An image forming apparatus

Abstract: An image forming apparatus includes image bearing member; a device for forming an electrostatic latent image on the image bearing member; a developing device for developing a latent image formed on the image bearing member, the device including a developing unit having a developer carrying member for carrying thereon and supplying a developer to the image bearing member; a moving device for moving the developing unit between an operative position and a non-operative position away therefrom; a supporting member for supporting the developing unit for rotation about an axis, the supporting member rotatably supporting the developing unit at one side and supporting the developing unit at the other side for rotation and movement in a predetermined direction; an urging device for urging the developing unit toward the image bearing member adjacent the other side; a driver for applying driving force to the developer carrying member of the developing unit; wherein when the developing unit rotates to be set to the operating position, and the driver is operated, the developing unit is urged toward the image bearing member about the axis.

Description: FIELD OF THE INVENTION AND RELATED ART The present invention relates to an image forming apparatus such as an electrophotographic machine or an electrostatic recording machine, or particularly to an image forming apparatus provided with developing means movable between its operative position and non-operative position. A movable developing apparatus having developing units wherein a developing unit is selectively moved between its operative position and non-operative position in relation to the image bearing member, is used in a multi-color image forming machine or in a full-color image forming machine. In a commercialized full-color electro-photographic copying machine, a rotary member supporting plural developing units is rotated to move the selected developing unit to the operative position. The rotary type developing device, however, requires a considerable space for the mounting thereof in the copying machine, and therefore, the copying apparatus necessarily becomes bulky. Additionally, the attitude of the developing units varies due to the rotation of the rotary member, with the result of higher liability of the developer scattering in the developing unit. In order to solve these problems, an arrangement is described in our patent specification No. EP-A-0381518 which proposes that plural developing units are supported on one movable base, which is moved in a horizontal plane to present the selected developing unit to the operative position. In this image forming apparatus, the movable base carrying the plural developing units is moved along a predetermined straight line, and the developing unit is shifted in a direction perpendicular to the movement direction of the movable base using the force for moving the movable base to place the developing unit at the operative position. The present invention relates to the image forming apparatus wherein the developing unit is moved to the operative position where it is positioned correctly, and is directed to the assured positioning. An object of the present invention to provide an image forming apparatus wherein the developing unit is correctly positioned at the operative position, so that the image quality is high without blurness of the image. According to the invention there is provided an image forming apparatus comprising a latent image bearing means and a developing means including a developing unit having a developer carrying member for carrying thereon and supplying a developer to said image bearing means and supporting means for supporting said developing unit for rotation about an axis, the developing unit being movable in a first direction between a position remote from the latent image bearing means and a position adjacent thereto, and being urged, by rotation about the axis, towards the latent image bearing means in a second direction, transverse to the first, into a developing position for developing a latent image on the latent image bearing means, characterised in that said supporting means includes a first part rotatably supporting said developing unit at one end thereof when considered longitudinally of said unit and a second part supporting said developing unit at the other end for rotation thereof and for movement toward and away from the latent image bearing means; and in that there are provided urging means for resiliently urging the developing unit toward the latent image bearing means adjacent said other end, and driving means for applying a driving force to the developing unit. The apparatus of the present invention aims to provide an image forming apparatus wherein the vibration occurring during movement of the developing unit to the developing position is minimized. In the accompanying drawings:- Figure 1 is a side view of a developing apparatus having a developing unit raising mechanism in an image forming apparatus according to a first embodiment of the present invention. Figure 2 shows a driving system for the developing apparatus of Figure 1. Figure 3 is a partial top plan view of the system shown in Figure 2. Figures 4 and 5 are side views illustrating a developing apparatus raising mechanism in an image forming apparatus according to a second embodiment of the present invention. Figure 6 is a rear view of the apparatus of Figure 5. Figure 7 is a sectional view of an image forming apparatus according to an embodiment of the present invention. Figure 8 is a perspective view of a movable base of the developing apparatus. Figure 9 is a partial side view as seen in a direction B in Figure 8. Figure 10 is a perspective view of a developing unit. Figure 11 is a perspective view of a toner conveying type for a toner supplier. Figure 12 is a perspective view of a toner supplier. Figure 13 is a perspective view of a developing unit raising mechanism. Figure 14 is a side view of the developing unit raising mechanism. Figure 15 is a front view of the developing unit and the photosensitive drum illustrating the relationship therebetween. Figure 16 is a side view of the developing unit raising mechanism. Figures 17 and 18 are side views illustrating a comparison example of a developing unit in an image forming apparatus. Figures 19 and 20 are side views of another comparison example of a developing unit in an image forming apparatus. Figure 21 is a side view of an embodiment of a developing unit raising mechanism in an image forming apparatus. DESCRIPTION OF THE PREFERRED EMBODIMENTS The preferred embodiment of the present invention will be described in conjunction with the accompanying drawings. Referring first to Figure 7, there is shown a full-color electrophotographic copying machine as an exemplary image forming apparatus according to an embodiment of the present invention. The general arrangements will first be described. The copying machine has an image bearing member in the form of a photosensitive drum rotatable in a direction indicated by an arrow x having a surface electrophotographic photosensitive layer, in this embodiment. The copying machine further comprises a primary charger 2 at the leftside of the photosensitive drum 1, a surface potentiometer 3 for measuring the surface potential of the photosensitive drum 1, a developing device 100 right below the photosensitive drum 1, a horizontally (left-right) movable developing device 100 right below the photosensitive drum 1, the developing device 100 having a plurality of developing units, namely, a developing unit 101M, a developing unit 101C, a developing unit 101Y and a developing unit 101BK, an image transfer device 5 at the upper right of the photosensitive drum 1, a pre-charger 6 for decreasing the deposition force between the photosensitive drum and the residual toner remaining on the surface of the photosensitive drum 1 after the image transfer to make the cleaning operation easier, at a position right above the photosensitive drum 1, and a cleaning device 7 at upper left of the photosensitive drum 1. At the upper position, there is an optical system 10 to project light information corresponding to the image of the original on the platen 8 onto the photosensitive drum 1 at an image exposure station 9 between the primary charger 2 and the surface potentiometer 3. The optical system 10 may be of any type, and in this embodiment, it comprises a first scanning mirror 11, second and third scanning mirrors 12 and 13 movable in the same direction as the first scanning mirror 11 and and at a speed which is one half that of the first scanning mirror, an imaging lens 14, a CCD 15 integral with B, G and R filters, a laser scanner unit 16 and fixed mirrors 17 and 18. The scanner unit 16 is of a known type which deflects the laser beam by a polygonal mirror, and the detailed description thereof is omitted for simplicity. In the optical system 10, the light source 19 for illuminating the original moves together with the first scanning mirror 11. Therefore, the reflected light image of the original scanned by the first, second and third scanning mirrors 11, 12 and 13 is passed through the lens 14 and is color-separated by the CCD 15 having B-G-R color separation filter, and is converted into electric signals. The original image information signals are subjected to the image information process including analog-digital conversion or the like, and are supplied to microprocessor unit (MPU) controlling the entire copying machine, as video signals. The MPU responsive to the signals oscillates a laser beam from the laser unit through the laser driver, and the laser beam thus generated is projected on the photosensitive drum 1 while being imagewisely rendered on and off, thus starting the copy sequential operation. To the right of the copying machine in this embodiment, there are an image fixing device 20 and a sheet feeding device 30. Between the transfer drum 5 and the fixing device 20 and between the transfer drum 5 and the sheet feeding device 30, there are transfer sheet conveying systems 25 and 35. With this structure, the photosensitive drum 1 is subjected to the charging, image exposure, developing, image transfer and cleaning operations by the primary charger 2, the optical system 10 and the scanner unit 16, the developing device 100, the transfer device 5 and the cleaning device 7, for the respective color components into which the image is separated by the CCD 15. The developing device 100, which will be described in detail hereinafter has the developing units 101M (magenta developing unit), 101C (cyan developing unit), 101Y (yellow developing unit) and 101BK (black developing unit), detachably mounted on a movable table 120 (Figure 3). They function to develop the respective color separated components. The transfer device 5 is typically provided with a transfer drum 5b having a gripper 5a for gripping the transfer sheet P (image receiving member) on the periphery thereof. The transfer device 5 catches the leading edge of the transfer sheet P fed through the transfer sheet conveying system 35 from the transfer sheet cassette 31 or 32 of the feeding device 30, and carries by the rotation thereof for transferring the respective color images from the photosensitive drum 1 to the transfer sheet carried thereon. In the image transfer zone, a transfer charger 5c is disposed inside the transfer drum 5. In this embodiment, the transfer sheet P is caught by the gripper, but this is not limiting, and it is possible that the transfer sheet P is electrostatically attracted on the transfer sheet carrying sheet 5b by electrostatic attracting means. After the respective visualized images, namely, the toner images are sequentially transferred onto the transfer sheet P, the transfer sheet P is released from the gripper 5a, and is separated from the transfer material carrying sheet of the transfer drum 5b by the separation charger 8 and the separation pawl 8′. Then, the transfer sheet P is conveyed to the image fixing device 20 by the transfer material conveying system 25. The toner image on the transfer sheet P is fixed on the transfer sheet by the fixing device 20, and the transfer sheet is discharged to the tray 23. In this embodiment, the leading edge of the transfer material P is mechanically gripped by a gripper. This is not limiting, and another structure is usable such as electrostatic attraction means by means of which the transfer material is electrostatically attracted on the transfer drum. The description will be made as to the developing device 100 according to this embodiment. The developing device 100 is reciprocable in a horizontal plane, namely left-right direction in the Figure. Each of the developing units is capable of being presented close to the photosensitive drum 1 right below the photosensitive drum 1 with a predetermined clearance. It is desired that each of the developing units is away from the photosensitive drum 1 when it is not at the developing position. Otherwise, when all of the developing units are moved in the vicinity of the photosensitive drum 1, all the developing units are close to the photosensitive drum 1 even when they are returned to the original position (home position) after the developing operations for the four colors are completed. Then, the toner erected on the developer carrying member, that is, the developing sleeve of each of the developing units is contacted to the photosensitive drum, with the result that the unnecessary toner is deposited on the photosensitive drum. When the unnecessary toner is deposited on the photosensitive drum, the toner is entirely or partly transferred onto the transfer drum 5 to contaminate the transfer drum 5, when there is no transfer material between the photosensitive drum 1 and the transfer drum 5b. Adjacent to the developing operation position for the photosensitive drum 1, there are disposed the surface potentiometer 3 and a baffle plate to prevent transfer of the foreign matter such as paper dust onto the developing sleeve of each of the developing units from the above. In order to maintain the predetermined clearance between the developing sleeve and such elements without contact therebetween, each of the developing units is desired to be away from the photosensitive drum 1 when the developing operation is not performed. The developing device 100 movable in the horizontal plane according to the present invention is such that each of the developing units is in the vicinity of the photosensitive drum 1 only during its developing operation when it is right below the photosensitive drum 1, and otherwise, it is away from the photosensitive drum 1. Referring to Figures 8 - 16, the developing device 100 will be described in further detail. In this embodiment, the developing units 101 (101M, 101C, 101Y and 101BK) have the same structure, but the colors of the developers therein are different. Referring to Figure 10, the developing unit has an elongated developer container 106 having a rectangular cross-section. In the developer container 106, a developing sleeve 102 having therein a magnet is rotatably supported. At the longitudinal opposite ends of the developing sleeve 102, there are mounted rollers 103 for functioning as spacers maintaining predetermined clearances from the photosensitive drum 1. To one of the longitudinal ends of the developing sleeve 102, a driving gear 104 for driving the developing sleeve 102 is mounted. The magnet within the developing sleeve 102 is correctly positioned in the developing sleeve 102 by a positioning plate 190 disposed at the other longitudinal end of the developing sleeve. At the opposite ends of the developer container 106, there are supporting shafts 105F and 105R coaxially in parallel with the axis of the developing sleeve 102. The supporting shafts are effective to position and mount the developing units 101 on the movable table 120 (Figure 8) which will be described hereinafter. Within the developer container 106, there are screws 107 and 108 (only the shafts thereof are shown) for stirring and conveying the developer in the container. The screws 107 and 108 are driven through the gear train (not shown) by the driving gear 104 of the developing sleeve 102. One of the screws (that is, the screw 107 in this embodiment) is projected to the front side from the developer container 106 at its one end, and the projected portion is connected with a conveying pipe 160 which is in turn connected with a toner supplier 200 (Figure 13), which will be described hereinafter. Above the conveying pipe 160, an elongated supply port 106a′ is formed for connection with the toner supplier 200. More particularly, as shown in Figure 11, the supply port 160a′ of the supply pipe 160a is coupled with a supply port 241a of a shutter 241 of the supply pipe 202 of the toner supplying device 200. Therefore, when the developing unit 101 moves, the supply port 241a is guided to the supply port 160a′ of the developing unit, following the horizontal movement of the unit. Then, the supply pipe 202 swings to permit normal supply of the toner. Figure 12 shows an example of the toner supply device 200. Figure 12 is a perspective view showing the general arrangement of the toner supply device 200 without details. The toner supply device 200 has containers 201M, 201C, 201Y and 201BK for containing the magenta developer, the cyan developer, the yellow developer and the black developer, respectively. Supply pipes 202M - BK containing therein respective screws are swingable mounted to the toner containers 201M - BK to supply the toner from the respective container to the developing units 101M, 101C, 101Y and 101BK. The toner supply port 241a of the shutter 241 of the supply pipe 202 is engaged with a part of the developing unit, more particularly, with the supply port 160a′, as described hereinbefore so as to supply the toner to the developer container 106. The toner container of the toner supply device 200, the supply pipes and other supplying devices are mounted on the supply system supporting plate 203. The supporting plate 203 is rotatably mounted to a supporting base 204 fixedly mounted on a front plate (not shown) of the copying machine by pins 205a and 205b. When the copying operation is to be effected, the supply system supporting plate 203 is rotated toward the front plate, and is fixed by screws or the like to the front plate at an end opposite from the rotational pivots. The bottom of the developer container 106 is provided with guiding legs 106b (106b1 and 106b2), 106c (Figure 1). The guiding legs are slidably engageable with a slide guide 121 mounted on he movable table which will be described hereinafter. Figure 8 is a perspective view of the entire developing device 100 of the copying machine from the front, left and upper position. In this Figure, each of the developing units 101 are omitted for the better understanding. The developing device 100 has the movable table 120 for carrying the developing units 101. The movable table 120 is provided with the slide guide 121 (121M, 121C, 121Y and 121BK) mounted thereto by screws. The slide guide 121 is engageable with the guiding legs 106b and 106c of the associated developing unit. It is slidable in the direction indicated by an arrow A. The movable table 120 has a rear supporting plate 122 and a front supporting plate 123 corresponding to each of the slide guides 121. The rear supporting plate 122 has an opening 122′ engageable with the rotational shaft 105R of he developing unit 101, and the front supporting plate 123 has an opening 123′ engageable with the rotational shaft 105F of the developing unit 101. After the developing unit 101 is inserted along the slide guide 121, and then the shaft 105R is engaged with the opening 122′ of the rear supporting plate 122, the shaft 105F is engaged with the opening 123′ of the front supporting plate 123, and thereafter, the front supporting plate 123 is mounted by screws to the base table. Referring also to Figure 9, the movable base table 120 is provided with a rail 127 at its rear end. The rail 127 is slidably engaged with a rail supporting table 128 mounted on a rear plate 50 fixed on a bottom plate 60 of the copying machine. On the other hand, to the front end of the base table 120, a roller 129 is rotatably supported through a supporting plate 130. The roller 129 has a bearing therein. The roller 129 is capable of rolling on the base plate 60. With the above structures, the movable base table 120 is movable relative to the base plate 60 and the rear plate 50. The driving of the movable table 120 is accomplished by transmitting the driving force of a stepping motor 132 mounted on the bottom plate 60 by an unshown supporting means to a rack 131 fixedly mounted on the base table 120 through a gear train mounted on the supporting means and comprising the gears 133, 134 and 135. The stepping motor 132, gears 133 - 135 are constituted as a unit, which is mounted on the bottom plate 60. In this embodiment, the developing device 100 is horizontally moved substantially in a tangential direction of the photosensitive drum to the photosensitive drum 1 together with the base table 120, so that a selected one of the developing units is moved to the position substantially right below the photosensitive drum 1, and then, it is raised toward the photosensitive drum 1 and fixed at the operating position with a predetermined clearance from the photosensitive drum 1. Figure 13 is a perspective view illustrating a mechanism for raising the developing unit to position it relative to the photosensitive drum 1. For the purpose of easier understanding, the movable base 120, the slide guide 121M for the developing unit 101M are shown by chain lines, and the slide guides 121C, 121Y and 121BK for the other developing units are omitted. To the backside of the slide guide 121M, a short rack 140M is fixed, and similar racks 140 are mounted to the slide guides 121C, 121Y and 121BK of the other developing units. In addition, to the backside of the base 120, a long rack 141 is fixedly mounted. On the other hand, the bottom plate 60 has a rotational shaft 142 rotatably supported by a supporting plate 143. To the shaft 142, a clutch gear 144 is mounted at a position for engagement with the backside rack 141 of the base 120. When a voltage is applied to the clutch gear 144, the rotation of the gear is transmitted to the shaft 142. To the shaft 142, a gear 145 is mounted at a position for engagement with the rack 140M at the backside of the side guide 121. The number of teeth Z144 of the clutch gear 144 is smaller than the number of the teeth Z145 of the gear 145. The reason for this will be described hereinafter. Adjacent to the gear 145, a ring 146 is fixedly mounted on the rotational shaft 142, and a coil spring 147 is mounted between the gear 145 and the ring 146. An end of the coil spring 147 is engaged with an unshown groove formed in the ring 146, and the other end, as shown in Figure 14, is engaged with a hole 145a formed in a side of the gear 145. The gear 145 is provided with a parallel pin groove 145b in the form of a sector, and the gear 145 is urged in the direction D by the coil spring 147 so that an end of the sector groove is abutted to the parallel pin. Figure 15 shows the relation between the developing unit 101 and the photosensitive drum 1 as seen in the direction E in Figure 10. When one of the developing units 101 is to perform its developing operation, it is rotated about the pins 105F and 105R until the abutment rolls 103 abut the photosensitive drum, so that it is raised until the gap between the photosensitive drum 1 and the developing sleeve 102 becomes a predetermined level. In the structure described above, when the main switch of the copying machine is actuated, the discrimination is made as to whether or not the base 120 is at the home positions or not. The discrimination is accomplished, as shown in Figure 8, by a sensor 181 constituted by a light emitting element and a light receiving element fixedly mounted on the bottom plate 60 and a light blocking plate 180 adjustably mounted to the base 120. If the base 120 is not at the home position, the stepping motor 132 rotates to move the base 120 until the light of the sensor 181 is blocked by the blocking plate 180, that is, until it reaches the home position. Referring to Figure 16, the further description will be made. The home position is set to be the position where the magenta developing unit 101M is away from right below the photosensitive drum 1 toward left as seen from the front by a distance l , where l is a distance between the pin 105F when the developing sleeve 102 is right below the photosensitive drum 1 and a pin 105F when it is at the home position, and is equal to the intervals between adjacent developing units. The position is adjusted by adjusting the light blocking plate 180. When a copy button is actuated to start the copying operation, the microprocessor unit (MPU) for controlling the operation of the main assembly of the copying machine supplies a signal to a pulse generator for the stepping motor 132 for driving the base 120 to displace the base 120 at a predetermined speed until the developing sleeve 102 of the magenta developing unit 101M is placed right below the photosensitive drum 1. Then, the pulse generator produces pulse signals corresponding to the movement distance and the moving speed, and the pulse signals are reformed and distributed by the driver of the stepping motor 132, so that the electric current is supplied to the coils of the stepping motor in a predetermined sequence for the respective pulses, by which the motor 132 is rotated. The stepping motor 132 stepwisely rotates by a predetermined angle in accordance with the number of input pulses, and the angular error is extremely small, and therefore, the stop position of the base 120 is very accurate, so that the influence to the image due to the variation in the stoppage accuracy is negligible. Together with movement of the base 120, the magenta developing unit 101M is raised to approach the photosensitive drum 1. The raising operation will be described in detail. Referring back to Figure 13, as described hereinbefore, the rack 141 and the clutch gear 144 are in meshing engagement with each other at the home position of the base 120. Immediately before the base 120 starts its movement, the clutch of the clutch gear 144 is shifted to an engaging position, so that the shaft 142 and the gear 145 rotate together with the movement of the base. During the movement of the base 120, the racks 140 fixedly mounted on the bottom of the slide guide 121 are brought into meshing engagement with the rotating gears 145. The number of teeth Z144 of the clutch gear 144 is smaller than the number of teeth Z145 of the gear 145. Therefore, if it is assumed that the position where the rack 140 start to mesh with the gear 145 is away by l ₁ (Figure 16, l ₁ < l ₂) the clutch gear 144 rotates through a distance l ₁ on a pitch circle thereof when the base 120 moves through the distance l ₁. The distance l ₁ is converted to a rotational angle of the shaft 142 as follows. α = l ₁/r144 (rad) where r144 is a radius of the pitch circle of the clutch gear 144. Therefore, the circumferential length on the pitch circle through which the gear 145 rotates, that is, the movement distance l ₂ of the slide guide 121M integral with the rack 140 meshed with the gear 145 is where r145 is a radius of the pitch circle of the gear 145. Here, Z145 > Z144, that is, r145 > r144, and therefore, l ₂ > l ₁. Thus, from the start to the end of the l ₁ movement of the base 120, the slide guide 121M slides relative to the base 121 by the distance l ₂ - l ₁. By the movement by l ₂ - l ₁ of the slide guide 121M relative to the base 120, as shown in Figure 16, the slide guide 121M pushes the leg 106b of the developing unit 101M which is rotatable on the base 120 about the pins 105F and 105R. As a result, the magenta developing unit 101M is raised in the direction B during the movement of the base 120 and is stopped at a position close to the photosensitive drum 1 at a position right below the photosensitive drum 1 by the abutment rollers 103. At the time of the raising movement, the leg 106C of the developer container 106 moves away from the slide guide 121M, as will be understood from the Figure, and therefore, as shown in Figure 8, the slide guide 121 has a cut-away portion 121M′ corresponding to the leg 106C to permit this movement. In order to urge the abutment rollers 103 to the photosensitive drum 1 with a predetermined pressure, the distance through which the developing unit 101M is raised in the direction B is d + α (α: positive), where d is a distance between the abutment rollers 103 and the surface of the photosensitive drum at the home position. To provide the proper raising distance, one skilled in the art determines the number of teeth of the gears 145 and 144 in consideration of the movement distance. When the developing unit 101M is positioned closely to the photosensitive drum 1 through the abutment roller 103 right below the photosensitive drum 1, the developing unit 101M receives a reaction force from the photosensitive drum 1, corresponding to the distance α. The gravity force applied to the developing unit is added to the reaction force. The sum of the forces tends to rotate the developing unit 101M about the pins 105F and 105R in the clockwise direction, and therefore, the slide guide 121M is pushed to the left (opposite from the direction A) by the leg 106b of the developer container. By this, the rack 104M meshed with the slide guide 121M tends to rotate the gear 145 in the counter clockwise direction. The gear 145, as described hereinbefore, is urged in the direction D by the coil spring 147. However, the coil spring flexes until the reaction force from the photosensitive drum 1 and the movement of the developing units are balanced with the urging force by the coil spring. The flexibility is provided by the parallel pin groove 145b in the sector form of the gear 145. Thus, briefly, the operation is as follows. From the point where rack 140 starts to mesh with gear 145, the base 120 moves through the distance 1 ₁. Accordingly, pin 105F must also move through distance 1 ₁. By the action of clutch gear 144, this rotates the shaft 142 through a certain angle, which also rotates gear 145 (driven by spring 147). However, because of the different number of teeth on gears 145 and 144, gear 145 drives short rack 140 through distance 1 ₂. Accordingly, leg 106b is driven through distance 1 ₂. Therefore, the bottom of developer container 106 moves further than the top of container 106 by the distance 1 ₂ - 1 ₁, so that the container 106 pivots round pin 105F, raising it up to the photosensitive drum 1. After the abutment rollers 103 have abutted against the photosensitive drum 1, no further upward rotation of the developer container 106 is possible. Therefore as base 120 continues to move, the gear 145 rotates with a slower angular velocity than gear 144 and shaft 142, and this difference in rotational velocity results in flexing of the coil spring 147, which thereby provides a reaction force which presses the abutment rollers 103 firmly against the photosensitive drum to locate the developer container 106. The wire diameter and the number of windings of the coil spring 147 are determined so that the reaction force from the photosensitive drum 1, that is the pressing force of the abutment rollers 103 to the photosensitive drum 1 is a predetermined proper force. The rotational shaft 142 is coupled with the stepping motor 132 through the clutch gear 144, the rack 141 and the rack 131. The self-retaining force of the stepping motor 132 acts on the rotational shaft 142, so that the correct movement of the base is accomplished without influence of the rotation of the gear 145. In the manner described above, when the developing unit 101M is raised, the abutment rollers 103 are press-contacted to the photosensitive drum 1 with a predetermined force, the developing sleeve 102 is placed close to the photosensitive drum 1 with a predetermined clearance therebetween. When the developing action for the magenta color is completed, the clutch gear 144 (Figure 13) is disengaged. This releases the rotation of the shaft 142, so that the fear 145 integral therewith becomes freely rotatable. Then, the reaction force from the photosensitive drum 1 and the gravity applied to the developing unit 101M causes the leg 106b of the developer container 106 to push the slide guide 121M back, that is, to the left. Thus, the developing unit 101M returns to the lower position shown at the left of Figure 16 to be away from the photosensitive drum 1. Subsequently, the stepping motor 132 rotates through a predetermined amount, and the clutch gear 144 is engaged, by which the cyan developing unit 101C is raised right below the photosensitive drum 1. Then, the developing sleeve 102 is driven to effect the cyan color developing operation. In the similar manner, the yellow and black developing operations, and the image transfer operation for the respective colors on the same transfer material, are performed, and then, the transfer sheet is separated from the transfer drum 5 by a separation charger 8 and the separation pawl 8′. It is then fixed by an image fixing apparatus and is discharged to the discharge tray 23, so that the copy operation is completed. As described in the foregoing, since the developing device is movable in a horizontal plane below the photosensitive drum, the developing device may be located below the transfer drum, and the developing sleeve is at the upper portion of the developer container. Therefore, the scattering of the toner and carrier can be suppressed much more than in conventional full-color electrophotographic copying machines. Thus, the necessity for preventing the toner and carrier scattering and for cleaning the apparatus can be eliminated, and in addition, a large rotatable developing unit frame member can be eliminated. As a result, a small size low cost full-color copying machine can be produced. In the developing apparatus of the above embodiments, the plural developing units are not supported on a rotatable frame, but are supported for horizontal movement, and therefore, the developing units are not reversed. For this reason, the behavior of the developer in the developing units is stabilized, whereby the control of the toner and carrier mixture ratio (T/C ratio) can be performed more accurately than in the conventional apparatus. This enables stabilized images to be produced at all times, which will be also desired by the users. When the above described advantageous structure is employed, the developing units are located below the photosensitive drum, and the selected developing unit is raised to the photosensitive drum. In this respect, the following problems have been found. Referring to Figure 15, the developing unit 101 are supported at four positions, more particularly, by the front side pin 105F, the rear side pin 105R, a front side abutment roller 103F and a rear side abutment roller 103R. If the supporting pins 105F and 105R are fixed, one of the abutment rollers 103F and 103R is not abutted as long as the drum axis and the developing unit axis are not completed parallel to each other. Even if special tools are used to establish parallelism between the axis of the photosensitive drum and the axis of the developing unit, they can not be completely parallel, so that it is difficult to assure the gap between the photosensitive drum 1 and the developing sleeve 102 on the order of several hundreds microns. Additionally, the assembling operation is difficult, and the number of parts for the adjustment is increased. The possibility of using a flexible structure for the developing unit particularly for the developer container is considered. However, it is difficult from the standpoint of properly sealing the magnet roller, assuring the sealing of the screw shaft and assuring the accuracy of the gap between the drum and the sleeve. Figure 17 shows a generally used solution to such a problem, in which one of the front and rear pins are supported with play. More particularly, the front side pin 105F is received by a bearing 109F′, and the bearing 109F′ is supported in a hole 109F″ elongated in the vertical direction, the hole being formed in a supporting plate 123 securedly mounted on the main assembly of the developing apparatus. However, the inventors' experiments have shown that the front side pin 105F does not rise from the bottom end of the elongated slot, as schematically shown in Figure 18, with the result that the abutment roller 103F did not contact the photosensitive drum. Figure 19 shows the analysis for the reason. As will be understood, the reason is that the raising force for the developing unit is in the direction tangential to the movement track of the developing unit rotation from the standpoint of most efficient rotational movement, so that the force for raising the pin 105F is hardly applied. Figure 20 shows an example of a solution, in which an upward force strong enough to bear the weight of the developing unit is applied at a position right below the pin 105F. Then, the component of the force effective for the rotation is reduced, so that the raising mechanism has to apply to the developing unit the force which is larger by the corresponding degree, with the result of increased load to the raising mechanism, so that it is most desirable. Figure 21 shows the structure by which the above described problems are avoided. In this structure, the bearing 109F′ supporting the front pin 105F is raised to the top end of the vertically elongated slot 109F″ against the weight of the developing unit 101 by the urging force of the spring 110. Since the front side pin 105F is at a position above the rear pin 105R, the front abutment roller 103F first abuts the photosensitive drum 1 when the developing unit swings. The force of the slide guide 121 to the leg 106b produces a moment about the pin 105F, and a force is applied to the photosensitive drum 1 by the abutment roller 103F, so that the developing unit 101 receives a reaction force in the downward direction. By the downward force, the pin 105F lowers against the raising force of the spring 110 until it is stopped when the rear abutment roller is abutted to the photosensitive drum so that it is supported at the three points, namely, the two abutment rollers and one of the pins. Using the above described mechanism, the developing device movable in the horizontal plane can be placed below the photosensitive drum, and any selected one of the developing units thereon can be shifted toward the photosensitive drum. However, the elastic force of the spring 110 for raising the pin 105F is required to be relatively large, and therefore, a correspondingly large force is required to be applied to the developing unit to overcome it. This will be described in further detail. With the U-shaped spring 110 as shown in Figure 21, it is difficult to provide a correct spring force. If the spring force is enough to provide the required raising force, the front pin is not fixed, so that the pivot moves when any swinging force is applied to the leg 106b of the developing unit, and therefore, the front abutment roller 103F is not abutted with sufficient abutment force, even to such an extent that the abutment roller 103F is not contacted to the drum. In order to assuredly raising it, the spring force of the spring 110 is desired to be sufficiently large with ample safety factor. The position of the front pin 109F′ at the time when the developing unit it raised may be closer to the top end of the elongated slot 109F″ or to the bottom end thereof, corresponding to the inclination of the rotational axis relative to the axis of the photosensitive drum. The spring force is required to be set such that the sufficient spring force can be provided even if the bearing 109F′ is close to the top end of the elongated slot 109″, and therefore, the expansion of the spring 110 is small. Therefore, when the bearing 109F′ is close to the bottom end of the elongated slot 109F″, and therefore, the expansion of the spring is large, the force required for expanding the spring 110 is further increased. For this reason, too, the spring force of the spring 110 tends to be large. In the raising operation of the developing unit described in conjunction with Figure 21, the front abutment roller 103F first abuts the drum, and then, the raising spring 110 is expanded by the moment provided by the pushing force of the slide guide 121 to the left 106b, by which the rotational axis becomes parallel to the axis of the photosensitive drum. As described hereinbelow, if the spring force of the raising spring 110 is large, the force required for rotating the developing unit is large, correspondingly, so that the load to the raising mechanism is increased. More particularly, in order to assure the mechanical strength of the racks and gears for driving the slide guide 121, high strength materials and large gear width are required with the result of increased cost and increased space. In this embodiment, the developing unit raising force is provided by the driving system for the movable base 102 for carrying the developing unit, and therefore, the load to the base driving motor is increased. If the raising force is further increased, the impact or shock caused upon abutment of the abutment rollers 103F and 103R to the photosensitive drum 1 becomes large with the result of non-uniform pitch of the image and misregistration of the color, thus degrading the image quality. In addition, the noise is increased. According to an embodiment of the present invention, the driving mechanism for the developing sleeve is improved to solve the above problems. Figures 1, 2 and 3 illustrate a developing apparatus 100 according to an embodiment of the present invention. Referring to Figure 3, in this embodiment, the rear supporting plate 124 supports the driving gears and clutches which will be described in detail hereinafter and also supports a supporting plate 126 for supporting the motor 125 for driving the developing sleeve. Figure 2 shows a driving system as seen from the front side of the rack 131, that is, in the direction C in Figure 8, and Figure 3 is a top plan view thereof. Referring to Figures 2 and 3, the output of the motor 125 in this embodiment is transmitted from its output gear 150 to the idler gear 152 mounted on the drive base 124, and further to a clutch gear 153M which has a rotational shaft 154 rotatably mounted on the drive base 124 and the supporting plate 155 and which selectively transmits the driving force to the developing sleeve 102. The driving force from the clutch gear 153M is transmitted through the rotational shaft 154 to the gear 158 mounted on the drive base 124. The gear 158 is meshed with the driving gear 157 for the developing sleeve to transmit the force thereto from the motor. At this time, the shaft 156 of the gear 157 is coaxial with a hole 122′ formed in the rear shaft supporting plate 122 cramped on the rear supporting plate 124. When the magenta developing unit 101M is inserted along the sliding guide 121M, the shaft 105R of the developing unit 101 is engaged with the hole 122′ of the rear supporting plate 122, as shown in Figure 3 (top plan view), by which the developing sleeve driving gear 157 and the gear 104 mounted to the end of the developing sleeve 102 are meshed with each other with a predetermined backlash. Figure 1 shows best the way of force transmission when the developing unit 101 receives the driving force, in this embodiment, Figure 1 shows the developing unit 101 as seen in the direction C in Figure 8. For the best understanding, the drive base 124 and the gears and clutches of the driving system are omitted with the exception that the developing sleeve driving gear 157. The developing unit 101 is moved to below the photosensitive drum 1 at the development operating timing, and simultaneously, it is urged to the slide guide 121 by force F1 and is rotated about an axis 200 (which corresponds to the pin 105R in Figure 10) to be raised. Then, the developing sleeve driving gear 157 which is mounted on the drive supporting plate 122 (Figure 3) coaxially with the pivot 200 drives the gear 104 of the developing unit, thus transmitting the driving force to rotate the developing sleeve. The gear 104 and the pivot 200 are positioned as shown in Figure 1. More particularly, the pivot 200 is disposed at the right side of the gear 104 in Figure 1 and at the same level as or above the center of the gear 104, and in addition, the rotational direction is such that the gear 104 rotates in the counterclockwise direction in Figure 1, by which the developing unit 101 receives force F2 from the developing sleeve driving gear 157 as shown in the Figure. The force F2 produces a clockwise moment about the pin 105R to assist the raising of the developing unit by the slide 121. In this embodiment, the other pin (the pin 105F in Figure 10) has the same structure as shown in Figure 21. The direct transmission of the rotational force to the developing unit at the supporting side (the pin 105R side) is effective to assuredly raise the developing unit. After the developing operation, the slide guide 121 and the developing sleeve driving gear 157 are released, upon which the developing unit 101 lowers with rotation by its weight. According to the driving system in the present invention, the raising force provided by the raising mechanism can be reduced in connection with the spring force of the raising spring 110 which functions only to assuredly abut the front side, and therefore, the load of the raising mechanism is reduced. In addition, the impact or shock upon the raising action can be reduced, so that the image misregistration or the non-uniform pitch is reduced, thus improving the image quality. Furthermore, the noise or vibration in the raising operation can be reduced. Figures 4 - 6 shows a further embodiment. In this embodiment, no particular mechanism is provided to rotate and raise the developing unit. When the developing unit is moved to a position below the photosensitive drum for the developing operation, an abutment member of the developing unit abuts the photosensitive drum or an abutment member correctly positioned relative to the photosensitive drum, by which the pivot of the developing unit urged upwardly is raised, so that the proper abutment pressure can be provided by the reaction. At the other pivot constructed for only rotation, the distance between the developing unit and the abutment member of the photosensitive drum is set to be so small that they are not contacted when the developing operation is not performed. Thus, only by the driving force for the developing sleeve driving gear, the rotational force is provided to abut them to each other. Referring to Figures 4 - 6, only the portions which are different in the previous embodiment will be described. Figure 4 shows the developing unit in this embodiment as seen from the front side. For the sake of simplicity, only one of the four developing units is shown. The apparatus of this embodiment does not have the mechanism for rotating the developing unit such as slide rails 121 or racks 140 as shown in Figures 8 and 13 of the previous embodiment. In place thereof, a stopper 192 is used to limit the rotation of the developing unit 101 to maintain the abutment roller 103F at a high position. In this structure, when the developing unit 101 is moved horizontally in the direction A by the movable base 120 not shown in Figure 4, the front abutment roller 103F abuts an abutment member 191F correctly positioned relative to the photosensitive drum 1 so as to lower the pin 109F′, when it reaches the operating position for the development, as shown in Figure 5. Figure 6 shows the developing unit as seen from behind, that is, in the opposite direction from Figures 4 and 5. As will be understood from Figure 6, an abutment member 191R is used at the rear side, similarly to the front side. When the developing unit is not driven, the abutment member 191R is slightly spaced apart from the abutment roller 103R. When the developing sleeve is driven by the driving gear 157 coaxial with the pivot 202 (corresponding to the pin 105R), the force F2 by the driving gear 157 rotates the developing unit against its weight, by which the abutment roller 103R abuts the abutment member 191R. At this time, the driving torque for the developing sleeve driving gear is required to be larger by the weight of the developing unit. According to this embodiment, the developing unit may be abutted to the photosensitive drum 1 assuredly and can be correctly positioned relative thereto without particular mechanism for raising the developing unit. The movement direction of the developing unit 101 toward and away from the photosensitive drum 1 is not limited to the horizontal direction, but may be vertical. In the foregoing description of the embodiments, a full-color copying machine has been taken, but the present invention is applicable to a multi-color copying machine or a monochromatic copying machine, if one or more developing units are movable between an operating position for developing the photosensitive drum and a non-operative position. As described in the foregoing, the developing unit of the image forming apparatus according to the embodiments of the present invention has plural pivots, one of which is movable and urged by urging means, and the pivots and a driving gear are so disposed that when the developing unit is moved, the developing unit is urged toward the image bearing member, by which the deformation of the urging member is assisted upon rotation of the developing unit. Therefore, the load imposed on the rotating mechanism can be reduced, so that the durability and reliability of the rotating mechanism is increased. In addition, the vibration during the rotation of the developing unit is reduced, and the correct positioning of the developing unit at the operating position is assured, which is effective to reduce the blurness and increases the image quality. While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth and this application is intended to cover such modifications or changes as may come within the scope of the following claims.

Claims: An image forming apparatus comprising a latent image bearing means (1) and a developing means (100) including a developing unit (101) having a developer carrying member (102) for carrying thereon and supplying a developer to said image bearing means (1) and supporting means (122, 123) for supporting said developing unit (101) for rotation about an axis (105F, 105R), the developing unit (101) being movable in a first direction (A) between a position remote from the latent image bearing means (1) and a position adjacent thereto, and being urged, by rotation about the axis (105F, 105R), towards the latent image bearing means (1) in a second direction (B), transverse to the first, into a developing position for developing a latent image on the latent image bearing means (1), characterised in that said supporting means (122, 123) includes a first part (122) rotatably supporting said developing unit (101) at one end thereof when considered longitudinally of said unit and a second part (123) supporting said developing unit (101) at the other end for rotation thereof and for movement toward and away from the latent image bearing means (1); and in that there are provided urging means (110) for resiliently urging the developing unit (101) toward the latent image bearing means (1) adjacent said other end, and driving means (125, 150, 152, 153M, 158, 157) for applying a driving force to the developing unit (101). An apparatus as claimed in claim 1, characterised in that said driving means is arranged to apply said driving force to the developing unit adjacent said one end for driving the developer carrying member (102) and also for urging the developing unit (101) toward the latent image bearing member (1). An apparatus according to claim 2, characterised in that said driving means includes a driving gear (157) which is coaxial with the axis for rotation of said developing unit (101), and said developing unit (101) includes a driving force receiving gear (104) mounted on said developer carrying member (102) and engaged with the driving gear (157) adjacent said one end. An apparatus according to claim 1, 2 or 3 characterised in that said developing means (100) has a plurality of such developing units (101M, C, Y, BK). An apparatus according to claim 4, characterised in that means (131-135) are provided for reciprocating the plural developing units (101M, C, Y, BK) along a rectilinear line in the first direction (A). An apparatus according to claim 5, characterised in that said plural developing units (101M, C, Y, BK) are arranged on one movable base (120). An apparatus according to claim 6, characterised in that said plural developing untis (101M, C, Y, BK) are movable in a substantially horizontal plane below said image bearing means (1). An apparatus according to any one of claims 4 to 7, characterised in that said plural developing units (101M, C, Y, BK) contain different color developers, respectively. An apparatus according to claim 8, characterised in that said image forming apparatus is capable of forming a full-color image. An apparatus according to any one of claims 4 to 9, characterised in that the developing position is common to said plural developing units (101M, C, Y, BK). An apparatus according to any one of claims 1-10 characterised in that the developing position is arranged beneath said image bearing means (1).

Inventors: HATAKEYAMA TAKUYA C O CANON KA, KOIKE MICHIRO C O CANON KABUSH, HATAKEYAMA, TAKUYA, C/O CANON KABUSHIKI KAISHA, KOIKE, MICHIRO, C/O CANON KABUSHIKI KAISHA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1990
Main classification: G03G15
IPC codes: G03G15