Sheet feeding device

Abstract: In a sheet feeding device for separating one sheet at a time from a stack of sheets (1) piled on a sheet feed tray (3) for feeding same to the next processing station, the distance L between a point (18) at which feeding means (4) for exerting a sheet feeding force on the uppermost sheet (1-a) of the stack of sheets (1) piled on the sheet feed tray (3) is brought into contact therewith and a point (19) at which separating means (5, 6) for offering resistance to the sheets (1) for separating them is brought into contact with the sheets (1) fed by the feeding means (4) or the relation between such distance L and a pressing force W exerted by the pressing means (2) to urge the stack of sheets (1) against the feeding means (4) is set in a range causing no buckling of the sheets (1) to occur between the feeding means (4) and the separating means (5, 6) during sheet feeding.

Description: Field of the Invention This invention relates to sheet feeding devices suitable for use with optical character read-out apparatus, printers, copy machines, etc., and more particularly it is concerned with a sheet feeding device of the type described capable of stably carrying out separation and feeding of sheets of below 55 kg paper. In this specification, the term "55 kg paper" refers to sheets of a size 788 mm × 1091 mm having a ream weight of 55 kgf in 1,000 sheets. Brief Description of the Drawings Figs. 1(a) and 1(b) are views showing the manner in which sheets are fed by a sheet feeding device of the prior art; Fig. 2 is a schematic perspective view of the sheet feeding device in its entirety according to the invention; Fig. 3 is a vertical sectional view of the essential portions of one embodiment of the sheet feeding device in conformity with the invention; Figs. 4 and 5 are views in explanation of the principles of the invention, Fig. 4 showing the manner in which a sheet is caused to buckling and Fig. 5 being a graph showing the buckling characteristic of a sheet; and Fig. 6 is a diagrammatic representation of the results of experiments showing the buckling characteristic of the sheets obtained with the sheet feeding device according to the invention. Description of the Prior Art There has in recent years been a demand to carry out rationalization of office work and various kinds of office automation equipment have been developed. The majority of office work is accounted for by paper work consisting of making and filing documents. To rationalize such work, it is important that input devices for reading the information recorded on a paper and output devices for printing out the results of calculation have their performance improved. For example, optical character read-out apparatus and various printers have important functions as input and output devices for office work. Meanwhile in this type of work, accumulation and transfer of information rely on sheets as a medium in many cases, and in practice the volume of sheets used in office work is increasing by leaps and bounds year by year. With this background, to use sheets of a small thickness for office work is an important requirement that should be met with a view to conserving natural resources and reducing office space. However, automatic sheet feeding devices of the prior art that have been developed for use in offices are only able to handle sheets of a large thickness such as sheets of over 55 kg paper. When the sheets used are lower in thickness, their rigidity is reduced and difficulties are experienced in handling them, with a result that the trouble of double feeding or sheet jamming occurs. Thus the aim of achieving rationalization of office work is defeated. For example, optical character read-out apparatus can handle without any trouble only those sheets of relatively high thickness and rigidity which are of 70-135 kg paper. On the other hand, there are two types of processes used in actual practice for separating one sheet at a time from a stack of sheets stored in a hopper and feeding them. One process relies on the force of friction. When these processes are used for feeding thin sheets, the following problems have been raised. In a process for attracting a sheet by means of a vacuum pump, thin sheets are air-permeable and not only one sheet but also more than two sheets are attracted by the force of vacuum, thereby causing double feeding to occur. A process is available which relies on subatmospheric pressure in attracting sheets for separating one sheet from the rest of the sheets. However, this process suffers a disadvantage in that a blower of a large capacity is required and the apparatus for working the process becomes large in size. In addition, the air makes a large noise, so that the requirements of reducing size and noise level cannot be met. Meanwhile a frictional separation mechanism used in many applications in copying apparatus, printers, etc., has also had the problems of sheet jamming, sheet bending and wrinkle formation due to a lack of rigidity in the sheets handled. As one example of the frictional separation mechanism, a device is disclosed in US-A-3981497. In the preamble of claim 1 a prior art is described as shown in Fig 1(a). As shown in Fig. 1(a), pickup feeding rollers R0 are in light pressing engagement with the uppermost sheet 1-a of a stack of sheets piled on a sheet feed tray A. The sheets fed by the pickup rollers R0 are separated one from another by separating means or a pair of rollers R1 and R2 located downstream of the pickup roller R0. In this construction, the uppermost sheet 1-a is fed by the pickup rollers R0 toward the supply roller R1. However, when the sheets handled are thin, the problem shown in Figs. 1(a) and 1(b) is raised. More specifically, the supply roller R1 rotates clockwise as shown in Fig. 1(a), but the friction member R2 in pressing engagement with the supply roller R1 remains stationary or rotates in the reverse direction to separate one sheet from another sheet as they are introduced between the two rollers R1 and R2. Thus the sheet 1-a fed by the pickup rollers R0 and moved leftwardly in Fig. 1(a) moves in sliding movement on a guide member G. However, if the leading end of the sheet 1-a abuts against the guide member G, its movement is interfered with. When the sheet is thick and has high rigidity, the rigidity of the sheet 1-a might overcome the frictional force of the friction member R2 to allow the leading end of the sheet 1-a to move leftwardly. However, when the sheet 1-a is thin and has low rigidity, the movement of the sheet 1-a is interfered because the frictional force of the friction member R2 is too high for the leading end of the sheet 1-a to move forwardly by overcoming it. That is, the first sheet 1-a buckles as shown, and if the pickup rollers R0 continue rotating, the trailing end portion of the first sheet 1-a alone is moved forwardly until the first sheet 1-a is warped between the pickup rollers R0 and the supply roller R1, resulting in a sheet jamming. If the first sheet 1-a develops buckling or jamming as aforesaid, the feeding force of the pickup rollers R0 is exerted on the second sheet 1-b with which the pickup rollers R0 are brought into contact, so that jamming of the sheets continuously occurs. Also, the first sheet 1-a exerts a force of friction on the second sheet 1-b to cause same to move leftwardly. Thus the first sheet 1-a ceases to function as a guide for the second sheet 1-b which buckles in the same manner as the first sheet 1-a, thereby intensifying the jamming phenomenon. Fig. 1(b) shows the manner in which the first sheet 1-a has avoided being brought to the condition shown in Fig. 1(a) and is held between the supply roller R1 and the friction member R2 to be conveyed forwardly. The first sheet 1-a is kept flat without being bent between rollers R0 and R1 as shown. However, the second sheet 1-b has a feeding force exerted thereon as friction occurs between it and the first sheet 1-a, but its leading end portion is held between the underside of the first sheet 1-a and the friction member R2 and unable to move. As a result, the second sheet 1-b may undergo deformation under the first sheet 1-a and develop buckling, until finally it may be bent near its leading end portion and develop jamming. There is a possibility that a similar phenomenon will occur with regard to the third sheet 1-c. The foregoing description refers to separating one sheet at a time from a stack of sheets to convey same forwardly In printers, the need arises to use a sheet unit comprising a plurality of carbon or noncarbon sheets. In this case, sheet units each comprising a plurality of sheets bonded to one another as by pasting at the leading end portions have to be fed one after another. This sheets of about 35 kg paper are generally used for this purpose. Thus when the first sheet of the uppermost sheet unit is fed by pickup rollers, the second and the following sheets of the uppermost sheet unit may not be moved by the friction between underlying sheets, so that the first sheet of the sheet unit may only be fed. As a result, a situation similar to that shown in Fig. 1(a) may occur thereby causing a sheet jamming to occur. All the phenomena described hereinabove are attributed to the fact that the sheets small in thickness and low in rigidity are liable to buckle. Summary of the Invention (1) Objects of the invention An object of the invention is to provide a sheet feeding device of high reliability capable of avoiding buckling or jamming of sheets in feeding them to the next processing station. Another object is to provide a sheet feeding device capable of accurately separating thin sheets thinner than 55 kg paper one by one by avoiding buckling or jamming in feeding them to the next processing station. (2) Statement of the Invention These objects are obtained according to the invention by a sheet feeding device as defined in claim 1 An Advantageous embodiment of this device is mentioned in dependent claim 2. Detailed Description of the Preferred Embodiment Fig. 2 shows the construction of a sheet feeding device according to the invention in its entirety. A stack of sheets 1 piled on a sheet feed tray 3 through springs 2 are separated into one sheet at a time by pickup feeding rollers 4 and by a supply roller 5 and a friction member 6 defining the separating means. The uppermost sheet 1-a of the stack of sheets 1 is in light contact with the pickup rollers 4, and the rollers 4 and 5 as well as a roller 12 connected to motors 7 and 8 through belts 9, 10 and 11 are rotated by the motors in the same direction to feed the sheet 1-a. Upon the motor 7 being actuated, the pickup rollers 4 and supply rollers 5 cooperate with each other to feed the uppermost sheet 1-a from the stack of sheets 1. Of the sheets moved leftwardly in the figure by a force of friction between the friction member 6 in pressing engagement with the supply roller 5 through a spring 13 and the supply roller 5, those which contact with the friction member 6 are interfered and the uppermost sheet 1-a alone which is brought into contact with the pickup rollers 4 and supply roller 5 is moved toward the downstream side. As a result, the stack of sheets 1 are separated one by one and transported by the pair of conveyor rollers 12 and 12' to the next processing station. The pickup rollers 4 are supported by a shaft 14 connected through a belt 11 to a shaft 15. A clutch 16 is mounted between the shaft 15 and the motor 7 to remove the drive forces exerted on the shafts 14 and 15 at a point in time at which the first sheet 1-a is held between the conveyor rollers 12 and 12'. A guide member 17 for guiding the stack of sheets 1 piled on the sheet feed tray 3 is provided, and the friction member 6 projects from the guide member 17 into pressing engagement with the supply roller 5. Fig. 3 shows the essential portions of one embodiment of the sheet feeding device in conformity with the invention. The point of contact 18 between the pickup rollers 4 and the stack of sheets 1 or the point at which a feeding force is exerted on the uppermost sheet 1-a and the point of contact 19 between the supply roller 5 and the friction member 6 which is the point at which a separating force is exerted on the sheets fed by the pickup rollers 4 located downstream of the point at which the feeding force is exerted on the uppermost sheet 1-a are separated by a distance L which is set at a level causing no buckling to occur between the pickup rollers 4 and the separating means during the time the sheets are fed to the next processing station. Research conducted by us has revealed that, when thin sheets thinner than 55 kg paper are handled, the distance L between the point at which a feeding force is exerted on the sheets and the point at which a separating force is exerted on the sheets that have been fed is preferably below 50 mm, to enable the sheet separating mechanism to satisfactorily function. The distance L which is preferably below 50 mm is set as a result of investigation into the buckling characteristic of sheets of different thicknesses and rigidity and based on the results of experiments on separation of sheets. The principles of separation according to the invention will now be described by referring to Figs. 4 and 5. As shown in Fig. 4, a force was exerted on a point spaced apart from the leading end of a sheet by a distance l to cause the sheet in a solid line position to be warped into a broken line position, and a reaction P produced when the buckling phenomenon occurred was measured. The measured reaction P was shown to have a characteristic represented by a solid line in Fig. 5. The solid line represents the buckling characteristic of a sheet of 55 kg paper. A study of a diagram in which the abscissa represents the distance l and the ordinate indicates the buckling reaction P shows that the smaller the distance l, the higher is the buckling reaction P. When the result of the test described hereinabove is applied to the separation mechanism shown in Fig. 3, it will be seen that it is necessary to reduce the pressing force with which the sheet 1 is forced against the pickup rollers 4 and to shorten the distance L between the pickup rollers 4 and the supply roller 5 or the distance L between the point 18 at which feeding force is exerted on the sheet 1 and the point 19 at which a separating force is exerted on the sheet 1 that has been fed. Referring to Fig. 5 again, it is possible to infinitely increase the value of l by reducing the force with which a sheet is fed by the pickup rollers 4. In actual practice, however, to feed a sheet by the pickup rollers 4 from a stack of sheets by overcoming a force of friction P p acting between between the sheets plus a force of friction R exerted by the friction member 6 on the leading end of the sheet, the device requires application of a force P F higher than a certain level (P F > p p ,+R). The force of friction P p acting between the uppermost sheet and the second sheet may vary depending on the thickness and size of the sheets. A sheet of 55 kg of a size A2 has a weight w of about 16 gf. The coefficient of friction µp between the sheets is generally 0.1 to 0.6, which coefficient Increases in the high humidity, now we assume that the coefficient of friction µp has a maximum value of 1.0 to cause the calculation for design to be more safe. Accordingly, P p may be represented by P p =w x µp=16 gf. On the other hand, the sheets fed by the pickup rollers 4 move on the surface of the guide member 17 in sliding movement. However, when they abut against the friction member 6, the force of friction R is exerted thereon to interfere with their movement. If the force of friction R becomes larger than the buckling reaction P of the sheets, jamming occurs. The force of friction R is greatly influenced by the angle at which the sheets abut against the friction member 6 and the coefficient friction (0.6 to 1.2) between the sheets and the friction member 6. The angle at which the sheets abut against the friction member 6 is decided by the dimensions and configurations of the guide member 17 and the friction member 6. In actual practice, deformation of sheets, such as bending, exerts influences on the angle. Experiments were conducted by us to obtain an optimum maximum force of friction R and it has been ascertained by the results and based on experiences that when the sheet handled is of 55 kg paper, the maximum friction force R is preferably about 30 gf. Thus the force with which the sheets are fed by the pickup rollers or the feeding force P F is 46 gf and the buckling reaction P corresponding to the feed force P F has a lower limit. More specifically, in Fig. 5, when the lower limit P₁ of the buckling reaction P is set at 46 gf, the value l₁ of the distance l is approximately 50 mm. In principle, the smaller the buckling reaction P₁, the greater can be made the value l₁ of the distance l (corresponding to the distance L in the sheet separation mechanism shown in Fig. 3). Referring to Fig. 3 again, it has been stated previously that the distance between the point 18 at which a feeding force is exerted on the sheet 1 by the pickup rollers 4 and the point 19 at which a separating force is exerted on the sheet 1 by the friction member 6 and the supply roller 5 is designated by L. It will be appreciated that, in view of the buckling characteristic of the sheet shown in Fig. 5, the higher the value of L, the more readily jamming of bending of the sheet occurs as a result of sheet buckling. Assume that the value of L has been decided. Then an allowable maximum value of a pressing force W with which the sheet 1 is forced against the pickup rollers 4 can be decided. Let the force (pressing force) with which the sheet 1 is forced against the pickup rollers 4 and the coefficient of friction between the sheets be denoted by W and µp respectively. Then a feeding force would be exerted on the second sheet 1-b under the uppermost sheet 1-a by the force of friction acting between them. At this time, a force of friction opposed to the aforesaid feeding force would be exerted on the underside of the second sheet 1-b because it is in contact with a third sheet 1-c below it. If the force of friction between any sheets remains constant at all times, the second sheet 1-b would be difficult to move. However, the coefficient of friction between the sheets does not remain constant because each sheet is differently processed at its upper- and undersides and a layer of air and/or bending or wrinkling exists between the sheets. Thus the second sheet 1-b usually moves as the uppermost sheet 1-a is fed by the pickup rollers 4. It a frictional feeding force essentially exerted on the second sheet 1-b is denoted by F p (=µpW), it would be evident in view of the buckling characteristic shown in Fig. 5 that bending or jamming of sheets would result unless the condition P > F p is satisfied. If the pressing force W were reduced, the frictional feeding force F p could be reduced and the condition P > F p could be satisfied. However, the value of L has a lower limit that is decided by design. Also, variations in the characteristic of the springs 2 for forcing the stack of sheets 1 against the pickup rollers 4 would occur. All things considered, it would be impossible to set the value of the pressing force W in the vicinity of zero, and there is, after all, an allowable minimum range for the values of allowable buckling reaction P. In Fig. 5, a dash-and-dot line representing the allowable range of values of P is shown in a straight line in approximation to the solid line. Thus the ranges of values of P and l that enable a mechanism feasible in actual practice can be essentially decided. More specifically, it will be seen that when the sheets handled are of 55 kg paper the following values can be optimally selected. The distance L (corresponding to l in Fig. 4) between the point 18 at which the pickup rollers 4 exerts a feeding force and the point 19 at which the supply roller 5 exerts a separating force may optimally be decided as below 50 mm. The pressing force W exerted by the pick up rollers 4 on the sheets 1 may optimally have a value such that the friction feeding force F p acting between the sheets would not exceed 500 gf. Also, the values of the distance L and the frictional feeding force F p may be selected to be in a region on the origin side of regions separated by the straight dash-and-dot line. It would appear that in Fig. 5 the condition P > F p is not satisfied because of the fact that the characteristic curve (solid line) is located closer to the origin than the straight line (dash-and-dot line). However, as can be clearly understood in view of the relation between the sheets 1-a and 1-b in Fig. 1(b), the sheet 1-a acts as a guide for the sheet 1-b and prevents deformation of the latter, so that in actual practice no buckling occurs in the range of the straight line. When the value of the frictional feeding force F p decided by the characteristic of the sheets has been selected, it is possible to decide upon the allowable range of values for the pressing force W by the formula W = F p /µp. Fig. 6 shows the results of experiments conducted on the buckling characteristic of sheets with regard to sheets of larger and smaller thicknesses than sheets of 55 kg paper which constituted the main objective of the experiments. The sheets serving as the objective of the experiments included those of 72 kg paper. 110 kg paper, 48 kg paper, 35 kg paper, 25 kg paper and 55 kg paper. In the diagram shown in Fig. 6, the abscissa represents the distance between the point at which the pickup rollers exert a feeding force on the sheets and the point at which the separating means exerts a separating force on the sheets, and the ordiniate indicates the frictional feeding force F p acting between the sheets of a stack of sheets pressed by feeding means. In the foregoing description, the pickup rollers have been described as being in the form of friction rollers From the foregoing description, it will be appreciated that the sheet feeding device according to the invention enables one thin sheet at a time to be fed by accurately separating them without the trouble of sheet bending or jamming occurring. The invention enables the sheets of a thickness smaller than 55 kg paper to be used in offices which have hitherto been difficult to handle by terminal equipment of office automation apparatus including OCR and printers. Thus the invention makes great contributions to the social effect like conservation of raw materials, reduction in paper costs for users and reduction in space required for storing sheets.

Claims: A sheet feeding device for separating one sheet at a time from a stack of sheets (1) piled on a sheet feed tray (3) and feeding same to the next processing station, comprising feeding means (4) comprising pickup rollers in frictional contact with the uppermost sheet (1-a) of the stack of sheets (1) piled on the sheet feed tray (3) for feeding same to separating means (5, 6); pressing means (2) for forcing the stack of sheets (1) piled on the sheet feed tray (3) against the feeding means (4); and separating means (5, 6) for offering a reaction force to the sheets (1) fed by the feeding means (4), said separating means (5, 6) comprising a supply roller (5) and a friction member (6) in pressing contact with the supply roller (5), characterized in that a pressing force W exerted by the pressing means (2) against the feeding means (4) is given by the following formula: W = F p /µp where F p designates a frictional feeding force acting between the sheets (1-a, 1-b) pressed by said pressing means (2), and µp designates a coefficient of friction between the sheets (1-a, 1-b), and that in a diagram wherein the abscissa represents a distance L between a point (18) at which the feeding means (4) exerts the feeding force on the sheets (1) and a point (19) at which the separating means (5, 6) exerts the reaction force on the sheets (1) and which is the point of contact between the supply roller (5) and the friction member (6) and the ordinate indicates said frictional feeding force F p , said distance L and said frictional feeding force F p are located in a range below the curves of figure 6 representing the buckling reaction force P of the separating means (5, 6) in dependence of the thickness of the sheets (1), whereby buckling of the sheets (1) to occur between the feeding means (4) and the separating means (5, 6) during sheet feeding is avoided and whereby said distance L and said frictional feeding force F p have lower limits that are decided by design. A sheet feeding device as claimed in claim 1, characterized in that said range is surrounded by a straight line connecting a point L = 50 mm and F p = 0 gf and a point of L = 0 mm and F p = 500 gf, the abscissa and the ordinate whereby 55 kg paper (sheets of a size of 788 mm x 1091 mm having a ream weight of 55 kgf in 1000 sheets) is used for the sheets.

Inventors: KAWAUCHI MASATAKA, SAIKI EISAKU, KAWAUCHI, MASATAKA, SAIKI, EISAKU
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1982
Main classification: B65H3
IPC codes: G03G15, B65H1, B65H3