Apparatus and method for controlling pressing force of separation roller

Abstract

There is provided a method in which a conveyance force of a feed roller applied to recording paper is maintained constant in a period from the start to the end of use of the feed roller and a separation roller. The separation roller pressing force control method according to the present invention is such that a pressing force of the separation roller applied to the feed roller is set to be low in a variable range (Sa) such as the start of use in which the coefficient of friction of the feed roller is high, so that the conveyance force applied to the recording paper is not too large. As the coefficient of friction gradually decreases in accordance with the use of the feed roller, the pressing force of the separation roller is made to be gradually raised, thereby increasing the conveyance force applied to the recording paper and retaining it in a stable area Sb. As a result, reduction in the coefficient of friction and the conveyance force in accordance with the use of the feed roller and the separation roller is canceled or offset by adjusting the pressing force of the separation roller applied to the feed roller, whereby the feed roller can always feed the sheets of recording paper at a constant conveyance force, thus making it possible to prevent defective conveyance and defective separation of the sheets of recording paper.

What is cliamed is:  
     
         1 . A separation roller pressing force control apparatus for controlling a pressing force of a separation roller applied to a feed roller, 
 said apparatus comprising:    a separation roller retention mechanism for retaining said separation roller in such a manner that said separation roller is pressed against said feed roller; and    a pressing force adjusting mechanism for maintaining a conveyance force of said feed roller applied to paper substantially constant by changing the pressing force of said separation roller in accordance with a change in the coefficient of friction of said feed roller.    
     
     
         2 . The separation roller pressing force control apparatus as set forth in  claim 1 , wherein said pressing force adjusting mechanism changes the pressing force of said separation roller applied to said feed roller by using a driving force of said feed roller.  
     
     
         3 . The separation roller pressing force control apparatus as set forth in  claim 2 , wherein said pressing force adjusting mechanism comprises: 
 a pressure arm having its one end held for rotation about a pressure arm support shaft so as to press said separation roller toward said feed roller;    a pressure spring having its one end attached to the other end of said pressure arm;    a movable arm having one end thereof held for rotation about a movable arm support shaft and the other end thereof to which is attached the other end of said pressure spring;    a speed reduction mechanism for performing transmission of the driving force of said feed roller; and    a pressure spring expansion mechanism adapted to receive the driving force transmitted from said speed reduction mechanism to set a tensile force of said pressure spring to a low value at the start of use of said feed roller, and to increase the tensile force of said pressure spring applied to said pressure arm in accordance with the increasing number of sheets of paper being fed thereby to gradually increase the pressing force applied to said separation roller by said pressure arm.    
     
     
         4 . The separation roller pressing force control apparatus as set forth in  claim 3 , wherein when the number of sheets of paper being fed has reached a threshold, said pressure spring expansion mechanism maintains the pressing force which is applied to said separation roller by said pressure arm at that time.  
     
     
         5 . The separation roller pressing force control apparatus as set forth in  claim 3 , wherein said pressure spring expansion mechanism comprises a screw shaft which is arranged to extend in a direction toward said pressure arm, and which is driven to rotate under the action of the driving force from the speed reduction mechanism, and a screw groove of an irregular pitch is formed on the outer periphery of said screw shaft, so that the one end of said movable arm is engageable with said screw groove.  
     
     
         6 . The separation roller pressing force control apparatus as set forth in  claim 5 , wherein said screw shaft has a start groove formed on the outer periphery thereof at an end portion thereof near said pressure arm, an end groove formed on the outer periphery thereof at an end portion thereof away from said pressure arm, and a screw groove formed on the outer periphery thereof between said start groove and said end groove, said screw groove starting from said start groove with a coarse pitch, and being connected with said end groove with its pitch becoming gradually finer and finer.  
     
     
         7 . The separation roller pressing force control apparatus as set forth in  claim 3 , wherein said pressure spring expansion mechanism comprises a rotating plate which is formed on its surface with a spiral groove extending from its center of rotation, and which is caused to rotate by the driving force from said speed reduction mechanism, and a screw groove of an irregular pitch is formed on the surface of said rotating plate, with the other end of said movable arm being engageable with said screw groove.  
     
     
         8 . The separation roller pressing force control apparatus as set forth in  claim 7 , wherein said rotating plate is formed on its surface with a spiral groove extending from its center of rotation and is caused to rotate by the driving force from said speed reduction mechanism, and said rotating plate has a start groove formed on its outermost periphery, and an end groove formed at its center of rotation, and a screw groove of an irregular pitch formed on its outer periphery between said start groove and said end groove, said screw groove starting from said start groove with a coarse pitch, and being connected with said end groove with its pitch gradually becoming finer and finer.  
     
     
         9 . A separation roller pressing force control method in which sheets of recording paper are passed between a feed roller and a separation roller with a pressing force of said separation roller applied to said feed roller being controlled so as to feed the sheets of recording paper one by one, 
 said method comprising:    retaining said separation roller so as to permit said separation roller to move toward said feed roller; and    maintaining the pressing force of said separation roller applied to said feed roller at a constant value by changing the pressing force of said separation roller applied to said feed roller by the use of a driving force of said feed roller so as to offset a change in the coefficient of friction of said feed roller which changes in accordance with the number of sheets of paper being fed.