Patent Publication Number: US-6340156-B1

Title: Sheet-transporting device and image-forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sheet-transporting device provided in various sheet-employing apparatuses, such as image-forming apparatuses and other office equipment. In such image-forming apparatuses, an appropriate transfer-type image-forming process (such as an electrophotographic-type image-forming process) or a direct-type image-forming process is used. Examples of the image-forming apparatus include printers, facsimile machines, and copying machines. 
     2. Description of the Related Art 
     FIG. 7 is a schematic structural view of a conventional image-forming apparatus comprising a sheet-feeding section and a register section which temporarily stops the leading edge of a recording sheet fed from the sheet-feeding section, forms a loop in the recording sheet to correct any oblique movement of the recording sheet, and sends the recording sheet to an image-forming section at a predetermined timing. The conventional image-forming apparatus roughly comprises an image-input section  90 R and an image-output section  90 P. 
     The image-input section  90 R is, for example, an image information photo-reader mechanism. Image information to be recorded is input to the image-input section  90 R, and variously processed. Then, an image signal carrying the image information is sent to the image-output section  90 P. The image-input section  90 R may be an optical device for exposing an image-carrying member of an image-forming section  91  of the image-output section  90 P with light carrying an image of an original by projecting and focusing the light thereon. 
     The image-output section  90 P is a transfer-type image-recording mechanism using a transfer drum  93 . It comprises the image-forming section  91 ; first to third sheet-feeding units  92 A to  92 C, serving as sheet-feeding sections; a sheet-transporting section  97  disposed in front of a register roller pair  96  serving as a register section; a transfer drum  93 ; a fixing unit  94 ; and a control unit  95 . The image-forming section  91  is used to form a transferable image, such as a toner image, on an image-carrying member, such as a photoelectric member, a dielectric member, or a magnetic member, by a suitable image-forming process, such as the electrophotographic-type image-forming process, an electrostatic-recording process, or a magnetic-recording process, in accordance with the image information input to the image-output section  90 P. The sheet-transporting section  97  transports a recording sheet P fed from any one of the sheet-feeding units  92 A to  92 C to the register roller pair  96 . The transfer drum  93  holds the recording sheet P transported from the register roller pair  96  and transports it to a transfer section T of the image-forming section  91 . The fixing unit  94  fixes the image to the recording sheet P onto which the image has been transfer at the transfer section T of the image-forming section  91 . The control unit  95  controls all of the operations of the image-forming apparatus. 
     The first and the second sheet-feeding units  92 A and  92 B are incorporated in the image-output section  90 P as top and bottom mechanisms for automatically separating sheets one by one for feeding. Starting from the topmost recording sheet, the recording sheets P loaded in and held by a sheet-feed cassette are fed one sheet at a time as a result of cooperation of a corresponding sheet-feed roller and a corresponding separating member based on a sheet-feed start signal. The third sheet-feeding unit  92 C is a manual sheet-feeding device. When it is used, a recording sheet P is inserted into the image-forming apparatus from a manual sheet-feed table, and is drawn into the image-forming apparatus in order to be fed to transfer section T. 
     When the image-forming apparatus starts to operate, a recording sheet P is fed from any one of the first to third sheet-feeding units  92 A to  92 C, and transported to the register roller pair  96  through the transporting section  97  by transporting roller pairs  98 A,  98 B, and  98 C, and a curling roller set  99 . The transporting roller pairs  98 A to  98 C and the curling roller set  99  can rotate in two different of rotational modes, or at two transporting speeds. In one mode, they rotate at ordinary transporting speeds. In the other mode, they rotate at speeds greater than the ordinary transporting speeds. 
     At the moment the edge of the recording sheet P fed from any one of the sheet-feeding units  92 A to  92 C is nipped by the corresponding first transporting roller pair (the transporting roller pair  98 A if the sheet has been fed from the first sheet-feeding unit  92 A; the transporting roller pair  98 B if the sheet has been fed from the second sheet-feeding unit  92 B; and the curling roller set  99  if the sheet has been fed from the third sheet-feeding unit  92 C) rotating at the ordinary transporting speed, the ordinary transporting speeds of the pairs of transporting rollers  98 A and  98 B and the pair of curling rollers  99  are switched to the higher transporting speeds in order to transport the sheet to the register roller pair  96  at a high speed. By this speed-controlling operation, the time from the start of the sheet-feeding operation to the start of the image-forming operation can be reduced compared to that when the recording sheet is transported only at the ordinary transporting speed. The rotational speed of each roller is changed by switching the rotational speed of a corresponding motor (not shown), such as a stepping motor. Each motor is directly connected to its corresponding roller. For example, when the transporting speed is to be quadrupled from 150 mm/s to 600 mm/s, the number of rotations of the driving motor of each roller is increased by a factor of  4 . In order to reduce loss of transporting speed, the speed switching is carried out in a short time period, that is, a time period of the order of 100 ms. 
     FIG. 4 illustrates the curling roller set  99  in detail. The pair  99  comprises two upper rollers  99 A and  99 B and one lower roller  99 C. The two upper rollers  99 A and  99 B are pressed against the lower roller  99 C by a spring or other biasing means (not shown). As shown in FIG. 4, when the recording sheet P passes between the two upper rollers  99 A and  99 B and the lower roller  99 C, the recording sheet P is curled upward into a convex shape. When the recording sheet P is curled, the recording sheet P is more easily attracted to the transfer drum  93 . 
     However, since the one lower roller  99 C is pressed by the two upper rollers  99 A and  99 B, the driving torque of the curling roller set  99  is larger than those of the transporting roller pairs  98 A and  98 B. For example, while the driving torques of these transporting roller pairs  98 A and  98 B are of the order of 1 kgf·cm, the driving torque of the curling roller set  99  is of the order of 2.5 kgf·cm. 
     An image corresponding to relevant image information is formed on the image-carrying member in the image-forming section  91 . In synchronism with an image-recording start signal, the register roller pair  96  starts to rotate, and the transporting section  97  returns to its recording-sheet-transporting state. The recording sheet P is transported from the register roller pair  96  to the rotating transfer drum  93 , and held by the outer face of the rotating transfer drum  93  in order to be transported to the transfer section T. This causes the portions of the image formed on the image-carrying member of the image-forming section  91  to be successively transferred onto the recording sheet P. 
     There are several methods by which the recording sheet P can be held by the outer face of the transfer drum  93 . The methods include electrostatic suction and air suction, towards a transporting medium, such as a thin dielectric film. 
     The timing at which the recording sheet P is transported by the register roller pair  96  is controlled such that when an edge of the image carried by the image-carrying member of the image-forming section  91  reaches the transfer section T, an edge of the recording sheet P is exactly at the transfer section T, whereby they are aligned (or subjected to registration). 
     The recording sheet P to which the image has been transferred at the transfer section T is separated from the outer face of the rotating transfer drum  93  and is transported to the fixing unit  94 . The image on the recording sheet P is fixed thereto by heat and/or pressure, after which the recording sheet P passes through a sheet-discharge section and is discharged to a sheet-discharge tray disposed outside the image-forming apparatus. At the fixing unit  94 , oil is applied to the recording sheet P in order to prevent it from being wound around the fixing rollers. 
     However, the conventional image-forming apparatus in which the rotational speeds of the sheet-transporting systems are controlled so that the rotational speeds are switched between two speeds has the following problems. 
     Since the rotational speeds of the transporting roller pairs  98 A and  98 B, and the curling roller set  99  are switched from low speeds to high speeds in about 100 ms, a very short time, large torques due to acceleration are produced during this short time. This may cause the rollers  99 A and  99 B, and the curling rollers  99  to rotate improperly due to large motor loads, or may cause sheet jamming resulting from the improper rotation. Since the inertial moments of the roller pairs  98 A,  98 B, and the curling roller set  99  are of the order of 2 kg·cm 2 , acceleration torques in the order of 1.2 kgf·cm are produced when an attempt is made to quadruple the transporting speeds from 150 mm/s to 600 mm/s in the switching time of the order of 100 ms. 
     As mentioned above, the driving torques of the transporting roller pairs  98 A and  98 B are in the order of 1 kgf·cm, and of the curling roller set  99  is in the order of 2.5 kgf·cm. At the moment the speeds are switched, the aforementioned acceleration torques are added to the torque values corresponding thereto, so that the required driving torques of the transporting roller pairs  98 A and  98 B are 2.2 kgf·cm, while the required driving torque of the roller set  99  is 3.7 kgf·cm. Therefore, the driving motor used to rotationally drive the curling roller set  99  needs to be larger and to have a larger output than the driving motors used to rotationally drive the transporting roller pairs  98 A and  98 B. 
     However, the high-output, large motor used to rotationally drive the curling roller set  99  is more costly than the motors used to rotationally drive the transporting roller pairs  98 A and  99 B. In addition, this high-output, large motor is used at only one location, so that failures tend to occur when purchasing parts or during assembly. Further, it takes up a lot of space, thereby increasing the size of the image-forming apparatus. 
     One possible way to overcome the above-described problems is by replacing each member rotationally supporting each curling roller with a driving-load-reducing member, such as a bearing. This reduces the curling roller loads to a level equal to those on the transporting rollers. 
     However, since bearings are expensive, mounting driving-load-reducing members results in an increase in total costs. As shown in FIG. 4, the two upper rollers  99 A and  99 B are relatively small-diameter rollers and are spatially disposed close together. Therefore, when bearings or the like are inserted into the structure of the curling roller set  99 , the structure becomes complicated. 
     Another way to overcome the above-described problems is by prolonging the time of switching the rotational speeds of the transporting roller pairs  98 A and  98 B, and the curling roller set  99  to, for example, a time in the order of 500 ms, as shown in FIG.  5 . There is a discussion related to this method in, for example, Japanese Patent Laid-Open Publication No. 6-312852. When this method is used, the acceleration torques are reduced to levels in of the order of 0.2 kgf·cm, so that the driving torques of the transporting roller pairs  98 A and  98 B are reduced to values of the order of 1.2 kgf·cm, and the driving torque of the curling roller set  99  is reduced to a value of the order of 2.7 kgf·cm. 
     However, in this case, as shown in FIG. 5, the times required for accelerating the transporting roller pairs  98 A and  98 B and the curling roller set  99 , are longer, so that the total amount that a recording sheet P is transported is reduced in correspondence with the delay times. Compared to the case where the transporting speeds are switched in 0.1 sec., the transportation of the recording sheet P is delayed by slightly less than 0.5 sec., so that it takes extra time to form an image, thus increasing the total image-forming time. In addition, during the switching between transporting speeds, the speeds actually become unstable as indicated by the solid lines in FIG. 8, so that prolonging the speed-switching time increases variables, such as the location to which the recording sheet P is transported. Therefore, failures caused by an unstable total controlling operation or sheet jamming caused by variations in the recording-sheet P location may result. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to make it possible to eliminate the problem of size increases of an image-forming apparatus resulting from the use of a large motor. It is another object of the present invention to make it possible to eliminate the problem of increases in cost resulting from the addition of, for example, a bearing. It is still another object of the present invention to make it possible to eliminate the problem of lengthening of the total image-forming time caused by prolonging the time of switching between recording-sheet transporting speeds. 
     To this end, according to one aspect of the present invention, there is provided a sheet-transporting device comprising: 
     first transporting means for transporting sheets; and 
     second transporting means for transporting the sheets at a location downstream from the first transporting means in a sheet-transporting direction, a driving torque of the second transporting means being greater than a driving torque of the first transporting means; 
     wherein a speed of the first transporting means and a speed of the second transporting means are each switchable between a first transporting speed and a second transporting speed which is greater than the first transporting speed; and 
     wherein a time required to switch the speed of the second transporting means from the first transporting speed to the second transporting speed is longer than a time required to switch the speed of the first transporting means from the first transporting speed to the second transporting speed. 
     According to another aspect of the present invention, there is provided a sheet-transporting device comprising: 
     sheet-feeding means for feeding sheets; 
     first transporting means disposed downstream from the sheet-feeding means in a sheet-transporting direction; and 
     second transporting means disposed downstream from the first transporting means in the sheet-transporting direction, with a driving torque of the second transporting means being greater than a driving torque of the first transporting means; 
     wherein when the sheets are being fed at a first transporting speed by the sheet-feeding means, the first transporting means and the second transporting means also rotate at the first transporting speed, and after a predetermined time from completion of sheet feeding, speed switching of the first transporting means and speed switching of the second transporting means from the first transporting speed to a second transporting speed that is greater than the first transporting means are started, with a time required to switch the speed of the second transporting means from the first transporting speed to the second transporting speed being longer than a time required to switch the speed of the first transporting means from the first transporting speed to the second transporting speed. 
     In accordance with yet another aspect of the present invention there is provided image forming apparatus which includes the above sheet transporting devices together with means for forming an image on the sheets transported thereby. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic sectional view of the structure of an image-forming apparatus comprising a sheet-transporting device to which the present invention is applied. 
     FIG. 2 is an enlarged schematic view of a transporting section of a transfer transporting unit in the image-forming apparatus. 
     FIG. 3 is an enlarged perspective view of a transfer drum of the transfer transporting unit in the image-forming apparatus. 
     FIG. 4 illustrates in enlarged form a curling roller section. 
     FIG. 5 shows a graph illustrating the relationship between the switchable roller transporting speed and the switching time. 
     FIG. 6 is an enlarged schematic view of a transportation path extending from a sheet-feeding section to an image-forming section. 
     FIG. 7 illustrates a conventional image-forming apparatus. 
     FIG. 8 shows a graph illustrating the relationship between the switchable roller transporting speed and the switching time in the conventional image-forming apparatus. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereunder, a detailed description will be given of embodiments of the image-forming apparatus comprising a sheet-transporting device to which the present invention is applied, with reference to the drawings. In the description below, an image-forming apparatus comprising a transfer drum for holding a sheet is taken as an example. 
     [First Embodiment] 
     A detailed description will be given of a first embodiment of the image-forming apparatus comprising a sheet-transporting device in accordance with the present invention, with reference to the drawings. FIG. 1 is a structural view of the first embodiment of the image-forming apparatus in accordance with the present invention. 
     The image-forming apparatus of the first embodiment is a color image-forming apparatus in which a transfer-type electrophotographic process is used. It comprises a digital color-image reader section  100  at the upper portion thereof, and a printer section  200  at the lower portion thereof. 
     (Reader section) 
     In the reader section  100 , an original  30  is placed on an original-placing glass  31 , and exposed and scanned using an exposure lamp  32 . The light carrying the image reflected from the original  30  is gathered at a full-color sensor  34  by a lens  33 , whereby a color-separation image signal is obtained. This signal travels through an amplifying circuit (not shown), is variously processed by a video-processing unit (not shown), and sent to the printer section  200 . 
     (Image-forming mechanism of the printer section) 
     Reference numeral  1  denotes a photosensitive drum serving as an image-carrying member. It is rotationally driven clockwise (as indicated by the arrow in FIG. 1) around the center axis thereof at a predetermined peripheral speed (or process speed). An exposure lamp  11 , a primary charger  2 , an exposing optical system  3 , such as a laser, an electrical potential sensor  12 , a developing device  4 , a transfer drum  5 , and a cleaner  6  are sequentially disposed in that order in the direction of rotation of the photosensitive drum  1  so as to oppose the outer peripheral surface of the photosensitive drum  1 . 
     The surface of the rotating photosensitive drum  1  is uniformly electrically charged by the exposing lamp  11  and the primary charger  2 . Then, the photosensitive drum  1  is exposed to light beams E (such as laser beams) modulated in accordance with the recording image signal through a polygon mirror  3 A, a lens  3 B, and a mirror  3 C of the optical system  3 , whereby an electrostatic latent image is formed on the photosensitive drum  1 . 
     The electrostatic latent image is developed by the developing device  4 , which comprises four developing device sections. The four developing device sections are a yellow developing device section  4 Y containing yellow developer or yellow toner, a cyan developing device section  4 C containing cyan developer or cyan toner, a magenta developing device  4 M containing magenta developer or magenta toner, and a black developing device  4 BK containing black developer or black toner. The developing operation is carried out by alternately bringing eccentric cams  24 Y,  24 C,  24 M, and  24 BK near the photosensitive drum  1 . 
     At the transfer section T, the portions of the toner image carried by the photosensitive drum  1  are successively transferred onto the surface of a recording sheet supplied to and held by the transfer drum  5 , as described later. 
     After the image transfer, the recording sheet is separated from the transfer drum  5 , and sent to the fixing unit  9  which fixes the transferred image on the recording sheet thereto. Then, the recording sheet is either discharged onto a sheet-discharge tray  10  disposed outside the image-forming apparatus or transported to a vertical sheet path  20  for two-side recording (described later) by a path-switching guide  19 . 
     The fixing unit  9  comprises a fixing roller  9 A and a presser roller  9 B. The fixing roller  9 A incorporates a heat source, such as a halogen lamp. The presser roller  9 B (which may also incorporate a heat source) presses against the roller  9 A. These two rollers  9 A and  9 B are used to fix the toner image by thermally fusing the toner on the recording sheet. Here, oil is applied to the upper fixing roller by an oil-applying roller in order to prevent the image from being transferred onto the upper fixing roller. However, oil may be applied to the recording sheet for the same purpose. 
     The fixing unit  9  also comprises a transporting belt  9 C; an entrance guide  9 D for guiding the recording sheet to a nip section between the rollers  9 A and  9 B; and a discharge roller  9 E for guiding the recording sheet that has been transported from the rollers  9 A and  9 B out of the image-forming apparatus. 
     (Sheet-feeding unit) 
     In the embodiment of the image-forming apparatus, the sheet-feeding unit section is formed at the lower side of the above-described image-forming mechanism. It includes four sheet-feeding sections. They are a first sheet-feeding section  41 A, a second sheet-feeding section  41 B, a third sheet-feeding section  41 C, and a fourth sheet-feeding section  41 D. Three of the four sheet-feeding sections, that is, the first to third sheet-feeding sections  41 A,  41 B, and  41 C, are disposed at the upper, intermediate, and lower portions of the sheet-feeding unit. They are mechanisms for automatically separating sheets one sheet at a time for feeding. The fourth sheet-feeding section  41 D is a mechanism for manually feeding sheets. 
     As shown in FIGS. 1 to  6 , starting from the topmost sheets, recording sheets P held by the first to third sheet-feeding sections  41 A,  41 B, and  41 C are successively separated one sheet at a time by corresponding pick-up rollers  42 A,  42 B, and  42 C, and fed by corresponding sheet-feed rollers  43 A,  43 B, and  43 C. When a sheet-feeding section  41 A,  41 B, or  41 C is specified, the pick-up roller of the specified sheet-feeding section  41 A,  41 B, or  41 C is driven to selectively execute sheet feeding from the specified sheet-feeding section  41 A,  41 B, or  41 C. 
     At the sheet-feeding sections  41 B and  41 C, the recording sheets P are fed at speeds of 150 mm/s. At the same time that the sheet feeding is carried out, corresponding transporting rollers  45 B and  45 C start to rotate at peripheral speeds of 150 mm/s. When the leading edge of the recording sheet P is nipped by transporting rollers disposed immediately behind the pick-up rollers  42 B and  42 C, the speeds of the transporting rollers  45 B and  45 C are changed to 600 mm/s. 
     As in conventional image-forming apparatus, the speeds of the transporting rollers are changed by switching the rotational speeds of motors (not shown), such as stepping motors, directly connected to the transporting rollers corresponding thereto. To quadruple the transporting speeds from 150 mm/s, or a first transporting speed, to 600 mm/s, or a second transporting speed, the numbers of rotations of the driving motors of the corresponding rollers are quadrupled. The speed-switching time is 100 ms to reduce loss of transportation time. 
     When, for example, sheet feeding is carried out from the lower sheet-feeding section  41 C, the speed of the transporting roller  45 C is changed to 600 mm/s when the transporting roller  45 C nips a recording sheet P. This causes the recording-sheet transporting speed to be changed to 600 mm/s at the same time. Simultaneously with the speed changes of the lower sheet-feeding section  41 C and the transporting roller  45 C, the speeds of the transporting rollers  45 B and  45 A are also changed to 600 mm/s in the same speed-switching time of 100 ms, and the transporting rollers  45 A and  45 B wait for the recording sheet P. During the speed switching, each motor load torque is equal to 2.2 kgf·cm, which is the sum of the ordinary load torque of each transporting roller, or 1 kgf·cm, and the acceleration torque produced at each transporting roller, or 1.2 kgf·cm. 
     (Transfer transporting unit) 
     The recording sheet P that has been fed from any one of the first to fourth sheet-feeding sections  41 A to  41 D and an intermediate tray  22  moves into the transfer transporting unit  50 . 
     The transfer transporting unit  50  roughly comprises a transporting section  50 A and the transfer drum  5 . Of the two sections, the transporting section  50 A will be described with reference to FIG. 2, which is a partial enlarged view thereof. The transfer transporting unit  50  comprises guides  48 A to  48 E, transporting rollers  46 A to  46 C, and  47 A and  47 B, a register roller pair  44 , and suction guides  48 F and  48 G. The guides  48 A to  48 E are used to guide a recording sheet P from a sheet-feeding unit  40 . The register roller pair  44  are used to send the recording sheet P to the transfer drum  5  (described later) based on an image-forming timing at the image-forming section. The suction guides  48 F and  48 G are used to guide the recording sheet P based on the aforementioned timing. 
     The transporting rollers  46 A,  46 B, and  46 C, being second transporting means, are curling rollers which are capable of curling the recording sheet P. At the same time that the sheet feeding is started, the curling rollers  46 A,  46 B, and  46 C start to rotate at 150 mm/s, or the first transporting speed. At the same time that the speed of the transporting roller  45 C starts to change, the speeds of the curling rollers  46 A,  46 B, and  46 C start to change to 600 mm/s, or the second transporting speed. The speed-switching time is 500 ms, which is longer than 100 ms. During speed switching, each motor load torque is 2.7 kgf·cm, which is the sum of the ordinary load torque of each of the curling rollers  46 A to  46 C, or 2.5 kgf·cm, and each acceleration torque, or 0.2 kgf·cm. Since the total load of each of the curling rollers  46 A to  46 C does not greatly differ from the load torque (equal to 2.2 kgf·cm) of each of the transporting rollers  45 A to  45 C at the time of speed switching, they can be driven by similar driving motors. 
     The recording sheet P moves between the curling transporting rollers  46 A to  46 C which are rotating at transporting speeds of 600 mm/s as a result of the speed switching. Therefore, the recording sheet P is transported towards the register roller pair  44  without being wrinkled due to improper sheet transportation resulting from differences in speed. 
     Thereafter, by the transporting rollers  47 A and  47 B and the guides  48 A to  48 E, the recording sheet P is transported to the register roller pair  44 . At this time, the register roller pair  44  is not rotating. An edge of the recording sheet P enters the nip section of the register roller pair  44 . After a certain time therefrom, the driving of the transporting rollers  47 A and  47 B is stopped. 
     (Transfer drum) 
     As shown in FIG. 3, the transfer drum  5  is formed by winding a recording-sheet-holding sheet film  55 S so as to form a cylindrical shape onto a frame  55 F. In the frame  55 F, annular members  55 A at both ends of the transfer drum  5  are coupled to each other by coupling members  55 B. The film  55 S is formed of, for example, PET (polyethylene terephthalate) or PVdF (polyvinylidene fluoride). 
     A suction roller  5 G (see FIG. 2) is supported at a location corresponding to where the recording sheet P transported from the register roller pair  44  is forced onto the transfer drum  5 . The suction roller  5 G is supported so that it comes into contact with and separates from the transfer drum  5 . A backup member and a suction charger  5 C (see FIG. 2) are disposed opposite the suction roller  5 G in order to oppose the pushing force of the suction roller  5 G, with the film  55 S being interposed between the suction roller  5 G and the charger  5 C. 
     A transfer charger  5 B (see FIG. 1) is disposed at the back side of the film  55 S, at an image transfer area (or transfer section) T where the photosensitive drum  1  and the transfer drum  5  oppose each other. Separating members (more specifically, a charge remover  5 H, a separating pawl  8 A, a film push-up roller  8 B, etc.) are disposed downstream from the transfer charger  5 B in order to separate the recording sheet P from the transfer drum  5 . Film charge removers  5 D and  5 E are disposed downstream from the separating members, with the sheet film  55 S being disposed therebetween. 
     A brush roller  14 , a corona discharger or a brush-type charge remover  15 , an oil-removing roller  16 , and a backup brush  17  are provided downstream of the film charge removers  5 D and  5 E. The brush roller  14  and the corona discharger or the brush-type charge remover  15  are used to clean off toner, sheet dust, or other contaminants stuck on the recording-sheet holding surface of the film  55 S. The oil-removing roller  16  and the backup brush  17  are used to clean off oil, used at the fixing unit  9 , that sometimes sticks onto the film surface through the recording sheet P during printing on two sides (described later). 
     The transfer drum  5  is constructed so that it can come into contact with and separate from the photosensitive drum  1 . Excluding the ordinary operation time, it is separated from the photosensitive drum  1  to prevent the transfer drum  5  and the photosensitive drum  1  from contacting each other for a long time, and to prevent the recording sheet P from coming into contact with and damaging the surface of the photosensitive drum  1  when eliminating jamming. In addition, the transfer transporting unit  50  is constructed so that it can be forwardly drawn out from the body of the image-forming apparatus by sliders  62 A and  62 B, thereby achieving reliable, easier jam elimination and maintenance. 
     (Color (multi-color) image recording) 
     When an image-forming-operation start signal is generated, a cam  71  undergoes half a rotation, causing the transfer drum  5  to rock so as to approach the photosensitive drum  1  and to move to an operating position. When it reaches the operating position, it rotates in synchronism with the photosensitive drum  1 . 
     Thereafter, based on the timing in which the image-forming section forms the image, the register roller pair  44  and the transporting rollers  47 A and  47 B start to rotate. The time of rotation start is set so that rotation starts when the recording sheet P and the toner image on the photosensitive drum  1  are exactly aligned at the image-transfer area T. 
     When the recording sheet P is forced onto and comes into contact with the transfer drum  5 , it is electrostatically attracted to the sheet film  55 S by corona discharge, occurring at the suction charger  5 C, and by the action of the suction roller  5 G. The suction roller  5 G is usually separated from the transfer drum  5  in order to reduce load. The suction roller  5 G is press-contacted against the transfer drum  5  only when the recording sheet P is forced onto the transfer drum  5 . 
     The transfer drum  5  rotates in synchronism with the photosensitive drum  1 , and the recording sheet P, being kept on the transfer drum  5 , is transported to the image-transfer area T. At the image-transfer area T, the toner image, formed on the photosensitive drum  1  by any one of the aforementioned image-forming processes, is transferred onto the surface of the recording sheet P by the transfer charger  5 B. Instead, the recording sheet P with the transferred image is transported to the separating section. 
     Since a one-color image is transferred by one transfer operation, when the image is a one-color image, the recording sheet P is separated from the transfer drum  5  as described below. On the other hand, when a color image (formed with four different colors when it is a full-color image) is to be formed, the recording sheet P, being kept on the transfer drum  5 , is rotated once and transported to the transfer area T again. It is not separated after one transfer operation. Instead, the next toner image is transferred onto the toner image formed by the first transfer operation. The transferring of a toner image onto the toner image or toner images already on the recording sheet P is repeated a necessary number of times. By carrying out transfer operations a multiple number of times, the toner images are synthesized to form the desired color (or full-color) image. The brush roller  14  and the other component parts at the transfer drum  5  are supported so that they can come into contact with and separate from the surface of the transfer drum  5 . They are separated from the surface of the transfer drum  5  at least when multiple transfer operations are being carried out. 
     After completion of the multiple transfer operations, the charge remover  5 H in the separating section reduces the force of attraction between the recording sheet P and the sheet film  55 S of the transfer drum  5 . Then, the recording sheet P is separated from the transfer drum  5  by the separating pawl  8 A and the film push-up roller  8 B. 
     The recording sheet P separated from the transfer drum  5  is transported to the fixing unit  9  by the transporting belt  9 C, and precisely guided on the entrance guide  9 D to the fixing roller nip section. Then, the toner image or the toner images are fixed to the surface of the recording sheet P by the heat generated from the fixing roller  9 A. After the fixing operation, the recording sheet is transported by the sheet-feed roller  9 E and discharged out of the image-forming apparatus. In the last step of the image-forming operation, the transfer drum  5  is separated from the photosensitive drum  1 , and the image-forming apparatus stops operating. 
     As can be understood from the foregoing description, according to the embodiment of the image-forming apparatus constructed so that the transporting speeds of the transporting roller pairs and the curling roller set can be changed in order to reduce the time required to transport a recording sheet to the image-forming section, the speed-switching times of the rollers with large rotational loads (or large driving torques), such as curling rollers, are made longer than the speed-switching times of the other rollers not having large rotational loads. This makes it possible to provide a low-cost, stable image-forming apparatus which does not use a large driving motor and a driving-load reducing means, such as a bearing. 
     In addition, the rollers whose speed-switching times are long, that is, the rollers, such as curling rollers, having large driving loads have their speeds changed at timings in which a recording sheet is not nipped. Therefore, it is possible to prevent the recording sheet from becoming wrinkled when it is being transported. 
     [Other Embodiments] 
     Although in the above-described embodiment a copying machine was used as an example of the image-forming apparatus, the image-forming apparatus may be, for example, a scanner, a printer, or a facsimile machine. By applying the present invention to a sheet-transporting device used in any one of these image-forming apparatuses, similar effects can be obtained. 
     Although in the above-described embodiment a sheet-transporting device for feeding recording sheets was used, a sheet-transporting device for transporting, for example, originals in sheet form from which information is read may also be used. Even in this case, similar effects can be obtained. 
     Although in the above-described embodiment an electrophotographic method was used as the recording method, other types of recording methods, such as the inkjet method, may also be used. 
     As can be understood from the foregoing description, according to the image-forming apparatus of the present invention in which the transporting speeds of the transporting means can be changed, the transporting-speed-switching times of the transporting means having large driving torques, such as curling rollers, are made longer than the transporting-speed-switching times of the rollers not having large driving torques. Therefore, it is possible to provide a low-cost, stable image-forming apparatus which uses neither a large driving motor nor a driving load reducing means, such as a bearing. 
     Further, since the speeds of the transporting means, such as curling rollers, having long speed-switching times or having large driving torques are changed at a timing in which a sheet is not nipped, it is possible to prevent wrinkles from becoming formed on the sheet being transported. 
     While the present invention has been described with respect to what is presently considered to be the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. The present invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.