Abstract:
A power transmission device includes a driving gear having gears over an entire circumferential surface thereof, a follower gear including an intermittent gear having a toothless portion, and a controller for controlling the follower gear to be engaged with or disengaged from the driving gear. Each of the driving gear and the follower gear is a helical gear.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The invention relates to a power transmission device for transmitting rotation by gears and a sheet feeding apparatus having a power transmission device.  
         [0002]     A power transmission device that transmits a continuous rotary movement through dividing it into rotary movements of a definite angle using an intermittent gear is used in a sheet feeding apparatus for conveying a sheet of paper.  
         [0003]     For example, in the publication of the unexamined patent application 2002-265076, it is disclosed a sheet feeding apparatus which conveys out sheets of paper one by one by a process such that, by the combination of a driving gear with a follower gear made up of an intermittent gear, a continuous rotary movement of the driving gear is converted into followed rotations of a definite angle, which are transmitted to the follower gear, and the rotary movements of a definite angle of the follower gear are transmitted to sheet feed rollers.  
         [0004]     In such a power transmission device, there is a problem that, because of shocks generated every time when the meshing of the gears transfers from a toothless state to a mesh state, shock sounds and wear or breakage of the gear teeth are generated.  
         [0005]     For a countermeasure of this, it has heretofore been put into practice to provide a shock absorbing elastic member at the meshing portion.  
         [0006]     Further, in the publication of the unexamined patent application 2002-265076, the generation of abnormal sounds and the breakage of the gear are prevented by making the shape of teeth of the follower gear such one as to be capable of reducing the pressing force at the time of meshing of the gears.  
         [0007]     As regards the means in which an elastic member is provided, there are a problem that the number of parts increases which results in a high cost and a low durability of the device, a problem that the low durability of the elastic member causes the durability of the device to be lowered, a problem that it is difficult to fit a small gear with an elastic member, etc.  
         [0008]     In the publication of the unexamined patent application 2002-265076, by the use of a means in which a notched slope is formed at the first tooth and the second tooth adjacent to a toothless portion at its downstream side in an intermittent gear, the abnormal sounds and the breakage of teeth are prevented. Because such a prevention means is to lower the strength of the teeth, it is insufficient for the prevention of the breakage of teeth, and the problem that a breakage of teeth occurs, for example, in a case where the torque of the drive force to be transmitted is large cannot be solved. Further, the means is insufficient for the prevention of the shock sound generated at the start of meshing.  
         [0009]     That is, at the time of transition into meshing from a toothless portion, also in a gear device described in the publication of the unexamined patent application 2002-265076, because the transition into a mesh state is made at the same time over the whole length of a tooth in the axial direction, the shock is large, and a shock sound is generated.  
         [0010]     As described above, the gear device in the publication of the unexamined patent application 2002-265076 is insufficient for both the purposes of the prevention of a shock sound and the prevention of a breakage of teeth.  
       SUMMARY OF THE INVENTION  
       [0011]     It is an object of this invention, to prevent, in a power transmission device using an intermittent gear, a breakage of a tooth concerned and generation of shock sounds, to solve the problems in a conventional power transmission device as described above.  
         [0012]     The above-mentioned object can be accomplished by any one of the structures described below.  
         [0013]     Structure 1: A power transmission device comprising a driving gear made up of a whole gear, a follower gear including an intermittent gear component having a toothless portion, and a control means for practicing the mesh and release of mesh for said set of follower gears, characterized by said driving gear and said follower gear each being made up of a helical gear.  
         [0014]     Structure 2: A power transmission device as set forth in the structure 1, characterized by further comprising an actuating means for meshing the aforesaid driving gear with the aforesaid follower gear when the aforesaid control means releases the mesh.  
         [0015]     Structure 3: A power transmission device as set forth in any one of the structures 1 to 3, characterized by comprising the follower gear made up of a first follower gear and a second follower gear meshing with said first follower gear.  
         [0016]     Structure 4: A power transmission device as set forth in the structure 1 or 2, characterized by the second follower gear including a whole gear component made up of a helical gear.  
         [0017]     Structure 5: A sheet feeding apparatus comprising a sheet feed roller to be driven by a power transmission device as set forth in any one of the structures 1 to 4 and the aforesaid follower gear. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1  is a conceptual drawing of a power transmission device of the embodiment of the invention;  
         [0019]      FIG. 2  is a partial view of an intermittent gear;  
         [0020]      FIG. 3  is a drawing showing a concrete example of a power transmission device of the embodiment of the invention;  
         [0021]      FIG. 4  is a drawing showing an example of a sheet feeding apparatus of the embodiment of the invention;  
         [0022]      FIG. 5  is a drawing showing another example of a sheet feeding apparatus of the embodiment of the invention;  
         [0023]      FIG. 6 ( a ) and  FIG. 6 ( b ) are drawings each showing another example of a first follower gear; and  
         [0024]      FIG. 7  is a cross-sectional view along the line VII-VII in  FIG. 2 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0025]      FIG. 1  is a conceptual drawing of a power transmission device of the embodiment of the invention, and  FIG. 2  is a partial view of an intermittent gear.  
         [0026]     In this description, a gear provided with teeth on the whole circumference is referred to as a whole gear for the purpose of discriminating it from an intermittent gear. In addition, the term “an intermittent gear” implies, as generally used, a gear having a toothless portion where no tooth is formed.  
         [0027]     In  FIG. 1 , the numeral  1  denotes a driving gear made up of a whole gear,  2  denotes a first follower gear made up of an intermittent gear component  2 A and a whole gear component  2 B, and  3  denotes a second follower gear made up of a whole gear.  
         [0028]     The drive gear  1  meshes with the intermittent gear component  2 A of the first follower gear  2 , and the whole gear component  2 B of the first follower gear  2  meshes with the second follower gear  3 .  
         [0029]     As shown by the arrow marks, power is transmitted from the drive gear  1  to the first follower gear  2 , and from the first follower gear  2  to the second follower gear  3 .  
         [0030]     The rotation of the driving gear  1  which is continuously rotating is transmitted to the intermittent gear component  2 A, and the first follower gear  2  makes a rotation of a definite angle. After the rotation of said definite angle, at the step when the first follower gear has rotated to a position where the toothless portion  2 C is at the mesh position with the driving gear, the transmission of power from the driving gear  1  to the first follower gear  2  is intercepted. Because the second follower gear  3  meshes with the whole gear component  2 B of the first follower gear  2 , the second follower gear  3  rotates following the rotation of the first follower gear  2 , and stops in compliance with its stop.  
         [0031]     Accordingly, to the second follower gear  3 , the driving force of the driving gear  1  for the rotation of the definite angle is transmitted. The mechanism of power transmission using an intermittent gear explained above is well known.  
         [0032]     In the invention, as shown in the drawings, the driving gear  1 , the intermittent gear component  2 A, the whole gear component  2 B, and the second follower gear each is made up of a helical gear.  
         [0033]     In a power transmission device having a structure as described above, as shown in  FIG. 2 , in the process of operation moving from a non-mesh state of the toothless portion  2 C to a mesh state, the teeth  21 ,  22 , and  23  start meshing at the leading portions  21   a ,  22   a , and  23   a  facing the toothless portion  2 C; that is, the whole tooth trace of one tooth is not simultaneously brought into mesh, as is done in a case where an intermittent gear made up of a spur gear is used. Accordingly, the shock at the time the operation moves from the state of no meshing of the toothless portion  2 C to the state of meshing is eased extremely satisfactorily. In addition, in  FIG. 2 , the solid lines represent the crests of the teeth and the broken lines represent the bottoms of the teeth.  
         [0034]     As the result, the generation of shock sounds and the breakage of teeth are sufficiently prevented. On top of it, force acting from the teeth of the drive gear  1  to the teeth of the first follower gear  2  is not directed perpendicularly to the tooth trace. That is, the force acting on the teeth of the intermittent gear component  2 A of the first follower gear  2  is decomposed into a component perpendicular to the tooth trace and a component parallel to the tooth trace. Therefore, breaking force against teeth is effectively weakened, and even in a case where the driving torque is large, the breakage of teeth is prevented.  
         [0035]      FIG. 3  shows a concrete example of a power transmission device of the embodiment of the invention.  
         [0036]     As already explained, the driving gear  1  meshes with the intermittent gear component  2 A of the first follower gear  2 , and the second follower gear  3  meshes with the whole gear component  2 B of the first follower gear  2 .  
         [0037]     In the first follower gear  2 , there are provided an engagement part  2 D having a step portion  2 Da and a pin  2 E. An engaging member  4  making up a control means engages with the engagement part  2 D. Further, to the pin  2 E, one end of a tension-type spring  5  as an actuating means whose another end is fixed at a fixed position is hooked. The engaging member  4  is supported rotatably around a shaft  4 A, and is rotated clockwise, driven by a solenoid  6  that makes up the control means.  
         [0038]     In the initial state shown in  FIG. 2  where the driving gear  1  is continuously rotating in the arrow mark direction, but the power of the driving gear  1  is not transmitted to the first follower gear due to the toothless portion  2 C facing the driving gear  1 , when an actuation signal is inputted to the solenoid  6  making up the control means, the engaging member  4  is rotated clockwise around the shaft  4 A, and the end portion becomes out of engagement with the step portion  2 Da. As the result, the first follower gear  2  is rotated counter-clockwise by the driving force of the spring  5 , which makes the driving gear  1  mesh with the intermittent gear component  2 A, to rotate the first follower gear  2 .  
         [0039]     At the step when the first follower gear has rotated up to a position of an angle where the toothless portion  2 C faces the driving gear  1 , the power transmission of the driving gear  1  is intercepted, the end portion of the engaging member  4  engages with the step portion  2 Da, and the first follower gear returns to the initial state shown in  FIG. 3 .  
         [0040]     That is, the first follower gear  2  and the second follower gear  3  stop in the initial state by an actuation signal after the rotation of the definite angle. Accordingly, every time an actuation signal is inputted to the solenoid  6 , the first follower gear  2  and the second follower gear  3  make the rotation of the definite angle.  
         [0041]      FIG. 4  shows an example of a sheet feeding apparatus of the embodiment of the invention having the power transmission device shown in  FIG. 3 .  
         [0042]     In  FIG. 4 , the numeral  10  denotes a sheet feed tray, and sheets D are stacked in the sheet feed tray  10  with their leading edges regulated by a regulation member  11 . The numeral  12  denotes a sheet feed roller, and as shown in the drawing, it is formed to have a half-moon cross-sectional shape with a part of a whole roller cut off. The numeral  13  denotes a conveyance roller, which rotates as shown by the arrow mark to convey a sheet D. The numeral  14  denotes a well-known separation roller having a torque limiter built in (not shown in the drawing). Thus, a separation and conveyance means for separating and conveying one sheet out of the stack of sheets D is made up of the conveyance roller  13  and the separation roller  14 .  
         [0043]     The shaft  12 A of the sheet feed roller  12  is the rotary shaft driven by the second follower gear  3  in  FIG. 2 .  
         [0044]     By the rotation of the shaft  12 A, the sheet feed roller  12  makes one rotation as shown by the arrow mark, to feed out a sheet D from the uppermost position of the stack. The fed sheet D is conveyed by the conveyance roller  13 , and if multiple-sheet feeding takes place, a single sheet on top is separated by the separation action of the separation roller  14 . Thus, the single sheet is conveyed by the separation and conveyance means.  
         [0045]      FIG. 5  shows another example of a sheet feeding apparatus of the embodiment of the invention.  
         [0046]     In  FIG. 5 , a sheet feed roller  22  is coupled to a conveyance roller  23  by a coupling plate  25 , while it receives a power transmitted from the conveyance roller  23  through a belt  24 , and is driven by the conveyance roller  23 . The conveyance roller  23  and a separation roller  24  having a torque limiter built in as the separation roller  14  shown in  FIG. 4  make up a separation and conveyance means. In addition, the conveyance roller  23  is fitted with a one-way clutch (not shown in the drawing), and rotates freely in a case where a driving force is applied by a sheet D in the direction shown by the arrow mark.  
         [0047]     At one end of the coupling plate  25 , there is provided an arm  26 , which engages with a plunger  28  of a solenoid  27 .  
         [0048]     Further, the shaft  23 A of the conveyance roller  23  is the driving shaft, and is driven by the second follower gear  3  shown in  FIG. 1  and  FIG. 2 .  
         [0049]     The state shown by the solid line represents a standby state, and the sheet feed roller  22  is not in contact with sheets D.  
         [0050]     When a sheet feed start signal is inputted to the solenoid  27 , the plunger  28  of the solenoid  27  ascends, the sheet feed roller  25  descends up to the position indicated by the broken line owing to gravity, to become in contact with a sheet D, and presses the sheet D with a constant pressure.  
         [0051]     Simultaneously with the descending of the sheet feed roller  22 , the shaft  23 A rotates, to rotate the conveyance roller  23  and the sheet feed roller  22 , and a sheet D is fed out. At the point of time when the leading edge of the sheet D is conveyed out by a specified length, as explained before, the driving of the shaft  23 A stops.  
         [0052]     At this step of stop, the leading edge of the sheet D is conveyed by a pair of conveyance roller at the next stage (not shown in the drawing). Accordingly, even if the shaft  23 A stops, the sheet D is conveyed by the pair of conveyance rollers at the next stage.  
         [0053]      FIG. 6 ( a ) and  FIG. 6 ( b ) each shows another example of the first follower gear.  
         [0054]     In the example shown in  FIG. 6 ( a ), a toothless portion  2 C is formed with border lines inclined by an angle θ against the direction of the axis of rotation as shown by  2 F. In the example shown in the drawing, the angle of inclination θ is determined to be a value of a little smaller than the angle of the tooth trace of the helical gear against the axis of rotation α.  
         [0055]     The example shown in  FIG. 6 ( b ) is a case where the toothless portion is provided with the angle of inclination θ of its border lines  2 F made to be of reverse sign in terms of positive or negative to the angle of the tooth trace against the axis of rotation α.  
         [0056]     By making the angle θ have a suitable value in  FIG. 6 ( a ) and  FIG. 6 ( b ), it is possible to make minimum the shock sounds and the wear and breakage of the teeth.  
         [0057]      FIG. 7  is a cross-sectional view along the line VII-VII in  FIG. 2  and shows an example of the shape of tooth.  
         [0058]     In  FIG. 7 , the tooth  22  is formed in a way such that its crest has a shape of a curve that gently rises in the neighborhood  22   a  of the border line  2 F between the toothless portion  2 C and toothed portion of the intermittent gear portion  2 A. Because the meshing with the driving gear proceeds in a manner as shown by the arrow mark W, the shock at the start of meshing is eased further as compared with the tooth  22  that is formed in such that its crest has a shape of a curve that sharply rises in the neighborhood  22   a.    
         [0059]     As the result, generation of shock sounds is prevented satisfactorily, and also wear and breakage of the teeth is prevented.  
         [0060]     Further, for example, it becomes possible to make a power transmission device using a small gear with a module of 0.6 or under.  
         [0061]     By an invention described in any one of the structures 1 to 5, it is possible to provide a power transmission device using an intermittent gear having a high durability, wherein generation of shock sounds and wear and breakage of its teeth are prevented without being accompanied by the increase of cost.  
         [0062]     Further, a power transmission device using a small gear is actualized, and on top of it, it becomes possible to transmit a driving force of a large torque.