Abstract:
The present invention provides a sheet feeding apparatus that has sheet feeding device for abutting against an upper surface of a sheet stacked and feeding out the sheet, biasing device for biasing the sheet feeding device toward the upper surface of the sheet, and lifting and lowering device for lifting and lowering the sheet feeding device, the lifting and lowering device including a holding member for engaging with the sheet feeding device and shifting the sheet feeding device in an up-and-down direction by a motor, and maintaining device for regulating the holding member so as to maintain the sheet feeding device at a position where the sheet feeding device is spaced apart from the upper surface of the sheet, in opposition to a biasing force of the biasing device.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sheet feeding apparatus and an image forming apparatus having such a sheet feeding apparatus, and more particularly, it relates to a sheet feeding apparatus in which a sheet is fed out by sheet feeding means provided above the sheet for lifting and lowering movements. 
     2. Related Background Art 
     In some of conventional image forming apparatuses such as printers, copying machines and the like, there is provided a sheet feeding apparatus in which a sheet is fed to an image forming portion by sheet feeding means such as a pick-up roller. Among such sheet feeding apparatuses, there is a sheet feeding apparatus in which the sheet feeding means is provided above the sheet for lifting and lowering movements in such a manner that, when the sheet is fed, the sheet feeding means is lowered to be urged against the sheet and, in this condition, the sheet is fed out by rotating the sheet feeding means, and, thereafter, the sheet feeding means is lifted to be separated from the sheet. 
     By the way, in such a sheet feeding apparatus, although means for lifting and lowering the sheet feeding means was generally comprised of a solenoid or a cam one revolution of which is controlled, recently, as the speed of the printer has been increased, it is required that the sheet feeding means be urged against the sheet at higher speed and also be separated from the sheet at higher speed after the feeding of the sheet. Further, as noise of recent printers has been reduced, it is required for avoiding usage of an actuator such as a solenoid generating great noise. 
     To this end, there has been proposed a sheet feeding apparatus in which, as the means for lifting and lowering the sheet feeding means, a lifting and lowering mechanism for directly lifting and lowering the sheet feeding means by using a pulse motor, for example, is provided. 
     FIG. 8 shows a construction of such a conventional sheet feeding apparatus using a pulse motor. In FIG. 8, the sheet feeding apparatus comprises pick-up rollers  1   a ,  1   b  as sheet feeding means provided for lifting and lowering movements, and rotations of the pick-up rollers around shafts  2   a ,  2   b  are controlled by drive sources (not shown). Further, the pick-up rollers  1   a ,  1   b  are rotatably held on ends of roller holders  3   a ,  3   b  which are held by an image forming apparatus (not shown) for rotations around shafts  4   a ,  4   b.    
     Incidentally, there are provided roller springs  5   a ,  5   b  as urging means for biasing the pick-up rollers  1   a ,  1   b  in anti-clockwise directions (along which the rollers are urged against an upper surface of a stached sheet P) via the roller holders  3   a ,  3   b  so that the pick-up rollers  1   a ,  1   b  are urged against the sheet (not shown) with predetermined pressure by biasing the pickup rollers  1   a ,  1   b  in the anti-clockwise directions by means of the roller springs  5   a ,  5   b.    
     Further, in FIG. 8, the reference numeral  6  denotes a rod as a holding member capable of moving in an up-and-down direction and adapted to hold the pick-up rollers  1   a ,  1   b  for lifting and lowering movements, and “M” denotes a pulse motor as a reversible motor. Rotation of the pulse motor M is transmitted to the rod  6  via a motor gear  12 , a first drive transmitting gear  11  as a first transmitting gear meshed with the motor gear  12 , a second drive transmitting gear  13  as a second transmitting gear integrally formed with the first drive transmitting gear  11 , and a rack gear  10  meshed with the second drive transmitting gear  13  and provided on one side of the rod  6 . Incidentally, the first and second drive transmitting gears  11 ,  13  are rotatably held on a rotary shaft  17 . 
     On the other hand, guide holes  6   c  extending in an up-and-down direction are formed in upper and lower portions of the rod  6 , so that, when the rotation of the pulse motor M is transmitted via the gear train comprised of the motor gear  12 , first and second drive transmitting gears  11 ,  13  and rack gear  10 , the rod  6  is shifted in the up-and-down direction while being guided by pins  7 ,  8  inserted in the guide holes  6   c.    
     Incidentally, support portions  6   a ,  6   b  for supporting the shafts  2   a ,  2   b  of the pick-up rollers  1   a ,  1   b  from below are protruded horizontally from lower and upper ends of the rod  6 . With this arrangement, when the rod  6  is shifted in the up-and-down direction, the pick-up rollers  1   a ,  1   b  are shifted in the up-and-down direction by the aid of the spring forces of the roller springs  5   a ,  5   b  or in opposition to the spring forces as the rod  6  is shifted. 
     Next, a sheet feeding operation of the sheet feeding apparatus having the above-mentioned construction will be explained. 
     In a waiting condition of the sheet feeding apparatus before it starts the sheet feeding operation, the rod  6  is held at a highest position or home position, so that the pick-up rollers  1   a ,  1   b  are positioned above the sheet. In this case, although the pulse motor M is not rotated, it is maintained in an exciting condition so that the rod  6  is held in the home position in opposition to the biasing forces of the roller springs  5   a ,  5   b.    
     On the other hand, when sheet feeding command is emitted from a controlling device (not shown) provided in the image forming apparatus, first of all, the pulse motor M is rotated in the anti-clockwise direction, and the anti-clockwise rotation is transmitted to the rod  6  via the motor gear  12 , first and second drive transmitting gears  11 ,  13  and rack gear  10 , with the result that the rod  6  is lowered. When the rod  6  is lowered in this way, the pick-up rollers  1   a ,  1   b  are also lowered together with the rod  6  by the biasing forces of the roller springs  5   a ,  5   b , thereby urging the pick-up rollers against the sheet P. 
     Incidentally, even after the pick-up rollers  1   a ,  1   b  are urged against the sheet in this way, the rod  6  is further lowered by a predetermined distance. Here, when the rod  6  is lowered in this way, since the pick-up rollers  1   a ,  1   b  abut against the sheet, the support portions  6   a ,  6   b  of the rod  6  are separated from the shafts  2   a ,  2   b  of the pick-up rollers  1   a ,  1   b . As a result, the pick-up rollers  1   a ,  1   b  are urged against the sheet P with predetermined abut pressure by the biasing forces of the roller springs  5   a ,  5   b.    
     After the pick-up rollers  1   a ,  1   b  are urged against the sheet P in this way, by rotating the pick-up rollers  1   a ,  1   b , the sheet P can be fed to the image forming portion (not shown). 
     On the other hand, when the sheet feeding operation is finished, the pulse motor M is rotated in a clockwise direction, with the result that the rod  6  is lifted and the support portions  6   a ,  6   b  abut against the shafts  2   a ,  2   b  of the pick-up rollers  1   a ,  1   b  from below. Further, thereafter, when the rod  6  is lifted, the pick-up rollers  1   a ,  1   b  are lifted together with the rod  6  in opposition to the biasing forces of the roller springs  5   a ,  5   b , thereby separating the pick-up rollers from the sheet P. Thereafter, the rod  6  is returned to the home position. In this case, by bringing the pulse motor M to the exciting condition, the rod  6  is held at the home position in opposition to the biasing forces of the roller springs  5   a ,  5   b.    
     By the way, the during such a sheet feeding operation, since the pick-up rollers  1   a ,  1   b  are biased so that they generally abut against the sheet P with load in the order of about 0.5 N to about 3 N, when the above-mentioned sheet feeding operation is effected, torque acting on the pulse motor M is changed as shown in FIG.  9 . Incidentally, in FIG. 9, the ordinate indicates the position of the rod  6  and the abscissa indicates torque acting on the motor shaft of the pulse motor M. 
     As apparent from FIG. 9, when the rod  6  is in the home position, maximum torque T max  acts on the pulse motor M to hold the rod  6  in the home position. Further, at the time when the pick-up rollers  1   a ,  1   b  are contacted with the sheet, although the torque is instantaneously decreased by an amount corresponding to the roller pressure, even in a condition HL that the rod  6  is lowered at the maximum extent, torque T min  corresponding to the weight of the rod  6  itself acts on the pulse motor M. 
     Accordingly, in the conventional sheet feeding apparatus using such a pulse motor M, the electrical power is required even in the waiting condition, and, since the maximum torque is great, a large torque motor is required, which is very disadvantageous in consideration of the power consumption. 
     On the other hand, proper backlash is provided between the first drive transmitting gear  11  and the motor gear  12 . Further, as shown in FIG. 10, proper backlash B is also provided between the second drive transmitting gear  13  and the rack gear  10  of the rod  6 . 
     However, in the conventional sheet feeding apparatus having the lifting and lowering means including such a pulse motor M and the gear train for transmitting the rotation of the pulse motor M to the rod  6 , since the pulse motor M must quickly be started and stopped reversibly and slow-up and slow-down control is effected on demand, due to the backlash B in the gears  10  to  13 , great discordant slapping noise is generated in meshed portions in the gear train, which makes reduction of noise difficult or impossible. 
     SUMMARY OF THE INVENTION 
     The present invention is made in consideration of the above-mentioned circumstances, and an object of the present invention is to provide a sheet feeding apparatus which can reduce power consumption and noise, and an image forming apparatus having such a sheet feeding apparatus. 
     To achieve the above object, the present invention provides a sheet feeding apparatus comprising sheet feeding means for abutting against an upper surface of a sheet stacked and feeding out the sheet, biasing means for biasing the sheet feeding means toward the upper surface of the sheet stack, and lifting and lowering means for lifting and lowering the sheet feeding means, wherein the lifting and lowering means comprises a holding member for engaging with the sheet feeding means and shifting the sheet feeding means in an up-and-down direction by a motor, and maintaining means for regulating the holding member so as to maintain the sheet feeding means at a position where the sheet feeding means is spaced apart from the upper surface of the sheet, in opposition to a biasing force of the biasing means. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a view showing a construction of a sheet feeding apparatus according to a first embodiment of the present invention; 
     FIG. 2 is an enlarged view showing main portions of the sheet feeding apparatus in a waiting condition; 
     FIG. 3 is an enlarged view showing a rack gear of a rod and a second drive transmitting gear in the sheet feeding apparatus; 
     FIG. 4 is an enlarged view showing main portions of the sheet feeding apparatus in a sheet feeding condition; 
     FIG. 5 is a view showing a relationship a position of the rod and torque acting on a pulse motor in the sheet feeding apparatus; 
     FIG. 6 is a view showing a construction of a sheet feeding apparatus according to a second embodiment of the present invention; 
     FIG. 7 is a view showing the sheet feeding apparatus in a waiting condition; 
     FIG. 8 is a view showing a construction of a conventional sheet feeding apparatus; 
     FIG. 9 is a view showing a relationship a position of the rod and torque acting on a pulse motor in the conventional sheet feeding apparatus; and 
     FIG. 10 is an enlarged view showing a rack gear of a rod and a second drive transmitting gear in the conventional sheet feeding apparatus. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will now be explained in connection with embodiments thereof with reference to the accompanying drawings. 
     FIG. 1 is a view showing a construction of a sheet feeding apparatus according to a first embodiment of the present invention. Incidentally, in FIG. 1, the same elements as those shown in FIG. 8 are designated by the same reference numerals. 
     Pick-up rollers  1   a ,  1   b  as sheet feeding means are disposed above sheet supporting means such as a sheet cassette or a deck (not shown) for supporting sheets P (not shown), for lifting and lowering movements. 
     A rod spring  15  as biasing means constituting maintaining means (described later) has one end locked to an upper end of a rod  6  as a holding means and the other end locked to a support portion  20  provided on an image forming apparatus (not shown). The rod  6  is biased by the rod spring  15  obliquely upwardly toward a first drive transmitting gear. 
     In a waiting condition of the sheet feeding apparatus before a sheet feeding operation is started, a spring force of the rod spring  15  is selected so that a pulling force (tensility) of the rod spring  15  is balanced with biasing forces of roller springs  5   a ,  5   b  when the rod  6  is in a highest position or home position as shown in FIG.  2 . Incidentally, elastic forces of the springs are selected so that resulting biasing force of the roller springs  5   a ,  5   b  for lowering the rod  6  becomes equal to or slightly smaller than the biasing force of the rod spring  15  or lifting the rod  6  upwardly. 
     By setting the spring force of the rod spring  15  in this way, the rod spring  15  acts as maintaining means for canceling the biasing forces (abutting forces) of the roller springs  5   a ,  5   b  acting on the rod  6  via shafts  2   a ,  2   b  of the pick-up rollers  1   a ,  1   b , so that torque is prevented from acting on a pulse motor M via a gear train comprised of a rack gear  10 , second drive transmitting gear  13 , first drive transmitting gear  11  and motor gear  12 . As a result, in the waiting condition, it is not required that the pulse motor M be maintained in an exciting condition. 
     FIG. 3 is an enlarged view showing the rack gear  10  and the second drive transmitting gear  13 . As shown in FIG. 3, a guide hole  6   c  provided in an upper portion of the rod  6  has a dimension greater than that of a pin  7 . Further, a shaft hole  13   a  of the second drive transmitting gear  13  and a shaft hole (not shown) of the first drive transmitting gear  11  has also dimensions greater than an outer diameter of a shaft  17 . 
     By forming the guide hole  6   c  of the rod  6  and the shaft holes  13   a  of the first and second drive transmitting gears  13 , play is generated in the rod  6  and the first and second drive transmitting gears  13 . With this arrangement, during the sheet feeding operation which will be described later, by the pulling force (urging force) of the rod spring  15  acting on the rod  6  as urging means directed toward the first and second drive transmitting gears, as shown in FIG. 3, the rack gear  10  and the second drive transmitting gear  13  are brought to a non-backlash condition. Incidentally, by having arrangement in this way, although not shown, a non-backlash condition is established between the first drive transmitting gear  11  and the motor gear  12 . 
     Next, a sheet feeding operation of the sheet feeding apparatus having the above-mentioned construction will be explained. 
     In the waiting condition of the sheet feeding apparatus before it starts the sheet feeding operation, the rod  6  is held at the highest position or home position, so that the pick-up rollers  1   a ,  1   b  are positioned above the sheet P. When the rod  6  is in the home position, since the pulling force of the rod spring  15  is balanced with the biasing forces of the roller springs  5   a ,  5   b , it is not required that the pulse motor M be maintained to the exciting condition. 
     On the other hand, when sheet feeding command is emitted from a controlling device (not shown) provided in the image forming apparatus, first of all, the pulse motor M is rotated in an anti-clockwise direction, and the anti-clockwise rotation is transmitted to the rod  6  via the motor gear  12 , first and second drive transmitting gears  11 ,  13  and rack gear  10 , with the result that the rod  6  is lowered. When the rod  6  is lowered in this way, the pick-up rollers  1   a ,  1   b  are also lowered together with the rod  6  by the biasing forces of the roller springs  5   a ,  5   b , thereby urging the pick-up rollers against the sheet P. 
     Incidentally, even after the pick-up rollers  1   a ,  1   b  are urged against the sheet P in this way, the rod  6  is further lowered by a predetermined distance. Here, when the rod  6  is lowered in this way, since the pick-up rollers  1   a ,  1   b  abut against the sheet P, support portions  6   a ,  6   b  of the rod  6  are separated from the shafts  2   a ,  2   b  of the pick-up rollers  1   a ,  1   b , as shown in FIG.  4 . As a result, the pick-up rollers  1   a ,  1   b  are urged against the sheet P with predetermined abut pressure by the biasing forces of the roller springs  5   a ,  5   b.    
     When the rod  6  is lowered in this way, since the biasing forces of the roller springs  5   a ,  5   b  does not act on the rod  6 , only the spring force of the rod spring  15  acts on the rod  6 , thereby pulling the rod  6  upwardly. 
     In order to hold the rod  6  pulled upwardly in a position shown in FIG. 4, the pulse motor M is excited while being stopped, until the sheet feeding operation is completed. As a result, the rod  6  is held and the pick-up rollers  1   a ,  1   b  are urged against the sheet P positively with predetermined pressure. And, one of the upper and lower pick-up rollers  1   a ,  1   b  is selectively driven by a driving force from a drive source (not shown), thereby feeding out the sheet P. 
     After the pick-up rollers  1   a ,  1   b  are urged against the sheet P in this way, by rotating the pick-up rollers  1   a ,  1   b , the sheet P can be fed to an image forming portion (not shown). 
     On the other hand, when the sheet feeding operation is finished, the pulse motor M is rotated in a clockwise direction, with the result that the rod  6  is lifted and the support portions  6   a ,  6   b  abut against the shafts  2   a ,  2   b  of the pick-up rollers  1   a ,  1   b  from below. Further, thereafter, when the rod  6  is lifted, the pick-up rollers  1   a ,  1   b  are lifted together with the rod  6  in opposition to the biasing forces of the roller springs  5   a ,  5   b , thereby separating the pick-up rollers from the sheet P. 
     Thereafter, the rod  6  is returned to the home position again. Incidentally, in this case, since the pulling force of the rod spring  15  is balanced with the biasing forces of the roller springs  5   a ,  5   b , it is not required that the pulse motor M be maintained to the exciting condition. 
     By the way, during the above-mentioned sheet feeding operation, as shown in FIG. 3, the rack gear  10  and the second drive transmitting gear  13  become a non-backlash condition by the pulling force of the rod spring  15  tending to pull the rod  6  toward the first and second drive transmitting gears. Further, a non-backlash condition is also established between the first drive transmitting gear  11  and the motor gear  12 . 
     By establishing the non-backlash conditions between the gears, slapping noise due to vibration generated every phase angle of the pulse motor M is not generated between the gears, with the result that the pick-up rollers  1   a ,  1   b  can be lifted and lowered silently. 
     FIG. 5 shows change in torque acting on the pulse motor M during the sheet feeding operation. Incidentally, in FIG. 5, the ordinate indicates a position of the rod  6  and the abscissa indicates torque acting on a motor shaft of the pulse motor M. 
     In this case, although a configuration of the change in torque becomes similar to the configuration shown in FIG. 9, in the illustrated embodiment, as apparent from FIG. 5, since the rod spring  15  and the roller springs  5   a ,  5   b  are balanced with each other when the rod  6  is in the home position, the torque becomes zero. 
     Further, although minimize torque Ta, is generated in a condition HL that the rod  6  is lowered at the maximum extent, an absolute value of the value thereof becomes greatly smaller than that of the torque T max  explained in connection with the prior art. Further, the motor is excited only during the sheet feeding operation. 
     In this way, by balancing the rod spring  15  with the roller springs  5   a ,  5   b  when the rod  6  is in the home position, i.e., by canceling the biasing forces (abut forces) of the roller springs  5   a ,  5   b  acting on the rod  6 , the pulse motor M can be excited only during the sheet feeding operation. As a result, since any electric current is not required in the waiting condition, power consumption can be minimized. Further, since the torque can be reduced by pulling the rod  6  by means of the rod spring  15 , a more compact motor having lower electric current can be used, thereby minimizing the power consumption. 
     Next, a second embodiment of the present invention will be explained. 
     FIG. 6 shows a construction of a sheet feeding apparatus according to a second embodiment of the present invention. Incidentally, in FIG. 6, the same elements as those shown in FIG. 1 are designated by the same reference numerals. 
     In FIG. 6, the sheet feeding apparatus includes a pad  30  abutting against a side of a rod  6  opposite to a side where a rack gear  10  is formed, and a spring  31  for urging the pad  30  against the rod  6 . By using the rod  6  toward first and second drive transmitting gears by means of urging means constituted by the spring  31  and the pad  30 , backlash between the rack gear  10  and the second drive transmitting gear  13  and backlash between the first drive transmitting gear  11  and the motor gear  12  can be eliminated. 
     Now, maintaining means for maintaining the rod in an upper position will be described. In FIG. 6, a spherical member  32  is protruded from a distal end of the rod  6 , and an engagement portion  33  is provided in a main body of an image forming apparatus (not shown) and serves to elastically engage by the spherical member  32  of the rod  6  when the rod  6  is in the home position. Maintaining means is constituted by the spherical member  32  and the engagement portion  33 . When the rod  6  is lifted from a lowered condition shown in FIG. 6 to the home position as shown in FIG. 7 by the rotation of the pulse motor M, as shown in FIG. 7, the engagement portion  33  is elastically engaged by the spherical member  32 , with the result that the rod  6  is held by the main body of the image forming apparatus via the engagement portion  33 . Incidentally, an engagement force (holding force) of the engagement portion  33  is set to a level which can be released by the driving force of the pulse motor M. 
     When the rod  6  is lifted up to the home position in this way, by engaging the spherical member  32  of the rod  6  by the engagement portion  33 , the biasing forces of the roller springs  5   a ,  5   b  acting on the rod  6  can be canceled. That is to say, by canceling the biasing forces (abutting forces) of the roller springs  5   a ,  5   b  acting on the rod  6  by means of the maintaining means comprised of the engagement portion  33  and the spherical member  32  of the rod  6 , the pulse motor M can be excited only during the sheet feeding operation. As a result, since any electric current is not required in the waiting condition, the power consumption can be minimized. 
     Incidentally, in the illustrated embodiment, while an example that the engagement portion  33  has a circular snap-fit configuration was explained, the present invention is not limited to such an example, but, the engagement portion may be of any type so long as it can engageably hold the rod  6 , and, for example, the engagement portion may hold the rod  6  magnetically.