Patent Publication Number: US-8538304-B2

Title: Belt drive apparatus and image forming apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority based on 35 USC 119 from prior Japanese Patent Application No. P2009-261250 filed on Nov. 16, 2009, entitled “BELT DRIVE APPARATUS AND IMAGE FORMING APPARATUS”, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a belt drive apparatus and an image forming apparatus. 
     2. Description of Related Art 
     In conventional image forming apparatus, such as printers, photocopiers, fax machines, and multifunction printers, or in an electrophotographic printer, for example, the surface of a photosensitive drum is uniformly charged by a charge roller and is exposed by an LED head to form an electrostatic latent image, the electrostatic latent image is developed by a developing unit to form a toner image on the photosensitive drum, the toner image is transferred onto a paper sheet, and the transferred toner image is fixed on the paper sheet. 
     A conventional electrophotographic printer is equipped with a belt drive apparatus to convey paper sheets to a transferring position located between the photosensitive drum and the transferring unit. The belt drive apparatus includes a drive roller, an idle roller, and an endless belt serving as a belt member stretched by the drive roller and the idle roller. When the drive roller rotates, the endless belt is made to run as the idle roller is driven accordingly, thereby conveying the paper sheet on the endless belt to the transferring position. 
     While the endless belt of the belt drive apparatus is running, the endless belt sometimes moves in the crosstrack direction of the belt and may snake back and forth in that direction. If this occurs, conveyance of the paper sheet becomes unstable, which causes unsatisfactory transfer of the developer image onto the paper sheet. As a consequence, the transferred image quality is degraded. 
     To address this problem, the snaking of the endless belt is prevented by providing a pulley at least at one end of either the drive roller or the idle roller (the idle roller, for example). 
       FIG. 2  is a sectional view illustrating an idle roller of the related art, and  FIG. 3  is a perspective view illustrating the idle roller of the related art. 
     These drawings show idle roller  200 , pulley  203  provided at one end of idle roller  200 , belt receiving portion  204  of pulley  203 , and endless belt  210 . 
     If endless belt  210  moves in its width direction and an edge portion of endless belt  210  is brought into contact with belt receiving portion  204  of pulley  203 , endless belt  210  is prevented from moving further in that direction. Accordingly, the snaking of endless belt  210  can be prevented (see, for example, JP 2006-162659A). 
     SUMMARY OF THE INVENTION 
     In the belt drive apparatus of JP 2006-162659A, an edge portion of endless belt  210  is brought into contact with a surface of belt receiving portion  204  of pulley  203  (hereafter, that surface is called belt receiving surface s 1 ). Belt receiving surface s 1  and the axis of the shaft for idle roller  200  make an angle of 90°. Accordingly, while endless belt  210  is running, the edge portion of endless belt  210  rubs against belt receiving surface s 1  both at position Pa where endless belt  210  reaches idle roller  200  (where endless belt  210  starts touching idle roller  200 ) and at position Pb where endless belt  210  leaves idle roller  200  (where endless belt  210  finishes touching idle roller  200 ). 
     Accordingly, at position Pa where endless belt  210  reaches idle roller  200 , the edge portion of endless belt  210  receives a force directed outward in the radial direction of pulley  203  whereas at position Pb where endless belt  210  leaves idle roller  200 , the edge portion of endless belt  210  receives a force directed inward in the radial direction of pulley  203 . Consequently, if, for example, endless belt  210  runs in a direction indicated by arrow A, flexures  212  are generated both at position Pa where endless belt  210  reaches idle roller  200  and at position Pb where endless belt  210  leaves idle roller  200 . In particular, at position Pb where endless belt  210  leaves idle roller  200 , endless belt  210  is drawn into rotating idle roller  200 . 
     Such flexures  212  are generated every time endless belt  210  makes a full rotation, so that an alternate load is applied to the vicinity of the edge portion of endless belt  210 . Accordingly, fatigue failure of endless belt  210 , that is, breakage of endless belt  210  caused by the fatigue of the material of endless belt  210 , is more likely to occur, and thus the durability of endless belt  210  is impaired. 
     This problem may be addressed by applying a reinforcement tape to reinforce the edge portion of endless belt  210 . However, the adhesive used for the reinforcement tape is likely to be affected by such factors as the temperature and the humidity of the use environment. For example, as the use of the reinforcement tape under high-temperature conditions tends to lower the adherence of the adhesive, the reinforcement tape may be displaced or may be removed from endless belt  210 . In addition, it is difficult to properly apply the reinforcement tape to the edge portion of endless belt  210 , resulting in higher manufacturing cost for the printer. 
     A first aspect of the invention is a belt drive apparatus including: a rotatable first roller; a rotatable second roller; a belt member stretched between the first and the second rollers so as to be capable of being conveyed by them; and a restraint member provided on at least one of the two ends of the second roller and including a belt receiving surface to be brought into contact with the edge portion of the belt member. The angle between the belt receiving surface and the axis of the second roller is in the range from 93° to 115°. 
     According to the first aspect, the lateral pressure that the edge portion of the belt member receives when the edge portion is brought into contact with the restraint member is reduced, and the shear stress generated in the belt member is reduced. Accordingly, material failure of the belt member is prevented. 
     In addition, a reduction is achieved both in the force directed outwards in the radial direction of the belt receiving surface and received by the belt member at the position where the belt member reaches the second roller and in the force directed inwards in the radial direction of the belt receiving surface and received by the belt member at the position where the belt member leaves the second roller. Accordingly, no flexure occurs in the belt member. Consequently, fatigue failure of the belt member is prevented. 
     In addition, the belt member is brought into uniform contact with the belt receiving surface. Accordingly, distortion can be prevented from occurring in the edge portion of the belt member. 
     Consequently, the durability of the belt member is improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating a principal portion of a transferring unit according to a first embodiment of the invention. 
         FIG. 2  is a sectional view illustrating a conventional idle roller. 
         FIG. 3  is a perspective view illustrating the conventional idle roller. 
         FIG. 4  is a conceptual diagram illustrating a printer according to the first embodiment of the invention. 
         FIG. 5  is a conceptual diagram illustrating the transferring unit according to the first embodiment of the invention. 
         FIG. 6  is a diagram illustrating a print pattern to be used when the durability of an endless belt of the first embodiment of the invention is assessed. 
         FIG. 7  is a diagram illustrating a pulley according to a modified example of the first embodiment of the invention. 
         FIG. 8  is a conceptual diagram illustrating a printer according to a second embodiment of the invention. 
         FIG. 9  is a conceptual diagram illustrating a transferring unit according to the second embodiment of the invention. 
         FIG. 10  is a conceptual diagram illustrating a pulley according to a third embodiment of the invention. 
         FIG. 11  is a conceptual diagram illustrating a printer according to a fourth embodiment of the invention. 
         FIG. 12  is a conceptual diagram illustrating a transferring unit according to the fourth embodiment of the invention. 
         FIG. 13  is a conceptual diagram illustrating a pulley according to the fourth embodiment of the invention. 
         FIG. 14  is a conceptual diagram illustrating a pulley according to a fifth embodiment of the invention. 
         FIG. 15  is an enlarged view illustrating a principal portion of the pulley according to the fifth embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Descriptions are provided hereinbelow for embodiments based on the drawings. In the respective drawings referenced herein, the same constituents are designated by the same reference numerals and duplicate explanation concerning the same constituents is omitted. All of the drawings are provided to illustrate the respective examples only. 
     Some embodiments of the invention are described in detail below by referring to the drawings. The following description is given by taking an electrophotographic printer as an exemplar image forming apparatus. 
       FIG. 1  is a diagram illustrating a principal portion of a transferring unit according to a first embodiment of the invention.  FIG. 4  is a conceptual diagram illustrating a printer according to the first embodiment of the invention.  FIG. 5  is a conceptual diagram illustrating the transferring unit according to the first embodiment of the invention. 
     As  FIG. 4  shows, printer  60  includes: a paper-sheet conveyance path (not denoted in the drawings); plural image forming units  61 Bk,  61 Y,  61 M, and  61 C; belt-type transferring unit  12 ; LED heads  69 ; register rollers  70 ; fixing unit  80 ; and the like. The paper-sheet conveyance path serves as a media conveyance path configured to convey the media (i.e., paper sheets P). Image forming units  61 Bk,  61 Y,  61 M, and  61 C are configured to form toner images as developer images of their respective colors (black, yellow, magenta, and cyan) in accordance with image data. Transferring unit  12  is provided so as to be opposed to photosensitive drums  65 , each of which serves as the image carrier of the corresponding one of image forming units  61 Bk,  61 Y,  61 M, and  61 C. Transferring unit  12  thus forms transferring positions for those colors with corresponding photosensitive drums  65 . Transferring unit  12  is configured to form color toner images by transferring, consecutively and one upon another, toner images of those colors formed on their respective photosensitive drums  65  onto the medium, that is, paper sheet P. LED heads  69  are provided so as to be opposed respectively to photosensitive drums  65  of image forming units  61 Bk,  61 Y,  61 M, and  61 C. Each LED head  69  serves as an exposure apparatus configured to form a latent image, i.e., an electrostatic latent image, by exposing the surface of corresponding photosensitive drum  65  to light. Register rollers  70  serve as conveyor unit configured to feed each paper sheet P that has been fed from paper-sheet cassette  64  and sent out to the paper-sheet conveyance path by feed roller R 1  serving as a sheet-feeder. Register rollers  70  feed each paper sheet P to the transferring positions at their respective proper times for image forming units  61 Bk,  61 Y,  61 M, and  61 C to form images. Fixing unit  80  serves as a fixing apparatus configured to fix, on paper sheet P, color toner images transferred to paper sheet P at their respective transferring positions. Fixing unit  80  includes heating roller  83  serving as a first rotating body and pressing roller  84  serving as a second rotating body. 
     All of image forming units  61 Bk,  61 Y,  61 M, and  61 C have identical structures. Each of image forming units  61 Bk,  61 Y,  61 M, and  61 C includes rotatable photosensitive drum  65 ; charge roller  67 ; development roller  66 ; cleaning blade  68 ; and the like. Charge roller  67 , development roller  66 , and cleaning blade  68  are provided in this order in the direction in which photosensitive drum  65  rotates. Charge roller  67  serves as a charging apparatus configured to charge uniformly the surface of photosensitive drum  65 . Development roller  66  serves as a developer carrier configured to form a developer image by developing the electrostatic latent image that has been formed by LED head  69 . Cleaning blade  68  serves as a first cleaning member included in a cleaning apparatus. 
     Transferring unit  12  is a belt-type transferring unit, and includes an unillustrated motor, drive roller  13 , idle roller  14 , endless belt  16 , transfer rollers  75 , cleaning blade  18 , and the like. The motor serves as a driving unit for image transfer. Drive roller  13  serves as a first roller which is coupled to the motor and which is made to rotate by the rotation of the motor. Idle roller  14  serves as a second roller which is driven to rotate by the rotation of drive roller  13 . Endless belt  16  is a belt member stretched between drive roller  13  and idle roller  14 , being capable of being conveyed in a path around drive roller  13  and idle roller  14 , and serving also as a transfer belt. Transfer rollers  75  serve as transfer members rotatably provided inside of the looped endless belt  16  so as to be opposed respectively to photosensitive drums  65 . Cleaning blade  18  serves as a second cleaning member which is provided in the vicinity of idle roller  14  so as to be in contact with the outside surface of endless belt  16 . 
     A tension (stretching force) of 6±10% kg is applied to endless belt  16  by an unillustrated tension providing apparatus. If drive roller  13  is made to rotate, idle roller  14  is driven to rotate. Transferring unit  12 , tension providing apparatus, and the like together form the belt drive apparatus configured to convey endless belt  16 . 
     Next, the operations of printer  60  are described. 
     First, paper sheet P supplied from paper-sheet cassette  64  is conveyed by register rollers  70 , and is then fed to the transferring positions by endless belt  16 . In the meanwhile, in each of image forming units  61 Bk,  61 Y,  61 M, and  61 C, the surface of photosensitive drum  65  is electrically charged by charge roller  67 , and is then exposed to light by LED head  69  to form an electrostatic latent image. Then, the electrostatic latent image is developed by development roller  66  to form a toner image of the corresponding color on photosensitive drum  65 . 
     Then, at the transferring positions, toner images of their respective colors—black, yellow, magenta, and cyan—are consecutively transferred one upon another onto paper sheet P by transfer rollers  75 . Thus, a color toner image is formed on paper sheet P. 
     Then, paper sheet P with color toner image formed thereon is conveyed to fixing unit  80 , where the color toner image on paper sheet P is fixed to paper sheet P by being heated and pressed. Consequently, a color image is formed. Then, paper sheet P is discharged from the main body of printer  60 , that is, out of the main body of the apparatus. 
     The residual toner, as the developer that still remains on each photosensitive drum  65  even after the toner image is transferred onto paper sheet P is scraped off and removed by cleaning blade  68 . In addition, toner, foreign objects (e.g., paper dust), and the like that stay on endless belt  16  after the fixing of the image are scraped off and removed by cleaning blade  18 . 
     While endless belt  16  is made to run by the belt drive apparatus, endless belt  16  sometimes moves in the crosstrack direction of endless belt  16  and, consequently, snakes back and forth laterally. If this occurs, conveyance of paper sheet P becomes unstable, which causes unsatisfactory transfer of each toner image onto paper sheet P. 
     To address this problem, the snaking of endless belt  16  is prevented by providing, at least at one end of either drive roller  13  or idle roller  14 , pulley  31  serving as a snaking-restraint member or as a rotating member. In this embodiment, pulley  31  is provided at least at one end of idle roller  14  as shown in  FIGS. 1 and 5 . 
     Next, pulley  31  is described. 
     As  FIG. 1  shows, idle roller  14  includes: roller main body  151 , which is the main body of idle roller  14 ; and rotary shafts  152 , which serve as the supporting shafts formed so as to extend along the axis of idle roller  14  and to protrude respectively from the two ends of roller main body  151 . Rotary shafts  152  are rotatably supported by unillustrated bearing portions, so that idle roller  14  is rotatable. The rotation of drive roller  13  moves endless belt  16 , which, in turn, causes idle roller  14  to rotate. To put it differently, along with the rotation of drive roller  13 , idle roller  14  is made to rotate by means of endless belt  16 . 
     Pulley  31  is provided on one of two rotary shafts  152  at a specified distance from roller main body  151 . In this embodiment, pulley  31  is fixed on rotary shaft  152 , but pulley  31  may be provided so as to be incapable of moving in the axial direction relative to rotary shaft  152  but capable of rotating in the rotating direction. 
     Pulley  31  includes core portion  154  and belt receiving portion  155 . Core portion  154  has the same diameter as that of roller main body  151 . Belt receiving portion  155  is formed so as to be adjacent to core portion  154 , to form a single unit with core portion  154 , and to serve as a conically-shaped flange portion. The diameter of belt receiving portion  155  increases at a position farther away from core portion  154 . 
     While endless belt  16  is running, pulley  31  is made to rotate together with idle roller  14 . Then, endless belt  16  moves in the crosstrack direction to bring its edge portion into contact with belt receiving portion  155 . The contact prevents endless belt  16  from moving further in that direction. Consequently, the snaking of endless belt  16  can be prevented. 
     In this embodiment, pulley  31  is provided on rotary shaft  152 , but pulley  31  may be provided on a different rotating member from any of rotary shafts  152  or on a supporting member to support endless belt  16 . In addition, in this embodiment, single pulley  31  is provided at one end of idle roller  14 , but two pulleys  31  may be provided respectively at both ends of idle roller  14 . 
     Note that the surface of belt receiving portion  155  of pulley  31  is brought into contact with the edge portion of endless belt  16 . The contact surface is referred to as belt receiving surface sa. The angle that belt receiving surface sa makes with the axis of idle roller  14  is denoted by θ (since the axis and the surface of roller main body  151  are parallel to each other, the angle θ is shown in  FIG. 1  as the angle that belt receiving surface sa makes with the surface of roller main body  151 ). 
     Next, endless belt  16  is described. 
     In the manufacturing of endless belt  16 , it is necessary, from the viewpoint of durability, mechanical characteristics, and the like, to use a material that can restrain, within a certain range, the deformation of endless belt  16  caused by the tension while endless belt  16  is running. In addition, the providing of pulley  31  to prevent endless belt  16  from snaking its way causes the edge portion of endless belt  16  to slide repetitively on belt receiving surface sa. Accordingly, by taking such sliding into consideration, it is necessary to use a material that can make edge portion of endless belt  16  less susceptible to wearing, bending, and cracking. 
     To this end, in this embodiment, polyamide-imide is used as a main material of endless belt  16 . 
     Carbon black of an appropriate amount is added to polyamide-imide to make polyamide-imide conductive and the mixture is mixed and agitated in an N-methylpyrrolidone solution. The resin material thus obtained is injected into a cylindrical mold. Then, while the mold is rotating, the mold is heated at a temperature that is not lower than 90° C. but is not higher than 120° C. for a predetermined time. Then, the mold is heated at a temperature that is not lower than 200° C. but is not higher than 350° C. for a predetermined time, and then the mold is cooled down. Thus formed in the mold is a raw pipe for the belt with a thickness of 100±10 μm and a circumferential length of 624±1.5 mm. Then, the raw pipe for belt is taken out of the mold and is cut so that each piece has a width of 228±0.5 mm. Thus obtained is endless belt  16  with a thickness of 100±10 μm, a circumferential length of 624±1.5 mm, and a width of 228±0.5 mm. 
     The rotating speed of the mold is set on the basis of the analysis result of the accuracy of thickness, the thickness itself, and the like of endless belt  16 . Specifically, the rotating speed is set not slower than 5 rpm but not faster than 1000 rpm, and is preferably set not slower than 10 rpm but not faster than 500 rpm. 
     Note that, endless belt  16  may be formed without rotating the mold. For example, a mold with a larger-diameter cylindrical portion and a smaller-diameter cylindrical portion is employed, and a resin material is injected into the gap between the larger-diameter cylindrical portion and the smaller-diameter cylindrical portion. Endless belt  16  may also be formed by applying a resin material onto the external surface of a cylindrical mold, or by immersing a cylindrical mold in a resin material. 
     In addition, endless belt  16  may be formed by the extrusion molding method, by the inflation molding method, or the like. In these cases, no solvent is needed to form endless belt  16 . 
     The polyamide-imide is a polymer with plural repeating units each of which is formed by binding an amide group and either a single or two imide groups by means of an organic group. Polyamide-imides are classified as aliphatic polyamide-imide or aromatic polyamide-imide depending on whether the organic group is aliphatic or aromatic. In this embodiment, an aromatic polyamide-imide is used. Note that the organic group of the aromatic polyamide-imide includes either a single or two benzene groups. 
     The polyamide-imide to be used is either one with imide rings that are completely closed or one that is still in the amide acid state with unclosed imide rings. To prevent the dimensional change rate of endless belt  16  from becoming too high, at least 50% or higher, or preferably 70% or higher, of the polyamide-imide that is still in the amide acid state is imidized. The ratio at which the polyamide-imide is imidized, i.e., imidization ratio is calculated, using Fourier-transform infrared spectrophotometer FT-IR (manufactured by PerkinElmer Co., Ltd.), from the intensity ratio between the absorption attributable to imide group (1780 cm −1 ) and the absorption attributable to benzene ring (1510 cm −1 ). 
     In general, endless belt  16  with a molecular structure that contains more aromatic rings, imide groups, and the like has a higher Young&#39;s modulus whereas endless belt  16  with a molecular structure that contains less aromatic rings, imide groups, and the like has a lower Young&#39;s modulus. 
     Note that, some other materials than polyamide-imide can be used for endless belt  16  as a material with which the deformation caused by the tension while endless belt  16  is running is restrained within a certain range. For example, some of the materials that can be used are such resins as polyimide (PI), polycarbonate (PC), polyamide (PA), polyether ether ketone (PEEK), polyvinylidene fluoride (PVdF), and ethylene-tetrafluoroethylene copolymer (ETFE) with a Young&#39;s modulus that is not smaller than 2.0 GPa, or preferably, that is not smaller than 3.0 GPa. Each of these resins may be used by itself. Alternatively, a mixture containing mainly these resins may also be used. 
     The solvent to be used when endless belt  16  is formed is determined appropriately depending upon which material is the main material of endless belt  16 . It is preferable to use organic polar solvents, particularly N,N-dimethylacetamides. Some examples of such preferable solvents are N,N-dimethylformamide, N,N-dimethylacetamide, N,N-diethylformamide, N,N-diethylacetamide, dimethylsulfoxide, N-methylpyrrolidone, pyridine, tetramethylene sulfone, and dimethyltetramethylene sulfone. Note that each of these organic polar solvents may be used either by itself or by being mixed with others. 
     As the carbon black, furnace black, channel black, ketjen black, acetylene black, or the like can be used. Each carbon black can be selected appropriately depending upon the necessary degree of conductivity. In this embodiment, the use of channel black, furnace black, or the like is particularly preferable. Note that carbon blacks subjected to a treatment to prevent oxidation and/or degradation, such as oxidation treatment, grafting treatment, or the like, and carbon blacks with improved dispersibility in the solvent may also be used. Each of these carbon blacks may be used either by itself or by being mixed with others. 
     The content of the carbon black is appropriately determined depending upon which kind of carbon blacks is to be used. In this embodiment, in view of the necessary mechanical strength and the like, the content of the carbon black is set not lower than 3 wt % of the composition-resin solid content of endless belt  16  but not higher than 40 wt %, preferably not lower than 5 wt % but not higher than 30 wt %, or more preferably not lower than 5 wt % but not higher than 25 wt %. 
     The necessary specularity of endless belt  16  can be obtained by adjusting appropriately the way of polishing the internal surface of the cylindrical mold. 
     The toners to be used are fabricated by the emulsion polymerization method and contain styrene-acryl copolymer as the main constituent. The toners to be used contain 9 parts by weight of paraffin wax, have an average particle size of 7 μm and a sphericity of 0.95. In this case, the transfer efficiency can be improved, and it is no longer necessary to apply any release agent to heating roller  83  of fixing unit  80 . In addition, the dot reproducibility, the resolution, and the like can be enhanced to produce a sharp image. Consequently, the image quality can be improved. 
     In addition, a piece of urethane rubber with a rubber hardness JIS A of 72° and with a thickness of 1.5 mm is used as cleaning blade  18 . Cleaning blade  18  is set so as to have a linear pressure of 4.3 g/mm against endless belt  16 . If cleaning blade  18  is formed with an elastic material such as urethane rubber, cleaning blade  18  can have higher ability of removing toners, foreign objects, and the like that adhere to the surface of endless belt  16  while the structure of printer  60  can be simplified, the size of printer  60  can be reduced, and the manufacturing cost of printer  60  can be reduced. In addition, among various rubber materials, urethane rubber is excellent in hardness, elasticity, wear resistance, mechanical strength, oil resistance, ozone resistance, and the like. 
     Note that, to secure certain cleaning ability, urethane rubber to be used as cleaning blade  18  as in the case of this embodiment has: a rubber hardness JIS A that is not lower than 60° but not higher than 90°, or preferably that is not lower than 70° but not higher than 85°; a breaking elongation that is not lower than 250% but not higher than 500%, or preferably that is not lower than 300% but not higher than 400%; a permanent elongation that is not lower than 1.0% but not higher than 5.0%, or preferably, that is not lower than 1.0% but not higher than 2.0%; a rebound resilience that is not lower than 10% but not higher than 70%, or preferably that is not lower than 30% but not higher than 50%. Each of these properties, that is, rubber hardness JIS A, breaking elongation, permanent elongation, and rebound resilience, can be measured by their respective measurement methods defined by JIS K6301. 
     In addition, the linear pressure of cleaning blade  18  against endless belt  16  is not smaller than 1 g/mm but not larger than 6 g/mm, or preferably is not smaller than 2 g/mm but not larger than 5 g/mm. A linear pressure that is smaller than 1 g/mm makes the adhesion of cleaning blade  18  to endless belt  16  insufficient and, accordingly, the cleaning tends to be done poorly. A linear pressure that is larger than 6 g/mm brings cleaning blade  18  and endless belt  16  into plane-to-plane contact with each other, resulting in an excessively large frictional resistance. Accordingly, the pressing force with which cleaning belt  18  is pressed against endless belt  16  becomes larger than the scraping power to remove toners, foreign objects, and the like that adhere to the surface of endless belt  16 . Consequently, the cleaning ability is impaired, filming phenomenon occurs, and various other inconveniences such as flipping-up are more likely to occur. 
     Each of drive roller  13  and idle roller  14  used has a diameter of 25 mm. However, a roller with a diameter that is not smaller than 10 mm but not larger than 50 mm may be used as long as the dimensions and the manufacturing cost of printer  60  permit it. 
     In the tension providing apparatus, a spring is used to give tension to endless belt  16 . In this embodiment, the tension providing apparatus gives a tension of 6±10% kg, but a tension that is not smaller than 2±10% kg but not larger than 8±10% kg may be given by taking account of the material of endless belt  16 , the motor used to make endless belt  16  run, and other factors. 
     In this embodiment, pulley  31  is provided to prevent endless belt  16  from snaking its way. Accordingly, if endless belt  16  moves in the crosstrack direction, and if its edge portion is brought into contact with pulley  31 , the edge portion receives a lateral pressure, that is, a certain pressure caused by the reaction force of pulley  31 . In this event, if a large shear stress is generated in endless belt  16 , endless belt  16  is broken by the stress acting on the material, that is, material failure of endless belt  16  occurs. 
     In addition, while endless belt  16  is running, the edge portion of endless belt  16  repetitively slides on belt receiving surface sa. Then, the edge portion of endless belt  16  rubs belt receiving surface sa both at a position where endless belt  16  reaches idle roller  14  and at a position where endless belt  16  leaves idle roller  14 . In this event, the edge portion of endless belt  16  receives a force directed outward in the radial direction of pulley  31  at the position where endless belt  16  reaches idle roller  14  while the edge portion of endless belt  16  receives a force directed inward in the radial direction of pulley  31  at the position where endless belt  16  leaves idle roller  14 , thus generating flexures. Accordingly, stress is concentrated on the vicinity of the edge portion of endless belt  16 , or to be more specific, concentrated on the inside area that is away from the edge portion by a distance that is not smaller than 5 mm but not larger than 10 mm, and thus alternate load is exerted. Consequently, fatigue failure of endless belt  16  occurs, so that the durability of endless belt  16  is impaired. 
     The shear stress and the alternate load becomes larger as the contact area of the edge portion of endless belt  16  with pulley  31  becomes larger and as the contact time of the edge portion of endless belt  16  with pulley  31  becomes longer. Larger shear stress and alternate load makes both material failure and fatigue failure of endless belt  16  more likely to occur. 
     To address this problem, the durability of endless belt  16  is assessed and determined by varying the angle θ that belt receiving surface sa of pulley  31  makes with the axis of idle roller  14 . 
     Next, the conditions for the durability assessment of endless belt  16  are described. 
       FIG. 6  is a diagram illustrating a print pattern to be used when the durability of the endless belt of the first embodiment of the invention is assessed. 
     To this end, a printer C5800n manufactured by Oki Data Corporation is used to print repetitively the print pattern shown in  FIG. 6  on paper sheets P of A4 size by changing the angle θ. In this event the running speed of endless belt  16 , that is, the linear speed of endless belt  16  is set at approximately 90 mm/sec. The print pattern includes horizontal lines of black, yellow, magenta, and cyan LBk, LY, LM, and LC. Each of horizontal lines LBk, LY, LM, and LC is printed with a printing density of 0.5% by assuming an ordinary job of printing text. 
     The printing is done under the condition of 3P/J (an action of printing three consecutive sheets followed by a 7-second pause). Printing of 60000 sheets and that of 100000 sheets are performed. Note that the printing of 60000 sheets is the rated lifetime of endless belt  16 . 
     Table 1 shows the results of assessing and determining the durability of endless belt  16 . 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 Durability 
                 Durability 
                   
               
               
                   
                   
                 Assessment 
                 Assessment 
                   
               
               
                   
                 θ (°) 
                 60000 (sheet) 
                 100000 (sheet) 
                 Determination 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Example 1 
                 75 
                 12000 (sheet) 
                 — 
                 x 
               
               
                   
                   
                 broken 
                   
                   
               
               
                 Example 2 
                 80 
                 24000 (sheet) 
                 — 
                 x 
               
               
                   
                   
                 broken 
                   
                   
               
               
                 Example 3 
                 85 
                 45000 (sheet) 
                 — 
                 x 
               
               
                   
                   
                 broken 
                   
                   
               
               
                 Example 4 
                 93 
                 60000 (sheet) 
                 100000 (sheet) 
                 Δ 
               
               
                   
                   
                 good 
                 warpage 
                   
               
               
                 Example 5 
                 95 
                 60000 (sheet) 
                 100000 (sheet) 
                 ∘ 
               
               
                   
                   
                 good 
                 good 
                   
               
               
                 Example 6 
                 100 
                 60000 (sheet) 
                 100000 (sheet) 
                 ∘ 
               
               
                   
                   
                 good 
                 good 
                   
               
               
                 Example 7 
                 105 
                 60000 (sheet) 
                 100000 (sheet) 
                 ∘ 
               
               
                   
                   
                 good 
                 good 
                   
               
               
                 Example 8 
                 110 
                 60000 (sheet) 
                 100000 (sheet) 
                 Δ 
               
               
                   
                   
                 good 
                 warpage 
                   
               
               
                 Example 9 
                 115 
                 60000 (sheet) 
                 100000 (sheet) 
                 Δ 
               
               
                   
                   
                 good 
                 warpage 
               
               
                   
               
            
           
         
       
     
     In Table 1, the determination result ∘ means that even after the printing of 60000 sheets and that of 100000 sheets, endless belt  16  is not broken and endless belt  16  is still in good condition. The determination result Δ means that minor warpage occurs in the printing of 100000 sheets but no warpage occurs until the printing of 60000 sheets is finished, that is, the state of endless belt  16  has no practical problem. The determination result x means that endless belt  16  is broken before the printing of 60000 sheets is finished. 
     Table 1 shows that if the angle θ is not smaller than 93° but not larger than 115°, or preferably not smaller than 95° but not larger than 105°, durability of endless belt  16  can be improved. 
     As the angle θ becomes smaller, the lateral pressure acting on the edge portion of endless belt  16  when the edge portion is in contact with pulley  31  becomes larger, and the shear stress generated in endless belt  16  becomes larger. Consequently, material failure of endless belt  16  becomes more likely to occur. 
     In addition, as the angle θ becomes smaller, the edge portion of endless belt  16  receives a larger force directed outward in the radial direction of belt receiving surface sa at the position where endless belt  16  reaches idle roller  14 , and a larger force directed inward in the radial direction of belt receiving surface sa at the position where endless belt  16  leaves idle roller  14 . Consequently, flexures becomes more likely to occur in endless belt  16 , and fatigue failure of endless belt  16  becomes more likely to occur. 
     In contrast, as the angle θ becomes larger, the contact between endless belt  16  and belt receiving surface sa becomes less uniform, so that warpage becomes more likely to occur in the edge portion of endless belt  16 . 
     Accordingly, in this embodiment, the angle θ is not smaller than 93° but not larger than 115°, or preferably not smaller than 95° but not larger than 105°. 
     In this embodiment, the lateral pressure acting on the edge portion of endless belt  16  when the edge portion is brought into contact with pulley  31  is reduced, and the shear stress generated in endless belt  16  is reduced. Accordingly, material failure of endless belt  16  is prevented. 
     In addition, of the entire belt receiving surface sa, the edge portion of endless belt  16  is in contact only with the portion that is adjacent to core portion  154 . Accordingly, the edge portion of endless belt  16  can receive a smaller force directed outward in the radial direction of belt receiving surface sa at the position where endless belt  16  reaches idle roller  14 , and a smaller force directed inward in the radial direction of belt receiving surface sa at the position where endless belt  16  leaves idle roller  14 . Consequently, no flexure occurs in endless belt  16 , so that fatigue failure of endless belt  16  is prevented. 
     In addition, endless belt  16  can be brought into uniform contact with belt receiving surface sa. Accordingly, warpage can be prevented from occurring in the edge portion of endless belt  16 . 
     Consequently, the durability of endless belt  16  is improved. 
       FIG. 7  is a diagram illustrating a pulley according to a modified example of the first embodiment of the invention. 
     Pulley  31  of this example includes conically-shaped belt receiving portion  155 . The diameter of belt receiving portion  155  increases at a position farther away from the smallest-diameter portion that has the same diameter as that of roller main body  151 . 
     Note that the surface of belt receiving portion  155  is brought into contact with the edge portion of endless belt  16 . The contact surface is referred to as belt receiving surface sa. The angle that belt receiving surface sa makes with the axis of idle roller  14  is denoted by θ (since the axis and the surface of roller main body  151  is parallel to each other, the angle θ is shown in  FIG. 7  as the angle that belt receiving surface sa makes with the surface of roller main body  151 ). 
     Also in this case, the angle θ is not smaller than 93° but not larger than 115°, or preferably not smaller than 95° but not larger than 105°. 
     Next, description is given of a second embodiment of the invention where pulley  31  is provided in the transferring unit. Note that, portions that have identical structures to those in the first embodiment are denoted by the same reference numerals that are used in the first embodiment. The effects of the first embodiment are incorporated in the second embodiment as to the effects of the invention attributable to such identical structures. 
       FIG. 8  is a conceptual diagram illustrating a printer according to the second embodiment of the invention.  FIG. 9  is a conceptual diagram illustrating the transferring unit according to the second embodiment of the invention. 
     In this embodiment, belt-type transferring unit  12  is provided so as to be opposed to photosensitive drums  65  serving as image carriers of image forming units  61 Bk,  61 Y,  61 M, and  61 C, respectively. 
     Transferring unit  12  includes a motor, drive roller  13 , idle roller  14 , tension roller  88 , endless belt  16 , transfer roller  89 , cleaning blade  18 , and the like. The motor serves as the driving unit for image transfer. Drive roller  13  serves as a first roller which is coupled to the motor and which is made to rotate by the rotation of the motor. Idle roller  14  serves as a second roller which is driven to rotate by the rotation of drive roller  13 . Tension roller  88  serves as a third roller which is driven to rotate by the rotation of drive roller  13 . Endless belt  16  is a belt member stretched by drive roller  13 , idle roller  14 , and tension roller  88 , and made to run in the direction indicated by the arrow. Endless belt  16  serves also as a transfer belt (intermediate transferring body). Transfer roller  89  serves as a transferring-position material rotatably provided outside of looped endless belt  16  so as to be opposed to tension roller  88 . Cleaning blade  18  serves as a second cleaning member which is provided in the vicinity of idle roller  14  so as to be in contact with the outside surface of endless belt  16 . Tension roller  88 , together with transfer roller  89 , is capable of moving endless belt  16  in a direction (outwards) so as to separate endless belt  16  away from image forming units  61 Bk,  61 Y,  61 M, and  61 C. Tension roller  88  gives endless belt  16  certain tension corresponding to the moving distance. Paper sheet P serving as a medium is conveyed between endless belt  16  and transfer roller  89 . 
     First transferring positions are formed between endless belt  16  and each of photosensitive drums  65  serving as image carriers of image forming units  61 Bk,  61 Y,  61 M, and  61 C. A second transferring position is formed between endless belt  16  and transfer roller  89 . 
     In image forming units  61 Bk,  61 Y,  61 M, and  61 C, toner images formed as developer images of those colors on their respective photosensitive drums  65  are consecutively transferred one upon another at their respective first transferring positions. Thus a color toner image is formed on endless belt  16 . While endless belt  16  is running, the color toner image is sent to the second transferring position, where the color toner image is transferred onto paper sheet P. 
     Pulley  31  serving as a snaking-restraint member or as a rotating member is provided at least at one end of idle roller  14  to prevent the snaking of endless belt  16 . 
     If endless belt  16  moves in the crosstrack direction, and if the edge portion is brought into contact with pulley  31 , the edge portion receives a lateral pressure, that is, a certain pressure caused by the reaction force of pulley  31 . In this event, if there are microscopic asperities on the contact surface, that is, on belt receiving surface sa, the edge portion receives an nonuniform lateral pressure. In addition, when endless belt  16  is manufactured by cutting the raw pipe for the belt so that each piece has a width of 228±0.5 mm, microscopic asperities are formed in the edge portion of the piece. As the number of the asperities increases, stress concentration occurs and endless belt  16  is more likely to be dragged by pulley  31  while endless belt  16  slides on pulley  31 . Accordingly, a larger shear stress occurs in endless belt  16 , and material failure of endless belt  16  occurs. 
     Next, description is given of a third embodiment of the invention that can prevent, more effectively, the material failure of endless belt  16 . Note that, portions that have identical structures to those in the first and the second embodiments are denoted by the same reference numerals that are used in the first and the second embodiments. The effects of the first and the second embodiments are incorporated in the third embodiment as to the effects of the invention attributable to such identical structures. 
       FIG. 10  is a conceptual diagram illustrating a pulley according to the third embodiment of the invention. 
     In this embodiment, belt receiving surface sa serving as the contact surface is coated with a fluorine coating material to form sliding-friction reducing film  156  that reduces the friction with the edge portion of endless belt  16 . The fluorine coating material is made of a fluorine containing material having both a low surface energy and a small friction coefficient, being UV-curable, and containing perfluoroalkyl group. The coating can be done first by applying the material with a spray gun to form a thin film and then by irradiating the thin-film coating material with ultraviolet rays with a UV irradiator to cure the coating material. 
     Some of the materials to be used as the fluorine containing material are tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), polytetrafluoroethylene (PTFE), and fluoroethylene vinyl ether polymer (FEVE). 
     The durability of endless belt  16  serving as a belt member such as a transfer belt is assessed and determined by varying the angle θ made by belt receiving surface sa on which sliding-friction reducing film  156  is formed with the axis of idle roller  14  serving as the second roller. The assessment and determination are done under the same assessment conditions and by the same determination method as those in the first embodiment. TRIBOGEAR14FV manufactured by Shinto Scientific Co., Ltd. is used to measure the friction coefficient. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                 Durability 
                 Durability 
                   
               
               
                   
                   
                 Assessment 
                 Assessment 
                   
               
               
                   
                 θ (°) 
                 60000 (sheet) 
                 100000 (sheet) 
                 Determination 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Example 1 
                 85 
                 60000 (sheet) 
                  82000 (sheet) 
                 x 
               
               
                   
                   
                 good 
                 broken 
                   
               
               
                 Example 2 
                 93 
                 60000 (sheet) 
                 100000 (sheet) 
                 ∘ 
               
               
                   
                   
                 good 
                 good 
                   
               
               
                 Example 3 
                 95 
                 60000 (sheet) 
                 100000 (sheet) 
                 ∘ 
               
               
                   
                   
                 good 
                 good 
                   
               
               
                 Example 4 
                 100 
                 60000 (sheet) 
                 100000 (sheet) 
                 ∘ 
               
               
                   
                   
                 good 
                 good 
                   
               
               
                 Example 5 
                 105 
                 60000 (sheet) 
                 100000 (sheet) 
                 ∘ 
               
               
                   
                   
                 good 
                 good 
                   
               
               
                 Example 6 
                 110 
                 60000 (sheet) 
                 100000 (sheet) 
                 ∘ 
               
               
                   
                   
                 good 
                 good 
                   
               
               
                 Example 7 
                 115 
                 60000 (sheet) 
                 100000 (sheet) 
                 ∘ 
               
               
                   
                   
                 good 
                 good 
                   
               
               
                   
               
            
           
         
       
     
     Table 2 shows that when sliding-friction reducing film  156  is formed on belt receiving surface sa, the setting of the angle θ not smaller than 93° but not larger than 115° can improve the durability of endless belt  16 . 
     Since sliding-friction reducing film  156  formed on belt receiving surface sa coats the microscopic asperities on belt receiving surface sa and has a small friction coefficient, the shear stress generated in endless belt  16  is reduced. 
     In this embodiment, since sliding-friction reducing film  156  is formed on belt receiving surface sa, the durability of endless belt  16  is improved furthermore. 
     Next, a fourth embodiment of the invention is described. Note that, portions that have identical structures to those in the first to the third embodiments are denoted by the same reference numerals that are used in the first to the third embodiments. The effects of the first to the third embodiments are incorporated in the fourth embodiment as to the effects of the invention attributable to such identical structures. 
       FIG. 11  is a conceptual diagram illustrating a printer according to the fourth embodiment of the invention. 
     In this embodiment, paper-sheet cassette  64  serving as a media container is provided detachably from apparatus main body  101  of printer  60 , and paper sheets P as the media are stored in paper-sheet cassette  64 . Media stacking plate  103  is provided in paper-sheet cassette  64  so as to be swingable (pivotable) about swing shaft sh 1 , and paper sheets P are stacked on media stacking plate  103 . 
     On the sheet-sending side, or in the front-end portion, of paper feeding tray  64 , lift-up lever  104  is swingably provided about swing shaft sh 2  that is coupled to motor  105  serving as the driving unit for media stacking. Swing shaft sh 2  is capable of engaging with or disengaging from motor  105  freely. When paper feeding tray  64  is set in apparatus main body  101 , lift-up lever  104  and motor  105  are engaged with each other. 
     If an unillustrated controller drives motor  105 , lift-up lever  104  moves pivotally, the leading end of lift-up lever  104  hits the floor of media stacking plate  103  to raise the front end of media stacking plate  103 , and the front end of paper sheet P stacked on media stacking plate  103  is lifted up. Once the front end of paper sheet P is lifted up to a predetermined height, lift-up detector  106  detects paper sheet P and sends a detection signal to the controller. Upon receiving the detection signal, the controller stops motor  105  to stop the pivotal movement of lift-up lever  104 . 
     In addition, media reeling-up unit  110  configured to feed paper sheets P by reeling up paper sheets P one by one is provided in the front-end portion of paper feeding tray  64 . Media reeling-up unit  110  includes pick-up roller  111 , feed roller  112 , and retard roller  113 . Pick-up roller  111  is provided so as to be in pressure contact with the front end of paper sheet P that has been raised up to the predetermined height. Pick-up roller  111  serves as a reel-up roller included in a reel-up member to reel-up paper sheets P. Feed roller  112  serves as a feed element provided to separate each of paper sheets P that have been reeled up by pick-up roller  111  from the others. Retard roller  113  serves as a retard element. Feed roller  112  and retard roller  113  form a separator apparatus. Media reeling-up unit  110  includes media-existence detector  114  and remaining-media detector  115 . Media-existence detector  114  is adjacent to lift-up detector  106  and detects whether there is or is not any paper sheet P. Remaining-media detector  115  is provided below lift-up detector  106  by a certain distance, and detects the amount of remaining paper sheets P. 
     Each paper sheet P having been reeled up by media reeling-up unit  110  and then separated from the others by feed roller  112  and retard roller  113  is sent to the conveyor path of first media conveyor unit  120 . In first media conveyor unit  120 , paper sheet P passes by media sensor  121  serving as a first media detector. After the leading end of paper sheet P is detected by media sensor  121 , paper sheet P is sent to conveyor-roller pair  122  serving as a first roller pair including rollers r 1  and r 2 . Upon detecting the leading end of paper sheet P, media sensor  121  sends a detection signal to the controller. Conveyor-roller pair  122  is driven to rotate by an unillustrated register motor serving as a first driving unit. 
     Subsequently, the paper sheet passes through conveyor-roller pair  122 , and then passes by entrance sensor  123  serving as a second media detector. After entrance sensor  123  detects the leading end of paper sheet P, paper sheet P is then sent to register-roller pair  124  serving as a second roller pair including rollers r 3  and r 4 . Register-roller pair  124  corrects the skew of paper sheet P. 
     Having passed through register-roller pair  124 , paper sheet P passes by print sensor  125  serving as a third media detector. After print sensor  125  detects the leading end of paper sheet P, paper sheet P is sent to image forming unit  100 . 
     Note that, entrance sensor  123  is provided near register-roller pair  124  and upstream of register-roller pair  124  in the conveying direction of paper sheet P. Print sensor  125  is provided near register-roller pair  124  and downstream of register-roller pair  124  in the conveying direction of paper sheet P. Upon detecting the leading end of paper sheet P, each of entrance sensor  123  and print sensor  125  sends a detection signal to the controller. 
     Image forming unit  100  includes: four image forming units  61 Y,  61 M,  61 C, and  61 Bk that are arranged in series. In addition, image forming unit  100  includes belt-type transferring unit  12 , LED heads  69 , and the like. Belt-type transferring unit  12  is provided so as to be opposed to photosensitive drums  65  serving respectively as image carriers of image forming units  61 Y,  61 M,  61 C, and  61 Bk. Transferring positions are formed between belt-type transferring unit  12  and each of photosensitive drums  65 . Toner images that are formed on photosensitive drums  65  as the developer images of their respective colors are transferred one after another onto paper sheet P as the medium to form a color toner image. LED heads  69  are provided so as to be opposed respectively to photosensitive drums  65 , and serve as exposure apparatus configured to expose the surfaces of their respective photosensitive drums  65  to form electrostatic latent images as latent images. 
     Transferring unit  12  includes a motor, drive roller  13 , idle roller  14 , endless belt  16 , transfer rollers  75 , cleaning blade  18 , toner box  19 , and the like. The motor serves as the driving unit for image transfer. Drive roller  13  serves as a first roller which is coupled to the motor and which is made to rotate by the rotation of the motor. Idle roller  14  serves as a second roller which is driven to rotate by the rotation of drive roller  13 . Endless belt  16  is a belt member stretched by drive roller  13  and idle roller  14 , being capable of running freely, and serving also as a transfer belt. Transfer rollers  75  serve as transferring-position materials that are rotatably provided inside of looped endless belt  16  so as to be opposed respectively to photosensitive drums  65 . Cleaning blade  18  serves as a second cleaning member which is provided in the vicinity of idle roller  14  so as to be in contact with the outside surface of endless belt  16 . Toner box  19  serves as a developer storage where the toner as developer scraped off by cleaning blade  18  are deposited and stored. 
     Tension is applied to endless belt  16  by a tension providing apparatus. If drive roller  13  is made to rotate, idle roller  14  is driven to rotate. Transferring unit  12 , the tension providing apparatus, and the like together form the belt drive apparatus configured to drive endless belt  16 . 
     The operation of image forming units  61 Y,  61 M,  61 C, and  61 Bk are synchronized with the motion of endless belt  16 . Toner images of those colors are transferred, one after another, onto paper sheet P on endless belt  16  to form a color toner image. Paper sheet P with color toner image formed in this way is sent to fixing unit  80  serving as a fixing apparatus. 
     Fixing unit  80  includes heating roller  83  serving as a first rotating body and pressing roller  84  serving as a second rotating body. In fixing unit  80 , color toner image on paper sheet P sent from image forming unit  100  is heated and pressurized to be melted, and thereby the color toner image is fixed on paper sheet P. After that, paper sheet P sent from fixing unit  80  then reaches separator  126  serving as a conveyance switcher configured to switch the discharging direction of each paper sheet P between an upward path and a straight path. Discharge-roller pairs  130  that are provided at predetermined plural positions on the conveyor passage are used to discharge paper sheet P. Paper sheet P is discharged onto stacker portion  131  formed in the top surface of apparatus main body  101  if the discharging direction is upward. If the discharging direction is straight, paper sheet P is discharged onto an unillustrated rear tray formed in a side surface of apparatus main body  101 . 
     Next, the belt drive apparatus is described. 
       FIG. 12  is a conceptual diagram illustrating the transfer unit according to the fourth embodiment of the invention.  FIG. 13  is a conceptual diagram illustrating a pulley according to the fourth embodiment of the invention. 
     As  FIG. 12  shows, the belt drive apparatus includes belt frame  90 , drive roller  13 , idle roller  14 , endless belt  16 , bearing unit  92 , pulley  31 , roller tilting lever  95 , and the like. Belt frame  90  serves as a supporting case. Bearing unit  92  serves as a supporting apparatus configured to support idle roller  14  so that idle roller  14  can rotate relative to belt frame  90  about axis sha. Pulley  31  is a rotating member and serves as a snaking-restraint member. Pulley  31  is provided at least at one end of idle roller  14 . In this embodiment, pulley  31  is provided at one end of idle roller  14 . Roller tilting lever  95  is provided between pulley  31  and bearing unit  92  so as to be adjacent to pulley  31 . Roller tilting lever  95  serves as a shaft-end-position changing apparatus configured to tilt axis sha. 
     Drive roller  13  is rotatably supported by belt frame  90 , and the surface of drive roller  13  is made of a material with a high friction coefficient. Gear  91  serving as a rotation transmitting element is provided on one end of drive roller  13 . When the motor is driven, the rotation is transmitted to drive roller  13  via gear  91 , and thereby drive roller  13  is made to rotate. 
     Idle roller  14  includes: roller main body  171 , which is the main body of idle roller  14 ; and rotary shafts  172 , which serve as the supporting shafts formed so as to extend along axis sha of idle roller  14  and to protrude respectively from the two ends of roller main body  171 . 
     Endless belt  16  is made of a belt of such a resin as polyamide-imide, and is formed to have a thickness of 0.1 mm. 
     Bearing unit  92  includes swingable supporting plate  181 , bearing  183 , spring  93 , and the like. Swingable supporting plate  181  is swingable relative to belt frame  90  about axis shb. Slot  182  is formed in swingable supporting plate  181 . Bearing  183  is capable of sliding freely in slot  182 , and rotatably supports rotary shaft  172 . Spring  93  is installed in slot  182 , and serves as an urging member to urge bearing  183  in a direction away from drive roller  13 . Spring  93  forms the tension providing apparatus, and the urging force of spring  93  generates a tension. 
     Pulley  31  is made to rotate on rotary shaft  172  together with idle roller  14 , and is movable freely relative to roller main body  171  in the axial direction of idle roller  14 . Roller tilting lever  95  is adjacent to pulley  31 , and is capable of pivoting about axis she that tilts from axis sha. In addition, roller tilting lever  95  holds rotary shaft  172 . To this end, hole  185  is formed in roller tilting lever  95 , and allows rotary shaft  172  to pass therethrough to hold rotary shaft  172 . 
     Pulley  31  includes core portion  154  and belt receiving portion  155 . Core portion  154  has the same diameter d 1  as that of roller main body  171 . Belt receiving portion  155  is formed so as to be adjacent to core portion  154  and to form a single unit with core portion  154 . Belt receiving portion  155  has a conical shape, and serves as a flange portion with a larger outer diameter d 2  than that of roller main body  171 . The diameter of belt receiving portion  155  increases at a position farther away from core portion  154 . In this embodiment the diameter d 1  is 24.6 mm whereas the diameter d 2  is 28 mm. 
     Next, the operations of the belt drive apparatus with the above configuration are described. 
     First, when the rotation of the motor is transmitted via gear  91  to drive roller  13  to make drive roller  13  rotate, endless belt  16  starts running along with the rotation of drive roller  13 . If endless belt  16  moves outwards (downwards in  FIG. 12 ) to bring the edge portion of endless belt  16  into contact with pulley  31 , pulley  31  is pushed by endless belt  16  and moves outwards to push roller tilting lever  95 . Then, roller tilting lever  95  rotates along a circle with axis shc as the center, and tilts axis sha of idle roller  14  to move upwards the end portion on pulley  31  side of idle roller  14 . Consequently, the outward movement of endless belt  16  is restricted. 
     If, in contrast, endless belt  16  moves inwards (upwards in  FIG. 12 ), pulley  31  moves inwards together with roller tilting lever  95 . Then, roller tilting lever  95  rotates along a circle with axis shc as the center, and tilts axis sha of idle roller  14  to move downwards the end portion on pulley  31  side of idle roller  14 . Consequently, the inward movement of endless belt  16  is restricted. 
     Belt receiving surface sa is the contact surface in belt receiving portion  155  of pulley  31 . The angle that belt receiving surface sa makes with axis sha of idle roller  14  is denoted by θ (since axis sha and the surface of core portion  154 , and axis sha and the surface of roller main body  171  are parallel to each other, the angle θ is shown in  FIG. 13  as the angle that belt receiving surface sa makes with the surface of core portion  154  and the surface of roller main body  171 ). The angle θ is not smaller than 93° but not larger than 115°, or preferably not smaller than 95° but not larger than 105°. In this embodiment, the angle θ is 100°. 
     In this embodiment, the lateral pressure acting on the edge portion of endless belt  16  when the edge portion is brought into contact with pulley  31  is reduced, and the shear stress generated in endless belt  16  is reduced. Accordingly, material failure of endless belt  16  is prevented. 
     In addition, of the entire belt receiving surface sa, the edge portion of endless belt  16  is in contact only with the portion that is adjacent to core portion  154 . Accordingly, the edge portion of endless belt  16  can receive a smaller force directed outward in the radial direction of belt receiving surface sa at the position where endless belt  16  reaches idle roller  14 , and a smaller force directed inward in the radial direction of belt receiving surface sa at the position where endless belt  16  leaves idle roller  14 . Consequently, no flexure occurs in endless belt  16 , so that fatigue failure of endless belt  16  is prevented. 
     In addition, endless belt  16  can be brought into uniform contact with belt receiving surface sa. Accordingly, warpage is prevented from occurring in the edge portion of endless belt  16 . 
     Consequently, the durability of endless belt  16  is improved. 
     Next, a fifth embodiment of the invention is described. Note that, portions that have identical structures to those in the first to the fourth embodiments are denoted by the same reference numerals that are used in the first to the fourth embodiments. The effects of the first to the fourth embodiments are incorporated in the fifth embodiment as to the effects of the invention attributable to such identical structures. 
       FIG. 14  is a conceptual diagram illustrating a pulley according to the fifth embodiment of the invention.  FIG. 15  is an enlarged diagram illustrating a principal portion of the pulley according to the fifth embodiment of the invention. 
     In this embodiment, pulley  31  serving as a snaking-restraint member or as a rotating member includes core portion  154  and belt receiving portion  155 . Core portion  154  has the same diameter d 1  as that of roller main body  171  ( FIG. 12 ). Belt receiving portion  155  is formed so as to be adjacent to core portion  154  and to form a single unit with core portion  154 . Belt receiving portion  155  has an approximately conical shape, and serves as a flange portion with a larger outer diameter d 2  than that of roller main body  171 . The diameter of belt receiving portion  155  increases at a position farther away from core portion  154 . In this embodiment, the diameter d 1  is 24.6 mm whereas the diameter d 2  is 28 mm. 
     The angle θ that belt receiving surface sa of belt receiving portion  155  of pulley  31  makes with axis sha of idle roller  14  serving as a second roller increases at a position farther away from core portion  154  and with a larger diameter. 
     In this embodiment, belt receiving surface sa includes first receiving surface sb with a smaller diameter and second receiving surface sc with a larger diameter. First receiving surface sb extends from point p 1  which is on belt receiving surface sa and which is adjacent to core portion  154 , to point p 2  which is away from point p 1  by a predetermined distance dx (in this embodiment dx=0.1 mm) in the axial direction. Second receiving surface sc extends from point p 2  to point p 3  which is a point on the outer perimeter of belt receiving portion  155 . Note that, of all the points on the outer perimeter of belt receiving portion  155 , the point that is the farthest from point p 1  is referred to as point p 4 . 
     The angle that first receiving surface sb makes with axis sha of idle roller  14  is referred to as the angle θ 1  whereas the angle that second receiving surface sb makes with axis sha of idle roller  14  is referred to as the angle θ 2 . The angle θ 1  is not smaller than 93° but not larger than 115°, or preferably not smaller than 95° but not larger than 105°. The angle θ 2  is larger than the angle θ 1 . In this embodiment, the angle θ 1  is 96° and the angle θ 2  is 135°. 
     Accordingly, the lateral pressure that the edge portion of endless belt  16  receives when the edge portion is brought into contact with pulley  31  is reduced and the shear stress generated in endless belt  16  is reduced. Consequently, material failure of endless belt  16  is prevented. 
     In addition, since second receiving surface sc is formed outside, in the radial direction, of first receiving surface sb, the edge portion of endless belt  16  is brought into contact with second receiving surface sc at a shallow angle at the position where endless belt  16  reaches idle roller  14 , and then endless belt  16  is guided towards first receiving surface sb to stay near point p 1 . 
     Accordingly, even if belt receiving surface sa tilts from central axis sha due to manufacture error and/or assemble error, endless belt  16  is guided from second receiving surface sc to first receiving surface sb, so that the snaking of endless belt  16  is reliably prevented. 
     In addition, at the position where endless belt  16  reaches idle roller  14 , the edge portion of endless belt  16  is brought into contact with second receiving surface sc at a shallow angle. Accordingly, no flexure occurs in endless belt  16 . Consequently, fatigue failure of endless belt  16  is prevented. 
     In addition, endless belt  16  can be brought into uniform contact with belt receiving surface sa. Accordingly, warpage is prevented from occurring in the edge portion of endless belt  16 . 
     Consequently, the durability of endless belt  16  is improved. 
     Note that, the foregoing description of the invention is based on an example of an electrophotographic printer, but the invention is applicable not only to printers but also to multifunction printers, fax machines, and the like. 
     In addition, the invention is applicable also to such endless belts as photosensitive belts, fixing belts, conveyor belts, and the like. 
     Note that, the invention is not limited to the foregoing embodiments. Various modifications can be made on the basis of the gist of the invention. Such modifications should not be excluded from the scope of the invention. 
     The invention includes other embodiments in addition to the above-described embodiments without departing from the spirit of the invention. The embodiments are to be considered in all respects as illustrative, and not restrictive. The scope of the invention is indicated by the appended claims rather than by the foregoing description. Hence, all configurations including the meaning and range within equivalent arrangements of the claims are intended to be embraced in the invention.