Patent Publication Number: US-8538310-B2

Title: Image transfer apparatus, image fixing apparatus, and registration apparatus which prevent a load torque variation upon entry or exit of a sheet into a nipping portion

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to image transfer apparatuses, image fixing apparatuses, and registration apparatuses in which a transfer material is transported into a nipping portion between a first rotating body rotated by a rotating force provided by a drive source and a second rotating body that is pressed against the first rotating body with a predetermined pressing force. 
     2. Description of the Related Art 
     In an electrophotographic image forming apparatus, a photosensitive material in the form of a drum or a belt is charged, and an electrostatic latent image is formed on the photosensitive drum or belt while it is rotated. The latent image is then visualized by causing toner to attach to it using a developing apparatus, whereby a toner image is formed on the image carrier, i.e., the drum or belt. The toner image is transferred onto the recording medium that is transported, such as a sheet of paper or an OHP film, either directly or via an intermediate transfer material that may be in the form of a belt. 
     In such an image forming apparatus, the transfer of the toner image on the image carrier onto the recording medium or the intermediate transfer material is carried out by an image transfer apparatus. 
       FIGS. 14A-14C  show an image transfer apparatus in a conventional image forming apparatus, such as disclosed in Japanese Patent No. 2883916 or Japanese Laid-Open Patent Applications 61-90167. 
     With reference to  FIG. 14A , the image transfer apparatus includes a displacement roller  201  disposed upstream of an image transfer position along a sheet transport path where an image is transferred by nipping a sheet. The displacement roller  201  is displaced downward as a sheet of transfer material  204  passes thereon, as shown in  FIG. 14B , whereby the pressing force of a pressure roller  203 , which is coupled to the displacement roller  201  via a connecting arm  202 , is adjusted depending on the amount of movement of the displacement roller  201 . In this way, the impact of the transfer material  204  as it enters the sheet-nipping image transfer position is reduced, as shown in  FIG. 14C . 
     Japanese Laid-Open Patent Application No. 6-274051 discloses a mechanism in which a gap is produced at a nipping portion by moving a pressure roller with a drive force provided by a drive unit. Specifically, an arm that supports the pressure roller is pressed down by the rotating force of an elliptic cam in order to produce a gap at the nipping portion in advance. The gap is subsequently eliminated and the pressure roller is pressed onto a drum upon entry of a sheet into the nipping portion between the roller and the drum. 
     Japanese Laid-Open Patent Application No. 2006-317627 discloses that, in order to reduce a load torque variation that occurs in a secondary transfer unit in an image forming apparatus due to the entry or exit of a thick sheet, the inertia of a pressure roller in the secondary transfer unit is minimized. 
     In the aforementioned technologies disclosed in Japanese Patent No. 2883916 and Japanese Laid-Open Patent Applications 61-90167, the presence of the displacement roller and the connecting arm increases the size of the transfer apparatus. In addition, because the transfer pressure varies depending on the thickness of the sheet, it is difficult to obtain appropriate transfer/nipping conditions. Since the purpose of nipping a sheet in a transfer apparatus or a fixing apparatus is to give a desired pressure to the toner image, a constant pressure needs to be imparted regardless of the thickness of the sheet. However, in the apparatuses according to these publications, nipping conditions vary depending on the depressing force of the connecting arm, which varies depending on the thickness of the sheet. As a result, a defective transfer or fixing may easily occur when a thick sheet is used. 
     In the aforementioned technology according to Japanese Laid-Open Patent Application No. 6-274051, in order to allow the arm to be moved at high speed against a pressing force, the actuator including the arm needs to have high rigidity and be able to provide a high torque. In recent years, the sheet transport speeds have been increased for productivity improvement purposes. There is also a demand to increase the image area on a sheet (i.e., reduction of the margin, or “borderless” image). Thus, in accordance with this technology, it is necessary to perform an operation of switching from a spaced-apart condition to a nipped condition in the nipping portion instantaneously. For example, in an image forming apparatus in which a sheet is transported at the speed of 200 mm/s, if it is desired to transfer an image onto the sheet 2 mm from its front edge, it is necessary to switch from a spaced-apart state to a nipped state approximately 0.01 second after the entry of the sheet into the nipping portion. However, it is difficult to realize a drive control mechanism that produces a torque with which the switching can be performed at such high speed. Furthermore, the impact upon pressing one roller onto another tends to produce vibrations. 
     In the aforementioned technology according to Japanese Laid-Open Patent Application No. 2006-317627, the pressing roller is pressed against an opposite roller via an elastic member. The pressing roller is required to have sufficient rigidity so that a uniform pressure can be imparted along the axis of the roller. Thus, there is a limit to which the inertia of the roller can be reduced. 
     In a conventional apparatus utilizing a pressing force, such as the aforementioned transfer apparatuses, in an electrophotographic apparatus as an image forming apparatus, a transfer material, such as a sheet of paper, may be transported to a nipping portion between a fixed roller and a pressing roller that is pressed against the fixed roller in order to transfer an image formed on the fixed roller onto the transfer material. Alternatively, in such a transfer apparatus, a transfer material may be passed through a nipping portion between a belt transported on a roller and another roller, in order to transfer an image on the belt onto the transfer material. In such transfer apparatuses, there is the problem that a load torque variation occurs when the transfer material enters or exits the nipping portion, resulting in a speed variation in the transfer material. 
     In the aforementioned conventional art according to Japanese Laid-Open Patent Application No. 6-274051, in which the roller and the drum are spaced apart in advance, a mechanism is required to produce a gap greater than the thickness of the transfer material. Such a separating mechanism requires a drive apparatus capable of producing a large torque. 
     SUMMARY OF THE INVENTION 
     It is a general object of the present invention to provide a novel and useful image transfer apparatus, fixing apparatus, and registration apparatus in which the aforementioned problems are eliminated. A more specific object is to prevent a load torque variation upon entry or exit of a sheet into a nipping portion while providing a sufficient nipping force required for image formation. 
     In one aspect, the invention provides an image transfer apparatus comprising a first rotating body that is rotated by a rotating drive force provided by a drive source; a second rotating body disposed near the first rotating body; and a pressing unit configured to press the second rotating body onto the first rotating body with a predetermined pressing force. A sheet of a transfer material is transported into a nipping portion between the first rotating body and the second rotating body, and an image formed on the first rotating body or a belt transported on the first rotating body is transferred onto the transfer material. The apparatus further includes a retaining unit configured to maintain a distance between the first rotating body and the second rotating body as long as the thickness of the transfer material that passes through the nipping portion remains the same. 
     In a preferred embodiment, the retaining unit comprises a one-way clutch mechanism that regulates the movement of the second rotating body by the pressing unit in a pressing direction while allowing the second rotating body to move freely in a direction opposite the pressing direction. The second rotating body is allowed to move freely in the pressing direction by a predetermined distance. 
     In another preferred embodiment, the retaining unit retains an outer diameter portion of the second rotating body. 
     In another preferred embodiment, the image transfer apparatus further comprises an arm member having a rotation center. The retaining unit is disposed on the arm member. 
     In another preferred embodiment, each of the first rotating body and the second rotating body includes a driving force transmitting unit configured to transmit a drive force from the same or an individual drive source. 
     In another aspect, the invention provides an image fixing apparatus comprising a first rotating body that is rotated by a rotating drive force provided by a drive source, the first rotating body including a heating member or supporting a heating member in the form of an endless belt; a second rotating body disposed near the first rotating body; and a pressing unit configured to press the second rotating body onto the first rotating body with a predetermined pressing force in order to fix a visible image on a transfer material that is transported through a nipping portion between the first rotating body and the second rotating body. The apparatus further includes a retaining unit configured to maintain a distance between the first rotating body and the second rotating body as long as the thickness of the transfer material that passes through the nipping portion remains the same. 
     In a preferred embodiment, the retaining unit comprises a one-way clutch mechanism that regulates the movement of the second rotating body by the pressing unit in a pressing direction while allowing the second rotating body to move freely in a direction opposite the pressing direction. The second rotating body is allowed to move freely in the pressing direction by a predetermined distance. 
     In another preferred embodiment, the retaining unit retains an outer diameter portion of the second rotating body. 
     In another preferred embodiment, the image fixing apparatus comprises an arm member having a rotation center. The retaining unit is disposed on the arm member. 
     In another preferred embodiment, each of the first rotating body and the second rotating body includes a driving force transmitting unit configured to transmit a drive force from the same or an individual drive source. 
     In another aspect, the invention provides a registration apparatus comprising a first rotating body that is rotated by a rotating drive force provided by a drive source; a second rotating body disposed near the first rotating body; and a pressing unit configured to press the second rotating body onto the first rotating body with a predetermined pressing force. A sheet of a transfer material is transported into a nipping portion between the first rotating body and the second rotating body, and an image formed on the first rotating body or a belt transported on the first rotating body is transferred onto the transfer material. The registration apparatus further includes a retaining unit configured to maintain a distance between the first rotating body and the second rotating body as long as the thickness of the transfer material that passes through the nipping portion remains the same. 
     In a preferred embodiment, the retaining unit comprises a one-way clutch mechanism that regulates the movement of the second rotating body by the pressing unit in a pressing direction while allowing the second rotating body to move freely in a direction opposite the pressing direction. The second rotating body is allowed to move freely in the pressing direction by a predetermined distance. 
     In another preferred embodiment, the retaining unit retains an outer diameter portion of the second rotating body. 
     In another preferred embodiment, the registration apparatus further comprises an arm member having a rotation center. The retaining unit is disposed on the arm member. 
     In another preferred embodiment, each of the first rotating body and the second rotating body includes a driving force transmitting unit configured to transmit a drive force from the same or an individual drive source. 
     In accordance with an embodiment of the invention, the amount by which a pressure roller is depressed upon entry of a transfer material, and the amount by which the pressure roller is pushed up upon exit of the transfer material, can be greatly reduced. Thus, the load torque variation on the transfer material transport drive system can be reduced, whereby the transfer material transport speed variation due to the load torque variation can be reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objects, features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of the invention, when read in conjunction with the accompanying drawings in which: 
         FIG. 1  schematically shows a photosensitive belt unit of an image forming apparatus; 
         FIG. 2  shows a graph indicating a transport position error of a photosensitive belt when a transfer material passes between an opposite roller and a transfer roller pressed against the opposite roller in a transfer apparatus; 
         FIG. 3  shows a diagrammatic illustration of a mechanism of the development of a torque load variation in the transfer apparatus; 
         FIG. 4  shows a graph indicating a calculated load torque variation; 
         FIG. 5A  shows a graph indicating a shift in load torque variation upon entry of a transfer material with a large thickness; 
         FIG. 5B  shows a graph indicating a shift in load torque variation upon entry of a transfer material with a small thickness; 
         FIG. 6  schematically shows an axial position regulating unit for a transfer roller; 
         FIG. 7A  shows a step in a sequence of operation for forming a gap between the shafts of rollers in an embodiment of the invention; 
         FIG. 7B  shows another step in the sequence of operation for forming a gap between the shafts of rollers; 
         FIG. 7C  shows another step in the sequence of operation for forming a gap between the shafts of rollers; 
         FIG. 8  shows a graph indicating a result of measuring the axial position of a transfer roller as a sheet of transfer material is passed; 
         FIG. 9  schematically shows a retaining mechanism according to another embodiment of the invention; 
         FIG. 10A  shows a roller-type one-way clutch according to an embodiment of the invention where the clutch is engaged; 
         FIG. 10B  shows the roller-type one-way clutch according to the embodiment of  FIG. 10A  where the clutch is released; 
         FIG. 11  shows an image transfer apparatus according to another embodiment of the invention; 
         FIG. 12  schematically shows an image forming apparatus according to another embodiment of the present invention; 
         FIG. 13  schematically shows an image forming apparatus according to another embodiment of the invention; 
         FIG. 14A  shows a conventional transfer material transport apparatus prior to the entry of a transfer material; 
         FIG. 14B  shows the conventional transfer material transport apparatus when the transfer material is travelling on a shifting roller; and 
         FIG. 14C  shows the conventional transfer material transport apparatus upon entry of the transfer material into a nipping portion. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereafter, embodiments of the present invention are described with reference to the drawings. 
     Based on a study conducted by the present inventors on the load torque variation that occurs when a transfer material enters a nipping portion between rollers, the following facts were observed: 
     (1) As the front end of the transfer material enters the nipping portion between the rollers, one of the rollers, i.e., the pressure roller, is moved against a pressing force, thereby increasing the load torque. 
     (2) As the rear end of the transfer material leaves the nipping portion between the rollers, the pressure roller as it is pressed by the pressing force pushes the transfer material in the transport direction, thereby reducing the load torque. 
     (3) Of the aforementioned load torque variations, a portion that exceeds an acceptable load torque range of the drive system as a whole, which is determined by a permitted torque of a drive source and the rigidity of the driving-force transmitting system including a series of gears for transmitting a drive force from the drive source to the rollers, causes a rotation variation in the rollers that affects an output image. 
     Conventionally, a method is proposed whereby a gap greater than the thickness of the transfer material is provided between the rollers, and the gap is eliminated to nip the transfer material upon its arrival between the rollers. However, there has been the problem of impact upon nipping the transfer material and also the need to provide a power source for forming the gap. The present inventors realized that actually it is not necessary to make the gap between the rollers greater than the thickness of the transfer material, and that the load torque can be controlled to stay within an acceptable load torque range even with a gap smaller than the transfer material thickness. Thus, an apparatus has been realized in which a sufficient roller center distance can be obtained without providing a drive mechanism for separating the two rollers apart. 
     In accordance with an embodiment of the present invention, a roller center distance is maintained so that the load torque produced by the vertical movement of a pressure roller upon entry or exit of the transfer material into or out of the nipping portion can be reduced regardless of the thickness of the transfer material. In this way, the load torque variation upon entry or exit of a sheet is prevented while a pressing force required for image formation is provided. 
     In the following, the phenomenon of load torque variation in an image forming apparatus and its mechanism are discussed.  FIG. 1  shows a photosensitive belt unit in the image forming apparatus. 
     In this photosensitive belt unit, a photosensitive belt  10  is extended on a drive roller  11 , a driven roller  12 , a tension roller  13 , and an opposite roller  14 . The photosensitive belt  10  is transported in the clockwise direction as the drive roller  11  is driven by a drive motor M 1 . Against the opposite roller  14 , a transfer roller  17  is pressed via the photosensitive belt  10  by being biased by a compression spring  16 , which is a biasing mechanism, thereby forming a nipping portion in a transfer apparatus  18 . 
     Between the photosensitive belt  10  and the transfer roller  17 , a transfer material transport path is formed along which a transfer material  20  is transported through the nipping portion from right to left in the drawing. Upstream of the transfer material transport path, there is provided a pair of register rollers  22  and  23 , forming a registration apparatus  21 . As one of the register rollers,  22 , is driven by a drive motor M 2 , the other register roller,  23 , which is biased toward the register roller  22  by a compression spring  24  as a biasing mechanism, is driven, thereby transporting the transfer material  20 . Downstream of the pair of register rollers  22  and  23 , there is disposed a pair of guide plates  25  that guide the transfer material  20  to the nipping portion of the transfer apparatus  18 . Downstream of the nipping portion of the transfer apparatus  18 , there is disposed a fixing apparatus  26 . The fixing apparatus  26  is composed of a heating roller  27  having a heat source, and a pressure roller  28  biased by a compression spring  29  and pressed against the heating roller  27 . 
     In this photosensitive belt unit, the transfer material  20 , which is fed from a sheet feeder unit which is not shown, once abuts against the register rollers  22  and  23  where it stands by. As the drive roller  11  is driven by the drive motor M 1 , the photosensitive belt  10  runs and while it runs, an image is formed thereon by an image forming apparatus (not shown). At a time determined with reference to the front end of the image on the photosensitive belt  10 , the register roller  22  is started by the drive motor M 2 , whereby the transfer material  20  is fed into the nipping portion of the transfer apparatus  18 . A toner image carried on the photosensitive belt  10  is then transferred onto the transfer material  20  by the pressing force of the compression spring  16  and a transfer bias applied to the transfer roller  17 . Thereafter, heat and pressure are applied to the transfer material  20  by the heating roller  27  and the pressure roller  28  in the fixing apparatus  26 , whereby the toner image is fixed on the transfer material  20 . 
     The phenomenon of load torque variation in the photosensitive belt unit is described.  FIG. 2  shows a graph indicating a transport position error of the photosensitive belt  10  when the transfer material  20  passes between the opposite roller  14  and the transfer roller  17  pressed against the opposite roller  14  in the transfer apparatus  18 . The horizontal axis shows time; the transfer material  20  enters the nipping portion between the rollers  14  and  17  at time  0 . The vertical axis shows the position variation, indicating the error or shift of the photosensitive belt transport distance with respect to a desired transport distance (position). As indicated by the oval in the graph, it is seen that the position error of the photosensitive belt  10  shifts in the negative direction upon entry of the transfer material  20  into the nipping portion. This means that the transport of the photosensitive belt  10  is delayed by the load torque variation due to the entry of the transfer material  20 . Such a position variation greatly affects the image being formed on the photosensitive belt  10 . 
     A similar phenomenon also occurs in the fixing apparatus  26 . Namely, upon entry of the transfer material  20  into the nipping portion in the fixing apparatus  26 , the rotation speed of the heating roller  27  drops due to the load torque variation (i.e., sudden increase in load torque). Simultaneously, the amount of transport of the transfer material  20  in the nipping portion in the fixing apparatus  26  also decreases. In the image forming apparatus, the fixing apparatus  26  is provided immediately after the transfer apparatus  18 . Upon entry of the transfer material  20  into the fixing apparatus  26 , the transfer material  20  is nipped by both the fixing apparatus  26  and the transfer apparatus  18 . Thus, the decrease in the amount of transport of the transfer material  20  greatly affects the image being transferred by the transfer apparatus  18 . 
     A similar phenomenon also occurs in the registration apparatus  21 . Specifically, a load torque variation occurs such that the load torque sharply drops as the transfer material  20  exits a nipping portion between the register rollers  22  and  23 . As a result, the rotation speed of the register rollers  22  and  23  increases, and so does the transport speed of the transfer material  20 . When the transfer material  20  is nipped by both the transfer apparatus  18  and the registration apparatus  21 , the image transferred onto the transfer apparatus  18  is greatly affected. 
     A transfer material transport apparatus according to an embodiment of the invention as described below includes a first rotating body, such as the opposite roller  14 , the register roller  22 , or the heating roller  27 ; a second rotating body that is movable into and out of contact with the first rotating body, such as the transfer roller  17 , the register roller  23 , or the pressure roller  28 ; and a biasing unit configured to bias the second rotating body onto the first rotating body, such as the compression spring  16 ,  24 , or  29 . The transport apparatus may be used in any of the transfer apparatus  18 , the registration apparatus  21 , and the fixing apparatus  26  in which the transfer material  20  is transported between the first rotating body and the second rotating body. 
     Hereafter, the mechanism of the development of the load torque variation is described. An analysis conducted by the present inventors revealed that the drop in the speed of the photosensitive belt  10  upon entry of the transfer material  20  is mainly due to the increase in load torque that is caused as the transfer roller  17  is moved downward by the transfer material  20  as it enters the nipping portion formed by the transfer roller  17  and the opposite roller  14 . On the other hand, when the transfer material  20  exits the nipping portion, the speed of the photosensitive belt  10  increases. This is mainly due to the decrease in the load torque caused by the transfer material  20  leaving the nipping portion, which results in the transfer roller  17  moving upward due to the pressing force applied to it, thereby pushing the transfer material  20  in the transport direction. The details are described below. 
     Hereafter, a description is given of the mechanism of the development of the torque load variation upon entry of the transfer material  20  in the transfer apparatus  18  as a transfer material transport apparatus.  FIG. 3  is a diagrammatic illustration of the mechanism of the development of the torque load variation in the transfer apparatus  18 . The diagram depicts mechanical relationships among various forces upon entry of the transfer material  20  into the nipping portion between the opposite roller  14 , which is the first rotating body, and the transfer roller  17 , which is the second rotating body. While in the foregoing description of the image forming apparatus there has been the photosensitive belt  10  between the opposite roller  14  and the transfer roller  17 , the photosensitive belt  10  is omitted in  FIG. 3  because its influence on the load torque variation is small. Its influence is small because the photosensitive belt  10 , which is wound on the opposite roller  14  by the tension provided by the tension roller  13 , slips little, and therefore the photosensitive belt  10  can be considered to move together with and be a part of the opposite roller  14  in a model. Namely, in the present model, the thickness of the photosensitive belt  10  is reflected in the diameter of the opposite roller  14 . 
     Initially, the relationship between the nipping portion between the opposite roller  14  and the transfer roller  17  and the transfer material  20  is described. The opposite roller  14  is fixed both in the horizontal direction and vertical direction; it is movable only in the rotating direction. 
     In the apparatus shown, the rotating shaft of the opposite roller  14  is supported by a bearing which is not shown. The bearing may be fixed to a casing of the image forming apparatus, or a casing of the photosensitive belt transport unit. On the other hand, the transfer roller  17  is movable in the vertical direction and the rotating direction, without its rotating shaft fixed. With the rotating shaft of the transfer roller  17 , the compression spring  16 , which is a biasing mechanism, is in contact as shown in  FIG. 1 . Thus, a pressing force a acts in the direction of the opposite roller  14 , whereby the transfer roller  17  is pressed against the opposite roller  14 . The transfer material  20  has a thickness t and enters into the nipping portion horizontally. The transfer material  20  as it is being transported has a thrust b. In the figure, the transfer material  20  is transported in the horizontal direction and comes into contact not with either the opposite roller  14  or the transfer roller  17  first but with them both simultaneously in the nipping portion. 
     With reference to  FIG. 3 , the balance of forces upon contact of the transfer material  20  with the rollers  14  and  17  (in a static mechanical equilibrium state) is described. First, the balance of forces in the rotating direction of the opposite roller  14  is described. The opposite roller  14  has a rotating force (rotating torque) c in the clockwise direction. The rotating torque c is supplied by a drive source (motor), which is not shown. In practice, the opposite roller  14  rotates at a desired average speed, and produces a predetermined load torque. In the present example, the static model is employed excluding the predetermined load torque in order to describe the load torque variation upon entry of the transfer material  20  into the nipping portion. The balance of forces in the rotating direction of the opposite roller  14  is related to a load torque variation ΔT which is expressed by:
 
 ΔT=R   1 ( F   1   +F   3 )   (1)
 
where:
 
     R 1  is the radius of the opposite roller  14 ; 
     F 1  is a frictional force d at the contact portion of the opposite roller  14  and the transfer material  20  in the rotating direction; and 
     F 3  is a frictional force e at the contact portion of the transfer roller  17  in the rotating direction. 
     The balance of forces in the rotating direction of the transfer roller  17 , which is driven by the opposite roller  14 , is expressed by:
 
R 2 F 3 ′=R 2 F 2    (2)
 
where:
 
     R 2  is the radius of the transfer roller  17 ; 
     F 2  is a frictional force f at the contact portion between the transfer roller  17  and the transfer material  20  in the rotating direction; and 
     F 3 ′ is a frictional force g at the contact portion of the opposite roller  14  in the rotating direction. 
     The balance of forces of the transfer material  20  in the horizontal direction is described. Upon contact of the transfer material  20  with both the opposite roller  14  and the transfer roller  17 , the balance of forces is expressed by:
 
 N   1  sin θ 1   +N   2  sin θ 2   =F   1 ′ cos θ 1   +F   2 ′ cos θ 2    (3)
 
where:
 
     N 1  is a normal force h at the contact portion between the opposite roller  14  and the transfer material  20 ; 
     N 2  is a normal force i at the contact portion between the transfer roller  17  and the transfer material  20 ; 
     F 1 ′ is a frictional force j at the contact portion between the opposite roller  14  and the transfer material  20  in the rotating direction; 
     F 2 ′ is a frictional force k at the contact portion between the transfer roller  17  and the transfer material  20  in the rotating direction; 
     θ 1  is an angle formed by a contact surface m of the transfer material and a line connecting the rotation center of the opposite roller  14  and a point of contact between the surface m and the opposite roller  14 ; and 
     θ 2  is an angle formed by the contact surface m of the transfer material and a line connecting the rotation center of the transfer roller  17  and a point of contact between the surface m and the transfer roller  17 . 
     When the transfer material  20  enters the transfer nipping portion horizontally, the contact surface m is parallel to a line connecting the rotation centers of the both rollers. Therefore, angles θ 3  and θ 1  and angles θ 4  and θ 2  are the same. 
     Similarly, the balance of forces in the vertical direction of the transfer material  20  is expressed by:
 
 N   1  cos θ 1   +F   1 ′ sin θ 1   =N   2  cos θ 2   +F   2 ′ sin θ 2    (4)
 
     The balance of forces with respect to the rotating shaft of the transfer roller  17  is described. 
     In the vertical direction, the following equation (5) is satisfied:
 
 P=N   2 ′ cos θ 2   +F   2  sin θ 2   +N   3    (5)
 
where:
 
     N 2 ′ is a normal force n at the contact portion between the transfer roller  17  and the transfer material  20 ; 
     N 3  is a normal force o at the contact portion between the opposite roller  14  and the transfer roller  17 ; and 
     P is a pressing force p of the transfer roller  17 . 
     A torque ΔT required for the transfer roller  17  to move away from the opposite roller  14  is determined. When the transfer roller  17  and the opposite roller  14  are spaced apart from each other, the following are satisfied:
 
N 3 =0, F 3 =0   (6)
 
     By modifying and substituting Equations (1) through (6), relationships between the torque ΔT and the pressing force or the fixing force of the transfer roller  17  are determined. The torque ΔT can be divided into a torque ΔT v  for the transfer roller  17  to move in the vertical direction, and a torque ΔT h  for it to move in the horizontal direction, as expressed by: 
                     Δ   ⁢           ⁢     T   v       =         PR   1     ⁢     sin   ⁡     (       θ   1     +     θ   2       )           cos   ⁢           ⁢     θ   2                 (   7   )                 Δ   ⁢           ⁢     T   h       =         N   4     ⁢     R   1     ⁢     sin   ⁡     (       θ   1     +     θ   2       )           sin   ⁢           ⁢     θ   2                 (   8   )               
where N 4  is a pressing force (fixing force) of the transfer roller  17  in the horizontal direction.
 
     The sum of the vertical component (Equation (7)) and the horizontal component (Equation (8)) is the load torque variation ΔT required for the transfer material  20  to be transported through the transfer nipping portion while pressing down the transfer roller  17 . When the load torque variation exceeds the torque (permitted torque) that can be supplied to the opposite roller  14  from the motor driving it, a rotation variation develops in the opposite roller  14 , resulting in a linear velocity variation in the photosensitive belt  10  and causing image degradation. 
     While the phenomenon upon entry of the transfer material has been described above, a similar phenomenon also occurs upon exit of the transfer material  20  out of the transfer nipping portion. Specifically, the load torque decreases due to the upward force P of the transfer roller  17 . Namely, when the rotating direction of the opposite roller  14  and the transport direction of the transfer material  20  shown in  FIG. 3  are reversed, a load torque variation occurs in a direction such that the signs become minus (opposite) in Equations (7) and (8). Thus, the position variation of the photosensitive belt  10  takes place toward the plus side. 
     In the above model, the opposite roller  14 , the transfer roller  17 , and the transfer material  20  are considered to be rigid bodies; the qualitative tendencies, however, do not change in the case of elastic bodies. 
       FIG. 4  shows a graph indicating the load torque variation calculated by Equations (7) and (8). The vertical axis shows the load torque variation, and the horizontal axis shows the angles θ 1  and θ 2  indicating the position of the transfer material  20  relative to center of the transfer nip. For convenience&#39; sake, the opposite roller  14  and the transfer roller  17  are considered to have the same diameter so that the two angles are equal at all times. 
     The angles θ 1 , θ 2  are alternate angles with respect to the angles θ 3 , θ 4 , respectively, of  FIG. 3 . These angles decrease as the rollers rotate and the transfer material  20  moves toward the center of the nipping portion. The calculation results indicate that the greater the angles, the greater the load torque. Namely, the load torque variation is the greatest immediately after the entry of the transfer material  20  into the transfer nipping portion, and decreases as the transfer material  20  is transported through the transfer nipping portion. 
       FIGS. 5A and 5B  show graphs indicating the shift in the load torque variation upon entry of the transfer material  20 .  FIG. 5A  is the case of a thick sheet of transfer material, while  FIG. 5B  is the case of a thin sheet of transfer material. The horizontal axis of the graph shows time; time t 1  is when the transfer material  20  enters the transfer nipping portion. The vertical axis shows the shift in the load torque applied to the rotating shaft of the opposite roller  14  over time. Load torque T ave  is a steady load torque when the opposite roller  14  is rotating at a desired constant angular velocity. A torque variation acceptable range, as indicated by a pair of horizontal broken lines, is the acceptable range of variation of the torque applied to the opposite roller  14 . The torque variation acceptable range may be determined by the torque acceptable value of the drive motor and the rigidity of the drive force transmitting system (and possibly an acceptable range of image degradation). When the load torque exceeds this range, the motor rotation speed decreases, or a deformation occurs in the drive force transmitting system (including gears, gear fastening portions, and shaft couplings). As a result, the speed variation in the photosensitive belt  10  and the speed variation in the transfer material  20  increases to such an extent the image quality is greatly degraded. 
     In  FIG. 5A , a result of calculating the load torque variation based on Equations (7) and (8) is indicated by a solid line, while an actual measurement result is indicated by a dotted line. Their tendencies are substantially identical, suggesting the validity of the mechanism of the development of the load torque variation described above. The difference between the calculated and measured values is due to the deformation of the elastic members on the surfaces of the transfer roller  17  and the opposite roller  14 . Reduction in the load torque variation can be expected by coating the roller surface with an elastic member, or making the roller members elastic bodies. 
     The reason why the load torque varies depending on the thickness of the transfer material is discussed below. When the thickness of the transfer material  20  is large ( FIG. 5A ), the load torque variation greatly exceeds the acceptable range. On the other hand, when the thickness is small ( FIG. 5B ), the load torque variation is small and within the acceptable range. 
     The difference in the shift in load torque variation depending on the thickness is explained with reference to the above static model analysis. The position of the contact surface m of the transfer material  20  upon entry into the roller pair shown in  FIG. 3  varies depending on the thickness t of the transfer material  20 . When the thickness t is smaller, the contact surface m of the transfer material  20  upon entry into the nipping portion is located closer to the center of the transfer nip position. As the thickness t increases, the contact surface m of the transfer material  20  is positioned more and more away from the center of the nipping portion. When the contact surface m of the transfer material  20  is closer to the center of the transfer nipping portion, angles θ 1  and θ 2  are smaller; as the contact surface m is located farther, these angles also become greater. Thus, a difference is caused in the load torque variation value based on Equations (7) and (8) upon entry into the nipping portion. Further, as the thickness t of the transfer material increases, the area in which the load torque variation is caused is extended. This is because the distance between the position of entry of the transfer material  20  into the nipping portion and the center of the nipping portion increases as the thickness of the transfer material  20  increases. 
     In accordance with an embodiment of the invention, the distance between the axes of the rollers is increased so that the contact surface m of the transfer material is located closer to the center of the nipping portion upon entry thereto (thereby reducing the angles θ 1  and θ 2 ). 
     Based on the above analysis made with reference to  FIG. 5 , it is seen that a high quality image output can be realized by keeping the load torque variation value upon entry of the transfer material into the nipping portion within an acceptable value. With reference to the static model of  FIG. 3 , by maintaining a constant position of the contact surface m of the transfer material upon entry regardless of the transfer material thickness, the load torque variation upon entry into the nipping portion can be kept to a constant value. Furthermore, by positioning the contact surface m of the transfer material upon entry into the nipping portion such that the load torque variation stays within an acceptable range regardless of the transfer material thickness, a high quality image output can be realized. 
     In practice, however, it is difficult to monitor and control the contact surface m of the transfer material upon entry into the nipping portion. Particularly, when an elastic member is used in the rollers, because the elastic member deforms over time in varying degrees depending on its environment, it becomes difficult to accurately locate the contact surface m upon entry into the nipping portion. Thus, in accordance with an embodiment of the present invention, a mechanism is employed to regulate the axial position of the transfer roller  17 , which is movable in the vertical direction. By regulating the axial position of the transfer roller  17  and thus the center distance between the opposite roller  14  and the transfer roller  17 , the position of the contact surface m of the transfer material upon entry into the nipping portion can also be changed. Thus, by controlling the axial position of the transfer roller  17 , the center distance between the opposite roller  14  and the transfer roller  17  is controlled so that the load torque variation stays within an acceptable range. 
     Based on the above analysis, an image transfer apparatus according to a first embodiment is described. In the image transfer apparatus of the present embodiment: 
     (1) The axial position of the transfer roller  17  is regulated. 
     (2) The axial position of the transfer roller  17  is set in an effective manner depending on the thickness of the transfer material (thus forming a gap between the shafts of the rollers). 
     (3) The axial position (ratchet interval) is regulated so that the load torque variation stays within an acceptable range. 
     (4) The axial position (ratchet backlash) is regulated so that a sufficient pressing force can be given to the transfer material. 
     Each of these features of the present embodiment is described in the following. 
     (1) Axial Position Regulation 
       FIG. 6  shows an example of an axial position regulating unit for the transfer roller  17 . This is a mechanism for maintaining a roller center distance when the transfer material is in the nipping portion, as described below. The retaining mechanism employs a one-way clutch that is freely movable in the roller-lowering direction while it is locked in the roller-raising direction. The example shown in  FIG. 6  employs a ratchet-type one-way clutch. The transfer roller  17  has a rotating shaft  32  that is supported by a bearing member  35  that is movable in the vertical direction. The pressing force of the compression spring  16  is transmitted to the transfer roller  17  via the bearing member  35 . To the bearing member  35 , a first clutch plate  34  is fixed. Opposite the first clutch plate  34 , there is disposed a second clutch plate  37  that can be moved into and out of contact with the first clutch plate  34 . The second clutch plate  37  can be horizontally moved by a plate guide member  36  vertically fixed to a casing (not shown). The second clutch plate  37  is biased by a clutch biasing mechanism  38  toward the first clutch plate  34 . On the opposing surfaces of the first and the second clutch plates  34  and  37 , saw-toothed projections are formed such that, when the first and the second clutch plates  34  and  37  are engaged, they can be locked. Specifically, the clutch mechanism regulates (locks) the movement of the transfer roller  17  in the biasing direction of the compression spring  16  (i.e., the upward direction in  FIG. 6 ). 
     On the other hand, in the direction opposite to the biasing direction of the compression spring  16  (i.e., in the downward direction in  FIG. 6 ), the transfer roller  17  is freely movable. In order to cause the second clutch plate  37  to be separated from the first clutch plate  34 , a release mechanism using a pull-type solenoid is used. A solenoid frame  43 , which is fixed to the casing not shown, contains a coil  41 , a fixed core, and a guide pipe  42 . When the coil  41  is energized, a movable core  40  is drawn into the solenoid frame  43  (i.e., to the right in  FIG. 6 ). The movable core  40  as it is drawn causes the second clutch plate  37  to be moved toward the solenoid against the force of the clutch biasing member, via a plate shaft  39  fixed to the second clutch plate  37 . Thus, the first clutch plate  34  is unlocked. 
     The clutch biasing mechanism  38  may employ a plate spring or a rubber material, other than the compression spring. The clutch-plate release mechanism may include a magnetic material fixed to the second clutch plate  37  so that the second clutch plate  37  can be drawn directly by the solenoid. 
     (2) Formation of a Gap Between Roller Shafts 
     A method of setting an effective axial position of the transfer roller  17  depending on the thickness of the transfer material  20  is described.  FIGS. 7A ,  7 B, and  7 C show a series of operations for forming a gap between the roller shafts in the present embodiment.  FIG. 7A  shows an initial state of the rollers  14 ,  17  prior to the passage of the transfer material  20 . As shown, the opposite roller  14  and the transfer roller  17  are in contact with each other, with the first clutch plate  34  engaged with the second clutch plate  37  by the clutch biasing mechanism  38 . In this initial state, a sheet of the transfer material  20  is test-transported in order to form a required gap between the roller shafts.  FIG. 7B  shows the transfer material  20  passing the nipping portion between the rollers  14 ,  17 . As the transfer material  20  is test-transported, the transfer roller  17  is moved downward. At the same time, the first clutch plate  34  also moves down while engaged under pressure with the second clutch plate  37 . As a result, a gap corresponding to the thickness of the transfer material is formed between the roller shafts. After the rear-end of the transfer material  20  has passed the nipping portion, the axial position is maintained as shown in  FIG. 7C . Because the first and the second clutch plates  34 ,  37  are still engaged under pressure, the upward movement of the transfer roller  17  is prevented. Thus, the gap corresponding to the thickness of the transfer material  20  can be maintained. In practice, however, there is some play or backlash in the ratchet intervals of the clutch portion or between the second clutch plate  37  and the plate guide member  36 , resulting in a smaller center distance than as shown in  FIG. 7B . By transporting the subsequent sheets of the transfer material  20  in the state of  FIG. 7C , the load torque variation that is caused can be greatly reduced. 
     When the transport of a series of sheets of the transfer material  20  with the same thickness is completed and another series of sheets of the transfer material  20  with a different thickness is to be transported, the first clutch plate  34  and the second clutch plate  37  are separated in a clutch releasing operation, whereby the initial state of  FIG. 7A  is resumed and a test transport is conducted again. Preferably, the clutch releasing operation is performed when the final transfer sheet with the initial thickness is in the nipping portion between the rollers; i.e., in the state of  FIG. 7B . This is because it is easier to move the second clutch plate  37  horizontally in the state of  FIG. 7B  than that of  FIG. 7C , so that the clutch can be released more smoothly. When the clutch is released in the state of  FIG. 7B , a load torque variation is caused upon exit of the rear end of the final sheet out of the nipping portion. However, the problem of image degradation does not easily occur once the image forming operation is completed. 
     While it is assumed in the example of  FIGS. 7A to 7C  that the roller members are made of metal or the like and that there is no deformation in the roller surfaces, the same effect can be obtained with the same operation when the roller members are made of elastic material such as rubber, with surface deformation. Any deformation that may occur in the elastic member on the roller surface or in the pressure roller shaft occurs in the same amount in response to the same pressing force, whether in the initial state of  FIG. 7A  or the nipping state of  FIG. 7B . Thus, the amount of change in the roller center distance from  FIG. 7A  to  FIG. 7B  still corresponds to the thickness of the transfer material  20 . Therefore, the same effect can be obtained by maintaining a gap between the roller shafts that corresponds to the thickness of the transfer material  20 . 
     The test transport of the first sheet of the transfer material  20  is described below. In the present embodiment, the test transport is conducted in order to form a gap between the roller shafts. At the time of the initial test transport, the load torque reducing effect of the present embodiment is not obtained. Thus, preferably, some measure is taken in light of the configuration of the image forming apparatus. In the following, two examples of such measure are described. 
     The first measure is to avoid the formation of an image during the test transport. The sheet after the test transport may be recycled by the user, or it may be fed back to a re-feeding path for automatic both-side printing. A re-feeding operation may be carried out by reversing the direction of feeding. 
     The second measure is to transport the first sheet and perform image formation at slower speed. In this case, a stepping motor or a DC servo motor may be used as the drive motor, and the motor shaft or the opposite roller  14  is rotated at constant speed. By performing a test transport at reduced speed, the load torque also varies slowly, so that it becomes possible for the motor to supply a torque sufficient for the load torque variation, thereby reducing the influence of the load torque variation. 
     In the axial position regulating unit, the ratchet pitch, i.e., the interval of the peaks of any two projections on the first clutch plate  34  and the second clutch plate  37 , should be no greater than a distance corresponding to the permitted torque variation range; the smaller the ratchet pitch, the better. The distance corresponding to the permitted torque variation range may be determined by conducting a transfer material transport experiment. In one such experiment, when the variation in the load torque applied to the opposite roller  14  was measured by passing multiple sheets of transfer material with different thicknesses, the variation stayed within the acceptable range up to a thickness of 200 μm. Accordingly, the movement of the transfer roller in the vertical direction upon the entry and exit of the transfer material should be equal to 200 μm or smaller. Therefore, the ratchet pitch should be 200 μm or smaller. In accordance with the present embodiment, the ratchet pitch is set to be approximately 70 μm by taking into consideration the backlash in the ratchet portion, as will be described below. A ratchet pitch greater than 70 μm may result in exceeding the permitted torque variation. 
     Between the second clutch plate  37  and the plate guide member  36 , a play, or a backlash, is provided. The backlash may be provided between the first clutch plate  34  and the bearing member  35 . During the transport of the transfer material  20  as shown in  FIG. 7B , the second clutch plate  37  contacts the lower surface portion of the plate guide member  36  as the transfer roller  17  is pressed down. After the transport as shown in  FIG. 7C , the second clutch plate  37  is moved upward by the transfer roller  17  by an amount corresponding to the backlash, so that the second clutch plate  37  contacts the upper surface portion of the plate guide member  36 . Such a backlash is provided in order to provide a desired pressing force during the transport of the transfer material in the state of  FIG. 7B . If there is no such backlash, the second clutch plate  37  would contact the upper surface portion of the plate guide member  36  where a pressing force would be applied, so that the pressing force applied to the transfer material would be reduced. Ideally, a backlash of as little as 1 μm can give a desired pressing force to the transfer material. In practice, however, the backlash needs to be set by taking into consideration the following variation factors (1) to (4) between the roller shafts: 
     (1) Variations due to changes in the environment (such as temperature and humidity) caused by the continuous passage of a large number of sheets of the transfer material. 
     (2) Variations due to different amounts of deformation of the elastic member around the roller surfaces that are caused by hardness variations. 
     (3) Variations due to the eccentricity of the rollers. 
     (4) Variations due to thickness variation in plural sheets of the transfer material of the same type, or variation in the thickness of an individual sheet. 
       FIG. 8  shows a graph of a result of measuring the axial position of the transfer roller  17  when feeding a copy paper into the image transfer apparatus. In the graph, the horizontal axis shows time and the vertical axis shows the roller axial position, with the roller axial position in the initial state of  FIG. 7A  taken as a reference (0) position. A time band  352  in  FIG. 8  indicates the time interval in which the transfer material passed the nipping portion as shown in  FIG. 7B . In the time band  352 , the transfer roller  17  is pushed down by approximately 460 μm, as indicated by a variation interval  351 , which corresponds to the thickness of the transfer material. Further, as indicated by a variation interval  353 , there is an axial position variation of approximately 110 μm that is due to the aforementioned multiple factors of the inter-axial variations. The backlash amount is set to be a variation peak-to-peak (P-P) value by taking into consideration the above variation factors in order to apply a desired pressing force to the transfer material at all times. Specifically, in the present embodiment, the backlash amount is set to be the P-P value of the variation, which is approximately 120 μm when environmental changes as well as the experimental data are considered. The amount of backlash may be made adjustable in order to handle low-quality transfer material having a large thickness variation. 
     The total of the ratchet pitch and the backlash amount is the maximum amount of lowering of the transfer roller  17  from the retained state of  FIG. 7A  to the time at which the transfer material passes as shown in  FIG. 7B . Thus, the total of the ratchet pitch and the backlash amount is designed to be less than 200 μm, which is the load torque variation acceptable range. In the present embodiment, where the ratchet pitch width is set to 70 μm and the backlash amount is set to 120 μm, the maximum amount by which the transfer roller is pushed up in the shaft-position-retained state of  FIG. 7C  as the transfer material passes the nipping portion is 190 μm, thus staying within the acceptable range. For example, when the aforementioned copy paper is fed, the transfer roller is pushed down by the maximum amount of variation in  FIG. 8 , or about 510 μm. Thus, the first clutch plate and the second clutch plate are relatively displaced by an amount corresponding to the six teeth of the ratchet, so that the transfer roller is retained at the shaft position of about −490 μm. However, because there is the backlash of 120 μm, the actual roller shaft position is about −370 μm. In the state where such an axial position is maintained, when the transfer material is transported, the amount of pushing-up of the transfer roller  17  varies within the variation width  353  of  FIG. 8 , or between about 20 to 140 μm, thus sufficiently staying within the acceptable range. 
     Hereafter, a roller outer-shape retaining mechanism according to a second embodiment is described. Of the axial position variations shown in  FIG. 8 , the variation component that occurs at the period of about 0.36 second is mainly due to the eccentricity in the opposite roller  14  and the transfer roller  17 . In an experiment, because the two rollers had the same diameter and had an eccentricity of approximately 50 to 60 μm each, the both variations were superposed upon each other, creating a variation width of approximately 110 μm. In order to provide a constant pressing force to the opposite roller  14  when there is such a center distance variation, the backlash amount is set to be not less than the aforementioned variation width. If the backlash amount is greater, it would be necessary to reduce the ratchet pitch of the clutch plates, which would require stricter machining or forming conditions and result in an increase in manufacturing cost. Thus, in order to overcome this problem, a retaining mechanism for the transfer roller  17  is adopted that is not readily affected by the roller eccentricity. 
       FIG. 9  schematically shows the retaining mechanism according to the second embodiment. In the present embodiment, a bearing portion  51  is adopted for retaining the outer-shape portion of the transfer roller. To the bearing portion  51 , the first clutch plate  34  is fixed, thereby forming a one-way clutch. By adopting the mechanism for retaining the outer-shape portion of the transfer roller  17 , the influence of the eccentricity in the transfer roller  17  can be reduced. When the position variation of the bearing portion  51  was measured, the variation at the period of 0.36 second was reduced by half. Thus, the ratchet pitch can be increased. In the present embodiment, the rotating shaft  32  of the transfer roller  17  is placed on a bearing portion  52 , and the transfer roller  17  is pressed against the opposite roller  14  by a compression spring  16 . 
     In the retained state of  FIG. 7C , when the surfaces of the opposite roller  14  and the transfer roller  17  are separated, the transfer roller  17 , which does not have its own drive source, stops rotating. Thus, preferably, a driving force transmitting mechanism is provided to rotate the transfer roller  17 . Upon entry of the transfer material into the nipping portion, a torque is required to sharply increase the speed of the transfer roller  17  from a stop condition to the transfer material transport speed, thus providing a factor for the transfer material transport speed variation. Thus, the driving force transmitting mechanism is installed so that the transfer roller  17  can rotate at the same speed as the opposite roller  14  even when the surface of the transfer roller  17  is spaced apart from that of the opposite roller  14 . Specifically, gears may be fixed to the shafts of the opposite roller  14  and the transfer roller  17  such that the rotation of the opposite roller  14  can be transmitted to the transfer roller  17 . The tooth height of the gears may be designed so that the gears can engage with each other when the both rollers are separated. Alternatively, a separate drive source may be provided for the transfer roller  17  and driven so that the transfer roller  17  can rotate at the same speed as the opposite roller  14 . 
     Alternatively to the ratchet-type, the one-way clutch may be of the roller type.  FIG. 10  schematically shows a roller-type one-way clutch. As a slide plate  53  moves in the upper direction, rollers  81  disposed in a housing  54  are wedged between the housing  54  and the slide plate  53 , thereby regulating the movement of the slide plate  53  in the upper direction. The regulation may be released by inserting a tapered clutch release nail  82  into the roller-wedged portion. By adopting such a roller-type clutch, it becomes possible to retain the position of the transfer roller in a stepless manner. The amount of wedging movement may be controlled by a backlash amount. In another example, the clutch mechanism may employ an electromagnetic mechanism that is well known. 
     While the foregoing description has been mainly concerned with the actions involved upon entry of the transfer material in the present embodiment, the same effects can be obtained with regard to the torque variation upon exit of the transfer material. 
     In the following, a third embodiment of the present invention is described. In the foregoing embodiment, the clutch mechanism is mounted on the bearing member for the rotating shaft or the outer-shape portion of the transfer roller  17 . When the acceptable amount of movement of the transfer roller based on the load torque acceptable variation range is 1 mm or smaller, it is necessary to set the ratchet pitch on the order of several tens to several hundreds of μm. It also becomes necessary to ensure sufficient rigidity for retaining the transfer roller against the pressing force. Thus, an arm member is adopted in order to increase the ratchet pitch and to reduce the rigidity required of the clutch. 
       FIG. 11  schematically shows an image transfer apparatus according to the third embodiment. An arm member  55  moves about a rotating shaft  56  and includes a bearing portion for supporting a rotating shaft  32  or an outer diameter portion of the transfer roller  17 . The arm member  55  also transmits the pressing force of the compression spring  16  to the transfer roller  17 . At the end of the arm member  55 , the first clutch plate  34  is disposed, thereby forming a one-way clutch. By thus employing an arm member, the amount of vertical movement of the transfer roller  17  and the amount of movement of the first clutch plate  34  increase in accordance with the ratio of the distance between the rotating shaft  32  of the transfer roller  17  and the rotating shaft  56  to the distance between the rotating shaft  56  of the arm member  55  and the first clutch plate  34 . The rigidity for retaining the position of the transfer roller  17  can also be reduced. Thus, a less expensive clutch mechanism can be adopted. 
     In the image transfer apparatus according to the third embodiment, a retaining operation similar to those of the first and the second embodiments is performed. Instead of the ratchet-type one-way clutch shown, a roller-type one-way clutch may be employed. Further, a rotary clutch may be adopted at the rotating shaft  56  of the arm member  55 . 
     Image Forming Apparatus 
     Hereafter, a description is given of an image forming apparatus in which a transfer apparatus and a fixing apparatus are used, with reference to the drawings. The image forming apparatus may be employed in a copy machine or a printer comprising a main component portion and a paper feed table retaining a large number of sheets, in which a scanner may be installed on the main component portion, or an automatic document feeder (ADF) may be further installed thereon.  FIG. 12  schematically shows the image forming apparatus. The image forming apparatus is a tandem-type electrophotographic apparatus using an intermediate transfer system. 
     The image forming apparatus includes an intermediate transfer belt  60  consisting of an endless belt which is an image carrier and an intermediate transfer material. The intermediate transfer belt  60  is extended across four supporting rotating bodies, i.e., support rollers  63 ,  78 ,  75 , and  76 , and is rotated in the anti-clockwise direction in the drawing. An intermediate transfer belt cleaning apparatus, not shown, is provided to the left of the support roller  63  for removing toner that remains on the intermediate transfer belt  60  after image transfer. Between the support rollers  75  and  76  along the belt movement direction, there is disposed a tandem image forming portion consisting of image forming units for the colors of yellow (Y), cyan (c), magenta (M), and black (K). Each of the image forming units includes an image carrier drum  62  that rotates in the clockwise direction, and a bias roller  68 . Around each of the image carrier drums  62 , which are photosensitive, there are disposed various apparatuses including a charging apparatus, a developing apparatus, and a cleaning apparatus, which are not shown. The individual image forming units are similarly configured, with the bias roller  68  disposed opposite the image carrier drum  62  across the intermediate transfer belt  60 . In the present embodiment, the support roller  78  is the drive roller. Under the tandem image forming portion, there is disposed an exposing apparatus  61  as a latent image forming unit. 
     Opposite the support roller  78  across the intermediate transfer belt  60 , there is disposed a secondary transfer roller  79  as a secondary transfer unit. The secondary transfer roller  79  is pressed against the support roller  78  via the intermediate transfer belt  60 , so that an image on the intermediate transfer belt  60  can be transferred onto a recording material sheet  35 . A load torque variation control mechanism including a nipping mechanism according to an embodiment of the invention is disposed at the rotating shaft of the secondary transfer roller  79 . Thus, the load torque variation at the secondary transfer apparatus nipping portion during sheet transport can be reduced, whereby the speed variation in the intermediate transfer belt  60  and the sheet transport speed variation can be reduced. Above the secondary transfer roller  79 , there is disposed a fixing apparatus  77  for fixing the image transferred onto the sheet  35 . A load torque variation control mechanism according to an embodiment that includes a guide slope member is also disposed at the rotating shaft of the roller for providing a pressing force in the fixing apparatus. Thus, the load torque variation caused as the sheet is transported to the nipping portion of the fixing apparatus  77  can be reduced, and also the transport speed variation upon entry of the sheet  35  into the nipping portion of the fixing apparatus  77  is reduced. Thus, the influence on the image during the secondary transfer upon entry of the sheet onto the fixing apparatus  77  in the transfer apparatus is reduced. The aforementioned support roller  78  also has a sheet transport function for transporting the sheet after image transfer to the fixing apparatus  77  by contacting and driving the secondary transfer roller  79 . It goes without saying that a secondary transfer apparatus consisting of a transfer belt or a contactless charger may be disposed. The transfer sheet  35  is inserted from below into the nipping portion between the support roller  78  and the transfer roller  79 , and a toner image is transferred onto the transfer sheet. 
     When making a copy using the above image forming apparatus, a manuscript may be placed on a manuscript holder of an ADF. Alternatively, the manuscript may be placed on a contact glass of a scanner by opening the ADF, and then the ADF may be closed to press down on the manuscript. Thereafter, as a start switch (not shown) is depressed, the manuscript, when placed on the ADF, is transported to the upper surface of the contact glass. On the other hand, when the manuscript is placed on the contact glass, the scanner is immediately operated. During the operation of the scanner, a light source emits light onto the manuscript, and the light reflected by the manuscript surface is further reflected and then passed through an imaging lens, and finally the content of the manuscript is read by a pickup sensor. Alternatively, digital image information may be received from a personal computer or a digital camera, for example. 
     In parallel to the reading of the manuscript or the reception of image information, the support roller  78  is rotated by a drive motor, not shown, as a drive source. Thus, the intermediate transfer belt  60  is rotated in the anti-clockwise direction in the drawing, whereby the remaining support rollers (driven rollers) are driven. Simultaneously, the photosensitive drum  62  as a latent image carrier in each of the image forming units is rotated and exposed, and the image thereon is developed in accordance with individual color information for yellow, cyan, magenta, and black, whereby a toner image (developed image) of an individual color is formed. The toner image on each of the photosensitive drums  62  is then transferred onto the intermediate transfer belt  60  successively in an overlapping manner, whereby a composed color image is formed on the intermediate transfer belt  60 . 
     In parallel to the above image formation, the sheet  35  is transported to the secondary transfer portion. Specifically, one of the paper feeding tables is selected, and sheets are picked one after another out of one of the feeding cassettes provided in multiple stacks in a paper bank. The sheets are separately fed into the feeding path by a separating roller, and then transported and guided by a transport roller onto the feeding path until the sheet abuts against register rollers. Alternatively, a paper-feed roller may be rotated to slide out sheets on a manual feed tray, and the sheets are separated by the separating roller one by one and fed onto a manual feeding path until the sheet abuts against the register rollers. The register rollers are rotated at a timing determined with reference to the composed color image on the intermediate transfer belt  60  in order to feed the sheet into the gap between the intermediate transfer belt  60  and the secondary transfer roller  79 , whereby the color image is transferred by the secondary transfer roller  79  onto the sheet. The sheet after image transfer is transported by the secondary transfer roller  79  and the opposite roller into the fixing apparatus  77 . In the fixing apparatus  77 , the transferred image is fixed by heat and pressure, and then ejected by an ejection roller and stacked on an ejected paper tray. 
     The intermediate transfer belt  60  after image transfer has the toner remaining thereon removed by the intermediate transfer belt cleaning apparatus (not shown) in order to prepare for the subsequent sequence of image formation in the tandem image forming portion. While the register rollers are generally grounded, a bias may be applied to them in order to remove the powdered paper material of the sheet. 
     It is also possible to make a black-and-white copy using the image forming apparatus. In this case, the intermediate transfer belt  60  is separated from the photosensitive drums  62  for the colors of yellow, cyan, and magenta by a unit which is not shown, and the photosensitive drums  62  for these three colors are temporarily deactivated. Image formation and transfer are carried out by bringing only the photosensitive drum  62  for black into contact with the intermediate transfer belt  60 . 
     Apart from the above-described tandem-type electrophotographic system using the intermediate transfer belt, the image forming apparatus may also be adapted to an electrophotographic system in which an intermediate transfer drum is used.  FIG. 13  shows an image forming apparatus according to another embodiment. In the present embodiment, a drive roller  118  has the function of an intermediate transfer drum. Specifically, the drive roller  118  retains and transports an image formed on a photosensitive drum  102 , and transfers the image onto a transfer material  35  at a nipping portion between the drive roller  118  and a secondary transfer roller  119 . A load torque variation control mechanism including an inter-roller nipping mechanism according to an embodiment of the invention is disposed at the rotating shaft of the secondary transfer roller  119 . Thus, the load torque variation at the secondary transfer nipping portion during sheet transport can be reduced, and also the development of the speed variation in the intermediate transfer drum and the sheet transport speed variation can be reduced. 
     As described above, in accordance with an embodiment of the present invention, a certain distance is maintained between two rollers during the transport of a transfer material. As a result, the amount by which the pressure roller is depressed upon entry of the transfer material can be greatly reduced, and the load torque variation in the transfer material transport drive system can be reduced. Thus, the transfer material transport speed variation caused by the load torque variation can be reduced. 
     In accordance with another embodiment of the present invention, a one-way clutch mechanism is adopted whereby a certain distance between the two rollers can be easily maintained by merely engaging clutch plates during the transport of the transfer material. Further, by setting a backlash such that the clutch plates can be moved freely by a predetermined distance, a desired pressing force can be applied to the transfer material at all times even when the distance between the two rollers varies due to roller eccentricity or the like. 
     In accordance with another embodiment of the present invention, a second rotating body is supported via its outer diameter portion rather than its rotating shaft, whereby a certain distance can be maintained between the two rollers without being affected by the eccentricity in the second rotating body. 
     In accordance with another embodiment of the present invention, an arm member is used whereby, based on the principle of leverage, the retaining torque applied to the clutch mechanism can be reduced compared with the case where the clutch mechanism is disposed on the rotating shaft of the second rotating body. Furthermore, because the amount of movement upon passage of the transfer material can be increased, it becomes possible to adopt an inexpensive clutch mechanism with greater ratchet intervals. 
     In accordance with another embodiment of the present invention, the two rollers can keep rotating at a transfer material transport speed even when a gap is produced between them by the retaining mechanism. Thus, no torque is required for accelerating one of the rollers that may be stopped or decelerating upon entry of the transfer material into the nipping portion. Thus, the transfer material can be transported stably. 
     In accordance with another embodiment of the present invention, the transport speed for the initial sheet of the transfer material is reduced, whereby the transfer material transport speed variation upon entry of the transfer material can be reduced by the feedback effect of the motor that is controlled to rotate at a constant speed. 
     In accordance with another embodiment of the present invention, the initial sheet of the transfer material is test-transported, whereby a certain retained state can be realized. During the test transport, no image formation is carried out and the sheet is fed back to the image forming portion, so that the transfer material is not wasted. 
     In accordance with another embodiment of the present invention, the initial sheet of the transfer material is test-transported in order to realize a certain retained condition. During the test transport, no image formation is performed, and the sheet is once transported to the nipping portion and then transported back and fed while image formation is performed. Thus, the transfer material is not wasted. 
     In accordance with another embodiment of the present invention, when transporting a sheet of the transfer material with a different thickness, the retention of distance between the rollers is released upon transport of a final sheet with the initial thickness when the torque applied to the retaining mechanism is small. Thus, the releasing operation can be conducted without damaging the clutch mechanism. 
     In accordance with another embodiment of the present invention, the load torque variation upon entry of the sheet into the fixing nipping portion is reduced, whereby a high-quality transferred image can be obtained without causing a rotation speed variation in the fixing roller. 
     Although this invention has been described in detail with reference to certain embodiments, variations and modifications exist within the scope and spirit of the invention as described and defined in the following claims. 
     The present application is based on the Japanese Priority Application No. 2007-223895 filed Aug. 30, 2007, the entire contents of which are hereby incorporated by reference.