Abstract:
There is provided an image forming apparatus including: a photoconductor; an intermediate transfer belt that is wrapped around and stretched between the photoconductor and a plurality of support rolls and is driven by the photoconductor; and a contact member that contacts the intermediate transfer belt, wherein the image forming apparatus satisfies the following expression (1)
 
2 T (e μθ −1)/(e μθ +1)&gt; F  and  T ≦396.9  N   (1),
wherein μ represents the coefficient of friction between the photoconductor and the intermediate transfer belt, T represents the initial tension of the intermediate transfer belt, θ represents the wrap angle of the intermediate transfer belt on the photoconductor, and F represents the load in a tangential direction applied from the contact member to the photoconductor via the intermediate transfer belt.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority under 35 USC 119 from Japanese Patent Application No. 2005-348235, the disclosure of which is incorporated by reference herein.  
       BACKGROUND  
       [0002]     1. Technical Field  
         [0003]     The present invention relates to an image forming apparatus disposed with an intermediate transfer belt that is wrapped around and stretched between plural support rolls and a photoconductor and is driven by the photoconductor.  
         [0004]     2. Related Art  
         [0005]     In 4-cycle full-color laser printers that use a single photoconductor to form a full-color toner image comprising yellow (Y), magenta (M), cyan (C), and black (K) on an intermediate transfer belt, the intermediate transfer belt is rotated to complete four laps, and the toner images are superposed one color at a time on the intermediate transfer belt each time the intermediate transfer belt completes one lap. At this time, in order to mutually align the plural toner images to be superposed on the intermediate transfer belt with high precision so as to suppress color shift, it becomes crucial to suppress fluctuations in the relative speed of the intermediate transfer belt with respect to the photoconductor drum and to suppress unevenness in pitch of the plural toner images to be superposed on the intermediate transfer belt, and various measures have been devised thus far.  
       SUMMARY OF THE INVENTION  
       [0006]     According to an aspect of the invention, there is provided an image forming apparatus including: a photoconductor; an intermediate transfer belt that is wrapped around and stretched between the photoconductor and a plurality of support rolls and is driven by the photoconductor; and a contact member that contacts the intermediate transfer belt, wherein the image forming apparatus satisfies the following expression (1)
 
2 T (e μθ −1)/(e μθ +1)&gt; F  and  T ≦396.9  N   (1),
 
         [0007]     wherein μ represents the coefficient of friction between the photoconductor and the intermediate transfer belt, T represents the initial tension of the intermediate transfer belt, θ represents the wrap angle of the intermediate transfer belt on the photoconductor, and F represents the load in a tangential direction applied from the contact member to the photoconductor via the intermediate transfer belt. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     An exemplary preferred embodiment of the present invention will be described in detail based on the following figures, wherein:  
         [0009]      FIG. 1  is a schematic sectional view showing an image forming apparatus pertaining to the exemplary embodiment of the present invention;  
         [0010]      FIG. 2  is a sectional view showing the stretched state of an intermediate transfer belt in the image forming apparatus pertaining to the exemplary embodiment of the present invention;  
         [0011]      FIG. 3  is a chart showing the material, dimensions, and strength of a photoconductor drum in the image forming apparatus pertaining to the exemplary embodiment of the present invention;  
         [0012]      FIG. 4  is a chart showing the material, dimensions, and strength of the intermediate transfer belt in the image forming apparatus pertaining to the exemplary embodiment of the present invention;  
         [0013]      FIG. 5  is a graph showing the relationship (under normal temperature and normal humidity) between the stretched time and the deterioration over time of the initial tension of the intermediate transfer belt in the image forming apparatus pertaining to the exemplary embodiment of the present invention;  
         [0014]      FIG. 6  is a graph showing the relationship (under high temperature and high humidity) between the stretched time and the deterioration over time of the initial tension of the intermediate transfer belt in the image forming apparatus pertaining to the exemplary embodiment of the present invention;  
         [0015]      FIG. 7  is a graph showing the relationship (under low temperature and low humidity) between the stretched time and the deterioration over time of the initial tension of the intermediate transfer belt in the image forming apparatus pertaining to the exemplary embodiment of the present invention;  
         [0016]      FIG. 8  is a graph showing the relationship between the initial elongation and temporal changes in the initial tension of the intermediate transfer belt in the image forming apparatus pertaining to the exemplary embodiment of the present invention;  
         [0017]      FIG. 9  is a graph showing measurement results of the coefficient of friction between the intermediate transfer belt and the photoconductor drum in the image forming apparatus pertaining to the exemplary embodiment of the present invention;  
         [0018]      FIG. 10  is a chart showing conditions for testing the necessary initial tension and the necessary initial elongation of the intermediate transfer belt in the image forming apparatus pertaining to the exemplary embodiment of the present invention;  
         [0019]      FIG. 11  is a chart showing conditions for testing the necessary initial tension and the necessary initial elongation of the intermediate transfer belt in the image forming apparatus pertaining to the exemplary embodiment of the present invention;  
         [0020]      FIG. 12  is a schematic diagram for describing the shift in toner images that have been primarily transferred onto the intermediate transfer belt;  
         [0021]      FIG. 13A  is a graph showing the amount of positional shift of the toner images in a sub-scanning direction when the circumference of the intermediate transfer belt at the time of being stretched is an integral multiple of the circumference of the photoconductor drum; and  
         [0022]      FIG. 13B  is a graph showing the amount of positional shift of the toner images in the sub-scanning direction when the circumference of the intermediate transfer belt is a non-integral multiple of the circumference of the photoconductor drum.  
     
    
     DETAILED DESCRIPTION  
       [0023]     Next, an exemplary embodiment of the present invention will be described on the basis of the drawings.  
         [0024]      FIG. 1  shows an image forming apparatus  10  pertaining to the exemplary embodiment of the present invention. The image forming apparatus  10  includes an image forming apparatus body  12 . An open/close cover  16  that is pivotable around a pivot support point  14  is disposed on an upper portion of the image forming apparatus body  12 . A paper supply unit  18  of one level, for example, is disposed in a lower portion of the image forming apparatus body  12 .  
         [0025]     The paper supply unit  18  includes a paper supply unit body  20  and a paper supply cassette  22  in which paper P is accommodated. A feed roll  24 , which feeds the paper P from the paper supply cassette  22 , and a retard roll  26 , which sorts the fed paper P one sheet at a time, are disposed in the upper vicinity of the deep end of the paper supply cassette  22 .  
         [0026]     A conveyance path  28  is a paper path from the feed roll  24  to a discharge port  30 . The conveyance path  28  is formed in the vicinity of the back side (right side in  FIG. 1 ) of the image forming apparatus body  12  substantially vertically from the paper supply unit  18  to a later-described fixing device  90 . A later-described secondary transfer roll  80  and a later-described secondary transfer backup roll  72  are disposed upstream of the fixing device  90  on the conveyance path  28 . A registration roll  32  is disposed upstream of the secondary transfer roll  80  and the secondary transfer backup roll  72 . Further, a discharge roll  34  is disposed in the vicinity of the discharge port  30  of the conveyance path  28 .  
         [0027]     The paper P is fed by the feed roll  24  from the paper supply cassette  22  of the paper supply unit  18  and sorted by the retard roll  26  such that just the uppermost sheet of paper P is guided to the conveyance path  28 . The paper P is temporarily stopped by the registration roll  32  and then passed at a timing between the later-described secondary transfer roll  80  and secondary transfer backup roll  72 , where a toner image is transferred to the paper P. Then, the fixing device  90  fixes the transferred toner image to the paper P, and the paper P is discharged by the discharge roll  34  from the discharge port  30  to a discharge unit  36  disposed in the upper portion of the open/close cover  16 . The portion of the discharge unit  36  near the discharge port  30  is low, and the discharge unit  36  slants such that it gradually becomes higher toward the front direction (left direction in  FIG. 1 ).  
         [0028]     A rotary developing device  38  is disposed in the substantially central portion, for example, of the image forming apparatus body  12 . The rotary developing device  38  includes a developer body  40  disposed with developers  42 Y to  42 K that respectively form toner images of the four colors of yellow (Y), magenta (M), cyan (C), and black (K), and the rotary developing device  38  rotates leftward (counter-clockwise in  FIG. 1 ) around a rotary developing device center  44 . The developers  42 Y to  42 K respectively include developing rolls  46 Y to  46 K and are pressed in the normal line direction of the developer body  40  by elastic bodies  48   a  to  48   d  such as coil springs, for example.  
         [0029]     A photoconductor drum  50  that rotates around a rotational support shaft  49 , for example, contacts the rotary developing device  38 , and the developing rolls  46 Y to  46 K are disposed such that part of their outer peripheries protrude (e.g., 2 mm) in the radial direction from the outer periphery of the developer body  40  in a state where they are not contacting the photoconductor drum  50 . Further, tracking rolls (not shown) that have diameters slightly larger than the diameters of the developing rolls  46 Y to  46 K are disposed at both ends of each of the developing rolls  46 Y to  46 K such that the tracking rolls rotate coaxially with the developing rolls  46 Y to  46 K. In other words, the developing rolls  46 Y to  46 K of the developers  42 Y to  42 K center around the rotary developing device center  44  and are disposed at the outer periphery of the developer body  40  at 90° intervals, the tracking rolls of the developing rolls  46 Y to  46 K contact flanges (not shown) disposed on both ends of the photoconductor drum  50  to form predetermined gaps between the developing rolls  46 Y to  46 K and the photoconductor drum  50 , and the developing rolls  46 Y to  46 K develop the latent image on the photoconductor drum  50  with toners of the respective colors.  
         [0030]     A charge device  52  including a charge roll, for example, that uniformly charges the photoconductor drum  50  is disposed below the photoconductor drum  50 . Further, an exposure device  60  that uses a light beam such as a laser beam to write the latent image onto the photoconductor drum  50  charged by the charge device  52  is disposed at the rear side of, and below, the rotary developing device  38 . Further, an intermediate transfer device  62 , which primarily transfers the toner images visualized by the rotary developing device  38  at a primary transfer position and conveys the toner images to a later-described secondary transfer position, is disposed above the rotary developing device  38 .  
         [0031]     The intermediate transfer device  62  includes an intermediate transfer belt  64 , a primary transfer roll  66 , a wrap-in roll  68 , a wrap-out roll  70 , a secondary transfer backup roll  72 , a brush backup roll  74 , and tension rolls  75  and  76 .  
         [0032]     The intermediate transfer belt  64  has elasticity and is stretched substantially flatly above the rotary developing device  38 . The edge of the upper surface side of the intermediate transfer belt  64  is stretched such that the edge is substantially parallel to the discharge unit  36  disposed in the upper portion of the image forming apparatus body  12 , for example. Further, the intermediate transfer belt  64  includes a primary transfer portion (photoconductor drum wrap region) that contacts the photoconductor drum  50  in a wrapped manner between the wrap-in roll  68  disposed upstream of the primary transfer roll  66  under the intermediate transfer belt  64  and the wrap-out roll  70  disposed downstream of the primary transfer roll  66 , and the intermediate transfer belt  64  is wrapped around a predetermined range of the photoconductor drum  50  so as to be driven by the rotation of the photoconductor drum  50 . For this reason, a dedicated drive source for causing the intermediate transfer belt  64  to be rotatingly driven becomes unnecessary, so that costs can be reduced.  
         [0033]     In this manner, the toner images on the photoconductor drum  50  are superposed and primarily transferred by the primary transfer roll  66  to the intermediate transfer belt  64  in the order of yellow (Y), magenta (M), cyan (C), and black (K), for example, and the intermediate transfer belt  64  conveys the primarily transferred toner images to the later-described secondary transfer roll  80 . It will be noted that the wrap-in roll  68  and the wrap-out roll  70  are separated from the photoconductor drum  50 .  
         [0034]     Further, the intermediate transfer belt  64  is stretched by the six rolls including the wrap-in roll  68 , the wrap-out roll  70 , the secondary transfer backup roll  72 , the brush backup roll  74 , and the tension rolls  75  and  76 , and the toner images on the photoconductor drum  50  are transferred to the intermediate transfer belt  64  by the primary transfer roll  66 .  
         [0035]     Moreover, a planar portion is formed at the back side (surface at the right side in  FIG. 1 ) of the intermediate transfer belt  64  by the tension roll  75  and the secondary transfer backup roll  72 . This planar portion is configured such that it serves as a secondary transfer portion and merges with the conveyance path  28 .  
         [0036]     The brush backup roll  74  assists a brush roll  86  in scraping off waste toner remaining on the intermediate transfer belt  64  after secondary transfer.  
         [0037]     Sensors  78  and  79 , such as reflective photosensors, for example, are disposed below the intermediate transfer belt  64 . The sensor  78  detects the density of the toner by reading the patches of the toner formed on the intermediate transfer belt  64 . Further, the sensor  79  detects the position of a belt position detection mark TRO formed on the intermediate transfer belt  64 .  
         [0038]     The secondary transfer roll  80  faces the secondary transfer backup roll  72  of the intermediate transfer device  62  with the conveyance path  28  sandwiched therebetween. In other words, the space between the secondary transfer roll  80  and the secondary transfer backup roll  72  serves as a secondary transfer position in the secondary transfer portion. The secondary transfer roll  80  is assisted by the secondary transfer backup roll  72  in secondarily transferring the toner images primarily transferred to the intermediate transfer belt  64  to the paper P at the secondary transfer position.  
         [0039]     Here, the secondary transfer roll  80  is configured to separate from the intermediate transfer belt  64  during a period of time when the intermediate transfer belt  64  completes three rotations—that is, during the period of time when the intermediate transfer belt  64  conveys the toner images of the three colors of yellow (Y), magenta (M), and cyan (C)—and to contact the intermediate transfer belt  64  after the toner image of black (K)has been transferred.  
         [0040]     It will be noted that the space between the secondary transfer roll  80  and the secondary transfer backup roll  72  is configured such that a predetermined electric potential difference arises, and when the secondary transfer roll  80  is given a high voltage, for example, the secondary transfer backup roll  72  is ground (GND).  
         [0041]     An intermediate transfer belt cleaner  82  is disposed in the vicinity of the intermediate transfer belt  64 . The intermediate transfer belt cleaner  82  includes a scraper  84 , the brush roll  86 , and a toner recovery bottle  88 , and swings around a rotational support shaft. The brush roll  86  scrapes off the waste toner on the intermediate transfer belt  64 . The scraper  84  scrapes and cleans off the waste toner adhering to the brush roll  86 . The toner recovery bottle  88  recovers the toner scraped off by the scraper  84 . The scraper  84  is composed of a thin plate of stainless steel, for example.  
         [0042]     The brush roll  86  is composed of a brush made of acrylic or the like that has been treated to make it electrically conductive, for example. Additionally, the brush roll  86  is configured to separate from the intermediate transfer belt  64  during the period of time when the intermediate transfer belt  64  conveys the toner images and to contact the intermediate transfer belt  64  at a predetermined timing.  
         [0043]     The fixing device  90  is disposed above the secondary transfer position. The fixing device  90  includes a heat roller  92  and a pressure roller  94 , causes the toner images secondarily transferred to the paper P by the secondary transfer roll  80  and the secondary backup roll  72  to be fixed to the paper P, and conveys the paper P to the discharge roll  34 .  
         [0044]     Incidentally, it is necessary for the plural toner images to be superposed on the intermediate transfer belt  64  to be mutually aligned with high precision, and for this reason, conditions such as the tension of the intermediate transfer belt  64  must be set such that the intermediate transfer belt  64  which is driven by the photoconductor drum  50  does not slip with respect to the photoconductor drum  50 . This point will be described below.  
         [0045]     As shown in  FIG. 2 , the secondary transfer roll  80  and the brush roll  86  contact the intermediate transfer belt  64 , and loads f 1  and f 2  from the secondary transfer roll  80  and the brush roll  86  to the photoconductor drum  50  via the intermediate transfer belt  64  are respectively applied to the intermediate transfer belt  64  in the tangential directions of the photoconductor drum  50 .  
         [0046]     Further, the intermediate transfer belt  64  is wrapped around the photoconductor drum  50  at a wrap angle θ and is driven to rotate the photoconductor drum  50  due to the frictional force with the photoconductor drum  50 . The coefficient of friction between the intermediate transfer belt  64  and the photoconductor drum  50  when the intermediate transfer belt  64  is driven to rotate the photoconductor drum  50  will be represented by μ. Further, an initial tension T is imparted to the intermediate transfer belt  64  at the point in time prior to rotation in order for the intermediate transfer belt  64  to be caused by the frictional force with the photoconductor drum  50  to rotate following the photoconductor drum  50 . Further, a load F (=f 1 + 2 ) in the tangential direction of the photoconductor drum  50  acts on the photoconductor drum  50  from the secondary transfer roll  80  and the brush roll  86  via the intermediate transfer belt  64  when the intermediate transfer belt  64  is driven to rotate the photoconductor drum  50 .  
         [0047]     Here, in order to ensure that the intermediate transfer belt  64  does not slip with respect to the photoconductor drum  50 , the wrap angle θ, the coefficient of friction μ, the initial tension T, and the load F are set such that they satisfy the following expression ( 1 ) based on Euler&#39;s belt theory.
 
2 T (e μθ −1)/(e μθ +1)&gt; F   (1)
 
         [0048]     Here, the initial tension T is set in consideration of the deflection of the photoconductor drum  50 . About 0.1 mm is reasonable for the allowable value of the deflection of the photoconductor drum  50  in consideration of the allowable amount of deformation in the sub-scanning direction in a straight line in the main scanning direction usually called BOW, and because the material, dimensions, and strength of the photoconductor drum  50  used in the present exemplary embodiment are as shown in the chart of  FIG. 3 , the initial tension T is set to 4.13 N or less.  
         [0049]     Further, an electrostatic suction force N arises between the photoconductor drum  50  and the intermediate transfer belt  64  due to applied voltage at the time the toner images are primarily transferred from the photoconductor drum  50  to the intermediate transfer belt  64 . Because this electrostatic suction force N also contributes to suppressing slippage of the intermediate transfer belt  64  with respect to the photoconductor drum  50 , the wrap angle θ, the coefficient of friction μ, the initial tension T, the load F, and the electrostatic suction force N may also be set such that they satisfy the following expression ( 2 ). In this case, the applicable range of the initial tension T can be widened in comparison to the above expression (1).
 
2 T (e μθ −1)/(e μθ +1)+ N (e μθ −1)θ&gt; F   (2)
 
         [0050]     Incidentally, the above expressions (1) and (2) must be stably satisfied regardless of temporal changes in the intermediate transfer belt  64  and fluctuations in the working environment. For this reason, it is necessary to set the initial tension T in consideration of a drop in the initial tension T resulting from creep deformation of the intermediate transfer belt  64  and fluctuations in the coefficient of friction μ resulting from environmental fluctuations. This point will be described below.  
         [0051]     First, the intermediate transfer belt  64  is stretched between the rolls, and the change in the initial tension T over time is measured. As a characteristic of the intermediate transfer belt  64  exemplified in the present exemplary embodiment, it is apparent from preliminary tests that the characteristic of the change in the initial tension T changes due to initial stretch conditions (initial elongation) and working environment conditions (temperature and humidity). Thus, the initial tension T is measured by changing the initial stretch conditions and environment conditions.  
         [0052]     It is apparent from preliminary tests that the characteristic of the change in the initial tension T does not change whether or not the intermediate transfer belt  64  is rotatingly driven while it is stretched.  
         [0053]     Further, the initial elongation refers to the percentage of increase in the circumference of the intermediate transfer belt  64 , from its initial state when the intermediate transfer belt  64  is in a natural state (an unstretched state) in normal temperature and humidity to the state when it is stretched between the rolls. When the circumference of the intermediate transfer belt  64  in the natural state is 100% and the circumference of the intermediate transfer belt  64  in the stretched state is 102%, then the initial elongation becomes 2% (=102%−100%).  
         [0054]     Further, in the following tests, the normal temperature normal humidity environment is 22° C with 55% relative humidity (RH), the high temperature high humidity environment is 28° C. with 85% relative humidity (RH), and the low temperature low humidity environment is 10° C. with 15% relative humidity (RH).  
         [0055]     Further, the material, dimensions, and strength of the intermediate transfer belt  64  used in these tests are as shown in the chart of  FIG. 4 .  
         [0056]     First, under normal temperature and normal humidity, the intermediate transfer belt  64  is rotated for 200 hours, with the initial elongation being set to 2%, 3%, and 4%, and the changes in the initial tension T during this time are measured. The measurement results are as shown in the graph of  FIG. 5 . Then, a map representing the correlation between the obtained stretch time x and the initial tension T is exponentially approximated. The approximations become like the following expressions (3) to (5).
 
 T= 46.09+18.69e −0.0246x (initial elongation 4%)  (3)
 
 T= 33.25+14.24e −0.0147x (initial elongation 3%)  (4)
 
 T= 24.87+6.179e −0.0118x (initial elongation 2%)  (5)
 
         [0057]     Further, under high temperature and high humidity, the intermediate transfer belt  64  is stretched for one month without being rotatingly driven with the initial elongation being 3% and 4%, and the changes in the initial tension T during this time are measured. The measurement results are as shown in the graph of  FIG. 6 . Then, a map representing the correlation between the obtained stretch time x and the initial tension T is exponentially approximated. The approximations become like the following expressions (6) and (7).
 
 T= 25.73+19.23e −0.022x (initial elongation 4%)  (6)
 
 T= 17.15+16.45e −0.0160x (initial elongation 3%)  (7)
 
         [0058]     Further, under low temperature and low humidity, the intermediate transfer belt  64  is stretched for one month without being rotatingly driven with the initial elongation being 3% and 4%, and the changes in the initial tension T during this time are measured. The measurement results are as shown in the graph of  FIG. 7 . Then, a map representing the correlation between the obtained stretch time x and the initial tension T is exponentially approximated. The approximations become like the following expressions (8) and (9).
 
 T= 67.05+14.64e −0.1688x (initial elongation 4%)  (8)
 
 T= 55.28+12.97e −0.1500x (initial elongation 3%)  (9)
 
         [0059]     Next, the lifespan of the intermediate transfer belt  64  is set to be two years (17,520 hours), and the initial tension T at the end of the lifespan is calculated from the above expressions (3) to (9). The calculation results are as shown in the graph of  FIG. 8 . Then, a map representing the correlation between the obtained initial elongation y at the end of the lifespan and the initial tension T is exponentially approximated. The approximations become like the following expressions (10) to (12).
 
 T= 11.76 y +19.99(low temperature low humidity)  (10)
 
 T= 10.61 y +2.912(normal temperature normal humidity)  (11)
 
 T= 8.581 y− 8.591(high temperature high humidity)  (12)
 
         [0060]     Usually, log approximation is used when approximating temporal changes such as these. When the stretch tension of the belt is constant, it is appropriate to use log approximation, but when the stretch tension of the belt is imparted by the initial elongation of the belt as described above and changes over time due to the elongation deformation of the belt, it is appropriate to use exponential approximation.  
         [0061]     Next, the coefficient of friction μ between the photoconductor drum  50  and the intermediate transfer belt  64  is measured with the environment conditions being changed to high temperature and high humidity, normal temperature and normal humidity, and low temperature and low humidity. Here, because it is predicted that the coefficient of friction μ will be greatly affected by the temporal changes of the surface state between the photoconductor drum  50  and the intermediate transfer belt  64 , when the change in the coefficient of friction μ from when the intermediate transfer belt  64  is brand new to when the intermediate transfer belt  64  has reached the end of its lifespan is measured in the preliminary tests, it is substantiated that the coefficient of friction μ become the lowest when the intermediate transfer belt  64  reaches the end of its lifespan and that the prediction is correct. For this reason, the coefficient of friction μ is measured in the state when the intermediate transfer belt  64  has reached the end of its lifespan, which is the state in which the intermediate transfer belt  64  to slip. The measurement results are as shown in the graph of  FIG. 9 .  
         [0062]     Then, the necessary initial tension T is calculated by substituting into the above expression (1) the obtained coefficient of friction μ, the design value (=15.68 N) of the load F in the tangential direction of the photoconductor drum  50 , and the wrap angle θ(=1.571 rad). As shown in the chart of  FIG. 10 , the calculation result becomes 54.92 N under normal temperature and normal humidity, 35.85 N under high temperature and high humidity, and 61.95 N under low temperature and low humidity.  
         [0063]     Then, the obtained initial tension T is substituted into the above expressions (10) to (12) to calculate the initial elongation. As shown in the chart of  FIG. 10 , the calculation result becomes 4.9% under normal temperature and normal humidity, 5.2% under high temperature and high humidity, and 3.6% under low temperature and low humidity.  
         [0064]     Consequently, regardless of environmental fluctuations, the initial elongation of the intermediate transfer belt  64  necessary in order to cause the intermediate transfer belt  64  to be stably driven by the photoconductor drum  50  is 5.2% or more.  
         [0065]     It will be noted that, as mentioned above, the upper limit of the initial elongation is 37.1% under normal temperature and normal humidity, 47.3% under high temperature and high humidity, and 32.1% under low temperature and low humidity, because the initial tension T is set to 396.9 N or less. Consequently, regardless of the environment, the upper limit of the initial elongation becomes 32.1% in order to set the initial tension T to 4.133 N or less.  
         [0066]     On the other hand, when the electrostatic suction force N arising between the photoconductor drum  50  and the intermediate transfer belt  64  is taken into consideration, the necessary initial tension T is calculated by substituting into the above expression (2) the obtained coefficient of friction μ, the design value (=15.68 N) of the load F in the tangential direction of the photoconductor drum  50 , the wrap angle θ (=1.571 rad), and the electrostatic suction force (=37.0 N). As shown in the chart of  FIG. 11 , the calculation result becomes 29.44 N under normal temperature and normal humidity, 9.055 N under high temperature and high humidity, and 36.71 N under low temperature and low humidity.  
         [0067]     Then, the obtained initial tension T is substituted into the above expressions (10) to (12) to calculate the initial elongation. As shown in the chart of  FIG. 11 , the calculation result becomes 2.5% under normal temperature and normal humidity, 2.1% under high temperature and high humidity, and 1.4% under low temperature and low humidity.  
         [0068]     Consequently, regardless of environmental fluctuations, the initial elongation of the intermediate transfer belt  64  necessary in order to cause the intermediate transfer belt  64  to be stably driven by the photoconductor drum  50  is 2.5% or more.  
         [0069]     As described above, by setting the initial elongation of the intermediate transfer belt  64  to 5.2% or more when the electrostatic suction force N is not considered, the initial tension T of the intermediate transfer belt  64  satisfies the above expression (1) and the intermediate transfer belt  64  is stably driven by the photoconductor drum  50 , regardless of temporal changes until the end of the normal lifespan and fluctuations in temperature and humidity in the normal working environment of the apparatus. Further, by setting the initial elongation of the intermediate transfer belt  64  to 2.5% or more when the electrostatic suction force N is considered, the initial tension T of the intermediate transfer belt  64  satisfies the above expression (2) and the intermediate transfer belt  64  is stably driven by the photoconductor drum  50 , regardless of temporal changes until the end of the normal lifespan and fluctuations in temperature and humidity in the normal working environment of the apparatus.  
         [0070]     Incidentally, unevenness in the circumferential speed of the photoconductor drum  50  resulting from the eccentricity of the photoconductor drum  50  can be cited as another factor that inhibits high-precision mutual alignment of the plural toner images to be superposed on the intermediate transfer belt  64 .  
         [0071]     Because a certain amount of eccentricity that is allowable in terms of manufacturing precision is present not only in the photoconductor drum  50  but also in roll-like rotators, speed unevenness in one rotational cycle arises in the circumferential speed even if the rotational speed is constant. For this reason, in the case of the photoconductor drum  50 , the pitch unevenness in the sub-scanning direction arises at the stage of exposure, and as shown in  FIG. 12 , the pitch unevenness in the sub-scanning direction arises in the toner images that have been primarily transferred to the intermediate transfer belt  64 .  
         [0072]     Here, by setting the circumference of the intermediate transfer belt  64  such that it is an integral multiple of the circumference of the photoconductor drum  50  in a state where the intermediate transfer belt  64  is wrapped around the wrap-in roll  68 , the wrap-out roll  70 , the secondary transfer backup roll  72 , the brush backup roll  74 , the tension rolls  75  and  76 , and the photoconductor drum  50 , the phases of the pitch unevenness of plural toner images to be superposed align (see the graph in  FIG. 13A ). Thus, the positional shift between the plural toner images to be superposed is suppressed.  
         [0073]     On the other hand, if the circumference of the intermediate transfer belt  64  is set such that it is a non-integral multiple of the circumference of the photoconductor drum  50  in a state where the intermediate transfer belt  64  is wrapped around the wrap-in roll  68 , the wrap-out roll  70 , the secondary transfer backup roll  72 , the brush backup roll  74 , the tension rolls  75  and  76 , and the photoconductor drum  50 , the phases of the pitch unevenness of the plural toner images to be superposed are shifted (see the graph in  FIG. 13B ). For this reason, positional shift between the plural toner images to be superposed occurs.  
         [0074]     Consequently, in the present exemplary embodiment, the circumference of the intermediate transfer belt  64  is set such that it is an integral multiple of the circumference of the photoconductor drum  50  in a state where the intermediate transfer belt  64  is wrapped around the wrap-in roll  68 , the wrap-out roll  70 , the secondary transfer backup roll  72 , the brush backup roll  74 , the tension rolls  75  and  76 , and the photoconductor drum  50 .  
         [0075]     Here, in order to generate the initial tension T in the intermediate transfer belt  64 , usually at least one stretch roll is made pivotable and the intermediate transfer belt is biased by a spring or the like. In this case, the initial tension T can be maintained at a constant, but the stretch roll ends up pivoting and the circumference of the intermediate transfer belt ends up changing. For this reason, the circumference of the intermediate transfer belt at the time of stretching cannot be maintained at an integral multiple of the circumference of the photoconductor drum.  
         [0076]     Thus, in the present exemplary embodiment, the positions of the rotational shafts of the wrap-in roll  68 , the wrap-out roll  70 , the secondary transfer backup roll  72 , the brush backup roll  74 , and the tension rolls  75  and  76  are made immovable and the intermediate transfer belt  64  is elastically deformed at the time of stretching, whereby the initial tension T is generated in the intermediate transfer belt  64 . Thus, the circumference of the intermediate transfer belt  64  at the time of stretching can be maintained at an integral multiple of the circumference of the photoconductor drum  50 , and the positional shift between the plural toner images to be superposed on the intermediate transfer belt  64  can be suppressed.