Patent Publication Number: US-6985684-B2

Title: Image forming apparatus having preset developer unit replacement positions and a locking device

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to an image forming apparatus, such as an electrostatic copying machine and a printer, comprising a developing device of a rotary developing type which includes a plurality of development units attached to a rotary for conducting a multicolor development such as a full-color development. More particularly, the present invention relates to an image forming apparatus in which the replacement of a development unit of a cartridge style is conducted at a preset replacement position. Further, the present invention relates to an image forming apparatus comprising a locking means for locking a rotary in such a position that the respective developer carriers of a plurality of development units are selectively set to a development position relative to a latent image carrier. 
   Among conventional image forming apparatuses such as electrostatic copying machines and printers, various image forming apparatuses of a type comprising a developing device which conducts a multicolor development such as a full-color development by a plurality of development units attached to a rotary have been created. In such an image forming apparatus, the rotary is rotated to sequentially bring the respective development rollers of the development units to a development position so as to sequentially develop latent images for the respective colors on a photoreceptor, thereby achieving the image formation. 
   Among image forming apparatuses of this type, an image forming apparatus has been proposed which is provided near the outer periphery of the rotary with a development unit driving gear to which the driving force of a development unit driving motor is transmitted for rotating a developer carrier such as a development roller of one of the development units, brought in the development position, while each development unit has an input gear to be selectively meshed with the development unit driving gear. By meshing the input gear with the development unit driving gear at the development position, the developer carrier is driven by driving force of the development unit driving motor so that a latent image is developed with toner carried by the developer carrier. 
   It is technically difficult to replenish toner to the rotating development unit. There is an idea of achieving the replenishment of toner by employing a single component developer and replacing a development unit of a compact cartridge type at a predetermined replacement position. In this case, it is required to provide well workability enabling the easy replacement of the development unit. Therefore, it is preferable that the replacement position is set at a non-development position where there is no possibility of damaging the latent image carrier, not in the vicinity of the latent image carrier and that the direction of detaching the development unit is set to the axial direction or the radial direction. 
   By the way, during the rotary with the plural development units mounted thereon is rotated, the input gears of the development units collide with the driving gear which is rotatably disposed near the outer periphery of the rotary, thus generating impact on the both gears due to the collision. In the normal image forming operation, the rotation of the rotary stops when the development unit reaches the development position. Accordingly, the input gear collides with the driving gear at extremely low speed to come in mesh with the driving gear. Though the impact during the collision is therefore relatively small, it is desired to further reduce the impact to achieve the smooth meshing between the both gears. 
   To reduce the impact due to collision between the both gears during rotation of the rotary in the normal image forming operation, Japanese Patent Publication No. H4-11030 discloses that a driving gear is supported to a pivotal shaft fixed at a predetermined position near the orbit of an input gear toward the development position such a manner as to allow a rocking movement of the driving gear by an elastic member, thereby reducing the impact due to collision between the input gear of the rotary side and the driving gear during the normal image forming operation and thus achieving the smooth meshing between the both gears. 
   Japanese Patent No. 3129875 discloses the rotation of a rotary for detecting the absence or presence of the development unit, that is, the rotation of the rotary in the non-image forming operation where the normal image forming operation is not conducted. 
   However, Japanese Patent Publication No. H4-11030 has neither disclosed nor suggested any rotation of the rotary in the non-image forming operation, for example, the rotation of the rotary for replacing a development unit as mentioned above. Therefore, the invention disclosed in Japanese Patent Publication No. H4-11030 is useful for reducing the impact due to collision between the input gear and the driving gear in the normal image forming operation, but not always useful for reducing the impact due to collision between the input gear and the driving gear during the rotation of the rotary in the non-image forming operation. 
   That is, in case of replaceable development units, the rotary is required to rotate to bring a development unit to be replaced to the replacement position in non-development position as mentioned above. Accordingly, the input gear of the development unit or the input gear of other development unit must pass the position where it collides with the driving gear. According to the rotation of the rotary during the operation for replacing development unit in the non-image forming operation, the input gear and the driving gear collide with each other so as to produce impact. The impact of this case affects the driving of the rotary even though the driving gear, disposed to allow its rocking movement because of the elastic member, evades away from the input gear. 
   In particular, in case of replacing a plurality of development units, the rotary is subjected to imbalance due to offset load generated in a state that one or more development units are removed. In this state, the impact is further increased to affect the driving of the rotary, thus not only further affecting the driving of the rotary but also increasing the load of the rotary driving motor. As the load of the rotary driving motor is increased, in case of employing a stepping motor as the rotary driving motor, the stepping motor may be stepped out, making the reliability poor. 
   It is desired to reduce the time required for replacing development unit as shorter as possible to achieve rapid replacement operation. However, as the rotary is rotated at a relatively high speed, the aforementioned impact is further increased. This is also a problem. 
   The above problems may be caused not only during the rotation of the rotary for replacing development unit but also during, for example, the rotation of the rotary for resetting and initializing the phase of the rotary after power-on. 
   Japanese Patent No. 3129875 has only disclosed the rotation of the rotary in the non-image forming operation, but has neither disclosed nor suggested the impact due to the collision between the input gear and the driving gear during this rotation of the rotary, that is, not considered such problems due to the impact as mentioned above. 
   On the other hand, as a method of sequentially positioning the rotary at a predetermined position and locking the rotation of the rotary at the position, the simplest method is utilizing the holding force of a driving motor for rotating the rotary. However, in view of the power consumption for holding operation and the holding capacity of the driving motor, it is preferable to position the rotary by some mechanical means without resort of holding force of the driving motor. In case that a single driving motor is used for driving both the development units and the rotary, it is required to provide two power transmission control means in the corresponding driving gear trains, respectively. When the driving motor drives either of the development units or the rotary, the power transmission control means in the corresponding driving gear train is connected and the power transmission control means in the other driving gear train is isolated. That is, since the power transmission control means in the rotary driving gear train is isolated when the development unit is driven after positioning of the rotary, it is impossible to utilize the holding force of the driving motor. Therefore, another mechanical means is required for positioning the rotary. Conventionally, it has been proposed that the positioning of the rotary is mechanically achieved by engaging a lever, attached to the body of the image forming apparatus, with a part of the rotary. 
   The lever is disposed such that it can move between an evacuation position where it does not position the rotary and a holding position where it is engaged with the rotary to position the rotary. In this case, in case of forming a full-color image from four colors, the operation of changing color among the four colors is required for sequentially developing electrostatic latent images of four colors on a latent image carrier. Since it is preferable to shorten the time required for this operation of changing color as shorter as possible, the lever is required to rapidly move between the evacuation position and the holding position. 
   Accordingly, in a general conventional manner, a solenoid and a spring are used for operating the lever and are operated under consideration of the rotational time of the rotary and the energizing time of the solenoid. That is, for setting the lever to the evacuation position, the solenoid is energized to produce solenoid force, thereby rotating the lever to the evacuation position. For setting the lever to the holding position, the energization of the solenoid is cancelled and the lever is rotated to the holding position by the biasing force of a spring. 
   The lever shifting means is not limited to the aforementioned means employing a solenoid. The lever may be rotated by utilizing a stepping clutch or a cam driven by another moving means. 
   In addition, proposed in Japanese Patent Publication No. H7-117784 is an image forming apparatus of a rotary developing type employing another mechanical means for locking the rotary which is composed of a cam, attached to the rotary, and a stopping means, attached to the body of the image forming apparatus. The stopping means is always in contact with the cam to act corresponding to the rotation of the cam, thereby conducting the positioning of the rotary. In the rotary positioning means disclosed in Japanese Patent Publication No. H7-117784, the cam is formed to have a profile having a relatively sharp inclined face when the stopping means positions the rotary to the development position and a relatively gentle inclined face when the rotary rotates away from the development position. 
   As for the positioning means using the solenoid to operate the lever, the time from energization of the solenoid to the actual movement of a plunger of the solenoid is constant as a characteristic of a solenoid to be used. The selection of the solenoid can be made based on its characteristics according to the type of usage of the image forming apparatus. 
   However, since the time from the cancellation of the energization to the actual release by the plunger is not constant, it is required to set enough time for moving the lever to the holding position for the rotary from the stop of the rotary at the predetermined position. Even a case employing a shifting means for the lever other than the means using the solenoid, such as the means for shifting the lever by using a stepping clutch or a cam as mentioned above, it is relatively easy to estimate the operation time, but it is necessary to set the operation time after the stop of the rotor because the lever should be shifted to the holding position after the stop of the rotary. This operation time for shifting the lever to the holding position directly affects the operation time of changing color to make the operation time of changing color relatively longer. As a result, there is a problem that it is difficult to improve the speed of multi-color development. 
   In the positioning means disclosed in Japanese Patent Publication No. H7-117784, since the stopping means is always in contact with the cam, it is required to increase the contact force of the stopping means relative to the cam as the speed of the rotary for the operation of changing color is increased. This is because if the rotation of the rotary is speeded up with week contact force of the stopping means relative to the cam, the stopping means can not follow the sharp change in profile of the cam so that the stopping means may bounce and may come off the cam in the development position. 
   However, as the contact force of the stopping means relative to the cam is increased, the frictional force between the stopping means and the cam is increased. This frictional force may disturb the smooth rotation of the rotary. That is, the positioning means has a problem that there is a limitation on speed-up of the rotation of the rotary. 
   The initializing operation for detecting the position of the rotary is conducted just after power-on of the image forming apparatus. Since the rotary passes the development position without stopping at the development position during its rotation for the initializing operation, a strong rotational moment acts on the rotary driving side due to the collision of the stopping means against the cam during the stopping means passes the development position. This strong rotational moment affects the rotation of the rotary. This is a problem due to the structure that the stopping means is always in contact with the cam. Further, the direction of the reaction during the development reaction is inevitably determined by the configuration of the cam. This is a problem that the degree of design freedom of the image forming apparatus must be smaller. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide an image forming apparatus in which the impact produced due to collision between an input gear of a development unit and a development unit driving gear during the rotation of a rotary in the non-image forming operation is effectively absorbed and the time required for rotation of the rotary in the non-image forming operation is reduced as shorter as possible, thus achieving the quick rotation of the rotary. 
   It is another object of the present invention to provide an image forming apparatus in which the speed for multi-color development is further improved by shortening the time required for positioning the rotary as shorter as possible so as to effectively shorten the time for operation of changing color. 
   It is still another object of the present invention to provide an image forming apparatus in which both development units and a rotary are driven by a single driving motor by controlling the transmission of driving force of the single driving motor, in which the speed for multi-color development is further improved by shortening the time required for positioning the rotary as shorter as possible so as to effectively shorten the time for operation of changing color, and the smooth rotation of the rotary is achieved. 
   It is further another object of the present invention to provide an image forming apparatus in which the adverse effect due to the reaction force during the development operation is reduced so as to increase the degree of design freedom of the image forming apparatus. 
   To achieve the aforementioned objects, the image forming apparatus of the present invention is an image forming apparatus comprising at least: a rotary, a rotary driving motor for rotating the rotary, a plurality of development units each of which is mounted to said rotary such that the development unit is replaceable at a replacement position and has a development roller for carrying developer and an input gear, a development unit driving motor for driving said development roller, a development unit driving gear which is disposed outside said rotary such that the development unit driving gear can be selectively meshed with one of said input gears and to which the driving force of said development unit driving motor is transmitted, and a control device for controlling the drive of said rotary driving motor and said development unit driving motor, wherein during the image forming operation, said development rollers of the respective development units are set at the development position relative to a photoreceptor by turns according to the rotation of said rotary and, at the development position, said input gears of the respective development units are meshed with said development unit driving gear so as to drive said development rollers with the driving force of said development unit driving motor by turns, thereby achieving multi-color development, and is characterized by further comprising a rotary driving motor control means which controls said rotary driving motor for the rotation of said rotary during the non-image forming operation such that the rotational speed of said rotary in a contact region where said input gear collides with said development unit driving gear is lower than the rotational speed of said rotary in a region other than said contact region. 
   As mentioned above, by the rotary driving motor control means, the rotation of the rotary during the non-image forming operation such that the rotational speed of the rotary in the contact region where the input gear collides with the development unit driving gear is set to be lower than the rotational speed of the rotary in a region other than the contact region, whereby the moving speed of the input gar in the contact region can be reduced. Therefore, when the input gear collides with the development unit driving gear during the rotation of the rotary in the non-image forming operation, the impact due to the collision can be absorbed. In this case, since the impact can be absorbed even though an impact absorbing means is not provided at the development unit driving gear side, the structure of the development unit driving gear side is simplified. 
   Accordingly, impact acting on a tooth of the input gear and a tooth of the development unit driving gear can be reduced, thereby preventing tips of the teeth from being damaged and thus effectively protecting the teeth. In addition, the load exerted on the driving unit driving motor can be reduced. Further, since the impact during collision between the input gear and the development unit driving gear is reduced, vibration transmitted to the development unit is also suppressed, thereby reducing the scattering of toner in the development unit due to the vibration. 
   In this case, since the impact during collision between the input gear and the development unit driving gear is reduced with keeping the rotational speed of the rotary on some level in the region other than the contact region, the time for necessary rotation of the rotary during the non-image forming operation can be shortened with protecting the teeth of both gears and suppressing the load of the development unit driving motor. 
   It is preferable that the rotational speed of the rotary in the region other than the contact region during the rotation of the rotary in the non-image forming operation is set to be lower than the maximum speed of the rotary rotating during the image forming operation. 
   The present invention is further characterized in that an impact absorbing means for absorbing an impact generated when said input gear collides with said development unit driving gear during the rotation of said rotary in the non-image forming operation is provided on said development unit driving gear side. 
   By adding the impact absorbing means provided on the development unit driving gear side, the impact during collision between the input gear and the development unit driving gear is further effectively absorbed. 
   The present invention is further characterized in that the rotational speed of said rotary in the region other than said contact region during the rotation of said rotary in the non-image forming operation is set to a first speed corresponding to the rotational speed of the maximum torque (T max ) of said development unit driving motor and the speed of said rotary when any one of the input gears passes said contact region during the rotation of said rotary in the non-image forming operation is set to a second speed lower than said first speed. 
   Since the rotational speed of the rotary in the region other than the contact region during the rotation of the rotary in the non-image forming operation is set to the first speed corresponding to the rotational speed of the maximum torque (T max ) of the development unit driving motor and the speed of the rotary when any one of the input gears passes the contact region during the rotation of the rotary in the non-image forming operation is set to the second speed lower than the first speed as mentioned above, the effective utilization of motor torque is achieved so that the necessary rotation of the rotary in the non-image forming operation can be reliably and rapidly conducted. 
   In addition, since the rotary driving motor is not required to have excess capacity even when the rotary driving motor is in the unbalanced state due to the offset load produced when one or more of the development units are removed from the rotary, the miniaturization of the rotary driving motor is achieved. Moreover, the vibration and noise generated when the rotary driving motor is driven can be reduced. 
   The present invention is further characterized in that the rotation of said rotary during said non-image forming operation is at least one of a group consisting of the rotation of said rotary for setting the development unit to be replaced at said replacement position during the operation of replacing said development unit, the rotation of said rotary for resetting and initializing the phase of said rotary after power-on, the rotation of said rotary for initializing the phase of said rotary after sudden power-down, and the rotation of said rotary for moving to the home position after the final development procedure. 
   Therefore, the time required for replacing development cartridge, the time required for initializing the rotary after power-on, the time required for initializing the phase of the rotary after sudden power-down, and the time required for moving to the home position after the final development procedure can be shortened. In addition, the impact during collision between the input gear and the development unit driving gear can be effectively absorbed. 
   The present invention also provides an image forming apparatus comprising a developing device of a rotary development type having a rotary on which a plurality of development units are mounted, and a locking means for positioning said rotary in order to selectively set said development units at a predetermined position and for locking the rotary at the predetermined position, wherein said locking means comprises a lockable position formed on said rotary side, a locking member which is movably disposed on the body of the image forming apparatus and has a locking position where the locking member is engaged with said lockable position to lock said rotary and an evacuation position where the locking member is not engaged with said lockable position, a shifting means for shifting said locking member to said evacuation position, and a biasing means for biasing said locking member to said locking position, and is characterized in that said locking member has a contact portion which can come in contact with said lockable position before the engagement with said lockable position according to the rotation of said rotary. 
   This structure ensures the contact of the lockable position with the contact portion of the locking member before the locking member provided on the body side of the image forming apparatus is engaged with the lockable position. Therefore, the movement of the locking member is synchronized with the rotation of the rotary so that the engagement between the locking member and the lockable portion is achieved at the same time that the rotary reaches the predetermined position. That is, since the start of the development operation can be allowed just after the stop of the rotary at the development position, the time required to obtain multi-color toner images by changing colors can be shortened, thus improving the image forming speed. The locking member is moved by the shifting means so that the engagement between the locking member and the lockable portion is cancelled. 
   The present invention is further characterized in that, in the driving means for said rotary, a driving pattern is set such that the rotary overruns said predetermined position when the driving means sets said rotary at the predetermined position. 
   By controlling the driving means of the rotary according to the driving pattern which is set such that the rotary overruns, the rotation of the rotary is ensured to overrun the predetermined position even when the rotary is subjected to offset load. Therefore, as the rotary reaches the predetermined position, the locking member and the lockable portion are engaged with each other. Accordingly, even when the rotary is subjected to offset load, the rotary is reliably positioned at the predetermined position. 
   The present invention is further characterized by further comprising a driving motor for driving said development unit and said rotary, and a power transmission control means for conducting the transmission and isolation of the driving force of said driving motor to at least one of said development unit and said rotary. 
   By the power transmission control means, the development units and the rotary can be selectively driven with the driving force of the single driving motor. 
   The present invention is further characterized in that said locking member further has a standby position on said locking position side and wherein said locking member is set at said standby position before said locking member is engaged with said lockable position. 
   Accordingly, the locking member is previously set to the standby position before the stop of the rotary. The moving amount to the locking position where the locking member positions the rotary can be reduced so that the time required for development by changing colors is shortened, thereby further improving the image forming speed. 
   The present invention is further characterized in that said lockable position is composed of convexities formed on the rotary, said locking member is a lock lever which is movably disposed to the body of said image forming apparatus, and that said lock lever has a concavity engageable with one of said convexities and has a contact portion with which the convexity comes in contact before engaging said concavity according to the rotation of said rotary, and side walls of said concavity are each formed in an arc of a circle of which center is equal to the rotational axis of said lock lever. 
   Accordingly, the convexity comes in contact with the contact portion of the lock lever before the concavity of the lock lever provided on the body of the image forming apparatus is engaged with the convexity provided on the rotary side. Therefore, the movement of the lock lever is synchronized with the rotation of the rotary so that the engagement between the convexity of the rotary side and the concavity of the lock lever is achieved at the same time that the rotary reaches the predetermined position. Since the concavity of the lock lever which is rotatably disposed on the body of the image forming apparatus is engaged with the convexity of the rotary side, the rotary can be securely held at the predetermined position by the lock lever. 
   Though a reaction force due to the development operation is exerted on the side wall of the concavity of the lock lever via the convexity, the lock lever is scarcely affected by the reaction force of the development operation, for example, scarcely subjected to a rotational force (rotational moment) due to the reaction force even though the reaction force acts on the side wall because the side wall of the concavity is formed in an arc of a circle of which center is equal to the rotational axis of the lock lever. Since the concavity of the lock lever does not come off the convexity due to the reaction force, the positioning of the rotary is secured. In addition, even when the development operation is suddenly stopped such as an irregular case, the positioning of the rotary is securely conducted. 
   Since the adverse effect due to the reaction force during the development operation is reduced, the degree of design freedom of the image forming apparatus is increased. 
   The present invention is further characterized in that said convexities are disposed corresponding to said development units, respectively, that said convexities are positioning convexities for development for positioning said rotary at said development position and positioning convexities for replacement for positioning said rotary at said replacement position, and that said lock lever has a concavity which can be selectively engaged with one of said convexities and has a contact portion with which the convexity comes in contact before engaging said concavity according to the rotation of said rotary. 
   Since the convexities are positioning convexities for development and positioning convexities for replacement, the rotary can be positioned at different positions of the development positions for the development units and the replacement positions for the development units. In the state where the rotary is set in the replacement position for the development unit, a component such as a developer carrier of the development unit to be replaced is spaced apart from the image carrier such as a photoreceptor, the image carrier may not be damaged by the development unit due to the replacement operation when the development unit is detached or attached to the rotary. 
   The present invention is further characterized in that the interval in the circumferential direction between said positioning convexity for development and said positioning convexity for replacement for one development unit is set to be smaller than the interval in the circumferential direction between said positioning convexity for replacement for said one development unit and the positioning convexity for development for the next development unit. 
   Even though the rotary is rotated at a relatively high speed for the image forming operation, enough period of time can be ensured until the convexity reaches the position where the convexity comes in contact with the contact portion of the lock lever. That is, before the convexity reaches the position where the convexity comes in contact with the contact portion of the lock lever, the contact portion of the lock lever is securely set in the contact position relative to the convexity. The increase in rotational speed of the rotary is achieved. 
   The present invention is further characterized in that the end of said each convexity is formed in an arc shape, that said contact portion is an inclined face with which the arc-shape end of said convexity can come in contact, wherein the start point of said inclined face is formed on an arc of a circle of which center is equal to the rotational axis of said rotary and which passes the ends of said convexities, and that, according to the rotation of said rotary, said convexity comes in contact with said inclined face and presses said inclined face before said convexity is engaged with said concavity. 
   That is, the arc-shaped end of the convexity provided on the rotary side comes in contact with the inclined face of the lock lever. Accordingly, disturbance produced when the end of the convexity collides with the lock lever is reduced so that the rotary is not affected by the collision and thus can smoothly continue to rotate. In addition, it can prevent the generation of noise due to the collision. In this case, since the start point of the inclined face is formed on an arc of a circle of which center is equal to the rotational axis of the rotary and which passes the ends of the convexities, the end of the convexity can smoothly move. Therefore, the disturbance can be further suppressed, thereby achieving further smooth rotation of the rotary and effectively preventing the noise due to collision. 
   In addition, since the end of the convexity is formed in an arc shape, the end of the convexity comes in contact with the lock lever at a point (or along a line). Accordingly, the contact friction due to the contact between the end of the convexity and the lock lever is reduced, thereby minimizing the effect on the rotary due to the contact friction. Therefore, further smooth rotation of the rotary is achieved. 
   The present invention is characterized in that said shifting means is a solenoid. 
   Since the solenoid is employed as the shifting means for moving the locking member, operation characteristic optimally corresponding to the quick movement of the lever can be obtained. Therefore, ensure and quick movement of the locking member can be achieved by the simple structure, thereby further optimally conducting the control of engagement and disengagement between the locking member and the lockable portion. 
   Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification. 
   The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an illustration schematically showing an image forming apparatus which is common to first through third embodiments of the present invention; 
       FIG. 2  is an illustration schematically showing a rotary-type developing device employed in an image forming apparatus of the first embodiment; 
       FIG. 3  is a block diagram of the control for a rotary driving motor and a development cartridge driving motor by a CPU in the first embodiment; 
       FIG. 4  is a graph showing the rotational speed curves of a rotary during the image forming operation and the non-image forming operation; 
       FIG. 5  is a graph showing the number of revolution (n)-torque (T) characteristics of a rotary driving motor; 
       FIG. 6  is an explanatory illustration showing a state where the rotary is subjected to rotational moment due to offset load in the direction (clockwise direction) opposite to the rotational direction (counter clockwise direction); 
       FIG. 7  is an explanatory illustration showing the state where the rotary is subjected to rotational moment due to offset load in the same direction as the rotational direction (counter clockwise direction); 
       FIG. 8  is an illustration schematically showing a rotary-type developing device employed in an image forming apparatus of the second embodiment; 
       FIG. 9  is an illustration schematically showing a driving system for a development cartridge in the rotary-type developing device shown in  FIG. 8 ; 
       FIG. 10  is an illustration schematically showing a driving system for a rotary in the rotary-type developing device shown in  FIG. 8 ; 
       FIG. 11  is an illustration schematically showing a locking means in the rotary-type developing device shown in  FIG. 8 ; 
       FIG. 12  is an illustration schematically showing a variation of the locking means; 
       FIG. 13  is an illustration for explaining a lock lever in the locking means shown in  FIG. 11 ; 
       FIG. 14  is a partially enlarged view of the locking means shown in  FIG. 13 . 
       FIG. 15  is an illustration showing one scene in the operation of the locking means shown in  FIG. 11  during the image forming operation; 
       FIG. 16  is a graph showing an example of the driving pattern of the driving motor for the operation of changing development color; 
       FIG. 17  is a graph showing an example of the driving pattern of the driving motor for the operation of replacing cartridge; 
       FIG. 18  is a graph showing the relation between the target angle according to the driving motor and the actual rotational angle of the rotary in the operation of replacing cartridge; 
       FIG. 19  is an illustration showing another scene in the operation of the locking means shown in  FIG. 11  during the image forming operation; 
       FIG. 20  is an illustration showing still another scene in the operation of the locking means shown in  FIG. 11  during the image forming operation; 
       FIG. 21  is an illustration showing further another scene in the operation of the locking means shown in  FIG. 11  during the image forming operation; 
       FIG. 22  is an illustration showing yet another scene in the operation of the locking means shown in  FIG. 11  during the image forming operation; 
       FIG. 23  is an illustration showing the state where the rotary is positioned by the locking means shown in  FIG. 11  during the operation of replacing development cartridge; and 
       FIG. 24  is a partially enlarged view similar to  FIG. 14 , but showing an image forming apparatus of the third embodiment of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereinafter, embodiments of the present invention will be described with reference to the attached drawings.  FIG. 1  is an illustration schematically showing a first embodiment of the image forming apparatus of the present invention,  FIG. 2  is an illustration schematically showing a rotary-type developing device employed in the image forming apparatus of the first embodiment, and  FIG. 3  is a block diagram of the control for a rotary driving motor and a development cartridge driving motor by a CPU in the first embodiment. 
   As shown in  FIG. 1 , an image forming apparatus  1  generally comprises an exposure device  2 , a rotary-type developing device  3 , a photoreceptor  4  on which an electrostatic latent image is formed by exposure of the exposure device  2  and is developed with toner from the rotary-type developing device  3  so as to obtain a visible toner image, an intermediate transfer medium  5  composed of an endless transfer belt, a primary transfer device  6  for primarily transferring the toner image on the photoreceptor  4  to the intermediate transfer medium  5 , a secondary transfer device  8  for secondarily transferring the toner image, primarily transferred on the intermediate transfer medium  5 , to a recording medium such as a paper sheet (in the following description, the recording medium will be referred to as a paper sheet)  7 , a paper feeding device  10  for feeding paper sheets  7  stacked in a sheet cassette  9  by a paper feeding roller  10   a , a fixing unit  11  for fixing the toner image secondarily transferred on the paper sheet  7 , and an outfeed tray  12  receiving the paper sheet  7  on which the given image is formed and fixed by the fixing unit  11 . 
   As shown in  FIG. 2 , the rotary-type developing device  3  of the first embodiment comprises a rotary  13  having a rotary frame  13   a  which is rotatably disposed, and development cartridges  14 ,  15 ,  16 ,  17  for respective colors of yellow (Y), magenta (M), cyan (C), and black (K) as development units supported on the rotary  13 . 
   As shown in  FIG. 2 , the development cartridges  14 ,  15 ,  16 ,  17  are arranged along the circumferential direction of the rotary  13  in the clockwise order at even intervals and comprise development rollers  14   a ,  15   a ,  16   a ,  17   a , development roller driving gears  14   b ,  15   b ,  16   b ,  17   b  each of which is disposed coaxially with and rotatably together with each of the development rollers  14   a ,  15   a ,  16   a ,  17   a , input gears  14   c ,  15   c ,  16   c ,  17   c  into which driving force of the development cartridge driving motor  56  is inputted, and power transmission gear mechanisms  14   d ,  15   d ,  16   d ,  17   d  for transmitting the driving force of the motor  56 , inputted in the corresponding input gears  14   c ,  15   c ,  16   c ,  17   c , to the corresponding development roller driving gears  14   b ,  15   b ,  16   b ,  17   b  with reducing speed. It should be understood that each development cartridge  14 ,  15 ,  16 ,  17  comprises a toner storage portion, a toner supply means for supplying toner in the toner storage portion to each development roller  14   a ,  14   a ,  16   a ,  17   a , and a toner regulating means for regulating the toner on the development roller  14   a ,  15   a ,  16   a ,  17   a  into a predetermined thickness to be carried to the photoreceptor  4 . 
   A development cartridge driving gear  19 , to which driving force from the development cartridge driving motor  56  is transmitted via a one-way clutch  18 , is rotatably disposed near the outer periphery of the rotary  13 . The development cartridge driving gear  19  can be selectively meshed with one of the input gears  14   c ,  15   c ,  16   c ,  17   c  so as to transmit the driving force to the meshed input gear. When the input gear of the development cartridge collides with the development cartridge driving gear  19 , the one-way clutch  18  allows the rotation of the development cartridge driving gear  19  in the clockwise direction in the drawing so as to absorb the impact during the collision. The one-way clutch  18  composes an impact absorbing means of the image forming apparatus  1  of the first embodiment. 
   In addition, a rotary driving gear  24  is rotatably disposed to be meshed with a gear (shown in the drawing as the same as the outer periphery of the rotary  13 , but is not limited to this configuration) which is coaxial with and rotatable together with the rotary  13 , in such a manner that the driving force from the rotary driving motor  55  is transmitted to the rotary driving gear  24 . 
   As show in  FIG. 3 , inputted into a CPU  51  are an image forming signal from an image forming operation command means  52  such as an image forming operation key (start key) on an operation panel of the image forming apparatus or an image forming command output means provided outside of the image forming apparatus for outputting image forming signal, a rotary rotation command for non-image forming operation from a rotary rotation command means  53  for non-image forming operation such as a rotary rotation command key for non-image forming operation on the operation panel of the image forming apparatus  1 , and a rotary position detecting signal from a rotary position detecting means  54  such as a publicly known phase detector for detecting the phase of the rotary  13  or the phase of the rotary driving motor  55  or a publicly known optical position detector for detecting the rotational position of the rotary  13 . The CPU  51  has a rotary driving motor control means  51   a  and a development cartridge driving motor control means  51   b  in such a manner as to output control signals from the control means  51   a  and  51   b  to the rotary driving motor  55  and the development cartridge driving motor  56 , respectively. 
   Therefore, the rotary  13  is driven to rotate by the rotary driving motor  55 , while the development cartridges  14 ,  15 ,  16 ,  17  are driven by the driving cartridge driving motor  56 . These motors  55 ,  56  are controlled for their driving by the central processing unit (CPU)  51  of the image forming apparatus  1 . 
   The rotary driving motor  55  and the development cartridge driving motor  56  can be composed of one common motor and a clutch. In the first embodiment, both the rotary driving motor  55  and the development cartridge driving motor  56  are provided. 
   Description will now be made as regard to the action of the image forming apparatus  1  of the first embodiment having the aforementioned structure during the image forming operation. In the state shown in  FIG. 1  and  FIG. 2 , the development roller  14   a  of the development cartridge  14  for yellow is in contact with the photoreceptor  4 , that is, the development cartridge  14  for yellow is set at the development position. When no image is developed (no image is formed), the rotary  13  is in a position other than the position shown in  FIG. 1  and  FIG. 2 , such that all of the development rollers  14   a ,  15   a ,  16   a ,  17   a  of the development cartridges  14 ,  15 ,  16 ,  17  are positioned apart from the photoreceptor  4 . When the image forming apparatus  1  is inoperative, movable members of the image forming apparatus  1  are stopped. 
   As an image forming signal is inputted into the CPU  51  from the image forming operation command means  52 , the image forming apparatus  1  starts its operation for forming an image. The CPU  51  rotates the photoreceptor  4  in the clockwise direction in  FIG. 1  and  FIG. 2  and activates the exposure device  2 . Then, the exposure device  2  exposes the photoreceptor  4  to light according to the image signal for yellow from CPU  51 , thereby forming an electrostatic latent image for yellow on the photoreceptor  4 . At the same time, the CPU  51  drives both the rotary driving motor  55  and the development cartridge driving motor  56  and also drives the intermediate transfer medium  5 . 
   Then, the driving force of the development cartridge driving motor  56  is transmitted to the development cartridge driving gear  19  via the one-way clutch  18  so as to rotate the development cartridge driving gear  19 . At the same time, the driving force of the rotary driving motor  55  is transmitted to the rotary driving gear  24  so as to rotate the rotary driving gear  24 . By the rotation of the rotary driving gear  24 , the rotary  13  is rotated in the counter clockwise direction so that the development cartridge  14  for yellow moves toward the development position shown in  FIG. 1  and  FIG. 2 . As the development cartridge  14  reaches the development position, the CPU  51  stops the driving of the rotary driving motor  55  according to a detected signal so that the rotation of the rotary  13  is stopped. 
   As the development cartridge  14  for yellow is set to the development position, the input gear  14   c  is meshed with the development cartridge driving gear  19 . Then, the driving force of the development cartridge driving motor  56  is inputted into the input gear  14   c  from the development cartridge driving gear  19  and is transmitted to the development roller  14   a  with a reduction in speed by the power transmission gear mechanism  14   d , thereby rotating the development roller  14   a . In addition, the development roller  14   a  is in contact with the photoreceptor  4 , that is, in the development position. 
   Then, the development roller  14   a  carries a predetermined amount of yellow toner to the photoreceptor  4  so as to develop the electrostatic latent image for yellow on the photoreceptor  4 , thereby forming a yellow toner image on the photoreceptor  4 . Further, the yellow toner image on the photoreceptor  4  is primarily transferred to the intermediate transfer medium  5  by the primary transfer device  6 . 
   As the transfer of the yellow toner image is finished, the CPU  51  drives the rotary driving motor  55  again so as to rotate the rotary  13  in the same direction again. Then, the development roller  14   a  is moved apart from the photoreceptor  4  and the input gear  14   c  is moved apart from the development cartridge driving gear  19 , thereby stopping the operation of the development cartridge  14 . That is, the rotation of the development roller  14   a  and the rotation of the input gear  14   c  are stopped. At about the time starting the rotation of the rotary  13 , the exposure device  2  exposes the photoreceptor  4  to light based on the image signal for magenta from the CPU  51 , thereby forming an electrostatic latent image for magenta on the photoreceptor  4 . 
   In the same manner as the aforementioned case for yellow, as the development cartridge  15  for magenta is set at the development position, the rotation of the rotary  13  is stopped and the input gear  15   c  is meshed with the development cartridge driving gear  19 . In addition, the development roller  15   a  is in contact with the photoreceptor  4 , that is, in the development position. Then, the development for magenta is conducted by the development cartridge  15  for magenta and a magenta toner image developed on the photoreceptor  4  is primarily transferred to the intermediate transfer medium  5 . 
   As the transfer of the magenta toner image is finished, the rotary  13  is rotated in the same direction again. Then, the development roller  15   a  is moved apart from the photoreceptor  4  and the input gear  15   c  is moved apart from the development cartridge driving gear  19 , thereby stopping the operation of the development cartridge  15 . That is, the rotation of the development roller  15   a  and the rotation of the input gear  15   c  are stopped. During this, the development cartridge driving gear  19  is kept in the rotating state. 
   According to the rotation of the rotary  13 , a cyan toner image and a black toner image are similarly formed on the photoreceptor  4  and are primarily transferred to the intermediate transfer medium  5  one by one. Therefore, the toner images for four colors primarily transferred to the intermediate transfer medium  5  are toned to form a full-color toner image. The full-color toner image on the intermediate transfer medium  5  is transferred to a paper sheet  7  by the secondary transfer device  8 . Then, the toner image transferred to the paper sheet  7  is fused by the fixing unit, thereby forming a full-color image on the paper sheet  7 . 
   During the image forming operation of the image forming apparatus  1  of the first embodiment, as the development by one of the development cartridges  14 ,  15 ,  16 ,  17  is finished, the rotary  13  is rotated for setting the next development cartridge to the development position. The rotational speed control (i.e. the rotation control of the rotary driving motor) is conducted to be the constant acceleration control as shown by a solid line in  FIG. 4 . 
   That is, for setting the next development cartridge to the development position after the development of one development cartridge, the rotational speed of the rotary  13  is controlled to be increased with the constant acceleration so that the rotational speed of the rotary  13  is linearly increased. As the rotational speed of the rotary  13  becomes the maximum speed (top speed) v max  which is previously determined, the rotational speed of the rotary  13  is controlled to be decreased with the constant acceleration so that the rotational speed of the rotary  13  is linearly decreased. When the next development cartridge reaches the development position, the rotary  13  is stopped. In this case, the top speed v max  is set to a relatively large value in order to speed up a sequence of settings of the development cartridges  14 ,  15 ,  16 ,  17  to the development position to shorten the image forming period of time. 
   By the way, as shown in  FIG. 2 , in the image forming apparatus  1  of the first embodiment, the body frame of the image forming apparatus is provided at a predetermined replacement position with a development cartridge replacement opening  45 . The development cartridge replacement opening  45  is formed to have a size allowing the withdrawal and insertion of the development cartridge in the axial direction of the rotary  13  (the direction perpendicular to the plane of the drawing sheet of  FIG. 2 ). 
   Now, the replacement of the development cartridge will be described. As the rotary  13  is rotated during the non-image forming operation where the image forming operation is not conducted, the development cartridge to be replaced is set to the replacement position. When the development cartridge is set to the replacement position, the development roller of the development cartridge is spaced apart from the photoreceptor  4  and the input gear is spaced apart from the development cartridge driving gear  19 . 
   In the state where the development cartridge to be replaced is set at the replacement position, the development cartridge is removed by withdrawing the development cartridge from the rotary  13  through the development cartridge replacement opening  45  in the axial direction of the rotary  13  and a new development cartridge is installed by inserting the new development cartridge into the rotary  13  through the development cartridge replacement opening  45  in the axial direction of the rotary  13 . 
   The rotational speed control of the rotary  13  for setting the development cartridge to be replaced at the replacement position is conducted as follows. For example, as a user runs a key for the replacement of the development cartridge corresponding to the color to be replaced on the operation panel in order to replace the development cartridge, the CPU  51  only shows the state on a display of the image forming apparatus when the development cartridge to be replaced is already at the aforementioned replacement position and does not drive the rotary driving motor  55 . 
   On the other hand, the CPU  51  drives the rotary driving motor  55  when the development cartridge to be replaced is not at the aforementioned replacement position so that the rotary  13  is controlled to rotate in the counter clockwise direction. After start of the rotation of the rotary  13 , the rotational speed of the rotary  13  is controlled to be increased with the constant acceleration similarly to the aforementioned image forming operation. As the rotational speed reaches a non-image forming operation speed v 1  which is lower than the aforementioned top speed v max , the rotary  13  is controlled to rotate at a constant speed of the non-image forming operation speed v 1  as shown by two-dot chain line in  FIG. 4 . 
   Further, when one development cartridge comes closer to the development position, that is, to a preset position just before the position where the input gear of the development cartridge collides with the development cartridge driving gear  19  during the rotation of the rotary  13 , the CPU  51  controls the rotary driving motor  55  to decrease its rotational speed with a constant acceleration according to the detection signal, whereby the rotational speed of the rotary  13  is controlled to be decreased with the constant acceleration. As the input gear of the development cartridge passes the development position, the CPU  51  controls the rotary driving motor  55  to increase its rotational speed with the constant acceleration according to the detection signal, whereby the rotational speed of the rotary  13  is controlled to be increased with the constant acceleration again. As the rotational speed then reaches a non-image forming operation speed v 1 , the rotary  13  is controlled to rotate at the constant speed of the non-image forming operation speed v 1 . At the non-image forming operation speed v 1  of the rotary  13 , the input gear passes the contact region relative to the development cartridge driving gear  19  and never collide with the development cartridge driving gear  19 . That is, the contact region is defined as a region from the aforementioned preset position to the position of the input gear when the rotational speed of the rotary  13  becomes the non-image forming operation speed v 1  by the rotational speed increasing control. 
   After that, as the development cartridge to be replaced comes closer to the replacement position, the rotary  13  is controlled to decrease its rotational speed. As the development cartridge to be replaced reaches the replacement position, the rotary  13  is stopped. 
   In this manner, the rotational speed of the rotary  13  during the replacement of the development cartridge is controlled to the constant non-image forming operation speed v 1  which is lower than the top speed v max  as the rotational speed of the rotary  13  during the image forming operation. In addition, when the input gear of the other development cartridge passes a region where the input gear collides with the development cartridge driving gear  19 , the rotational speed of the rotary is controlled to be decrease to a speed which is further lower than the non-image forming operation speed v 1 . According to this rotational speed control of the rotary  13 , the rotary is controlled to be decelerated from the non-image forming operation speed v 1  when passing the region where the development roller collides with the photoreceptor  4   
   If the input gear of the development cartridge reaches the contact region before the rotational speed of the rotary  13  is increased to the non-image forming operation speed v 1 , the rotation of the rotary  13  is controlled as follows. 
   That is, when the rotational speed of the rotary  13  in this case is higher than the minimum speed v 2 , the rotation of the rotary  13  is controlled such that the rotational speed is decreased to the minimum speed v 2  and is kept at the minimum speed v 2  during the input gear passes the contact region. After the input gear passes the development position, the rotation of the rotary  13  is controlled to increase its rotational speed similarly to the aforementioned case. When the rotational speed reaches the non-image forming operation speed v 1 , the speed is kept at a constant speed of this speed v 1 . When the development cartridge to be replaced reaches the replacement position, the rotary  13  is stopped. 
   When the rotational speed of the rotary  13  in the aforementioned case is lower than the minimum speed v 2 , the rotation of the rotary  13  is controlled such that the rotational speed is increased to the minimum speed v 2  until the input gear reaches the development position during the input gear passes the contact region. After the rotational speed becomes the minimum speed v 2 , the rotational speed is then kept at the minimum speed v 2 . After that, the rest of the control after the speed increasing control is conducted in the same manner as mentioned above. If the input gear reaches the development position before the rotation of the rotary  13  becomes the minimum speed v 2 , the rest of the control after the speed increasing control is directly conducted in the same manner as mentioned above. 
   It is preferable that the aforementioned non-image forming operation speed v 1  and the minimum speed v 2  at which the input gear of the development cartridge passes the contact region relative to the development cartridge driving gear  19  are determined as follows. There is a case where two of the development cartridges are removed and the rotary  13  is thus subjected to rotational moment due to offset load in the direction (clockwise direction) opposite to the rotational direction (counter clockwise direction) as shown in  FIG. 6  and, on the other hand, there is a case where two of the development cartridges are removed and the rotary  13  is thus subjected to rotational moment due to offset load in the same direction as the rotational direction as shown in  FIG. 7 . When the rotary  13  is commanded to be rotated in the counter clockwise direction in order to replace at least one of the remaining development cartridges in either case of the above cases, it is preferable to ensure the rotation of the rotary  13  and to set the development cartridge to be replaced at the replacement position in a short time. 
   To achieve these, in the image forming apparatus  1  of the first embodiment, the number of revolution (n)-torque (T) characteristics of the rotary driving motor  55  can be effectively utilized as shown in  FIG. 5 . That is, the non-image forming operation speed v 1  is set corresponding to the number of revolution n 1  of the motor which is smaller than the number of revolution n 0  of the motor at the maximum torque T max  in the number of revolution (n)-torque (T) of the rotary driving motor  55 , while the minimum speed v 2  is set corresponding to the number of revolution n 2  of the motor which is smaller than the number of revolution n 1  of the motor. 
   By effectively utilizing the motor torque with keeping the rotational speed of the rotary  13  on some level for replacing the development cartridge as mentioned above, the development cartridge can be rapidly and securely set to the replacement position. 
   According to the image forming apparatus  1  of the first embodiment, the rotary is set to move the input gear of the development cartridge at the constant non-image forming operation speed v 1 , which is lower than the top speed v max  for the image forming operation, in a region other than the contact region relative to the development cartridge driving gear  19 , and to move the input gear at a speed, which is further lower than the non-image forming operation speed v 1 , in the aforementioned contact region. Therefore, even when the input gear of the development cartridge collides with the development cartridge driving gear  19  during the rotational operation of the rotary  13  for the replacement of the development cartridge, the impact due to the collision can be effectively and efficiently absorbed. 
   Accordingly, impacts acting on the teeth of the input gear and the teeth of the development cartridge driving gear  19 , respectively, can be reduced, thereby achieving further effective protection of these teeth and reducing the load applied on the rotary driving motor  55 . In addition, since the impact during the collision between the input gear and the development cartridge driving gear  19  is absorbed, vibration transmitted to the development cartridge is inhibited, thereby reducing the scattering of toner in the development cartridge due to the vibration. 
   In this case, since the impact can be absorbed with keeping the rotational speed of the rotary  13  on some level for replacing the development cartridge as mentioned above, the time required for replacing the development cartridge can be shortened with protecting the teeth and reducing the load on the rotary driving motor  55 . 
   Further, since the non-image forming operation speed v 1  and the minimum speed v 2  at which the input gear passes the contact region are set, by utilizing the number of revolution (n)-torque (T) characteristics of the rotary driving motor  55 , to correspond to the first and second torques T 1 , T 2  which are smaller than the maximum torque (T max ) of the rotary driving motor  55 , effective utilization of motor torque can be achieved and the development cartridge to be replaced can be rapidly and securely set to the replacement position. 
   Further, the impact absorbing means composed of the one-way clutch  18  provided on the development cartridge driving gear  19  side is employed in addition to the above structure, whereby the impact during collision between the input gear and the development cartridge driving gear  19  can be further effectively absorbed. 
   Particularly, in case where rotational moment due to offset load acts on the rotary  13  in the direction opposite to the rotational direction of the rotary  13  as shown in  FIG. 6 , when the teeth of the input gear collide with the teeth of the development cartridge driving gear  19 , moment due to reaction during this collision is added to the rotational moment due to offset load to act on the rotary  13  in the direction opposite to the rotational direction of the rotary  13 . In this case, the load of the rotary driving motor  55  can be securely inhibited. Therefore, the miniaturization of the rotary driving motor  55  is achieved because the rotary driving motor  55  in the unbalanced state due to the offset load is not required to have excess capacity. In addition, the vibration and noise generated when the rotary driving motor  55  is driven can be reduced. 
   On the other hand, in case where two of the development cartridges are removed and rotational moment due to offset load acts on the rotary  13  in the same direction as the rotational direction as shown in  FIG. 7 , the impact when the tooth of the input gear collides with the tooth of the development cartridge driving gear  19  must be greater. Also in this case, the teeth can be securely protected. 
   Though the non-image forming operation speed v 1  is set to be lower than the top speed v max  for the image forming operation in the aforementioned first embodiment, the non-image forming operation speed v 1  may be set to be the same as the top speed v max . In this case, it is preferable that the absolute value of the acceleration for the speed reduction control at the contact region is set to be larger than that of the first embodiment. Also in this case, substantially the same effect as the first embodiment can be obtained. In short, it is preferable that the non-image forming operation speed v 1  is set to be a value lower than the top speed v max  during the image forming operation. 
   Depending on the size and weight of the development cartridge and the capacity of the rotary driving motor  55 , the speed reduction control at the contact region may be omitted as shown by a chain line in  FIG. 4  while the non-image forming operation speed v 1  is set to be a value lower than the top speed v max . Also in this case, substantially the same effect as the first embodiment can be obtained. 
   Though the rotational speed control of the rotary  13  is conducted with a constant acceleration in the aforementioned first embodiment, the acceleration may vary with time. 
   Though the one-way clutch  18  is provided as the impact absorbing means on the development cartridge driving gear  19  side in the aforementioned first embodiment, the impact absorbing means may be composed of a spring. That is, the development cartridge driving gear  19  is biased to a meshing position at which it is meshed with the input gear, whereby the development cartridge driving gear  19  can escape from the meshing position relative to the input gear by the pressing force of the input gear when the input gear collides with the development cartridge driving gear  19 . 
   Alternatively, a driving means such as a solenoid may be employed. In this case, the development cartridge driving gear  19  is normally set to the meshing position relative to the input gear and, if necessary, set to an evacuation position where it is not meshed with the input gear by the driving means. The aforementioned control may be adopted to an image forming apparatuses having this structure. In this case, when the rotary  13  is rotated for conducting the replacement of the development cartridge, the development cartridge driving gear  19  is set to the evacuation position by the driving means. However, if the development cartridge driving gear  19  can not be set to the evacuation position due to the driving means failure or the like so that the development cartridge driving gear  19  is still at the meshing position, the aforementioned control of the image forming apparatus  1  of the first embodiment can take effect. 
   That is, the present invention can be adopted to any image forming apparatus in which the input gear of the development cartridge collides with the development cartridge driving gear  19  or there is a possibility that the input gear of the development cartridge may collide with the development cartridge driving gear  19 . 
   Further, though the non-image forming operation speed v 1  and the minimum speed v 2  are set utilizing the number of revolution (n)-torque (T) characteristics of the rotary driving motor in the above, the non-image forming operation speed v 1  and the minimum speed v 2  may be set in any other method. In case of using a stepping motor as the driving motor  55 , the non-image forming operation speed v 1  and the minimum speed v 2  may be set by suitably selecting the exciting method. 
   Furthermore, though the contact region is defined as a region from the preset position just before the position where the input gear collides with the development cartridge driving gear  19  to the position of the input gear when the rotational speed of the rotary  13  becomes the non-image forming operation speed v 1  by the rotational speed increasing control in the first embodiment, the image forming apparatus of the present invention is not limited thereto. For example, the contact region is defined as a region from a first preset position before the position where the input gear collides with the development cartridge driving gear  19  to a second preset position after the position where the input gear departs apart from the development cartridge driving gear  19  by the rotational speed increasing control. 
   Moreover, though the rotation of the rotary  13  during the non-image forming operation is referred to as the rotation of the rotary  13  for the replacement of the development cartridge in the aforementioned first embodiment, the present invention is not limited thereto. The rotation of the rotary  13  during the non-image forming operation includes any rotation of the rotary  13  during the non-image forming operation not the image forming operation, for example, a case that, in an image forming apparatus using a motor such as a stepping motor of which phase becomes unknown when the power of the image forming apparatus is broken on a sudden, the rotary driving motor  55  of which phase is unknown is rotated for detecting the phase of the motor  55 , a case that the rotary driving motor  55  is rotated for initializing the image forming apparatus  1 , and a case that the rotary driving motor  55  is rotated for moving the rotary to its home position after the final development procedure. 
     FIG. 8  is an illustration schematically showing a rotary-type developing device employed in an image forming apparatus of the second embodiment,  FIG. 9  is an illustration schematically showing a driving system for a development cartridge in the rotary-type developing device shown in  FIG. 8 ,  FIG. 10  is an illustration schematically showing a driving system for a rotary in the rotary-type developing device shown in  FIG. 8 . In the second embodiment, the same components as the components of the first embodiment are marked with the same reference numerals so that the detail description of such component will be omitted. 
   The basic structure of the image forming apparatus of the second embodiment is the same as that of the aforementioned first embodiment shown in  FIG. 1 . In the first embodiment, the rotary  13  is driven to rotate by the rotary driving motor  55  and the development cartridges  14 ,  15 ,  16 ,  17  are driven to rotate by the development cartridge driving motor  56 . As shown in  FIG. 8  and  FIG. 9 , in the image forming apparatus  1  of the second embodiment, however, the rotary  13  and the development cartridges  14 ,  15 ,  16 ,  17  are driven to rotate by a single driving motor  21 . 
   In the image forming apparatus of the second embodiment, the development cartridge driving gear  19  is connected to a motor output gear  22  fixed to a rotational shaft  21   a  of the driving motor  21  via a power transmission gear mechanism  20  so that the driving force of the driving motor  21  is transmitted to the development cartridge driving gear  19  with a reduction in speed by the power transmission gear mechanism  20 . 
   Further, similarly to the first embodiment, a one-way clutch  18 , not shown in  FIG. 8  and  FIG. 9 , is incorporated into the development cartridge driving gear  19 . For driving the rotary  13 , even when the development cartridge driving gear  19  is meshed with one of development drive input gears  14   c ,  15   c ,  16   c ,  17   c , the one-way clutch  18  runs idle relative to the direction corresponding to the rotational direction of the rotary  13  so that the driving force is not transmitted to the development drive input gear  14   c ,  15   c ,  16   c ,  17   c . On the other hand, for driving selectively one of the development cartridges  14 ,  15 ,  16 ,  17  (that is, for the development operation), the one-way clutch  18  is locked (connected) relative to the rotational direction corresponding to the developing direction so that the driving force is transmitted to the selected one of the development drive input gears  14   c ,  15   c ,  16   c ,  17   c.    
   As shown in  FIG. 10 , a rotary driving gear  23  is disposed on one end side of the rotary  13  coaxially with the rotary  13  such that the rotary driving gear  23  can rotate together with the rotary  13 . In addition, a rotary driving gear  24  to which the driving force from the driving motor  21  is transmitted is rotatably disposed to be meshed with the rotary driving gear  23 . The rotary driving gear  24  is connected to the motor output gear  22  of the driving motor  21  through a power transmission gear mechanism  25  and an electromagnetic clutch  26  as the power transmission control means. In this case, an input gear  26  of the electromagnetic clutch  26  is meshed with the motor output gear  22 . As the electromagnetic clutch  26  is turned on to establish the connection, the driving force of the driving motor  21  is transmitted to the rotary driving gear  24  with a reduction in speed by the power transmission gear mechanism  20 . 
   By the way, as shown by three-dot chain line in  FIG. 8 , the image forming apparatus  1  of this embodiment is provided with the development cartridge replacement opening  45  which is formed at a predetermined replacement position of a body frame (not shown) thereof. The development cartridge replacement opening  45  is formed to have a size allowing the withdrawal and insertion of the development cartridge in the axial direction of the rotary  13  (the direction perpendicular to the plane of the drawing sheet of  FIG. 8 ). 
   As shown in  FIG. 11 , near the other end of the rotary  13 , a locking means  27  is provided for stopping the rotary  13  at a predetermined position and retaining the rotary at the stop position. The locking means  27  comprises pairs of convexities (lockable positions)  28 ,  29 ;  30 ,  31 ;  32 ,  33 ;  34 ,  35  fixed to the rotary frame  13   a  corresponding to the respective mounting portions for the development cartridges  14 ,  15 ,  16 ,  17 , a lock lever  38  as a locking member being rotatable about a pivot  37  and being provided at an end with a concavity  36  which can be selectively engaged with one of these convexities  28 ,  29 ;  30 ,  31 ;  32 ,  33 ;  34 ,  35 , a lock lever spring  39  as a biasing member which is connected to the end opposite to the end with the concavity  36  of the lock lever  38  and always biases the lock lever  38  in a direction engaging the concavity  36  with one of the convexities  28 ,  29 ;  30 ,  31 ;  32 ,  33 ;  34 ,  35 , a solenoid (shifting means)  40  which exerts solenoid force to the lock lever  38  against the spring force of the lock lever spring  39  in a direction dragging the concavity  36  away from the one of the convexities  28 ,  29 ;  30 ,  31 ;  32 ,  33 ;  34 ,  35  when it is activated, and a stopper  41  which restricts the rotation of the lock lever  38  in the clockwise direction in  FIG. 11  to retain the lock lever  38  at a standby position. 
   The aforementioned shifting means is not limited to the solenoid and may be, for example, a cam  40 ′. That is, as shown in  FIG. 12 , the cam  40 ′ is designed such a manner that the cam  40  always in contact with a lock lever  38 ′ and that, for retaining the lock lever  38 ′ at the evacuation position, a bulge of the cam surface of the cam  40 ′ pushes up the lock lever  38 ′ so as to move the lock lever  38 ′ to the evacuation position. The means for driving the cam  40 ′ may be a driving means for driving other component(s) mounted in the image forming apparatus or an independent driving means for exclusively driving the cam  40 ′. In this case, the rotational force of the driving means may be transmitted to the cam  40 ′ via a stepping clutch in such a manner as to caracole the cam  40 ′ by a full rotation of the driving means. However, for rapidly moving a lever between two positions of the standby position and the evacuation position, i.e. the shifting means is preferably a solenoid because of its suitable operational characteristics and its relatively simple structure. 
   As shown in  FIG. 13 , the convexities  28 ,  29 ;  30 ,  31 ;  32 ,  33 ;  34 ,  35  are formed to project in the radial direction of the circle of which center is equal to the rotation axis of the rotary  13  and to have arc-like ends. 
   As for each pair of convexities  28 ,  29 ;  30 ,  31 ;  32 ,  33 ;  34 ,  35  corresponding to the mounting portion for each development cartridge  14 ,  15 ,  16 ,  17 , the convexity  28 ;  30 ;  32 ;  34  as one of each pair is a convexity for stopping the rotary  13  when reaching the development position of the corresponding development cartridge  14 ,  15 ,  16 ,  17  and retaining the rotary  13  at the stopped position, that is, a positioning convexity for development of the development cartridge, while the convexity  29 ;  31 ;  33 ;  35  as the other one of each pair is a convexity for stopping the rotary  13  when reaching the replacement position for the corresponding development cartridge  14 ,  15 ,  16 ,  17  and retaining the rotary  13  at the stopped position, that is, a positioning convexity for replacement of the development cartridge. 
   As shown in  FIG. 11 , the positioning convexities  29 ;  31 ;  33 ;  35  for replacement are disposed on the downstream side of the positioning convexities  28 ;  30 ;  32 ;  34  for development in the rotational direction of the rotary and are spaced apart from the positioning convexities  28 ;  30 ;  32 ;  34  for development at predetermined intervals “a” in the circumferential direction, respectively, thereby preventing the convexities  29 ;  31 ;  33 ;  35  from colliding with the lock lever  38  even when the lock lever  38  is set at the standby position to ensure the engagement of the concavity  36  of the lock lever  38  with the positioning convexity  28 ;  30 ;  32 ;  34 , as will be described later. 
   Further, the positioning convexities  29 ;  31 ;  33 ;  35  for replacement corresponding to the mounting portions of the development cartridges  14 ,  15 ,  16 ,  17  are spaced apart from the positioning convexities  30 ;  32 ;  34 ;  28  for development of the development cartridges  15 ,  16 ,  17 ,  14  next to the development cartridges  14 ,  15 ,  16 ,  17  on the downstream side in the rotational direction of the rotary  13  at predetermined intervals “b” in the circumferential direction, respectively. The aforementioned interval “a” and the interval “b” are set such that the interval “a” is smaller than the interval “b” (interval “a”&lt;interval “b”). 
   In the image forming apparatus  1  of a rotary development type of the second embodiment, the rotary  13  is rotated at a relatively high speed during the image forming operation because it is required to rapidly set the rotary at the development positions in order to rapidly change the color in order of the development cartridges  15 ,  16 ,  17 ,  14 , while the rotary  13  is rotated at a relatively low speed during the non-image forming operation without the image forming operation because, for example, in a case of development cartridge replacement, it is not required to set as rapidly as the image forming operation for setting the development cartridge to be replaced at the replacement position. By setting the interval “a” and the interval “b” to establish Interval “a”&lt;Interval “b”, the development cartridge  15 ,  16 ,  17 ,  14  can be securely set at the development position or the replacement position. 
   The lock lever  38  has the locking position where the concavity  36  is engaged with the concavity of the rotary  13  so as to lock the rotary, the evacuation position where the concavity  36  is not engaged with the concavity, and the standby position set near the locking position. 
   As shown in  FIG. 13  and  FIG. 14 , both side walls  36   a ,  36   b  of the concavity  36  of the lock lever  38  are each formed to extend along an arc as a part of a circle of which center is equal to the rotation axis of the lock lever  38 , i.e. the axis of the pivot  37 . In addition, corners between outer surface of the lock lever  38  and the both side walls  36   a ,  36   b  of the concavity  36  are formed as rounded portions. As shown in  FIG. 14 , when the concavity  36  is engaged with the convexity, the side wall  36   a  and the arc face of the end of the convexity are in contact with each other at a point (along a line, exactly). During this, the reaction during the development operation of the rotary-type developing device  3  is transmitted from the end of the convexity to the side wall  36   a  and is thus received by the side wall  36   a.    
   The lock lever  38  is provided with an inclined face (contact portion)  42  which is formed in an outer periphery on a side facing the rotary  13  to incline toward the rotary  13  in a direction from the pivot  37  side toward the concavity  36  and is formed continuously from the concavity  36 . In this case, the start point  42   a  of the inclined face  42  is formed on a path of rotation of the end of the convexity, that is, an arc of a circle of which center is equal to the rotation axis of the rotary  13 . Concretely, as shown in  FIG. 15 , a contact point  42   a  between the inclined face  42  in the outer periphery of the side facing the rotary of the lock lever  38  and a linear portion  43  nearer to the pivot  37  than the inclined face  42  is formed on an arc of a circle  44  of which center is equal to the rotation axis of the rotary  13  and which has contacts with the inclined face  42  and the linear portion  43  in such a manner as to make smooth connection between the inclined face  42  and the linear portion  43  at the contact point  42   a . Accordingly, the end of the convexity can smoothly move when the contact portion of the end of the convexity with the lock lever  38  transfers from the linear portion  43  to the inclined face  42  of the lock lever  38 . 
     FIG. 15  shows a state that the lock lever  38  is at the standby position where the lock lever  38  is in contact with the stopper  41  shown in  FIG. 11 . At this point, the linear portion  43  of the lock lever  38  is identical to the tangent of the circle  44 . 
   The pivot  37  of the lock lever  38  is positioned on a tangent c of a circle of which center is equal to the rotation axis of the rotary  13  and which passes the center of a circular arc of the end of the convexity engaged with the concavity  36  in a state that the concavity  36  is completely engaged with the convexity so that the rotary  13  is stopped and retained as shown in  FIG. 13 . 
   Hereinafter, driving patterns of the driving motor  21  and control patterns of the rotary  13  for changing color and for replacing the cartridge in the image forming apparatus of this embodiment will be described. 
     FIG. 16  is a diagram showing an example of the driving pattern of the driving motor during operation of changing development color,  FIG. 17  is a diagram showing an example of the driving pattern of the driving motor during operation of detecting cartridge, and  FIG. 18  is a diagram showing the relation between the target angle and the actual rotational angle during operation of detecting cartridge. 
   In the image forming apparatus of this embodiment, by rotating the rotary-type developing device  3  to overrun the stop position in a state that the lock lever  38  as a locking member is biased by the lock lever spring  39 , the concavity  36  of the lock lever  38  is engaged with one of the convexities  28 ,  29 ;  30 ,  31 ;  32 ,  33 ;  34 ,  35  of the rotary frame  13   a . In this manner, the stopping control is conducted. In this case, the overrunning amount of the rotary-type developing device  3  is set such that the overrunning amount for the stopping control for replacing the development cartridge  2  is larger than that for the stopping control for changing the development color. In addition, the driving pattern of the driving motor  21  for the operation for replacing the development cartridge is set to ensure the overrunning of the rotary  13  even in a state that the maximum offset load is exerted in the reverse rotational direction of the rotary  13 , this state meeting a condition making it difficult for the rotary to overrun. The driving pattern of the driving motor  21  is set to ensure the overrunning of the rotary  13  in a state that the four color cartridges are mounted so that little offset load is applied during the operation for changing color. 
   That is, for ensuring the overrunning, the target angle of the driving motor  21  must be set to increase the rotational amount of the driving motor  21  to move the rotary beyond the predetermined stop position. However, if the rotational amount is increased simply, there is a possibility of deviation because the driving motor  21  continues to rotate even after the engagement of the lock lever at the stop position. 
   Therefore, as a measure for ensuring the overrunning in the image forming apparatus  1  of this embodiment, an acceleration in the decelerating direction (i.e. deceleration) is drastically applied to the rotary  13  to overcome the rotational force due to the offset load when positioning the rotary after driving the rotary. 
   In this case, in view of mechanical strength of the apparatus and the noise, it is preferable that the rotary  13  is driven calmly at a lowest possible speed for the initializing operation and the cartridge replacement operation, because there is no limitation in operation speed for these operations. However, when the rotary  13  is driven at the low speed, the rotational speed of the rotary  13  is not increased so that only little rotational inertia is produced. 
   In the image forming apparatus  1  of the second embodiment, the rotary  13  is rotated at a constant speed until reaching a predetermined stop position and the driving speed of the motor  21  is significantly zeroed, not slowly reduced, at the moment it reaches the stop position. At this moment, in theory, infinitely large negative acceleration is applied to the rotary  13  so that inertial force sufficiently overcoming the offset load of the rotary  13  is generated, whereby the rotary  13  tends to overrun the predetermined stop position because of backlash of the driving gear train. 
   In the cartridge detecting operation just after a new cartridge is mounted during the cartridge replacement operation, it is desired to agitate toner. For this, it is preferable to rotate the rotary  13  at a high speed. However, the addition of an exclusive driving pattern (sequence) for agitating toner makes the software production complex. By diverting the driving pattern for changing development color as the driving pattern for agitating toner, the number of man-hour for producing the software can be reduced. However, since the operation for changing development color is based on the assumption that all of the cartridges for the respective colors are mounted, the driving pattern for changing development color is not made for applying sufficient inertia when the rotary is stopped. The operation for replacing cartridge has a possibility that, depending on the presence or absence of cartridges, a strong offset load may be applied. In this embodiment, the driving pattern for changing development color is diverted with a slight change for intensifying the final end of the deceleration pattern in order to apply enough inertia to the rotary  13  when the rotary is stopped. 
   For example, in a driving mechanism in which when the pulse value is 300, the motor step is 1.8 degree/p, and reduction ratio is 6, the rotational angle of the driving motor  21  becomes 540 degree and the rotational angle of the rotary becomes 90 degrees (the rotational angle of a single developing device among four color developing devices), the driving mechanism is accelerated from a driving frequency around 200 to the highest driving frequency (i) in the order of 1600 during the normal operation for changing development color, taking 0.15 sec of time as shown in  FIG. 16  and, reversely, is decelerated from the highest frequency to a driving frequency about 300, taking substantially the same period of time as the aforementioned period of time so that the driving pattern having an angle shape and a triangular shape is employed. Since this driving pattern is based on the assumption that the cartridges of four colors are mounted, the driving motor  21  is stopped while being accelerated at a final position to minimize the overrunning amount in consideration of the load on the driving motor  21 . 
   In this case, however, as an offset load of the rotary  13  is increased, the rotary  13  is stopped with error in stopping angle of the rotary  13  within a range of backlash of the driving gear train of the rotary. That is, the driving motor  21  is controlled to rotate at an angle of 90 degree as the target angle. Accordingly, the rotary  13  is rotate in a range of (90±E′) degrees, wherein E′ is an error in stop position between the actual stop position of the rotary  13  and the predetermined stop position of the rotary  13  when it is rotated at an angle of 90 degree and is smaller than E (E′&lt;E) as the maximum error when the rotary  13  is rotated at an angle of 90 degree. 
   On the other hand, the operation of replacing cartridge employs a driving pattern in which the driving mechanism is accelerated to the maximum speed (i), in the same manner as the normal operation of changing development color, as shown in  FIG. 17 . When the driving mechanism is at the maximum speed (i), a predetermined pulse (10 pulse) is applied, the total pulse remains unchanged, and the predetermined pulse (10 pulse) added at the highest speed (i) is eliminated at the final position (ii). In this driving pattern, the total pulse remains unchanged and the rotational angle also remains unchanged. However, the predetermined pulse (10 pulse) is added so as to shorten the intervals of pulses and shorten the total rotational time. In this case, load is exerted on the driving motor  21 . However, there is no strength-wise problem because the frequency of replacing cartridges is smaller than the frequency of changing development colors. 
   According to this driving pattern, the state when the maximum offset load is exerted on the rotary  13  in the reverse direction meets a condition making it difficult for the rotary to overrun when the driving motor  21  is stopped. In this case, as shown in  FIG. 18 , the lock is released at 0.075 sec before the start of the drive so that the motor rotates by itself (iii). According to the start of the drive, the motor rotates so that the actual rotational angle lags behind the target angle by 1-1.5 degree (iv). When the target angle becomes 90 degree, the drive is stopped so that the motor starts to overrun because of its inertia (v). As the lock is engaged (vi), the motor shifts in a range allowed by the backlash of the lock and is stopped (vii). Thus, the motor  21  is turned off. 
   Hereinafter, description will now be made as regard to the action during the image forming operation of the image forming apparatus  1  of the second embodiment having the aforementioned structure. The state shown in  FIG. 1  and  FIG. 8  is a state that the development roller  14   a  of the development cartridge  14  for yellow is in contact with the photoreceptor  4 , i.e. that the development cartridge  14  for yellow is set to the development position. On the other hand, during the non-development (non-image forming operation), the rotary  13  is at the home position shown in  FIG. 11  and all of the development rollers  14   a ,  15   a ,  16   a ,  17   a  of the development cartridges  14 ,  15 ,  16 ,  17  are retained apart from the photoreceptor  4 . 
   When the image forming apparatus  1  is inactive, the movable members of the image forming apparatus  1  stop. The solenoid  40  is not excited and is thus inactive, the lock lever  38  is set at the standby position where it is in contact with the stopper  41  shown in  FIG. 11  by the biasing force of the lock lever spring  39 . Further, the driving motor  21  of the rotary  13  is controlled for its drive according to the driving pattern as shown in  FIG. 16  for changing color and according to the driving pattern as shown in  FIG. 17  for replacing cartridge. 
   As an image forming signal is inputted from the image forming operation command means  22  into the CPU  51  of the image forming apparatus  1 , the operation of the image forming apparatus  1  for forming an image is started so that the CPU  51  is driven to rotate the photoreceptor  4  in the clockwise direction in  FIG. 1  and  FIG. 8  and the exposure device  2  is activated. Then, the exposure device  2  exposes the photoreceptor  4  to light according to the image signal for yellow from the CPU  51 , thereby forming an electrostatic latent image for yellow on the photoreceptor  4 . At the same time, the CPU  51  turns on the electromagnetic clutch  26  to establish the connection, and drives the driving motor  21  and also drives the intermediate transfer medium  5 . 
   Then, the driving force of the driving motor  21  is transmitted to the development cartridge driving gear  19  via the motor output gear  22  and the power transmission gear mechanism  20  with a reduction in speed, whereby the development cartridge driving gear  19  is rotated. At the same time, the driving force of the driving motor  21  transmitted to the rotary driving gear  23  via the motor output gear  22 , the electromagnetic clutch  26 , the power transmission gear mechanism  25 , and the rotary driving gear  24 , whereby the rotary  13  is rotated in the counterclockwise direction in  FIG. 1  and  FIG. 8  or in the clockwise direction in  FIG. 11 . Then, the convexity  28  is moved in the clockwise direction in  FIG. 11 . 
   Since the development cartridge driving gear  19  is provided with a one-way clutch, even when the development cartridge driving gear  19  is meshed with any one of the development drive input gears  14   c ,  15   c ,  16   c ,  17   c , the driving force of the driving motor  21  is not transmitted to the development drive input gear meshed with the development cartridge driving gear  19  because the one-way clutch runs idle relative to the rotational direction of the rotary  13 . 
   As shown in  FIG. 15 , as the convexity  28  moves in the clockwise direction after the end of the convexity  28  collides with the start point  42   a  of the inclined face  42  of the lock lever  38 , the end of the convexity  28  presses the inclined face  42  of the lock lever  38 . Since the start point  42   a  is formed on an arc of the circle  44  of which center is equal to the rotation axis of the rotary  13 , the convexity  28  can smoothly collide with the lock lever  38  and the end of the convexity  28  can smoothly move relative to the inclined face  42  as shown in  FIG. 19 . Accordingly, little disturbance produced when the end of the convexity  28  collides with the lock lever  38  is produced so that the rotary  13  is not affected by the collision and thus can smoothly continue to rotate. In addition, little noise due to the collision is generated. Moreover, the end of the convexity  28  is formed in an arc shape. Since the end of the convexity  28  comes in contact with the lock lever  38  at a point (exactly, along a line because of the thickness of the convexity  28  and the thickness of the lock lever  38 ), the contact friction is very small, thereby minimizing the effect on the rotary  13  due to the contact friction. Therefore, the rotation of the rotary  13  is further smooth. 
   As the end of the convexity  28  presses the inclined face  42 , the lock lever  38  is rotated about the pivot  37  against the biasing force of the lock lever spring  39  in the counter clockwise direction in  FIG. 19 . As the rotational angle of the driving motor  21  becomes the target angle of 90 degrees, the CPU  51  stops the drive of the driving motor  21 . However, the rotary  13  rotates in the clockwise direction because of its inertia. Since the offset load is small because the cartridges for four colors are mounted, the rotary  13  can overrun the stop position so that the rotational angle becomes (90+E′) degrees. Therefore, as the rotary  13  further rotates in the clockwise direction in  FIG. 19 , the end of the convexity  28  is positioned to face the concavity  36 . By the biasing force of the lock lever spring  39 , the lock lever  38  pivots in the clockwise direction in  FIG. 19  and comes in the locking position where the concavity  36  is engaged with the convexity  28 . Just after that, the CPU  51  turns off the electromagnetic clutch  26  to isolate the rotor driving gear train. In this state, therefore, the driving force of the driving motor is not transmitted to the rotary driving gear  24 . 
   Since the concavity  36  of the lock lever  38  is engaged with the convexity  28  and the driving force of the driving motor  21  is not transmitted, the lock lever  38  stops the rotary  13  and retains the rotary  13  at this position. That is, the rotary  13  is positioned by the lock lever  38 . Since the lock lever  38  is switched from the standby position set near the locking position to the locking position, the lock lever  38  can rapidly lock the rotary  13  with small amount of pivotal movement and can ensure the locking of the rotary  13  because the concavity  36  is engaged with the convexity  28 . 
   As a process of achieving the engagement between the concavity  36  of the lock lever  38  and the convexity  28 , the concavity  36  moves along the arc of the end of the convexity  28  to engage the convexity  28 . Therefore, there is no possibility of bounce of the lock lever  38  so that the concavity  36  is smoothly engaged with the convexity  28 . In addition, since the corners of the concavity  36  are rounded, the concavity  36  is further smoothly engaged with the convexity  28 . 
   In the state the rotary  13  is positioned by the engagement between the concavity  36  of the lock lever  38  and the convexity  28 , the development cartridge  14  for yellow is set at the development position as shown in  FIG. 1  and  FIG. 8 . That is, the development roller  14   a  is in contact with the photoreceptor  4 . Then, the input gear  14   c  is meshed with the development cartridge driving gear  19 , the CPU  51  drives the driving motor  21 , and the one-way clutch is locked (connected) relative to the rotational direction of the driving motor  21  for the development operation. Therefore, the driving force of the driving motor  21  is inputted from the development cartridge driving gear  19  to the input gear  14   c  and is further transmitted to the development roller driving gear  14   b  with a reduction in speed by the power transmission gear mechanism  14   d , whereby the development roller  14   a  is rotated. 
   Therefore, the development roller  14   a  carries a predetermined amount of yellow toner to the photoreceptor  4  so that the development for yellow of an electrostatic latent image on the photoreceptor  4  is performed, thereby forming an yellow toner image on the photoreceptor  4 . During this development operation, a reaction force due to the development operation exerted on the lock lever  38  via the convexity  28  acts in a tangential direction of a circle of which center is equal to the rotation axis of the rotary  13  and which passes the center of the circular arc of the end of the convexity  28 . 
   However, since the center of the pivot  37  as the rotation axis of the lock lever  38  is positioned on the tangent and the side wall  36   a  of the concavity  36  is formed on an arc of a circle of which center is the axis of the pivot  37 , the reaction force due to the development operation never produces rotational force (rotational moment) of the lock lever  38 . Therefore, there is no possibility of disengagement of the concavity  36  from the convexity  28  due to rotation of the lock lever  38  during the development operation, and the rotary  13  can be positioned securely by the lock lever  38  such that the development cartridge  14  is in the development position. In this case, since the side walls  36   a ,  36   b  of the concavity  36  are each formed to extend along an arc as a part of a circle of which center is equal to the axis of the pivot  37 , the positioning of the rotary  13  is securely conducted regardless of which side wall  36   a  or  36   b  is subjected to the reaction force due to the development operation, and the positioning of the rotary  13  is securely conducted even when the development operation is suddenly stopped such as an irregular case. 
   By the way, in the operation for changing color, the driving motor  21  is controlled according the driving pattern shown in  FIG. 16 . This driving pattern is based on the assumption that the cartridges of four colors are mounted and that the offset load is small. However, for example, when the toner consumption of one of the cartridges for four colors is risen so that the amount of the toner in the cartridge is considerably less than those of the other cartridges or when one of the cartridges is not mounted, the offset load of the rotary  13  may be relatively increased. In such a case, the rotary  13  may not reach the stop position (rotational position at an angle of 90 degrees) in the range of the backlash of the driving gear train, that is, stops after rotating only (90−E′) degrees. 
   In this case, however, since the driving gear train on the development side is engaged, the rotary  13  further rotates in its rotational direction due to the reaction force of the development operation at the start of the development operation conducted continuously after the stop of the driving motor  21  of the rotary  13 , whereby the convexity  28  of the rotary  13  is engaged with the concavity  36  of the lock lever  38  and the rotary  13  is positioned at the development position for yellow (this positioning method of the rotary  13  in this case is the same as the positioning method disclosed in Japanese Patent Unexamined Publication No. 2002-311713). 
   The yellow toner image carried by the photoreceptor  4  is primarily transferred to the intermediate transfer medium  5  by the primary transfer device  6 . As the primary transfer of the yellow toner image is finished, the CPU  51  stops the driving motor  21  and, at the same time, turns on and connects the electromagnetic clutch  26  again. At this point, the operation of the development cartridge  14  is stopped. Just after that, the CPU  51  excites the solenoid  40  so that the plunger  40   a  is pulled by the solenoid force of the solenoid  40 , whereby the lock lever  38  is rotated about the pivot  37  in the counter clockwise direction in  FIG. 20  against the biasing force of the lock lever spring  39 . Then, the concavity  36  is disengaged from the convexity  28 , that is, the engagement between the concavity  36  and the convexity  28  is released, so that the lock lever  38  is in the evacuation position where the concavity  36  and the convexity  28  are not engaged with each other. After that, the CPU  51  drives the driving motor  21  so that the rotary  13  is rotated again in the clockwise direction in  FIG. 21  by the driving force of the driving motor  21  in the same manner as described above. 
   At about the time starting the rotation of the rotary  13 , the exposure device  2  exposes the photoreceptor  4  to light based on the image signal for magenta from the CPU  51 , thereby forming an electrostatic latent image for magenta on the photoreceptor  4 . 
   According to the rotation of the rotary  13 , the positioning convexity  29  for replacement passes the concavity  36  as shown in  FIG. 22 . As the convexity  29  deviates from the region of the lock lever  38  according to the further rotation of the rotary  13 , the excitation of the solenoid  40  is cancelled so that the plunger  40   a  is released from the solenoid force. Then, the lock lever  38  is rotated about the pivot  37  in the clockwise direction in  FIG. 22  by the biasing force of the lock lever spring  39  so that the lock lever  38  comes in contact with the stopper  41  and becomes in the standby position again similarly to the state shown in  FIG. 11 . 
   The rotary  13  is rotated at a relatively high speed for the image forming operation as mentioned above. Since the interval “a” in the circumferential direction between the positioning convexity  28  for development and the positioning convexity  29  for replacement is set to be smaller than the interval “b” in the circumferential direction between the convexity  29  and the positioning convexity  30  for development of the next development cartridge  15  for magenta, enough period of time can be ensured until the convexity  30  reaches the position where the convexity  30  comes in contact with the start point  42   a  of the inclined face  42  of the lock lever  38 . That is, a long period of time of releasing the plunger  40   a  can be estimated. Therefore, even if the time from the restrained state where the solenoid  40  is excited and the plunger  40   a  is pulled by the solenoid force to the released state is uncertain and long, the lock lever  38  can be securely set at the standby position before the convexity  30  reaches the position where it comes in contact with the start point  42   a.    
   For setting the time for releasing the plunger  40   a  of the solenoid as longer as possible, it is preferable that the interval “a” is set to be the minimum value possible in a range allowing the engagement of the concavity  36  of the lock lever  38  with the positioning convexity  29 ;  31 ;  33 ;  35  for replacement and the interval “b” is therefore set to be large. 
   As the rotary  13  is further rotated in the clockwise direction in  FIG. 11 , similarly to the aforementioned case for yellow as shown in  FIG. 20 , the concavity  36  of the lock lever  38  is engaged with the positioning convexity  30  for development so that the rotary  13  is positioned and the development cartridge  15  for magenta is set at the development position. 
   As the development cartridge  15  for magenta is set at the development position, the rotary  13  is stopped from rotating and is retained at the position, the input gear  15   c  is meshed with the development cartridge driving gear  19 , and the development roller  15   a  comes in contact with the photoreceptor  4 . Then, the development for magenta is conducted by the development cartridge  15  for magenta and a magenta toner image developed on the photoreceptor  4  is primarily transferred to the intermediate transfer medium  5 . 
   As the primary transfer of the magenta toner image is finished, the CPU  51  stops the driving motor  21  and, at the same time, turns on and connects the electromagnetic clutch  26  again similarly to the aforementioned case for yellow. At this point, the driving of the development cartridge  15  is stopped. Just after that, the solenoid  40  is excited so that the concavity  36  is disengaged from the positioning convexity  30  for development and the rotary  13  is rotated in the same direction. 
   Further, similarly to the aforementioned case for yellow, the cancellation of excitement of the solenoid  40  sets the lock lever  38  to the standby position in the same manner as shown in  FIG. 11 . After that, in the same manner, the concavity  36  of the lock lever  38  is engaged with the positioning convexity  32  for development of the development cartridge  16  for cyan, whereby the development for cyan is conducted by the development cartridge  16  for cyan and a cyan toner image developed on the photoreceptor  4  is primarily transferred to the intermediate transfer medium  5 . Further, the concavity  36  of the lock lever  38  is engaged with the positioning convexity  34  for development of the development cartridge  17  for black, whereby the development for black is conducted by the development cartridge  17  for black and a black toner image developed on the photoreceptor  4  is primarily transferred to the intermediate transfer medium  5 . 
   Accordingly, toner images of four colors primarily transferred on the intermediate transfer medium  5  are toned so as to form a full-color toner image. The full-color toner image on the intermediate transfer medium  5  is transferred to a paper sheet  7  by the secondary transfer device  8 . Then, the toner image transferred to the paper sheet  7  is fused by the fixing unit, thereby forming a full-color image on the paper sheet  7 . 
   Hereinafter, description will now be made as regard to the operation for replacing any of the development cartridges  14 ,  15 ,  16 ,  17 . For example, as a user runs a key for the replacement of the development cartridge on the operation panel of the image forming apparatus  1 , the CPU  51  drives the driving motor  21 , turns on the electromagnetic clutch  26 , and excites the solenoid  40 . Then, similarly to the case as shown in  FIG. 21  and  FIG. 22 , the lock lever  38  is retained by the solenoid  40  at a position where the concavity  36  is not engaged with the convexity and the rotary  13  is rotated in the clockwise direction in  FIG. 21  and  FIG. 22 . During the operation for replacing the development cartridge, the rotary is rotated at a relatively lower speed than that for the image forming operation. 
   Just before the development cartridge to be replaced, for example, the development cartridge  14  for yellow reaches the replacement position, that is, just before the positioning convexity  29  for replacement corresponding to the development cartridge  14  reaches the position where it engages the concavity  36  of the lock lever  38 , the CPU  51  cancel the excitement of the solenoid  40 . Then, in the same manner as described above, the lock lever  38  is rotated about the pivot  37  in the clockwise direction, i.e. the direction toward the standby position, by the biasing force of the lock lever spring  39  so that the positioning convexity  29  for replacement of the development cartridge  14  comes in contact with the inclined face  42  of the lock lever  38 . According to further rotation of the rotary  13 , as the positioning convexity  29  presses the inclined face  42 , the lock lever  38  is rotated about the pivot  37  in the counter clockwise direction in the same manner as mentioned above. 
   Since the driving pattern of the driving motor  21  for the operation for replacing the development cartridge is set based on the assumption that the maximum offset load is exerted in the reverse rotational direction of the rotary  13 , this state meeting a condition making it difficult for the rotary to overrun, the rotary  13  overruns the stop position without fail. 
   Therefore, as the positioning convexity  29  for replacement reaches the position corresponding to the concavity  36 , the lock lever  38  is rotated about the pivot  37  in the clockwise direction by the biasing force of the lock lever spring  39  so that the concavity  36  is engaged with the convexity  29  as shown in  FIG. 23  in the same manner as mentioned above. Accordingly, the rotary  13  is stopped from rotating and is retained at the position. Though the excitation of the solenoid  40  is cancelled just before the positioning convexity  29  for replacement reaches the position where it engages the concavity  36 , the rotary  13  is rotated at a relatively lower speed during the replacement operation so that the concavity  36  is securely engaged with the convexity  29  even if the time from the restrained state where the plunger  40   a  of the solenoid  40  is pulled by the solenoid force to the released state is uncertain and long. 
   In this manner, in the state that the rotary  13  is positioned, the development cartridge  14  to be replaced is set at the replacement position. When the development cartridge  14  is set at the replacement position, the development roller  14   a  of the development cartridge  14  is spaced apart from the photoreceptor  4  and the input gear  14   c  is spaced apart from the development cartridge driving gear  19 . 
   In the state that the development cartridge  14  to be replaced is set at the replacement position, the development cartridge  14  is removed by withdrawing the development cartridge  14  from the rotary  13  through the development cartridge replacement opening  45  in the axial direction of the rotary  13  and a new development cartridge is installed by inserting the new development cartridge into the rotary  13  through the development cartridge replacement opening  45  in the axial direction of the rotary  13 . Since the development roller of the development cartridge to be replaced is spaced apart from the photoreceptor  4  during the replacement of the development cartridge, the photoreceptor  4  may not be damaged by the development cartridge during the replacement operation. 
   In the image forming apparatus  1  of the second embodiment, the convexity comes in contact with the inclined face  42  of the lock lever  38  before the concavity of the lock lever  38  is engaged with the convexity formed on the rotary  13  according to the rotation of the rotary  13 . Therefore, the lock lever  38  is moved according to the rotation of the rotary  13  so that the engagement between the concavity  36  of the lock lever  38  and the convexity of the rotary  13  is completed simultaneously with the arrival of the rotary  13  at the predetermined position. That is, the start of the development operation is allowed immediately after the rotary is stopped at the development position, thereby shortening the time required for obtaining toner images of four colors by changing color and, as a result, improving the speed of forming a full-color image. 
   Since the solenoid  40  is employed as the means for evacuating the lock lever  38 , the lock lever  38  is reliably and quickly operated. 
   During the rotation of the rotary  13 , the lock lever  38  is previously set at the standby position before the stop of the rotary  13 . From this state, the rotary  13  is restrained and positioned by operating the lock lever  38  simultaneously with the stop of the rotary  13 . Therefore, the moving amount of the lock lever  38  from the standby position to the position restraining the rotary  13  is small. This shortens the time required for obtaining toner images of four colors by changing color and, as a result, improving the speed of forming a full-color image. 
   Since the convexity formed on the rotary  13  side is designed to come in contact with the inclined face  42  of the lock lever  38 , little disturbance produced when the end of the convexity  28  collides with the lock lever  38  is produced so that the rotary  13  is not affected by the collision and thus can smoothly rotate. In addition, little noise due to the collision is generated. 
   Particularly, the start point  42   a  of the inclined face  42  is formed on an arc of a circle of which center is equal to the rotation axis of the rotary  13  and which passes the ends of the convexities, the end of the convexity can smoothly move when the contact portion of the end of the convexity with the lock lever  38  transfers from the linear portion  43  to the inclined face  42  of the lock lever  38 . 
   Moreover, since the end of the convexity is formed in an arc shape and is designed such that the end of the convexity comes in contact with the lock lever  38  at a point (or along a line), the contact friction is very small, thereby reducing the effect on the rotary  13  due to the contact friction. Therefore, the rotation of the rotary  13  is further smooth. 
   In addition, as a process of achieving the engagement between the concavity  36  of the lock lever  38  and the convexity  28 , the concavity  36  moves along the arc of the end of the convexity  28  to engage the convexity  28 . Therefore, there is no possibility of bounce of the lock lever  38  so that the concavity  36  can be reliably engaged with the convexity  28  and the rotary can be reliably restrained at the stop position. 
   During this development operation, a reaction force due to the development operation exerted on the lock lever  38  via the convexity acts in a tangential direction of a circle of which center is equal to the rotation axis of the rotary  13  and which passes the center of the circular arc of the end of the convexity. Since the rotational axis of the lock lever  38  is positioned on the tangent and the side wall  36   a  of the concavity  36  is formed on an arc of a circle of which center is the axis of the pivot  37 , the reaction force due to the development operation never produces rotational force (rotational moment) of the lock lever  38 . Therefore, there is no possibility of disengagement of the concavity  36  from the convexity  28  due to rotation of the lock lever  38  during the development operation, and the rotary  13  can be positioned securely by the lock lever  38 . 
   In this case, since the side walls  36   a ,  36   b  of the concavity  36  are each formed to extend along an arc as a part of a circle of which center is equal to the rotational axis of the lock lever  38 , the positioning of the rotary  13  is securely conducted regardless of which side wall  36   a  or  36   b  is subjected to the reaction force due to the development operation, and the positioning of the rotary  13  is securely conducted even when the development operation is suddenly stopped such as an irregular case. 
   Further, the positioning convexities  28 ,  30 ,  32 ,  34  for development and the positioning convexities  29 ,  31 ,  33 ,  35  for replacement are provided to corresponding to the development cartridges  14 ,  15 ,  16 ,  17 , respectively, and the intervals “a” between each pair of the positioning convexities  28 ,  30 ,  32 ,  34  for development and the positioning convexities  29 ,  31 ,  33 ,  35  for replacement is set to be smaller than the interval “b” in the circumferential direction between the positioning convexity  29 ,  31 ,  33 ,  35  for replacement and the positioning convexity  30 ,  32 ,  34 ,  28  for development of the next development cartridge  15 ,  16 ,  17 ,  14 , enough period of time can be ensured until the convexity reaches the position where it comes in contact with the start point  42   a  of the inclined face  42  of the lock lever  38 . That is, a long period of time of releasing the plunger  40   a  can be estimated. Therefore, even if the time from the restrained state where the plunger  40   a  is pulled by the solenoid force of the solenoid  40  to the released state is uncertain and long as mentioned above, the lock lever  38  can be securely set at the standby position before the convexity reaches the position where it comes in contact with the start point  42   a.    
   When the rotary  13  tends to rotate (90+E′) degrees beyond (overrun) the stop position (the 90-degree rotation position) within a range of backlash or when the rotary  13  stops after rotating only (90−E′) degrees within the range of backlash, i.e. stops before reaching the stop position during the operation of changing color, the final stopping rotation angle becomes always 90 degrees, achieving the high-precision positioning. In addition, since the high-precision positioning is achieved, the start angle falls in exactly the same phase relative to the next rotation of the rotary. Therefore, even if there is difference in rotational angle due to the backlash of the rotary drive gear train as mentioned above, the difference can be always cancelled, thereby ensuring the stable rotation of the rotary  13 . 
     FIG. 24  is a partially enlarged view similar to  FIG. 14 , but showing a third embodiment of the image forming apparatus of the present invention. It should be noted that the same components as those of the aforementioned embodiments are marked with the same numerals, so description of such components will be omitted. 
   Though the ends of the convexities  28 ,  29 ,  30 ,  31 ,  32 ,  33 ,  34 ,  35  are each formed in an arc shape in the second embodiment, each of convexities  28 ,  29 ,  30 ,  31 ,  32 ,  33 ,  34 ,  35  is provided at its end with a rotary  46  such as a rotatable roller rotatably disposed in the image forming apparatus  1  of the third embodiment. In this case, the rotational axis of the rotary  46  is set at the same position as the center of the arc of the end of the convexity. 
   According to the image forming apparatus  1  of the third embodiment, the convexity comes in contact with the lock lever  38  via the rotary  46 , thereby significantly reducing the friction due to the contact. Therefore, the effect on the rotary  13  due to the contact friction is further reduced and the disengagement of the convexity from the concavity of the lock lever  38  is extremely facilitated. In addition, at least the surface of the rotary  46  is composed of an elastic member having such elasticity capable of keeping the function of positioning the rotary  13 , thereby exhibiting buffering effect when the lock lever  38  is engaged with the convexity and when the lock lever  38  is disengaged from the convexity. 
   Other structure and other works and effects of the image forming apparatus of the third embodiment are the same as those of the aforementioned second embodiment. 
   Though four development cartridges  14 ,  15 ,  16 ,  17  are mounted to the rotary  13  in the first through third embodiments, the number of development cartridges is not limited to four and a plurality of development cartridges may be mounted in the image forming apparatus of the present invention. 
   Though a single driving motor  21  is commonly provided for driving the rotary and for driving the development cartridge in the aforementioned second and third embodiments, driving motors may be separately provided for driving the rotary and for driving the development cartridge, respectively, similarly to the first embodiment. 
   Though the transmission and isolation of the driving force of the driving motor to the rotary  13  is controlled by the electromagnetic clutch  26  as the power transmission control means in the aforementioned second and third embodiments, the transmission and isolation of the driving force of the driving motor to the development cartridge may be controlled by the electromagnetic clutch, and both the transmission and isolation of the driving force of the driving motor to the development cartridge and the transmission and isolation of the driving force of the driving motor to the rotary  13  may be controlled by the electromagnetic clutches. 
   Though the driving pattern during the operation of changing color shown in  FIG. 16  is based on the assumption that the cartridges of four colors are mounted and that the offset load is small in the aforementioned second and third embodiments, the driving pattern may be based on the assumption that the maximum offset load is exerted in the reverse rotational direction of the rotary  13 , this state meeting a condition making it difficult for the rotary to overrun, at the stop of the driving motor  21 . In this case, the rotary  13  inevitably overruns the stop position to securely engage the convexity of the rotary  13  with the concavity  36  of the lock lever even for the operation of changing color. Therefore, the rotary  13  can be reliably set at the stop position (that is, the development position).