Abstract:
A sheet turning device in a sheet transporting path for receiving a sheet transported in a predetermined direction turning the sheet and transporting the sheet in the predetermined direction. A pair of rollers are provided with a belt extending between the rollers for turning the sheet. A switch is provided for stopping the sheet transported onto the lower inner surface of the sheet turning belt. A driving shaft circulates the sheet turning belt to transport the sheet kept on the lower inner surface of the sheet turning belt in a direction orthogonal to the predetermined direction. A pressing member is provided for stopping and pressing the sheet on the upper inner surface of the sheet turning belt, and a sheet transporting roller for transporting the sheet pressed by the pressing member in the predetermined direction.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present imvention relates to a device for turning a sheet of paper and, more specifically, to such a device usable for copying on both sides of a sheet of paper in a copying machine. 
     2. Disclosure of the Background Art 
     An image formation apparatus, such as a copying machine or a laser printer, capable of forming an image on both sides of a sheet of paper is commercially available. In such an image formation apparatus, the sheet must be turned. For that purpose, a switchback type sheet turning device as shown in FIG. 6, for example, is commonly employed. The sheet turning apparatus is provided with a sheet transporting path 52, which makes a loop-shaped path with a printing unit 51 therein, to transport the sheet in the counterclockwise direction in the figure. The sheet is turned in the course of the circulation in the sheet transporting path 52 by rollers 53 which rotates in both the forward and reverse directions. Specifically, after printed with an image on one side in the printing unit 51, the sheet is transported to the rolllers 53 and moved by the rollers 53 until the tail portion of the sheet is nipped by the rollers 53. Then, the sheet is stopped once and transported with its original tail portion first by reverse rotation of the rollers 53. Since the sheet is turned by the transportation through which the original tail portion of the sheet goes first, the other side of the sheet is then formed with an original image when the sheet is transported through the printing unit 51 again. After that, the sheet with two images on both sides is discharged onto a sheet receiver 54 by the forward rotation of the rollers 53. 
     When a two-sided copy is carried out with the above device turning the sheet, the directions of the images on both sides of the sheet may be opposite to each other. For example, in the case where a direction of the sheet transportation is the same as that of the images, an image shown in a solid line in FIG. 7 is made on the first face of the sheet which is transported in the direction of an arrow A shown in FIG. 7. However, after the rollers 53 turns the sheet so that the tail of the sheet goes first, the sheet is transported in direction of an arrow B. Therefore, the direction of the image formed on a second face of the sheet is opposite to that of the image on the first face as can be seen from an illustration of a broken line in FIG. 7. 
     In order to solve the above disadvantage, image information stored in memory can be read from the end of the information in, for example, a laser printer. In this manner, however, the control must be complicated. 
     Transporting a sheet in a direction orthogonal to the transporting direction may come to turn the sheet as proposed in Japanese Examined Patent Publication No. 22787/1980. According to the description of the publication, first a sheet having an image on one side is turned so that the first character train in the image goes first, that is, the sheet is turned in a direction opposite to the predetermined direction when the image is automatically copied on both sides of the sheet through a copying machine. Second after the sheet goes through a fuser, the sheet is received by the sheet turning device. In the device, the sheet is transported in a direction orthogonal to the predetermined direction so that one side of the sheet goes first, and then the sheet is turned. Third, the sheet is transported in the predetermined direction so that the original head of the sheet goes first, and then the sheet is turned. Fourth, the sheet is transported in order to copy an image on the second face of the sheet. Fifth, the sheet having two images on both sides is received from the fuser by the sheet turning device and turned. After that, the sheet is transported to a sorter. 
     SUMMARY OF THE INVENTION 
     The present invention provides a sheet turning device in a sheet transporting path for receiving a sheet transported in a predetermined direction, turning the sheet and transporting the sheet in the direction, comprising a pair of rollers disposed on opposite sides of the sheet transporting path along the predetermined direction; a belt extending between the rollers for turning the sheet; means for stopping the sheet transported onto the lower inner surface of the belt; means for circulating the belt to transport the sheet kept on the lower inner surface of the belt in a direction orthogonal to the predetermined direction so that the sheet turns; a pressing member for stopping and pressing the sheet which is turning and moving, on the upper inner surface of the belt; and a sheet transporting roller for pressing the sheet against the pressing member to transport the sheet pressed by the pressing member in the predetermined direction in spite of pressing the sheet against the upper inner surface by the pressing member. 
     Since the device according to the present invention has the pair of rollers and the sheet turning belt therebetween in the sheet transporting path, the transported sheet is moved upward by the inner surface of the belt in a direction orthogonal to the predetermined direction so that it is turned. The turning of the sheet is carried out not in a direction identical with the predetermined direction but in a direction orthogonal to the predetermined direction, so that the sheet can be turned without changing the head and tail of the sheet. 
     When the sheet is turned, the sheet moves upward on the inner surface of the sheet turning belt, so that the sheet must be on the upper inner surface of the belt. In order to keep the sheet, the device according to the present invention has the pressing member for pressing the upper inner surface of the sheet turning belt. Since the pressing member is placed inside the sheet turning belt, the device can be miniaturized. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 to 3 are views showing a constitution of a sheet turning device which is an embodiment according to the present imvention: FIG. 1 is plan view, 
     FIG. 2 is an elevational view and 
     FIG. 3 is a side view seen from the right; 
     FIGS. 4(A) to 4(E) are views presented for explaining a sequential steps by which a sheet is turned in the sheet turning device; 
     FIG. 5 is a view showing a main portion of a laser printer applied to the sheet turning device; 
     FIG. 6 is a view showing a sheet turning mechanism of a prior art embodiment; and 
     FIG. 7 is a view showing an example of an image produced when a sheet is turned by the prior art sheet turning device. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 5 is a schematic view showing a constitution of a main portion of a laser printed L having a sheet turning device which is an embodiment according to the present invention. 
     The laser printer L comprises a printing unit 1, a sheet circulating unit 2, a sheet turning unit (a sheet turning device) 3, a sheet tray 4 and a sheet receiver 5. 
     The printing unit 1 includes a photoconductive drum 11 made of photosensitive substance and a processig unit provided on the peripheral portion of the drum 11 for producing an image. The printing unit 1 produces an image on the upper side of a sheet fed from the sheet tray 4 or the sheet circulating unit 2. An electrifying device 12 for uniformly establishing static electric charge on the surface of the photoconductive drum 11, a laser unit 13 for directing light to the charged photoconductive drum 11 to make a latent image thereon, a developing unit 14 for depositing toner on the latent image and making it visible by development, a transferring unit 15 for transferring the image of toner to paper and a cleaner 17 for eliminating the toner remaining on the photoconductive drum 11 are disposed in that order, surrounding the photoconductive drum 11, to produce an image with toner on a sheet of paper. The image of toner is fused on the sheet by a fuser 16, and then the sheet is transported to the sheet circulating unit 2. 
     The sheet circulating unit 2 receives the sheet from the fuser 16, transports the sheet in the counterclockwise direction so as to pass it under the printing unit 1 and further transports it through the sheet turning device 3 to the printing unit 1 again. The sheet circulating unit 2 is provided with a flap 21 close to the fuser 16. The flap 21 sorts sheets fed from the fuser 16 upward and downward by virtue of a solenoid 21a and a return spring 21b. A sheet guided upward is transported to the sheet receiver 5, and a sheet guided downward is transported to the printing unit 1 again through the sheet turning device 3. The sheet turning device turns the sheet. The sheet turning device has a microswitch 22 serving as a sheet stopper and a feeding roller 23 in its inlet. 
     FIGS. 1 to 3 are views showing a constitution of the sheet turning device 3: FIG. 1 is a plan view, FIG. 2 is an elevational view and FIG. 3 is a side view seen from the right. 
     Arrow C in those figures denotes a predetermined direction in which the sheet circulating unit 3 transports a sheet. Reference character D in FIG. 1 denotes a sheet transporting path in the sheet circulating unit 3. 
     A pair of rollers 31a, 31b are disposed on opposite sides of the sheet transporting path D in a direction corresponding to the predetermined direction C. The rollers 31a, 31b are connected to a driving system (not shown), such as a driving shaft of a photoconductive drum serving as belt driving means and rotated in the counterclockwise direction in FIG. 3. Belts, more specifically four sheet turning belts, 32a to 32d extend between the rollers 31a, 31b at an interval to each other. These belts 32a to 32d are rotated in the direction orthogonal to the predetermined direction C (namely, in the counterclockwise direction in FIG. 3) by the rotation of the rollers 31a, 31b. Between the belts 32b, 32c, a sheet transporting roller 33 is positioned. The sheet transporting roller 33 (referred to simply as &#34;transporting roller 33&#34; below), which is rotated by a driving system not shown in the drawings, transports the sheet whose head and tail are reversed in the predetermined direction C. 
     A pressed 34 is mounted close to and inside the upper extension of each of the belts 32a to 32d between the rollers 31a, 31b. The presser 34 may be a plate of resin such as acrylic and is pressed against the upper extension of each of the belts 32a to 32d from the inside by pressing members including springs 35. The sheet is sent in a space between the presser 34 and the belts 32a to 32d by the rotation of the belts 32a to 32d. The transporting roller 33 rotates in contact with the upper surface of the presser 34 when the sheet is transported. The transporting roller 33 has semicircular pickup rollers 33a. Usually the transporting roller 33 does not come in contact with the presser 34, because each of the pickup rollers 33a is positioned with a straight part of its peripheral surface being opposite to the presser 34. However, in the case where the sheet sent to the presser 34 is transported in the predetermined direction C, the transporting roller 33 rotates, and a arc portion each of the pickup rollers 33a comes in contact with the presser 34. The sheet on the presser 34 is transported in the predetermined direction C by friction force against the pressing force that the presser 34 produces by pressing the upper inside extension of each of the belts 32a to 32d. 
     An end 34a of the pressser 34 is slightly bent down close to the roller 31a which comes in contact with the sheet and serves as a paper guide. Because of the end 34a, the sheet transported from the sheet turning device along the inner surface of each of the belts 32a to 32d is assuredly guided to a space between the presser 34 and the belts 32a to 32d without deviating from the presser 34. On the other hand, an end 34b of the presser 34 is bent up close to the roller 31b which does not come in contact with the sheet and serves as a guard. The end 34b stops the sheet sent through the space between the presser 34 and the belts 32a to 32d. A microswitch 36 is mounted close to the end 34b to sense the sheet kept at the end 34b. 
     Further, a paper guide 37 is held slightly above the inner surface of the lower extension of each of the belts 32a to 32d along the predetermined direction C. The paper guide 37 prevents the belts 32a to 32d extending orthogonal to the predetermined direction C from catching the sheet from the sheet circulating unit 2. The sheet is sent along the paper guide 37. 
     FIGS. 4(A) to 4(E) are views showing sequential steps of the operation of the sheet turning device. According to convenience of explanation, FIGS. 4(A) and 4(E) are elevational views, and FIGS. 4(B) to 4(D) are side views. 
     First, a sheet 40 printed with an image on one side is fed to the sheet turning device 3. The sheet 40 is sent in a direction corresponding to an arrow C along the paper guide 37 and is stopped on the inner surface of the lower extension of each of the belts 32a to 32d in accordance with a stop of the feeding roller 23 a predetermined period of time after the microswitch 22 turns off (the tail of the sheet passes the microswitch 22). At this time, the sheet 40 has a face having the image positioned on the surface of each of the belts 32a to 32d (namely, the sheet 40 lies on the face having the image) (see FIG. 4(A)). 
     When the rollers 31a, 31b are rotated in the counterclockwise direction, the sheet 40 begins to move along the inner surface of the extension of each of the belts 32a to 32d (FIG. 4(B)) and is sent to the space between the presser 34 and the belts 32a to 32d (FIG. 4(C)). The transportation of the sheet 40 is stopped by the end portion 34b of the presser 34, and simultaneously the microswitch 36 turns on (FIG. 4(D)). In accordance with the turning on of the microswitch 36, the rollers 31a, 31b are stopped. At this time, the sheet 40 has the face having no image positioned on the surface of the belts 32a to 32d, namely, the sheet 40 is turned compared to the original state shown in FIG. 4(A). 
     Lastly, the feeding roller 33 rotates to move the sheet 40 on the presser 34 in the predetermined direction C (FIG. 4(E)) to the printing unit 1 so that the sheet 40 lies on the face having no image therein. 
     Thus, the sheet turning device 3 can turn a sheet in the above mentioned manner. In this sheet turning device 3, the head and tail of the sheet is kept unchanged, and therefore there is no need to rearrange an original image upside down. The sheet turning device 3 can be advantageously miniaturized because it does not require many trays and sheets are transported along and inside the sheet turning belts unlike a device disclosed in Japanese Examined Patent Publication No. 22787/1980. 
     Although there are four of the sheet turning belts 32a to 32d extending between the rollers 31a, 31b in this embodiment, one belt may be substituted for them. An advantage of using a plurality of belts is that troubles such as a jam are easily settled. 
     As has been described, the sheet turning device 3 turns a sheet without changing the head and tail of the sheet with regard to a predetermined direction in which sheets are transported, and therefore there is no possibility that an image is produced upside down when the sheet turning device 3 is employed in an imaging machine or the like. Additionally, there is another advantage that the sheet turning device can be miniaturized because the sheet is turned by transporting it along the inner surface of the belts extending between the rollers.