Abstract:
A sheet feed mechanism includes a chassis for supporting a stack of sheets; a top sheet engaging member for engaging a top most sheet of the stack; a stack engaging structure, a first end of the stack engaging structure engaging the stack of sheets to bias a top most sheet of the stack of sheets against the top sheet engaging member, the stack engaging structure being supported from the chassis by a resilient member; a friction surface extending perpendicularly from a second end of the stack engaging structure opposite to the first end; a lock mechanism having a lock arm hingedly connected to the chassis, the lock mechanism including a biased contact foot for engaging the friction surface to retard a rectilinear movement of the stack engaging structure; and an actuator for engaging and disengaging the contact foot from the friction surface.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation of U.S. application Ser. No. 12/505,520 filed Jul. 19, 2009, which is a continuation of U.S. application Ser. No. 11/482,981 filed Jul. 10, 2006, now issued U.S. Pat. No. 7,571,906, all of which are herein incorporated by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a mechanism for moving a stack of sheet material. In particular, the invention is a mechanism for lifting a stack of sheet media for feeding individual sheets into a feed path. 
       CO-PENDING APPLICATIONS 
       [0003]    The following applications have been filed by the Applicant with application No. 11/482,981: 
         [0000]    
       
         
               
               
               
               
               
             
           
               
                   
               
             
             
               
                 7,637,588 
                 7,648,222 
                 7,669,958 
                 7,607,755 
                 7,699,433 
               
               
                 7,658,463 
                 7,530,663 
                 7,467,846 
                 7,669,957 
                 11/482,963 
               
               
                 11/482,956 
                 7,695,123 
                 11/482,974 
                 7,604,334 
                 11/482,987 
               
               
                 7,708,375 
                 7,695,093 
                 7,695,098 
                 7,722,156 
                 7,703,882 
               
               
                 7,510,261 
                 7,658,792 
                 7,709,633 
                 11/482,985 
                 11/482,980 
               
               
                 11/482,967 
                 11/482,966 
                 11/482,988 
                 7,681,000 
                 7,530,446 
               
               
                 7,654,645 
                 11/482,977 
                 7,571,906 
                 7,703,903 
                 7,645,034 
               
               
                 7,637,602 
                 7,645,033 
               
               
                   
               
             
          
         
       
     
         [0004]    The disclosures of these co-pending applications are incorporated herein by reference. 
       CROSS REFERENCES TO RELATED APPLICATIONS 
       [0005]    Various methods, systems and apparatus relating to the present invention are disclosed in the following US Patents/Patent Applications filed by the applicant or assignee of the present invention: 
         [0000]    
       
         
               
               
               
               
               
             
           
               
                   
               
             
             
               
                 6,750,901 
                 6,476,863 
                 6,788,336 
                 7,249,108 
                 6,566,858 
               
               
                 6,331,946 
                 6,246,970 
                 6,442,525 
                 7,346,586 
                 7,685,423 
               
               
                 6,374,354 
                 7,246,098 
                 6,816,968 
                 6,757,832 
                 6,334,190 
               
               
                 6,745,331 
                 7,249,109 
                 7,197,642 
                 7,093,139 
                 7,509,292 
               
               
                 7,685,424 
                 10/866,608 
                 7,210,038 
                 7,401,223 
                 7,702,926 
               
               
                 7,716,098 
                 7,364,256 
                 7,258,417 
                 7,293,853 
                 7,328,968 
               
               
                 7,270,395 
                 7,461,916 
                 7,510,264 
                 7,334,864 
                 7,255,419 
               
               
                 7,284,819 
                 7,229,148 
                 7,258,416 
                 7,273,263 
                 7,270,393 
               
               
                 6,984,017 
                 7,347,526 
                 7,357,477 
                 7,465,015 
                 7,364,255 
               
               
                 7,357,476 
                 11/003,614 
                 7,284,820 
                 7,341,328 
                 7,246,875 
               
               
                 7,322,669 
                 7,445,311 
                 7,452,052 
                 7,455,383 
                 7,448,724 
               
               
                 7,441,864 
                 7,506,958 
                 7,472,981 
                 7,448,722 
                 7,438,381 
               
               
                 7,441,863 
                 7,438,382 
                 7,425,051 
                 7,399,057 
                 7,695,097 
               
               
                 7,686,419 
                 11/246,669 
                 7,448,720 
                 7,448,723 
                 7,445,310 
               
               
                 7,399,054 
                 7,425,049 
                 7,367,648 
                 7,370,936 
                 7,401,886 
               
               
                 7,506,952 
                 7,401,887 
                 7,384,119 
                 7,401,888 
                 7,387,358 
               
               
                 7,413,281 
                 6,623,101 
                 6,406,129 
                 6,505,916 
                 6,457,809 
               
               
                 6,550,895 
                 6,457,812 
                 7,152,962 
                 6,428,133 
                 7,204,941 
               
               
                 7,282,164 
                 7,465,342 
                 7,278,727 
                 7,417,141 
                 7,452,989 
               
               
                 7,367,665 
                 7,138,391 
                 7,153,956 
                 7,423,145 
                 7,456,277 
               
               
                 7,550,585 
                 7,122,076 
                 7,148,345 
                 7,470,315 
                 7,572,327 
               
               
                 7,416,280 
                 7,252,366 
                 7,488,051 
                 7,360,865 
                 7,438,371 
               
               
                 7,465,017 
                 7,441,862 
                 7,654,636 
                 7,458,659 
                 7,455,376 
               
               
                 6,746,105 
                 11/246,687 
                 7,645,026 
                 7,322,681 
                 7,708,387 
               
               
                 11/246,703 
                 7,712,884 
                 7,510,267 
                 7,465,041 
                 11/246,712 
               
               
                 7,465,032 
                 7,401,890 
                 7,401,910 
                 7,470,010 
                 11/246,702 
               
               
                 7,431,432 
                 7,465,037 
                 7,445,317 
                 7,549,735 
                 7,597,425 
               
               
                 7,661,800 
                 7,712,869 
                 7,156,508 
                 7,159,972 
                 7,083,271 
               
               
                 7,165,834 
                 7,080,894 
                 7,201,469 
                 7,090,336 
                 7,156,489 
               
               
                 7,413,283 
                 7,438,385 
                 7,083,257 
                 7,258,422 
                 7,255,423 
               
               
                 7,219,980 
                 7,591,533 
                 7,416,274 
                 7,367,649 
                 7,118,192 
               
               
                 7,618,121 
                 7,322,672 
                 7,077,505 
                 7,198,354 
                 7,077,504 
               
               
                 7,614,724 
                 7,198,355 
                 7,401,894 
                 7,322,676 
                 7,152,959 
               
               
                 7,213,906 
                 7,178,901 
                 7,222,938 
                 7,108,353 
                 7,104,629 
               
               
                 7,303,930 
                 7,401,405 
                 7,464,466 
                 7,464,465 
                 7,246,886 
               
               
                 7,128,400 
                 7,108,355 
                 6,991,322 
                 7,287,836 
                 7,118,197 
               
               
                 7,575,298 
                 7,364,269 
                 7,077,493 
                 6,962,402 
                 7,686,429 
               
               
                 7,147,308 
                 7,524,034 
                 7,118,198 
                 7,168,790 
                 7,172,270 
               
               
                 7,229,155 
                 6,830,318 
                 7,195,342 
                 7,175,261 
                 7,465,035 
               
               
                 7,108,356 
                 7,118,202 
                 7,510,269 
                 7,134,744 
                 7,510,270 
               
               
                 7,134,743 
                 7,182,439 
                 7,210,768 
                 7,465,036 
                 7,134,745 
               
               
                 7,156,484 
                 7,118,201 
                 7,111,926 
                 7,431,433 
                 7,018,021 
               
               
                 7,401,901 
                 7,468,139 
                 7,128,402 
                 7,387,369 
                 7,484,832 
               
               
                 7,448,729 
                 7,246,876 
                 7,431,431 
                 7,419,249 
                 7,377,623 
               
               
                 7,328,978 
                 7,334,876 
                 7,147,306 
                 7,721,948 
                 7,079,712 
               
               
                 6,825,945 
                 7,330,974 
                 6,813,039 
                 6,987,506 
                 7,038,797 
               
               
                 6,980,318 
                 6,816,274 
                 7,102,772 
                 7,350,236 
                 6,681,045 
               
               
                 6,728,000 
                 7,173,722 
                 7,088,459 
                 7,707,082 
                 7,068,382 
               
               
                 7,062,651 
                 6,789,194 
                 6,789,191 
                 6,644,642 
                 6,502,614 
               
               
                 6,622,999 
                 6,669,385 
                 6,549,935 
                 6,987,573 
                 6,727,996 
               
               
                 6,591,884 
                 6,439,706 
                 6,760,119 
                 7,295,332 
                 6,290,349 
               
               
                 6,428,155 
                 6,785,016 
                 6,870,966 
                 6,822,639 
                 6,737,591 
               
               
                 7,055,739 
                 7,233,320 
                 6,830,196 
                 6,832,717 
                 6,957,768 
               
               
                 7,456,820 
                 7,170,499 
                 7,106,888 
                 7,123,239 
                 10/727,162 
               
               
                 7,377,608 
                 7,399,043 
                 7,121,639 
                 7,165,824 
                 7,152,942 
               
               
                 10/727,157 
                 7,181,572 
                 7,096,137 
                 7,302,592 
                 7,278,034 
               
               
                 7,188,282 
                 7,592,829 
                 10/727,180 
                 10/727,179 
                 10/727,192 
               
               
                 10/727,274 
                 7,707,621 
                 7,523,111 
                 7,573,301 
                 7,660,998 
               
               
                 10/754,536 
                 10/754,938 
                 10/727,160 
                 7,171,323 
                 7,278,697 
               
               
                 7,369,270 
                 6,795,215 
                 7,070,098 
                 7,154,638 
                 6,805,419 
               
               
                 6,859,289 
                 6,977,751 
                 6,398,332 
                 6,394,573 
                 6,622,923 
               
               
                 6,747,760 
                 6,921,144 
                 10/884,881 
                 7,092,112 
                 7,192,106 
               
               
                 7,457,001 
                 7,173,739 
                 6,986,560 
                 7,008,033 
                 7,551,324 
               
               
                 7,222,780 
                 7,270,391 
                 7,195,328 
                 7,182,422 
                 7,374,266 
               
               
                 7,427,117 
                 7,448,707 
                 7,281,330 
                 10/854,503 
                 7,328,956 
               
               
                 10/854,509 
                 7,188,928 
                 7,093,989 
                 7,377,609 
                 7,600,843 
               
               
                 10/854,498 
                 10/854,511 
                 7,390,071 
                 10/854,525 
                 10/854,526 
               
               
                 7,549,715 
                 7,252,353 
                 7,607,757 
                 7,267,417 
                 10/854,505 
               
               
                 7,517,036 
                 7,275,805 
                 7,314,261 
                 7,281,777 
                 7,290,852 
               
               
                 7,484,831 
                 10/854,523 
                 10/854,527 
                 7,549,718 
                 10/854,520 
               
               
                 7,631,190 
                 7,557,941 
                 10/854,499 
                 10/854,501 
                 7,266,661 
               
               
                 7,243,193 
                 10/854,518 
                 10/934,628 
                 7,163,345 
                 7,465,033 
               
               
                 7,452,055 
                 7,470,002 
                 7,722,161 
                 7,475,963 
                 7,448,735 
               
               
                 7,465,042 
                 7,448,739 
                 7,438,399 
                 11/293,794 
                 7,467,853 
               
               
                 7,461,922 
                 7,465,020 
                 7,722,185 
                 7,461,910 
                 11/293,828 
               
               
                 7,270,494 
                 7,632,032 
                 7,475,961 
                 7,547,088 
                 7,611,239 
               
               
                 11/293,819 
                 11/293,818 
                 7,681,876 
                 11/293,816 
                 7,448,734 
               
               
                 7,425,050 
                 7,364,263 
                 7,201,468 
                 7,360,868 
                 7,234,802 
               
               
                 7,303,255 
                 7,287,846 
                 7,156,511 
                 10/760,264 
                 7,258,432 
               
               
                 7,097,291 
                 7,645,025 
                 10/760,248 
                 7,083,273 
                 7,367,647 
               
               
                 7,374,355 
                 7,441,880 
                 7,547,092 
                 10/760,206 
                 7,513,598 
               
               
                 10/760,270 
                 7,198,352 
                 7,364,264 
                 7,303,251 
                 7,201,470 
               
               
                 7,121,655 
                 7,293,861 
                 7,232,208 
                 7,328,985 
                 7,344,232 
               
               
                 7,083,272 
                 7,621,620 
                 7,669,961 
                 7,331,663 
                 7,360,861 
               
               
                 7,328,973 
                 7,427,121 
                 7,407,262 
                 7,303,252 
                 7,249,822 
               
               
                 7,537,309 
                 7,311,382 
                 7,360,860 
                 7,364,257 
                 7,390,075 
               
               
                 7,350,896 
                 7,429,096 
                 7,384,135 
                 7,331,660 
                 7,416,287 
               
               
                 7,488,052 
                 7,322,684 
                 7,322,685 
                 7,311,381 
                 7,270,405 
               
               
                 7,303,268 
                 7,470,007 
                 7,399,072 
                 7,393,076 
                 7,681,967 
               
               
                 7,588,301 
                 7,249,833 
                 7,524,016 
                 7,490,927 
                 7,331,661 
               
               
                 7,524,043 
                 7,300,140 
                 7,357,492 
                 7,357,493 
                 7,566,106 
               
               
                 7,380,902 
                 7,284,816 
                 7,284,845 
                 7,255,430 
                 7,390,080 
               
               
                 7,328,984 
                 7,350,913 
                 7,322,671 
                 7,380,910 
                 7,431,424 
               
               
                 7,470,006 
                 7,585,054 
                 7,347,534 
                 7,441,865 
                 7,469,989 
               
               
                 7,367,650 
                 7,469,990 
                 7,441,882 
                 7,556,364 
                 7,357,496 
               
               
                 7,467,863 
                 7,431,440 
                 7,431,443 
                 7,527,353 
                 7,524,023 
               
               
                 7,513,603 
                 7,467,852 
                 7,465,045 
               
               
                   
               
             
          
         
       
     
         [0006]    The disclosures of these applications and patents are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0007]    Sheet material is typically supplied and stored in stacks. To use the individual sheets, they first need to be separated from each other. The paper feed systems in printers, scanners, copiers or faxes are a common examples of the need to sequentially feed individual sheets from a stack into a paper feed path. Given the widespread use of such devices, the invention will be described with particular reference to its use within this context. However, this is purely for the purposes of illustration and should not be seen as limiting the scope of the present invention. It will be appreciated that the invention has much broader application and may be suitable for many systems involving the handling of stacked sheet material. 
         [0008]    Printers, copiers, scanners, faxes and the like, sequentially feed sheets of paper from a stack in the paper tray, past the imaging means (e.g. printhead), to a collect tray. There are many methods used to separate single sheets from the stack. Some of the more common methods involve air jets, suction feet, rubberized picker rollers, rubberized pusher arms and so on. In the systems that use a pick up roller or pusher arm, it is important to control the force with which the roller touches the top sheet of the stack to drive, push or drag it off the top. The friction between the top sheet and the pusher or roller needs to exceed the friction between the top sheet and the sheet underneath. Too much force can cause two or more sheets to be drawn from the stack (known as ‘double picks’), and too little will obviously fail to draw any sheets. 
         [0009]    Sheet feed mechanisms should also be relatively simple, compact and have low power demands. For example, consumer expectations in the SOHO (Small Office/Home Office) printer market are directing designers to reduce the desktop footprint, improve feed reliability for a variety of paper grades while maintaining or reducing manufacturing costs. 
       SUMMARY OF THE INVENTION 
       [0010]    According to an aspect of the present invention, a sheet feed mechanism includes a chassis for supporting a stack of sheets; a top sheet engaging member for engaging a top most sheet of the stack; a stack engaging structure, a first end of the stack engaging structure engaging the stack of sheets to bias a top most sheet of the stack of sheets against the top sheet engaging member, the stack engaging structure being supported from the chassis by a resilient member; a friction surface extending perpendicularly from a second end of the stack engaging structure opposite to the first end; a lock mechanism having a lock arm hingedly connected to the chassis, the lock mechanism including a biased contact foot for engaging the friction surface to retard a rectilinear movement of the stack engaging structure; and an actuator for engaging and disengaging the contact foot from the friction surface. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    Specific embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which: 
           [0012]      FIGS. 1 to 5  is a diagrammatic illustration of one embodiment of the invention at various stages of its operation; 
           [0013]      FIG. 6  is a diagrammatic illustration of another embodiment of the invention; 
           [0014]      FIG. 7  is a perspective view of an inkjet printer and paper feed tray for use with the invention; 
           [0015]      FIG. 8  is a perspective of the printer shown in  FIG. 1  with the paper feed tray and the outer housings removed to expose the components of the invention; 
           [0016]      FIG. 9  is a perspective of the invention shown in  FIG. 8  with the majority of the unrelated printer components removed; 
           [0017]      FIG. 10  is a perspective of the components of the present invention shown in  FIG. 9  with unrelated components of the printer removed; 
           [0018]      FIG. 11  is an elevation showing the drive motor, lock arm and lock surface in isolation; 
           [0019]      FIG. 12  is the elevation of  FIG. 11  at the fully unlocked stage of its operating cycle and with one side of the lock arm removed; 
           [0020]      FIG. 13  is the elevation shown in  FIG. 11  at the re-locking stage of its operating cycle; 
           [0021]      FIG. 14  is a perspective of the drive motor, lock arm and lock surface at the fully unlocked stage of its operation; 
           [0022]      FIG. 15  is an elevation of one side of the lock arm and the lock surface in isolation; and, 
           [0023]      FIG. 16  is an elevation of the drive motor, lock arm and lock surface returned to the start of the operative cycle. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0024]      FIGS. 1 to 5  show one form of the sheet feed mechanism in a diagrammatic form for ease of understanding. The sheet feed mechanism  1  is typically used in a larger device such as a printer or the like and would likely have its chassis  2  integrated with that of the printer. The sheet feed mechanism  1  lifts the stack of sheets  4  to the picker roller  6  that draws a single sheet into the printer sheet feed path (not shown). Instead of a picker roller, the sheet feed mechanism could also lift the stack toward a suction shoe or other sheet engaging means. 
         [0025]    Referring to  FIG. 1 , the stack  4  is inserted into the designated part of the device such as the paper tray of the printer (not shown) while the lift arm  8  is in a lowered position. The lift arm  8  is biased upwards by the lift spring  10  but is held in the lowered position by the lock mechanism  12 . The lock mechanism  12  is at the distal end of the lock arm  14  which is hinged to the chassis  2  at the same hinge axis  16  as the lift arm  8 . The lock mechanism releasably secures the lock arm  14  to the lift arm  8  via the friction surface  18 . The lock mechanism  12  abuts the cam  20  to prevent the lock arm  14  and the lift arm  8  from rotating upwards because of the biasing force of the lift spring  10 . 
         [0026]    Referring to  FIG. 2 , the cam  20  rotates clockwise in response to a paper feed request signal from the printer. The cam  20  is positioned within a U-shaped member  22  of the lock mechanism  12 . The U-shaped member  22  is hinged to the lock arm  14  at the hinge  24 . The hinge  24  is on the cross piece  26  separating the engagement arm  28  and the disengagement arm  30  on either side of the ‘U’. The contact foot  32  is attached to the cross piece  26  on the opposite side of the lock hinge  24  to the disengagement arm  30  to form a first class lever. Rotating the cam  20  clockwise uses the friction generated between the cam  20  and the engagement arm  28  to urge the contact foot  32  into firmer engagement with the friction surface  18 . This helps to avoid any slippage between the contact foot and the friction surface before the cam  20  engages the disengagement arm  34 . Slippage can allow the lift arm  8  to press the top-most sheet  40  onto the picker roller  6  before other components in the printer feed path are ready to receive a sheet. 
         [0027]    As the cam  20  rotates out of engagement with the engagement arm  28 , the lift spring  10  pushes the lift arm  8 , locking surface  18  and locking arm  14  upwards until the bearing surface  34  abuts the stop  36  on the chassis  2 . The cam  20  continues to rotate until it contacts the disengagement arm  30 . Further rotation presses the disengagement arm  30  towards the bearing surface  34  against the bias of the lock spring  38 . This actuates the lever to lift the contact foot  32  out of engagement with the friction surface  18 . This unlocks the lift arm  8  from the lock arm  14 . This allows the lift spring  10  to elevate the stack  4  until the top-most sheet  40  engages the picker roller  6  and is drawn away from the remainder of the stack. 
         [0028]    Referring to  FIG. 3 , the cam  20  continues to rotate and allow the lock spring  38  to push the disengagement arm  30  away from the bearing surface  34 . This in turn re-engages the contact foot  32  with the friction surface  18  to lock the lock arm  14  and the lift arm  8  together. The picker roller  6  continues to draw the top-most sheet  40  from the stack  4 . 
         [0029]    Turning to  FIG. 4 , the cam  20  rotates into contact with the engagement arm  28  to add to the force with which the contact foot  32  presses onto the friction surface  18 . At this point, the cam  20  also starts to push the engagement arm  28  and therefore the lock arm  14  and lift arm  8  clockwise against the bias of the lift spring  10 . Accordingly, the stack  4  starts to drop away from the picker roller  6  before it draws the new top-most sheet  42  off the stack  4 . 
         [0030]      FIG. 5  shows the sheet feed mechanism at the completion of its operative cycle. The cam  20  rotates until the high point is in contact with the engagement arm  28 . This pushes the lock arm  14  and the lift arm  8  back through a set angle of rotation. In turn, the stack  4  retracts from the picker roller  6  by a predetermined distance. This distance does not alter regardless of the grade (or thickness) of paper in the stack. Because of this, the lift spring  10  need only compress a small amount and therefore the energy consumed by the mechanism as it indexes through the stack is reduced. Furthermore, as the stack  4  depletes, it weighs less but the spring  10  also decreases its force biasing the stack against the picker roller  6  because it is less compressed. This keeps the force pressing successive top-most sheets against the picker roller substantially uniform. 
         [0031]      FIG. 6  is a diagrammatic illustration of another embodiment of the sheet feed mechanism  1 . In this embodiment, the hinged lift arm is replaced with a lift structure  44  that has rectilinear movement instead of rotational. The friction surface  18  is on an arm that extends upwardly to be parallel with the direction of travel of the lift structure  44 . The lock arm  14  is again hinged to the chassis  2  and has a bearing surface  34  with lock spring  38  to bias the contact foot  32  into locking engagement with the friction surface  18 . The disengagement arm  30 , lock hinge  24  and the contact foot  32  again form a first class lever. 
         [0032]    The embodiment shown does not use a U-shaped member but instead configures the lock arm  14  to act as the engagement arm  28  as well. When the cam  20  contacts the engagement arm  28 , it rotates anti-clockwise about the hinge  16 . The contact foot  32  maintains locking engagement with the friction surface  18  because the spring  38  continues to bias the disengagement arm  30  in a clockwise direction despite the rotation of the engagement arm in an anti clockwise direction. 
         [0033]    In fact the bearing surface  34  rotating anti clockwise tends to maintain the gap bridged by the spring  38  so that the biasing force remains relatively uniform. 
         [0034]    The embodiment shown in  FIG. 6  demonstrates that the invention can adopt many different configurations to suit specific functional requirements and space limitations. Ordinary workers in this field will also appreciate that the cam may be replaced by the solenoid actuator or pneumatic/hydraulic actuators. Any dual action actuator that contacts the disengagement arm and the engagement arm in succession will be suitable for the purposes of this invention. 
         [0035]      FIG. 7  shows the invention incorporated into a SOHO printer. The printer  46  has a paper feed tray  48  for receiving a ream of blank paper (not shown). The paper feed assembly in the printer draws sheets sequentially from the stack placed in the feed tray  48  and directs it then through a C-shaped paper path past a printhead. After printing the pages are collected from a collection tray (not shown) on top of the feed tray  48 . 
         [0036]    The lift arm  8  is positioned directly beneath the picker roller  6  with the distal end  50  of the lift arm positioned beneath the leading edge of the stack of sheets (not shown). Initially the lifter arm is held in a fully depressed configuration so that its distal end is flush with the paper support platen  52  in the feed tray  48 . The lift arm  8  is forced into this initial position using the lift arm reset lever  54  described in greater detail below. 
         [0037]    Turning to  FIG. 8 , the feed tray and outer housing have been removed for clarity. Again the lift arm  8  is in its lowered initial position so that the distal end  50  lies beneath the leading edge of the paper stack. Coil spring  10  biases the lifter arm upwards about the hinge shaft  16 . However the lock mechanism (described below) holds the lifter arm in its initial position until the actuator responds to a request for a sheet. 
         [0038]    In  FIG. 9  more components of the printer have been removed to expose the lock mechanism. Hinge shaft  16  extends from the lifter arm  8  through the lock spring  10  to the locking assembly  56 . On the outer most end of the hinge shaft  16  is the reset arm  58 , which is connected to the reset lever  54  via the connecter rod  60 . The reset arm  58  is mounted to the hinge at shaft  16  via a ratchet engagement that locks the shaft and arm together when rotating clockwise that allows the arm to rotate anti-clockwise while the shaft remains fixed. In this way the user simply depresses the lifter arm reset lever  54  to draw down the reset arm  58  and therefore the lifter arm  8  against bias of the spring  10 . 
         [0039]    Also shown in  FIG. 9 , is the cam drive motor  62  with its output worm drive  64  meshed with the drive gear  66  mounted on the cam shaft  68 . One side of the lock arm  14  is also shown and this is described in greater detail below. 
         [0040]      FIG. 10  shows the feed mechanism with further components removed for clarity. The lock arm  14  has two side plates  70  and  72  mounted to the hinge shaft  16 . The distal ends of the side plates  70  and  72  are connected by the abutment block  74  positioned to abut the stop  36  secured to the printer chassis. Mounted between the side plates  70  and  72  is the arcuate friction arm  18  and the U-shaped member  22 . The side plates  70  and  72  are rotateably mounted to the hinge shaft  16  while the arcuate friction arm  18  is fixed to the shaft  16 . 
         [0041]    Referring to  FIG. 11 , the cam  20  is shown between the sides of the U-shaped member  22 . In response to a sheet feed request, the cam  20  starts rotating clockwise along the engagement arm  28 . It will be appreciated that the contact foot is urged into engagement with the arcuate friction arm  18  by any friction between the cam  20  and the engagement arm  28 . This is because the contact foot is between side plates  70  and  72  (not shown), to the right of the lock mechanism hinge  24 . Of course the lock spring  38  also pushes the contact foot into locking engagement. 
         [0042]      FIG. 12  shows the locking assembly in the unlocked condition. The locking assembly  56  is shown with the side plate  70  removed. The cam  20  has rotated to press against the disengagement arm  30  of the U-shaped member  22 . The cam  20  initially pushes the entire assembly  56  such that it rotates into engagement with the stop  36 . After engaging the stop  36  the cam then rotates the U-shaped member anti-clockwise about the lock mechanism hinge  24 . This lifts the contact foot  32 , or rather simply unweights it from the arcuate surface on the arcuate friction arm  18 . With the arcuate friction arm now free to rotate it is urged in an anti-clockwise direction by hinge shaft  16 . Hinge shaft  16  is under the torque provided by the lifter spring  10  (see  FIG. 10 ). Not shown in  FIG. 12  is the elevation of the paper stack by the lifter arm  8  once the arcuate friction arm has been unlocked. The lift arm  8  continues to elevate the stack of paper until the top most sheet engages the picker roller  6 . 
         [0043]      FIG. 14  shows the locking assembly in its unlocked condition in perspective. The U-shaped member  22  is rotated about the lock mechanism hinge  24  such that the disengagement arm  30  compresses the lock spring  38  against the abutment block  74 . The contact foot  32  is levered out the engagement from the arcuate friction arm  18  to allow the lift arm  8  (see  FIG. 10 ) to raise the paper stack. 
         [0044]      FIG. 13  shows the locking mechanism  56  as the U-shaped member returns to the lock position. The cam  20  continues to rotate clockwise and allows the U-shaped member  22  to also rotate under the action of the lock spring  38 . It should be noted that at this stage abutment block  74  is still against the stop  36 . Furthermore, the paper stack is still pressed against the picker roller, which would still be drawing the top most sheet from the stack. 
         [0045]    The locked configuration of the U-shaped member  22  and the arcuate friction arm  18  is best shown in  FIG. 15 . It can be clearly seen that the disengagement arm  30 , the lock mechanism hinge  24  and the contact foot  32  form a first class lever whereby the biasing force of the lock spring  38  is amplified at the contact foot  32  by virtue of the mechanical advantage provided by the lever. 
         [0046]      FIG. 16  shows the locking assembly returned to its initial configuration. The cam  20  has rotated back into engagement with the engagement arm  28  to rotate the entire assembly  56  about the hinge shaft  16 , a small distance away from the stop  36 . As the arcuate friction arm  18  and the lock arm  14  are now locked together the hinge shaft  16  is forced to rotate by the cam shaft  20 . This in turn rotates the lift arm  8  (see  FIG. 10 ) then by retracting the paper stack a small distance from the picker roller  6 . As the cam need only retract paper a very small distance from the surface of the picker roller in order to prevent it from drawing more sheets from the stack, the power load on the cam drive motor  62  is relatively low. Furthermore, the distance that the stack retracts from the thicker roller will always remain uniform regardless of the grade of paper inserted in paper feed tray. This improves the versatility of the overall feed mechanism. 
         [0047]    The invention has been described here by way of example only. Still workers in this field will readily recognise many variations and modifications, which do not depart from the spirit and scope of the broad invented concept.