Skew correcting device and image forming apparatus

Abstract: In an obliquely traveling sheet correcting device with a simple arrangement capable of reducing the cost and size thereof and enhancing an obliquely traveling sheet correcting accuracy, shutter members 15 are turnably disposed to the rotary shaft 18 of free conveyer rollers 13 constituting a pair of conveyer roller 13, 14, the leading edge of a sheet S conveyed by a pair of conveyer rollers 5 is stopped by being abutted against the collision surfaces 20a of the shutter members 15, the shutter members 15 are turned and retracted when the sheet forms a predetermined loop to thereby permit the sheet S to pass therethrough. There is also provided an image forming apparatus provided with the obliquely traveling sheet correcting device.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an obliquely traveling sheet correcting device and an image forming apparatus. Description of the Related Art In general, a position where an image is recorded with respect to the position of a sheet (hereinafter, referred to as "recording accuracy") is an important element of image quality in image forming apparatuses. Therefore, there are proposed various types of obliquely traveling sheet correcting means for enhancing the recording accuracy of conventional image forming apparatuses. For example, there is widely employed an obliquely traveling sheet correcting device of a resist roller system which is arranged such that a pair of resist rollers are disposed in the direction perpendicular to a sheet conveying direction at a position nearest to an image forming unit upstream in the sheet conveying direction (hereinafter, simply referred to as "upstream") and the oblique travel of a sheet is corrected by causing the leading edge of the sheet to be abutted against the nip portion between the pair of resist rollers. An example of a conventional image forming apparatus provided with an obliquely traveling sheet correcting device using the resist roller system will be described using FIG. 19. In FIG. 19, the leading edge of a sheet S fed from a sheet cassette 100 to a feed roller 101 is abutted against the nip portion between a pair of resist rollers 102 which have previously been stopped by a clutch or the like and the sheet S forms a curved loop as shown in FIG. 19 by the further rotation of the feed roller 101, whereby the oblique travel of the sheet is corrected by the stiffness of the sheet itself. A resist sensor 103 is disposed upstream of the pair of resist rollers 102 in the vicinity thereof. After the leading edge of the sheet S is detected by the resist sensor 103, the pair of resist rollers 102 are rotated to cause a photoreceptor drum 105 disposed in a process cartridge 104 to convey the sheet S to a confronting image forming unit at a timing at which a predetermined period of time, during which the sheet S forms the loop and the oblique travel thereof is corrected as described above, has elapsed. In the above prior art, however, the pair of resist rollers 102 and the feed roller 101 must be driven in rotation at a predetermined timing to cause the sheet S to form a predetermined loop between the pair of resist rollers 102 and the feed roller 101. As a result, a drive control system for them is complex and requires a high degree of control accuracy. Thus, a countermeasure such as the provision of a high performance clutch with a gear train is required. Further, the pair of resist rollers 102 must be composed of a metal roller or the like having a low frictional resistance to reduce the frictional resistance made when the sheet S enters the nip portion between the pair of resist rollers 102. However, when the pair of resist rollers 102 are composed of metal, a pressurizing mechanism must be provided with the pair of resist rollers 102 to compensate for the insufficient conveying force of the sheet S. Since the pressurizing mechanism is relatively expensive and increases an occupying space in the device, there is the problem of the size of the device being increased as well as the cost thereof is made expensive. An obliquely traveling sheet correcting device comprising the features contained in the preamble of claim 1 is known from document US-A-4 844 443. The stopping means of this known device is held in the first position by means of a latching means. While the stopping means is in its first position, it stops the leading edge of the sheet which is conveyed by the first conveyer means. Thereby, the sheet is registered so that the orientation of an obliquely traveling sheet is corrected. While the stopping means abuts against the leading edge of the sheet, rollers of the first conveyer means slip on the sheet. The release of the stopping means is timed in relation to the arrival of a toner image on a moving surface at a location downstream of the second conveyer means and is achieved by momentarily pivoting the latching means such that it no longer holds the stopping means in its first position. Thereafter, the stopping means is moved to its second position by a frictional driving force generated between the. stopping means and a rotating shaft. The release of the stopping means permits the sheet to be advanced into the second conveyer means. This known device necessarily requires a drive control system for actuating the latching means, i.e. for releasing the stopping means from its first position. This drive control system is complex and requires a high degree of control accuracy. Moreover, the known device requires special surface characteristics of the rollers of the first conveyer means. Finally, after the stopping means has been released from its first position, it no longer has a registering or orientation correcting effect on the sheet, or at least its correcting effect is not reliable. Document US-A-3 761 075 discloses a document conveying mechanism having a stopping means. This stopping means is moved out of its first position by means of a mechanical drive means and does not continue to abut against the leading edge of the conveyed sheet until the leading edge is clamped by a conveyer means. Document US-A-4 025 187 also discloses a device comprising the features contained in the preamble of claim 1. This known device comprises the stopping means which, in its first position, stops the leading edge of the conveyed sheet. The stopping means is pivoted by means of a cam and follower arrangement from its first position to its second position. Document JP-A-59 194 953 discloses a transportation apparatus which comprises a stopping means which moves from its first position to its second position by being pushed by a sheet conveyed by a first conveyer means. After the sheet has moved the stopping means to the second position, the sheet slides along the lower surface of the tip end of the stopping means before it is clamped by a second conveyer means. This results in that the correct orientation of the sheet achieved by the abutment between the leading edge of the sheet and the stopping means is not ensured until the sheet is clamped by the second conveyer means. SUMMARY OF THE INVENTION It is an object of the present invention to provide an obliquely traveling sheet correcting device which solves the problems associated with the known device according to document US-A-4 844 443 and which has a simple arrangement, reduces the cost and size of the device and enhances the sheet oblique travel correcting accuracy. Moreover, it is an object of the present invention to provide an image forming apparatus provided with such an obliquely traveling sheet correcting device. These objects are achieved by the obliquely traveling sheet correcting device according to claim 1 and by the image forming apparatus according to claim 22. Advantageous developments of the device according to claim 1 and of the apparatus according to claim 22 are defined in the dependent claims 2 to 21 and 23 to 26. If the device is designed according to claim 7, the urging means exhibits an urging force causing the leading edge of the sheet to be guided by the collision surface of the stopping means to be abutted against the leading edge of the sheet, and the leading edge of the sheet conveyed by the first conveyer means is stopped by the stopping means and oblique travel of the sheet is corrected by the stiffness of the sheet itself through the loop formed by the sheet. When the stiffness of the sheet which is in the state that it cannot escape anywhere acts on the stopping means, the stiffness of the sheet is greater than the urging force of the urging means for urging the stopping means. Since the stiffness is greater than the urging force, the sheet swings and retracts the stopping means while the leading edge of the sheet is guided by the collision surface of the stopping means against which the leading edge thereof is abutted, so that the sheet is conveyed by the second conveyer means passing through the stopping means. If the device is designed according to claim 15, the alignment accuracy of the stopping means with the second conveyer means is enhanced by the arrangement. When a sheet conveying velocity is changed by a conveying resistance and the tolerance of the parts constituting the first conveyer means and the like which exist upstream of the stopping means, the sheet may swing and retract the stopping means without forming a loop and the first conveyer means cannot exhibit a force which causes the sheet to pass through the stopping means. In such a case, the difference of the sheet conveying velocity can be absorbed on the upstream side in such a manner that the sheet is forced to form a predetermined loop by a space, a flapper member, an elastic member or the like disposed upstream of the stopping means to thereby exhibit predetermined stiffness. The leading edge of the sheet can be prevented from being damaged when the leading edge is abutted against the stopping means by forming the abutting surface of the stopping means to an arc surface having a predetermined radius or a curved surface with the R surface made to the end thereof. Further, no trace of swing of the sheet is partially formed by forming the collision surface surface of the stopping means of a plurality of members to be abutted against the leading edge of the sheet in the direction perpendicular to the sheet conveying direction. Further, the reaction force acting on the sheet by the stopping means is necessary until the sheet corrects the oblique travel by the stiffness of the sheet itself, whereas it is advantageous that the reaction force is smaller to convey the sheet after the leading edge of the sheet passes through the stopping means. Therefore, the reaction force to be acted on the sheet is preferably composed of the moment resulting from the dead weight of the stopping means itself, so that the reaction force acting on the sheet by the stopping means can be gradually reduced. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross sectional view illustrating a first embodiment of an obliquely traveling sheet correcting device according to the present invention and an image forming apparatus provided with the obliquely traveling sheet correcting device; FIG. 2 is a perspective view showing the arrangement of the obliquely traveling sheet correcting device of the first embodiment; FIG. 3 is a plan view showing the arrangement of the obliquely traveling sheet correcting device of the first embodiment; FIG. 4 is a view illustrating the operation of the obliquely traveling sheet correcting device of the first embodiment; FIG. 5 is a view illustrating the operation of the obliquely traveling sheet correcting device of the first embodiment; FIG. 6 is a view illustrating the operation of the obliquely traveling sheet correcting device of the first embodiment; FIG. 7 is a plan view illustrating how the obliquely traveling sheet correcting device of the first embodiment operates with a different sheet width; FIG. 8(a) is a horizontal cross sectional view showing the outside shape of the collision surface of a shutter member of the first embodiment and FIG. 8(b) is a horizontal cross sectional view showing the outside shape of the collision surface of the shutter member of a second embodiment; FIG. 9 is a vertical cross sectional view showing the outside shape of the collision surface of the shutter member of a third embodiment; FIG. 10 is a plan view showing the arrangement of the obliquely traveling sheet correcting device of a fourth embodiment; FIG. 11 is a cross sectional view illustrating the arrangement of the obliquely traveling sheet correcting device of the fourth embodiment; FIG. 12 is an enlarged perspective view of the A 1 portion in FIG. 10; FIG. 13 is a plan view showing the arrangement of the obliquely traveling sheet correcting device of a fifth embodiment; FIG. 14(a) and FIG. 14(b) are cross sectional views illustrating the arrangement of the obliquely traveling sheet correcting device of the fifth embodiment; FIG. 15 is a cross sectional view showing the arrangement of the image forming apparatus provided with the obliquely traveling sheet correcting device of a sixth embodiment; FIG. 16 is a cross sectional view showing the arrangement of the image forming apparatus provided with the obliquely traveling sheet correcting device of an seventh embodiment; FIG. 17 is a cross sectional view showing the arrangement of the image forming apparatus provided with the obliquely traveling sheet correcting device of a eighth embodiment; FIG. 18 is a cross sectional view illustrating the main portion of the portion A 3 in FIG. 17; and FIG. 19 is a view illustrating a conventional image forming apparatus. DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of an obliquely traveling sheet correcting device according to the present invention and an image forming apparatus provided with the obliquely traveling sheet correcting device will be specifically described with reference to the drawings. FIG. 1 is a cross sectional view describing the first embodiment of the obliquely traveling sheet correcting device according to the present invention and the image forming apparatus provided with the obliquely traveling sheet correcting device, FIG. 2 is a perspective view showing the arrangement of the obliquely traveling sheet correcting device of the first embodiment, FIG. 3 is a plan view showing the arrangement of the obliquely traveling sheet correcting device of the first embodiment, FIG. 4 to FIG. 6 are views describing operation of the obliquely traveling sheet correcting device of the first embodiment, and FIG. 7 is a plan view describing how the obliquely traveling sheet correcting device of the first embodiment cope with a different sheet width. First, the entire arrangement of the obliquely traveling sheet correcting device according to the present invention and the image forming apparatus provided with the obliquely traveling sheet correcting device will be described with reference to FIG. 1. Sheets S composed of paper, synthetic resin or the like accommodated in a feed unit U are drawn out by a feed roller 1 and fed while being separated one by one by a separator (not shown). Each of the sheets S fed by the feed roller 1 is conveyed by a pair of conveyer rollers 5 as first sheet conveyer means and further conveyed by a pair of sheet conveyer rotary bodies 13, 14 acting as second sheet conveyer means which is disposed at a position downstream of a sheet conveying direction (hereinafter, simply referred to as "downstream"), so that the sheet S is introduced to an image forming unit which is confronted by a photoreceptor drum 8 as an image carrier rotary body composed of an electrophotographic photoreceptor and disposed in a process cartridge 7. On the other hand, a laser scanner 6 controlled by a control unit 4 forms a latent image on the photoreceptor drum 8 the surface of which is uniformly charged by an electrizer by scanning a laser beam according to image information. Then, a toner image is formed to the latent image by a developing unit provided with the process cartridge 7. The sheet S is conveyed between the photoreceptor drum 8 and an image transfer electrizer 12 in synchronism with the rotation of the photoreceptor drum 8, and the toner image formed on the photoreceptor drum 8 by the image transfer electrizer 12 is transferred to the sheet S. The sheet S to which the image is transferred is conveyed to a fixing unit 11, and after the toner image is fixed to the sheet S by heat and pressure from the fixing unit 11, the sheet is discharged to the outside of the apparatus by pairs of discharge rollers 2, 3. Sensors 9, 10, which act as sheet sensing means, are respectively disposed between the pair of conveyer rollers 5 and the pair of conveyer rollers 13, 14, and downstream of the pair of conveyer rollers 13, 14 in the vicinity thereof and upstream of the photoreceptor drum 8 in a sheet conveying direction (hereinafter, simply referred to as "upstream"), and an obliquely traveling sheet correcting device, which is shown in FIG. 2 to FIG. 7 and will be described later in detail, is provided with the pair of conveyer rollers 13, 14. As shown in FIG. 4 to FIG. 6, the sensor 10 is disposed downstream but in the vicinity of the pair of sheet conveyer rotary bodies 13, 14, and a sensor flag 10a which is turnable about a shaft 10b is disposed such that one end of the sensor flag 10a projects through a sheet conveyer passage 21b disposed downstream of the pair of conveyer rotary bodies 13, 14 in a first state when the sheet S does not pass therethrough. The other end of the sensor flag 10a can turn to the position of the first state where it shuts off the light passage of a sensing unit 10c composed of a photosensor (see FIG. 4 and FIG. 5) and to the position of a second state where the sheet S is engaged with one end of the sensor flag 10a projecting to the sheet conveyer passage 21b and turns the sensor flag 10a in the direction of the arrow a of FIG. 6 about the shaft 10b to cause the other end of the sensor flag 10a to open the light passage of the sensing unit 10c (see FIG. 6). Next, the arrangement of the first embodiment of the obliquely traveling sheet correcting device will be described using FIG. 2 to FIG. 7. As shown in FIG. 2, the pair of sheet conveyer rotary bodies 13, 14, which are disposed such that the rotational axes thereof are parallel with the direction of the rotational axis of the photoreceptor drum 8, are subdivided into a plurality of sections in a direction perpendicular to a sheet conveying direction. That is, the sheet conveyer rotary bodies 14 is composed of a plurality of the subdivided conveyer rollers 14b which are securely fixed to a rotary shaft 14a and rotated integrally therewith and the sheet conveyer rotary body 13 is composed of a plurality of subdivided free rollers 13a inopposing relation to the conveyer rollers 14b. The free conveyer rollers 13a are fixed to a rotary shaft 18 which is journaled by a not shown main body frame and act as the sheet conveyer rotary body 13. A plurality of auxiliary free rollers 19 each having an outside diameter smaller than that of the free conveyer rollers 13a are rotatably engaged with the rotary shaft 18, to which the free conveyer rollers 13a are fixed in the direction of the rotary shaft 18, and are positioned between the free conveyer rollers 13a. Further, each of a plurality of shutter members 15 which serve as stopping means, and which are integrally connected to one another by a connecting member 16, is turnably engaged with each of the auxiliary free rollers 19 on the outer periphery threreof. As shown in FIG. 3, the sheet S is conveyed using the center thereof as a conveying reference and the shutter members 15 are aligned with the free conveyer rollers 13 by the auxiliary free rollers 19 in such a manner that the shutter member regulating surfaces 19a of the auxiliary free rollers 19 disposed on both the sides of the free conveyer rollers 13a disposed at a center C is abutted against and slides on the abutting surfaces 15h of the shutter members 15 engaged with the auxiliary free rollers 19. With the above arrangement, since the shutter members 15 are not in contact with the free conveyer rollers 13a in the direction of the rotary shaft 18, the free conveyer rollers 13a and the shutter members 15 can turn independently of each other. Therefore, the turning motion of the shutter members 15 is not prevented by the free conveyer rollers 13a. Although this embodiment is arranged by using the auxiliary free rollers 19 to separate the free conveyer rollers 13a from the shutter members 15, it may be arranged by regulating the position of each shutter member 15 in the direction of the rotary shaft 18 by, for example, an E type stop ring (so-called E ring) or the like. The plurality of shutter members 15 can swing about the rotary shaft 18 between the position of a first attitude where the collision surfaces 20a of the shutter members 15 are abutted against the leading edge of the sheet S conveyed by the plurality of conveyer rollers 5 and can lock the leading edge thereof (see FIG. 4, FIG. 5) and the position of a second attitude where the sheet S is permitted to pass through the shutter members 15 (see FIG. 6) and the phases of the turning motion of the respective shutter members 15 about the rotary shaft 18 are caused to coincide with one another by the connecting member 16. The shutter members 15 maintain the first attitude shown in FIG. 2 and FIGs. 4 and 5 at all times by the gravity of the shutter members 15 and the connecting member 16 acting thereon. That is, in this embodiment, urging means for urging the shutter members 15 to the position of the first attitude makes use of the torque which is conveyed to the shutter members 15 by the gravity resulting from the dead weight of the connecting member 16 for connecting the shutter members 15 with respect to the rotary shaft 18 as the center of turn of the connecting member 16. The shutter members 15 are provided with the collision surfaces 20a which act as sheet leading edge collision surfaces which can lock the sheet S by abutting against the leading edge thereof just before the sheet S enters the nip portion between the conveyer rollers 14b and the free conveyer rollers 13a. When the shutter members 15 are located at the position of the first attitude, the collision surfaces 20a of the shutter members 15 are located upstream of the nip portion between the conveyer rollers 14b and the free conveyer rollers 13a. Note, although the plurality of shutter members 15 are connected by the connecting member 16 in the first embodiment, the shutter members 15 may alternatively be integrally formed with the connecting member 16. When the sheet S is conveyed by the pair of conveyer rollers 5 and, for example, obliquely enters the pair of conveyer rotary bodies 13, 14 as shown in FIG. 3, the sheet S reaches the image forming unit while being conveyed in the oblique attitude in the absence of the shutter members 15 and the connecting member 16, thus an image transferred to the sheet S is recorded obliquely with respect to the sheet S. In the first embodiment, however, since the shutter members 15 and the connecting member 16 are arranged as described above and provided with the pair of conveyer rotary bodies 13, 14, the oblique travel of the sheet S can be corrected by an action to be described later. First, the leading edge of the sheet S on the advancing side thereof (lower side in FIG. 3) is abutted against the collision surfaces 20a of the shutter members 15 disposed at the positions corresponding to the leading edge. At this timing, the leading edge of the sheet S cannot push and turn the shutter members 15 against the gravity acting thereon by the dead weight of the connecting member 16. When the sheet S is further conveyed by the pair of conveyer rollers 5, the leading edge A of the sheet S on the advancing side thereof is stopped in the state that the leading edge A is abutted against the collision surfaces 20a of the shutter members 15 and the leading edge B of the sheet S on the trailing side thereof are stopped by being sequentially abutted against the collision surfaces 20a of the shutter members 15 which are disposed at the positions corresponding to the leading edge B. When the sheet S is further conveyed by the pair of conveyer rollers 5, the sheet S forms a curved loop as shown in FIG. 5, so that the oblique travel of the sheet S is corrected by the action of the stiffness of the sheet S itself so that the leading edge thereof is made parallel with the direction of the rotational axis of the pair of conveyer rotary bodies 13, 14. Then, the sheet S forms a predetermined loop in the sheet conveyer passage 21a disposed upstream of the pair of conveyer rotary bodies 13, 14 in the vicinity thereof as shown in FIG. 5. When the loop portion of the sheet S is abutted against guide member 21d or 21c constituting the sheet conveyer passage 21a to thereby regulate the further increase of the loop, a force for turning the shutter members 15 about the rotary shaft 18 in the direction of the arrow b in FIG. 5 and retracting the same is produced for the first time by the stiffness of the sheet S. The leading edge of the sheet S enters the nip portion between the conveyer rollers 14b and the free conveyer rollers 13a while abutting and pushing against the shutter members 15. When the sheet S is conveyed while being clamped by the nip portion between the conveyer rollers 14b and the free conveyer rollers 13a, the shutter members 15 are pushed away by the sheet S and moved to a retracting position as shown in FIG. 6. The leading edge of the sheet S advances in the state that it is abutted against the shutter members 15 at least until it is clamped by the nip portion between the conveyer rollers 14b and the free conveyer rollers 13a. With this operation, since the sheet S is clamped by the nip portion between the conveyer rollers 14b and the free conveyer rollers 13a in the state that the oblique travel thereof is corrected, the oblique travel corrected state is maintained even if shutter members 15 retract. Then, since the sheet S is engaged with the sensor flag 10a of the sensor 10 disposed downstream of the pair of conveyer rotary bodies 13, 14 in the vicinity thereof to thereby turn the sensor flag 10a about the shaft 10b in the direction of the arrow a of FIG. 6 and open the light passage of the sensing unit 10c, the sensor 10 detects that the sheet S has passed. When the sensor 10 detects the sheet S, the laser scanner 6 forms a latent image on the surface of the photoreceptor drum 8, a toner image is formed on the photoreceptor drum 8 by a series of processing operation effected by the process cartridge 7 and the toner image which is formed on the surface of the photoreceptor drum 8 is transferred to the sheet S conveyed in synchronism with the rotational motion of the photoreceptor drum 8. Since the sensor flag 10a actuates the sensing unit 10c so as to start the image forming operation of the image forming means in response to the information detected by the sensing unit 10c as described above, a timing at which an image is formed can be maintained constant regardless of a period of time necessary for the sheet S to pass through the shutter members 15. That is, even if the period of time during which the sheet S passes through the shutter members 15 is different in correspondence to various types of the sheet S having different stiffness, the timing at which the image is formed can be maintained constant. Since the leading edge of the sheet S is parallel to the rotational axis of the pair of conveyer rotary bodies 13, 14 at the time by the above oblique travel correcting operation, when a sufficient conveying force is applied to the sheet S by the pair of conveyer rotary bodies 13, 14, the sheet S can be conveyed to the photoreceptor drum 8 without traveling obliquely, so that the image to be transferred to the sheet S can be recorded without inclining with respect to the sheet S. The length of the sheet S being used in the direction perpendicular to the sheet conveying direction (hereinafter, referred to as "width of sheet S") is relatively large as shown in FIG. 7 (the sheet S is shown by the solid line in FIG. 7), the oblique travel of the sheet S is corrected mainly by the two shutter members 15a, 15d which are disposed corresponding to both the side edges of the sheet S in the vicinity thereof to act on the leading edge of the sheet S as described above. Further, when the width of the sheet S being used is relatively small and cannot be caught by the shutter members 15a and 15d (the sheet S shown by the two-dot-and-dash line in FIG. 7), the oblique travel of the sheet S is corrected by the shutter members 15b, 15c which are located to the center side with respect to the shutter members 15a, 15d. When the intervals between the plurality of shutter members 15 corresponding to the width of the sheet S are set as large as possible, the error of correcting angle of the leading edge of the sheet S with respect to the direction of the rotational axis of the pair of conveyer rollers 13, 14 is reduced. Thus, although it is preferable to dispose the shutter members 15 in the vicinity of both the side edges of the sheet S being conveyed to obtain an oblique travel correcting capability of pinpoint accuracy to the sheet S, it is also preferable to provide the plurality of shutter members 15 by disposing them in the vicinity of the center of conveyance C of the sheet S so that the oblique travel of the sheet S having a relatively small width can be corrected. It is preferable to make the intervals of the two shutter members 15b, 15c nearest to the center C of the conveyer passage of the sheet S on both the sides thereof smaller than the minimum width of the sheet S to be used to the image forming apparatus. The oblique travel correcting effect to the sheet S can be maintained in such a manner that the sheet S is conveyed by being clamped between the nip portion between the conveyer rollers 14b and the free conveyer rollers 13a just after the oblique travel of the sheet S is corrected by the shutter members 15. This correcting operation is realized by that the distance between the collision surfaces 20a of the shutter members 15 and the nip portion between the conveyer rollers 14b and the free conveyer rollers 13a is minimized in a possible range as shown in this embodiment and the leading edge of the sheet S is locked by being abutted against the collision surfaces 20a just before the leading edge of the sheet S, which entered the nip portion between the conveyer rollers 14b and the free conveyer rollers 13a, is clamped by the nip portion. The urging means for urging the shutter members 15 to the position of the first attitude is arranged by using the gravity balance acting on the dead weight of the connecting member 16. This arrangement is advantageous because it is simple in construction and also because when the shutter members 15 are turned by being pushed by the sheet S and the sheet S passes through the shutter members 15, the reaction force from the shutter members 15 to the sheet S is not increased. In particular, since the gravitational force exerted on shutter members 15 by the connecting member 16 are set in this case so that when the shutter members 15 are swung by being pushed by the sheet S, the reaction force which the sheet S receives from the shutter members 15 is made weaker than the reaction force which the sheet S receives from the shutter members 15 when the leading edge of the sheet S is abutted against the shutter members 15 for the first time, the sheet S is liable to pass through the shutter members 15 by turning and retracting them. This arrangement is preferable because the occurrence of a jam is prevented. That is, since a gravity vector acting on the center of gravity of the shutter members 15 and the connecting member 16 as a whole approaches the rotary shaft 18 as the shutter members 15 turn clockwise, a counterclockwise moment is made weak, thus the reaction force to the sheet is also made weak. Further, the oblique travel of the sheet S having various types of stiffness can be corrected by changing the weight of the connecting member 16 in correspondence to the stiffness of various types of the sheet S. Although the shutter members 15 and the connecting member 16 are arranged to turn about the rotational axis of the free conveyer rollers 13a in this embodiment, they may be arranged to turn about the rotary shaft 14a of the conveyer rollers 14b. Next, a second embodiment of the obliquely traveling sheet correcting device according to the present invention and the image forming apparatus provided with the obliquely traveling sheet correcting device will be described using FIG. 8(a) and FIG. 8(b). FIG. 8(a) is a horizontal cross sectional view showing the outside shape of the collision surface 20a of the shutter member of the first embodiment and FIG. 8(b) is a horizontal cross sectional view showing the outside shape of the collision surface 20b of the shutter member of the second embodiment. Note, components arranged similarly to those of the first embodiment are denoted by the same numerals and the description thereof is omitted. Since the collision surface 20a of the shutter member 15 formes a plane in the first embodiment as shown in FIG. 8(a), the edge of the collision surface 20a is made to a corner. Thus there is a possibility that when the leading edge of a sheet S is abutted against the corner, the leading edge of the sheet S will be damaged by it. To prevent the above drawback, the second embodiment is arranged such that the collision surface 20b of the shutter member 15 has an arc shaped surface having a predetermined radius or a curve surface with an R surface at a corner as shown in FIG. 8(b). As a result, when the sheet S is conveyed obliquely and the leading edge of the sheet S is abutted against the collision surface 20b of the shutter member 15, the leading edge is not abutted against a corner contrary to the first embodiment, thus the leading edge of the sheet S is not damaged. Since the R surface or the arc surface of the collision surface 20b is formed to have a predetermined curvature so that when the sheet S achieves the maximum oblique travel supposed by the obliquely traveling sheet correcting device, the leading edge of the sheet S is not abutted against the curvature changing portion at the boundary between both the side edges of the collision surface 20b and the R surface or the arc surface, a sheet oblique travel correcting capability is compatible with a damage preventing effect to the sheet S in an optimum state. The other aspects of the second embodiment are similar to that of the first embodiment and advantages similar to that of the first embodiment can be obtained. Next, a third embodiment of the obliquely traveling sheet correcting device according to the present invention and the image forming apparatus provided with the obliquely traveling sheet correcting device will be described using FIG. 9. FIG. 9 is a vertical cross sectional view showing the outside shape of the collision surface 20c of the shutter member of the third embodiment. Note, components arranged similarly to those of the first embodiment are denoted by the same numerals and the description thereof is omitted. In the first embodiment, when the shutter member 15 is located at the position of the first attitude, the collision surface 20a of the shutter member 15 is formed in the direction perpendicular to the surface of the sheet S being conveyed (as shown in FIG. 5) and the collision surface 20a is across the surface of the sheet S which collides against the collision surface 20a at approximately 90°. In this case, there is a possibility that the leading edge of the sheet S escapes to the lower side of the FIG. 5 from the collision surface 20a depending upon how the loop of the sheet S is formed. To prevent this drawback, the third embodiment is arranged such that when the shutter member 15 is located at the position of the first attitude, the collision surface 20c of the shutter member 15 is formed in the direction of an acute angle below 90° in the clockwise direction of FIG. 9 from the side from which the sheet S is conveyed with respect to the surface of the sheet S being conveyed and the collision surface 20c is across the surface of the sheet S to be collided thereagainst at an acute angle below 90° as shown in FIG. 9. With this arrangement, the leading edge of the sheet S drops into a groove 20d as a holding portion formed by the collision surface 20c and the edge 15e of the shutter member 15 and does not escape from the collision surface 20c contrary to the aforesaid case, so that the leading edge of the sheet S is securely locked. As a result, the oblique travel of the sheet S can be corrected. The other arrangement of the third embodiment is similar to that of the first embodiment and an advantage similar to that of the first embodiment can be obtained. Next, a fourth embodiment of the obliquely traveling sheet correcting device according to the present invention and the image forming apparatus provided with the obliquely traveling sheet correcting device will be described using FIG. 10 to FIG. 12. FIG. 10 is a plan view showing the arrangement of the obliquely traveling sheet correcting device of the fourth embodiment, FIG. 11 is a cross sectional view describing the arrangement of the obliquely traveling sheet correcting device of the fourth embodiment and FIG. 12 is a perspective enlarged view of the portion A 1 in FIG. 10 and shows the main portion of a reaction force switching means of the urging means. Note, components arranged similarly to those of the first embodiment are denoted by the same numerals and the description thereof is omitted. As shown in FIG. 10, the fourth embodiment is arranged such that a twisted coil spring 24 acting as the urging means for urging the shutter member 15 to the position of the above first attitude is additionally disposed to the obliquely traveling sheet correcting device arranged similarly to that of the first embodiment. The twisted coil spring 24 is slidingly engaged with the rotary shaft 18 with the end 24b thereof on the side of the connecting member 16 being secured to the connecting member 16 as well as the end 24a thereof on the side of a device main body frame 17 being suitably selectively locked to first and second projections 23a, 23b acting as reaction force switching means disposed to the frame 17. For example, the end 24a of the twisted coil spring 24 on the frame 17 side is secured to the first projection 23a in the case of a sheet S having normal stiffness. With the above arrangement, the shutter member 15 is urged in the direction of the arrow c in FIG. 11 at all times by the twisted coil spring 24. Further, an urging force F acting in the direction of the arrow c in FIG. 11 can be increased by switching the end 24a of the twisted coil spring 24 on the frame 17 side to the second projection 23b in the above arrangement. Thus, a suitable urging force F can be obtained by selectively locking the end 24a of the twisted coil spring 24 on the frame 17 side to the first and second projections 23a, 23b according to the stiffness of the sheet S being used. Since the sheet S can be conveyed with the oblique travel thereof corrected by the shutter member 15 approximately similarly to the first embodiment, the detail of the correcting operation is not described here. As the sheet S, various types of the sheet S ranging from thin paper having a weight of about 50g/m 2 to thick paper having a weight of about 120g/m 2 will be used. When the sheet S is the thin paper, the stiffness of the sheet is weak, and therefore the sheet S is liable to form a loop even if the urging force F of the shutter member 15 is small so that the oblique travel of the sheet S can be easily corrected. Whereas, since the stiffness of the sheet S is weak, when the urging force F produced by the twisted coil spring 24 is set larger than necessary, the sheet S has difficulty passing through the shutter member 15 and a problem of jam and the like arises. Further, when the sheet S is the thick paper, it is difficult to form a loop when the urging force F of the shutter member 15 is weak because the sheet S has strong stiffness and thus the oblique travel of the sheet S may not be perfectly corrected. Thus a problem arises in that the sheet S turns and retracts the shutter member 15 and passes therethrough. If it is assumed that the urging force F is set to such a degree that even the sheet S having the weakest stiffness among the sheets S to be used turns and retracts the shutter member 15 and passes therethrough when the end 24a of the twisted coil spring 24 on the frame 17 side is locked to the first projection 23a in this embodiment, when the sheet S having relatively strong stiffness such as the thick paper and the like is conveyed in this state, the amount of correction of the oblique travel of the sheet S is made smaller than the amount of correction made to the thin paper. At the time, when the end 24a of the twisted coil spring 24 on the frame 17 side is switched to the second projection 23b, the urging force F is increased and a large amount of correction of the oblique travel can be secured even to the thick paper. With the arrangement of the fourth embodiment as described above, it is possible to exhibit a large oblique travel correcting capability to the sheet S such as the thick paper and the like having relatively strong stiffness while securing a conveying capability to the sheet S having relatively weak stiffness such as the thin paper and the like. Note, although the fourth embodiment adjusts the urging force F produced by the twisted coil spring 24 by changing the position to which the end 24a of the twisted coil spring 24 on the frame side 17 is locked, the present invention is not limited to the above arrangement but it can be arranged by using various other means capable of adjusting the urging force F for urging the shutter member 15 to the position of the first attitude. Next, a fifth embodiment of the obliquely traveling sheet correcting device according to the present invention and the image forming apparatus provided with the obliquely traveling sheet correcting device will be described using FIG. 13 and FIG. 14. FIG. 13 is a plan view showing the arrangement of the obliquely traveling sheet correcting device of the fifth embodiment and FIG. 14(a) and FIG. 14(b) are cross sectional views describing the arrangement of the obliquely traveling sheet correcting device of the fifth embodiment. Note, components arranged similarly to those of the first embodiment are denoted by the same numerals and the description thereof is omitted. As shown in FIG. 13, the fifth embodiment is arranged such that the obliquely traveling sheet correcting device arranged similarly to that of the first embodiment is provided with a sensor flag 16a integrally therewith to actuate a sensing unit 10c acting as a sheet sensing means. Provided with an end of the rotary shaft 18 is the sensor flag 16a which is engaged with the rotary shaft 18 and journaled with respect to the rotary shaft 18 as well as securely fixed to the connecting member 16. The sensor flag 16a can be turned integrally with the connecting member 16 about the rotary shaft 18 and when the shutter member 15 turns, the sensor flag 16a is turned integrally with the shutter member 15 in synchronism therewith. The sensing unit 10c constituting sheet sensing means is disposed at a position confronting the sensor flag 16a likewise the first embodiment. As shown in FIG. 14(a), when the shutter member 15 is located at the position of a first attitude where it is abutted against the leading edge of a sheet S to thereby stop the leading end of sheet S, the sensor flag 16a is set at a position where the light passage of the sensing unit 10c is opened. Further, as shown in FIG. 14(b), when the shutter member 15 is located at the position of a second attitude where it permits the sheet S to pass therethrough, the sensor flag 16a is set to a position where the light passage of the sensing unit 10c is shut off. In the above arrangement, after the leading edge of the sheet S conveyed by the pair of conveyer rollers 5 as in the first embodiment is locked by being abutted against the collision surface 20a of the shutter member 15 and the oblique travel thereof is corrected by the formation of a loop, the shutter member 15 is turned by being pushed upward and retracted so that the sheet S passes therethrough. At the time, the sensor flag 16a is turned about the rotary shaft 18 by the same angle as that of the shutter member 15 in the direction of the arrow b in FIG. 14(a) to thereby shut off the light passage of the sensing unit 10c. The laser scanner 6 (see Fig. 1) forms a latent image on the surface of a photoreceptor drum 8 in response to the information detected by the sensor 10 and a toner image is formed on the surface of the photoreceptor drum 8 by a series of processing operation effected by a process cartridge 7. The toner image which is formed on the surface of the photoreceptor drum 8 is transferred to the sheet S conveyed in synchronism with the rotational motion of the photoreceptor drum 8. Although a plurality of the shutter members 15 are connected by the connecting member 16 and the sensor flag 16a is securely fixed to the connecting member 16 in the fifth embodiment, the shutter members 15, the connecting member 16 and the sensor flag 16a may be integrally formed. The sensor flag 16a being disposed to the connecting member 16 for connecting the shutter members 15, the sensor flag 16a associating with the turning operation of the shutter members 15 actuates the sensing unit 10c and the image forming operation of image forming means is started in response to the information detected by the sensing unit 10c in the aforesaid embodiment. Therefore, a timing at which an image is formed can be maintained constant regardless of a period of time necessary for the sheet S to pass through the shutter members 15. Moreover, since a portion of the shutter members 15 also serves as a portion of the sheet sensing means, the number of parts can be reduced to thereby lower the cost of the device. Next, a sixth embodiment of the obliquely traveling sheet correcting device according to the present invention and the image forming apparatus provided with the obliquely traveling sheet correcting device will be described using FIG. 15. FIG. 15 is a cross sectional view showing the arrangement of the image forming apparatus provided with the obliquely traveling sheet correcting device of the sixth embodiment. The components arranged similarly to those of the first embodiment are denoted by the same numerals and the description thereof is omitted. The sixth embodiment has a plurality of the obliquely traveling sheet correcting devices which are arranged similarly to that of the first embodiment and disposed on a conveyer passage for conveying a sheet S. In the image forming apparatus of the sixth embodiment shown in FIG. 15, feed units U1, U2 are arranged at two stages located upward and downward. Then, a pair of conveyer rotary bodies 13A, 14A provided with a shutter member 15A are disposed upstream of a process cartridge 7 in the vicinity thereof as in the first embodiment. A pair of conveyer rotary bodies 13B, 14B provided with a shutter member 15B and the like as the obliquely traveling sheet correcting device arranged similarly to that of the first embodiment are also disposed downward of the joint point of the sheet conveyer passages from the feed units U1, U2 of the main body of the device in the vicinity thereof. A pair of conveyer rotary bodies 13C, 14C provided with a shutter member 15C and the like as the obliquely traveling sheet correcting device arranged similarly to that of the first embodiment are disposed to the lower feed unit U2 which has a relatively long conveyer passage for the sheet S up to an image forming unit confronted by a photoreceptor drum 8 disposed to the process cartridge 7. Further, sensors 9, 10 acting as sensing means are disposed downstream of the pair of conveyer rotary bodies 13A, 14A and the pair of conveyer rotary bodies 13B, 14B in the vicinity thereof likewise the first embodiment, respectively. The other arrangements of the sixth embodiment are similar to that of the first embodiment and advantages similar to that of the first embodiment can be obtained. With the above arrangement, the sheet S which was drawn out from the feed unit U2 by a feed roller 1a acting as first sheet conveyer means first forms a loop between the feed roller 1a and the pair of conveyer rotary bodies 13C, 14C acting as second sheet conveyer means to thereby correct the oblique travel thereof, then is conveyed downstream and further forms a loop between the pair of conveyer rotary bodies 13C, 14C acting as first sheet conveyer means this time and the pair of conveyer rotary bodies 13B, 14B acting as second sheet conveyer means to thereby correct the oblique travel thereof again. Thereafter, the sheet S is conveyed downstream and further forms a loop between the pair of conveyer rotary bodies 13B, 14B acting as the first sheet conveyer means tins time and the pair of conveyer rotary bodies 13A, 14A acting as second sheet conveyer means to thereby correct the oblique travel thereof once more again and then conveyed to an image forming unit confronted by the photoreceptor drum 8 disposed to the process cartridge 7. Further, the sheet S which was drawn out from the feed unit U1 by a feed roller 1b acting as first sheet conveyer means first forms a loop between the feed roller 1b and the pair of conveyer rotary bodies 13B, 14B acting as second sheet conveyer means to thereby correct the oblique travel thereof, then is conveyed downstream and further forms a loop between the pair of conveyer rotary bodies 13B, 14B acting as first sheet conveyer means this time and the pair of conveyer rotary bodies 13A, 14A acting as second sheet conveyer means to thereby correct the oblique travel thereof again. Thereafter, the sheet S is conveyed to the image forming unit opposing the photoreceptor drum 8 disposed in the process cartridge 7. With these arrangements, even if the length of the conveyer passage of the sheet S from the feed unit where the sheet S is accommodated to the process cartridge 7 acting as the image forming means is relatively long, since the plurality of obliquely traveling sheet correcting devices are disposed at various positions, the oblique travel of the sheet S can be corrected on the conveyer passage of the sheet S as necessary. As a result, even if the sheet S being conveyed is caused to travel obliquely by a conveying resistance and the like arising in the sheet conveyer passage, the oblique travel can be corrected at the predetermined positions when necessary before the amount of the oblique travel is accumulated and increased, and thus image recording accuracy can be further enhanced. Although the sixth embodiment shows an example that the obliquely traveling sheet correcting devices according to the present invention are disposed at the three positions, the desired number of the obliquely traveling sheet correcting devices may also be disposed to other desired positions. Next, a seventh embodiment of the obliquely traveling sheet correcting device according to the present invention and the image forming apparatus provided with the obliquely traveling sheet correcting device will be described using FIG. 16. FIG. 16 is a cross sectional view showing the arrangement of the image forming apparatus provided with the obliquely traveling sheet correcting device of the eighth embodiment. Note, components arranged similarly to those of the first embodiment are denoted by the same numerals and the description thereof is omitted. In the above respective embodiments, a sheet S must form the amount of a loop which is necessary to correct the oblique travel of the sheet S in the state that the leading edge of the sheet S is abutted against the shutter member 15 and stopped by it. Since the amount of the loop is the same as the amount of the oblique travel of the sheet S to be corrected, when the amount of the oblique travel of the sheet S is large, a predetermined amount of the loop must be stably secured. In FIG. 16, when the leading edge of the sheet S conveyed by a pair of conveyer rollers 5 acting as first sheet conveyer means is stopped by abutting against the collision surface 20a of the shutter member 15 disposed to the pair a conveyer rotary bodies 13, 14 acting as the second sheet conveyer means, the sheet S forms a loop so as to be separated from a conveyer guide surface G 1 acting as a sheet guide with a predetermined curvature interposed between the pair of conveyer rollers 5 and the pair of conveyer rotary bodies 13, 14. When there is no member for guiding the sheet S on a side confronting the conveyer guide surface G 1 at the time the predetermined amount of the loop is formed, there is a problem that the urging force causes the sheet S to increase the size the loop endlessly and eventually buckles so that it cannot enter the nip portion between the conveyer rotary bodies 13, 14, and a jam occurs. The seventh embodiment, for solving the above problem, is arranged such that the sheet S whose leading edge is stopped by the shutter member 15 is caused to form a stable loop so that the sheet S can securely pass through the shutter member 15 by turning and retracting the shutter member 15 by pushing it by the stiffness thereof. In FIG. 16, a flapper member 22, which confronts the conveyer guide surface G 1 acting as a sheet guide having a predetermined curvature as well as has a guide surface 22a formed to have a curvature corresponding to that of the conveyer guide surface G 1 , is turnably disposed between the pair of conveyer rollers 5 and the pair of conveyer rotary bodies 13, 14 so as to approach and retract from the conveyer guide surface G 1 . The center of turn of the flapper member 22 is disposed in the vicinity of a line extending from the nip portion between the pair of conveyer rollers 5 as shown in FIG. 16. The conveyance of the sheet S is guided by setting the flapper member 22 to the position shown by the solid line of FIG. 16 where the flapper member 22 does not open until the leading edge of the sheet S conveyed along the conveyer guide surface G 1 enters the shutter member 15. Then, the time when the leading edge of the sheet S is stopped by abutting against the shutter member 15 and the sheet S forms a loop, the flapper member 22 is opened by being turned to the position shown by the broken line in FIG. 16 so that the sheet S can form the loop which is sufficient to correct the oblique travel thereof. At least two sets of the flapper members 22 are disposed on the same line in the width direction of the sheet S in correspondence to the width of the sheet S being conveyed so that they can be abutted against at least both the edges of the sheet S being conveyed in the vicinity thereof in a sheet conveying direction. As a result, the flapper members 22 can absorb the difference between the amounts of a loop on the right and left sides of the sheet S as well as prevent that the loop is excessively grown on the contrary and the leading edge of the sheet S is apart from the collision surface 20a of the shutter member 15 or enters the nip portion between the pair of conveyer rotary bodies 13, 14 before the oblique travel of the sheet S is not completely corrected. Since the flapper member 22 exerts a force for pushing the loop of the sheet S by being abutted against the sheet S by the dead load thereof, it can sufficiently exhibit an effect even if it is light in weight. Further, as another arrangement, there may be provided a space, where the sheet S can form a loop in a direction toward which the curvature thereof is increased from the vicinity of the line extending from the nip portion between the pair of conveyer rollers 5, in the region between the pair of conveyer rollers 5 and the pair of conveyer rotary bodies 13, 14 making use of the curvature of the conveyer guide surface G 1 shown in FIG. 16 18 so that the sheet S forms the loop necessary to sufficiently correct the oblique travel thereof by being guided by the space. Further, an elastic member by which the loop formed by the sheet S is guided may be disposed in confrontation with the conveyer guide surface G 1 in place of the flapper member 22 so that the sheet S can form the loop necessary to sufficiently correct the oblique travel thereof by being guided by the elastic member. Next, an eighth embodiment of the obliquely traveling sheet correcting device according to the present invention and the image forming apparatus provided with the obliquely traveling sheet correcting device will be described using FIG. 17 and FIG. 18. FIG. 17 is a cross sectional view showing the arrangement of the image forming apparatus provided with the obliquely traveling sheet correcting device of the ninth embodiment and FIG. 18 is a view illustrating the main portion of the portion A 3 in FIG. 17. Note, components arranged similarly to those of the first embodiment are denoted by the same numerals and the description thereof is omitted. The eighth embodiment enhances the drawing-up property of a sheet S when it is discharged by disposing a shutter member 15 constituting the obliquely traveling sheet correcting device to a pair of discharge rollers 3. In particular, a sheet S to which a toner image is fixed by being subjected to heat and press processing in a fixing unit 11 is liable to be curled and if the sheet S is discharged in the curled state, the drawing-up property thereof is lowered. To cope with this problem, the drawing-up property and loading property of the sheet S can be enhanced when the sheet S is discharged in such a manner that the oblique travel of the sheet S is corrected by forming a loop thereto in front of the pair of discharge rollers 3 as well as an action for removing the curl of the sheet S is added by the cooperation of the shutter member 15 with the pair of discharge rollers 3. As shown in FIG. 18, the shutter member 15 arranged approximately similarly to that of the first embodiment is mounted to the rotary shaft 3c of one of the pair of discharge rollers 3 acting as second sheet conveyer means so that the shutter member 15 can turn about the rotary shaft 3c with respect thereto and the other end of an urging spring 25, one end of which is securely fixed to a device main body frame 17, is engaged with one end of the shutter member 15. As shown by the solid line in FIG. 18, the shutter member 15 is set to the position of a first attitude where the shutter member 15 is abutted against the leading edge of the sheet S, which is conveyed by a pair of discharge rollers 2 acting as first sheet conveyer means, by the urging force of the urging spring 25 and can stop the leading edge thereof. Then, when the leading edge of the sheet S conveyed by the pair of discharge rollers 2 turns the shutter member 15 about the rotary shaft 3c and retracts the same by pushing it, the shutter member 15 is set to the position of a second attitude where an abutting portion 15f formed to one end of the shutter member 15 is abutted against the device main frame 17 and permits the sheet S to pass through the shutter member 15. Further, there are provided between the pair of discharge rollers 2 and the pair of discharge rollers 3 a conveyer guide surface G 2 acting as a sheet guide having a predetermined curvature and a conveyer guide surface G 3 for forming a space having a shape 26 corresponding to the shape of a loop formed by the sheet S opposing the conveyer guide surface G 2 . In these conveyer guide surfaces G 2 and G 3 , the conveyer guide surface G 3 forms the space 26 which expands in the left direction in FIG. 18 from the line extending from the nip portion between the pair of discharge rollers 2, so that the loop of the sheet S can be positively formed by the space 26. With the above arrangement, when the sheet S is conveyed by the pair of discharge rollers 2 and the leading edge of the sheet S is stopped by being abutted against the collision surface 20a of the shutter member 15 held at the position of the first attitude by the urging force of the urging spring 25, the sheet S forms a loop in the space 26. Then, when the loop of the sheet S is abutted against the conveyer guide surface G 3 , the leading edge of the sheet S turns the shutter member 15 about the rotary shaft 3c by pushing it to thereby retract it, so that the sheet S passes through the shutter member 15 and enters the nip portion between the pair of discharge rollers 3 so that it is conveyed by being clamped by the nip portion of discharge rollers 3. At the time, the shutter member 15 is held at the position of the second attitude shown by the broken line of FIG. 18 and the angle  between the direction toward which the sheet S enters the nip portion between the pair of discharge rollers 3 and the squeezing surface 15g of the shutter member 15 is set to a predetermined angle. With this arrangement, the curl of the sheet S is removed in such a manner that the sheet S is squeezed by the action effected between the nip portion between the pair of discharge rollers 3 and the squeezing surface 15g of the shutter member 15 and then the sheet S is discharged to the outside of the device. Since the direction of the curl formed to the sheet S is different depending upon various types of the image forming apparatus, the correction of the oblique travel and the removal of the curl of the sheet S can be executed at one time in correspondence to the various types of the image forming apparatus by suitably changing the urging force of the urging spring 25, the squeezing angle , the regulating direction of the shutter member 15 and the like in correspondence to the various types of the image forming apparatus. In the eighth embodiment, since the obliquely traveling sheet correcting device exhibits a curl removing function in addition to the oblique travel correcting function for the sheet S, the drawing-up property and the loading property of the sheet S after the sheet S is discharged is enhanced. Although the eighth embodiment shows the case that the obliquely traveling sheet correcting device is disposed near the portion of the image forming device from which the sheet S is discharged, the present invention is not limited thereto but a similar advantage can be obtained by disposing one or a plurality of the obliquely traveling sheet correcting devices at desired positions on a conveyer passage on which the sheet S is conveyed. Although the above respective embodiments exemplify the case that they are applied to the image forming apparatus provided with electrophotographic image forming means as image forming means, they may be also applied to an image forming apparatus provided with image forming means comprising an ink jet head or a thermal head as other arrangement. Since the present invention has the arrangement and function as described above and need not use electric control such as the start/stop control of conveyer rollers such as a pair of resist rollers used in the aforesaid prior art, the number of sheet sensing means and the like such as sensors and the like for detecting a sheet can be minimized, and the mechanism for correcting the oblique travel of a sheet can be simply arranged at a low cost and a space can be saved. Since the urging force of the shutter member to the sheet is adjustable, a sheet conveying capability and an oblique travel correcting capability can be secured regardless of a type of the sheet. The number of components can be reduced by using a portion of the shutter member also as a portion of the sheet sensing means and a cost can be reduced by it as well as a timing at which the image forming means starts to record an image can be maintained constant regardless of a period of time necessary for the sheet to pass through the shutter member. Since the space where the sheet forms a loop is secured upstream of the shutter member, a large oblique travel correcting capability can be exhibited. The sheet oblique travel preventing function can be enhanced by disposing a plurality of the obliquely traveling sheet correcting devices according to the present invention at predetermined positions of the sheet conveyer passage. Further, the drawing-up property and the loading property of the sheet can be enhanced when the sheet to which an image has been formed is discharged by the provision of the sheet curl removing function in addition to the sheet oblique travel correcting function. 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 scope of the appended claims.

Claims: An obliquely traveling sheet correcting device, comprising: first conveyer means (2; 5) for conveying a sheet (S); second conveyer means (3; 13, 14) disposed downstream of said first conveyer means (2; 5) for conveying the sheet (S) while clamping it; and stopping means (15) movable between a first position where it stops the leading edge of the sheet (S) conveyed by said first conveyer means (2, 5) and a second position where it permits the sheet conveyed by said first conveyer means (2; 5) to pass therethrough, wherein when said stopping means (15) is located at the first position, it abuts, by means of a collision surface (20a, 20b, 20c) of the stopping means (15), against the leading edge of the sheet (S) upstream of said second conveyer means (3; 13, 14), characterized in that said stopping means (15) moves from the first position to the second position by being pushed by the sheet (S) conveyed by said first conveyer means (2; 5), and in that the stopping means (15), while moving to the second position, moves thereto while continuing to abut against the leading edge of the sheet (S) at least until the leading edge of the sheet is clamped by the second conveyer means (3; 13, 14). A device according to claim 1, wherein said second conveyer means includes a pair of rotary bodies (3; 13, 14) for conveying the sheet (S) while clamping it. A device according to claim 1, wherein said stopping means (15) is turnably supported and is acted upon by gravity such that the stopping means (15) generates torque in a direction for forcing back the leading edge of the sheet (S) conveyed by said first conveyer means (5). A device according to claim 3, wherein the action of gravity on said stopping means (15) is such that the more said stopping means (15) is turned as it is pushed by the sheet (S), the weaker the torque in the direction for forcing back the sheet is made. A device according to claim 1, wherein said stopping means (15) includes a holding unit (20d) for holding the leading edge of the sheet (S). A device according to claim 5, wherein said holding unit includes a groove (20d) formed to permit the leading edge of the sheet (S) to enter. A device according to claim 1 or 5, wherein said stopping means (15) is disposed upstream to and in the vicinity of said second sheet conveyer means (3; 13, 14) wherein the movement of said stopping means (15) between said first and second positions is a swinging movement, and wherein the device further comprises an urging means (16, 24, 25) for urging said stopping means (15) to the first position. A device according to claim 7, wherein the collision surface (20a, 20b, 20c) of said stopping means (15) comprises at least three members to be abutted against the leading edge of the sheet (S), said members disposed at at least three positions along a line running perpendicular to the sheet conveying direction, and at least one position of the members being disposed at the center of the width of the sheet (S) being conveyed. A device according to claim 7, wherein the collision surface (20b) of said stopping means (15) to be abutted against the leading edge of the sheet (S) is composed of an arc surface having a predetermined radius. A device according to claim 7, wherein when said stopping means (15) is set to the first position where it stops the leading edge of the sheet (S), the collision surface (20c) of said stopping means (15) to be abutted against the leading edge of the sheet has a surface which is across the surface of the sheet at an acute angle less than 90°. A device according to claim 7, wherein said stopping means (15) is arranged such that when the sheet (S) swings said stopping means (15) by pushing it, the reaction force which the sheet (S) receives from the stopping means (15) weakens as the leading edge of the sheet causes said stopping means to swing. A device according to claim 11, wherein the reaction force which said urging means (16) causes said stopping means (15) to act on the sheet is generated by gravity acting on said stopping means. A device according to claim 7, wherein said urging means (24) includes reaction force switching means (23a, 23b) for adjusting the reaction force acting on the sheet (S) by said stopping means (15). A device according to claim 7, wherein said second sheet conveyer means (3) has a sheet oblique travel correcting action and a sheet curl removing action in cooperation with said stopping means (15). A device according to claim 7, wherein said second sheet conveyer means (3; 13, 14) includes a sheet conveyer rotary body (13) and said stopping means (15) is swingable about the center of rotation of said sheet conveyer rotary body (13). A device according to claim 15, wherein: said sheet conveyer rotary body (13) is divided into a plurality of portions each disposed on the same axis; said stopping means (15) is swingingly supported between the divided portions of said sheet conveyer rotary body (13); and further comprising an auxiliary free roller (19), wherein said stopping means (15) is positioned with respect to said sheet conveyer rotary body (13) to permit said sheet conveyer rotary body (13) and said stopping means (15) to independently turn, said auxiliary free roller (19) being disposed between the divided portions of said sheet conveyer rotary body (13) and said stopping means (15). A device according to one of claims 1 to 7, wherein a space having a shape (26) corresponding to the shape of a loop which is formed by the sheet (S) by the action of said stopping means (15) is provided on a sheet conveyer passage between said first sheet conveyer means (2) and said second sheet conveyer means (3). A device according to claim 7, further comprising a sheet guide (G 1 ) having a shape corresponding to the shape of a loop which is formed by the sheet (S) by the action of said stopping means (15) and disposed in a sheet conveying passage between said first sheet conveyer means (5) and said second sheet conveyer means (13, 14), and a flapper member (22) disposed at a position corresponding to said sheet guide (G 1 ) for assisting the formation of the loop of the sheet (S). A device according to claim 18, wherein said flapper member (22) is disposed at at least two positions on a line in the direction perpendicular to the sheet conveying direction in correspondence to the width of the sheet conveyed and abutts against at least at both the edges of the sheet (S) being conveyed in the sheet conveying direction. A device according to one of claims 1 to 7, further comprising a sheet guide (G 1 ) having a shape corresponding to the shape of a loop formed by the sheet (S) by the action of said stopping means (15), said sheet guide positioned between said first sheet conveyer means (5) and said second sheet conveyer means (13, 14), and an elastic member disposed at a position corresponding to said sheet guide (G 1 ) for assisting the formation of the loop of the sheet (S). A device according to claims 7 to 20, comprising a plurality of said stopping means (15) disposed on a conveyer passage through which the sheet (S) is conveyed. An image forming apparatus, comprising: an obliquely traveling sheet connecting device according to claim 1 or one of claims 7 to 14; and image forming means (6, 7, 8, 12) for forming an image on the conveyed sheet (S). An image forming apparatus according to claim 22, further comprising sheet sensing means (10) disposed in the vicinity of and downstream of said stopping means (15) in the sheet conveying direction and upstream of said image forming means in the sheet conveying direction. An image forming apparatus according to claim 23 wherein said stopping means (15) cooperates with said sheet sensing means (10). An image forming apparatus according to claim 24, wherein a portion (16a) of said stopping means (15) also acts as a portion of said sheet sensing means (10). An image forming apparatus according to claim 22 or claim 23 wherein said second sheet conveyer means includes an image carrier rotary body (8) of said image forming means (6, 7, 8, 12) or a transfer rotary body confronting said image carrier rotary body.

Inventors: AMANO MASAO, KAWAI TOSHIHARU, KUDO KAZUHIDE, AMANO, MASAO, KAWAI, TOSHIHARU, KUDO, KAZUHIDE
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1997
Main classification: B65H9
IPC codes: B41J13, B65H9