hooked on the bearing arms 40a of the inner charge roller bearing 40.

This tension spring 44 is a so-called tension spring, and even in an unloaded state where no external load is applied, the wire spaces between adjacent coils in the longitudinal direction of the tension spring coil section 44b are narrowed to form a tightly packed coil shape. Therefore, even in an unloaded state, the tension spring 44 is under initial tension, which is a force that causes the coils in the tension spring coil section 44b to tightly pack together.

The length of the entire tension spring 44 in the longitudinal direction (i.e., the direction in which the tension spring coil portion 44b expands and contracts) in the natural state in which the tension spring 44 is not tensioned is defined as the spring natural length L1. Specifically, this spring natural length L1 is the length along the longitudinal direction from the inside of the bend of the hook-shaped end of one tension spring arm portion 44a to the inside of the bend of the hook-shaped end of the other tension spring arm portion 44a. Furthermore, the length of the tension spring coil portion 44b in the stretching direction in the natural state is defined as the coil portion natural length L0. Furthermore, the length of the tension spring arm portion 44a in the stretching direction is defined as the arm portion length La. Specifically, this arm portion length La is the length along the longitudinal direction from the end of the tension spring coil portion 44b to the inside of the bend of the hook-shaped end of the tension spring arm portion 44a. This arm length La is a constant value regardless of the stretched or contracted state of the tension spring coil portion 44b. Furthermore, the outer diameter of the tension spring coil portion 44b is the outer diameter R. This outer diameter R is a constant value regardless of the stretched or contracted state of the tension spring coil portion 44b.

Furthermore, the length of the center of the entire tension spring 44 in the stretching direction when the tension spring 44 is in a bent and stretched state is defined as the spring center length in use L2 (Figure 4). Specifically, this spring center length in use L2 is the length along the longitudinal direction from the inside of the bend of the hook-shaped end of one tension spring arm 44a, through the center of the entire tension spring 44, to the inside of the bend of the hook-shaped end of the other tension spring arm 44a when in use. Furthermore, the length of the inner part of the bend of the entire tension spring 44 in the stretching direction when the tension spring 44 is in use is defined as the spring inner length in use L4 (Figure 4). Here, the inner part of the bend is the side of the outer periphery of the tension spring coil part 44b in the tension spring 44 that abuts against the cleaning roller bearing 42. That is, the in-use spring inner length L4 is the length of a line extending from both ends of a curve along the inner periphery of the bend of the tension spring coil portion 44b toward the bearing arm 40a to the end of the bearing arm 40a on the opposite side from the tension spring coil portion 44b. Therefore, in the in-use state, the length of the inner part of the bend in the tension spring 44 is shorter than the length of the outer part of the bend. The curve indicating the in-use spring center length L2 and the curve indicating the in-use spring inner length L4 are in a parallel state. The in-use spring center length L2 can be calculated by image analysis using a projector to analyze the axis distance measured using a non-contact three-dimensional measuring device described later and the part shapes of the inner charging roller bearing 40 and the cleaning roller bearing 42.

8, the angle indicating the degree of bending of the tension spring 44, which indicates the extent to which the tension spring 44 is wound around the cleaning roller bearing 42, is defined as the winding angle θ. Here, the straight line perpendicular to the imaginary end face of the circular portion on one end of the tension spring coil portion 44b is defined as the coil portion one end side extension line LN1. Also, the straight line perpendicular to the imaginary end face of the circular portion on the other end of the tension spring coil portion 44b is defined as the coil portion other end side extension line LN2. The winding angle θ is the angle on the side away from the cleaning roller bearing 42, of the two angles formed by the coil portion one end side extension line LN1 and the coil portion other end side extension line LN2. Therefore, the winding angle θ becomes smaller as the tension spring 44 is wound loosely around the cleaning roller bearing 42 so that one end of the tension spring arm 44a and the other end are separated, whereas the winding angle θ becomes larger as the tension spring 44 is wound tightly around the cleaning roller bearing 42 so that one end of the tension spring arm 44a and the other end are closer to each other.

Using this winding angle θ, the inner length L4 of the spring during use can be calculated using the following formula (1).

L4=L2-0.5π×R×(θ/180)……(1)

When the winding angle θ is 180 degrees, the inner length L4 of the spring during use can be calculated using the following formula (2).

L4=L2-0.5π×R...(2)

In this embodiment, the bearing arm 40a is positioned so that the center length of the tension spring 44 is 20.0 mm when the winding angle θ is 160 degrees, the outer diameter of the cleaning roller bearing 42 is 3.0 mm, and the axial distance between the central axis of the charging roller core 32a of the charging roller 32 and the central axis of the cleaning roller core 34a of the cleaning roller 34 is 7.4 mm.

Table TB1 in Figure 11 shows the characteristics of the seven types of tension springs 44 used in this embodiment. The material of these tension springs 44 was SUS304-WPB, the wire diameter d was 0.26 mm, the outer diameter R was 1.9 mm, and the arm length La was fixed at 3.45 mm to create each tension spring 44. The spring constant k is calculated from the material, wire diameter d, number of turns Na, and outer diameter R of the tension spring 44. The natural length L0 of the coil is calculated by wire diameter d x number of turns Na. The natural length L1 of the spring is calculated by natural length L0 of the coil + arm length La x 2.

[1-4-1-2-4. Relationship between center distance and bearing load]
In this measurement, using the image forming apparatus 1 and charging device 16 configured as described above, a tensile test was performed in which the cleaning roller bearing 42, around which the tension spring 44 is wound, was pulled so as to gradually move the cleaning roller bearing 42 away from the bearing arm 40a, thereby measuring the relationship between the axial distance between the charging roller 32 and the cleaning roller 34 and the bearing load, which is the load exerted by the tension spring 44 on the cleaning roller bearing 42.

The center distance was measured using a non-contact three-dimensional measuring device NH-5Ns (manufactured by Mitaka Koki Co., Ltd., objective lens x50). Specifically, in this measurement, the center axis of the charging roller core metal 32a of the charging roller 32 and the center axis of the cleaning roller core body 34a of the cleaning roller 34 were obtained from the circular arc shapes of the outer circumference of the charging roller 32 and cleaning roller 34 for a rotation angle of 30 degrees, and the center distance (i.e. the pulling distance) between the center axis of the charging roller 32 and the center axis of the cleaning roller 34 was measured.

The bearing load, which is the load exerted by the tension spring 44 on the cleaning roller bearing 42, was measured using a low-load universal material testing machine 5543A (manufactured by Instron Japan Co., Ltd.).

The measured centerline distance and bearing load are shown in the centerline distance bearing load measurement result line ML1 of graph GR1 in Figure 13, with centerline distance on the X-axis and bearing load on the Y-axis.

When the tension spring 44 is bent and the axis distance is short and within range A, the inner spring length L4 during use is shorter than the natural spring length L1. When within range A, the outer side of the bend of the tension spring 44 is stretched compared to its natural state, but the inner side of the bend of the tension spring 44 is the same as its natural state and is not stretched. In other words, in range A, the inner side of the bend of the tension spring 44 cannot be contracted any further. In this way, until the tension spring 44 is pulled with a force exceeding the initial tension, the outer side of the bend of the tension spring 44 is stretched compared to its natural state, but the inner side of the bend of the tension spring 44 is the same as its natural state and is not stretched.

When the center distance increases, in range A, the extension of the tension spring 44 is small even when a bearing load is applied until it reaches a point where it enters range B, i.e., until the initial tension is applied. Also, when the center distance increases, in range A, where the initial tension is not exceeded, the bearing load also changes relatively greatly according to the amount of change in the center distance.

On the other hand, when the tension spring 44 is bent and the cleaning roller bearing 42 is pulled beyond the initial tension from range A and enters range B, the inner spring length L4 during use becomes longer than the natural spring length L1. Within range B, both the outer and inner sides of the bend of the tension spring 44 are stretched compared to the natural state. In range B, the tension spring 44 behaves according to its spring constant. If the tension spring 44 is not used in a bent state but is used in a state in which it expands and contracts along a straight line, the tension spring 44 is usually used so that the characteristics within range B are utilized. Also, in range B, the amount of change in bearing load according to the amount of change in the center distance is smaller than in range A.

Furthermore, when the tension spring 44 is bent, the inner length L4 of the spring during use is equal to the natural length L1 of the spring when it is on the boundary between ranges A and B. The boundary between ranges A and B is the position of the inflection point Pi, where the slope of the axis distance bearing load measurement result line ML1 changes significantly (details will be described later).

In this way, the tension spring 44 has the characteristic that when the tension spring 44 is bent and used to bias an object on the inside of the bend, an inflection point Pi appears at the point where the inside of the bend begins to stretch, at which point the slope of the change in bearing load relative to the change in axis distance (i.e., the slope of the axis distance bearing load measurement result line ML1) changes significantly.

[1-4-1-3. Configuration of Exposure Apparatus]
The exposure device 18 ( FIG. 1 ) is a device that forms an electrostatic latent image on the surface (surface portion) of the photosensitive drum 14 by irradiating and exposing the surface of the photosensitive drum 14 with irradiation light. The exposure device 18 is supported, for example, on the housing of the image forming apparatus 1. The exposure device 18 is configured to include, for example, a plurality of light sources that emit irradiation light, and a lens array that forms an image of the irradiation light on the surface of the photosensitive drum 14. Note that, as each of these light sources, for example, a light emitting diode (LED) or a laser element can be given.

[1-4-1-4. Configuration of developing roller]
The developing roller 20 is a member that carries toner on its surface for developing an electrostatic latent image, and is disposed so as to contact the surface (circumferential surface) of the photosensitive drum 14. The developing roller 20 has, for example, a metal shaft and a semiconductive urethane rubber layer that covers the outer periphery (surface) of the metal shaft. The developing roller 20 rotates at a predetermined peripheral speed, for example, in the opposite direction to the photosensitive drum 14. Toner contained in a toner supply unit 22 is supplied to the developing roller 20 by a supply roller or the like.

[1-4-1-5. Configuration of toner supply unit]
The toner supply units 22 are containers that accommodate the toners of the respective colors described above. That is, black toner, cyan toner, magenta toner, and yellow toner are accommodated in the toner supply units 22 of the developing units 12K, 12C, 12M, and 12Y, respectively.

[1-4-1-6. Configuration of cleaning blade]
One end of the cleaning blade 24 contacts the surface of the photosensitive drum 14, and scrapes off toner remaining on the surface of the photosensitive drum 14 without being transferred to the transfer belt 26. The cleaning blade 24 is made of, for example, a flexible rubber material or a plastic material.

[1-5. Configuration of transfer unit]
The transfer section 5 has a transfer belt 26, a driven roller 27, a drive roller 28, a transfer roller 29, and a cleaning blade 30. The image forming section 4 and the transfer section 5 form an image forming unit 13 that forms a developer image (toner image) on the medium P while transporting the medium P.

Transfer rollers 29 are provided for each of the developing units 12K, 12C, 12M, and 12Y, and electrostatically transfer the toner images of each color formed in the developing units 12K, 12C, 12M, and 12Y onto the medium P. A plurality of transfer rollers 29 are arranged to face the photosensitive drums 14 of the developing units 12K, 12C, 12M, and 12Y across the transfer belt 26. Note that the plurality of transfer rollers 29 have substantially the same configuration. In addition, each transfer roller 29 arranged to face the developing units 12C, 12M, and 12Y is spaced a fixed distance from the photosensitive drum 14 during monochrome printing.

The transfer belt 26 transports the medium P transported by the transport unit 3 to the fixing unit 6. The transfer belt 26 is stretched by a drive roller 28 and a driven roller 27, and rotates counterclockwise in FIG. 1. The drive roller 28 and the driven roller 27 transport the transfer belt 26. The cleaning blade 30 scrapes off the toner adhering to the transfer belt 26.

[1-6. Configuration of Fixing Unit]
The fixing unit 6 applies heat and pressure to the toner image on the medium P transported from the transfer belt 26, thereby fixing the toner image to the medium P. The fixing unit 6 has, for example, a heating roller 46 and a pressure roller 48 disposed opposite each other across the transport path along which the medium P is transported. The heating roller 46 has a heater therein. The pressure roller 48 is urged against the heating roller 46 during the fixing process to form a nip portion.

[1-7. Configuration of control mechanism]
Here, the control mechanism of the image forming apparatus 1 will be described with reference to Fig. 10. The image forming apparatus 1 is provided with a control unit 50, a receiving memory 52, an image data editing memory 54, an operation unit 56, a group of sensors 58, and a power supply circuit 60 as a control mechanism. The image forming apparatus 1 is further provided with a drive motor 62. The drive motor 62 drives the photosensitive drum 14 (Fig. 1).

The control unit 50 has an interface (I/F) control unit 64, a main control unit 66, an exposure device control unit 18S, a fixing control unit 6S, a transport motor control unit 3S, and a drive control unit 62S. The main control unit 66 is composed of a microprocessor, a ROM, a RAM, an input/output port, etc., and controls the entire processing operation in the image forming device 1, for example, by executing a predetermined program. Specifically, the main control unit 66 receives print data and control commands from the I/F control unit 64, and performs printing operations by controlling the exposure device control unit 18S, the fixing control unit 6S, the transport motor control unit 3S, and the drive control unit 62S. The I/F control unit 64 receives print data and control commands from an external device such as a personal computer (PC), or transmits signals related to the status of the image forming device 1.

The receiving memory 52 temporarily stores print data sent from an external device such as a PC via the I/F control unit 64. The image data editing memory 54 receives the print data stored in the receiving memory 52 and stores image data that is the edited version of the print data. The operation unit 56 has, for example, LED lamps for displaying information such as the status of the image forming device 1, and an input unit (buttons and a touch panel) for the user to give instructions to the image forming device 1. The sensor group 58 includes various sensors that monitor the operating status of the image forming device 1, such as a medium P position detection sensor, a temperature and humidity sensor, a print density sensor, and a toner remaining amount detection sensor.

The exposure device control unit 18S sends the image data recorded in the image data editing memory 54 to the exposure device 18, and controls the drive of the exposure device 18. The fixing control unit 6S controls the voltage applied to the fixing unit 6 when the toner image transferred to the medium P is fixed to the medium P. The transport motor control unit 3S controls the operation of the transport unit 3 (hopping roller 8, transport roller pairs 10 and 11) when the transport unit 3 transports the medium P. The drive control unit 62S controls the operation of the drive motor 62.

The power supply circuit 60 has a charging roller power supply 32V, a developing roller power supply 20V, a toner supply unit power supply 22V, and a transfer roller power supply 29V. Here, the charging roller power supply 32V, the developing roller power supply 20V, the toner supply unit power supply 22V, and the transfer roller power supply 29V apply voltages based on instructions from the main control unit 66 to the charging roller 32, the developing roller 20, the toner supply unit 22, and the transfer roller 29, respectively. When the developing roller power supply 20V applies a voltage to the developing roller 20, the toner carried by the developing roller 20 is developed into the electrostatic latent image formed on the surface of the photosensitive drum 14. In addition, when the toner supply unit power supply 22V applies a voltage to the toner supply unit 22, the toner is supplied from the toner supply unit 22 to the developing roller 20. In addition, when the charging roller power supply 32V applies a voltage to the charging roller 32, the surface of the photosensitive drum 14 is charged. In addition, when a voltage is applied to the transfer roller 29 by the transfer roller power supply 29V, the toner image developed on the surface of the photosensitive drum 14 is transferred to the medium P.

[1-8. Evaluation of tension springs]
The image forming apparatus 1 and the charging device 16 having the above-described configuration were used to study the tension spring 44. In this study, as shown in Fig. 11, seven different types of tension springs 44, No. 1, No. 2, No. 3, No. 4, No. 5, No. 6, and No. 7, were created, and evaluations were performed during continuous testing for each type, and the nip amount of the cleaning roller 34 and the load applied to the cleaning roller bearing 42 were measured.

[1-8-1. Continuous test]
In this evaluation, the printed image was evaluated. The printed image was evaluated for the presence or absence of black spots (color spots) and density unevenness after 32 cycles of the charging roller, and the evaluation results are shown in Table TB2 of FIG. 12. The measurement conditions are as follows.

- 1% Duty (print image density), continuous printing at 3 pages/job - 10,000 sheets printed in an environment with a temperature of 23°C and humidity of 55%,
After that, print 10,000 sheets in an environment with a temperature of 28°C and humidity of 80%.
After that, print 10,000 sheets in an environment with a temperature of 10°C and a humidity of 20%.
・A total of 30,000 sheets printed ・After continuous printing, image quality was checked under an environment of 10[℃] temperature and 20[%] humidity ・Image check pattern: blank paper (0[%] Duty (print image density)), 2by2 (600dpi), solid

The print image density here is a value that represents the proportion of the number of pixels to which developer is transferred to the medium P out of the total number of pixels when an image is broken down into pixels. For example, printing with an area ratio of 100% when performing full solid printing on the printable range of a specified area (such as one revolution of the photosensitive drum 14 or one page of a print medium) is called a print image density of 100%, and printing that corresponds to an area of 1% of this print image density of 100% is called a print image density of 1%. When the print image density DPD is expressed mathematically using the number of dots used Cm, the number of rotations Cd, and the total number of dots CO, it can be expressed as in the following formula (3).

DPD [%] = Cm/(Cd x CO) x 100...(3)

However, the number of dots used Cm is the number of dots actually used to form an image while the photosensitive drum 14 rotates Cd times, and is the total number of dots exposed by the exposure device 18 while forming the image. The total number of dots CO is the total number of dots per rotation of the photosensitive drum 14, that is, the total number of dots that can be potentially used when forming an image, regardless of whether or not they are exposed, while the photosensitive drum 14 rotates once. In other words, the total number of dots CO is the total number of dots used when forming a solid image in which developer is transferred to all pixels. Therefore, the value (Cd x CO) represents the total number of dots that can potentially be used when forming an image while the photosensitive drum 14 rotates Cd times.

In Table TB2, the presence or absence of black dots (color dots) is indicated by a symbol "o" if black dots do not occur, and by a symbol "x" if black dots occur. The presence or absence of density unevenness is indicated by a symbol "o" if density unevenness does not occur, and by a symbol "x" if density unevenness occurs. These black dots (black dust) occur on the medium P at the rotation period of the charging roller 32 when the nip force from the cleaning roller 34 to the charging roller 32 is too strong, and the dust of the adhesive 34c, which is an insulator in the cleaning roller 34, adheres to the charging roller 32 and the photosensitive drum 14 cannot be charged. The density unevenness occurs on the medium P at the rotation period of the charging roller 32 when the nip force from the cleaning roller 34 to the charging roller 32 is insufficient, and the cleaning roller 34 is unable to remove all the external additives that have peeled off from the charging roller 32.

Also, although not shown in Table TB2, if the nip force from the cleaning roller 34 to the charging roller 32 is too strong, and dirt from the adhesive 34c of the cleaning roller 34 adheres to the charging roller 32 and causes black spots, the black spots may be stretched onto the charging roller 32 by the nip pressure, causing vertical bands.

[1-8-2. Measurement of nip amount of cleaning roller]
In this measurement, a non-contact three-dimensional measuring device NH-5Ns (manufactured by Mitaka Koki Co., Ltd., objective lens ×50) was used to measure how much the cleaning roller 34 was nipped to the charging roller 32. Specifically, in this measurement, the central axis of the charging roller core metal 32a of the charging roller 32 and the central axis of the cleaning roller core body 34a of the cleaning roller 34 were obtained from the circular arc shapes of the outer periphery of the charging roller 32 and the cleaning roller 34 for a rotation angle of 30 degrees, and the axial distance between the central axis of the charging roller 32 and the central axis of the cleaning roller 34 was measured, and the shortened axial distance was considered to be equivalent to the crushing amount B of the cleaning roller 34 shown in FIG. 9, and the crushing amount B of the cleaning roller 34 was calculated.

At this time, since the hardness of the charging roller elastic layer 32b of the charging roller 32 is sufficiently harder than the cleaning roller elastic layer 34b of the cleaning roller 34, it is considered that only the cleaning roller 34 is recessed, and the amount of crushing of the charging roller elastic layer 32b of the charging roller 32 is ignored.

Furthermore, when the thickness of the cleaning roller elastic layer 34b of the cleaning roller 34 is thickness A, the nip amount of the cleaning roller 34 is calculated by crush amount B/thickness A, and the evaluation results of crush amount B and nip amount B/A are shown in Table TB2 in FIG. 12.

[1-8-3. Measurement of the load applied to the cleaning roller bearing]
In this measurement, a low-load universal material testing machine 5543A (manufactured by Instron Japan Co., Ltd.) was used to measure the bearing load, which is the load applied to the cleaning roller bearing 42 by the tension spring 44.

[1-8-4. Relationship between nip amount and bearing load]
In this evaluation, the calculated nip amount (deformation amount B/wall thickness A) and the measured bearing load are shown in graph GR2 of Fig. 14, with the nip amount on the X-axis and the bearing load on the Y-axis. Note that graph GR2 shows the nip amount and bearing load for tension springs 44 No. 2, No. 3, No. 6, and No. 7, and omits the nip amount and bearing load for tension springs 44 No. 1, No. 4, and No. 5. Also, only the nip amount and bearing load for tension spring 44 No. 3 is shown as a representative in nip amount-bearing load measurement result line ML2 of graph GR3 of Fig. 15.

As shown in graph GR3, in the range X where nip amount B/A is 0 to 0.53, the slope of the change in bearing load relative to the change in nip amount B/A (i.e., the slope of nip amount bearing load measurement result line ML2) is relatively gentle. Also, in the range Y where nip amount B/A is 0.53 to 0.83, the slope of the change in bearing load relative to the change in nip amount B/A (i.e., the slope of nip amount bearing load measurement result line ML2) is greater than in range X. Furthermore, in the range Z where nip amount B/A is 0.83 to 1.0, the bearing load hardly changes relative to the change in nip amount B/A.

This is because in range Z, the axial distance between the charging roller 32 and the cleaning roller 34 is narrow, causing the tension spring coil portion 44b to slacken, resulting in a state in which no bearing load is applied to the cleaning roller bearing 42. On the other hand, when the nip amount B/A decreases and moves from range Z to range Y, a bearing load begins to be applied to the cleaning roller bearing 42, but the inside of the bend of the tension spring coil portion 44b is slightly loose and has not yet stretched. When the nip amount B/A further decreases and moves from range Y to range X, the inside of the bend of the tension spring coil portion 44b begins to stretch at the boundary between range X and range Y, and the load of the tension spring 44 is applied to the cleaning roller bearing 42.

The approximation line ALx in range X and the approximation line ALy in range Y in the nip amount bearing load measurement result line ML2 intersect at the inflection point Pi. In other words, the slope of the approximation line ALx and the approximation line ALy changes at the inflection point Pi on the X-axis value. Thus, in the tension spring 44, an inflection point Pi of the slope is seen on the nip amount bearing load measurement result line ML2 at the boundary between ranges X and Y, where the inner side of the bend starts to stretch as it transitions from range Y to range X. Such an inflection point Pi is seen in each tension spring 44. The position of the inflection point Pi in each tension spring 44 is shown in Table TB2 in FIG. 12.

In this way, the tension spring 44 has the characteristic that when the tension spring 44 is bent and used to bias an object on the inside of the bend, an inflection point Pi appears at the point where the inside of the bend begins to stretch, at which point the slope of the change in bearing load relative to the change in nip amount B/A (i.e., the slope of the nip amount bearing load measurement result line ML2) changes significantly.

[1-8-5. Judgment result]
As shown in FIG. 12, the positions of the inflection points Pi of the tension springs 44 of No. 1 and No. 2 are larger than the nip amount of 0.51. The tension spring 44 is formed by the elastic layer 34b of the cleaning roller 34 repeatedly contracting and expanding due to the strong nip pressure between the charging roller 32 and the cleaning roller 34 and the rotation of the charging roller 32. A part of the adhesive 34c used for bonding the cleaning roller core 34a came up onto the surface of the cleaning roller elastic layer 34b and adhered to the surface of the charging roller 32, causing black spots on the charging roller 32.

The position of the inflection point Pi of the tension springs 44 of No. 3, No. 4, No. 5 and No. 6 is within the range of 0.07≦nip amount≦0.51. Therefore, when the center distance of the tension springs 44 of No. 3, No. 4, No. 5 and No. 6 is shortened and the nip amount moves from the range Y beyond the inflection point Pi to the range X, the load load increases rapidly with respect to the shortening of the center distance (i.e., the decrease in the nip amount). Therefore, the tension springs 44 of No. 3, No. 4, No. 5 and No. 6 can prevent the cleaning roller 34 from being pressed against the charging roller 32 more strongly than a certain level even if the center distance is shortened and the nip amount increases. This allows the tension springs 44 of No. 3, No. 4, No. 5 and No. The tension spring 44 No. 6 prevents a portion of the adhesive 34c from rising to the surface of the cleaning roller elastic layer 34b, and prevents the adhesive 34c from adhering to the surface of the charging roller 32. Thus, the tension springs 44 No. 3, No. 4, No. 5, and No. 6 can suppress the occurrence of black spots and uneven density in the charging roller 32 cycle. Furthermore, the tension springs 44 No. 3, No. 4, No. 5, and No. 6 can suppress the occurrence of black spots, and therefore the occurrence of the vertical bands described above.

The tension spring 44 of No. 7 has an inflection point Pi of 0 or less, so the nip pressure between the charging roller 32 and the cleaning roller 34 is weak, and a strong load cannot be obtained even if the axis distance increases when the charging roller 32 is rotated. Therefore, the tension spring 44 of No. 7 does not obtain sufficient nip pressure between the charging roller 32 and the cleaning roller 34, and the cleaning properties of the surface of the charging roller 32 cannot be obtained sufficiently. As a result, the tension spring 44 of No. 7 causes the surface of the charging roller 32 to become contaminated with external additives, causing uneven density of the charging roller 32.

In this way, the charging device 16 can suppress the occurrence of image defects when the inflection point Pi is set in the range of 0.07≦nip amount≦0.51 by using a tension spring 44 with an inflection point Pi of 0.07 or more. In this way, the charging device 16 is designed to set the inflection point Pi of the bearing load in an appropriate range for the nip amount.

[1-9. Effects, etc.]
In the above configuration, the charging device 16 urges the cleaning roller 34 toward the charging roller 32 by winding the inside of the bend of the tension spring 44, which is a bent tension coil spring, around the cleaning roller bearing 42, which is coaxial with the cleaning roller 34, to apply a nip force. If the nip force is too strong, the dirt on the cleaning roller 34, such as aggregates of the cleaned toner additives and residues of the adhesive 34c, which is generated during the manufacturing process when the sponge is bonded to the cleaning roller core 34a, seeping out from the cleaning roller elastic layer 34b, will adhere to the charging roller 32. On the other hand, if the nip force is too weak, the cleaning roller 34 will separate from the charging roller 32, and an appropriate nip amount will not be obtained, resulting in unevenness in the cycle of the charging roller 32. In this way, if the cleaning roller 34 is not in contact with the charging roller 32 with an appropriate pressure contact force, the cleaning performance will decrease and printing defects will occur.

In contrast, in the charging device 16, when the natural length of the tension spring 44 is defined as the spring natural length L1 and the inner length of the tension spring 44 when wound around the cleaning roller bearing 42 in a bent state is defined as the spring inner length during use L4, the spring natural length L1 = spring inner length during use L4.

Therefore, even if the cleaning roller 34 momentarily separates from the charging roller 32 while the charging roller 32 is rotating and the natural spring length L1 becomes smaller than the inner spring length during use L4, the tension spring 44 stretches as a whole, momentarily increasing the urging force, so that the tension spring 44 immediately urges the cleaning roller 34 toward the charging roller 32, thereby preventing a decrease in cleaning performance.

On the other hand, in the charging device 16, even if the cleaning roller 34 momentarily presses strongly against the charging roller 32 while the charging roller 32 is rotating and the natural spring length L1 is greater than the inner spring length during use L4, the inclination of the approximation line ALy is greater than that of the approximation line ALx (Figure 15), and the entire tension spring 44 cannot contract any further, so the biasing force (bearing load) drops sharply. This prevents dirt adhering to the cleaning roller 34 from adhering to the charging roller 32, thereby preventing the occurrence of printing defects.

This allows the charging device 16 to bring the cleaning roller 34 into contact with the charging roller 32 with an appropriate pressure, preventing deterioration of cleaning performance and poor printing.

Nowadays, miniaturization of the image forming apparatus 1 is an important issue, and there is a demand for miniaturization of the charging device 16, including the charging roller 32 and cleaning roller 34. The charging device 16 can be made smaller by reducing the diameter of the cleaning roller 34, but by reducing the thickness of the charging roller elastic layer 32b to reduce the diameter, the adhesive 34c used to bond the charging roller elastic layer 32b and the charging roller core metal 32a is transferred to the surface of the charging roller 32, which causes a problem of poor image quality during the charging roller 32 cycle.

In response to this, the charging device 16 is designed so that if the cleaning roller 34 momentarily presses strongly against the charging roller 32 while the charging roller 32 is rotating, the tension spring 44 as a whole cannot contract any further, and the biasing force drops suddenly. This allows the charging device 16 to prevent the adhesive 34c from being transferred to the surface of the charging roller 32. This allows the image forming device 1 to reduce the size of the cleaning roller 34 while suppressing the occurrence of printing defects, and therefore allows the charging device 16 including the cleaning roller 34 to be reduced in size.

Here, if the nip amount bearing load measurement result line ML2 shown in FIG. 15 enters a range larger than the inflection point Pi, but the slope remains the approximation line ALx rather than the approximation line ALy, the bearing load does not decrease easily even if the force becomes smaller than the initial tension, despite the fact that the center distance is smaller due to the large nip amount. In this case, the nip force becomes too strong, and dirt on the cleaning roller 34 adheres to the charging roller 32.

In response to this, the charging device 16 uses the tension spring 44 in a bent state, so that when the inflection point Pi is exceeded, the amount of bearing load reduction relative to the increase in nip amount increases suddenly, as shown by the approximate line ALy, relative to the approximate line ALx. As a result, even if the cleaning roller 34 momentarily presses strongly against the charging roller 32 while the charging roller 32 is rotating, the charging device 16 can instantaneously reduce the bearing load and prevent dirt adhering to the cleaning roller 34 from adhering to the charging roller 32, thereby preventing the occurrence of printing defects.

Furthermore, the charging device 16 uses a tension spring 44 with an inflection point Pi of a nip amount of 0.07 or more, so that the position of the inflection point Pi is within the range of 0.07≦nip amount≦0.51. As a result, the charging device 16 can adjust the nip amount appropriately, and can suppress deterioration of cleaning performance and printing defects.

According to the above configuration, the charging device 16 is provided with a charging roller 32 that is rotatable and contacts and charges the surface of the photosensitive drum 14 as a charged member, a cleaning roller 34 that is rotatable and cleans the surface of the charging roller 32, a tension spring 44 that is expandable and contractible in the longitudinal direction and brings the cleaning roller 34 and the charging roller 32 into contact, and a bearing arm 40a that holds one end side and the other end side of the tension spring 44 in a bent state and brings the outer periphery of the tension spring 44 into contact with the cleaning roller 34, and where the natural length of the tension spring 44 is the spring natural length L1 and the inner length of the tension spring 44 in the bent state is the spring inner length in use L4, the spring natural length L1 = spring inner length in use L4.

As a result, when the cleaning roller 34 momentarily separates from the charging roller 32, the charging device 16 stretches the entire tension spring 44, momentarily increasing the urging force, and the tension spring 44 immediately urges the cleaning roller 34 toward the charging roller 32, thereby preventing a decrease in cleaning performance. On the other hand, when the cleaning roller 34 momentarily strongly presses against the charging roller 32, the charging device 16 prevents the entire tension spring 44 from contracting any further, causing a sudden decrease in the urging force, thereby preventing dust adhering to the cleaning roller 34 from adhering to the charging roller 32 and preventing the occurrence of printing defects.

In this way, the charging device 16 can bring the cleaning roller 34 into contact with the charging roller 32 with an appropriate pressure, preventing deterioration of cleaning performance and poor printing.

2. Second embodiment
[2-1. Configuration of Image Forming Apparatus]
The image forming apparatus 101 (FIG. 1) according to the second embodiment differs from the image forming apparatus 1 according to the first embodiment in that it has a charging device 116 instead of the charging device 16, but is otherwise configured in the same manner.

[2-2. Configuration of charging device]
As shown in FIG. 16 in which the same reference numerals are used for the members corresponding to those in FIG. 2, the charging device 116 differs from the charging device 16 of the first embodiment in that a gear 34g is added and that a cleaning roller 134 is provided in place of the cleaning roller 34, but otherwise the charging device 116 is configured similarly.

Gear 34g is fixed to one end of cleaning roller core 34a, and rotates cleaning roller core 34a by transmitting driving force from a motor (not shown). As a result, charging device 116 generates a difference in peripheral speed between the surface of charging roller 32 and the surface of cleaning roller 134. As a result, charging device 116 can have even stronger cleaning properties compared to charging device 16 by rubbing the surface of charging roller 32 with the surface of cleaning roller 134.

The cleaning roller 134 (FIG. 5) is different from the cleaning roller 34 according to the first embodiment in that it has a cleaning roller elastic layer 134b instead of the cleaning roller elastic layer 34b, but is otherwise configured in the same manner. As shown in FIG. 17, in which the same reference numerals are used for the members corresponding to those in FIG. 6, in the second embodiment, the cleaning roller elastic layer 134b is wound around the surface of the cleaning roller core 34a at intervals in the longitudinal direction.

The charging device 116 according to the second embodiment can achieve the same effects as the charging device 16 according to the first embodiment.

3. Other embodiments
In the above-mentioned first embodiment, the cleaning roller elastic layer 34b is wound around the surface of the cleaning roller core 34a with a small gap in the longitudinal direction, and in the second embodiment, the cleaning roller elastic layer 134b is wound around the surface of the cleaning roller core 34a with a small gap in the longitudinal direction. The present invention is not limited to this, and the point is that the cleaning roller elastic layer 34b or 134b is configured to clean the surface of the charging roller 32, so long as it can function as the cleaning roller 34 or 134.

In the above embodiment, the cleaning roller 34 or 134 and the charging roller 32 perform circular motion, i.e., rotate, in only one direction around the rotation axis. The present invention is not limited to this, and it is sufficient that the cleaning roller 34 or 134 and the charging roller 32 at least rotate around a predetermined axis.

Furthermore, in the above-described embodiment, the present invention has been described as being applied to the image forming apparatus 1 or 101 of a so-called direct transfer type, in which a toner image is directly transferred from the photosensitive drum 14 to the medium P. The present invention is not limited to this, and the present invention may also be applied to an image forming apparatus of a so-called intermediate transfer type (or secondary transfer type), in which a toner image of each color is transferred from the photosensitive drum 14 to an intermediate transfer belt in a sequentially overlapping manner, and the toner image is transferred from this intermediate transfer belt to the medium P.

Furthermore, in the above-mentioned embodiment, the present invention has been described as being applied to the image forming apparatus 1 or 101 that uses a developer used in a one-component development method. The present invention is not limited to this, and the present invention may also be applied to an image forming apparatus that uses a developer in a two-component development method, in which a carrier and a toner are mixed together and friction between the carrier and the toner is used to impart an appropriate amount of charge to the toner.

Furthermore, in the above-described embodiment, the present invention has been described as being applied to an image forming apparatus 1 or 101 that has four developing units 12 and forms a color image using toner of four colors. The present invention is not limited to this, and the present invention may be applied to an image forming apparatus that has three or less or five or more developing units 12 and forms an image using toner of a predetermined number of colors.

Furthermore, in the above-described embodiment, the present invention has been described as being applied to the image forming apparatus 1 or 101, which is a single-function printer. The present invention is not limited to this, and may be applied to image forming apparatuses having various other functions, such as an MFP (Multi Function Peripheral) that has the functions of a copier or facsimile machine. The present invention may also be applied to various electronic devices that form images on a medium P, such as paper, using a developer by electrophotography.

Furthermore, the present invention is not limited to the above-mentioned embodiment or other embodiments. In other words, the scope of application of the present invention extends to embodiments that arbitrarily combine the above-mentioned embodiment with part or all of the other above-mentioned embodiments, or to embodiments that extract parts of the above-mentioned embodiment.

Furthermore, in the above-mentioned embodiment, the charging device 16 or 116 is configured as a charging device by the charging roller 32 as a charging member, the cleaning roller 34 or 134 as a cleaning member, the tension spring 44 as an elastic member, and the bearing arm 40a as a holding member. The present invention is not limited to this, and the charging device may be configured by a charging member, a cleaning member, an elastic member, and a holding member having various other configurations.

The present invention can be used when printing images on media using an electrophotographic image forming device.

1: Image forming apparatus, 2: Medium supply section, 3: Transport section, 4: Image forming section, 5: Transfer section, 6: Fixing section, 7: Cassette, 8: Hopping roller, 10, 11: Transport roller pair, 12: Development section, 13: Image forming unit, 14: Photosensitive drum, 16, 116: Charging device, 18: Exposure device, 20: Development roller, 22: Toner supply section, 24: Cleaning blade, 26: Transfer belt, 27: Driven roller, 28: Drive roller, 29: Transfer roller, 30: Cleaning blade, 32: Charge roller, 32a: Charge roller core metal, 32b: Charge roller elastic layer, 34, 134: Cleaning roller, 34a: Cleaning roller core body, 34b, 134b: Cleaning roller elastic layer, 34c: Adhesive, 34g: Gear, 36: Outer charge roller bearing, 38: Compression spring, 4 0: inner charging roller bearing, 40a: bearing arm, 42: cleaning roller bearing, 44: tension spring, 44a: tension spring arm, 44b: tension spring coil, 46: heating roller, 48: pressure roller, 50: control unit, 52: receiving memory, 54: image data editing memory, 56: operation unit, 58: sensor group, 60: power supply circuit, 62: drive motor, 64: I/F control unit, 66: main control unit, 18S: exposure device control unit, 6S: fixing control unit, 3S: conveying motor control unit, 62S: drive control unit, 32V: charging roller power supply, 20V: developing roller power supply, 22V: toner supply unit power supply, 29V: transfer roller power supply, La: arm length, L0: coil natural length, L1: spring natural length, L2: spring center length when in use, L4: spring inner length when in use, θ: winding angle, P: medium.

Claims (9)

a charging member which is rotatable and contacts the surface of a member to be charged to charge the member;
a cleaning member that is rotatable and cleans the surface of the charging member;
an elastic member that is stretchable in a longitudinal direction and that brings the cleaning member and the charging member into contact with each other;
a holding member that holds one end side and the other end side of the elastic member in a bent state in a longitudinal direction and brings an outer periphery of the elastic member into contact with the cleaning member,
A charging device, characterized in that, when a natural length of the elastic member is L1 and an inner length of the elastic member in a bent state is L4, L1=L4. 2. The charging device according to claim 1, wherein when a layer thickness of the cleaning member is A, an amount of deformation of the cleaning member caused by the application of a nip force by the elastic member is B, and a nip amount is B/A, an inflection point at which a slope of an amount of change in the load on the cleaning member relative to an amount of change in the nip amount changes is in a range where the nip amount is 0.07 or more. 3. The charging device according to claim 2, wherein the inflection point is within a range of 0.07≦B/A≦0.51. The elastic member is
4. The charging device according to claim 1, wherein, when the charging device is used to contact the cleaning member in a bent state, a slope of a change in load on the cleaning member relative to a change in nip amount between the cleaning member and the charging member has a nonlinear characteristic. The elastic member is
5. The charging device according to claim 4, wherein when the charging device is used in contact with the cleaning member in a bent state, a slope of a change in the load on the cleaning member relative to a change in the nip amount changes around an inflection point. When the length of the center of the elastic member in the bent state is L2 and the outer diameter of the coil portion of the elastic member is R, the inner length L4 of the elastic member in the bent state is calculated by the following formula (1): L4 = L2 - 0.5π × R (1)
6. The charging device according to claim 1, wherein the charging member is a roller. When the winding angle θ is defined as the angle between a line perpendicular to an imaginary end face on one end of the coil portion of the elastic member and a line perpendicular to an imaginary end face on the other end of the coil portion , the inner length L4 of the elastic member in a bent state can be calculated by the following formula (2): L4 = L2 - 0.5π x R x (θ/180) ... (2)
6. The charging device according to claim 1, wherein the charging member is a roller. 8. An image forming unit comprising the charging device according to claim 1. An image forming apparatus comprising the image forming unit according to claim 8 .

Images